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The Sheahan Diamond Literature Reference Compilation - Technical Articles based on Major Region - India
The Sheahan Diamond Literature Reference Compilation is compiled by Patricia Sheahan who publishes on a monthly basis a list of new scientific articles related to diamonds as well as media coverage and corporate announcements called the Sheahan Diamond Literature Service that is distributed as a free pdf to a list of followers. Pat has kindly agreed to allow her work to be made available as an online digital resource at Kaiser Research Online so that a broader community interested in diamonds and related geology can benefit. The references are for personal use information purposes only; when available a link is provided to an online location where the full article can be accessed or purchased directly. Reproduction of this compilation in part or in whole without permission from the Sheahan Diamond Literature Service is strictly prohibited. Return to Diamond Region Index
Sheahan Diamond Literature Reference Compilation - Scientific Articles by Author for all years
Each article reference in the SDLRC is tagged with one or more key words assigned by Pat Sheahan to highlight the main topics of the article. In addition most references have been tagged with one or more region words. In an effort to make it easier for users to track down articles related to a specific region, KRO has extracted these region words and developed a list of major region words presented in the Major Region Index to which individual region words used in the article reference have been assigned. Each individual Region Report contains in chronological order all the references with a region word associated with the Major Region word. Depending on the total for each reference type - technical, media and corporate - the references will be either in their own technical, media or corporate Region Report, or combined in a single report. Where there is a significant number of technical references there will be a technical report dedicated to the technical articles while the media and corporate references are combined in a separate region report. References that were added in the most recent monthly update are highlighted in yellow within the Region Report. The Major Region words have been defined by a scale system of "general", "continent", "country", "state or province" and "regional". Major Region words at the smaller scales have been created only when there are enough references to make isolating them worthwhile. References not tagged with a Region are excluded, and articles with a region word not matched with a Major Region show up in the "Unknown" report.
Kimberlite - diamondiferous
Lamproite - diamondiferous
Lamprophyre - diamondiferous
Other - diamondiferous
Kimberlite - non diamondiferous
Lamproite - non diamondiferous
Lamprophyre - non diamondiferous
Other - non diamondiferous
Kimberlite - unknown
Lamproite - unknown
Lamprophyre - unknown
Other - unknown
Future Mine
Current Mine
Former Mine
Click on icon for details about each occurrence. Works best with Google Chrome.
CITATION: Faure, S, 2010, World Kimberlites CONSOREM Database (Version 3), Consortium de Recherche en Exploration Minérale CONSOREM, Université du Québec à Montréal, Numerical Database on consorem.ca. NOTE: This publicly available database results of a compilation of other public databases, scientific and governmental publications and maps, and various data from exploration companies reports or Web sites, If you notice errors, have additional kimberlite localizations that should be included in this database, or have any comments and suggestions, please contact the author specifying the ID of the kimberlite: [email protected]
Examination of a Remarkable Red Sandstone from the Junction of the Diamond Limestone and Sandstone at Nurmoor in the Kurnool Territory, Southern India.
Asiatic Soc. Bengal Journal, Vol. 13, PP. 336-338.
Gems and Precious Stones. Lectures on the Results of the Great Exhibition of 1851, Delivered Before the Society of Arts, Manufacturers, and Commerce at the Suggestion of H.r.h. Prince Albert.
The Applications of Geology to the Arts and Manufacturers. Being Six Lectures on Practical Geology, Delivered Before The Society of Arts, As a Part of the Cantor Series of Lectures for 1865.
The Koh-i-nur Diamond its Romance and History with Special Notes by Her Majesty the Queen. Also the Curious History of The Celebrated Pitt Diamond with Special Notes by the Ex-empress Eugenie.
Report on the Preliminary Investigation for Diamonds in The banganapalle Conglomerate and Tungabhadra Krishna River Gravels in Parts of Kurnool and Mahaboolonagar Districts.
India Geological Survey Program Report, FOR 1967-1968
A Report on the Diamond Investigation, Krishna Gravel Areas and Reconnaissance Work in Srisailam and Penner River Areas, Krishna, Kurnool and Cuddapah Districts
India Geological Survey Program Report, FOR 1969-1970
A Report on the Assessment of Diamond Resources, Krishna River Gravels and Reconnaissance Work of Ustapalle and Mellwalli Areas, Krishna District, A.p.
India Geological Survey Program Report, FOR 1970-1971
Gem News. the Golconda "d" Diamond Resurfaces. John Sinkankas Provides Information on Diamonds in Thailand. a Diamond Is Melted in Laboratory. Punch Jones Diamond.
Gems And Gemology, Vol. 32, WINTER PP. 242-243.
India, Thailand, Russia, United States, Great Lakes
Gems news:India -Tanna and Chatapur areas. Japan -largestsyntheticdiamond. South Africa - Diamond inclusions in pyrope.Sri Lanka - geological exploration d
Gems and Gemology, Vol. 22, No. 1, Spring pp. 54-55
Relationships between crustal contamination and crystallization in continental flood basalt magmas with special reference to the Deccan Traps Of the Westrn Ghats, In
Earth and Planetary Science Letters, Vol. 84, No. 1, June pp. 59-68
Investigation for apatite and other associated minerals in the Tamar-Porapahar shear zone in Perulia district and the petrological studies of Association carbonatites
Records of the Geological Survey of India, extended abstracts of progress, Vol. 122, pt. 3, p. 45
Study of the carbonatite complex in Ampavalli area near Sunki, Koraputdistrict, and Khariar area in Kalahandidistrict, Orissa with special ref. toR.E.E.
Records of the Geological Survey of India, extended abstracts of progress, Vol. 122, pt. 3, p. 47
Mineralogical composition and geographical distribution of African and Brazilian periatlantic laterites. the influence of continental drift and tropical paleoclimes
Journal of Sth. African Earth Sciences, Vol. pp. 283-295
A note on the appearance of inclusions in garnets supposedly from India and problems relating to disclosure of origin: using this as an elementary example.
The South African Gemologist, Vol. 6, No. 2, June, pp. 11-13.
Preliminary survey and assessment for locating source rocks and to find out potential area for diamond occurrence in Mahanadi, Ong, Tel and Suktel basins...
Geological Society of India Records, Vol. 131,3, pp.229-31.
Regional survey to identify potential blocks for occurrence of kimberlite/lamproite pipes in Indravati River Basin, Koraput and Nabarangapur Districts, Orissa
Records of the Geological Survey of India, Vol. 133, 3, eastern 1998-1999, pp.191-3.
Experimental determination of calcite solubility in H2O NaCl solutions at deep crust/upper mantle pressures and temperatures: implications for metasomatic processes
Comments on: pattern of occurrence of kimberlite pipes based on gravity and magnetic anomalies in Wajrakarur Lattavaram region, Andhra Pradesh by Vasanthi and Mallic
Journal of the Geological Society of India, Vol. 60, 4, Sept. pp. 350-352.
Regional survey to identify potential blocks for occurrence of kimberlite/lamproite pipes in Indravati Basin, Koraput and Nawarangpur district, Orissa.
Records of the Geological Survey of India, Vol. 132, 3, eastern 1997-1998, pp.209-11.
A petrological and geochemical appraisal of the Mesoproterozoic Diamondiferous Majhgawan pipe of central India: evidence for transitional kimberlite - orangeite ( group II kimberlite) - lamproite rock type.
Evidence for Proterozoic collision from airborne magnetic and gravity studies in s Granulite terrain, signatures of recent tectonic activity in Palghat Gap.
Roy, A., Sarkar, A., Jeyakumar, S., Aggrawal, S.K., Ebihara, M., Satoh, H.
Late Archean mantle metasomatism below eastern Indian Craton: evidence from trace elements, REE geochemistry and Sr Nd O isotope systematics of ultramafic dykes.
Proceedings National Academy of Sciences India , Vol. 113, 4, pp. 649-666. Ingenta 1045680437
Roy, A., Sarkar, A., Jeyakumar, S., Aggrawal, S.K., Ebihara, M., Satoh, H.
Late Archean mantle metasomatism below eastern Indian craton: evidence from trace elements, REE geochemistry and Sr Nd O isotope systematics of ultramafic dykes.
Proceedings National Academy of Sciences India , Vol. 113, 4, pp. 649-665.
Santosj, M., Tanaka, K., Yokoyama, K., Collins, A.S.
Late Neoproterozoic Cambrian felsic magmatism along transcrustral shear zones in southern India: U Pb electron microprobe ages implications for amalagamtion
Srivastava, R.K., Heaman, L.M., Sinha, A.K., Shihua, S.
Emplacement age and isotope geochemistry of Sung Valley alkaline carbonatite complex, Shillong Plateau, northeastern India: implications for primary carbonate
Electrical signatures due to thermal anomalies along mobile belts reactivated by the trail and outburst of mantle plume: evidences from the Indian subcontinent.
Chalapathi Rao, N.V., Burgess, R., Anand, M., Mainkar, D.
Evidence for a Phanerozoic (478 Ma) Diamondiferous kimberlite emplacement epoch in the Indian Shield from 40 Ar/ 39Ar dating of the Kodomali kimberlite: implications ....
Geological Society of India, Bangalore November Meeting Group Discussion on Kimberlites and Related Rocks India, Abstract p. 103-106.
India, Bastar Craton, Rodinia
Tectonics - Kodomali, Pan African , Geothermometry
Geochemistry and origin of the Proterozoic kimberlites, ultramafic and ultrapotassic magmatic rocks from Indravati Basin in Bastar Craton, central India.
Geological Society of India, Bangalore November Meeting Group Discussion on Kimberlites and Related Rocks India, Abstract p. 94-97.
Dongre, A., Kamde, G., Chalapathi Rao, N.V., Kale, H.S.
Is megacrystic/xenocrystic ilmenite entrainment in the source magma responsible for the non-Diamondiferous nature of the Maddur-Kotakonda-Narayanpet kimberlites
Geological Society of India, Bangalore November Meeting Group Discussion on Kimberlites and Related Rocks India, Abstract p. 72.
Geology, petrology, geochemistry and mineral chemistry of new kimberlite fields in the Wajrakarur kimberlite field, Anantapur district, Andhra Pradesh.
National Seminar on Exploration Survey, Geological Society of India Special Publication, No. 58, pp. 593-602.
Mesoproterozoic Diamondiferous ultramafic pipes at Majkgawan and Hinota, Panna area, central India: key to the nature of sub-continental lithospheric mantle...
Journal of Earth System Science, Vol. 115, 1m pp. 161-183.
Sharma, R., Muthry, Ch.V.V.S., Nagaraju, B.V., Gouda, H.C., Singh, R.K.
Interpretation of aeromagnetic dat a of Panna and adjoining areas for evaluating of structural patterns favourable for emplacement of KCRs and depth magnetics
Geological Society of India, Bangalore November Meeting Group Discussion on Kimberlites and Related Rocks India, Abstract p. 121-122.
India, Madhya Pradesh, Aravalli Bundelkhand Craton
Geochemistry and petrogenesis of neoarchean high Mg low Ti mafic igneous rocks in an intracratonic setting, central India craton: evidence for boninite magmatism.
Geochemistry and petrogenesis of neoArchean high Mg low Ti mafic igneous rocks in and intracratonic setting, Central India craton: evidence for boninite magmatism.
The Jungel Valley re-visited: evidence from the lamprophyres for the presence of a Paleoproterozoic carbonate rich metasomatised mantle in Mahakoshal belt
Geological Society of India, Bangalore November Meeting Group Discussion on Kimberlites and Related Rocks India, Abstract p. 123.
India, Madhya Pradesh, Aravalli Bundelkhand Craton
Electrical signatures due to thermal anomalies along mobile belts reactivated by the trail and outburst of mantle plume: evidence from the Indian subcontinent.
Journal of Applied Geophysics, Vol. 58, 4, April, pp. 313-320.
Hetenyl, G., Cattin, R., Brunet, F., Bollinger, L., Vergne, J., Nabalek, J.L., Diament, M.
Density distribution of the India plate beneath the Tibetan plateau: geophysical and petrological constraints on kinetics of lower crustal eclogitization
Earth and Planetary Science Letters, Vol. 264, 1-2, pp. 226-244.
Building blocks and crustal archtitecture of Indian peninsular shield: cratons and fold belts and their interaction based on geophysical and geological data.
Journal of the Geological Society of India, Vol. 68, 6, pp. 1037-1057.
Petrology, geochemistry and tectonic significance of Paleoproterozoic alkaline lamprophyres from the Jungel Valley, Mahakostal supracrustal belt, Central India.
Mineralogy and Petrology, Vol. 89, 3-4, pp. 189-215.
Geochemistry of distinct mafic intrusive rocks from Darba-Kukanar and Kerlapal-Sukma-Mokhp southern Bastar Craton: further dat a on the Early Precambrian mafic magmatism of central India.
Journal of the Geological Society of India, Vol. 69, 6, pp. 1176-1188.
Babu, E.V.S.S.K., Griffin, W.L., Mukherjee, A., O'Reilly, S.Y., Belousova, E.A.
Combined U Pb and Lu Hf analysis of megacrystic zircons from the Kalyandurg 4 kimberlite pipe, S. India: implications for the emplacement age and HF isotopic..
New policy likely to be unfolded by the Indian Ministry of Mines is set to attract a foreign direct investment (FDI) of INR5 Trillion within 5 - 6 years.
Petrology and geochemistry of Diamondiferous Mesoproterozoic kimberlites from Wajrakarur kimberlite field, eastern Dharwar craton, southern India: genesis and constraints on mantle
Contributions to Mineralogy and Petrology, Vol. 157, 2, pp. 245-265.
Das, S., Nasipuri, P., Bhattachaya, A., Swaminathan, S.
The thrust contact between the Eastern Ghats belt and the adjoining Bastar craton, Eastern India: evidence from mafic granulites and tectonic implications.
Training in diamond exploration. Course organized by Geological Society of India , Rio Tinto India et al. Leaders Mitchell and Coopersmith. Brief overview.
Journal of Geological Society of India, Vol. 71, April pp. 453=458.
Alkaline magmatism along the southeastern margin of the Indian shield: implications for regional geodynamics and constraints on craton-eastern Ghats belt suturing.
Griffin, W.L., Kobussen, A.F., Babu, E.V.S.S.K., O'Reilly, S.Y., Norris, R., Sengupta, P.
A translithospheric suture in the vanished 1 Ga lithospheric root of South India: evidence from contrasting lithospheric sections in the Dharwar Craton.
Chromite and PGE deposits of Mesoarchean ultramafic mafic suites within the greenstone belts of the Singhbhum Craton, India: implications for mantle heterogeneity and tectonic setting.
Journal of the Geological Society of India, Vol. 73, 1, pp. 36-51.
Srivastava, R.K., Chalapathi Rao, N.V., Sinha, A.K.
Cretaceous potassic intrusives with affinities to aillikites from Jharia area: magmatic expression of metasomatically veined and thinned lithospheric mantle
Srivastava, R.K., Chalapathi Rao, N.V., Sinha, A.K.
Cretaceous potassic intrusives with affinities to aillikites from Jharia area: magmatic expression of metasomatically veined and thinned lithospheric mantle
Chalapathi Rao, N.V., Anand, M., Dongre, A., Osborne, I.
Carbonate xenoliths hosted by the Mesoproterozoic Siddanpalli kimberlite cluster ( Eastern Dharwar craton): implications for the geodynamic evolution of
International Journal of Earth Sciences, Vol. 99, pp. 1791-1804.
Mineralogy and geochemistry of kimberlites NK-2 and KK-6 Narayanpet kimberlite field, eastern Dharwar Craton, southern India: evidence for transitional ...
The Canadian Mineralogist, Vol. 47, 5, pp. 1117-1135,
Chalapathi Rao, N.V., Lehmann, B., Mainkar, D., Belyatsky, B.
Petrogenesis of the end Cretaceous Diamondiferous Behradih kimberlite, central India: implication for the plume lithosphere interactions in the Bastar craton?
International Dyke Conference Held Feb. 6, India, 1p. Abstract
Chalapathi Rao, N.V., Lehmann, B., Mainkar, D., Belyatsky, B.
Petrogenesis of the end Cretaceous Diamondiferous Behradih orangeite pipe: implications for mantle plume - lithosphere interaction in the Bastar craton, India.
Contributions to Mineralogy and Petrology, Vol. 161, pp. 721-742.
Metamorphic evolution of the Naga Hills eclogite and blueschist northeast India: implications for early subduction of the Indian Plate under Burma microplate
Journal of Metamorphic Geology, Vol. 28, 2, pp. 209-225.
Eastern promise... financial,political and industrial developments have seen China, India and Japan's fortunes rise. Rivalry... constructive or destructive?
Griffin, W.L., Kobussen, A.F., Babu, E.V.S.S.K., O'Reilly, S.Y., Norris, R., Sengupta, P.
A translithospheric suture in the vanished 1 Ga lithospheric root of South India: evidence from contrasting lithosphere sections in the Dharwar craton.
Brief - comments on a meeting held by Rio Tinto on their Diamond & Minerals.... regrouped in a new division but still targeted for growth ( India mentioned).
Geochemistry of mafic dikes in the Singhbhum Orissa craton: implications for subuction related metasomatism of the mantle beneath the eastern Indian craton.
International Geology Review, Vol. 52, 1, pp. 79-94.
Major element composition of concentrate garnets in Proterozoic kimberlites from the eastern Dharwar Craton, India: implications on sub-continental lithospheric mantle.
Journal of Asian Earth Sciences, Vol. 39, 6, pp. 578-588.
India's changing place in global Proterozoic reconstructions: a review of geochronologic constraints and paleomagnetic poles from the Dharwar Bundelk hand and Marwar
Journal of Geodynamics, Vol. 50, 3-4, pp. 224-242.
Ratre, K., De Waele, B., Kumar, Biswal, T., Sinha, S.
Shrimp geochronology for the 1450 Ma Lakhna dyke swarm: its implication for the presence of Eoarchean crust in the Bastar Craton and the 1450-517 Ma depositional age
Journal of Asian Earth Sciences, Vol. 39, 6, pp. 565-577.
Saha, L., Pant, N.C., Pati, J.K., Upadhyay, D., Berndt, J., Bhattacharya, A., Satynarayanan, M.
Neoarchean high pressure margarite phengite muscovite chlorite corona mantle corundum in quartz free high Mg, Al phlogopite chlorite schists from the Bundelkhand
Contributions to Mineralogy and Petrology, in press available, 20p.
Petrology and mineral chemistry of a major picrite dyke from Peddakudala Velpula area, in southwestern part of Proterozoic Cuddapah Basin, Andhra Pradesh, India.
International Dyke Conference Held Feb. 6, India, 1p. Abstract
Map of distinct early Precambrian mafic dyke swarms from the central Indian Bastar Craton and their possible relation with Paleosupercontinent and Large Igneous Province.
International Dyke Conference Held Feb. 6, India, 1p. Abstract
Bose, S., Dunkley, D.J., Dasgupta, S., Das, K., Arima, M.
India-Antarctica-Australia-Laurentia connection in the Paleoproterozoic-Mesoproterozoic revisited: evidence from new zircon U Pb and monzazite chemical age data
Geological Society of America Bulletin, Vol. 123, 9/10 pp. 2031-2049.
Enigmatic association of the carbonatite and alkali pyroxenite along the Northern Shear Zone, Purulia, West Bengal: a saga of primary magmatic carbonatite.
Journal of Geological Society of India, Vol. 76, 5, pp.399-402.
Geochemistry of alkaline basalts and associated high Mg basalts from the 2.7 Ga Penakacherla Terrane, Dharwar Craton, India: an Archean depleted mantle OIB array
Chalapathi Rao, N.V., Lehmann, B., Belousova, E., Frei, D., Mainkar, D.
Petrology, bulk rock geochemistry, indicator mineral composition and zircon U-Pb geochronology of the end Cretaceous Diamondiferous Mainpur orangeites, Bastar Craton, Central India.
10th. International Kimberlite Conference Feb. 6-11, Bangalore India, Abstract
Origin and diamond prospectivity of Mesoproterozoic kimberlites from the Narayanpet field, eastern Dharwar Craton southern India: insights from groundmass mineralogy, bulk chemistry and perovskite oxybarometry.
The geology and geochemistry of the Wadagera kimberlite and the characteristics of the underlying subcontinental lithospheric mantle, Dharwar Craton, India
10th. International Kimberlite Conference Feb. 6-11, Bangalore India, Abstract
Paleomagnetic and geochronological studies of the mafic dyke swarms of Bundelk hand craton, central India: implications for the tectonic evolution and paleogeographic reconstructions.
PGE geochemistry of Diamondiferous and non-Diamondiferous kimberlites from eastern Dharwar craton, southern India: implications for understanding the nature of the mantle below Dharwar.
10th. International Kimberlite Conference Held Bangalore India Feb. 6-11, Poster abstract
New paleomagnetic and rock magnetic results on Mesoproterozoic kimberlites from the Eastern Dharwar craton, southern India: towards constraining India's position in Rodinia.
Petrology, bulk-rock geochemistry, indicator mineral composition and zircon U-Pb geochronology of the end-Cretaceous Diamondiferous Mainpur orangeites, Bastar craton, central India.
Proceedings of the 10th. International Kimberlite Conference, Vol. 1, Special Issue of the Journal of the Geological Society of India,, Vol. 1, pp. 93-121.
Platinum group elements (PGE) geochemistry of Deccan orangeites, Bastar craton, central India: implication for a non-terrestrial origin for irridium enrichment at the K-Pg boundary.
Journal of Asian Earth Sciences, Vol. 84, pp. 24-33.
Mesoproterozoic U-Pb ages, trace element and Sr-Nd isotopic composition of perovskite from kimberlites of the Eastern Dharwar craton, southern India: distinct mantle sources and a Wide spread 1.1 Ga Tectonomagmatic event.
Tuffisitic kimberlite from Eastern Dharwar craton, Undraldoddi area, Raichur District, Karnataka, India.
Proceedings of the 10th. International Kimberlite Conference, Vol. 2, Special Issue of the Journal of the Geological Society of India,, Vol. 2, pp. 109-128.
Isotopic and trace element geochemistry of alkalic mafic ultramafic carbonatitic complexes and flood basalts in NE India: origin in a heterogeneous Kerguelen plume.
Geochimica et Cosmochimica Acta, Vol. 115, pp. 46-72.
Petrology of P-5 and P-13 kimberlites from Lattavaram kimberlite cluster, Wajrakarur kimberlite field, Andhra Pradesh, India: reclassification as lamproites.
Proceedings of the 10th. International Kimberlite Conference, Vol. 1, Special Issue of the Journal of the Geological Society of India,, Vol. 1, pp. 183-194.
The geology and geochemistry of the Wadagera kimberlite and the characteristics of the underlying subcontinental lithospheric mantle, Dharwar craton, India.
Proceedings of the 10th. International Kimberlite Conference, Vol. 1, Special Issue of the Journal of the Geological Society of India,, Vol. 1, pp. 167-181.
Diamonds from the Behradih kimberlite pipe, Bastar craton, India: a reconnaissance study.
Proceedings of the 10th. International Kimberlite Conference, Vol. 1, Special Issue of the Journal of the Geological Society of India,, Vol. 1, pp. 309-316.
Mandal, N., Charavarty, K.H., Borah, K., Rai, S.S.
Is a cation ordering transition of the Mg-Fe olivine phase in the mantle responsible for the shallow mantle seismic discontinuity beneath the Indian craton?
Radhakrishna, T., Krishnendu, N.R., Balasubramonian, G.
Nd-Hf isotope systematics of megacrysts from the Mbuji-Mayi kimberlites, D.R. Congo: evidence for a metasomatic origin related to kimberlite interaction with the cratonic lithosphere mantle.
Petrology of lamproites from the Nuapada lamproite field, Bastar craton, India.
Proceedings of the 10th. International Kimberlite Conference, Vol. 1, Special Issue of the Journal of the Geological Society of India,, Vol. 1, pp. 137-165.
Smith, C.B., Haggerty, S.E., Chatterjee, B., Beard, A., Townend, R.
Kimberlite, lamproite, ultramafic lamprophyre, and carbonatite relationships on the Dharwar Craton, India: an example from the Khaderpet pipe, a Diamondiferous ultramafic with associated carbonatite intrusion.
Smith, C.B., Haggerty, S.E., Chatterjee, B., Beard, A., Townend, R.
Kimberlite, lamproite, ultramafic lamprophyre, carbonatite relationships on the Dharwar Craton, India; and example from the Khaderpet pipe, a Diamondiferous ultramafic with associated carbonatite intrusion.
Lamproites from the eastern margin of the Bhandara craton, Orissa, India: an exploration case study.
Proceedings of the 10th. International Kimberlite Conference, Vol. 2, Special Issue of the Journal of the Geological Society of India,, Vol. 2, pp. 129-141.
Inetgration of geophysical and geological dat a of kimberlites in Narayanpet-Maddur field, Andhra Pradesh, India.
Proceedings of the 10th. International Kimberlite Conference, Vol. 2, Special Issue of the Journal of the Geological Society of India,, Vol. 2, pp. 229-239.
New paleomagnetic and rock magnetic results on Mesoproterozoic kimberlites from the Eastern Dharwar craton, southern India: towards constraining India's position in Rodinia.
Qualitative analysis of mafic dyke swarms and kimberlites from morphological and geophysical signatures, NW of Proterozoic Cuddapah basin, eastern Dharwar craton.
Journal of the Geological Society of India, Vol. 83, 3, pp. 235-251.
Chalapathi Rao, N.V., Kumar, A., Sahoo, S., Dongre, A.N., Talukdar, D.
Petrology and petrogenesis of Mesoproterozoic lamproites from the Ramadugu field NW margin of the Cuddapah basin, eastern Dharwar craton, southern India.
Platinum-group elements ( PGE) geochemistry of Deccan orangeites, Bastar craton, central India: implication for a non-terrestrial origin for iridium enrichment at the K-Pg boundary.
Journal of Asian Earth Sciences, Vol. 84, Apr. 15, pp. 24-33.
Paleo/Mesoproterozoic tectonism in the northern fringe of the western Dharwar craton ( India): its relevance to Gondwanaland and Columbia supercontinent reconstructions.
Geological Society of London Special Publication: Continent formation through time., No. 389, pp. 135-163.
India
Geotectonics
Abstract: Field and geochemical studies combined with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating set important constraints on the timing and petrogenesis of volcanic rocks of the Neoarchaean Kadiri greenstone belt and the mechanism of crust formation in the eastern Dharwar craton (EDC). The volcanic rocks are divided into three suites: tholeiitic basalts, calc-alkaline high-Mg# andesites and dominant dacites-rhyolites. The basalts (pillowed in places) show flat rare earth element (REE) and primordial mantle-normalized trace element patterns, but have minor negative Nb and Ta anomalies. They are interpreted as mantle plume-related oceanic plateau basalts whose source contained minor continental crustal input. The andesites are characterized by high Mg# (0.66-0.52), Cr and Ni, with depletion of high-field strength elements (HFSE) and enrichment of light REE (LREE) and large-ion lithophile elements (LILE). They were probably derived from a metasomatized mantle wedge overlying a subducted slab in a continental margin subduction zone. The dacites-rhyolites are silicic rocks (SiO2 = 61-72 wt%) with low Cr and Ni, K2O/Na2O mostly 0.5-1.1, highly fractionated REE patterns, enrichments of LILE and distinctly negative HFSE anomalies. One rhyolite sample yielded a zircon U-Pb age of 2353 ± 32 Ma. This suite is similar to potassic adakites and is explained as the product of deep melting of thickened crust in the arc with a significant older crustal component. Collision between a continental margin arc with an oceanic plateau followed by slab break-off, upwelling of hot asthenosphere and extensive crustal reworking in a sustained compressional regime is proposed for the geodynamic evolution of the area. This is in corroboration with the scenario of EDC as a Neoarchaean hot orogen as suggested recently by some workers.
Journal of the Geological Society of India, Vol. 85, 5, pp. 537-546.
India, Rajasthan
Calciocarbonatite
Abstract: A wide spectrum of calciocarbonatites and associated alkaline rocks are exposed around Kamthai, Rajasthan. The mineralogical studies exhibit a bimodal distribution of REE minerals. The southeastern block has carbocernaite and the eastern block has bastnaesite ± ancylite / synchysite as the dominant REE minerals followed by parisite and other accessory minerals. Calcite is the most abundant of the gangue phase, followed by biotite, albite, k-feldspar and iron oxide / hydroxide. Minor and trace gangue phases include pyrite, ilmenite, apatite, siderite, ankerite, amphibole, pyroxene, strontianite, barite, ilmenite / pyrophanite, celestine, clay minerals, sphalerite, pyrochlore, fluorite and Mn-rich phases including hollandite and Mn-Fe oxides. Calciocarbonatite occuring as intrusive veins, sills/dykes and plug, is perhaps a product of crystallization of a primary carbonatite melt generated at upper mantle. The first phase magmatic calciocarbonatite is alvikite type, rich in carbocernaite whereas second hydrothermal phase, sovite type, is enriched in bastnaesite. The absence of supergene activity and minerals (crandalite, florencite, gorceixite) indicate minor role of secondary enrichment. A rift-related mechanism, thermal equivalent to Deccan flood basalt (65±2 Ma), invoking "Reunion plume - continental hot spot" might have triggered Tertiary alkaline magmatism.
Abstract: We report mineral chemistry and whole-rock major and trace-element geochemistry for a recent find of Mesoproterozoic (~1.4 Ga) lamproites from the Garledinne (Banganapalle) cluster, south-western part of the Paleo-Mesoproterozoic Cuddapah Basin, southern India. The Garledinne lamproites occur as WNW-ESE-trending dykes that have undergone varying degree of pervasive silicification and carbonate alteration. Nevertheless, their overall texture and relict mineralogy remain intact and provide important insights into the nature of their magmas. The lamproite dykes have porphyritic to weakly porphyritic textures comprising pseudomorphed olivine macrocrysts and microphenocrysts, titanian phlogopite microphenocrysts, spinel having a compositional range from chromite to rarely magnesiochromite, Sr-rich apatite and niobian rutile. The Garledinne and other Cuddapah Basin lamproites (Chelima and Zangamarajupalle) collectively lack sanidine, clinopyroxene, potassic richterite, and titanite and are thus mineralogically distinct from the nearby Mesoproterozoic lamproites (Krishna and Ramadugu) in the Eastern Dharwar Craton, southern India. The strong correlation between various major and trace elements coupled with high abundances of incompatible and compatible trace elements imply that alteration and crustal contamination have had a limited effect on the whole-rock geochemistry (apart from K2O and CaO) of the Garledinne lamproites and that olivine fractionation played an important role in their evolution. The Garledinne lamproites represent small-degree partial melts derived from a refractory (previously melt extracted) peridotitic mantle source that was subsequently metasomatised (enriched) by carbonate-rich fluids/melts within the garnet stability field. The involvement of multiple reservoirs (sub-continental lithospheric mantle and asthenosphere) has been inferred in their genesis. The emplacement of the Garledinne lamproites is linked to extensional events, across the various Indian cratons, related to the break-up of the Proterozoic supercontinent of Columbia.
Subduction related origin of eclogite xenoliths from the Wajrakarur kimberlite field, Eastern Dharwar craton, southern India: constraints from petrology and geochemistry.
Geochimica et Cosmochimica Acta, Vol. 166, pp. 165-188.
Abstract: The P-12 "para-kimberlite" from Wajrakarur consists of forsteritic olivine, Al-Na-poor diopside, Fe-Ti-rich, Al-poor phlogopite, K-Ti-richterite, spinel, perovskite, cymrite, apatite, barite, Ba-Sr- bearing calcite, gittinsite, witherite, strontianite, and hydrogrossular (hydrogarnet). The rock also contains small clasts consisting dominantly of calcite, with lesser Ba-Sr-bearing calcite, cymrite, barite, strontianite, witherite, apatite, and hydrogrossular. Two generations of forsteritic olivine (Fo80-93) crystals are present: common phenocrystal-to-microphenocrystal; and rare anhedral macrocrystic olivines. Phlogopite occurs as microphenocrysts and as groundmass poikilitic plates with inclusions of spinel, perovskite, apatite, and chlorite pseudomorphs (after pyroxene). Phlogopites also occur as reaction rims around olivine crystals. The phlogopites have extremely low Al2O3 (2.2-3.8 wt.%), moderate-to-high FeO (6.9-16 wt.%), TiO2(1.9-4.6 wt.%), and Na2O (0.4-2.7 wt.%) contents and are enriched in fluorine (up to 6.0 wt.%) and considered to be tetraferriphlogopite. The pyroxenes occur in five parageneses as: (1) phenocrysts and microphenocrysts; (2) small slender crystals(<30 µm) forming part of the groundmass; (3) the cores of richterite crystals; (4) reaction products replacing earlier-formed olivine; (5) acicular crystals mantling carbonate clasts. These pyroxenes do not differ significantly in composition and are all diopsides with minor variation in their TiO2, Al2O3, Na2O contents. Titanian-potassium richterite commonly occurs as: (1) groundmass poikilitic plates; (2) small prismatic crystals (<30 µm); (3) reaction rims on olivine and pyroxene crystals. Groundmass poikilitic richterites commonly enclose pyroxene and apatite. Perovskites have a bimodal size distribution. Small (<20 µm) euhedral perovskites are scattered throughout the groundmass, whereas larger (100-300 µm) subhedral-to-euhedral perovskites are patchily-zoned and commonly broken. Micro-clasts consisting of accumulations of perovskite with phlogopite and apatite are also present. Spinels occur as large atoll crystals and small (<20 µm), euhedral-to-subhedral crystals, scattered throughout the groundmass. Some small spinel crystals are also present in the rims of olivine and pyroxene crystals. Atoll spinels are up to 100 µm in size, commonly with single and double cores. Atoll spinels are typically associated with perovskites. The euhedral-to-subhedral small spinels are ulvospinels. The atoll spinels have cores of titanian aluminous magnesiochromite with rims of magnesian titaniferous magnetite. The spinels have compositions which evolve along the lamproite-spinel compositional trend. Zoned calcite crystals occur as residual phases. Late stage residual calcite and carbonate clasts host prismatic cymrite crystals which are interpreted as pseudomorphs after potassium feldspar and/or barite. Subhedral-to-euhedral gittinsite and its Sr-analog are reported for the first time from the groundmass carbonate-chlorite mesostasis of a lamproite. Square-to-rectangular crystals of cymrite and hydrogrossular occur in the carbonate clasts and groundmass material. Barite anhedra commonly occur in the carbonate clasts together with witherite, strontianite, and Ba-Sr-bearing calcite. The texture and compositions of olivine, phlogopite, spinel, and K-Ti-richterite, together with the presence of cymrite pseudomorphs, possibly after potassium feldspar, demonstrate that this intrusion is a bona fide olivine lamproite and not a kimberlite. It is postulated that this, and other lamproites, located adjacent to the Eastern Ghats Mobile Belt, are derived by extensional decompressional melting of ancient subduction zones underlying the cratonic regions.
Abstract: The present study contributes new constraints on, and definitions of, the reconstructed plate margins of India and Madagascar based on flexural isostasy along the Western Continental Margin of India (WCMI) and the Eastern Continental Margin of Madagascar (ECMM). We have estimated the nature of isostasy and crustal geometry along the two margins, and have examined their possible conjugate structure. Here we utilize elastic thickness (Te) and Moho depth data as the primary basis for the correlation of these passive margins. We employ the flexure inversion technique that operates in spatial domain in order to estimate the spatial variation of effective elastic thickness. Gravity inversion and flexure inversion techniques are used to estimate the configuration of the Moho/Crust-Mantle Interface that reveals regional correlations with the elastic thickness variations. These results correlate well with the continental and oceanic segments of the Indian and African plates. The present study has found a linear zone of anomalously low-Te (1-5 km) along the WCMI (~1680 km), which correlates well with the low-Te patterns obtained all along the ECMM. We suggest that the low-Te zones along the WCMI and ECMM represent paleo-rift inception points of lithosphere thermally and mechanically weakened by the combined effects of the Marion hotspot and lithospheric extension due to rifting. We have produced an India-Madagascar paleo-fit representing the initial phase of separation based on the Te estimates of the rifted conjugate margins, which is confirmed by a close-fit correlation of Moho geometry and bathymetry of the shelf margins. The matching of tectonic lineaments, lithologies and geochronological belts between India and Madagascar provide an additional support for the present plate reconstruction.
Abstract: Field setting, petrography, geochemistry and available radiometric ages of Proterozoic mafic dykes from the northern Bastar craton have helped to identify four sets of mafic dykes; two Paleoproterozoic [viz. NW-SE North Bastar dykes (NBD) and ENE-WSW Dongargarh-Chhura dykes (DCD)] and two Mesoproterozoic [viz. 1.42 Ga ENE-WSW Bandalimal dykes (BDD) and 1.44 Ga N-S Lakhna dykes (LKD)]. Their petrographic and geochemical characteristics are very distinct and suggest their derivation from different mantle melts. Chemistry of all the four sets suggests different petrogenetic histories and samples of each distinct set are co-genetic nature. The NBD, the DCD and the BDD samples are sub-alkaline tholeiitic in nature, whereas the LKD samples show alkaline nature. Very distinct REE patterns are observed for all the four sets again suggesting their different petrogenetic histories. Geochemical comparison between the studied samples and mafic dyke samples of southern and central parts of the Bastar craton suggests very different picture for the northern Bastar craton. Only one set of northern Bastar dykes, i.e. the NBD, matches with BD1 dykes; no other dyke sets match with any of the dyke swarms identified in southern and central Bastar craton. Geochemically it is not straightforward to confirm crustal contamination, however, on the other hand, possibility of crustal contamination cannot be ruled out completely. A petrogenetic model based on trace element data suggests that all the four sets are derived from different mantle melts. The NBD and the DCD are probably generated within spinel stability field, whereas the BDD and the LKD may be derived from melts generated within garnet stability field. Available geological and geochemical data support the emplacement of studied dykes in a stable continental rift tectonic setting, however earlier intrusions have chemistry similar to N-MORB. The available geological, geochemical and geochronological data on the four indentified sets of mafic dykes from the northern Bastar craton indicate their relation to the assembly and break-up of Columbia supercontinent.
Geophysical Journal International, Vol. 203, pp. 910-926.
India
Geophysics - seismics
Abstract: We present a comprehensive study of thickness and composition of the crust; and the nature of crust-mantle boundary beneath Southern India using P-wave receiver function from 119 seismic stations. Data from distributed network of seismograph location encompass geological domains like mid to late Archean Dharwar craton, Archean and Proterozoic metamorphic terrains, Proterozoic basin, rifted margins and escarpments, and Deccan volcanics. Except for the mid to lower crust exhumed Archean terrains (of West Dharwar and Southern Granulite) all other geological domains have crustal thickness in the range 33-40 km. In the western Dharwar, crustal thickness increases from ?40 km in the north to over 50 km in the south. The Archean domain of granulite terrain is thicker (40-45 km) and more mafic compared to its counterpart in south deformed at 550 Ma. Most of the crustal blocks have low to moderate Vp/Vs (1.72-1.76) representing a felsic to intermediate composition. Exception to the above include Archean granulite terrain with high Vp/Vs (1.76–1.81) suggestive of more mafic crust beneath them. When accounted for the paleo burial depth of 15-25 km, the study suggests a possible Himalaya-Tibet like scenario beneath the mid-late Archean in southwestern Dharwar and north granulite terrain whose deeper crust has progressively densified. This led to a gradational crust-mantle transition that is otherwise sharp elsewhere. The study suggests a more homogenized and felsic nature of the Precambrian crust beneath the terrains formed after 2.6 Ga, possibly due to delamination of the mafic lower crust. Our study does not suggest any distinction between late Archean and Proterozoic crust. The Deccan volcanism at 65 Ma does not appear to have altered the crustal character beneath it and is similar to the adjoining late Archean east Dharwar craton. The western Ghat escarpment and the coastal plain formed due to separation of India from Madagascar are underlain by mafic lower crust.
Journal of Asian Earth Sciences, Vol. 105, pp. 300-319.
India
Boninites
Abstract: High MgO volcanic rocks having elevated concentrations of Ni and Cr are potential hosts for platinum group elements (PGE) owing to their primitive mantle origin and eruption at high temperatures. Though their higher PGE abundance is economically significant in mineral exploration studies, their lower concentrations are also valuable geochemical tools to evaluate petrogenetic processes. In this paper an attempt has been made to evaluate the PGE geochemistry of high MgO volcanic rocks from two greenstone belts of western and eastern Dharwar Craton and to discuss different mantle processes operative at diverse geodynamic settings during the Neoarchean time. The Bababudan greenstone belt of western and Gadwal greenstone belt of eastern Dharwar Cratons are dominantly composed of high MgO volcanic rocks which, based on distinct geochemical characteristics, have been identified as komatiites and boninites respectively. The Bababudan komatiites are essentially composed of olivine and clinopyroxene with rare plagioclase tending towards komatiitic basalts. The Gadwal boninites contain clinopyroxene, recrystallized hornblende with minor orthopyroxene, plagioclase and sulphide minerals. The Bababudan komatiites are Al-undepleted type (Al2O3/TiO2 = 23-59) with distinctly high MgO (27.4-35.8 wt.%), Ni (509-1066 ppm) and Cr (136-3036 ppm) contents. These rocks have low ?PGE (9-42 ppb) contents with 0.2-2.4 ppb Iridium (Ir), 0.2-1.4 ppb Osmium (Os) and 0.4-4.4 ppb Ruthenium (Ru) among Iridium group PGE (IPGE); and 1.4-16.2 ppb Platinum (Pt), 2.8-19 ppb Palladium (Pd) and 0.2-9.8 ppb Rhodium (Rh) among Platinum group PGE (PPGE). The Gadwal boninites are high-Ca boninites with CaO/Al2O3 ratios varying between 0.8 and 1.0, with 12-24 wt.% MgO, 821-1168 ppm Ni and 2307-2765 ppm Cr. They show higher concentration of total PGE (82-207 ppb) with Pt concentration ranging from 13 to 19 ppb, Pd between 65 and 180 ppb and Rh in the range of 1.4-3 ppb compared to the Bababudan komatiites. Ir, Os and Ru concentrations range from 0.6 to 2.2 ppb, 0.2 to 0.6 ppb and 1.4 to 2.6 ppb respectively in IPGE. The PGE abundances in Bababudan komatiites were controlled by olivine fractionation whereas that in Gadwal boninites were influenced by fractionation of chromite and sulphides. The Al-undepleted Bababudan komatiites are characterized by low CaO/Al2O3, (Gd/Yb)N, (La/Yb)N, with positive Zr, Hf, Ti anomalies and high Cu/Pd, Pd/Ir ratios at low Pd concentrations suggesting the derivation of parent magma by high degrees (>30%) partial melting of mantle under anhydrous conditions at shallow depth with garnet as a residual phase in the mantle restite. The komatiites are geochemically analogous to Al-undepleted Munro type komatiites and their PGE compositions are consistent with Alexo and Gorgona komatiites. The S-undersaturated character of Bababudan komatiites is attributed to decompression and assimilation of lower crustal materials during magma ascent and emplacement. In contrast, the higher Al2O3/TiO2, lower (Gd/Yb)N, for Gadwal boninites in combination with negative Nb, Zr, Hf, Ti anomalies and lower Cu/Pd at relatively higher Pd/Ir and Pd concentrations reflect high degree melting of refractory mantle wedge under hydrous conditions in an intraoceanic subduction zone setting. Higher Pd/Ir ratios and S-undersaturation of these boninites conform to influx of fluids derived by dehydration of subducted slab resulting into high fluid pressure and metasomatism of mantle wedge.
Geological Magazine, Rapid communication Oct. 14p.
India
Tectonics, geochronology
Abstract: A comprehensive study based on U-Pb and Hf isotope analyses of zircons from gneisses has been conducted along the western part (Babina area) of the E–W-trending Bundelkhand Tectonic Zone in the central part of the Archaean Bundelkhand Craton. 207Pb-206Pb zircon ages and Hf isotopic data indicate the existence of a felsic crust at ~ 3.59 Ga, followed by a second tectonothermal event at ~ 3.44 Ga, leading to calc-alkaline magmatism and subsequent crustal growth. The study hence suggests that crust formation in the Bundelkhand Craton occurred in a similar time-frame to that recorded from the Singhbhum and Bastar cratons of the North Indian Shield.
Abstract: We analyzed satellite gravity and geoid anomaly and topography data to determine the 3D lithospheric density structure of the Singhbhum Protocontinent. Our density model shows that distinct vertical density heterogeneities exist throughout the lithosphere beneath the Singhbhum Protocontinent. The crustal structure identified includes a lateral average crustal density variation from 2800 to 2890 kg/m3 as well as a relatively flat Moho at 35-40 km depth in Singhbhum Protocontinent and Bastar Craton. A similar Moho depth range is found for the Mahanadi, Damodar, and Bengal basins. In the northern part of the area, Moho undulates between more than 40 km under the confluence of Mahanadi-Damodar Gondwana basins and the Ganga foreland basin, and 36-32 km under the Eastern Ghats Mobile belt and finally reaches 24 km in the Bay of Bengal. The lithosphere-asthenosphere boundary (LAB) across the Singhbhum Protocontinent is at a depth of about 130-140 km. In the regions of Bastar Craton and Bengal Basin, the LAB dips to about 155 ± 5 km depth. The confluence of Mahanadi and Damodar Gondwana basins toward the north-west and the foreland Ganga Basin toward the north are characterized by a deeper LAB lying at a depth of over 170 and 200 km, respectively. In the Bay of Bengal, the LAB is at a shallower depth of about 100-130 km except over the 85 0E ridge (150 km), and off the Kolkata coast (155 km). Significant density variation as well as an almost flat crust-mantle boundary indicates the effect of significant crustal reworking. The thin (135-140 km) lithosphere provides compelling evidence of lithospheric modification in the Singhbhum Protocontinent. Similarities between the lithospheric structures of the Singhbhum Craton, Chhotanagpur Gneiss Complex, and Northern Singhbhum Mobile Belt confirm that the repeated thermal perturbation controlled continental lithospheric modification in the Singhbhum Protocontinent.
Arabian Journal of Geosciences, Vol. 8, no. 11, pp. 9373-9388.
India
Deposit - Narayanpet
Abstract: In the present study, an attempt was made to analyse the reflectance spectra of kimberlites to evaluate its potential as key in remote sensing based spatial mapping. The spectral profiles of kimberlite samples were collected within the visible-near infrared-shortwave infrared (VNIR-SWIR) electromagnetic domain. In this regard, we analysed the reflectance spectra of three kimberlite pipes (having variable mineralogy) of Narayanpet kimberlite field (NKF) based on the comparative analysis of spectral features of kimberlite samples with the spectral features of their dominant constituent minerals. The relative abundances of each of the constituent minerals were confirmed using semiquantitative mineralogical data from X-ray diffraction analysis. This was supplemented with petrographical data as reference. We found that the absorption features imprinted in the reflectance spectra of kimberlites were mineralogically sensitive. These spectral features were imprinted by spectral features of serpentine, olivine, and calcite depending on the relative dominance of these minerals in kimberlites. With regard to understand the spectral behaviour of weathered residue of kimberlite for targeting buried kimberlite, we also attempted a comparative analysis of spectral profiles of in-situ soil developed above the pipes with the spectra of respective kimberlites in NKF area. While comparing aforementioned spectra, it was observed that the spectral signatures of NKF kimberlites were broadly translated to the in-situ soil. Further, we compared the spectral profiles of selected NKF kimberlites with the spectra of three distinct kimberlite pipes of Wajrakarur kimberlite field (WKF) characterised with similar mineralogy with respect to the selected NKF pipes. Relative dominance of constituent minerals (i.e., serpentine, olivine, calcite, etc.) in these pipes was taken as reference to identify the mineralogical similarity of the pipes of both the field. It was observed that the spectral profiles of NKF and WKF kimberlites were highly correlated with regard to wavelength of diagnostic absorption features. Finally, we also made an attempt to understand the effect of spectral mixing, in spectral separation of kimberlites and associated granite-granodiorite gneiss (i.e., Dharwar Gneiss). It was seen that the spectral contrast of kimberlite and gneiss was dependent on the relative size of the pipe with respect to pixel or ground sampling diameter of spectral data acquisition. Study confirmed the diagnostic nature of reflectance spectra of pipes along with their mineralogical sensitiveness and spatial integrity. It also highlighted how spectral mixing can influence the spectral feature based remote detection of kimberlites.
Mineralogy and Petrology, in press available, 25p.
India
Lamproites - Nuapada field
Abstract: We report the mineralogy, bulk-rock geochemistry, 40Ar/39Ar (whole-rock) age and radiogenic (Sr and Nd) isotope composition of an ultrapotassic dyke from Sakri (Nuapada lamproite field) located at the tectonic contact between the easternmost margin of the Bastar craton and Eastern Ghats Mobile Belt, India. The Sakri dyke has a mineralogy which strongly resembles a lamproite sensu stricto (viz.,Ti-rich phlogopite, Na-poor diopside, Fe-rich sanidine, ulvospinel trend and Sr-rich apatite). However, its bulk-rock major element geochemical characteristics (viz., extreme silica-undersaturated nature) resemble sensu lato kamafugite from Toro Ankole, Uganda, East African Rift, and Alto Paranaiba Province, Brazil. The Sakri dyke also displays certain compositional peculiarities (viz., high degree of evolution of mica composition from phlogopite to biotite, elevated titanium and aluminum in clinopyroxene and significantly lower bulk Mg#) when compared to the ultrapotassic rocks from various Indian cratons. 40Ar/39Ar dating gave a plateau age of 1045?±?9 Ma which is broadly similar to that of other Mesoproterozoic (i) lamproites from the Bastar and Bundelkhand cratons, and (ii) kimberlites from the Eastern Dharwar craton. Initial bulk-rock Sr (0.705865-0.709024) and Nd (0.511063-0.511154) isotopic ratios reveal involvement of an ‘enriched’ source region with long-term incompatible element enrichment and a depleted mantle (TDM) Nd model age of 2.56 Ga straddling the Archaean-Proterozoic chronostratigraphic boundary. The bulk-rock incompatible trace element ratios (Ta/Yb, Th/Yb, Rb/Ba and Ce/Y) of the Sakri ultrapotassic dyke negate any significant influence of crustal contamination. Small-degree melting (1 to 1.5 %) of a mixed garnet-facies and spinel-facies phlogopite lherzolite can account for its observed REE concentrations. Whereas the emplacement of the Sakri ultrapotassic dyke is related to the amalgamation of the supercontinent of Rodinia, its overlapping geochemical characteristics of lamproite and kamafugite (also displayed by two other lamproites of the Nuapada field at Amlidadar and Parkom) are linked to the emplacement in a unique geological setting at the craton-mobile belt contact and hence of geodynamic significance.
Journal of Geophysical Research, Vol. 120, 10, pp. 6698-6721.
India
Geophysics - craton
Abstract: Magnetotelluric-derived two-dimensional lithospheric resistivity structure of the western Dharwar craton (WDC) and adjoining Coorg block indicates isolated low-resistivity zones in the crust and three striking upper mantle conductive features within the highly resistive Archean lithosphere. The crustal conductors in the WDC show good spatial correlation with the exposed supracrustal rocks conformable with the relic schist belt channels having conductive mineral grains. Conductive zones within the Coorg crust might be related to the relatively young (933?Ma) metamorphic processes in the area and/or possible fluids derived from the Cretaceous passage of Reunion plume in the proximity of Coorg area. A near-vertical conductive structure extending from the lower crust into the upper mantle coincides with the transition zone between Coorg and WDC. This is interpreted as the suture zone between the two tectonic blocks and provides evidence for the individuality of the two Archean terrains. An anomalous upper mantle conductive zone found beneath the craton nucleus may indicate a modified cratonic lithosphere. This could have been derived due to the collision between Coorg and WDC and possibly survived by the subsequent multiple episodes of melt and fluid infiltration processes experienced in the region. Thick (~190?km) and preserved lithosphere is mapped at the eastern segment of WDC. Resistive lithosphere of ~125?km thickness is imaged for the Coorg block.
Abstract: A comprehensive study based on U-Pb and Hf isotope analyses of zircons from gneisses has been conducted along the western part (Babina area) of the E-W-trending Bundelkhand Tectonic Zone in the central part of the Archaean Bundelkhand Craton. 207Pb-206Pb zircon ages and Hf isotopic data indicate the existence of a felsic crust at ~ 3.59 Ga, followed by a second tectonothermal event at ~ 3.44 Ga, leading to calc-alkaline magmatism and subsequent crustal growth. The study hence suggests that crust formation in the Bundelkhand Craton occurred in a similar time-frame to that recorded from the Singhbhum and Bastar cratons of the North Indian Shield.
Mineralogy and Petrology, in press available, 13p.
India
Deposit - Wajrakur
Abstract: Although Ti-rich garnets are commonly encountered in the groundmass of many alkaline igneous rocks, they are comparatively rare in kimberlites. Here we report on the occurrence of Ti-rich garnets in the groundmass of the P-15 and KL-3 kimberlites from the diamondiferous Wajrakarur field in the Eastern Dharwar craton of southern India. These garnets contain considerable Ti (11.7-23.9 wt.% TiO2), Ca (31.3-35.8 wt.% CaO), Fe (6.8-15.5 wt.% FeOT) and Cr (0.04-9.7 wt.% Cr2O3), but have low Al (0.2-5.7 wt.% Al2O3). In the case of the P-15 kimberlite they display a range in compositions from andradite to schorlomite, with a low proportion of grossular (andradite(17.7-49.9)schorlomite(34.6-49.5)-grossular(3.7-22.8)-pyrope(1.9-10.4)). A few grains also contain significant chromium and represent a solid solution between schorlomite and uvarovite. The Ti-rich garnets in the KL-3 kimberlite, in contrast, are mostly schorlomitic (54.9?90.9 mol %) in composition. The Ti-rich garnets in the groundmass of these two kimberlites are intimately associated with chromian spinels, perhaps suggesting that the garnet formed through the replacement of spinel. From the textural evidence, it appears unlikely that the garnets could have originated through secondary alteration, but rather seem to have formed through a process in which early magmatic spinels have reacted with late circulating, residual fluids in the final stages of crystallization of the kimberlite magma. Raman spectroscopy provides evidence for low crystallinity in the spinels which is likely to be a result of their partial transformation into andradite during their reaction with a late-stage magmatic (kimberlitic) fluid. The close chemical association of these Ti-rich garnets in TiO2-FeO-CaO space with those reported from ultramafic lamprophyres (UML) is also consistent with results predicted by experimental studies, and possibly implies a genetic link between kimberlite and UML magmas. The occurrence of Ti-rich garnets of similar composition in the Swartruggens orangeite on the Kaapvaal craton in South Africa, as well as in other kimberlites with an orangeitic affinity (e.g. the P-15 kimberlite on the Eastern Dharwar craton in southern India), is inferred to be a reflection of the high Ca- and high Ti-, and the low Al-nature, of the parent magma (i.e. Group II kimberlites).
Journal of The Geological Society of India, Vol. 87, 2, pp. 127-131.
India
Lamproite
Abstract: A singular outcrop of a lamproite dyke is located ~1.5 km south-west of Chintalapalle village at the NW margin of the Cuddapah basin, eastern Dharwar craton, southern India.. The dyke trends E-W and is emplaced within the granitic rocks belonging to the peninsular gneissic complex. The lamproite dyke has a porphyritic to weakly porphyritic texture comprising microphenocrysts of sanidine, and potassic richterite set in a groundmass rich in carbonate, and chlorite with rutile and titanate as accessory phases. This new occurrence of lamproite is located mid-way between the well-known Narayanpet kimberlite field towards the west and the Ramadugu and Vattikod lamproite fields in east. The Chintalapalle lamproite dyke, together with those from Vattikod, Ramadugu, Krishna and Cuddapah basin lamproite fields, constitute a wide spectrum of ultrapotassic magmatism emplaced in and around the Palaeo-Mesoproterozoic Cuddapah basin in southern India.
Global Business and Organizational Excellence, Vol. 35, 3, pp. 6-17.
India
Cutting and polishing industry
Abstract: As informal firms in emerging markets are expanding their role in global supply chains, managers of formal multinational organizations are increasingly relying on their services. Yet, little is known about the organizational aspects of enterprises in the informal sector. An investigation of informal firms engaged in the cutting and polishing of diamonds (CPD) in Surat, India, the world's hub of diamond manufacturing, reveals that in the absence of well-defined strategies, structures, and processes, the intangible aspect of organizing—specifically, organizational culture—governs business practices. Despite the strong clan-like orientation of these firms and a culture focused on loyalty, trust, team work, and consensus, the study found evidence of hierarchical characteristics and market-driven leadership. Coupled with insightful observations of the overall Indian CPD sector, these findings can help guide managers in planning strategies for effective partnerships with informal firms, regardless of their industry.
Mineralogy and Petrology, in press available, 25p.
India
Deposit - Sakri Nuapada
Abstract: We report the mineralogy, bulk-rock geochemistry, 40Ar/39Ar (whole-rock) age and radiogenic (Sr and Nd) isotope composition of an ultrapotassic dyke from Sakri (Nuapada lamproite field) located at the tectonic contact between the easternmost margin of the Bastar craton and Eastern Ghats Mobile Belt, India. The Sakri dyke has a mineralogy which strongly resembles a lamproite sensu stricto (viz.,Ti-rich phlogopite, Na-poor diopside, Fe-rich sanidine, ulvospinel trend and Sr-rich apatite). However, its bulk-rock major element geochemical characteristics (viz., extreme silica-undersaturated nature) resemble sensu lato kamafugite from Toro Ankole, Uganda, East African Rift, and Alto Paranaiba Province, Brazil. The Sakri dyke also displays certain compositional peculiarities (viz., high degree of evolution of mica composition from phlogopite to biotite, elevated titanium and aluminum in clinopyroxene and significantly lower bulk Mg#) when compared to the ultrapotassic rocks from various Indian cratons. 40Ar/39Ar dating gave a plateau age of 1045?±?9 Ma which is broadly similar to that of other Mesoproterozoic (i) lamproites from the Bastar and Bundelkhand cratons, and (ii) kimberlites from the Eastern Dharwar craton. Initial bulk-rock Sr (0.705865-0.709024) and Nd (0.511063-0.511154) isotopic ratios reveal involvement of an ‘enriched’ source region with long-term incompatible element enrichment and a depleted mantle (TDM) Nd model age of 2.56 Ga straddling the Archaean-Proterozoic chronostratigraphic boundary. The bulk-rock incompatible trace element ratios (Ta/Yb, Th/Yb, Rb/Ba and Ce/Y) of the Sakri ultrapotassic dyke negate any significant influence of crustal contamination. Small-degree melting (1 to 1.5 %) of a mixed garnet-facies and spinel-facies phlogopite lherzolite can account for its observed REE concentrations. Whereas the emplacement of the Sakri ultrapotassic dyke is related to the amalgamation of the supercontinent of Rodinia, its overlapping geochemical characteristics of lamproite and kamafugite (also displayed by two other lamproites of the Nuapada field at Amlidadar and Parkom) are linked to the emplacement in a unique geological setting at the craton-mobile belt contact and hence of geodynamic significance.
Mineralogy and Petrology, Vol. 110, 2, pp. 295-307.
India
Deposit - Wajrakarur
Abstract: Although Ti-rich garnets are commonly encountered in the groundmass of many alkaline igneous rocks, they are comparatively rare in kimberlites. Here we report on the occurrence of Ti-rich garnets in the groundmass of the P-15 and KL-3 kimberlites from the diamondiferous Wajrakarur field in the Eastern Dharwar craton of southern India. These garnets contain considerable Ti (11.7-23.9 wt.% TiO2), Ca (31.3-35.8 wt.% CaO), Fe (6.8-15.5 wt.% FeOT) and Cr (0.04-9.7 wt.% Cr2O3), but have low Al (0.2-5.7 wt.% Al2O3). In the case of the P-15 kimberlite they display a range in compositions from andradite to schorlomite, with a low proportion of grossular (andradite(17.7-49.9)schorlomite(34.6-49.5)-grossular(3.7-22.8)-pyrope(1.9-10.4)). A few grains also contain significant chromium and represent a solid solution between schorlomite and uvarovite. The Ti-rich garnets in the KL-3 kimberlite, in contrast, are mostly schorlomitic (54.9?90.9 mol %) in composition. The Ti-rich garnets in the groundmass of these two kimberlites are intimately associated with chromian spinels, perhaps suggesting that the garnet formed through the replacement of spinel. From the textural evidence, it appears unlikely that the garnets could have originated through secondary alteration, but rather seem to have formed through a process in which early magmatic spinels have reacted with late circulating, residual fluids in the final stages of crystallization of the kimberlite magma. Raman spectroscopy provides evidence for low crystallinity in the spinels which is likely to be a result of their partial transformation into andradite during their reaction with a late-stage magmatic (kimberlitic) fluid. The close chemical association of these Ti-rich garnets in TiO2-FeO-CaO space with those reported from ultramafic lamprophyres (UML) is also consistent with results predicted by experimental studies, and possibly implies a genetic link between kimberlite and UML magmas. The occurrence of Ti-rich garnets of similar composition in the Swartruggens orangeite on the Kaapvaal craton in South Africa, as well as in other kimberlites with an orangeitic affinity (e.g. the P-15 kimberlite on the Eastern Dharwar craton in southern India), is inferred to be a reflection of the high Ca- and high Ti-, and the low Al-nature, of the parent magma (i.e. Group II kimberlites).
International Journal of Earth Sciences, Vol. 105, 4, pp. 1087-1107.
India
Not specific to diamonds
Abstract: We have carried out radioelemental (232Th, 238U, 40K), petrological and geochemical analyses on granitoids and gneisses covering major rock formations of the Bundelkhand craton, central India. Our data reveal that above characteristics are distinct among granitoids (i.e. pink, biotite and grey granitoids) and gneisses (i.e. potassic and sodic types). Pink granitoid is K-feldspar-rich and meta-aluminous to per-aluminous in character. Biotite granitoid is meta-aluminous in character. Grey granitoid is rich in Na-feldspar and mafic minerals, granodiorite to diorite in composition and meta-aluminous in character. Among these granitoids, radioelements (Th, U, K) are highest in pink granitoid (45.0 ± 21.7 ppm, 7.2 ± 3.4 ppm, 4.2 ± 0.4 %), intermediate in biotite granitoid (44.5 ± 28.2 ppm, 5.4 ± 2.8 ppm, 3.4 ± 0.7 %) and lowest in grey granitoid (17.7 ± 4.3 ppm, 4.4 ± 0.6 ppm, 3.0 ± 0.4 %). Among gneisses, potassic-type gneisses have higher radioelements (11.8 ± 5.3 ppm, 3.1 ± 1.2 ppm, 2.0 ± 0.5 %) than the sodic-type gneisses (5.6 ± 2.8 ppm, 1.3 ± 0.5 ppm, 1.4 ± 0.7 %). Moreover, the pink granitoid and the biotite granitoid have higher Th/U (6 and 8, respectively) compared to the grey granitoid (Th/U: 4), implying enrichment of Th in pink and biotite granitoids relative to grey granitoid. K/U among pink, biotite and grey granitoids shows little variation (0.6 × 104, 0.6 × 104, 0.7 × 104, respectively), indicating relatively similar increase in K and U. Therefore, mineralogical and petrological data along with radioelemental ratios suggest that radioelemental variations in these lithounits are mainly related to abundances of the radioactive minerals that have formed by the fractionation of LILE from different magma sources. Based on present data, the craton can be divided into three distinct zones that can be correlated with its evolution in time and space. The central part, where gneisses are associated with metavolcanics of greenstone belt, is characterized by lowest radioelements and is the oldest component. The southern part, dominated by pink granitoid, is characterized by highest radioelements and is the youngest part. The northern part, dominated by grey and biotite granitoid, is characterized by moderate radioelements.
Abstract: A team of researchers recently discovered an ancient relic hidden within Earth: a tectonic plate resting beneath the southern Indian Ocean. Scientists have found other tectonic plates that sank below Eurasia and North America, but here Simmons et al. describe the unique structure of this newly discovered slab, which they named the Southeast Indian Slab (SEIS). The slab has at least one feature scientists have rarely seen before: It maintains its slab-like structure all the way from the upper mantle near Earth’s crust down to the region where the mantle meets the planet’s superheated core. The Farallon plate beneath North America is a well-known example of this—but it was expected to exist and sank much more recently than the SEIS. In addition, not only does the SEIS traverse the entire mantle, but it also becomes more vertical along one end, so much so that it stands almost vertically between the crust and core along the eastern edge, whereas the western portion is more horizontal. Researchers can make out structures beneath Earth’s crust by examining the speed at which seismic waves generated by earthquakes and similar Earth-shattering events—known as P and S waves—travel through Earth. Here the researchers used wave data from 12,607 seismic events dating back to the 1960s, collected by 7783 seismic stations around the world, to develop the model that identified the ancient slab. Once this tectonic slab was identified, the team looked at the region’s tectonic history over millions of years to determine where and when this plate was on the surface. They determined that the slab was once along the eastern portion of the early supercontinent of Gondwana. Then, sometime during the Triassic or Jurassic period, which stretched from 250 million years ago to 145 million years ago, the slab plunged underneath another plate. They further concluded that the subduction, or the sinking of the Southeast Indian Slab beneath another plate, terminated around 130 to 140 million years ago in the Mesozoic era, around the same time that the tectonic plates under eastern Gondwana began to separate and split up the continent. Tectonic plates usually sink down into the mantle at a rate of about 1 centimeter per year or more; they don’t necessarily melt but instead bunch up at the base of the mantle and eventually assimilate or become undetectable as their temperature increases. However, if the researchers accurately estimated the timing of their newly discovered slab’s subduction, this slab must have stalled in a transition zone before descending deeper down into the mantle, allowing the slab to persist in the mantle longer than any other known plate.
International Geology Review, Vol. 58, 13, pp. 1596-1615.
India
Boninites
Abstract: Nd-isotope and lithogeochemistry of an early Palaeoproterozoic high-Si high-Mg boninite -norite (BN) suite of rocks from the southern Bastar craton, central India, are presented to understand their nature, origin, and tectonic setting of emplacement. Various types of evidence, such as field relationships, radiometric metamorphic ages, and the global distribution of BN magmatism, suggest emplacement in an intracratonic rift setting, commonly around 2.4 -2.5 Ga. On the basis of geochemistry these high-Si high-Mg rocks are classified as high-Ca boninites, high-Mg norites, and high-Mg diorites. Nd-isotope data indicate that the high-Mg norite and the high-Mg diorite samples are similar, whereas the high-Ca boninites have a different isotopic character. The high-Mg norite and the high-Mg diorite samples have younger TDM model ages than the high-Ca boninites. Geochemical and Nd-isotopic characteristics of the studied rocks indicate some prospect of crustal contamination; however, the possibility of mantle metasomatism during ancient subduction event cannot be ignored. Trace-element modelling suggests that the high-Ca boninites may have crystallized from a magma generated by a comparatively greater percentage of melting of a lherzolite mantle source than the source for the other two varieties. Furthermore, the high-Ca boninite rocks are most likely derived from an Archaean subduction process (the Whundo-type), whereas the other two types are the products of the interaction of subduction-modified refractory mantle wedge and a plume, around the Neoarchaean -Palaeoproterozoic boundary. The emplacement of the high-Mg norites and the high-Mg diorites may be linked to crustal thickening and associated cratonization at the end of the Archaean.
India's fast Mesozoic drift linked to continental mantle lithosphere delamination: new insights from (U-Th)/He thermochronology of Dharwar craton kimberlites.
Journal of the Geological Society of India, Vol. 87, 6, pp. 709-715.
India
Carbonatite
Abstract: The Siriwasan carbonatite-sill along with associated alkaline rocks and fenites is located about 10 km north of the well-known Amba Dongar carbonatite-alkaline rocks diatreme, in the Chhota Udaipur carbonatite-alkaline province. Carbonatite has intruded as a sill into the Bagh sandstone and overlying Deccan basalt. This resulted in the formation of carbonatite breccia with enclosed fragments of basement metamorphics, sandstone and fenites in the matrix of ankeritic carbonatite. The most significant are the plugs of sovite with varied mineralogy that include pyroxene, amphibole, apatite, pyrochlore, perovskite and sphene. REE in sovites is related to the content of pyrochlore, perovskite and apatite. The carbon and oxygen isotopic compositions of some sovite samples and an ankeritic carbonatite plot in the "mantle box" pointing to their mantle origin. However, there is also evidence for mixing of the erupting carbonatite magma with the overlying Bagh limestone. The carbonatites of Siriwasan and Amba Dongar have the same Sr and Nd isotopic ratios and radiometric age, suggesting the same magma source. On the basis of available chemical analyses this paper is aimed to give some details of the Siriwasan carbonatites. The carbonatite complex has good potential for an economic mineral deposit but this is the most neglected carbonatite of the Chhota Udaipur province.
Proceedings National Academy of Sciences India , Vol. 82, 3, July special issue pp. 515-536.
India
Kimberlites, lamproites
Abstract: Major highlights of researches carried out on kimberlites, lamproites, lamprophyres, carbonatites, other alkaline rocks and mafic dykes from the Indian shield during 2012-2016 are presented. New findings involving field mapping, petrology, geochemistry (including high quality mineral based in situ isotopic studies) and geophysics have provided remarkable insights on the mode of their occurrence, timing of emplacement, mineralogy and bulk-rock composition, redox conditions, relative contribution of the lithosphere and asthenosphere, as well as their economic potential. Several large-scale geodynamic aspects such as plume-lithosphere interactions, ancient subduction events, layered structure of the sub-continental lithospheric mantle, spatial extent of the Precambrian large igneous provinces and supercontinent configurations could be unraveled from these studies on deep-mantle derived small-volume magmatic rocks.
Abstract: Blue diamonds are among the rarest and most valuable of naturally occurring gemstones. In this study, 12 rough naturally-sourced type IIb diamonds were subjected to HPHT annealing, three different irradiation energies, and then all were stepwise annealed from 200 °C to 1100 °C and the optical defects were documented by changes in phosphorescence and photoluminescence spectroscopy. Several optical features that are removed from natural type IIb diamonds by HPHT processing, such as 3H, 648.2 nm peak, 776.4 nm peak, and 660 nm band (red) phosphorescence, can be reintroduced into these diamonds with subsequent electron irradiation and annealing at low-to-moderate temperatures. The thermal stability of these centers along with their spatial distribution provided additional insights into their configuration and distinguished them from nitrogen-bearing diamonds.
Abstract: We report groundmass perovskite U -Pb (SIMS) ages, perovskite Nd isotopic (LA-ICPMS) composition and bulk-rock geochemical data of the Timmasamudram diamondiferous kimberlite cluster, Wajrakarur kimberlite field, in the Eastern Dharwar craton of southern India. The kimberlite pipes gave similar Mesoproterozoic ages of 1086 ± 19 Ma (TK-1, microcrystic variant) and 1119 ± 12 Ma (TK-3). However, a perovskite population sampled from the macrocrystic variant of TK-1 gave a much younger Late Cretaceous age of ca. 90 Ma. This macrocrystic kimberlite phase intrudes the Mesoproterozoic microcrystic phase and has a distinct bulk-rock geochemistry. The Nd-isotope composition of the ~ 1100 Ma perovskites in the cluster show depleted ?Nd(T) values of 2.1 ± 0.6 to 6.7 ± 0.3 whereas the ~ 90 Ma perovskites have enriched ?Nd(T) values of ? 6.3 ± 1.3. The depleted-mantle (DM) model age of the Cretaceous perovskites is 1.2 Ga, whereas the DM model age of the Proterozoic perovskites is 1.2 to 1.5 Ga. Bulk-rock incompatible trace element ratios (La/Sm, Gd/Lu, La/Nb and Th/Nb) of all Timmasamudram kimberlites show strong affinity with those from the Cretaceous Group II kimberlites from the Bastar craton (India) and Kaapvaal craton (southern Africa). As the Late Cretaceous age of the younger perovskites from the TK-1 kimberlite is indistinguishable from that of the Marion hotspot-linked extrusive and intrusive igneous rocks from Madagascar and India, we infer that all may be part of a single Madagascar Large Igneous Province. Our finding constitutes the first report of Cretaceous kimberlite activity from southern India and has significant implications for its sub-continental lithospheric mantle evolution and diamond exploration programs.
Abstract: The nature of crustal and lithospheric mantle evolution of the Archean shields as well as their subsequent deformation due to recent plate motions and sustained intraplate geodynamic activity, has been a subject of considerable interest. In view of this, about three decades ago, a new idea was put forward suggesting that out of all shield terrains, the Indian shield has an extremely thin lithosphere (?100 km, compared to 250-350 km, elsewhere), apart from being warm, non-rigid, sheared and deformed. As expected, it met with scepticism by heat flow and the emerging seismic tomographic study groups, who on the contrary suggested that the Indian shield has a cool crust, besides a coherent and thick lithosphere (as much as 300-400 km) like any other shield. However, recently obtained integrated geological and geophysical findings from deep scientific drillings in 1993 Killari (Mw: 6.3) and 1967 Koyna (Mw: 6.3) earthquake zones, as well as newly acquired geophysical data over other parts of Indian shield terrain, have provided a totally new insight to this debate. Beneath Killari, the basement was found consisting of high density, high velocity mid crustal amphibolite to granulite facies rocks due to exhumation of the deeper crustal layers and sustained granitic upper crustal erosion. Similar type of basement appears to be present in Koyna region too, which is characterized by considerably high upper crustal temperatures. Since, such type of crust is depleted in radiogenic elements, it resulted into lowering of heat flow at the surface, increase in heat flow contribution from the mantle, and upwarping of the lithosphere-asthenosphere boundary. Consequently, the Indian shield lithosphere has become unusually thin and warm. This study highlights the need of an integrated geological, geochemical and geophysical approach in order to accurately determine deep crust-mantle thermal regime in continental areas.
Abstract: The Kumta and Mercara suture zones welding together Archean crustal blocks in western peninsular India offer critical insights into Precambrian continental juxtapositions and the crustal evolution of eastern Gondwana. Here we present the results from an integrated study of the structure, geology, petrology, mineral chemistry, metamorphic P-T conditions, zircon U-Pb ages and Lu-Hf isotopes of metasedimentary rocks from the two sutures. The dominant rocks in the Kumta suture are greenschist- to amphibolite-facies quartz-phengite schist, garnet-biotite schist, chlorite schist, fuchsite schist and marble. The textural relations, mineral chemistry and thermodynamic modelling of garnet-biotite schist from the Kumta suture indicate peak metamorphic P-T conditions of ca. 11 kbar at 790 °C, with detrital SHRIMP U-Pb zircon ages ranging from 3420 to 2547 Ma, ?Hf (t) values from ? 9.2 to 5.6, and TDMc model ages from 3747 to 2792 Ma. The K-Ar age of phengite from quartz-phengite schist is ca. 1326 Ma and that of biotite from garnet-biotite schist is ca. 1385 Ma, which are interpreted to broadly constrain the timing of metamorphism related to the suturing event. The Mercara suture contains amphibolite- to granulite-facies mylonitic quartzo-feldspathic gneiss, garnet-kyanite-sillimanite gneiss, garnet-biotite-kyanite-gedrite-cordierite gneiss, garnet-biotite-hornblende gneiss, calc-silicate granulite and metagabbro. The textural relations, mineral chemistry and thermodynamic modelling of garnet-biotite-kyanite-gedrite-cordierite gneiss from the Mercara suture indicate peak metamorphic P-T conditions of ca. 13 kbar at 825 °C, followed by isothermal decompression and cooling. For pelitic gneisses from the Mercara suture, LA-ICP-MS U-Pb zircon ages vary from 3249 to 3045 Ma, ?Hf (t) values range from ? 18.9 to 4.2, and TDMc model ages vary from 4094 to 3314 Ma. The lower intercept age of detrital zircons in the pelitic gneisses from the Mercara suture ranges from 1464 to 1106 Ma, indicating the approximate timing of a major lead-loss event, possibly corresponding to metamorphism, and is broadly coeval with events in the Kumta suture. Synthesis of the above results indicates that the Kumta and Mercara suture zones incorporated sediments from Palaeoarchean to Mesoproterozoic sources and underwent high-pressure metamorphism in the late Mesoproterozoic. The protolith sediments were derived from regions containing juvenile Palaeoarchean crust, together with detritus from the recycling of older continental crust. Integration of the above results with published data suggests that the Mesoproterozoic (1460-1100 Ma) Kumta and Mercara suture zones separate the Archean (3400-2500 Ma) Karwar-Coorg block and Dharwar Craton in western peninsular India. Based on regional structural and other geological data we interpret the Kumta and Mercara suture zones as extensions of the Betsimisaraka suture of eastern Madagascar into western India.
Abstract: Lamproites are mineralogically complex rocks and their bulk rock geochemistry is not for characterization of their parental magmas (Mitchell and Bergman, [1]). Characterization is best accomplished by consideration of their typomorphic mineralogy. We have investigated nine dykes from Vattikod (VL1:Vl8 and VL10). The mineral assemblage and their compositions are comparable to those of lamproites in terms of the presence of phlogopite (Ti-rich, Al-poor phlogopite and tetraferriphlogopite); amphiboles (potassic-arfvedsonite, potassic-richterite, potassic-ferro-richterite, potassic-katophorite, Ti- rich potassic-katophorite, Ti-rich potassic-magnesio-katophorite); Al-poor pyroxene; feldspars (K- feldspar, Ba-K-feldspar and Na-feldspar), spinels (chromite-magnetite and qandilite-ulvÖspinel-ZnFe2O4). These dykes have also undergone varied degrees of deuteric alteration as shown by the development of secondary phases such as titanite, allanite, hydro-zircon, calcite, chlorite, quartz and cryptocrystalline SiO2. We have classified the Vattikod dyke on the basis of their typomorphic major mineralogy in conjunction with alteration affects as: Group 1 (VL1); Group 2 (VL2 and VL3); Group 3 (VL4 and VL5); Group 4 (VL6, VL7 and VL8); and Group 5 (VL10). Group 2 dykes are pseudoleucite-amphibole-lamproite; Group 3 dykes are pseudoleucite-phlogopite-lamproite; Group 4 dykes are pseudoleucite-phlogopite- amphibole-lamproite. The Group 1 dyke is completely altered and the precursor mineralogy cannot be identified. Group 5 dyke is also extensively altered but contains fresh euhedral apatite microphenocrysts together with pseudomorphs after leucite and is classified as a pseudoleucite-apatite-(phlogopite?) lamproite. It is suggested that the Vattikod lamproites represent a spectrum of modal variants of lamproite produced by the differentiation and crystallization of a common parental peralkaline potassic magma. The near-linear disposition of Deformed Alkaline Rocks and Carbonatites commonly known as DARC’s (Burke and Khan, [2]) and lamproites in eastern India implies a relationship with subduction-related processes (Das Sharma & Ramesh, [3]; Gurmeet Kaur & Mitchell, [4]). We propose that the Vattikod and other lamproites in eastern India emplaced at 1100-1450 Ma are possible manifestations of subduction- related alkaline magmatism along the Eastern Ghats Mobile Belt, in contrast to extension-related anorogenic lamproite magmatism related to supercontinent(s) break-up as has been suggested for Ramadugu and other Dharwar Craton lamproites.
Acta Geologica Sinica, Vol. 90, July abstract p. 191.
India
Deposit - Lattavaram
Abstract: Utility of geochemistry in mineral exploration is known since more than half-a-century. In reconnaissance diamond exploration, regolith geochemistry is a well known tool worldwide and helps in distinguishing bedrock geology in hard rock terrains. More than 100 kimberlite pipes were discovered so far in the Eastern Dharwar Craton (EDC) of South India by various public and private organizations. Within the EDC, majority of diamondiferous pipe clusters occur in Anantapur District of Andhra Pradesh state in India. Lattavaram kimberlite cluster (LKC) is one among them in this district and four pipes in this cluster are reported to be diamondiferous along with two kimberlite bodies at Muligiripalli. The kimberlite pipes at Lattavaram occur in semi-circular shape whereas a kimberlite body exposed at Muligiripalli village occurs as a dyke within the granitoid country. The pipe 4 at Lattavaram exhibits conspicuous difference in the soil color and texture i.e., greenish color and mottled texture when compared with the reddish brown compact residual soils of granitic country rock. In the Indian context, soil regolith geochemistry is not a popular subject in search of kimberlites. It is observed that little or no literature is available on the utility of regolith geochemistry applied to kimberlite or diamond exploration in India. An attempt is made in this study, to demonstrate and understand the spatial surface geochemical signatures using residual soil geochemistry on known kimberlite pipes viz., 3 and 4 of LKC. Spatial and statistical analysis of trace and rare earth elements revealed that certain elements show predominance in the vicinity of the kimberlite pipes which can act as an exploration guide in distinguishing kimberlitic rocks within a granitoid country. These elements show distinct variation in their dispersion in the soil which can be attributed to basement lithology. It is observed that elements like Cr, Ni, Co, Cu, Nb, Zr, Ti, Ba and rare earth elements (REE) are significantly enriched in the mottled zone and calcretized duricrust relative to the country rock granitoids. A suite of trace elements comprising those associated with ultramafic rocks (Cr, Co & Ni) and felsic rocks (Nb, La, Sm and P) can readily distinguish the ultramafic/kimberlitic regolith from that derived from granitoid or felsic rocks which can be used as an exploration path finder.
Abstract: New mineralogical and bulk-rock geochemical data for the recently recognised Mesoproterozoic (ca. 1100 Ma) and late Cretaceous (ca. 90 Ma) kimberlites in the Timmasamudram cluster (TKC) of the Wajrakarur kimberlite field (WKF), Eastern Dharwar Craton, southern India, are presented. On the basis of groundmass mineral chemistry (phlogopite, spinel, perovskite and clinopyroxene), bulk-rock chemistry (SiO2, K2O, low TiO2, Ba/Nb and La/Sm), and perovskite Nd isotopic compositions, the TK-1 (macrocrystic variety) and TK-4 (microcrystic variety) kimberlites in this cluster are here classified as orangeites (i.e. Group II kimberlites), with geochemical characteristics that are very similar to orangeites previously described from the Bastar Craton in central India, as well as the Kaapvaal Craton in South Africa. The remaining kimberlites (e.g., TK-2, TK-3 and the TK-1 microcrystic variant), are more similar to other 1100 Ma, Group I-type kimberlites of the Eastern Dharwar Craton, as well as the typical Group I kimberlites of the Kaapvaal Craton. Through the application of geochemical modelling, based on published carbonated peridotite/melt trace element partition coefficients, we show that the generation of the TKC kimberlites and the orangeites results from low degrees of partial melting of a metasomatised, carbonated peridotite.
Abstract: Continental crust is buoyant compared with its oceanic counterpart and resists subduction into the mantle. When two continents collide, the mass balance for the continental crust is therefore assumed to be maintained. Here we use estimates of pre-collisional crustal thickness and convergence history derived from plate kinematic models to calculate the crustal mass balance in the India-Asia collisional system. Using the current best estimates for the timing of the diachronous onset of collision between India and Eurasia, we find that about 50% of the pre-collisional continental crustal mass cannot be accounted for in the crustal reservoir preserved at Earth’s surface today—represented by the mass preserved in the thickened crust that makes up the Himalaya, Tibet and much of adjacent Asia, as well as southeast Asian tectonic escape and exported eroded sediments. This implies large-scale subduction of continental crust during the collision, with a mass equivalent to about 15% of the total oceanic crustal subduction flux since 56 million years ago. We suggest that similar contamination of the mantle by direct input of radiogenic continental crustal materials during past continent-continent collisions is reflected in some ocean crust and ocean island basalt geochemistry. The subduction of continental crust may therefore contribute significantly to the evolution of mantle geochemistry.
International Geology Review, Vol. 58, 12, pp. 1461-1480.
India
Carbonatite
Abstract: The South Indian Granulite Terrane (SGT) is a collage of Archaean to Neoproterozoic age granulite facies blocks that are sutured by an anastomosing network of large-scale shear systems. Besides several Neoproterozoic carbonatite complexes emplaced within the Archaean granulites, there are also smaller Paleoproterozoic (2.4 Ga, Hogenakkal) carbonatite intrusions within two NE-trending pyroxenite dikes. The Hogenakkal carbonatites, further discriminated into sövite and silicate sövite, have high Sr and Ba contents and extreme light rare earth element (LREE) enrichment with steep slopes typical of carbonatites. The C- and O-isotopic ratios [?13CVPDB = ?6.7 to ?5.8‰ and ?18OVSMOW = 7.5-8.7‰ except a single 18O-enriched sample (?18O = 20.0‰)] represent unmodified mantle compositions. The ?Nd values indicate two groupings for the Hogenakkal carbonatites; most samples show positive ?Nd values, close to CHUR (?Nd = ?0.35 to 2.94) and named high-?Nd group while the low-?Nd group samples show negative values (?5.69 to ?8.86), corresponding to depleted and enriched source components, respectively. The 87Sr/86Sri ratios of the two groups also can be distinguished: the high-?Nd ones have low 87Sr/86Sri ratios (0.70161-0.70244) while the low-?Nd group shows higher ratios (0.70247-0.70319). We consider the Nd-Sr ratios as primary and infer derivation from a heterogeneous mantle source. The emplacement of the Hogenakkal carbonatites may be related to Paleoproterozoic plume induced large-scale rifting and fracturing related to initiation of break-up of the Neoarchean supercontinent Kenorland.
Journal of Geological Society of India, Vol. 88, 5, pp. 539-548.
India
Craton, Bundelkhand
Abstract: Two Archaean supracrustal complexes are documented in the Bundelkhand craton viz. 1) the Central Bundelkhand greenstone complex (formed the Babina and Mauranipur belts), and 2) the Southern Bundelkhand metasedimentary (paragneisses, schists) complex (formed the Girar belt). The Central Bundelkhand greenstone complex consists of (i) an early assemblage composed of basic-ultrabasic, felsic volcanics, banded iron formation (BIF), and (ii) a late assemblage which contains felsic volcanics. The U-Pb zircon ages of felsic volcanics from the early assemblage, formed in an arc-type subduction setting, are 2813 Ma (Mesoarchaean). The U-Pb zircon ages of felsic volcanics from the late assemblage, which also occur in subduction settings as an active continental marginal type, are 2542 Ma (Neoarchaean). The Girar belt, located in the southern part of the craton, consists generally of metasedimentary rocks namely quartzites and BIFs. Quartzites are represented by fuchsite- and hematite- bearing quartz arenite and lesser quartz pebble conglomerates that have been subjected to low-grade metamorphism. BIF is represented by thinly-bedded quartz-hematite (±magnetite) rocks. Some lenses of marble and chlorite schist are present at the contact between the two formations. BIF is fairly rich in Cr and Ni, poor in Zr, Hf, Ba, Th, Sr, Yb and Lu, and displays a distinct positive Euanomaly (Eu/Eu* = 1.14-2.46). Zircons from quartzites give two U-Pb ages: 3432±9.7 Ma and 3252±6.4 Ma. The Sm-Nd isotope study of quartzite from the Girar belt shows that the TDM is 3.29 Ga. This TDM correlates well with the U-Pb ages of zircon and indicates that the continental crust in the Girar area began to form in the Paleoarchaean (3.4-3.2 Ga). The Central Bundelkhand greenstone complex was thus formed in a subduction-accretion setting in Mesoarchaean (ca 2.8 Ga) and Neoarchaean (2.54 Ga) time, and the Southern Bundelkhand metasedimentary complex originated in a sedimentary basin on the old continental crust apparently at ca ~ 2.7 Ga.
Abstract: There exist three mainstream opinions regarding the timing of the initial collision between the Indian and Eurasian continents, namely, 65±5, 45±5, and 30±5 Ma. Five criteria are proposed for determining which tectonic event was related to the initial collision between India and Asia: the rapid decrease in the rate of plate motion, the cessation of magmatic activity originating from the subduction of oceanic crust, the end of sedimentation of oceanic facies, the occurrence of intracontinental deformation, and the exchange of sediments sourced from two continents. These criteria are used to constrain the nature of these tectonic events. It is proposed that the 65±5 Ma tectonic event is consistent with some of the criteria, but the upshot of this model is that the magmatic activity originating from the Tethyan subduction since the Mesozoic restarted along the southern margin of the Asian continent in this time after a brief calm, implying that the subduction of the Neotethys slab was still taking place. The magmatic activity that occurred along the southern margin of the Asian continent had a 7-Myr break during 72-65 Ma, which in this study is interpreted as having resulted from tectonic transformation from subduction to transform faulting, indicating that the convergence between the Indian and Asian continents was once dominated by strike-slip motion. The 30±5 Ma tectonic event resulted in the uplift of the Tibetan Plateau, which was related to the late stage of the convergence between these two continents, namely, a hard collision. The 45±5 Ma tectonic event is in accordance with most of the criteria, corresponding to the initial collision between these two continents.
Abstract: In 2015, a field team from GIA visited the Indian city of Jaipur to capture the full scope of its gem and jewelry industry: colored stone cutting, wholesale trading, jewelry design, manufacturing, and retail. The authors documented the current state of the industry from a manufacturing as well as a business perspective. The results substantiated many of the team's prior assessments but also brought to light recent developments with far-reaching effects. The impact of vertical integration, consolidation, globalization, and jewelry television retail far exceeded expectations. Once known as a colored stone manufacturing center, Jaipur has rapidly climbed the value chain into jewelry manufacturing and retail by successfully incorporating experience and tradition with technology and innovation.
Cr-diopsides from Lattavaram and Kalyanadurgam kimberlites, Anatapur district, Andhra Praseh, southern India: inferences from loam sampling.
Department of Geophysics, University College of Science Osmania University, Hyderabad 500 007, March 16, 17, Role of Geophysics in Earth and Environmental studies: special emphasis on mineral exploration 1p. Abstract
National Seminar on Strategic trends and future perspectives in the development of natural resources of Telangana state, Kakatiya University, Abstract Volume, 1, March 30,31 pp. 18-19.
India, Telangana
Lamproites
Abstract: The authigenic carbonates which occur in arid and semi-arid regions of the world are commonly referred to as calcretes or caliche or kankar. These are pedogenic calcretes which occur in association with soil forming the residual regolith. Many rock types produce calcretes upon weathering and denudation, but calcrete derived from certain rocks acts as an exploration guide. Calcrete is a prominent sampling medium in countries like Australia and South Africa whereas it is not so popular in the Indian context. Kimberlites, being ultrapotassic in nature and owing to the enrichment of olivine, serpentine an calcite, often produce calcrete duricrust as a capping on the outcrops. The calcretes derived from kimberlites contain relict kimberlitic xenocrystic minerals like pyrope, ilmenite, Cr-diopside, pseudomorphs of olivine, phlogopite etc. unlike those derived from other rock types. The calcretes derived from granitoid rocks significantly contain minerals like chert, quartz, semi-weathered feldspar etc. Recently more than fifteen lamproites have been discovered at Vattikodu and Chintalapalli and one lamprophyre at Bayyaram of Telangana state, by the Geological Survey of India, unraveling new panorama that the state has a substantial potential for occurrence of more kimberlite clan rocks. Perhaps for the first time, an attempt has been made here to test the geochemical affinity of calcretes from various locations within Nalgonda district, which is endowed mostly with granitic terrain and Cuddapah sedeimentaries in the southern part. About sixteen samples have been collected from the in-situ regolith, spread in the granite-mafic dyke terrain, with an omission of calcretes occurring in transported black soil areas. The samples were geochemically analysed for major and trace elements for a preliminary study. The data has been compared with published geochemical data of lamproites of Ramadugu Field, to understand their geohchemical association. The calcretes are low in SiO2 (33.92-45.1wt %), high in K2O (1.07-2.21wt %) and CaO (0.78-13.61wt %). When compared to other major elements, MgO displays low concentration and K2O has a higher concentration than Na2O. The trace elements are found to be enriched in some of the samples collected in close vicinity of known lamproite occurrences. The samples show a high degree of chemical weathering, alteration and compositional variation indices. It is observed that enrichment of elements like Cr, Nb, Ni indicates, similar to parent kimberlite/lamproite rock, favourable targets for further ground exploration in virgin areas. In the present study, two samples, towards five kilometers southeast of Vattikodu Lamproite Field, possess higher Nb (>25ppm) concentration, which stand out as explorable targets for further ground investigations. Further field investigations such as geological mapping, pitting, petrography and geochemistry on these two locations are in progress to ascertain whether or not these two targets unveil new kimberlites/lamproites in the area.
Abstract: The Central Indian region is having complex geology covering the Godavari Graben, the Bastar Craton (including the Chhattisgarh Basin), the Eastern Ghat Mobile Belt, the Mahanadi Graban and some part of the Deccan Trap, the Northern Singhbhum Orogen and the Eastern Dharwar Craton. The region is well covered by reconnaissance scale aeromagnetic data, analyzed for the estimation of basement and shallow anomalous magnetic sources depth using scaling spectral method. The shallow magnetic anomalies are found to vary from 1 to 3 km whereas magnetic basement depth values are found to vary from 2 to 7 km. The shallowest basement depth of 2 km corresponds to the Kanker granites, a part of the Bastar Craton, whereas deepest basement depth of 7 km is for the Godavari Basin and the southeastern part of the Eastern Ghat Mobile Belts near the Parvatipuram Bobbili fault. The estimated basement depth values correlate well with the values found from earlier geophysical studies. The earlier geophysical studies are limited to few tectonic units whereas our estimation provides detailed magnetic basement mapping in the region. The magnetic basement and shallow depth values in the region indicate complex tectonic, heterogeneity and intrusive bodies at different depth which can be attributed to different thermo-tectonic processes since Precambrian.
Abstract: The Paleo-Mesoproterozoic Zhongtiao aulacogen in the North China Craton and Cuddapah basin in the Indian Craton, have both been interpreted as intra-continental rift formed by a mantle plume that led to the breakup of Columbia supercontinent, but the mechanism has not been completely deciphered. In this paper, the mechanism of the Zhongtiao aulacogen and Cuddapah basin related to initial breakup of Columbia has been evaluated with 2D elastic finite element models of the North China Craton and the Indian Craton. The trajectories of the horizontal maximum principal compressive stress of the best-fit model fit well with the trends of dyke swarms in the North China Craton and the Indian Craton. When the other three models generated were compared with the best-fit model, it can be found that a mantle plume beneath the Zhongtiao and Cuddapah areas played the most vital role in developing the Zhongtiao aulacogen, Cuddapah basin and initial breakup of Columbia supercontinent. The boundary subduction forces, including the northern margin of the NCC, the northwest and southwest margins of the Indian Craton are indispensable factors for the rifting and breakup, whereas the mechanical properties have little influence on these modeling results. The initial breakup of Columbia supercontinent might have been resulted from the coupling between a mantle plume upwelling and some plate tectonic forces.
Abstract: Two Neoproterozoic carbonatite suites of spatially related carbonatites and associated silicate alkaline rocks from Sevattur and Samalpatti, south India, have been investigated in terms of petrography, chemistry and radiogenic–stable isotopic compositions in order to provide further constraints on their genesis. The cumulative evidence indicates that the Sevattur suite is derived from an enriched mantle source without significant post-emplacement modifications through crustal contamination and hydrothermal overprint. The stable (C, O) isotopic compositions confirm mantle origin of Sevattur carbonatites with only a modest difference to Paleoproterozoic Hogenakal carbonatite, emplaced in the same tectonic setting. On the contrary, multiple processes have shaped the petrography, chemistry and isotopic systematics of the Samalpatti suite. These include pre-emplacement interaction with the ambient crustal materials with more pronounced signatures of such a process in silicocarbonatites. Calc-silicate marbles present in the Samalpatti area could represent a possible evolved end member due to the inability of common silicate rocks (pyroxenites, granites, diorites) to comply with radiogenic isotopic constraints. In addition, Samalpatti carbonatites show a range of C–O isotopic compositions, and ?13CV-PDB values between + 1.8 and + 4.1‰ found for a sub-suite of Samalpatti carbonatites belong to the highest values ever reported for magmatic carbonates. These heavy C–O isotopic signatures in Samalpatti carbonatites could be indicative of massive hydrothermal interaction with carbonated fluids. Unusual high-Cr silicocarbonatites, discovered at Samalpatti, seek their origin in the reaction of pyroxenites with enriched mantle-derived alkali-CO2-rich melts, as also evidenced by mantle-like O isotopic compositions. Field and petrographic observations as well as isotopic constraints must, however, be combined with the complex chemistry of incompatible trace elements as indicated from their non-uniform systematics in carbonatites and their individual fractions. We emphasise that, beside common carriers of REE like apatite, other phases may be important for incompatible element budgets, such as mckelveyite–(Nd) and kosmochlor, found in these carbonatites. Future targeted studies, including in-situ techniques, could help further constrain temporal and petrologic conditions of formation of Sevattur and Samalpatti carbonatite bodies.
Abstract: New mineralogical and bulk-rock geochemical data for the recently recognised Mesoproterozoic (ca. 1100 Ma) and late Cretaceous (ca. 90 Ma) kimberlites in the Timmasamudram cluster (TKC) of the Wajrakarur kimberlite field (WKF), Eastern Dharwar Craton, southern India, are presented. On the basis of groundmass mineral chemistry (phlogopite, spinel, perovskite and clinopyroxene), bulk-rock chemistry (SiO2, K2O, low TiO2, Ba/Nb and La/Sm), and perovskite Nd isotopic compositions, the TK-1 (macrocrystic variety) and TK-4 (Macrocrystic variety) kimberlites in this cluster are here classified as orangeites (i.e. Group II kimberlites), with geochemical characteristics that are very similar to orangeites previously described from the Bastar Craton in central India, as well as the Kaapvaal Craton in South Africa. The remaining kimberlites (e.g., TK-2, TK-3 and the TK-1 microcrystic variant), are more similar to other 1100 Ma, Group I-type kimberlites of the Eastern Dharwar Craton, as well as the typical Group I kimberlites of the Kaapvaal Craton. Through the application of geochemical modelling, based on published carbonated peridotite/melt trace element partition coefficients, we show that the generation of the TKC kimberlites and the orangeites results from low degrees of partial melting of a metasomatised, carbonated peridotite. Depleted mantle (TDM) Nd perovskite model ages of the 1100 Ma Timmasamudram kimberlites show that the metasomatic enrichment of their source regions are broadly similar to that of the Mesoproterozoic kimberlites of the EDC. The younger, late Cretaceous (ca. 90 Ma) TK-1 (macrocrystic variant) and TK-4 kimberlites, as well as the orangeites from the Bastar Craton, share similar Nd model ages of 1100 Ma, consistent with a similarity in the timing of source enrichment during the amalgamation of Rodinia supercontinent. The presence of late Cretaceous diamondiferous orangeite activity, presumably related to the location of the Marion hotspot in southern India at the time, suggests that thick lithosphere was preserved, at least locally, up to the late Cretaceous, and was not entirely destroyed during the breakup of Gondwana, as inferred by some recent geophysical models.
Abstract: Manganilmenite is found to be associated with the magnetite ore body of Pokphur area in the Nagaland ophiolites, North East India. There is perhaps no earlier description of the mineral from the Indian subcontinent. It occurs as an accessory mineral with magnetite and Fe-chlorite (chamosite). Electron probe micro-analytical data reveal that the mineral contains 5.6–8.5 wt% MnO and traces of MgO, ZnO and Cr2O3, while the TiO2 content remains within narrow limits of 50–53 wt%. The calculated pyrophanite end-member varies from 13% to 18%. Although the magnetite body of Pokphur has been reasonably proved to be a hydrothermally altered product of basic and ultrabasic igneous rocks, and most of the minerals in the magnetite body are supergene in nature, different end-member compositions of mangan–ilmenite indicate that it has originally crystallized with the basic suite of rocks and has survived the alteration process with only marginal effects. Since manganilmenite has been considered as a diamond indicator mineral and ophiolites are a newly documented host of microdiamonds elsewhere in the world, the presence of manganilmenite in the Pokphur magnetite hints towards occurrence of microdiamonds in the ophiolite suite of rocks of the Indo-Myanmar ranges.
Role of Geophysics in Earth and Environmental Studies , March 1p. Abstract
India
mineralogy
Abstract: A comparison of major element content in Cr-diopside mineral grains, from loam samples, of two kimberlite pipes each from Lattavaram (P-3 and P-4) and Kalyanadurgam (KL-1 and KL-2) clusters of Wajrakarur Kimberlite Field (WKF) has been presented here. The two selected Lattavaram pipes are well exposed whereas the Kalyanadurgam pipes are concealed under 1.5 to 2 meter thick alluvium and calcrete, which is endowed with easily identifiable kimberlitic indicator minerals (KIMs). The indicator minerals are mantle derived xenocrystic types like pyrope garnet, Cr-diopside, ilmenite, chromite and olivine which provide inferences on their petrogenesis. It is observed that Cr-diopside is a prominent mineral constituent in both these locations and plays a vital role in reconnaissance diamond exploration. Surface in-situ loam sampling was carried out and ~15 kg of sample has been collected, to segregate heavy minerals which were concentrated by panning and jigging. In total, 66 Cr-diopside grains in total; 26 from Lattavaram and 40 from Kalyanadurgam were picked under the microscope and studied for their major element geochemistry by EPMA. In Lattavaram area, 25 grains were identified to be of C5 class and one grain belongs to C3. In Kalyandurgam, it is observed that 39 grains belong to CP5 category and one grain to C2 class. The range of Cr2O3 weight% for Lattavaram samples is 0.94- 2.8 and that for Kalyanadurgam samples is 0.54- 6.34. It is envisaged that the entries of Fe, Al, Na, Ca, and Cr into the clinopyroxene structure are strongly affected by the P-T-X conditions during mineral crystallization. It is observed that the mantle derived kimberlitic Cr-diopside is low in Fe-content relative to that of crustal rocks. This study revealed that Cr-diopsides of investigated pipes are of kimberlitic nature and plot in the diamond inclusion field thereby signifying the prospectivity of the pipes.
Abstract: The Samchampi (26° 13?N: 93° 18?E)-Samteran (26° 11?N: 93° 25?E) alkaline complex (SSAC) occurs as an intrusion within Precambrian basement gneisses in the Karbi-Anglong district of Assam, Northeastern India. This intrusive complex comprises a wide spectrum of lithologies including syenite, ijolite-melteigite, alkali pyroxenite, alkali gabbro, nepheline syenite and carbonatite (nepheline syenites and carbonatites are later intrusives). In this paper, we present new major, trace, REE and Sr-Nd isotope data for different lithologies of SSAC and discuss integrated petrological and whole rock geochemical observations with Sr-Nd isotope systematics to understand the petrogenetic evolution of the complex. Pronounced LILE and LREE enrichment of the alkaline-carbonatite rocks together with steep LREE/HREE profile and flat HREE-chondrite normalized patterns provide evidence for parent magma generation from low degree partial melting of a metasomatized garnet peridotite mantle source. LILE, HFSE and LREE enrichments of the alkaline-silicate rocks and carbonatites are in agreement with the involvement of a mantle plume in their genesis. Nb-Th-La systematics with incompatible trace element abundance patterns marked by positive Nb-Ta anomalies and negative K, Th and Sr anomalies suggest contribution from plume-derived OIB-type mantle with recycled subduction component and a rift-controlled, intraplate tectonic setting for alkaline-carbonatite magmatism giving rise to the SSAC. This observation is corroborated by enriched 87Sr/86Srinitial (0.705562 to 0.709416) and 143Nd/144Ndinitial (0.512187 to 0.512449) ratios for the alkaline-carbonatite rocks that attest to a plume-related enriched mantle (~ EM II) source in relation to the origin of Samchampi-Samteran alkaline complex. Trace element chemistry and variations in isotopic data invoke periodic melting of an isotopically heterogeneous, metasomatized mantle and generation of isotopically distinct melt batches that were parental to the different rocks of SSAC. Various extents of plume-lithosphere interaction also accounts for the trace element and isotopic variations of SSAC. The Srinitial and Ndinitial (105 Ma) isotopic compositions (corresponding to ?Nd values of ? 6.37 to ? 1.27) of SSAC are consistent with those of Sung Valley, Jasra, Rajmahal tholeiites (Group II), Sylhet Traps and Kerguelen plateau basalts.
Abstract: A mineralogical study of the hypabyssal facies, late Cretaceous macrocrystic pulse of TK1 intrusion and the Mesoproterozoic aphanitic pulse of TK4 intrusion in the Wajrakarur Kimberlite Field of southern India shows that the rocks contain macrocrysts of forsteritic olivine, phenocrysts and microphenocrysts of Al–Na-poor diopside and phlogopite set in a groundmass mainly of Al–Na-poor diopside and phlogopite. Other groundmass minerals are spinel, perovskite and fluorapatite in TK1, and spinel, titanite, chlorite, calcite and gittinsite in TK4. K-richterite and perovskite occur only as inclusions in phlogopite and titanite, respectively in TK4. Late-stage deuteric phases include pyrite and barite in TK1, and strontianite, chalcopyrite, galena and pentlandite in TK4. Diopside microphenocrysts in TK4 exhibit oscillatory zoning with characteristics of diffusion controlled magmatic growth. TK1 spinels show magmatic trend 2 that evolves from magnesiochromite and culminates in titaniferous magnetite, whereas TK4 spinels are less evolved with magnesiochromite composition only. TK1 phlogopites show a simple compositional trend that is typical of lamproite micas, while four distinct growth zones are observed in TK4 phlogopites with the following compositional characteristics: zone I: high Cr2O3 and TiO2 and low BaO; zone II: low Cr2O3; zone III: low TiO2 and high BaO; zone IV: low BaO. Forsterite contents and trace element concentrations reveal two xenocrystic core populations and one magmatic rim population for TK1 olivines. Mineralogically, both TK1 and TK4 are classified as diopside–phlogopite lamproites rather than archetypal kimberlites. The two lamproites are considered to have formed from the same parent magma but crystallised under distinct oxygen fugacity conditions. With elevated content of Fe3 + in phlogopite, spinel and perovskite, TK1 appears to have crystallised in a relatively high oxygen fugacity environment. Multiple growth generations of phlogopite, spinel and fluorapatite in TK4 indicate a complex evolutionary history of the magma. Close spatial and temporal associations of Mesoproterozoic kimberlites and lamproites in southern India can possibly be explained by a unifying model which accounts for the generation of diverse magmas from a range of geochemical resevoirs in a continental rift setting.
Abstract: The first comprehensive and authoritative history of the Koh-i Noor, arguably the most celebrated and mythologised jewel in the world. On 29 March 1849, the ten-year-old Maharajah of the Punjab was ushered into the magnificent Mirrored Hall at the centre of the great Fort in Lahore. There, in a public ceremony, the frightened but dignified child handed over to the British East India Company in a formal Act of Submission to Queen Victoria not only swathes of the richest land in India, but also arguably the single most valuable object in the subcontinent: the celebrated Koh-i Noor diamond.
Abstract: A mineralogical study of the hypabyssal facies, late Cretaceous macrocrystic pulse of TK1 intrusion and the Mesoproterozoic aphanitic pulse of TK4 intrusion in the Wajrakarur Kimberlite Field of southern India shows that the rocks contain macrocrysts of forsteritic olivine, phenocrysts and microphenocrysts of Al–Na-poor diopside and phlogopite set in a groundmass mainly of Al–Na-poor diopside and phlogopite. Other groundmass minerals are spinel, perovskite and fluorapatite in TK1, and spinel, titanite, chlorite, calcite and gittinsite in TK4. Krichterite and perovskite occur only as inclusions in phlogopite and titanite, respectively in TK4. Late-stage deuteric phases include pyrite and barite in TK1, and strontianite, chalcopyrite, galena and pentlandite in TK4. Diopside microphenocrysts in TK4 exhibit oscillatory zoning with characteristics of diffusion controlled magmatic growth. TK1 spinels show magmatic trend 2 that evolves from magnesiochromite and culminates in titaniferous magnetite, whereas TK4 spinels are less evolved with magnesiochromite composition only. TK1 phlogopites show a simple compositional trend that is typical of lamproite micas, while four distinct growth zones are observed in TK4 phlogopites with the following compositional characteristics: zone I: high Cr2O3 and TiO2 and low BaO; zone II: low Cr2O3; zone III: low TiO2 and high BaO; zone IV: low BaO. Forsterite contents and trace element concentrations reveal two xenocrystic core populations and one magmatic rim population for TK1 olivines. Mineralogically, both TK1 and TK4 are classified as diopside–phlogopite lamproites rather than archetypal kimberlites. The two lamproites are considered to have formed from the same parent magma but crystallised under distinct oxygen fugacity conditions. With elevated content of Fe3+ in phlogopite, spinel and perovskite, TK1 appears to have crystallised in a relatively high oxygen fugacity environment. Multiple growth generations of phlogopite, spinel and fluorapatite in TK4 indicate a complex evolutionary history of the magma. Close spatial and temporal associations of Mesoproterozoic kimberlites and lamproites in southern India can possibly be explained by a unifying model which accounts for the generation of diverse magmas from a range of geochemical resevoirs in a continental rift setting.
Journal of Asian Earth Sciences, in press available, 47p.
India, Africa, Madagascar
tectonis
Abstract: It has long been recognised that Madagascar was contiguous with India until the Late Cretaceous. However, the timing and nature of the amalgamation of these two regions remain highly contentious as is the location of Madagascar against India in Gondwana. Here we address these issues with new U-Pb and Lu-Hf zircon data from five metasedimentary samples from the Karwar Block of India and new Lu-Hf data from eight previously dated igneous rocks from central Madagascar and the Antongil-Masora domains of eastern Madagascar. New U-Pb data from Karwar-region detrital zircon grains yield two dominant age peaks at c. 3100 Ma and c. 2500 Ma. The c. 3100 Ma population has relatively juvenile ?Hf(t) values that trend toward an evolved signature at c. 2500 Ma. The c. 2500 Ma population shows a wide range of ?Hf(t) values reflecting mixing of an evolved source with a juvenile source at that time. These data, and the new Lu-Hf data from Madagascar, are compared with our new compilation of over 7000 U-Pb and 1000 Lu-Hf analyses from Madagascar and India. We have used multidimensional scaling to assess similarities in these data in a statistically robust way. We propose that the Karwar Block of western peninsular India is an extension of the western Dharwar Craton and not part of the Antananarivo Domain of Madagascar as has been suggested in some models. Based on ?Hf(t) signatures we also suggest that India (and the Antongil-Masora domains of Madagascar) were palaeogeographically isolated from central Madagascar (the Antananarivo Domain) during the Palaeoproterozoic. This supports a model where central Madagascar and India amalgamated during the Neoproterozoic along the Betsimisaraka Suture.
Abstract: Stable calcium isotopic composition (44/40Ca) of silicate rock standards show limited variability [c.f., 1] although, fractionation between co-exiting ortho- and clino-pyroxenes have been reported [2]. Variability in 44/40Ca in Hawaiian shield stage tholeiites have been interpreted as evidence of subducted ancient marine carbonates, with very low 44/40Ca, into the Hawaiian plume [3]. Carbonatites are unique mantlederived carbonate-bearing igneous rocks with limited spatial but wide temporal occurrences. Few available measurements (n=5) of 44/40Ca in whole rock and leached carbonatites show a 0.2 ‰ range but broadly overlapping values with mantle-derived silicate rocks from different tectonic settings [1,4]. However, boron isotopic composition of global carbonatites suggest the contribution of subducted crustal component to the mantle source of relatively young carbonatites (<300 Ma old) [5], a signature which should potentially be traceable using Ca isotopes. We report 44/40Ca of global carbonatites ranging in age from Proterozoic to recent. The samples were analyzed using a 43Ca- 48Ca double spike on a Thermo Fischer Triton Plus Thermal Ionization Mass Spectrometer (TIMS) at IISc. 44/40Ca in the carbonatites (n = 11) range from 0.47 - 0.97 ‰ (w.r.t. SRM 915a). Our external reproducibility, estimated from multiple analyses of NIST standards SRM 915a, SRM 915b and seawater (NASS6), is better than 0.1 ‰ (2SD). 44/40Ca of the ~65 Ma old Ambadongar carbonatites of India, associated with eruption of the Deccan Traps, show correlations with Nb/Yb, K/Rb as well as with Sr/Nb, Sr/Zr. These variations suggest the role of phlogopite versus amphibole in the mantle source as well as subducted carbonates in controlling the 44/40Ca of these carbonatites.
Abstract: The nitrogen isotopic composition of mantle samples lie between -15 to -5 ‰ and, is different from subducted sediments that varies between +6 to +15 ‰. As a result, N can be an excellent tracer of a subducted component [e.g. 1]. We have studied N from Indian carbonatites of adjacent locations in Hogenakal (2700 Ma) and Sevattur (770 Ma) with established mantle origin. We have also studied two diamonds representative of the Witwatersrand basin (2900–2700 Ma) and Jagersfontein (1100-1700 Ma) belonging to the Kapvaal supergroup. Both India and the Kapvaal craton formed part of a supercontinent that persisted as coherent units until the breakup of Pangea. Comparing the N in these diamonds with the carbonatites will put constraints on differences in their sources and the role of subduction in their generation. The nitrogen isotopic composition of the carbonatites are comparable ranging between -3 to +13 ‰, although value as low as -22 ‰ is observed from an apatite from Hogenakal. The N contents vary from 130 to 6000 ppb. Of the diamonds, Jagersfontein have ?15N of +2.3 and +9.3 ‰, while those from the Witwatersrand basin are –2.3 and 0 ‰. Their nitrogen lie between 500-900 ppm. These results have important implications for their formation and sources.
Abstract: EPMA data are obtained from the P-5 kimberlite from the Wajrakarur field in the Eastern Dharwar craton of southern India (EDC). The studied sample consists of xenocrysts and xenoliths set in a variable grain size groundmass of olivine (with two textures: rounded-anhedral and subhedraleuhedral), phlogopite, perovskite, spinel, pyroxene, spinel and spinel containing Ti-garnet core. Ti-rich garnet associated with spinel is a rare occurrence in kimberlites. Two types of spinel have been identified (a) fine grained (<80 ?m) and compositionally non titaniferous, and (b) large macrocrysts (>100 ?m) having replacement cores having distinctly Ti-rich (TiO2 up to 28.51 wt %) compositions. Spinel is an abundant phase varying from <20 to >300 ?m in size, mostly subhedral to euhedral in shape. Pipe-5 has atolland necklace-textured spinels in addition to the euhedral groundmass spinels. Apart from individual grains in ground mass spinel there are also spinel intergrowths with perovskite (no apparent reaction texture observed), and sieve-like intergrowths. The composition of groundmass spinel is extensively used as petrogenetic indicator mineral (Roeder and Schulze 2008). Ti-garnets contain significant Ti (21.25-28.51wt.% TiO2), Ca (15.45-27.69 wt.% CaO), Fe (2.62-24.46 wt.% FeO) and low Cr (0.08-1.52 wt.% Cr2O3) and low Al (1.40-3.87 wt.% Al2O3). Ti- garnets and their paragenetic relationships to spinel are considered here as vital petrogenetic indicators of metasomatic fluids (Dongre et al., 2016; Cheng et al., 2014), and textural association with spinel shows that Ti-garnet formed when early crystallizing spinel interacted with residual melt during magma crystallization.
Abstract: We have studied noble gases in carbonates and apatites from three carbonatites of South India, namely Hogenakal (2400 Ma), Sevattur (770 Ma) and Khambamettuu (523 Ma) by vacuum crushing. Apatite has also been analysed by pyrolysis. Vacuum crushing mostly releases the trapped gas components. The ratios 21Ne/20Ne, 22Ne/20Ne and 40Ar/36Ar increase with progressive crushing due to preservation of different composition gases in smaller inclusions released in later steps. This heterogeneity of isotopic composition of fluid inclusions is a consequence of the involvement of magmas carrying different noble gas signatures. The inclusions with lower ratios suggest the presence of a subducted atmospheric component, while the higher 21Ne/20Ne, 22Ne/20Ne and 40Ar/36Ar can be attributed to the presence of an enriched lithospheric mantle component. In addition, very minor trapped gases from less degassed, deeper mantle may also be present but overprinted by lithospheric and/or nucleogenic components. We propose that these carbonatites were generated only in an advanced stage of magmatism when this lithospheric component overwhelmed any contribution from the deeper mantle source. The lithospheric mantle underwent enrichment during an ancient subduction process through mantle metasomatism manifested in nucleogenic/radiogenic isotopic ratios of 21Ne/20Ne, 22Ne/20Ne and 40Ar/36Ar. The apatites analysed by pyrolysis clearly show nucleogenic 21Ne from 18O(?,n) reaction. We have demonstrated the potential of using U,Th–21Ne systematics as a thermo-chronometer in conjunction with the established U,Th–4He and U–136Xe clocks. While for Hogenakal, the U,Th–21Ne age of 845 ± 127 Ma is in agreement with the age of emplacement of other adjacent younger carbonatites, syenites and alkali granites, for the Sevattur apatite (738 ± 111 Ma) it indicates the crystallisation age.
Abstract: We report preliminary data for sulfur isotopes from two spatially related Neoproterozoic carbonatite complexes in Tamil Nadu, S India, with the aim of getting further insights into their magmatic and/or post-emplacement histories [1]. The major sulfide phase in these rocks is pyrite, with minor chalcopyrite, whereas sulfate occurs as barite. A bimodal distribution of G34Ssulfide is found for Samalpatti (13.5 to 14.0‰), and Sevattur (?2.1 to 1.4‰) carbonatites. A significantly larger range of G34Ssulfide values is found for the associated Samalpatti silicate rocks (?5.2 to 7.4‰) relative to Sevattur pyroxenites and gabbros (?1.1 to 2.1‰). High G34Ssulfide values for Samalpatti carbonatites are unsual [2,3] but could reflect hydrothermal post-emplacement modification [1] of S isotopes. The low G34Ssulfide values for Sevattur may represent a mantle source signature. The G34Ssulfate is uniformly positive for both complexes, with most data falling in a narrow range (5.7 to 7.8‰) and one datum for a pyroxenite yielding more positive G34Ssulfate = 13.3‰. Data for '33S varies outside of analytical uncertainty (?0.07 to 0.04‰), indicating contribution from a source with a surface-derrived component. The small range of '33S values does not allow us to determine whether these sources contain S fractionated by biogeochemical (mass-dependent) or photochemical (mass-independent, pre GOE) processes. Data for '36S is positive, and varies within uncertainty (0.28 ± 0.15‰). Variations of this magnitude have been observed in other localities, and are not diagnostic of any unique source or process. The sulfur isotope data imply addition of crustal sulfur to Samalpatti. In contrast, sulfur from Sevattur has a mantle-like G34S but '33S with anomalous character. These observations support the idea of a different evolutionary story for these complexes, possibly more complex than previously thought.
Abstract: Calcium isotope compositions are presented for two suites of carbonatites and associated alkaline silicate rocks from Neoproterozoic Sevattur and Samalpatti complexes in Tamil Nadu, South India. Despite their geographic proximity, the mean G44/40Ca values are different for Sevattur (G44/40Ca = 0.69 r 0.10‰, n = 7) and Samalpatti (0.81 r 0.16‰, n = 5). The former suite is derived from an enriched mantle source without significant post-emplacement modifications [1] and its Ca isotope composition falls to the lower end of Ca isotope range reported for mantle-derived rocks [2]. Some carbonatites from Samalpatti show a 44Ca-enriched signature which could reflect large-scale low-temperature modification, recognized also by their 13C–18O-enriched isotope systematics and sizeable loss of REE, when compared to pristine carbonatites from the area [1]. This is also consistent with albite–epidote metasomatic sample and shistose pyroxenite from Samalpatti, both showing a 44Ca-depleted signature. Leaching experiments confirm a systematic G44/40Ca offset with isotopically light carbonate relative to bulk sample [also 3]. Pyroxenites from Samalpatti are isotopically heavier than accompanying unmodified carbonatites and their G44/40Ca values fall into the mantle range. In contrast, pyroxenite and phosphate from Sevattur have a G44/40Ca value identical with associated carbonatites, indicating a homogeneous mantle source for the latter complex. For K-rich syenites and monzonites, 40K-decay corrections need to be considered for the intrinsic mass-dependent isotope fractionations considering the Neoproterozoic age and high K/Ca character of some samples.
Earth and Planetary Science Letters, Vol. 474, pp. 59-67.
Mantle, India-Eurasia
subduction
Abstract: We present three-dimensional numerical models to investigate the dynamics of continental collision, and in particular what happens to the subducted continental lithosphere after oceanic slab break-off. We find that in some scenarios the subducting continental lithosphere underthrusts the overriding plate not immediately after it enters the trench, but after oceanic slab break-off. In this case, the continental plate first subducts with a steep angle and then, after the slab breaks off at depth, it rises back towards the surface and flattens below the overriding plate, forming a thick horizontal layer of continental crust that extends for about 200 km beyond the suture. This type of behaviour depends on the width of the oceanic plate marginal to the collision zone: wide oceanic margins promote continental underplating and marginal back-arc basins; narrow margins do not show such underplating unless a far field force is applied. Our models show that, as the subducted continental lithosphere rises, the mantle wedge progressively migrates away from the suture and the continental crust heats up, reaching temperatures >900?°C. This heating might lead to crustal melting, and resultant magmatism. We observe a sharp peak in the overriding plate rock uplift right after the occurrence of slab break-off. Afterwards, during underplating, the maximum rock uplift is smaller, but the affected area is much wider (up to 350 km). These results can be used to explain the dynamics that led to the present-day crustal configuration of the India–Eurasia collision zone and its consequences for the regional tectonic and magmatic evolution.
Abstract: The paper presents new palaeomagnetic results and reassesses complete set of published palaeomagnetic results on the lamproite intrusions in the Gondwana formations of the Eastern India. Altogether eleven sites register reliable characteristic magnetisations corresponding to the c. 110 Ma emplacement age of the lamproites. A mean ChRM is estimated with D = 331.3°; I = ?62.4° (?95 = 6.2°, k = 55; N = 11). The palaeomagnetic pole of ? = 14.9°: ? = 287.6° (A95 = 8.4°) is established for the lamproites and it averaged the secular variation and confirms to the Geocentric Axial Dipole (GAD). The pole compares remarkably well with the grand mean pole reported for the Rajmahal traps that are attributed to represent location of the Kerguelen mantle plume head. The palaeolatitudes transferred to Rajmahal coordinates (25.05°: 87.84°) are situated ?6° north of the present location of the Kerguelen hotspot location. The interpretations are consistent with earlier suggestions of southward migration of the plume based on palaeomagnetic results of Site 1138 of the ODP Leg 183 and with the predictions of numerical models of global mantle circulation.
Abstract: The Neoproterozoic Sevattur and Samalpatti alkaline– carbonatite complexes in S India were supposedly emplaced into regional metagranite at ~800 Ma [1]. Both complexes are close to each other (~4 km apart), with a similar NE–SW elongated oval shape arranged along NE–SW trending lineament formed by the Koratti–Attur tectonic zone [2]. Both complexes share a similar setting with central syenite intrusion mantled with a discontinuous ring and/or crescentshaped suites of carbonatites, pyroxenites, gabbros, and dunites. In contrast to identical tectonic position and similar structure, the two complexes differ significantly in geochemistry and Sr–Nd–Pb–O–C isotope compositions. The Sevattur suite is derived from an enriched mantle source without significant post-emplacement modification whilst extensive hydrothermal overprint by crustal fluids must have occurred to result in the observed 13C–18O-enriched systematics reported for the Samalpatti carbonatites [3]. Some Samalpatti pyroxenites, though, show a clear mantle signature [3]. We report preliminary K–Ar age-data, that indicate a prolonged period of the magmatic activity in this area. Sevattur gabbro and pyroxenite (both Bt-fraction) as well as one Samalpatti Cr-rich silicocarbonatite (Amp-fraction) yielded the range of ages at 700–800 Ma, consistent with previous reports [see 3 for details]. The new K–Ar data from syenites display significantly younger ages of 560–576 Ma for Samalpatti and 510–540 Ma for Sevattur, regardless of the mineral fraction used (Bt or Kfs). The K–Ar results are being supplemented by systematic U–Pb analyses of zircons. If proven true, the age disparity would have profound consequences on our understanding of carbonatite evolution.
Abstract: Deformed Alkaline Rocks and Carbonatites (DARCs) are markers of suture zones where continents have rifted apart and later amalgamated [1]. Petrological and geochronological data indicates that parts of India and East Antarctica may have been involved in several episodes of collision and breakup during the assembly of past supercontinents [2]. DARCs at the eastern margin of the Eastern Ghats Province (EGP) in India preserve the record of these amalgamation and breakup events. It is thought that the Napier Complex of East Antarctica collided with the Dharwar Craton of India at ca. 1.60 Ga forming the central and eastern Indian shield [3]. New zircon U-Pb ages from DARCs at the EGP margin show that the alkaline complexes (Kamakhyanagar: 1350±14 Ma Rairakhol: 1379±6 Ma; Khariar: 1478±5 Ma; Koraput: 1387±34 Ma; Kunavaram: 1360±5 Ma; Jojuru: 1352±6 Ma) were emplaced in a narrow time interval. The alkaline magmatism marks an episode of rifting in the Indo-Antarctic continental fragment, correlatable with breakup of the Columbia supercontinent. Metamorphic zircon from the alkaline rocks furnish age populations at 917-950 Ma, 792- 806 Ma and 562-569 Ma. The 917-950 Ma ages are correlated with the closure of an oceanic basin between the Ruker Terrane of East Antarctica and the Indian Shield during the assembly of the Rodinia supercontinent. This led to the collision of the Ruker Terrane with the combined India-Napier Complex producing the Grenville-age EGPRayner Complex orogen [2, 3]. The 792-806 Ma ages record the disintegration of Rodinia when Greater India started to break away from East Antarctica [4]. In the early Paleozoic, India reconverged towards Antarctica and Australia during Gondwanaland assembly. The 562-569 Ma zircon ages date the resulting collisions during Pan-African orogenesis.
Geological Society of London Special Publication, Vol. 457, pp. 339-351.
India
Tectonics
Abstract: In this short paper, we outline the potential links between India and the East Antarctica region from Enderby Land to Princess Elizabeth Land using the Mesozoic East Gondwana configuration as a starting point. Palaeomagnetic data indicate that East Gondwana did not exist prior to the Ediacaran-Cambrian. Early Neoproterozoic (1050-950 Ma) deformation in East Antarctica and along the Eastern Ghats Province in India marks the initial contact between the two regions. Volcanism in the Kerguelen hotspot led to final break-up of India and East Antarctica in the Cretaceous. Although connections between the Archaean and Proterozoic provinces of India and East Antarctica have been proposed, the current record of large igneous provinces (or dyke swarms), palaeomagnetic data and geochronology do not show a consistently good match between the two regions.
Abstract: Mineralogy and geochemistry of the Udirpikonda lamprophyre, located within the Mesoproterozoic diamondiferous Wajrakarur kimberlite field (WKF), towards the western margin of the Paleo-Mesoproterozoic Cuddapah basin are presented. The lamprophyre is characterised by a panidiomorphic-porphyritic texture imparted by clinopyroxene, olivine and biotite set in a groundmass of feldspar and spinel. Olivine occurs as the microphenocrysts with a composition range of Fo87-78. Clinopyroxenes display reverse as well as oscillatory optical zoning and are diopsidic in nature with a variation in the composition from core (Wo47 En28 Fs20Ac5) to rim (Wo46En41Fs11Ac3). Biotite (Mg# < 0.6) is the only mica present and spinels are titano-magnetites showing ulvospinel- magnetite solid solution. Plagioclase is the dominant feldspar with a variable compositional range of An41-8Ab82-56Or33-3. Based on the mineralogy, the lamprophyre can be classified to be of calc-alkaline variety but its geochemistry display mixed signals of both alkaline and calc-alkaline lamprophyres. K2O/Na2O ranges from 1.49 to 2.79, making it distinctly potassic and highlights its shoshonitic character. Moderate Mg# (60-65), Ni (110-200 ppm) and Cr (110-260 ppm) contents in the bulk-rock indicate substantial fractional crystallization of olivine and clinopyroxene. Fractionated chondrite normalized REE patterns (average (La/Yb)N = 37.56) indicates involvement of an enriched mantle source from within the garnet stability field whereas slightly negative Ta-Nb-Ti and Hf anomalies displayed on the primitive mantle normalized multi-element spider gram highlight involvement of a subducted component in the mantle source. Given the spatial disposition of the studied lamprophyre, the age of the emplacement is considered to be coeval with WKF kimberlites (~ 1.1 Ga) and the initial 143Nd/144Nd (0.510065-0.510192) and 87Sr/86Sr (0.705333-0.706223) are strikingly similar to those observed for the Smoky Butte lamproites, Montana, USA. Fluid-related subduction enrichment of the mantle source is apparent from the enriched ratios of La/Nb, Ba/Nb and (Hf/Sm)N, (Ta/La)N < 1. Petrogenetic modelling reveals melt generation from 1 to 2% partial melting of an enriched mantle source that subsequently underwent fractional crystallization. Our study provides geochemical and isotopic evidence for a sub-continental lithospheric mantle (SCLM) modified by subduction and asthenospheric upwelling in the Eastern Dharwar Craton. The partial melting of a resulting heterogeneous Eastern Dharwar Craton SCLM to generate Udiripikonda lamprophyre and Wajrakarur kimberlites has been attributed to the Mesoproterozoic regional lithospheric extension event.
Abstract: Re-Os and platinum group element analyses are reported for peridotite xenoliths from the 533 Ma Venetia kimberlite cluster situated in the Limpopo Mobile Belt, the Neoarchaean collision zone between the Kaapvaal and Zimbabwe Cratons. The Venetian xenoliths provide a rare opportunity to examine the state of the cratonic lithosphere prior to major regional metasomatic disturbance of Re-Os systematics throughout the Phanerozoic. The 32 studied xenoliths record Si-enrichment that is characteristic of the Kaapvaal lithospheric mantle and can be subdivided into five groups based on Re-Os analyses. The most pristine group I samples (n = 13) display an approximately isochronous relationship and fall on a 3.28 ± 0.17 Ga (95 % conf. int.) reference line that is based on their mean TMA age. This age overlaps with the formation age of the Limpopo crust at 3.35-3.28 Ga. The group I samples derive from ?50 to ?170 km depth, suggesting coeval melt depletion of the majority of the Venetia lithospheric mantle column. Group II and III samples have elevated Re/Os due to Re addition during kimberlite magmatism. Group II has otherwise undergone a similar evolution as the group I samples with overlapping 187Os/188Os at eruption age: 187Os/188OsEA, while group III samples have low Os concentrations, unradiogenic 187Os/188OsEA and were effectively Re-free prior to kimberlite magmatism. The other sample groups (IV and V) have disturbed Re-Os systematics and provide no reliable age information. A strong positive correlation is recorded between Os and Re concentrations for group I samples, which is extended to groups II and III after correction for kimberlite addition. This positive correlation precludes a single stage melt depletion history and indicates coupled remobilisation of Re and Os. The combination of Re-Os mobility, preservation of the isochronous relationship, correlation of 187Os/188Os with degree of melt depletion and lack of radiogenic Os addition puts tight constraints on the formation and subsequent evolution of Venetia lithosphere. First, melt depletion and remobilisation of Re and Os must have occurred within error of the 3.28 Ga mean TMA age. Second, the refractory peridotites contain significant Re despite recording >40 % melt extraction. Third, assuming that Si-enrichment and Re-Os mobility in the Venetia lithospheric mantle were linked, this process must have occurred within ?100 Myr of initial melt depletion in order to preserve the isochronous relationship. Based on the regional geological evolution, we propose a rapid recycling model with initial melt depletion at ?3.35 Ga to form a tholeiitic mafic crust that is recycled at ?3.28 Ga, resulting in the intrusion of a TTG suite and Si-enrichment of the lithospheric mantle. The non-zero primary Re contents of the Venetia xenoliths imply that TRD model ages significantly underestimate the true depletion age even for highly depleted peridotites. The overlap of the ?2.6 Ga TRD ages with the time of the Kaapvaal-Limpopo collision is purely fortuitous and has no geological significance. Hence, this study underlines the importance of scrutiny if age information is to be derived from whole rock Re-Os analyses.
Journal of Asian Earth Sciences, in press available, 17p.
India
geodynamics
Abstract: The Pitepani volcanic suite of the Dongargarh Supergroup, central India comprises of a calc-alkaline suite and a tholeiitic suite, respectively. The rare earth element (REE) patterns, mantle normalized plots and relict clinopyroxene chemistry of the Pitepani calc-alkaline suite are akin to high-Mg andesites (HMA) and reveal remarkable similarity to the Cenozoic Setouchi HMA from Japan. The Pitepani HMAs are geochemically correlated with similar rocks in the Kotri-Dongargarh mobile belt (KDMB) and in the mafic dykes of the Bastar Craton. The rationale behind lithogeochemical correlations are that sanukitic HMAs represent fore-arc volcanism over a very limited period of time, under abnormally high temperature conditions and are excellent regional and tectonic time markers. Furthermore, the tholeiitic suites that are temporally and spatially associated with the HMAs in the KDMB and in the mafic dykes of the Bastar Craton are classified into: (a) a continental back-arc suite that are depleted in incompatible elements, and (b) a continental arc suite that are more depleted in incompatible elements, respectively. The HMA suite, the continental back-arc and continental arc suites are lithogeochemically correlated in the KDMB and in the mafic dykes of the Bastar Craton. The three geochemically distinct Neoarchaean magmatic suites are temporally and spatially related to each other and to an active continental margin. The identification of three active continental margin magmatic suites for the first time, provides a robust conceptual framework to unravel the Neoarchaean geodynamic evolution of the Bastar Craton. We propose an active continental margin along the Neoarchaen KDMB with eastward subduction coupled with slab roll back or preferably, ridge-subduction along the Central Indian Tectonic Zone (CITZ) to account for the three distinct magmatic suites and the Neoarchean geodynamic evolution of the Bastar Craton.
Proceedings of XXXIV held Aug. 4-9. Perchuk International School of Earth Sciences, At Miass, Russia, 1p. Abstract
India
mineral chemistry
Abstract: The present study examines an unexplored catchment (60 Sq. Km.) at Nutimadugu village, in Anantapur district, on the right bank of Penna River (Fig.1), to attest the occurrence of kimberlite indicator minerals (KIMs). The left bank is famous for diamondiferous kimberlites known as Timmasamudram Kimberlite Cluster. About eight stream sediment samples (~15kg in weight), were collected from the catchment with an approximate sample sapcing of one kilometer. The -1mm fraction of the stream sediments was reduced to heavy mineral concentrate (HMC) using traditional techniques. The selected mineral grains were probed for major elements using electron probe microanalysis (EPMA). Majority (75%) are chromite grains. Uvaravite and grossularite garnets and few grains of ilmenite are also present. The chromite grains plot in the world wide kimberlitic chromite field, with few in the diamond inclusion field (Fig.2 & 3). The garnet grains plot in the G12 field and found to be wehrlitic in character (Fig.4 & 5). On the contrary, the ilmenite grains plot in the non-kimberlitic field (Fig. 6), which might have been derived from the supracrustal rocks occurring to the east of the catchment. Thus this study highlights that the catchment stands as a priority target for further field investigations.
Khanna, T.C., Subba Rao, D.V., Bizimis, M., Satyanarayanan, M., Krishna, A.K., SeshaSai, V.V.
~2.1 Ga intraoceanic magmatism in the central India tectonic zone: constraints from the petrogenesis of ferropicrites in the Mahakoshal suprarcustal belt.
Thesis, Phd. Osmania University 258p., http://shodhganga.inflibnet.ac.in/handle/10603/178609.
India, Andhra Pradesh
deposit - Lattavaram, Anumpalli
Abstract: The thesis is organized into six chapters. The first chapter enunciates general concepts of kimberlite geology covering literature, previous work, definitions, classification, mode of occurrence, regional geological history, global and Indian occurrences of kimberlites. The second chapter elucidates the common principles and practices applied and adopted in diamond exploration applicable to the Indian context, to some extent exemplifying the kimberlite clusters of the study area. A six-stage exploration strategy, applicable to Indian geological scenario, has been proposed. The third chapter portrays the general geological setting of the study area comprising Lattavaram and Anumpalli kimberlite clusters along with spatial morphologies of the pipes and various field geological characteristics illustrated through field photographs. The fourth chapter describes various mineralogical and petrographic characteristics observed in the pipes and their associated calcretes as well of the study area emphasising their genetic significance. The fifth chapter characteristically articulates the whole rock geochemistry with the aid of major, trace and rare earth element analyses to depict the classification of the pipes under study. This chapter also demonstrates calcrete geochemistry of calcretes associated with the kimberlite pipes of the study area in detail, perhaps for the first time. The sixth chapter describes the petrogenetic inferences including source region, partial melting, temperature, density and viscosity etc., derived from geochemical analyses and thereby demonstrates the diamond prospectivity of Lattavaram and Anumpalli kimberlite pipes. In a nutshell, this research work aims to present a detailed account of petrography, geochemistry, petrogenesis and diamond prospectivity of kimberlites from Lattavaram and Anumpalli clusters in light of recently discovered pipes. For the first time, petrographic and geochemical analyses of kimberlitic calcretes are presented and interpreted.
Russian Mineralogical Society 200th. Anniversary meeting Oct. 10-13., 4p. Abstract pdf
India
deposit - Pipe-5, Wajrakakarur
Abstract: In an attempt to study the kimberlite indicator minerals (KIMs), loam sampling in the close vicinity of the pipes has been carried out on some kimberlites of the WKF. In this paper, preliminary results of KIMs derived out of loam sampling on one of the WKF pipes at Muligiripalli (pipe-5) are presented. Surface loam sampling has been carried out in the topographically low-lying areas in the close proximity of the pipe outcrop. Four composite samples, at a spacing of ~100 meters, weighing 25 kg each have been collected and sieved through 1 mm mesh. Approximately 5 kg of ‘-1’ fraction has been obtained after sieving which is further subjected to coning and quartering. The material has been subjected to heavy mineral (HM) separation using a manual jig. The HM further screened through Frantz’s isodynamic separator to separate magnetic mineral grains. The HM assemblage includes ilmenite, chromite, Cr-diopside, olivine and sphene. ). To ascertain the kimberlitic nature of the HM grains and thereby to check diamondiferous nature of this pipe, additonal loam sampling to obtain more number of mineral grains to carry out Electron Microprobe Analysis (EPMA) is planned.
Abstract: Potassic and ultrapotassic magmatism from deep lithospheric sources in intra-cratonic settings can be the signal of subsequent voluminous mafic magmatism and the formation of Large Igneous Provinces (LIPs) triggered by mantle plumes. Here we report for the first time, precise zircon U-Pb age data from a suite of lamproites in the Bastar Craton of central India that mark the onset of Paleoproterozoic rifting and culminating in the formation of extensive mafic dyke swarms as the bar codes of one of the major LIP events during the Precambrian evolution of the Indian shield. The lamproites from the Nuapada field occur as dismembered dykes and are composed of phenocrysts and microphenocrysts of altered olivine together with microphenocrysts of phlogopite and magnetite within a groundmass of chlorite and calcite with accessory rutile, apatite and zircon. The rocks compositionally correspond to olivine phlogopite lamproite and phlogopite lamproite. Geochemical features of the lamproites correlate with their counterparts in Peninsular India and other similar suites elsewhere in the world related to rift settings, and also indicate OIB-like magma source. The associated syenite shows subduction-related features, possibly generated in a post-collisional setting. Magmatic zircon grains with high Th/U ratios in the syenite from the Nuapada lamproite form a coherent group with an upper intercept age of 2473 ± 8 Ma representing the timing of emplacement of the magma. Zircon grains in three lamproite samples yield four distinct age groups at ca. 2.4 Ga, 2.2 Ga, 2.0 Ga and 0.8 Ga. The 2.4 Ga group corresponds to xenocrysts entrained from the syenite whereas the 2.2 Ga group is considered to represent the timing of emplacement of the lamproites. The ca. 2.0 Ga zircon grains correlate with the major thermal imprint associated with mafic magmatism and dyke emplacement in southern Bastar and the adjacent Dharwar Cratons. A few young zircon grains in the syenite and lamproites show a range of early to middle Neoproterozoic ages from 879 to 651 Ma corresponding to younger thermal event(s) as also represented by granitic veins cutting across these rocks and extensive silicification. Zircon Lu-Hf isotope data suggest magma derivation from a refertilized Paleo-Mesoarchean sub-continental lithospheric mantle, or OIB-type sources. The differences in Hf-isotope composition among the zircon grains from different age groups indicate that the mantle sources of the lamproite are heterogeneous at the regional scale. A combination of the features from geochemical and zircon Hf isotope data is consistent with asthenosphere-lithosphere interaction during the lamproite magma evolution. The timing of lamproite emplacement in central India correlates with the global 2.2 Ga record of LIPs. We link the origin of the related mantle plume to the recycling of subducted slabs associated with the prolonged subduction-accretion history prior to the Neoarchean cratonization, as well as the thermal blanket effect of the Earth’s oldest supercontinent. Pulsating plumes and continued rifting generated voluminous dyke swarms across the Bastar and Dharwar Cratons, forming part of a major global rifting and LIP event.
Ackerman, L., Magna, T., Rapprich, V., Upadhyay, D., Kratky, O., Cejkova, B., Erban, V., Kochergina, Y.V., Hrstka, T.
Contrasting petrogenesis of spatially related carbonatites from Samalpatti and Sevattur, Tamil Nadu, India: insights from trace element and isotopic geochemistry.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 31-33.
India
deposit - Samalpatti, Sevattur
Abstract: The Tamil Nadu region in southern India hosts several carbonatite bodies (e.g., Hogenakal, Samalpatti, Sevattur, Pakkanadu-Mulakkadu) which are closely associated with alkaline silicate rocks such as syenites, pyroxenites or dunites (e.g, Kumar et al., 1998; Schleicher et al., 1998; Srivastava, 1998). This is in contrast to the carbonatite occurrences in north-western India associated with the Deccan Trap basalts (e.g., Amba Dongar) or Proterozoic Newania dolomitic carbonatites. We have studied two, spatially related, Neoproterozoic carbonatite-silico(carbonatite) suites in association with alkaline silicate rocks (e.g., pyroxenite, gabbro) from Sevattur and Samalpatti in terms of petrography, chemistry and radiogenic-stable isotopic compositions in order to provide constraints on their genesis and evolution. In these two bodies, several different carbonatite types have been reported previously with striking differences in their trace element and isotopic compositions (Srivastava, 1998; Viladkar and Subramanian, 1995; Schleicher et al., 1998; Pandit et al., 2002). Collected data for previously poorly studied calcite carbonatites from the Sevattur representing the first carbonatite magmas on this locality, indicate similar geochemical characteristics to those of dolomitic carbonatites, such as high LREE/HREE ratios, very high Sr and Ba contents, large amounts of apatite and magnetite, identical Sr-Nd-C-O isotopic compositions. Thus, they were derived from an enriched mantle source without significant post-emplacement modifications through crustal contamination and hydrothermal overprint, in agreement with previous studies (e.g., Schleicher et al., 1998). Detailed microprobe analyses revealed that high levels of some incompatible elements (e.g., REE, Y, Sr, Ba) cannot be accounted by matrix calcite hosting only significant amounts of SrO (~0.6-1.2 wt.%). On the other hand, abundant micro- to nano-scale exsolution lamellae and/or inclusions of mckelveyite-(Nd) appear to host a significant fraction of LREE in parallel with apatite. Distribution of Sr is most likely influenced also by common but heterogeneously dispersed barite and strontianite. Newly acquired as well as detailed inspection of available geochemical data permits distinguish two different types of carbonatites in Samalpatti: (1) Type I similar to Sevattur carbonatites in terms of mineralogy, trace element and radiogenic-stable isotopic compositions and (2) Type II with remarkably low concentrations of REE and other incompatible trace elements, more radiogenic Sr isotopic compositions and extremely variable C–O isotopic values. The petrogenesis of the Type II seems to be intimately associated with the presence of silicocarbonatites and abundant silicate mineral domains. Instead of liquid immiscible separation from a silicate magma, elevated SiO2 contents observed in silico-carbonatites may have resulted from the interaction of primary carbonatitic melts and crustal rocks prior to and/or during magma emplacement. Arguments for such hypothesis include variable, but radiogenic Sr isotopic compositions correlated with SiO2 and other lithophile elements (e.g., Ti, Y, Zr, REE). Calc-silicate marbles present in the Samalpatti area could represent a possible evolved crustal end member for such process due to the inability of common silicate rocks (pyroxenites, granites, diorites) to comply with radiogenic isotopic constraints. The wide range of C-O isotopic compositions found in Samalpatti carbonatites belong to the highest values ever reported for magmatic carbonates and can be best explained by massive hydrothermal interaction with carbonated fluids. Unusual high-Cr silicocarbonatites were discovered at Samalpatti forming centimetre to decimetre-sized enclaves enclosed in pyroxenites with sharp contacts at hand specimen scale. Detailed microprobe analyses revealed peculiar chemical compositions of the Mgamphibole with predominantly sodic composition embaying and replacing Na-Cr-rich pyroxene (kosmochlor), accompanied by the common presence of Cr-spinel and titanite. Such association have been reported for hydrous metasomatism by Na-rich carbonatitic melts at upper mantle conditions (Ali and Arai, 2013). However, the mineralogy and the mode of occurrence of Samalpatti Mg–-r-rich silicocarbonatites argue against such origin. We explain the petrogenesis of these rocks through the reaction of pyroxenites with enriched mantle-derived alkali-CO2-rich melts, as also evidenced by mantle-like O and Hf isotopic compositions.
Balasubramani, S., Sahoo, P., Bhattacharya, D., Rengarajan, M., Thangavel, S., Bhatt, A.K., Verma, M.B., Nanda, L.K.
A note on anomalous concentration of scandium in the Pakkanadu alkaline complex, Salem District, Tamil Nadu, India.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 46.
India
alkaline rocks
Abstract: Pakkanadu Alkaline complex (PAC) of Neoproterozoic age is located at the southwestern end of Dharmapuri rift/shear zone on the northern part of southern granulitic terrain in Tamil Nadu, India. PAC mainly comprises carbonatite-syenitepyroxenite suite of rocks. Syenite is the predominant rock exposed on the eastern and western part of the explored area with enclaves of pyroxenite and dunite. The carbonatite (sovite) occurs as thin veins/bands and discontinuous lenticular bodies intrusive into highly deformed biotite schist that is considered as the fenitised product of pyroxenite traceable over a strike length of 1.5 km. Petromineralogical study of the biotite schist, pyroxenite containing carbonatite rock and carbonatite indicated presence of monazite, allanite, sphene and betafite as the main radioactive minerals occurring as inclusion within biotite or as discrete mineral grains. Other ore minerals are apatite, thorite, titanite, rutile and barite. Chloritisation, hematitisation, silicification and calcitisation are the main wall rock alteration observed in pyroxenite and syenite. Sub-surface exploration carried out by Atomic Minerals Directorate (AMD) in PAC revealed that biotite schist (n=166) contains anomalously high concentration of Scandium (11-1275 ppm, av.161 ppm), REE (67-58275 ppm, av. 14836 ppm,) and V (5-620 ppm, av. 127 ppm, with carbonatite veins and syenite (n=149) contain scandium (10-462 ppm, av.71 ppm,), REE (18-57510 ppm, av. 4106 ppm) and V (1-285 ppm, av. 48 ppm). In these rocks, LREE (12.5-57670 ppm, av. 9617 ppm, n=315) shows enrichment over HREE (7.1-774 ppm, av. 173 ppm, n=315). The concentration of Scandium (Av. 166 and 71 ppm in biotite schist and syenite respectively) is anomalous as compared to its crustal abundance (22 ppm). Geochemical analyses of the rock indicate that the radioactive biotite schist, pyroxenite containing carbonatite veins generally shows higher Sc and REE concentrations as compared to those of the other rocks (syenite). However, there is no significant correlation between REE and Sc. The higher concentration of scandium in PAC is possibly due to selective partitioning of it into minerals like apatite, pyrochlore, allanite, monazite and other REE bearing phases, apart from its concentration in the ferromagnesian minerals. Scandium rarely concentrates in nature as independent ore mineral. The demand for the metal is very high due to multiple high value commercial uses as an alloy with aluminum, specifically in aerospace and automobile industry, besides, in solid oxide fuel cells (SOFC) in electrical industries. Eight boreholes drilled as part of the preliminary subsurface exploration in PAC, covering an area of 0.05 sq. km, indicated an elevated Scandium content of about 6 times that of the average crustal abundance.
Major, trace element compositions and Nd, Sr and stable Ca isotopic compositions of carbonatites and alkaline silicate rocks of the Amba Dongar carbonatite complex, India: role of mantle mineralogy and subducted carbonates.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 18.
India
deposit - Amba Dongar
Abstract: Carbonatites, with limited spatial but wide temporal occurrences, are unique igneous rocks with more than 50% modal carbonate. Geochemically carbonatites are characterized by high La/Yb(N) and enrichment in the Ba, Sr concentrations and superchondritic whole-rock Nb/Ta (~35) and Zr/Hf ratios (~60). Most of the global carbonatites are associated with the alkaline silicate rocks and their origin is highly debated. To understand the petrogenesis of carbonatites, we present geochemical and isotopic data, including the first ever measurements of Ca stable isotopes in the ~65 Ma old carbonatites and associated silicate rocks occurring in the Amba Dongar carbonatitic complex in Western India We have performed a detail geochemical and isotopic investigation of the carbonatites and associated silicate rocks occurring in the Amba Dongar carbonatitic complex in Western India. The analysed carbonatites are primarily calcitic and ankeritic in nature. The associated silicate rocks can be classified into two groups, the first being highly Krich and alkaline while the second group is tholeiitic in nature. The high La/Yb(N) ratio of carbonatites are suggestive of low degree partial melting while the enrichment in large ion lithophile elements (LILE) of the silicate rocks suggest that these rocks are derived from a LILE enriched upper mantle source. The carbonatites and the associated alkaline silicate rocks from the Amba Dongar carbonatite complex show overlapping Nd-Sr isotopic compositions with the tholeiitic rocks from the Phenai Mata complex, located approximately 16 km NW of Amba Dongar. Variability in ? 44/40Ca in Hawaiian shield stage tholeiites have been interpreted as evidence of subducted ancient marine carbonates, with very low -44/40Ca, into the Hawaiian plume (Huang et al., 2010). Boron isotopic composition of global carbonatites suggests that subducted crustal components contributed to the mantle source of relatively young carbonatites (<300 Ma old) (Hulett et al., 2016), a signature which should potentially be traceable using Ca isotopes. We report -44/40Ca of carbonatites and associated alkaline silicate rocks from the Amba Dongar complex. The samples were analyzed using a 43Ca-48Ca double spike on a Thermo Fischer Triton Plus Thermal Ionization Mass Spectrometer (TIMS) at IISc. ?44/40Ca in these rocks show a significant variation (~0.6 ‰- (w.r.t. SRM 915a), which is much larger than the variation observed in limited analyses of global carbonatites (Amini et al., 2009). Our external reproducibility, estimated from multiple analyses of NIST standards SRM 915a, SRM 915b and seawater (NASS6), is better than 0.1 - (2SD). ?44/40Ca of the ~65 Ma old Amba Dongar carbonatites shows correlations with Nb/Yb, K/Rb as well as with Sr/Nb, Sr/Zr. These variations suggest the role of phlogopite versus amphibole in the mantle source as well sas subducted carbonates in controlling the ?44/40Ca of these carbonatites.
Gautam, I., Bhutani, R., Balakrishnan, S., Chatterjee, A., George, B.G., Ray, J.S.
142Nd/144Nd of alkaline magmas in Phenai Mat a complex, Chhota Udaipur, Deccan flood basalt province.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 14.
India
alkaline rocks
Abstract: The 65 million year old alkaline plug at Phenai Mata Complex, in Chota Udaipur sub province, is often linked to the last pulse of the Deccan volcanism. However, many believe that the Deccan-Reunion mantle plume that was responsible for the generation of flood basalts might not have been the source of Phenai Mata. It, however, could have acted as a heat source for these magmas derived from the subcontinentallithospheric- mantle (SCLM). Since the SCLM is generally considered to be a nonconvective mantle domain it has the potential to preserve some of the geochemical evidence of the early silicate Earth differentiation, e.g., 142Nd anomaly. In search of such signatures we analysed alkali basalts from the complex for their 142Nd/144Nd using high precision thermal ionization mass spectrometry. Whereas the geochemical characterization of these samples confirmed the lithospheric origin of their source magmas, their ? 142Nd compositions are found to be normal with respect to terrestrial standards. We infer that either the mantle source of Phenai Mata does not represent a true non-convective mantle or it is too young to retain any evidence of 146Sm decay.
Noble gas composition of Indian carbonatites ( Amba Dongar, Siriwasan): implications on mantle source compositions and late stage hydrothermal processes.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 10.
India
deposit - Amba Dongar, Siriwasan
Abstract: Within a stepwise crushing study we determined the noble gas composition of several calcite separates, one aegirine and one pyrochlore-aegirine separate of the carbonatite ring dyke complex of Amba Dongar and carbonatite sill complex of Siriwasan, India. Both carbonatites are related to the waning stages of volcanic activity of the Deccan Igneous Province ca. 65 Ma ago. Major observations are a clear radiogenic 4He* and nucleogenic 21Ne* imprint related to insitu production from U and Th in mineral impurities, most likely minute apatite grains. However, in first crushing steps of most calcites from Amba Dongar a well-resolvable mantle neon signal is observed, with lowest air-corrected mantle 21Ne/22Ne-compositions equivalent to the Réunion hotspot mantle source. In case of the aegirine separate from Siriwasan we found a neon composition similar to the Loihi hotspot mantle source. We conclude that previously derived models of a lithospheric mantle source containing recycled components in generation of the carbonatitic magmas from Amba Dongar are obsolete. Instead, the mantle source of both investigated carbonatite complexes is related to a primitive mantle plume source that we tentatively ascribe to the postulated Deccan mantle plume. If, as is commonly suggested, the present location of the Deccan mantle plume source is below Réunion Island, the currently observed more nucleogenic neon isotopic composition of the Réunion hotspot might be obliterated by significant upper mantle contributions, similar to Kilauea Volcano, Hawai’i. In addition, compared with other carbonatite complexes worldwide a rather significant contribution of atmospheric noble gases is observed. This is documented in cut-off 20Ne/22Ne-ratios of ca. 10.2 (Amba Dongar) and 10.45 (Siriwasan) and cut-off 40Ar/36Ar-ratios of about 1500. This atmospheric component likely had been added at shallow levels during the emplacement process. However, understanding the late-stage interaction between atmospheric gases and magmatic mantle fluids still requires further investigation.
Jadhav, G.N., Viladkar, S.G., Goswami, R., Badhe, K.
Fluid melt inclusions petrography of primary calcites from carbonatites of Amba Dongar, Gujarat India.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 15.
India
deposit - Amba Dongar
Abstract: The Amba Dongar Carbonatite complex consists of sovites which are dominantly composed of calcite along with pyrochlore, phlogopite, apatite, barite, ankerite and haematite and minor opaques such as magnetite, chalcopyrite and pyrite. Two distinct types of texture are present in these carbonatites- a mosaic of equigranular calcite crystals and porphyritic texture. Silicate melt inclusions are observed in primary minerals viz. apatite and calcites. These are small droplets of silicate melt entrapped during the growth of the minerals. In this case carbonatite-alkaline silicate melt inclusions are entrapped predominantly in calcite crystals. Dominantly these calcite host minerals are predominantly containing fluid inclusions along with halite, sylvite and minor nahcolite as daughter crystals. The presence of calcite with nahcolite indicates the coexistence of a Ca-rich, alkali-bearing carbonatite melt phase. The melt inclusions are heated upto 1100 °C and the carbonate melt inclusions appear to be homogenized around 950 °C. This fall within the range of melting temperature of a carbonatite melt. In addition to these, three types of fluid inclusions were also observed in host calcite they are i) monophase, ii) biphase and iii) polyphase types of fluid inclusions. The fluid inclusions contain CO2 gas, Li-K carbonate phases and fergusonite based on Micro-Laser- Raman. Carbon dioxide is the dominant gas phase in most of the fluid inclusions, indicating high temperature and deep mantle source(?). The fluid inclusions have formed from a primary mother liquor that has separated out from the early formed carbonatitic melt. This fluid was either formed just after the formation of melt inclusions or during simultaneous crystallization from a carbonatitic or to be more precise carbonatiticpegmatite melt(?).The presence of both melt and fluid inclusions in these primary calcite host minerals indicates the presence of a carbonatitic-pegmatitic fluid, which must have got separated out from the early formed carbonatite-alkaline silicate magma.
Carbonatites of India: part 1. Field relations, petrology, mineralogy and economic aspects.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 1-2.
India
carbonatites
Abstract: Carbonatites of India have been reviewed by Krishnamurthy (1988; 2008) and Viladkar (2001). The present review in two parts incorporates all the developments in the field of carbonatites from India since 1963. Carbonatites of India occur in some well-defined geological environments and structural set-ups, and belong to four age groups namely, Palaeoproterozoic, Neoproterozoic, Cretaceous and Palaeocene. The Proterozoic ones are found in the three shield areas, namely southern (e.g., Hogenakal, Sevathur, Samalpatti, Pakkanadu, Khambammettu and Munnar), eastern (e.g. Beldi-Kutni and others) and north-western (e.g., Newania) India, often associated with deep faults and shear zones that may define terrain boundaries (e.g. carbonatites of Tamil Nadu between the Dharwar granite-greenstone schist belt and the southern Indian granulite zone). The Cretaceous and Palaeocene ones (e.g., Amba Dongar, Sirivasan, Sung Valley, Samchampi, Sarnu-Dandali-Kamthai and others) have been found to be related to the flood basalt provinces of Rajmahal, Sylhet (eastern and north-eastern India) and the Deccan (western India). Based on the field relations and associated rock types, the carbonatite-alkaline rock complexes can be grouped into four major types, namely: (a) syenite-dominated complexes with subordinate pyroxenites ± dunites (e.g. Sevathur, Samalpatti, Pakkanadu, and Samchampi); (b) pyroxenite/gabbro dominated ± dunite, ijolite, melteigite with minor syenite (e.g. Sung Valley, Swangre; Mer-Mundwara); (c) carbonatite dominated ringcomplexes or dykes with minor nephelinite and phonolite (e.g. Amba Dongar, Sarnu- Dandali, Kamthai); (d) Sheet-like, minor dykes and veins of carbonatites either alone or with syenites (e.g., Newania, Kunavaram, Eichuru, Munnar and others). Carbonatitekimberlite- lamproite-lamprophyre association has been clearly seen in the Precambrian Wajrakarur kimberlite field (e.g. Chelima dykes and Khaderpet cluster, Andhra Pradesh) and in the Jungal Valley (Mahakhoshal Group, Uttar Pradesh). Such an association from the Cretaceous Deccan basalt province has been shown to exist from Kutch, Gujarat and the Chhatishgargh-Odhisha areas. A wide variety of fenites, notably the syenitic types comprising sodic, sodic-potassic, and potassic variants have been noticed from several complexes, such as Amba Dongar, Newania, Sevattur, Samchampi, and Sung Valley. Fenitisation is attributed to both carbonatite and alkaline rocks as at Amba Dongar, Sevattur, Sung Valley, and Samchampi or to carbonatite alone (e.g. Newania and others).Among the carbonatite types, sovites (calcitic types) are the most common in most of the localities. Beforsitic (dolomitic) and ankeritic/sideritic types occur in complexes which manifest well developed differentiation trends that range from sovite to beforsite or to ankeritic and sideritic types, as exemplified by complexes such as Amba Dongar, Sevattur, Samalpatti, Newania and Sung Valley. Associated alkaline rocks, as mentioned above, enable the grouping of the complexes into four types. Heterogeneity in terms of structures, mineralogy, and chemistry is characteristic of many carbonatite bodies. Apart from the dominant carbonate-minerals such as calcite, dolomite, ankerite and siderite in the major carbonatite types, a variety of minor minerals have also been found in them. Early phase apatite-magnetite and silicate minerals (olivine, aegirineaugite, ritcherite, riebeckite, phlogopite and others) are well-developed in deep-seated plutonic complexes such as Sevattur, Newania, Sung Valley, Samalpatti, Pakkanadu, and Hogenekal. Some uncommon carbonatite types include those containing Fe-Nb rutile and benstonite from Samalpatti and eschynite, monazite, cerianite, celestite, and allanitebearing types from Pakkanadu, and magnesite from Newania. Minerals of economic importance, often in workable concentrations, occur in several complexes. These include: 1. REE minerals consisting of bastnaesite-(La) and daqingshanite-(Ce), bastnaesite-(Ce), ancylite and synchysite occur at Kamthai; bastnaesite and parasite from ankeritic carbonatites at Amba Dongar; bastnaesite-(Ce), ancylite-(Ce), belovite-(Ce), and britholite-(Ce) at Sung Valley. REE also occur as substituted elements in apatite in many complexes. 2. Pyrochlore - often uraniferous, occur at Sevathur, Sung Valley, Newania and Samchampi; 3. Apatite and/or phosphatic rocks (e.g. Beldih-Kutni, Samchampi, Sung, Sevathur and Newania). 4. Ti-magnetite/ hematite deposit at Samchampi. In addition a large fluorite deposit occurs at Amba Dongar and both vermiculite and apatite are mined from the fenitised-pyroxenite envelope to the north of the Sevathur carbonatite-complex. Evaluation of field association of pyroxenite-fenites in carbonatite-syenite association along with development of carbo-thermal and/or pegmatitic and skarn-rock facies in some complexes such as Samalpatti and Pakkanadu in Tamil Nadu suggests strong possibilities of Sc mineralization in some (e.g. 0.02% Sc from Pakkanadu pyroxenite) or Sc along with possible HREE associations.
Carbonatites of India: part 2. Geochemistry, stable and unstable isotopes and petrogenesis.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 26-28.
India
carbonatites
Abstract: Geochemically carbonatites and genetically associated alkaline rocks represent an anomalous association of both large-ion lithophile (LIL) elements including the highfield strength (HFS) elements group such as Sr, Ba, Zr, Nb, REE, Y, Sc, Th, and U (excluding Rb) often from trace (< 0.1%) to minor/major components (> 0.1-1%) besides Ca, Mg, Fe, Mn, Si, Ti, Al, P, Na, K and CO2 in major components. Extreme heterogeneity in terms of elemental abundances is in fact a characteristic feature, often at a single outcrop level, in many carbonatite complexes (e.g. Amba Dongar, Sevathur, Sung Valley). Such apparent chemical diversity is related to the mineralogical heterogeneity that is not uncommon in many carbonatite complexes, leading to diverse mineral prefixes in carbonatite types such as apatite-sovite, apatite-magnetite soviet, riebeckite beforsite, silico-carbonatite and numerous other types (e.g. Sevathur, Samalpatti and Pakkanadu). The most diagnostic geochemical character of carbonatites stem from their geochemical features, especially the higher abundances of LIL and HFS elements, often the highest among the diverse igneous rock types as also compared to the primitive mantle or sedimentary or metamorphosed limestone/or marble or calc-silicate rocks. This has been shown from several studies of Indian carbonatites (Krishnamurthy, 1988; Schleicher et. al. 1998 and others). Radiogenic and stable isotopic ratios have been used since the mid 1990’s on Indian carbonatites which range in age from mid Proterozoic to Cretaceous in both rift related settings and associated with large igneous provinces, apparently related to deep mantleplumes, to provide constraints on the evolution of the sub-continental mantle through time. Various mantle reservoirs like HIMU (A mantle source enriched in U and Th believed to be due to recycling of ancient altered oceanic crust into the mantle), DMM (Depleted MORB mantle), EM1 (Enriched Mantle 1, generated either by recycling of lower crustal material or enrichment by mantle metasomatism) and EM2 (Enriched Mantle 2, possibly formed by recycling of continentally derived sediment, or ocean island crust into the mantle by subduction processes) with distinct isotopic signatures in the Sr- Nd-Pb isotopic space have been invoked to explain the observed variations in isotopic ratios in carbonatites worldwide (Zindler and Hart, 1984 and others). Stable isotopes of Indian carbonatites have been comprehensively reviewed by Ray and Ramesh (2009). Based on ?13C and ?18O variations, carbonatites have been grouped by them into: 1. Primary, unaltered ?18O values (5.3-7.5‰) which indicate mantle signatures that ensue from batch crystallization under plutonic conditions, as observed at Hogenakal, Sung Valley and Samchampi. ?13C values, however, appear to be more enriched (-6 to - 3.1‰) than expected for the mantle. Such a feature of enrichment probably happened sometime around ~2.4 Ga, as a sequel to metasomatism by fluids derived from recycled oceanic crust through subduction that carried enriched carbon of lithospheric mantle. 2. Secondary, altered carbonatites’ (e.g. mainly Amba Dongar and others) showing wide variations in ?13C and ?18 O values apparently results from low temperature alteration by either meteoric water or CO2-bearing aqueous fluids. The values of ??Sr (+5.3 to +7.8), ??Nd ( +1.7 to + 2.3) and initial Pb ratios (19.02, 15.67 and 39.0) for the Sung Valley complex and ?Sr (+3.0 to + 9.3) and ?Nd (+0.45 to +2.3) and initial Pb ratios ( 206Pb/204Pb= 19.12, 207Pb/204Pb= 15.66 and 208Pb/204Pb= 39.56) for the Samchampi alkaline complex are well constrained and indicate that they have originated from isotopically similar source regions that are characterised by somewhat higher Rb/Sr ratio relative to bulk earth, minor LREE depletion with respect to CHUR and time integrated enhancement of the U/Pb ratio relative to bulk earth. However, carbonatites from Sirivasan and Amba Dongar (Srivatsava and Taylor, 1996, Simonetti et al., 1995, Ray and Ramesh, 2006) indicate higher values with ?Sr = +14.6 to +21.8, ?Nd = -0.6 to -1.84 and measured 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of 19.0, 15.6 and 39.3 and indicate greater enrichment in terms of higher Rb/Sr ratios and LREE enrichment with respect to CHUR. Differences in the north eastern complexes and western complexes are also seen in the stable isotopic data wherein, data for both Sung Valley and Samchampi are constrained with average values of -3.1 ± 0.1‰ for ?13C and 6.33 ± 0.2‰ and -3.1 ± 0.2‰ for ?13C and 7.34 ± 0.7‰ for ?18O respectively whereas data from Amba Dongar and Sirivasan have ?13C of -2.6 to -8.6 ‰ and ?18O of 7.62 to 26.8 ‰. Heterogeneous mantle source has been proposed for the Hogenakal carbonatites with two groups one having high ??Nd and low ??Sr and the other having low ??Nd and high ??Sr. Carbonatites from Sevattur are more enriched with ??Sr (22 to 23), ??Nd ( -5.1 to -5.7) and ?13C ( -4.8 to -6.2‰) and ?18O (6.7 to 7.6 ‰) (Schleicher et.al., 1996, Pandit., et al. 2016). Petrogenetic models of the different carbonatite complexes are reviewed in the light of geochemical and isotopic characteristics. These include models that invoke mantle plumes of both the Kerguelen (e.g. Sung Valley and Samchampi) and Reunion (e.g. Amba Dongar, Sarnu-Dandali and others related to the Deccan volcanism) and their influence on the subcontinental lithosphere. Enriched mantle sources have been indicated for many of the Proterozoic complexes of Tamil Nadu. Evaluations of the different carbonatite complexes in terms of the three known genetic models, listed as follows, have also been elucidated. These include: (a) Direct partial melts from enriched, carbonatedperidotitic sources; (b. Immiscible carbonate and silicate magma after differentiation of the primary, carbonated peridotitic magma; (c) Extreme stage of differentiation of the ultra-alkaline, nephelinite magma. Such approaches also lead us to understand the temporal evolution of the mantle source regions of carbonatites of India since Palaeoproterozoic times. The petrogenetic link between carbonatite-kimberlite-lamproitelamprophyre in the Indian scenario is also briefly reviewed.
Abstract: The Central Indian region has a complex geology covering the Godavari Graben, the Bastar Craton (including the Chhattisgarh Basin), the Eastern Ghat Mobile Belt, the Mahanadi Graben and some part of the Deccan Trap, the northern Singhbhum Orogen and the eastern Dharwar Craton. The region is well covered by reconnaissance-scale aeromagnetic data, analysed for the estimation of basement and shallow anomalous magnetic sources depth using scaling spectral method. The shallow magnetic anomalies are found to vary from 1 to 3 km, whereas magnetic basement depth values are found to vary from 2 to 7 km. The shallowest basement depth of 2 km corresponds to the Kanker granites, a part of the Bastar Craton, whereas the deepest basement depth of 7 km is for the Godavari Basin and the southeastern part of the Eastern Ghat Mobile Belt near the Parvatipuram Bobbili fault. The estimated basement depth values correlate well with the values found from earlier geophysical studies. The earlier geophysical studies are limited to few tectonic units, whereas our estimation provides detailed magnetic basement mapping in the region. The magnetic basement and shallow depth values in the region indicate complex tectonic, heterogeneity, and intrusive bodies at different depths, which can be attributed to different thermo-tectonic processes since Precambrian.
Magna, T., Rapprich, V., Wittke, A., Gussone, N., Upadhyay, D., Mikova, J., Pecskay, Z.
Calcium isotope systematics and K-Ar and U-Pb temporal constraints on the genesis of Sevattur Samalpatti carbonatite silicate alkaline complexes.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 34-35.
India
deposit - Samalpatti, Sevattur
Abstract: We present the first systematic survey of Ca isotope compositions in carbonatites and associated silicate rocks from Samalpatti and Sevattur, two Neoproterozoic complexes in Tamil Nadu, south India. Despite their close geographic proximity, their genesis and post-emplacement histories differ (Ackerman et al. 2017). The Sevattur complex appears to have been derived from an enriched mantle source with a limited post-magmatic disturbance. In contrast, carbonatites from Samalpatti show a record of extensive late-stage post-magmatic overprint, also apparent from unusually heavy C-O isotope compositions in a sub-suite of carbonatites (Ackerman et al. 2017). The mean ?44/40Ca = 0.69 ± 0.10‰ is slightly lighter than the average of fertile, unmetasomatized peridotites at ?44/40Ca = 0.95 ± 0.05‰ (Kang et al. 2017). This difference may attest to the general difference between carbonates and silicates (see Kang et al. 2017). It could also reflect Ca isotope fractionation between isotopically heavy silicate and isotopically light carbonate (e.g., John et al. 2012), though to a somewhat minor extent. This is supported by leaching experiments in this study where the extent of silicate-carbonate fractionation (44/40Casilicate-carbonate) has been investigated. The values at ~0.1-0.2‰ are expectedly lower than those reported earlier (~0.6‰; John et al. 2012) and may reflect high-temperature Ca isotope fractionation. The variability in ?44/40Ca values of carbonatites and silico-carbonatites from the Samalpatti complex is larger (0.70- 1.14‰) and appears to be in accord with extensive post-emplacement disturbance. Significant loss of REE and 13C-18O-enriched signature are combined with high ?44/40Ca values, which could reflect massive exchange with metasomatic aqueous fluids. The 40Kdecay correction was applied to K-rich rocks (syenites, monzonites). Given the antiquity of the complex dated at ca. ~800 Ma (Schleicher et al. 1997) and considering high-K/Ca character of some rocks, the resulting ?44/40Ca800 Myr correction was up to ~+1.2‰. In this regard, it is crucial to constrain the age history of the entire region. The nearby Hogenakal carbonatite body was dated at ~2.4 Ga which is much older than Rb-Sr and Sm-Nd age of Sevattur (Kumar et al. 1998) from the same fault system. We have acquired K-Ar mineral (K-feldspar, biotite, amphibole) and U-Pb zircon data from Sevattur and Samalpatti. The K-Ar ages span a range between ~800 and ~510 Ma (~800 Ma for amphiboles and biotites from silico-carbonatites and mafic silicate rocks and ~570-510 Ma for K-feldspars and biotites from syenites), dating two high-grade regional tectono-thermal overprint events, documented earlier. The complex nature of this process is indicated by concordant U-Pb zircon age at ~2.5 Ga yielded for a melatonalite, for which K-Ar biotite age of ~802 Ma was measured. This fits into the age bracket of basement of the Eastern Dharwar Craton. The age distribution bimodality at ~2.5 Ga and ~800 Ma has been found for several other samples, suggesting a pulsed thermal history of the area, associated with a significant overprint by fluids likely derived from the local crust. Particularly high U concentrations in zircons (thousands ppm), combined with a range of K-Ar ages, attest to such multi-episodic history.
LREE and Nb multi metal potentiality of the Amba Dongar carbonatite complex, Chhota Udepur district, Gujarat.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 43-44.
India
deposit - Amba Dongar
Abstract: Rare earth elements (REE) are used in science innovations, due to their unique magnetic, fluorescent and chemical properties. REE are key components in rnany technological devices, like hybrid rechargeable batteries, catalysts, glass polishing, magnets, lasers, TV colour components, superconductors, ceramics etc. They are in great demand for hybrid cars, CD, cameras and high end defence systems. Similarly, niobium (Nb) finds its usage in diverse high tech applications including atomic energy. With increasing technological applications of REE and Nb, their global demand has enhanced over the years. To keep pace with the current demand, many carbonatite complexes in India including the Amba Dongar were revisited to assess their REE and Nb content. Amba Dongar is a classic carbonatite-alkalic rock complex of the Deccan basalt plateau and is emplaced in close proximity to Narmada rift zone. The main rock types of carbonatite affinity include sovite (calcium carbonatite), ankerite (Fe-Mg•Mn carbonatite), siderite (Fe carbonatite), carbonatite breccia (mixed rock. fragments with carbonate cement) etc. Sovite forms a large ring-dyke (nearly 1.5 km dia.) surrounding an incomplete ring of carbonatite breccia. Plugs of ankeritic carbonatite intrude the sovite. To assess rare metal and REE potential of the carbonatite complex geological and radiometric surveys followed by core drilling were carried out in western part of the complex. Rocks of carbonatite affinity have been intercepted in all the boreholes upto a maximum drilled depth of 150 m. It is for the first time that presence of carbonatite and carbonatite breccia has been reported below central basalt in the Amba Dongar complex. Continuity of carbonatites beyond the drilled depth is inferred. Petromineralogical and X-Ray Diffraction studies indicated presence of REE minerals such as monazite, thorite, cerite, synchisite and bastnasite. Besides, rare earth fluorocarbonates, parisite, florencite, barite, strontianite and columbite have also been reported by earlier investigators. Fairly good amount of pyrochlore (Nb mineral) is also present in all the variants of carbonatite. Detailed chemical analysis core at 1 m interval and of composite samples from every borehole was carried out. The results indicate homogeneity of mineralisation in the entire column upto an explored vertical depth of 120 m. Except a few lean zones, the entire column hosts REE mineralisation of the order of >1% ?REE. Some zones have indicated REE mineralisation of the order of >4 % also. Major element analysis of a composite sample representing a small block (400 m x 100 m x 113 m) indicates 14.69% SiO2, 10.57% Fe2O3, 7 21% MgO, 32.23% CaO, 2.77%, Al2O3, 1.48% P2O5, 2.13% MnO, 0.84% FeO, 0.37% TiO2, 0.95% Na2O, 1.35% K2O, and 23.50% LOI. 1.16% LREE (including 161 ppm HREE), 215 ppm Y, 650 ppm Nb, 310 ppm Th and 467 ppm V appear to be of economic significance. Additionally, presence of high content of Ba (2.65%), Sr (0.50%), Pb (530 ppm), F (1.95%) and Zn (1248 ppm) is also important. Taking into consideration these results, resource estimation of a small block of 400 m x 100 m (0.04 sq. km) with an average depth of 113 m was carried out Inferred REE resources ~140000 tonnes contained in 12.00 million tonne ore have been estimated with an average grade of 1.16% REE. Additionally, this block contains 9,600 tonnes Nb2O5 at an average grade of 0 08 % Nb2O5. These values indicate high potential of Amba Dongar carbonatite complex.
Journal of Asian Earth Sciences, in press available, 11p.
India
geophysics - seismics
Abstract: The time-bound crustal evolution and subsequent deformation of the Cuddapah basin, Nellore Schist Belt and Eastern Ghats terrain of Eastern Dharwar Craton, which have undergone sustained geodynamic upheavals since almost 2.0 billion years, remain enigmatic. An attempt is made here to integrate newly available potential field data and other geophysical anomalies with deep seismic structure, to examine the generative mechanism of major crustal features, associated with this sector. Our study indicates that the initial extent of the Cuddapah basin sedimentation may have been much larger, extending by almost 50-60?km west of Tadipatri during Paleoproterozoic period, which subsequently shrank due to massive erosion following thermal uplift, caused by SW Cuddapah mantle plume. Below this region, crust is still quite warm with Moho temperatures exceeding 500?°C. Similarly, Nallamalai Fold Belt rocks, bounded by two major faults and extremely low gravity, may have occupied a large terrain in western Cuddapah basin also, before their abrasion. No geophysical signatures of thrusting are presently seen below this region, and thus it could not be an alien terrain either. In contrast, Nellore Schist Belt is associated with strikingly high positive gravity, possibly caused by a conspicuous horst structure and up dipping mafic crustal layers underneath, that resulted due to India-east Antarctica collision after the cessation of prolonged subduction (1.6-0.95?Ga). Further, the crustal seismic and gravity signatures would confirm presence of a totally distinct geological terrain east of the Cuddapah basin, but the trace of Eastern Ghats Belt is all together missing. Instead, all the geophysical signatures, point out to presence of a Proterozoic sedimentary terrain, east of Nellore Schist Belt. It is likely that the extent of Prorerozoic sedimentation was much larger than thought today. In addition, presence of a seismically detected Gondwana basin over Nellore Schist Belt, apart from some recently discovered similar subsurface Gondwana occurrences in intracratonic parts, would indicate that Dharwar Craton was rifting even during Gondwana period, thereby challenging the long held view of cratonic stability.
Paleoproterozoic carbonatites Newania and Hogenkkal: geochemical and isotoopic characteristics.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 24.
India
deposit - Newania. Hogenkkal
Abstract: Several carbonatite occurrences ranging in age from Proterozoic to Phanerozoic are known and reported from India. Only two amongst them, the Hogenakkal and Newania carbonatites are Paleoproterozoic in age. The Newania carbonatite body, emplaced within Mesoarchean Untala Granite, comprises predominantly dolomiticcarbonatite and minor ankeritic and sövitic phases, in the chronological order. Only the dolomitic-carbonatites have mantle like C- and O-isotopic characteristics and some authors argue that the ankeritic phase is its alteration product while sövite represents late stage calcite veins. The Pb - Pb geochronology yields high ratios and 2.23 Ga age for the dolomitic carbonatite and extremely high Pb/Pb ratios resulting in an unrealistic 1550 Ma age for the ankeritic phase. Melt residence in an intermediate upper mantle/lower crust reservoir, prior to emplacement, has been postulated for the high Pb/Pb ratios observed in dolomitic carbonatites. The 2.4 Ga Hogenakkal carbonatites occur in the northern part of the Southern Granulite Terrane as small isolated bodies hosted within two sub-parallel pyroxenite dykes within a milieu of Archean charnockites. These are fine to mediumgrained and named as sövite, silicate sövite and melanocratic silicate sövite, based upon calcite or calcite + pyroxene as dominant mineralogy. The C- and O-isotopic ratios for all the varieties are remarkably consistent and represent unmodified mantle values (-5.8 to - 6.7‰ V-PDB and 7.2 to 8.7‰ V-SMOW, respectively). However, diversity in Sr- and Nd-isotopic characteristics suggest both enriched and depleted source characteristics and call for a heterogeneous mantle beneath the SGT. Both the groups do indicate presence of an enriched mantle component during late Archean
Abstract: Systematic closely spaced geological traverses conducted in the year 2010, in Lattavaram Kimberlite Cluster (LKC) of Anantapur district, Andhra Pradesh, India, have led to the discovery of a new kimberlite pipe outcrop in the river bed of Balkamthota Vanka (name of the stream used by local farmers) at its confluence with Penna River, close to Pennahobilam. This new pipe occurs at a distance of 1.5 km in NE direction to hitherto reported pipes-5 and 13 occurring at Muligiripalli and Tummatapalli respectively in the LKC of the Wajrakarur Kimberlite Field (WKF). With this pipe, the total number of kimberlite pipes in the WKF raises to 48, considering all the kimberlites discovered by various public and private organizations so far. Preliminary petrography, geochemistry, petrogenetic aspects and diamond prospectivity of the new occurrence have been presented here. Mineralogically, the kimberlite constitutes olivine macrocrysts, serpentinsed olivine psuedomorphs with xenocrystic ilmenite, phlogopite, perovskite, magnetite, Cr-diopside, garnet along with calcite veins. The kimberlite is classified as hypabyssal macrocrystic calcite- phlogopite kimberlite. Mineralogically, the new kimberlite pipe appears as archetypal Group- I kimberlite however, geochemically; the kimberlite shows character of both Group- I and II varieties, more close to lamproitic character. Although it is too early to comment, based on limited analyses carried out in this study, the diamond potentiality of this pipe is not encouraging; it is noteworthy that it highly warrants detailed investigations involving bulk rock geochemistry and drilling to assess its definite geochemical status, petrogenesis and diamond potentiality.
Platinum group element and rhenium osmium geochemistry of selected carbonatites from India, USA and East africa.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 22-23.
India, United States, Africa, East Africa
carbonatites
Abstract: Carbonatites and associated alkaline silicate rocks might have potential economic impact for a large variety of metals such as Cu, Ni, Fe and platinum-group elements (PGE - Os, Ir, Ru, Pd, Pt) as it is demonstrated in South Africa (Phalaborwa; Taylor et al. 2009) or Brazil (Ipanema; Fontana 2006). In addition, determined PGE contents along with Re-Os isotopic compositions may also provide important information about PGE fractionation during the genesis of upper mantle-derived carbonatitic melts and nature of their sources. Nevertheless, the existing PGE data for carbonatites are extremely rare, limited mostly to Chinese localities and they are not paralleled by Re-Os isotopic data (Xu et al. 2008). Therefore, in this study, we present the first complete PGE datasets together with Re-Os determinations for a suite of selected carbonatite bodies worldwide. We have chosen eight carbonatite sites with different alkaline rock association, age and geotectonic position. Among these, the youngest samples are from East African rift system and include Oldoinyo Dili, Tanzania with an age spanning from ~0 to 45 Ma; same as Tororo and Sukulu in Uganda (Woolley and Kjarsgaard 2008). These carbonatites are in association with pyroxenites and nepheline syenites. Another young carbonatitic complex is Amba Dongar in west India with Cretaceous age of ~65 Ma associated with alkaline volcanic rocks such as trachybasalts within Deccan Traps (Sukheswala and Udas 1963). Proterozoic bodies are represented by Iron Hill, USA carbonatites associated with pyroxenite, melitolite and ijolite with age ranging from ~520 to 580 Ma (Nash 1972). These carbonatites are famous for their intensive and varied fenitization. Last and the oldest carbonatites in this study comes from Samalpatti and Sevattur, South India having the age of ~800 Ma (Schleicher et al. 1997) and outcropping as small bodies within alkaline rocks such as pyroxenite, syenite and gabbro. The PGE concentrations and Re-Os isotopic ratios were determined by standard methods consisting of decarbonatization using HCl, decomposition of samples in Carius Tubes in the presence of reverse aqua regia and spikes (isotopic dilution), separation of Os by CHCl3 followed by N-TIMS measurements and Ir, Ru, Pd, Pt, Re isolation by anion exchange chromatography followed by ICP-MS measurements. All analysed carbonatites exhibit extremely low PGE contents (? PGE up to 1 ppb), even in the samples with high S contents (up to 1.5 wt. %). Such values are much lower than other determined so far for upper mantle-derived melts such as basalts, komatiites, etc. (Day et al. 2016). Such signatures indicate very low partitioning of PGE into carbonatitic melts and/or early separation of PGE-bearing fraction. Elements from iridium-group I-PGE; Os, Ir and Ru; mostly < 0.1 ppb) are distinctly lower compared to palladiumgroup elements and Re (PPGE; Pt, Pd, Re; mostly > 0.1 ppb) with some rocks being largely enriched in Re (up to ~6 ppb). Most of the analysed carbonatites exhibit progressive enrichment from Os to Re and consequently, PdN/ReN < 0.1 except south India carbonatites and associated alkaline rocks (> 0.30). Rocks analysed so far for Os have OsN/IrN up to 6.2 that might suggest that the carbonatites might concentrate Os over Ir. The highest HSEtot contents have been found in Mg-Cr-rich silicocarbonatites from South India (up to 40 ppb) and taking into account their only slightly radiogenic 187Os/188Os ratios (0.14-0.57), these rocks represents mixture of CO2-rich alkaline mantle melts and country rocks. Very high concentrations of HSE have been also found in magnetite separated from Fe-carbonatite from Amba Dongar, India (0.2-0.5 ppb of I-PGE and 0.9-9 ppb of P-PGE). The 187Os/188Os ratios determined so far for carbonatites from South India vary from 0.24 to 6.5 and calculated ?Os values range from +100 up to +5000. Such wide range of values suggest extremely heterogenous source of the melts and/or possible contamination by 187Os-rich crustal materials.
Carbonatite occurrences in Munnar area, Kerala, southern India.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 36-37.
India
carbonatites
Abstract: Carbonatites, usually associated with alkaline complexes and emplaced within continental rifting environment, are the rarest of all the igneous rocks. Carbonatite and alkaline intrusive complexes, as well as their weathering products, are the primary sources of REEs (Long et.al. 2010). Carbonatites are defined by the International Union of Geological Sciences (IUGS) system of igneous rock classification as having more than 50 modal percent primary carbonate minerals, such as calcite, dolomite, and ankerite, and less than 20 percent SiO2 (Le Maitre, 2002). Southern India has several carbonatite occurrences and the alkaline complex of Munnar in southern India comprises of an alkali granite plutons with minor patches of charnockite, syenite and carbonatite emplaced within Precambrian gneisses (Nair et.al., 1983, 1984; Santosh et.al., 1987, Nair et.al., 1984). Gneissic layering and foliation are apparent in all but the least deformed granitic rocks in the study area. The Munnar granite body is situated in the western part of the Madurai block in Southern Granulite Terrane (SGT) of Peninsular India, within the newly defined Western Madurai Domain. The complex is spatially related to the intersection zone of Karur-Kambam-Painavu-Trissur lineament. The alkali granite of the complex has been dated at 740±30 my (Odom, 1982) and 804±6 Ma (Brandt et. al., 2014). Present study deals with examining the nature of the carbonatites and takes a relook at its major and REE contents, and for the first time, looks at the stable isotope signatures of these rocks, in an attempt to check whether these rocks are indeed carbonatites. The geology and geochemistry of the rock types in and around Munnar area have been mapped with special focus on carbonatites. Extensive field mapping was carried out and a base map was prepared and all the geological and structural features were recorded in the base map. Intra- and inter-relationships of various rock units were examined. Field photographs of interesting geological features have been recorded. Carbonatites in Munnar area are exposed as two minor patches. The one which occurs towards north of the Munnar town and is seen as patches, lens and veins of 30 cm to 1 m thickness, cutting coarse grained syenite which occurs as a NW- SE along a body. Exposures are found about 15 km from Munnar on the Udumalpet road. The second exposure occurs towards the east of the Munnar town, near at the Ellapatty estate 24 km from Munnar on the road to top station where coarse grained cabonatites occurs as lenticular bodies up to 1.5 m thick within granite. In both the localities, the carbonatite bodies show sharp and discordant margins with absence of any pseudomorphs within them. Fenitisation is characterised by the development of pink K-feldspar megacrystals in the country rock at the contact. The carbonatites are fresh and homogenous and represent two varieties. A coarse grained holocrystalline type and yellowish calcite crystals constituting 90% of the rock, with pyroxene apatite and magnetite correspond to sovite (Streckeisen, 1979). The second variety which contains highly coarse calcite crystals (up to 1 cm) and associated dolomite with mafic minerals constituting 30% of the rock corresponds to alvikite. The sovite exhibits an interlocking crystals mosaic of calcite in thin section. The calcite crystals of alvikite show exsolution blebs of dolomite. The major mafic component in both varieties is aegirine-augite which forms euhedral- subhedral laths (Santosh et al., 1984). The opaque phase is dominantly magnetite. Rarely phlogopite, biotite and minor laths of albite are also noted, small crystals of euhedral apatite are found occluded in calcite grain although alkaline complexes with carbonatite of Munnar devoid of related mafic differentiates like gabbros or lamprophyres may be considered unique. The immiscibility of carbonatitic and alkalic silicate liquids can be physically explained as the separation of a less viscous carbonate liquid from a more viscous polymerized silicate phase. The carbonate liquid would be lower in density because of higher content of H2O and this contrast in density could cause phase separation due to earth’s gravitational field alone (Moller et al., 1980). The pre-requisite to establish separation of immiscible silicate-saturated carbonatite liquid and the associated carbonate-saturated silicate melt is achieved as follows; Large-scale volatile outgassing occurs during crustal wrapping and distention prior to rifting which trigger mantle degassing (Bailey, 1974). An imprint of such large-scale volatile influx is recognised in the Kerala region (Nair et al., 1984). Rapid ascent volatiles enriched in CO2 liberated from the mantle cause partial melting at shallower levels of the mantle.
Primary carbonate-silicate association in the pelletal lapilli: first direct evidence of carbonated peridotitic mantle source for Amba Dongar carbonatites, Deccan igneous province, India.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 30.
India
deposit - Amba Dongar
Abstract: Pelletal lapilli are discrete sub-spherical clasts with a central fragment mantled by a rim of probable juvenile origin. They typically range in size from <1 mm – 60 mm, and occur as accessory components of pipe-filling volcaniclastic rocks such as kimberlites, carbonatites, kamafugites, melilitites and orangeites. They have been variously referred to as ‘tuffacitic lapilli’, ‘spherical or elliptical lapilli’, ‘Pele’s tears’, ‘spinning droplets’, ‘cored lapilli’, or ‘concentric shelled lapilli. Their presence has been increasing reported from extrusive carbonatite complexes world over; for e.g. Umbria and Latium, Central Italy, Abruzzo and Lucaniae, Southern Italy; Fort Portal and Katwe Kikorongo, Uganda; West Qinling, China; Campo de Calatrava, Spain, along with others. I am reporting here occurrence of pelletal and cored lapilli in the proximity of Amba Dongar carbonatite complex. The lapillus comprises of abundant phenocrysts of olivine, which are typically rimmed by dark brown thick coating. Such phenocrysts (oikocrysts) acquire remarkable roundedness, many of these show central circular fracture. Some of the phenocrysts are broken yet preserving their thick-brown rim. This feature is accentuated where such olivine oikocrysts are welded over groundmass mafics (typically clinorpyroxenes). However, most conspicuous thing is the presence of chrome spinel, which is dispersed as minor octahedra within the olivine, but not found in association with other minerals implying that olivine and chrome spinels are primary phases within the lapilli. There are other smaller droplets forming cored lapilli, which are dominantly composed of carbonates. Mineral chemistry of different phases indicate presence of forsteric (Fo89.20-67.34: Fa32.10-10.71: Tp0.46-0.04); chromium-rich spinel (Chromite55.34-24.86, Spinel31.88-10.16, Magnetite34.13-8.68, Mag. Chromite24.45-0.00, Ulvospinel23.21-0.00, Mag. Ulvospinel7.00-0.00, diopsidic clinopyroxene (Wo45.36-41.41: En40.92-50.60: Fs7.98-15.46), ilmenite (Il76.79-75.10:Ge13.08-14.60:Py1.72-1.08:He9.22-7.26), calcic-plagioclase (An69.36-47.90:Ab48.32- 29.27:Or3.78-1.37), and chlorite (ripidolite/brunsvigite). Discovery of pelletal and cored lapilli in the proximity of Amba Dongar carbonatite complex thus provide first direct evidence of the carbonated peridotitic mantle source for Amba Dongar carbonatites.
Carbonatite breccia: a neglected unit in Amba Dongar distreme, Gujarat, India.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 38.
India
deposit - Amba Dongar
Abstract: Carbonatite breccia forms a major unit in the carbonatite-alkalic diatreme of Amba Dongar. In addition to the innermost part of the ring structure, it also forms small and large plugs outside ring structure in form of discontinuous ring around sövite. It is mainly composed of rounded to sub-angular fragments of basement metamorphics, Bagh sandstones, pre-carbonatite basalt, nephelinite and sövite set in the carbonatitic matrix. Besides rock fragments it also shows presence xenocrystal minerals. Carbonatite breccia is later invaded by sövite, alvikite and ankeritic carbonatite. Thorite, pyrochlore, barite, apatite, fluorite and small amount of REE-minerals were introduced in carbonatite breccia by these later intrusives. Zircon, however seems have been caught up from metamorphic gneisses. Microprobe analyses of all these minerals are given here. In places, carbonatite breccia has been silicified by invading hydrothermal solutions rich in fluorite and silica.
Petrographic studies in understanding carbonatites.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 48-49.
India
carbonatites
Abstract: Carbonatites are mantle derived carbonate rich rocks of igneous origin. Carbonatites are often spatially associated with alkaline rocks and typically confined to continental rift related tectonic setting. Mineralogically, carbonatites are predominantly composed of primary carbonate minerals (calcite, dolomite), while, oxides, hydroxides, silicates, phosphate and sulphide minerals are also found as associated minerals in variable amounts. Although geochemical and isotope geology studies significantly contribute to understand the genetic aspects of these rare REE rich rocks of economic significance; petrographic studies with the aid of polarizing microscope play a critical role in (i) identification of the constituent minerals (ii) ascertain the relative abundance of various minerals and (iii) to recognise the textures. All these three aspects along with geochemical, isotope and mineral inclusion studies are extremely important to understand carbonatite petrogenesis. As per the IUGS classification scheme, the primary carbonate minerals [calcite CaCO3, dolomite (Ca, Mg) CO3, ankerite Ca (Fe, Mg, Mn) (CO3)2] constitute > 50 % by mode in carbonatites, while the SiO2 is < 20 % (Le Maitre, 2002). Though the primary mineralogy in carbonatite is variable, petrographic studies do help in establishing the presence of REE rich phases like apatite and pyrochlore; presence of mineral phases like phlogopite, perovskite, olivine, fluorite (transmitted light) and opaque oxides; eg. magnetite (reflected light) in carbonate rich rock with crystalline texture; as an initial stage for identification of a carbonatite. Based on the chemical composition, the carbonatites are classified as calciocarbonatites, magnesiocarbonatites and ferrocarbonatites (Woolley, 1982); the calciocarbonatites are further classified as sovite and alvikite (Le Bas, 1999). Based on the mineralogical-genetic criteria, carbonatites are divided into primary carbonatites and carbothermal residua (Mitchell, 2007). Petrographic studies help to initially identify the chemically distinct calciocarbonatites; sovite or alvikite. Sovite is texturally coarse grained, while alvikite is relatively fine grained. The coarse grained nature of the calciocarbonatites (average grain size of the carbonate minerals ranging from 1 to 5 mm) coupled with their equigranular nature makes them texturally distinct. Calcite and dolomite can be distinguished with the aid of staining techniques (Dickson, 1965). Staining technique will be useful for rapid estimation of the modal contents of the carbonate phases (calcite, ferroan calcite, dolomite,) in carbonatites. Though less abundant, the ferrocarbonatites are charecterised by the presence of clearly relatively large magnetite grains. Often the porphyritic appearance in the ferrocarbonatites is due to the presence of aggregates of celadonite and phlogopite leaving olivine and pyroxene as relict phases. Late stage magmatic-hydrothermal fluids can play a role in alteration of the textural and mineralogy in carbonatites (Duraiswami and Shaikh, 2014). Study the primary magmatic inclusions in silicates phases in carbonatites with the aid of optical and scanning electron microscopy provide critical information to understand the petrogenetic aspects of carbonatites (e.g. Nisbett and Kelly, 1977). Petrographic studies also contribute in identification of textures indicating crystal-melt interaction in carbonatites (Sesha Sai and Sengupta, 2017). Field and laboratory studies leading to chronological understanding of the geotectonic events in a given area, along with petrographic analyses with detailed mineralogical and textural descriptions, not only contribute to understand the fundamental aspects of carbonatites, but also form a solid substratum to build an acceptable petrogenetic model, by synthesising the information obtained by the geochemical, isotope geology and mineral inclusion studies.
Shitole, A., Sant, D.A., Parvez, I.A., Rangarajan, G., Patel, S., Viladkar, S.G., Murty, A.S.N., Kumari, G.
Shallow seismic studies along Amba Dongar to Sinhada ( longitude 74 3 50E) transect, western India.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 16.
India
deposit - Amba Dongar
Abstract: The microtremor method is applied to map subsurface rheological boundaries (stratigraphic, faults and plutons) is based on strong acoustic impedance across contrasting density of rock/ sediment/ weathered interfaces up to shallow depths along longitude 74° 3'50" E from village Amba Dongar (latitude: 21° 59'N) up to Sinhada village (latitude: 22° 14' N). The 30 km long transect exposes variety of rocks viz., unclassified granite gneisses and metasediments (Precambrian age); sediments belonging to Bagh Group (Late Cretaceous); alkaline - carbonatite plutons and lava flows belonging to Deccan Traps (Late Cretaceous). In all, sixty stations were surveyed along the longitude 74° 3'50" E with spacing of 500 m. H/V spectral ratio technique reveals four rheological interfaces identified by resonant frequencies (fr) ranges 0.2213 to 0.7456 Hz (L1), 1.0102 to 3.076 Hz (L2), 4.8508 to 21.0502 Hz (L3), and 24.5018 to 27.1119 Hz (L4). L1 represents interface between plutons, Precambrian basement rocks; L2 represents interface between Bagh sediments, Deccan Traps and intrusives whereas L3 and L4 captures depth of top most weathered profile. We estimate the depth range for L1 L2 L3 and L4 using equation (h = 110.18fr?1.97) derived based on Deep Banni Core (1764 m deep from surface: DGH record). Deep Banni Core has a distinct interface between Mesozoic rocks and Precambrian basement. The depths are further compared with terrain-based equation. Further, the overall results from the present study are compared with seismic refraction studies along Phangia-Kadipani (NGRI Technical Report, 2003). The subsurface profile across longitude 74° 3'50" E educe faults that bound Bagh Group of rocks with Deccan Trap and Precambrian. We identify two plutons underneath three zones of intrusive viz., Amba Dongar Carbonatite Complex (Station 1 to 8), Tiloda Alkaline (station 33 to 44) and Rumadia Alkaline (station 46 to 51). The present study demarcates the presence of depression over Amba Dongar hill (station 1 to 3), filled by post carbonatite basalt earlier reported by Viladkar et al., (1996 and 2005) suggesting caldera morphology. Similarly, studies identify intrusive-pluton interfaces one, below the Amba Dongar hill, and second between village Tiloda and Rumadia at depth of ~500 m from the surface. Microtremor survey further depicts both basement morphology and thickness of Bagh Group and Deccan Traps.
Nd, Sr, Pb and B isotopic investigation of carbonatite/alkaline centers in west central India: insights into plume driven vs lithospheric controlled magmatism.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 17.
India
carbonatites
Abstract: The exact origin of carbonatite magmas remains debatable as there are two main hypothesis proposed; one relates magmatism to asthenospheric upwellings and/or mantle plumes, whereas the other argues for generation from metasomatized lithosphere. However, proponents of the latter rarely describe in detail the origin of the metasomatic agents required to generate the high concentrations of rare earth and highly incompatible elements present in carbonatite magmas. In a recent study, Boron isotopic signatures of carbonatite complexes worldwide, ranging in age from ~2600 to ~65 million years old, indicate greater input of recycled (subducted), crustal material and plume activity with increasing geologic age of the Earth. More positive Boron isotopic values with increasing geologic time were attributed to the change of Earth’s geodynamics to a modern style of plate tectonics. In this study, the radiogenic (Sr, Nd, Pb) and B isotope systematics of carbonatites and alkaline rocks from west-central India are reported and discussed with reference to the plume-lithosphere interaction model previously proposed for the generation of Deccan-related alkaline centers in this region of the Indian sub-continent
Rare earth element applications, market outlook and Indian perspectives.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p.41-42.
India
rare earths
Abstract: Rare earth elements (REE) are a group of seventeen chemical elements that occur together in the periodic table. The group consists of yttrium and the 15 lanthanide elements (lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium). Promethium does not occur in nature because it is highly and the only lanthanide which has no stable (or even long-lived) isotopes. The most stable isotope of the element is Promethium-145, a half-life of 17.7 years. Due to its very less half-life most of the Promethium might have exhausted within first 10,000 years of formation of Earth. Scandium is found in most rare earth element deposits and is sometimes classified as a rare earth element. The rare earth elements are all metals, and the group is often referred to as the "rare earth metals." These metals have many similar properties, and that often causes them to be found together in geologic deposits. They are also referred to as "rare earth oxides" because many of them are typically sold as oxide compounds. The rare earths elements are classified into two groups; Cerium Group and Yttrium group, the former consists of light rare earths ( Sc, La, Ce, Nd, Pr, Pm, Sm and Eu ). The later is composed of (Y, Gd,Tb, Dy, Ho, Er, Tm, Yb and Lu). In nature the minerals of cerium group are different then the Yttrium group. The most common light rare earth minerals are monazite and bastnasite and for the heavy rare earth element the mineral is xenotime. Rare earth elements are not as "rare" as their name implies. Thulium and lutetium are the two least abundant rare earth elements - but they each have an average crustal abundance that is nearly 200 times greater than the crustal abundance of gold. However, these metals are very difficult to mine because it is unusual to find them in concentrations high enough for economical extraction. The most abundant rare earth elements are cerium, yttrium, lanthanum and neodymium. They have average crustal abundances that are similar to commonly used industrial metals such as chromium, nickel, zinc, molybdenum, tin, tungsten, and lead. Again, they are rarely found in extractable levels. Because of their unique magnetic, luminescent, and electrochemical properties, these elements help make many technologies perform with reduced weight, emissions, and energy consumption, and give them greater efficiency, performance, miniaturization, speed, durability, and thermal stability. Rare earth-enabled products and technologies help to fuel global economic growth, maintain high standards of living, and save lives. Rare earth elements are used extensively in aerospace and defense, health care, clean energy, electronics, transportation and vehicles, catalysts, polishing industry etc. Global resources of rare earths are about 120 Mt, China tops with 44 Mt, Vietnam and Brazil both 22Mt each, Russia 18 Mt, India 6.9 Mt, Australia 3.4 Mt, USA 1.4 Mt, Greenland 1.5 Mt, Malawi, 0.136 Mt, South Africa 0.86 Mt. The major producers today are China and Australia. China producing about 105,000 tons and Australia, 14000 tons, Russia, 3000 tons, India, 1700 tons, Brazil, 1100 tons, Thailand 800 tons, Malaysia, 300 tons( based on 2016 data). In 2016, excess global supply caused prices for many rare-earth compounds and metals to decline, and China continued to dominate the global supply. In China, the rare-earth mining production quota for 2016 was set at 105,000 tons, unchanged from 2015.The major reason of price decline was the illegal mining of rare earths in China which cause pollution and other financial losses. Now China is clamping down on mining as part of a campaign to tackle pollution and tighten control of its massive industrial complex. Various measures to curb production have already driven up prices of aluminum, steel, and now rare earths. Praseodymium-neodymium oxide, a raw material for the metal, has almost doubled this year, Neodymium surged by nearly a third in August alone and is up 81 percent in 2017. Demand for some rare earths may exceed supply in the second half after the crackdown on illegal mines. The global demand for automobiles, consumer electronics, energy-efficient lighting, permanent magnets and catalysts is expected to rise rapidly over the next decade. Rare earth magnet demand is expected to increase, as is the demand for rechargeable batteries. New developments in medical technology are expected to increase the use of surgical lasers, magnetic resonance imaging, and positron emission tomography scintillation detectors. Rare earth elements are heavily used in all of these industries, so the demand for them should remain high. So far EVs and renewable energy from clean technology point of view are concerned the rare earths join with niche metals including lithium and cobalt as beneficiaries of rapid growth in the electric vehicle industry and in renewable energy in the form of permanent magnets used in gearless turbines. By 2020, the REE demand in EVs will increase from 2000 tons per year to 7000 tons per year in 2020 and 12000 tons by 2024. In India, Indian Rare Earth Ltd is planning to produce 10,000 tons REO per year. The carbonatite hosted REE deposit with non-monazite sources has also been identified in Barmer district of Rajasthan.
Abstract: The Archean Yilgarn Craton in Western Australia is intruded by numerous mafic dykes of varying orientations, which are poorly exposed but discernible in aeromagnetic maps. Previous studies have identified two craton-wide dyke swarms, the 2408?Ma Widgiemooltha and the 1210?Ma Marnda Moorn Large Igneous Provinces (LIP), as well as limited occurrences of the 1075?Ma Warakurna LIP in the northern part of the craton. We report here a newly identified NW-trending mafic dyke swarm in southwestern Yilgarn Craton dated at 1888?±?9?Ma with ID-TIMS U-Pb method on baddeleyite from a single dyke and at 1858?±?54?Ma, 1881?±?37 and 1911?±?42?Ma with in situ SHRIMP U-Pb on baddeleyite from three dykes. Preliminary interpretation of aeromagnetic data indicates that the dykes form a linear swarm several hundred kilometers long, truncated by the Darling Fault in the west. This newly named Boonadgin dyke swarm is synchronous with post-orogenic extension and deposition of granular iron formations in the Earaheedy basin in the Capricorn Orogen and its emplacement may be associated with far field stresses. Emplacement of the dykes may also be related to initial stages of rifting and formation of the intracratonic Barren Basin in the Albany-Fraser Orogen, where the regional extensional setting prevailed for the following 300?million years. Recent studies and new paleomagnetic evidence raise the possibility that the dykes could be part of the coeval 1890?Ma Bastar-Cuddapah LIP in India. Globally, the Boonadgin dyke swarm is synchronous with a major orogenic episode and records of intracratonic mafic magmatism on many other Precambrian cratons.
Carbonatites-alkaline rocks, and associated economic mineral deposits: a view from beneficiation.
Carbonatite-alkaline rocks and associated mineral deposits , Dec. 8-11, abstract p. 3.
India
carbonatites
Abstract: Among the known 20 carbonatite-alkaline rock associations in India, eight contain potentially economic deposits, major among them are: fluorite at Amba Dongar, Gujaratand hematite, Nb, apatite etc. at Samchampi Complex, Assam, Amba Dongar complex is estimated to host 11.6 million tonnes of fluorite ore. In the broad region of the entire Amba Dongar-Mogra-Sedivasan Carbonatite complex, fluorite mineralization has been reported, particularly on the northern and southern flanks as isolated pockets. Starting from 1964, numerous agencies like Geological Survey of India, Directorate of Geology & Mining and, finally GMDC have carried out exploration activities from time to time. The deposit is hydrothermal, mainly in form of vug filling, cavity filling and dissemination and exhibits large variations in grade, in terms of CaF2, CaCO3 and SiO2% as well as in thickness. This being the only commercially viable fluorite deposit in India, GMDC set up a 500 TPD Beneficiation Plant to produce acid/metallurgical grades in 1970. Typical problems of Fine dissemination of major part of fluorite grains, intimate association of fluorite with silica, interstitial presence of apatite in fluorite matrix are some major issues giving rise to difficulties encountered in upgrading the ore. Major setback for the Project has been deteriorating grade of 30% CaF2 at the top to current 20% CaF2 after excavating 90 meters. Having recently found more selective Collector of fluorite from silica and carbonate, an operation of 1000 TPD is now under implementation. Current mine is associated with sovite carbonatite as overburden which are reported to have appreciable amount of RE Elements like Nb, La, Ce etc. simultaneous development of which can provide a strong base for enhancing commercial aspects of the combined Project.
Journal of the Geological Society of India, Vol. 90, 5, pp. 591-594.
India
carbonatite
Abstract: The present paper for the first times gives details of pyroxenesövites of Amba Dongar and discusses significance of these pyroxenes in evolution of carbonatite magma in Amba Dongar. Calciocarbonatite (sövite) forms the major mass of carbonatite in Amba Dongar complex. It shows large variation in texture and mineral composition and has complex evolutionary history. Three types of compositional variations are observed in sövite samples, (1) monomineralic sövites are coarse grained with 99% calcite, (2) sövites with abundant apatite, barite, pyrochlore, magnetite and zirconolite and (3) silico-sövite with of clinopyroxene and phlogopite. In the crystallization history of various sövite types, silico-sövite seems to have crystallized as an earlier phase and was later caught up in major sövite mass. Both, phlogopite-sövite and pyroxene-sövite are coarse grained and exhibit hypidiomorphic texture. Phlogopite is strongly zoned with Mg-rich core to Fe-rich rims. Pyroxenes also exhibit zoning with decrease in Ca and Mg and increase in Fe and Na from core to rim. In general composition of clinopyroxene varies from diopsidic to aegirine-augite. Pyroxenesövites show good concentration of Ba, Sr, Nb and LREE. Elevated concentrations of LREE are found in two aegirine-sövites.
Abstract: Potassic and ultrapotassic magmatism from deep lithospheric sources in intra-cratonic settings can be the signal of subsequent voluminous mafic magmatism and the formation of Large Igneous Provinces (LIPs) triggered by mantle plumes. Here we report for the first time, precise zircon U-Pb age data from a suite of lamproites in the Bastar Craton of central India that mark the onset of Paleoproterozoic rifting and culminating in the formation of extensive mafic dyke swarms as the bar codes of one of the major LIP events during the Precambrian evolution of the Indian shield. The lamproites from the Nuapada field occur as dismembered dykes and are composed of phenocrysts and microphenocrysts of altered olivine together with microphenocrysts of phlogopite and magnetite within a groundmass of chlorite and calcite with accessory rutile, apatite and zircon. The rocks compositionally correspond to olivine phlogopite lamproite and phlogopite lamproite. Geochemical features of the lamproites correlate with their counterparts in Peninsular India and other similar suites elsewhere in the world related to rift settings, and also indicate OIB-like magma source. The associated syenite shows subduction-related features, possibly generated in a post-collisional setting. Magmatic zircon grains with high Th/U ratios in the syenite from the Nuapada lamproite form a coherent group with an upper intercept age of 2473 ± 8 Ma representing the timing of emplacement of the magma. Zircon grains in three lamproite samples yield four distinct age groups at ca. 2.4 Ga, 2.2 Ga, 2.0 Ga and 0.8 Ga. The 2.4 Ga group corresponds to xenocrysts entrained from the syenite whereas the 2.2 Ga group is considered to represent the timing of emplacement of the lamproites. The ca. 2.0 Ga zircon grains correlate with the major thermal imprint associated with mafic magmatism and dyke emplacement in southern Bastar and the adjacent Dharwar Cratons. A few young zircon grains in the syenite and lamproites show a range of early to middle Neoproterozoic ages from 879 to 651 Ma corresponding to younger thermal event(s) as also represented by granitic veins cutting across these rocks and extensive silicification. Zircon Lu-Hf isotope data suggest magma derivation from a refertilized Paleo-Mesoarchean sub-continental lithospheric mantle, or OIB-type sources. The differences in Hf-isotope composition among the zircon grains from different age groups indicate that the mantle sources of the lamproite are heterogeneous at the regional scale. A combination of the features from geochemical and zircon Hf isotope data is consistent with asthenosphere-lithosphere interaction during the lamproite magma evolution. The timing of lamproite emplacement in central India correlates with the global 2.2 Ga record of LIPs. We link the origin of the related mantle plume to the recycling of subducted slabs associated with the prolonged subduction-accretion history prior to the Neoarchean cratonization, as well as the thermal blanket effect of the Earth's oldest supercontinent. Pulsating plumes and continued rifting generated voluminous dyke swarms across the Bastar and Dharwar Cratons, forming part of a major global rifting and LIP event.
Geological Society of London Special Publication, No. 463, pp. 137-169.
India
carbonatite
Abstract: The genetic connection between Large Igneous Province (LIP) and carbonatite is controversial. Here, we present new major and trace element data for carbonatites, nephelinites and Deccan basalts from Amba Dongar in western India, and probe the linkage between carbonatite and the Deccan LIP. Carbonatites are classified into calciocarbonatite (CaO, 39.5-55.9 wt%; BaO, 0.02-3.41 wt%; ?REE, 1025-12 317 ppm) and ferrocarbonatite (CaO, 15.6-31 wt%; BaO, 0.3-7 wt%; ?REE, 6839-31 117 ppm). Primitive-mantle-normalized trace element patterns of carbonatites show distinct negative Ti, Zr-Hf, Pb, K and U anomalies, similar to that observed in carbonatites globally. Chondrite-normalized REE patterns reveal high LREE/HREE fractionation; average (La/Yb)N values of 175 in carbonatites and approximately 50 in nephelinites suggest very-low-degree melting of the source. Trace element modelling indicates the possibility of primary carbonatite melt generated from a subcontinental lithospheric mantle (SCLM) source, although it does not explain the entire range of trace element enrichment observed in the Amba Dongar carbonatites. We suggest that CO2-rich fluids and heat from the Deccan plume contributed towards metasomatism of the SCLM source. Melting of this SCLM generated primary carbonated silicate magma that underwent liquid immiscibility at crustal depths, forming two compositionally distinct carbonatite and nephelinite magmas.
Journal of the Geological Society of India, Vol. 91, 2, pp. 135-146.
India
Prakasam alkaline province
Abstract: Three distinct alkaline magmas, represented by shonkinite, lamprophyre and alkali basalt dykes, characterize a significant magmatic expression of rift-related mantle-derived igneous activity in the Mesoproterozoic Prakasam Alkaline Province, SE India. In the present study we have estimated emplacement velocities (ascent rates) for these three varied alkaline magmas and compared with other silicate magmas to explore composition control on the ascent rates. The alkaline dykes have variable widths and lengths with none of the dykes wider than 1 m. The shonkinites are fine- to medium-grained rocks with clinopyroxene, phologopite, amphibole, K-feldspar perthite and nepheline as essential minerals. They exhibit equigranular hypidiomorphic to foliated textures. Lamprophyres and alkali basalts characteristically show porphyritic textures. Olivine, clinopyroxene, amphibole and biotite are distinct phenocrysts in lamprophyres whereas olivine, clinopyroxene and plagioclase form the phenocrystic mineralogy in the alkali basalts. The calculated densities [2.54-2.71 g/cc for shonkinite; 2.61-2.78 g/cc for lamprophyre; 2.66-2.74 g/cc for alkali basalt] and viscosities [3.11-3.39 Pa s for shonkinite; 3.01-3.28 Pa s for lamprophyre; 2.72-3.09 Pa s for alkali basalt] are utilized to compute velocities (ascent rates) of the three alkaline magmas. Since the lamprophyres and alkali basalts are crystal-laden, we have also calculated effective viscosities to infer crystal control on the velocities. Twenty percent of crystals in the magma increase the viscosity by 2.7 times consequently decrease ascent rate by 2.7 times compared to the crystal-free magmas. The computed ascent rates range from 0.11-2.13 m/sec, 0.23-2.77 m/sec and 1.16-2.89 m/sec for shonkinite, lamprophyre and alkali basalt magmas respectively. Ascent rates increase with the width of the dykes and density difference, and decrease with magma viscosity and proportion of crystals. If a constant width of 1 m is assumed in the magma-filled dyke propagation model, then the sequence of emplacement velocities in the decreasing order is alkaline magmas (4.68-15.31 m/sec) > ultramafic-mafic magmas (3.81-4.30 m/sec) > intermediate-felsic magmas (1.76-2.56 m/sec). We propose that SiO2 content in the terrestrial magmas can be modeled as a semi-quantitative “geospeedometer” of the magma ascent rates.
Abstract: For centuries, the Koh-i-Noor, or Mountain of Light, has been a diamond of exceptional renown in the East as well as in the West. Several legends circulate regarding this stone, and among these are tales of its origin and the way it received its name. This article attempts to verify the authenticity of these accounts and shows that the true origin of the diamond's name is connected to its appearance achieved through the faceting style known today as the Mughal cut. Although the provenance of this cut has thus far not been determined, this article proposes that it possibly originated in the 16th century in Goa, India, through the Gujaratis and under the influence of European diamond cutters. Various lines of evidence suggest that the Koh-i-Noor may have been worked in the 16th century by an Indian specialist in the Vijayanagara Empire.
Journal of Asian Earth Sciences, Vol. 157, pp. 218-234.
India
Craton
Abstract: The Pitepani volcanic suite of the Dongargarh Supergroup, central India comprises of a calc-alkaline suite and a tholeiitic suite, respectively. The rare earth element (REE) patterns, mantle normalized plots and relict clinopyroxene chemistry of the Pitepani calc-alkaline suite are akin to high-Mg andesites (HMA) and reveal remarkable similarity to the Cenozoic Setouchi HMA from Japan. The Pitepani HMAs are geochemically correlated with similar rocks in the Kotri-Dongargarh mobile belt (KDMB) and in the mafic dykes of the Bastar Craton. The rationale behind lithogeochemical correlations are that sanukitic HMAs represent fore-arc volcanism over a very limited period of time, under abnormally high temperature conditions and are excellent regional and tectonic time markers. Furthermore, the tholeiitic suites that are temporally and spatially associated with the HMAs in the KDMB and in the mafic dykes of the Bastar Craton are classified into: (a) a continental back-arc suite that are depleted in incompatible elements, and (b) a continental arc suite that are more depleted in incompatible elements, respectively. The HMA suite, the continental back-arc and continental arc suites are lithogeochemically correlated in the KDMB and in the mafic dykes of the Bastar Craton. The three geochemically distinct Neoarchaean magmatic suites are temporally and spatially related to each other and to an active continental margin. The identification of three active continental margin magmatic suites for the first time, provides a robust conceptual framework to unravel the Neoarchaean geodynamic evolution of the Bastar Craton. We propose an active continental margin along the Neoarchaen KDMB with eastward subduction coupled with slab roll back or preferably, ridge-subduction along the Central Indian Tectonic Zone (CITZ) to account for the three distinct magmatic suites and the Neoarchean geodynamic evolution of the Bastar Craton.
Earth Planetary Science Letters, Vol. 492, pp. 186-196.
India
carbonatite
Abstract: Within a stepwise crushing study we determined the noble gas composition of several calcite separates, one aegirine and one pyrochlore-aegirine separate of the carbonatite ring dyke complex of Amba Dongar and carbonatite sill complex of Siriwasan, India. Both carbonatites are related to the waning stages of volcanic activity of the Deccan Igneous Province ca. 65 Ma ago. Major observations are a clear radiogenic 4He and nucleogenic 21Ne imprint related to in situ production from U and Th in mineral impurities, most likely minute apatite grains, or late incorporation of crustal fluids. However, in first crushing steps of most calcites from Amba Dongar a well-resolvable mantle neon signal is observed, with lowest air-corrected mantle 21Ne/22Ne-compositions equivalent to the Réunion hotspot mantle source. In case of the aegirine separate from Siriwasan we found a neon composition similar to the Loihi hotspot mantle source. This transition from a mantle plume signal in first crushing step to a more nucleogenic signature with progressive crushing indicates the presence of an external (crustal) or in situ nucleogenic component unrelated and superposed to the initial mantle neon component whose composition is best approximated by results of first crushing step(s). This contradicts previous models of a lithospheric mantle source of the carbonatitic magmas from Amba Dongar containing recycled crustal components which base on nucleogenic neon compositions. Instead, the mantle source of both investigated carbonatite complexes is related to a primitive mantle plume source that we tentatively ascribe to the postulated Deccan mantle plume. If, as is commonly suggested, the present location of the Deccan mantle plume source is below Réunion Island, the currently observed more nucleogenic neon isotopic composition of the Réunion hotspot might be obliterated by significant upper mantle contributions. In addition, compared with other carbonatite complexes worldwide a rather significant contribution of atmospheric noble gases is observed. This is documented in cut-off 20Ne/22Ne-ratios of ca. 10.2 (Amba Dongar) and 10.45 (Siriwasan) and cut-off 40Ar/36Ar-ratios of about 1500. This atmospheric component had been added at shallow levels during the emplacement process or later during hydrothermal alteration. However, understanding the late-stage interaction between atmospheric gases and magmatic mantle fluids still requires further investigation.
Abstract: The mineralogy of nine recently discovered dykes (VL1:VL8 and VL10) in the vicinity of Vattikod village, Nalgonda district in Telangana State is described. The mineral assemblage present and their compositions are comparable to those of bona fide lamproites in terms of the presence of phlogopite (Ti-rich, Al-poor phlogopite and tetraferriphlogopite); amphiboles (potassic-arfvedsonite, potassic-richterite, potassic-ferro-richterite, potassic-katophorite, Ti-rich potassic-katophorite, Ti-rich potassic-magnesio-katophorite); Al-poor clinopyroxenes; feldspars (K-feldspar, Ba-K-feldspar and Na-feldspar) and spinels (chromite-magnetite and qandilite-ulvöspinel-franklinite). These dykes have undergone diverse and significant degrees of deuteric alteration as shown by the formation of secondary phases such as: titanite, allanite, hydro-zircon, calcite, chlorite, quartz and cryptocrystalline SiO2. On the basis of their respective mineralogy: the VL4 and VL5 dykes are classified as pseudoleucite-phlogopite lamproite; VL2 and VL3 dykes as pseudoleucite-amphibole-lamproite; and VL6, VL7 and VL8 as pseudoleucite-phlogopite-amphibole-lamproite. VL10 is extensively altered but contains fresh euhedral apatite microphenocrysts together with pseudomorphs after leucite and is classified as a pseudoleucite-apatite-(phlogopite?) lamproite. The mineralogy of the Vattikod lamproite dykes is compared with that of the Ramadugu, Somavarigudem and Yacharam lamproite dykes which also occur in the Ramadugu lamproite field. The lamproites from the Eastern Dharwar Craton are considered as being possible expressions of ancient subduction-related alkaline magmatism along the Eastern Ghats mobile belt.
Journal of the Geological Society of India, Vol. 91, pp. 135-146.
India
Alkaline - Prakasam
Abstract: Three distinct alkaline magmas, represented by shonkinite, lamprophyre and alkali basalt dykes, characterize a significant magmatic expression of rift-related mantle-derived igneous activity in the Mesoproterozoic Prakasam Alkaline Province, SE India. In the present study we have estimated emplacement velocities (ascent rates) for these three varied alkaline magmas and compared with other silicate magmas to explore composition control on the ascent rates. The alkaline dykes have variable widths and lengths with none of the dykes wider than 1 m. The shonkinites are fine- to medium-grained rocks with clinopyroxene, phologopite, amphibole, K-feldspar perthite and nepheline as essential minerals. They exhibit equigranular hypidiomorphic to foliated textures. Lamprophyres and alkali basalts characteristically show porphyritic textures. Olivine, clinopyroxene, amphibole and biotite are distinct phenocrysts in lamprophyres whereas olivine, clinopyroxene and plagioclase form the phenocrystic mineralogy in the alkali basalts. The calculated densities [2.54-2.71 g/cc for shonkinite; 2.61-2.78 g/cc for lamprophyre; 2.66-2.74 g/cc for alkali basalt] and viscosities [3.11-3.39 Pa s for shonkinite; 3.01-3.28 Pa s for lamprophyre; 2.72-3.09 Pa s for alkali basalt] are utilized to compute velocities (ascent rates) of the three alkaline magmas. Since the lamprophyres and alkali basalts are crystal-laden, we have also calculated effective viscosities to infer crystal control on the velocities. Twenty percent of crystals in the magma increase the viscosity by 2.7 times consequently decrease ascent rate by 2.7 times compared to the crystal-free magmas. The computed ascent rates range from 0.11-2.13 m/sec, 0.23-2.77 m/sec and 1.16-2.89 m/sec for shonkinite, lamprophyre and alkali basalt magmas respectively. Ascent rates increase with the width of the dykes and density difference, and decrease with magma viscosity and proportion of crystals. If a constant width of 1 m is assumed in the magma-filled dyke propagation model, then the sequence of emplacement velocities in the decreasing order is alkaline magmas (4.68-15.31 m/sec) > ultramafic-mafic magmas (3.81-4.30 m/sec) > intermediate-felsic magmas (1.76-2.56 m/sec). We propose that SiO2 content in the terrestrial magmas can be modeled as a semi-quantitative "geospeedometer" of the magma ascent rates.
Abstract: Simultaneous acquisition of detrital zircon Pb-Pb ages and trace element abundances from grains collected across the Indian craton, spanning ?3 b.y., reveals prominent shifts in Eu/Eu* and light and middle to heavy rare earth element ratios. These shifts correspond to a ca. 3.0-2.2 Ga interval of crustal thickening during Indian craton formation, followed by a period wherein arc magmatism occurred along thinner craton margins from ca. 1.9 to 1.0 Ga, with arc magmatism concentrated along attenuated continental margins after ca. 1.0 Ga. Similar temporal shifts in trace element concentrations are recognized in global whole-rock compilations. We propose that the post-1.0 Ga increase in juvenile magmatism reflects a switch to lateral arc terrane accretion as the primary style of continental growth over the past billion years.
Journal of the Geological Society of India, Vol. 91, 3, pp. 263-272.
India
shonkinite
Abstract: Formation of the fragments of the wall-rock during dyking is one of the important manifestations of instantaneous magmatic events. This process is well documented at shallower depths of Earth’s crust but not at deeper levels. In this paper the in situ xenoliths of host rock nepheline syenite within a micro-shonkinite dyke emplaced at mid-crustal depths is described and the fractal theory applied to evaluate origin of the xenoliths. The nepheline syenite xenoliths are angular to oval shaped and sub-millimetre to ~50 cm long. The xenoliths are matrix supported with clasts and matrix being in equal proportions. Partly detached wall-rock fragments indicate incipient xenolith formation, which suggested that the model fragmentation processes is solely due to dyke emplacement. Fractal analytical techniques including clast size distribution, boundary roughness fractal dimension and clast circularity was carried out. The fractal data suggests that hydraulic (tensile) fracturing is the main process of host rock brecciation. However, the clast size and shape are further affected by postfragmentation processes including shear and thermal fracturing, and chemical erosion. The study demonstrates that dyking in an isotropic medium produces fractal size distributions of host rock xenoliths; however, post-fragmentation processes modify original fractal size distributions.
Journal of Asian Earth Sciences, Vol. 157, pp. 302-316.
India
geophysics - seismics
Abstract: The time-bound crustal evolution and subsequent deformation of the Cuddapah basin, Nellore Schist Belt and Eastern Ghats terrain of Eastern Dharwar Craton, which have undergone sustained geodynamic upheavals since almost 2.0 billion years, remain enigmatic. An attempt is made here to integrate newly available potential field data and other geophysical anomalies with deep seismic structure, to examine the generative mechanism of major crustal features, associated with this sector. Our study indicates that the initial extent of the Cuddapah basin sedimentation may have been much larger, extending by almost 50-60?km west of Tadipatri during Paleoproterozoic period, which subsequently shrank due to massive erosion following thermal uplift, caused by SW Cuddapah mantle plume. Below this region, crust is still quite warm with Moho temperatures exceeding 500?°C. Similarly, Nallamalai Fold Belt rocks, bounded by two major faults and extremely low gravity, may have occupied a large terrain in western Cuddapah basin also, before their abrasion. No geophysical signatures of thrusting are presently seen below this region, and thus it could not be an alien terrain either. In contrast, Nellore Schist Belt is associated with strikingly high positive gravity, possibly caused by a conspicuous horst structure and up dipping mafic crustal layers underneath, that resulted due to India-east Antarctica collision after the cessation of prolonged subduction (1.6-0.95?Ga). Further, the crustal seismic and gravity signatures would confirm presence of a totally distinct geological terrain east of the Cuddapah basin, but the trace of Eastern Ghats Belt is all together missing. Instead, all the geophysical signatures, point out to presence of a Proterozoic sedimentary terrain, east of Nellore Schist Belt. It is likely that the extent of Prorerozoic sedimentation was much larger than thought today. In addition, presence of a seismically detected Gondwana basin over Nellore Schist Belt, apart from some recently discovered similar subsurface Gondwana occurrences in intracratonic parts, would indicate that Dharwar Craton was rifting even during Gondwana period, thereby challenging the long held view of cratonic stability.
Journal of Applied Geology and Geophysics (IOSR), Vol. 6, 2, pp. 55-69. pdf
India
shoshonite
Abstract: Field geological, petrographic and geochemical characteristics of a lamprophyre intrusion, presumably of plug-type, at Sivarampet (SPL), occurring within the Wajrakarur kimberlite field (WKF) to the west of Cuddapah basin, are presented and discussed. The lamprophyre intrusion occurs as brecciated outcrop with angular country rock granitoid clasts and also it forms stringers/veinlets within the granitic country rock. The melanocratic rock displays panidiomorphic/porphyritic texture, typical of lamprophyres, comprising clinopyroxene, biotite, phlogopite set in a groundmass of feldspar, magnetite and spinel. Plagioclase is dominant feldspar. The K2O/Na2O ratio ranges from 1.55 to 1.89 wt %, making it distinctly potassic and brings out its shoshonitic behaviour. The fractionated chondrite normalised patterns of REE (with average (La/Yb)N= 21.01 ppm) implies involvement of an enriched mantle source while depleted values of Nb, Hf, Th and U concentrations indicate prevalence of subducted component in the mantle source. The concentrations of Rb, Sr and Ba indicate presence of phlogopite in the source. Based on the mineral assemblages, the SPL can be classified as calc-alkaline variety; however, its geochemistry shows characteristics of both alkaline and calc-alkaline varieties. The moderate Mg# (52 to 55.6) and low concentration of Ni (95.61 to 112.4 ppm) in the bulk rock indicate a low degree of partial melting of magmatic fluid from enriched asthenospheric mantle which underwent fractionation of olivine and pyroxene, subsequently producing lamprophyre magma. Recent discovery of diamonds in shoshonitic lamprophyres of Canada, appeals further investigations on diamondiferous nature of similar rock types of the WKF.
Periodico di Mineralogia, Vol. 86, pp. 213-228. pdf
India
deposit - Balkamthota Vanka
Abstract: Systematic closely spaced geological traverses conducted in the year 2010, in Lattavaram Kimberlite Cluster (LKC) of Anantapur district, Andhra Pradesh, India, have led to the discovery of a new kimberlite pipe outcrop in the river bed of Balkamthota Vanka (name of the stream used by local farmers) at its confluence with Penna River, close to Pennahobilam. This new pipe occurs at a distance of 1.5 km in NE direction to hitherto reported pipes-5 and 13 occurring at Muligiripalli and Tummatapalli respectively in the LKC of the Wajrakarur Kimberlite Field (WKF). With this pipe, the total number of kimberlite pipes in the WKF raises to 48, considering all the kimberlites discovered by various public and private organizations so far. Preliminary petrography, geochemistry, petrogenetic aspects and diamond prospectivity of the new occurrence have been presented here. Mineralogically, the kimberlite constitutes olivine macrocrysts, serpentinsed olivine psuedomorphs with xenocrystic ilmenite, phlogopite, perovskite, magnetite, Cr-diopside, garnet along with calcite veins. The kimberlite is classified as hypabyssal macrocrystic calcite- phlogopite kimberlite. Mineralogically, the new kimberlite pipe appears as archetypal Group- I kimberlite however, geochemically; the kimberlite shows character of both Group- I and II varieties, more close to lamproitic character. Although it is too early to comment, based on limited analyses carried out in this study, the diamond potentiality of this pipe is not encouraging; it is noteworthy that it highly warrants detailed investigations involving bulk rock geochemistry and drilling to assess its definite geochemical status, petrogenesis and diamond potentiality.
The P3 kimberlite and P4 lamproite, Wajrakur kimberlite field, India: mineralogy, and major and minor element compositions of olivines as records of their phenocrystic vs xenocrystic origin.
Journal of the Geological Society of India, Vol. 91, pp. 395-399.
India
Alkaline - Mundwara
Abstract: The occurrence of a rare mantle-derived chrome-diopside megacryst (~8 mm), containing inclusions of olivine, in a lamprophyre dyke from the late Cretaceous polychronous (~100 - 68 Ma) Mundwara alkaline complex of NW India is reported. The olivine inclusions are forsteritic (Fo: 85.23) in composition, and their NiO (0.09 wt%) and CaO (0.13 wt%) contents imply derivation from a peridotitic mantle source. The composition of the chrome diopside (Cr2O3: 0.93 wt ) (Wo45.27 En48.47 Fs5.07 and Ac1.18) megacryst is comparable to that occurring in the garnet peridotite xenoliths found in diamondiferous kimberlites from Archaean cratons. Single pyroxene thermobarometry revealed that this chrome diopside megacryst was derived from a depth range of ~100 km, which is relatively much deeper than that of the chrome-diopside megacrysts (~40-50 km) reported in spinellherzolite xenoliths from the alkali basalts of Deccan age (ca. 66- 67 Ma) from the Kutch, NW India. This study highlights that pre- Deccan lithosphere, below the Mundwara alkaline complex, was at least ~100 km thick and, likely, similar in composition to that of the cratonic lithosphere.
Russian Geology and Geophysics, Vol. 59, pp. 512-524.
Africa, Australia, South America, India
craton
Abstract: This is a synopsis of available data the on crustal structure and properties of thirteen Archean cratons of Gondwanaland (the cratons of Africa, Australia, Antarctica, South America, and the Indian subcontinent). The data include estimates of surface area, rock age and lithology, Moho depth, thickness of lithosphere and sediments, as well as elevations, all summarized in a table. The cratons differ in size from 0.05 x 106 km2 (Napier craton) to 4 x 106 km2 (Congo craton) and span almost the entire Archean period from 3.8 to 2.5 Ga. Sediments are mostly thin, though reach 7 km in the Congo and West African cratons. Elevations above sea level are from 0 to 2 km; some relatively highland cratons (Kaapvaal, Zimbabwe, and Tanzanian) rise to more than 1 km. On the basis of regional seismic data, the Moho map for cratons has been improved. The Moho diagrams for each craton are constructed. The analysis of the available new data shows that the average Moho depth varies from 33 to 44 km: Pilbara (33 km), Grunehogna (35 km), Sao Francisco (36 km), Yilgarn (37 km), Dharwar (38 km), Tanzanian (39 km), Zimbabwe (39 km), Kaapvaal (40 km), Gawler (40 km), Napier (40 km), West Africa (40 km), Congo (42 km), and Amazon (44 km) cratons. The Moho depth within the cratons is less uniform than it was assumed before: from 28 to 52 km. The new results differ significantly from the earlier inference of a relatively flat Moho geometry beneath Archean cratons. According to the new data, early and middle Archean undeformed crust is characterized by a shallow Moho depth (28-38 km), while late Archean or deformed crust may be as thick as 52 km.
Abstract: The generation, preservation and destruction of continental crust on Earth is of wide interest in understanding the formation of continents, cratons and supercontinents as well as related mineral deposits. In this contribution, we integrate the available field, petrographic, geochronologic, elemental Nd-Hf-Pb isotope data for greenstones, TTG gneisses, sanukitoids and anatectic granites from the Dharwar Craton (southern India). This review allows us to evaluate the accretionary processes of juvenile crust, mechanisms of continental growth, and secular evolution of geodynamic processes through the 3600-2500?Ma window, hence providing important insights into building of continents in the Early Earth. The Dharwar Craton formed by assembly of micro-blocks with independent thermal records and accretionary histories. The craton can be divided into three crustal blocks (western, central and eastern) separated by major shear zones. The western block contains some of the oldest basement rocks with two generations of volcano-sedimentary greenstone sequences and discrete potassic plutons whereas the central block consist of older migmatitic TTGs, abundant younger transitional TTGs, remnants of ancient high grade supracrustal rocks, linear volcanic-dominated greenstone belts, voluminous calc-alkaline granitoids of sanukitoid affinity and anatectic granites. In contrast, the eastern block comprises younger transitional TTGs, abundant diatexites, thin volcanic-sedimentary greenstone belts and calc-alkaline plutons. Published geochronologic data show five major periods of felsic crust formation at ca. 3450-3300?Ma, 3230-3150?Ma, 3000-2960?Ma, 2700-2600?Ma, and 2560-2520?Ma which are sub-contemporaneous with the episodes of greenstone volcanism. U-Pb ages of inherited zircons in TTGs, as well as detrital zircons together with Nd-Pb-Hf isotope data, reveal continental records of 3800-3600?Ma. The U-Pb zircon data suggest at least four major reworking events during ca. 3200?Ma, 3000?Ma, 2620-2600?Ma, and 2530-2500?Ma corresponding to lower crustal melting and spatially linked high grade metamorphic events. The TTGs are sub-divided into the older (3450-3000?Ma) TTGs and the younger (2700-2600?Ma) transitional TTGs. The older TTGs can be further sub-divided into low-Al and high-Al groups. Elemental and isotopic data suggest that the low-Al type formed by melting of oceanic island arc crust within plagioclase stability field. In contrast, the elemental and isotopic features for the high-Al group suggest derivation of their magmatic precursor by melting of oceanic arc crust at deeper levels (55-65?km) with variable garnet and ilmenite in residue. The transitional TTGs likely formed by melting of composite sources involving both enriched oceanic arc crust and sub-arc mantle with minor contamination of ancient crustal components. The geochemical and isotopic compositions of granitoids with sanukitoid affinity suggest derivation from enriched mantle reservoirs. Finally, anatectic granites were produced by reworking of crustal sources with different histories. In the light of the data reviewed in this contribution, we propose the following scenario for the tectonic evolution of the Dharwar Craton. During 3450-3000?Ma, TTGs sources (oceanic arc crust) formed by melting of down going slabs and subsequent melting of such newly formed crust at different depths resulted in TTG magmas. On the contrary, by 2700?Ma the depth of slab melting increased. Melting of slab at greater depth alongside the detritus results in enriched melts partly modified the overlying mantle wedge. Subsequent melting of such newly formed enriched oceanic arc crust and surrounding arc-mantle generated the magmatic precursor to transitional TTGs. Finally at ca. 2600-2560?Ma, eventual breakoff of down going slab caused mantle upwelling which induced low degree (10-15%) melting of overlying enriched mantle at different depths, thereby, generating the sanukitoid magmas which upon emplacement into the crust caused high temperature metamorphism, reworking and final cratonization.
Journal of the Indian Institute of Science, 15p. available in press
India
magmatism
Abstract: We report rare occurrence of lamprophyre xenoliths within a host lamprophyre from the Dongargaon area, Deccan Large Igneous Province, NW India. The lamprophyre xenoliths are distinct in texture (grain size) as well in mineralogy from those of their host rock. The clinopyroxene (diopside) in the xenoliths is depleted in Ca and Mg but substantially enriched in Fe compared to those in the host lamprophyre. Mica in the xenoliths is a phlogopite whereas that present in the host rock is compositionally a biotite; spinels in the host lamprophyre are relatively enriched in TiO2. As the host lamprophyre dyke has been dated to be of Eocene (ca. 55 Ma) age, the entrained lamprophyre xenoliths are inferred to represent an earlier pulse of lamprophyre emplacement. The recurrent lamprophyre emplacement in this domain is consistent with the recently brought out polychronous nature of Late Cretaceous alkaline magmatism at the Mundwara and Sarnu Dandali complexes in the NW India and is related to the extensional events linked with the reactivation of the Narmada rift zone.
Current Science, Vol. 114, 6, Mar. 25, pp. 1299-1303.
India
legal
Abstract: Scholarly journals play an important role in maintaining the quality and integrity of research by what they publish. Unethical practices in publishing are leading to an increased number of predatory, dubious and low-quality journals worldwide. It has been reported that the percentage of research articles published in predatory journals is high in India. The University Grants Commission (UGC), New Delhi has published an 'approved list of journals', which has been criticized due to inclusion of many substandard journals. We have developed a protocol with objective criteria for identifying journals that do not follow good publication practices. We studied 1336 journals randomly selected from 5699 in the university source component of the 'UGC-approved list'. We analysed 1009 journals after excluding 327 indexed in Scopus/Web of Science. About 34.5% of the 1009 journals were disqualified under the basic criteria because of incorrect or non-availability of essential information such as address, website details and names of editors; another 52.3% of them provided false information such as incorrect ISSN, false claims about impact factor, claimed indexing in dubious indexing databases or had poor credentials of editors. Our results suggest that over 88% of the non-indexed journals in the university source component of the UGC-approved list, included on the basis of suggestions from different universities, could be of low quality. In view of these results, the current UGC-approved list of journals needs serious reconsideration. New regulations to curtail unethical practices in scientific publishing along with organization of awareness programmes about publication ethics at Indian universities and research institutes are urgently needed.
Abstract: Investigation of large scale suture zones in old continental interiors offers insights into the evolution of continents. The Dharwar Craton (DC) and the Southern Granulite Terrain(SGT) of the Indian shield represent large segments of Precambrian middle to lower crust and preserve a geological record spanning from Mesoarchean to Cambrian. This study illuminates the deep structure of the Palghat-Cauvery Shear Zone System (PCSS) and the Palghat-Cauvery Suture Zone (PCSZ) that comprise crustal-scale structures related to multiple episodes of orogeny, crust formation and reworking. We utilize here 3202 high quality P-receiver functions computed using new data from a 23 station seismic network operated by us. Results show a thick (>38?km) mafic (Poisson's ratio >0.25) crust beneath the SGT. The change in crustal thickness is gradual, with a shallower Moho towards the south of PCSZ. We found little evidence for drastic changes in crustal thickness across prominent shear zones like the PCSZ and Moyar-Bhavani. Few seismic stations located along these boundaries have shown evidence for dipping reflectors around 8-20?km depth, with strikes matching well with the trends of surface geological sutures. We opine that these suture zones do not show indications of a terrane boundary. However, a drastic change in the crustal thickness is observed around the prograde metamorphic transition zone or broadly, the "Fermor line", which separates rocks of Chanockitic (Orthopyroxene bearing granitoid) and non-Charnockitic (Orthopyroxene-free granitoid) mineral assemblage, further north beneath the DC. We suggest that thicknening of crust north of Moyar-Attur Shear Zone (MASZ) and around Fermor line is related to subduction processes operative during the Precambrian.
Springer Nature , book review in Mineralium Deposita diamonds mentioned p. 6-7. of review
India
diamonds
Abstract: Nonrenewable natural resources - metallic and non-metallic minerals, industrial rocks and energy resources (both organic and inorganic), have been treated in a holistic manner in this book, including two important resources (soil and water), not commonly covered in most books on this topic. For the uninitiated reader, an introductory chapter looks into some basic definitions as well as nature and characteristics of mineral deposits followed by a chapter on the different crustal processes that produce the various ore deposits in the endogenous and exogenous environments. The strength of the book lies in its critical treatment of the genetic processes of the mineral deposits, their classification and the geodynamic context of metallogeny, and coverage of sustainable development of mineral deposits with special reference to various socio-economic as well as regulatory and environmental issues that face the Indian mining industry today. The text is punctuated with examples of Indian deposits, balanced with classical deposits around the world, to cater to the interests of Indian students and the international readership. This is a book for advanced undergraduate and post-graduate students of Geology, Environmental Sciences and Natural Resource Management.
Abstract: Investigation of large scale suture zones in old continental interiors offers insights into the evolution of continents. The Dharwar Craton (DC) and the Southern Granulite Terrain(SGT) of the Indian shield represent large segments of Precambrian middle to lower crust and preserve a geological record spanning from Mesoarchean to Cambrian. This study illuminates the deep structure of the Palghat-Cauvery Shear Zone System (PCSS) and the Palghat-Cauvery Suture Zone (PCSZ) that comprise crustal-scale structures related to multiple episodes of orogeny, crust formation and reworking. We utilize here 3202 high quality P-receiver functions computed using new data from a 23 station seismic network operated by us. Results show a thick (>38?km) mafic (Poisson's ratio >0.25) crust beneath the SGT. The change in crustal thickness is gradual, with a shallower Moho towards the south of PCSZ. We found little evidence for drastic changes in crustal thickness across prominent shear zones like the PCSZ and Moyar-Bhavani. Few seismic stations located along these boundaries have shown evidence for dipping reflectors around 8-20?km depth, with strikes matching well with the trends of surface geological sutures. We opine that these suture zones do not show indications of a terrane boundary. However, a drastic change in the crustal thickness is observed around the prograde metamorphic transition zone or broadly, the “Fermor line”, which separates rocks of Chanockitic (Orthopyroxene bearing granitoid) and non-Charnockitic (Orthopyroxene-free granitoid) mineral assemblage, further north beneath the DC. We suggest that thicknening of crust north of Moyar-Attur Shear Zone (MASZ) and around Fermor line is related to subduction processes operative during the Precambrian.
Journal of Asian Earth Sciences, Vol. 183, pp. 43-53.
India
craton
Abstract: oriented, 280?km long profile (from Yellapura to Sindhanur) with 22 magnetotelluric stations. Regional strike directions, estimated were ?5° and 13° for the crust and the lithospheric mantle respectively. Our results indicate in western Dharwar craton, presence of low resistivity zones in the crust besides two significant upper mantle conductive features within the highly resistive Archaean lithosphere. We analyze the available geophysical data that include heat flow, seismic tomography and magnetotellurics (MT) from the Dharwar craton. Our inference supports to the existence of a thick lithosphere. A thickness of more than 200?km is estimated for the lithosphere beneath the Dharwar craton by our magnetotelluric model. The study has brought out the presence of lithospheric upper mantle conductive features in the depth range of 100-200?km bounded to the west part of the magnetotelluric profile. Significant variations in conductivity are seen on either side of the Chitradurga shear zone. The conductive feature in the depth range 120-150?km is related with kimberlite melts and the conductive nature in the depth range 160-200?km is explained by refertilization process, as craton passed over the Marion (ca. 90?Ma) hotspot.
Abstract: Trends in concentration of selected trace elements in residual soils on four known diamondiferous kimberlite pipes (3, 4, 8 and 9) occurring at Lattavaram within the Wajrakarur Kimberlite Field (WKF) is attempted for the first time. The pipes 3 and 4 are exposed whereas the 8 and 9 are concealed under calcrete and colluvium. For this purpose, elements like Nb, Cr, Ni, Co, Zr, Mg, Sr and La are used to understand their concentrations in the kimberlitic soils in comparison with background granitic soils. It is observed that the soils on kimberlite pipes show conspicuous enrichment of elements such as Cr, Co, Nb, Ni, Mg and Sr when compared to soils in the country rock granitoid. However, no much variation in the elements La and Zr patterns between the kimberlitic and background soils is noticed. The high pulse in trace elements in kimberlitic soils is attributed to the presence of primary kimberlitic minerals and their weathered products in the soil. This particular aspect of pedogeochemistry is envisaged to be useful as an exploration tool in search of kimberlites in cratonic parts of southern India. An enrichment of Nb content upto 45 ppm in residual soils may be considered as anamolous in the craonic parts of Indian subcontinent, which needs to be confirmed and taken forward in conjunction with high resolution geological mapping, geophysics followed up by drilling for confirmation of kimberlite/lamproite occurrence.
Abstract: Petrographic, mineral chemical and whole-rock geochemical characteristics of two newly discovered lamproitic dykes (Dyke 1 and Dyke 2) from the Sidhi Gneissic Complex (SGC), Central India are presented here. Both these dykes have almost similar sequence of mineral-textural patterns indicative of: (1) an early cumulate forming event in a deeper magma chamber where megacrystic/large size phenocrysts of phlogopites have crystallized along with subordinate amount of olivine and clinopyroxene; (2) crystallization at shallow crustal levels promoted fine-grained phlogopite, K-feldspar, calcite and Fe-Ti oxides in the groundmass; (3) dyke emplacement related quench texture (plumose K-feldspar, acicular phlogopites) and finally (4) post emplacement autometasomatism by hydrothermal fluids which percolated as micro-veins and altered the mafic phases. Phlogopite phenocrysts often display resorption textures together with growth zoning indicating that during their crystallization equilibrium at the crystal-melt interface fluctuated multiple times probably due to incremental addition or chaotic dynamic self mixing of the lamproitic magma. Carbonate aggregates as late stage melt segregation are common in both these dykes, however their micro-xenolithic forms suggest that assimilation with a plutonic carbonatite body also played a key role in enhancing the carbonatitic nature of these dykes. Geochemically both dykes are ultrapotassic (K2O/Na2O: 3.0 -9.4) with low CaO, Al2O3 and Na2O content and high SiO2 (53.3 -55.6 wt.%) and K2O/Al2O3 ratio (0.51 -0.89) characterizing them as high-silica lamproites. Inspite of these similarities, many other features indicate that both these dykes have evolved independently from two distinct magmas. In dyke 1, phlogopite composition has evolved towards the minette trend (Al-enrichment) from a differentiated parental magma having low MgO, Ni and Cr content; whereas in dyke 2, phlogopite composition shows an evolutionary affinity towards the lamproite trend (Al-depletion) and crystallized from a more primitive magma having high MgO, Ni and Cr content. Whole-rock trace-elements signatures like enriched LREE, LILE, negative Nb-Ta and positive Pb anomalies; high Rb/Sr, Th/La, Ba/Nb, and low Ba/Rb, Sm/La, Nb/U ratios in both dykes indicate that their parental magmas were sourced from a subduction modified garnet facies mantle containing phlogopite. From various evidences it is proposed that the petrogenesis of studied lamproitic dykes stand out to be an example for the lamproite magma which attained a carbonatitic character and undergone diverse chemical evolution in response to parental melt composition, storage at deep crustal level and autometasomatism.
Abstract: The sequence of replacement in groundmass perovskite and spinel from SK-1 and SK-2 kimberlites (Eastern Dharwar craton, India) has been established. Two types of perovskite occur in the studied Indian kimberlites. Type 1 perovskite is found in the groundmass, crystallized directly from the kimberlite magma, it is light rare-earth elements (LREE)-rich and Fe-poor and its ?NNO calculated value is from ?3.82 to ?0.73. The second generation of perovskite (type 2 perovskite) is found replacing groundmass atoll spinel, it was formed from hydrothermal fluids, it is LREE-free and Fe-rich and has very high ?NNO value (from 1.03 to 10.52). Type 1 groundmass perovskite may be either replaced by anatase or kassite along with aeschynite-(Ce). These differences in the alteration are related to different f(CO2) and f(H2O) conditions. Furthermore, primary perovskite may be strongly altered to secondary minerals, resulting in redistribution of rare-earth elements (REE) and, potentially, U, Pb and Th. Therefore, accurate petrographic and chemical analyses are necessary in order to demonstrate that perovskite is magmatic before proceeding to sort geochronological data by using perovskite. Ti-rich hydrogarnets (12.9 wt %-26.3 wt % TiO2) were produced during spinel replacement by late hydrothermal processes. Therefore, attention must be paid to the position of Ca-Ti-garnets in the mineral sequence and their water content before using them to classify the rock based on their occurrence.
Mineralogy and Petrology, 10.1007/s00710-018-0616-5 13p.
India
ilmentite
Abstract: Indian kimberlites occur in the Bastar craton (Central India) and in the Eastern Dharwar craton (EDC) Southern India. Nearly 100 kimberlite pipes have been discovered in the Eastern Dharwar craton of southern India, and they are distributed in three distinct fields: 1) the southern Wajrakarur kimberlite field (WKF); 2) the northern Narayanpet kimberlite field (NKF); and 3) the Raichur kimberlite field (RKF) (Chalapathi Rao et al, 2013). Nine kimberlites have been selected for this study: three came from the Siddanpalli cluster of RKF (SK-1, SK-2 and SK-3); other six kimberlites came from WKF, from Chigicherla (CC-4 and CC-5), Kalyandurg (KL-3 and KL-4), Lattavaram (P-3) and Mulligripally (P-5). The kimberlite emplacement took place during the Mesoproterozoic, around 1.1 Ga (Chalapathi Rao et al., 2013). Ilmenite is one of the classic diamond indicator minerals (DIMs) and for long it has been used as a guide for kimberlite exploration. The aim of this study is to evaluate the petrogenetic information that can be provided from the textural and geochemical study of the different ilmenite generations present in the Indian kimberlites studied in this work.
Abstract: The formation of transitional agpaitic rocks is not a well understood process as there are few studies of miaskitic to agpaitic transitions. The Mesoproterozoic Sushina Hill complex (India) provides a suitable site to investigate these "transitions" as this complex hosts diverse miaskitic and agpaitic nepheline syenites, together with syenites containing exotic mineral assemblages. In this study, we have used mineralogical and geochemical data to describe the evolution of the transitional agpaitic rocks occurring at Sushina Hill. In common with other occurrences, high field strength elements (HFSE) in miaskitic nepheline syenites are mainly sequestered by primary zircon and magnetite. In contrast, the major HFSE carriers in agpaitic nepheline syenites (agpaitic unit-I) are late-magmatic eudialyte and rinkite-(Ce) - nacareniobsite-(Ce), formed at T between 825° - 784ºC and aSiO2 in the range of 0.41 - 0.44. With decreasing temperature (? 575ºC) and aSiO2(0.30), coupled with an increase in aH2O, this assemblage has undergone extensive subsolidus alteration leading to the decomposition of late-magmatic eudialyte to wöhlerite - marianoite, alkali-zirconosilicates (catapleiite/gaidonnyaite, hilairite), and pectolite - serandite. Decomposition of late-magmatic eudialyte resulted in a more alkaline fluid by increasing the a(Na+)/a(Cl-) ratio, facilitating crystallization of hydrothermal eudialyte replacing late-magmatic eudialyte. Crystallization of hydrothermal eudialyte leads to evolving fluids which are less alkaline, resulting in the crystallization of a transitional agpaitic assemblage of pyrochlore + zircon + niobokupletskite + wadeite in agpaitic unit-II in the temperature range 547º - 455ºC with aSiO2 in the range 0.27 - 0.25. Regional scale deformation contemporaneous with the subsolidus alteration stage leads to separation of the evolving fluid from the system, resulting in extensive albitization, with superposition of a new miaskitic-like assemblage in syenite I in the form of late-stage zircon - magnetite - xenotime - monazite-(Ce) upon the early assemblage of primary zircon and magnetite. During deformation, syenite unit-II composed of eudialyte - albite - aegirine was also formed and considered as a later stage pegmatitic offshoot of agpaitic unit I. The mineralogical changes are also complemented by variations in the bulk-rock composition in which the total REE, Nb, U and Th concentrations increase in order from: miaskitic unit ? agpaitic unit I ? syenite unit II, -I ? agpaitic unit II at constant Zr concentration. This suggests that the REE-Nb are mainly mobilized in agpaitic unit-II during the agpaitic - to - transitional agpaitic assemblage transformation in a relatively less alkaline environment.
Journal of Earth System Science, Vol. 127, 6, pp. 76- doi:10.1007/s12040-018-0988-2
India
cratons
Abstract: The northern part of the Nellore-Khammam schist belt and the Karimnagar granulite belt, which are juxtaposed at high angle to each other have unique U-Pb zircon age records suggesting distinctive tectonothermal histories. Plate accretion and rifting in the eastern part of the Dharwar craton and between the Dharwar and Bastar craton indicate multiple and complex events from 2600 to 500 Ma. The Khammam schist belt, the Dharwar and the Bastar craton were joined together by the end of the Archaean. The Khammam schist belt had experienced additional tectonic events at ?1900 and ?1600 Ma. The Dharwar and Bastar cratons separated during development of the Pranhita-Godavari (P-G) valley basin at ?1600 Ma, potentially linked to the breakup of the Columbia supercontinent and were reassembled during the Mesoproterozoic at about 1000 Ma. This amalgamation process in southern India could be associated with the formation of the Rodinia supercontinent. The Khammam schist belt and the Eastern Ghats mobile belt also show evidence for accretionary processes at around 500 Ma, which is interpreted as a record of Pan-African collisions during the Gondwana assembly. From then on, southern India, as is known today, formed an integral part of the Indian continent.
Geophysical Research Abstracts EGU , Vol. 19, EGU2017-12945-2 1p. Abstract
India
lamproites
Abstract: The Mesoproterozoic Banganapalle Lamproite Field of southern India comprises four lamproite dykes which have intruded the Tadpatri Shale of the Cuddapah platformal sedimentary sequence. Mineralogical study of the dyke no. 551/110/4 shows that the rock has an inequigranular texture with megacrysts and macrocrysts of possibly olivine which are completely pseudomorphed by calcite and quartz due to pervasive hydrothermal and/or duteric alteration. Phenocrysts and microphenocrysts of phlogopite are highly chloritised with occasional preservation of relicts. The groundmass is dominated by calcite with subordinate amounts of phlogopite (completely chloritised), diopside, apatite, rutile and spinel. Other minor phases in the groundmass include titanite, allanite, monazite, zircon, barite, carboceranite, pyrite, pyrrhotite, chalcopyrite, galena, sphalerite, heazlewoodite, and pentlandite. Spinel occurs in three textural types: (i) xenocrysts showing homogeneous composition; (ii) phenocrysts and microphenocrysts with continuous compositional zoning from the core to the rim; and (iii) groundmass crystals with distinct growth zones marked by discontinuous compositional zoning from the core to the rim. Four growth zones (zones I-IV) of spinel are recognized. Phenocrysts and microphenocrysts are designated as zone I spinels which have 55.0-65.7 wt% Cr2O3, 2.7-7.2 wt% Al2O3, <0.4 wt% TiO2, and record a decrease in Al/(Al+Cr) from the core to the rim. Zone II spinels either occur as overgrowth rims on xenocrystal and zone I spinels or form cores to zone III rims in discrete grains, and have higher TiO2 (1.2-3.6 wt%), lower Al2O3 (1.2-2.9 wt%) and similar Cr2O3 (55.0-63.8 wt%) contents compared to zone I spinels. Zone III spinels either occur as overgrowth rims on xenocrystal and zone II spinels or form cores to zone IV rims in discrete grains, and contain higher Al2O3 (5.7-10.2 wt%), lower Cr2O3 (45.9-56.0 wt%) and similar TiO2 (1.6-3.4 wt%) compared to zone II spinels. Overgrowth rims of zone II and zone III spinels locally exhibit oscillatory zoning with characteristics of diffusion controlled magmatic growth. Zone IV spinels are marked by low Cr2O3 (17.4-25.5 wt%) and Al2O3 (1.6-2.0 wt%), and high Fe2O3 (28.8-35.4 wt%) and TiO2 (4.0-7.1 wt%) contents. Xenocrystal spinels are distinguished from magmatic spinels by high Al2O3 content (11.3-22.4 wt%) and uniform composition of individual grains. The wide range of composition and the zonation pattern of magmatic spinels suggest that the mineral was on the liquidus through most part of the lamproite crystallisation. The abrupt changes in composition between the zones indicate hiatus in crystallisation and/or sudden changes in the environmental conditions, resulting from crystallisation of associated minerals and periodic emplacement of certain elements into the magma. Diopside occurs in groundmass segregations and has low contents of Na2O (<0.77 wt%), Al2O3 (<1.2 wt%), Cr2O3 (<0.25 wt%) and TiO2 (<1.7 wt%), although higher values of TiO2 (up to 3.0 wt%) are locally encountered. Phenocrystal phlogopite has Mg/(Mg+Fe2+) ratios in the range of 0.76-0.83, and a Cr-rich composition (3.2-3.6 wt% Cr2O3) that indicates its crystallisation at mantle pressures. Co-precipitation of this phlogopite with phencocrystal spinel can explain the observed Al-Cr zoning in the latter.
Abstract: The Wajrakarur kimberlite field (WKF) records >45 pipes so far, majority being diamodiferous. In addition to pipe-10 (Anumpalli) and 11 (Dibbasanipalli) discovered by the Geological Survey of India, of late, Rio Tinto Group has discovered three more outcropping pipes in this area (east of Dibbasanipalli, west of Anumpalli and Khaderpet) and termed all these five pipes as Anumpalli kimberltie cluster (AKC). The AKC pipes contain crustal granitoid xenoliths. The Khaderpet and Dibbasanipalli east pipes show effects of fenitisation in the country rock granitoids and are intensely chloritised kimberlite granite breccias; however, the former is unique in having its association with carbonatite (sovite) intrusion. Petrographically, the AKC kimberlites exhibit inequigranular texture resulted by anhedral to subhedral olivine macrocrysts and pseudomorphs, phenocrysts of Crdiopside, ilmenite, perovskite and minor or no amounts of phologopite with two generations of olivine within a finegrained matrix of same mineral phases. Based on the major element geochemistry the AKC pipes are classified as Group- I archetypal. Based on trace element modeling, the AKC pipes appear to be originated form garnet lherzolite source with residual garnet of 0.5 to 5%, associated with stable continental and/or orogenic area and remarkably belong to non-subduction environment. The observed LREE enrichment and low HREE cencentrations in the AKC, is consistent with inferior degrees of partial melting (0.1 to 2%). The AKC pipes appear to have originated from a hydrous magma enriched in volatiles. Exploration evidences support that kimberlites of the AKC are diamodiferous. The calculated diamond grade (DG) values of the AKC pipes are high (3.43 to 8.48) which are inversely proportional to the TiO2 content. In the binary diagram of Ta and Sc (ppm), the AKC pipes plot in the field of ‘Fe-Ti diamondiferous kimberlites’. In the Fe2O3 (wt %) vs. Y (ppm) diagram, the AKC pipes plot in the ‘prospective’ field. The diamondiferous nature of the AKC pipes indicates the conditions of diamond preservation at metastable phases in crustal environment during rapid ascent of kimberlite melt to the surface from the deep mantle, which is supported by low density and ultralow viscosity of these intrusions.
Contributions to Mineralogy and Petrology, Vol. 173, doi.org/10.1007/ s00410-018-1493-y 27p.
India
lamproites
Abstract: Numerous lamproite dykes are hosted by the Eastern Dharwar Craton, southern India, particularly towards the northwestern margin of the Cuddapah Basin. We present here a comprehensive mineralogical and geochemical (including Sr and Nd isotopic) study on the lamproites from the Vattikod Field, exposed in the vicinity of the well-studied Ramadugu lamproite field. The Vattikod lamproites trend WNW-ESE to NW-SE and reveal effects of low-temperature post-magmatic alteration. The studied lamproites show porphyritic texture with carbonated and serpentinized olivine, diopside, fluorine-rich phlogopite, amphibole, apatite, chromite, allanite, and calcite. The trace-element geochemistry (elevated Sr and HFSE) reveals their mixed affinity to orogenic as well as anorogenic lamproites. Higher fluorine content of the hydrous phases coupled with higher whole-rock K2O highlights the role of metasomatic phlogopite and apatite in the mantle source regions. Trace-element ratios such as Zr/Hf and Ti/Eu reveal carbonate metasomatism of mantle previously enriched by ancient subduction processes. The initial 87Sr/86Sr-isotopic ratios (calculated for an assumed emplacement age of 1350 Ma) vary from 0.7037 to 0.7087 and ?Nd range from ??10.6 to ??9.3, consistent with data on global lamproites and ultrapotassic rocks. We attribute the mixed orogenic-anorogenic character for the lamproites under study to multi-stage metasomatism. We relate the (1) earlier subduction-related enrichment to the Paleoproterozoic amalgamation of the Columbia supercontinent and the (2) second episode of carbonate metasomatism to the Mesoproterozoic rift-related asthenospheric upwelling associated with the Columbia breakup. This study highlights the association of lamproites with supercontinent amalgamation and fragmentation in the Earth history.
Abstract: Kyanite-eclogite xenoliths from Wajrakarur are considered as remnants of subducted ocean-floor crust. Here trace element concentration and isotopic data are presented in garnet (Grt) and kyanite (Ky) from xenoliths KL-2 E1-E4, characterized by. We use the precise 87Sr/86Sr host kimberlite groundmass perovskite ratio (0.70312-0.70333, as a proxy for the extent of kimberlitic magma infiltration at 1.1 Ga. The xenolithic Grt and Cr-rich (upto 1506 ppm) Ky have more radiogenic 87Sr/86Sr values than kimberlite, at 1.1 Ga, of 0.703829-0.705203 and 0.703811-0.704502, respectively. Furthermore, the Grt and Ky 143Nd/144Nd ratios, at 1.1 Ga, are 0.509321-0.511372 and 0.510951-0.511156, respectively, and are distinctly lower than those of the host kimberlite (0.511870-0.512290). This indicates that the infiltration of kimberlitic fluid has not altered the 87Sr/86Sr and 143Nd/144Nd ratios in the Grt and Ky, and therefore their isotope compositions must be inherited and predate the kimberlite magma generation event at 1.1 Ga. Trace elements in Grt and Ky indicate extreme metasomatism (Sr in Grt 104-296 ppm, in Ky 672-8713 ppm [limit Sr<2ppm] and Nb in Grt 0.64-1.78 ppm, in Ky 1.7-4.54 ppm [limit Nb<0.5ppm]). The xenoliths underwent at least one major melting event inferred from extreme depletions in Re, Os and 177Os/178Os ratios [5]. Their mantle-like ?18O values (Grt 5.3-5.4‰, Ky 5.3-5.9‰), positive Eu anomalies in both Grt and Ky (similar to Group 1 HREE-depleted garnets of) suggests that the protolith likely was a chromite-bearing leucogabbro, emplaced as a high-pressure cumulate at the crust-mantle boundary, which was later eclogitized due to deep-seated subduction and underwent episodes of extreme melting and metasomatism before 1.1 Ga and at least before 1.7 Ga, as inferred from their youngest Re depletion dates.
Earth and Planetary Science Letters, Vol. 502, pp. 166-173.
India
subduction
Abstract: Tectonic plate velocities predominantly result from a balance between the potential energy change of the subducting slab and viscous dissipation in the mantle, bending lithosphere, and slab-upper plate interface. A range of observations suggest that slabs may be weak, implying a more prominent role for plate interface dissipation than previously thought. The shallow thrust interface is commonly assumed to be weak due to an abundance of fluids and near-lithostatic pore fluid pressures, but little attention has been paid to the influence of the deeper, viscous interface. Here we show that the deep interface viscosity in subduction zones is strongly affected by the relative proportions of sedimentary to mafic rocks that are subducted to depth. Where sediments on the down-going plate are sparse, the deep interface is dominated by mafic lithologies that metamorphose to eclogites, which exhibit viscosities 1-2 orders of magnitude higher than the asthenospheric mantle, and reduce subduction plate speeds. In contrast, where sediments are abundant and subducted to depth, the deep interface viscosity is 1-2 orders of magnitude lower than the asthenospheric mantle, thus allowing significantly faster plate velocities. This correlation between subduction plate speed and deep sediment subduction may help explain dramatic accelerations (or decelerations) in convergence rates, such as the acceleration documented for India-Asia convergence during the mid-Cenozoic.
Abstract: In this contribution, we present detailed field, petrography, mineral chemistry, and geochemistry of newly identified high-Si high-Mg metavolcanic rocks from the southern part of the ~3.3 Ga Holenarsipur greenstone belt in the western Dharwar craton, India. The rocks occur as conformable bands that were interleaved with the mafic-ultramafic units. The entire volcanic package exhibits uniform foliation pattern, and metamorphosed under greenschist to low grade amphibolite facies conditions. The rocks are extremely fine grained and exhibit relict primary igneous textures. They are composed of orthopyroxene and clinopyroxene phenocrysts with serpentine, talc, and amphibole (altered clinopyroxene). Cr-spinel, rutile, ilmenite, and apatite occur as disseminated minute grains in the groundmass. The mineralogical composition and the geochemical signatures comprising of high SiO2 (~53 wt. %), Mg# (~83), low TiO2 (~0.18 wt. %), and higher than chondritic Al2O3/TiO2 ratio (~26), reversely fractionated heavy rare earth elements (REE) (GdN/YbN ~ 0.8), resulting in concave-up patterns, and positive Zr anomaly, typically resembled with the Phanerozoic boninites. Depletion in the high field strength elements Nb, and Ti relative to Th and the REE in a primitive mantle normalized trace element variation diagram, cannot account for contamination by pre-existing Mesoarchean continental crust present in the study area. The trace element attributes instead suggest an intraoceanic subduction-related tectonic setting for the genesis of these rocks. Accordingly, the Holenarsipur high-Si high-Mg metavolcanic rocks have been identified as boninites. It importantly indicates that the geodynamic process involved in the generation of Archean boninites, was perhaps not significantly different from the widely recognized two-stage melt generation process that produced the Phanerozoic boninites, and hence provides compelling evidence for the onset of Phanerozoic type plate tectonic processes by at least ~3.3 Ga, in the Earth’s evolutionary history.
Abstract: Lamprophyres from the greenstone belts play a crucial role in deciphering tectonic and geodynamic processes operating during the Archean. This study presents a comprehensive mineralogical and geochemical study of three lamprophyre dykes with calc-alkaline to shoshonitic affinities from the Neoarchean Kadiri schist belt, eastern Dharwar craton, southern India. These rocks display porphyritic-panidiomorphic texture, typical of the lamprophyres with amphibole (magnesio-hornblende) as phenocrysts, biotite as microphenocrysts and feldspar, epidote, titanite and apatite confined to the groundmass. Alteration of biotite to chlorite is observed along with mild deformation in the amphibole phenocrysts. Based on mineralogy and major oxide geochemistry, these rocks are classified as the calc-alkaline lamprophyres. Higher Ba/Nb and low Nb/La points to their derivation from an enriched lithospheric mantle source and higher Th/Yb ratio along with negative TNT (Ti-Nb-Ta) and Zr-Hf anomalies on the primitive mantle (PM) normalized multi-element diagram indicates dehydrated fluids from the foundering slab could be the possible metasomatic agent. Fractionated HREE ratios (GdN/YbN >1.9) and higher SmN/YbN suggests that the source region lies in the garnet stability field. Higher than PM Rb/Sr along with positive correlation between K/La and Rb/La reveals presence of metasomatic phlogopite in the source region. Strong negative initial ?Nd along with radiogenic 87Sr/86Sr ratios further support an enriched mantle reservoir involved in their genesis. Non-modal batch melting (1-5%) of a mixed source (phlogopite-garnet peridotite) assuming 5% mixing of subducted sediment with ambient mantle wedge (depleted mantle) satisfies the multi-element concentration pattern shown by the Kadiri lamprophyres. The source enrichment can be linked to the accretion-related growth of Dharwar craton and its schist belts during Neoarchean. Our study shows that a majority of lamprophyres associated with the Archean greenstone belts display a shoshonitic character; this highlights the role of subduction-related processes in the growth and evolution of the greenstone belts .
Journal of the Geological Society of India, Vol. 92, 2, pp. 141-144.
India
deposit - Amba Dongar
Abstract: In the Amba Dongar diatreme, "ferrocarbonatite" is not a single unit of late differentiate of calciocarbonatite magma but it is a family with variation on field occurrence, mineralogy and chemistry of each unit. The family includes dikes of ankeritic carbonatites (phase I and II), plugs of ankeritic carbonatite within sövite ring dike, dikes of sideritic carbonatite in ankeritic carbonatite plug and rödberg veins. Their intrusive relations are very clear in the field and each phase has characteristic mineralogy and trace and REE geochemistry. According to the nomenclature suggested by Harmer and Gittins (1997) majority of "ferrocarbonatites" of Amba Dongar plot in field of "ferruginous calciocarbonatite" and only siderite and rödberg plot in the field of "ferrocarbonatite". Within these family members, their trace and REE show clear increase from early phase to last phase of sideritic carbonatite. The present short communication discusses various aspects of "ferrocarbonatites".
Diamond & Related Materials, Vol. 89, pp. 322-329.
Russia, India
deposit - Mir, Panna
Abstract: The optical features of milky type IaB diamonds were studied systematically by non-destructive approaches including FTIR, photoluminescence (PL), and cathodoluminescence (CL) spectroscopy. From 97 type IaB diamonds ranging from 0.2?ct to ~100?ct submitted to GIA's New York laboratory for screening, we found that all the milky type IaB diamonds consistently displayed the hydrogen-related defect with an absorption line at 3107?cm?1, and ~96% of them were accompanied by a weaker line at 3085.4?cm?1, which is undetectable in most non-milky diamonds. Most of the diamond samples display no platelet defect or a very tiny residual platelet peak with a position at larger wavenumber in milky diamonds than in non-milky counterparts. “Amber center” with a weak but sharp absorption line at 4168.8?cm?1 has been observed in ~73% of the milky diamonds and ~24% of the non-milky ones. Photoluminescence (PL) results reveal that several defects with ZPLs at 490.7, 536, 575.9 and 612.4?nm are more common in milky type IaB diamonds than non-milky ones. A zero-phonon line (ZPL) at 536?nm has been confirmed by PL mapping and CL spectra as a product of plastic deformation, and it might be linked with the H4 center (N4V2 defect). A ZPL at 490.7?nm could be related to a nitrogen-vacancy complex. The defects present more often in milky IaB diamonds are generally associated with plastic deformation. The presence of a hydrogen-related peak at 3085.4?cm?1 and a 536?nm center would help effectively distinguish IaB diamonds with subtle milky areas from their non-milky counterparts.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 201-222.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 223-236.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 191-200.
Abstract: The ~1100 Ma CC2 and P13 lamproite dykes in the Wajrakarur Kimberlite Field (WKF), Eastern Dharwar Craton, and ~65 Ma Kodomali and Behradih lamproite diatremes in the Mainpur Kimberlite Field (MKF), Bastar Craton share a similar mineralogy, although the proportions of individual mineral phases vary significantly. The lamproites contain phenocrysts, macrocrysts and microcrysts of olivine set in a groundmass dominated by diopside and phlogopite with a subordinate amount of spinel, perovskite, apatite and serpentine along with rare barite. K-richterite occurs as inclusion in olivine phenocrysts in Kodomali, while it is a late groundmass phase in Behradih and CC2. Mineralogically, the studied intrusions are classified as olivine lamproites. Based on microtextures and compositions, three distinct populations of olivine are recognised. The first population comprises Mg-rich olivine macrocrysts (Fo89-93), which are interpreted to be xenocrysts derived from disaggregated mantle peridotites. The second population includes Fe-rich olivine macrocrysts (Fo82-89), which are suggested to be the product of metasomatism of mantle wall-rock by precursor lamproite melts. The third population comprises phenocrysts and overgrowth rims (Fo83-92), which are clearly of magmatic origin. The Mn and Al systematics of Mg-rich olivine xenocrysts indicate an origin from diverse mantle lithologies including garnet peridotite, garnet-spinel peridotite and spinel peridotite beneath the WKF, and mostly from garnet peridotite beneath the MKF. Modelling of temperatures calculated using the Al-in-olivine thermometer for olivine xenocrysts indicates a hotter palaeogeotherm of the SCLM beneath the WKF (between 41 and 43 mW/m2) at ~1100 Ma than beneath the MKF (between 38 and 41 mW/m2) at ~65 Ma. Further, a higher degree of metasomatism of the SCLM by precursor lamproite melts has occurred beneath the WKF compared to the MKF based on the extent of CaTi enrichment in Fe-rich olivine macrocrysts. For different lamproite intrusions within a given volcanic field, lower Fo olivine overgrowth rims are correlated with higher phlogopite plus oxide mineral abundances. A comparison of olivine overgrowth rims from the two fields shows that WKF olivines with lower Fo content than MKF olivines are associated with increased XMg in spinel and phlogopite and vice versa. Melt modelling indicates relatively Fe-rich parental melt for WKF intrusions compared to MKF intrusions. The Ni/Mg and Mn/Fe systematics of magmatic olivines indicate derivation of the lamproite melts from mantle source rocks with a higher proportion of phlogopite and/or lower proportion of orthopyroxene for the WKF on the Eastern Dharwar Craton compared to those for the MKF on the Bastar Craton. This study highlights how olivine cores provide important insights into the composition and thermal state of cratonic mantle lithosphere as sampled by lamproites, including clues to elusive precursor metasomatic events. Variable compositions of olivine rims testify to the complex interplay of parental magma composition and localised crystallisation conditions including oxygen fugacity variations, co-crystallisation of groundmass minerals, and assimilation of entrained material.
Society of Economic Geology Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits, Special Publication no. 20, pp. 237-252.
Abstract: The ~1100?Ma CC2 and P13 lamproite dykes in the Wajrakarur Kimberlite Field (WKF), Eastern Dharwar Craton, and ~65?Ma Kodomali and Behradih lamproite diatremes in the Mainpur Kimberlite Field (MKF), Bastar Craton share a similar mineralogy, although the proportions of individual mineral phases vary significantly. The lamproites contain phenocrysts, macrocrysts and microcrysts of olivine set in a groundmass dominated by diopside and phlogopite with a subordinate amount of spinel, perovskite, apatite and serpentine along with rare barite. K-richterite occurs as inclusion in olivine phenocrysts in Kodomali, while it is a late groundmass phase in Behradih and CC2. Mineralogically, the studied intrusions are classified as olivine lamproites. Based on microtextures and compositions, three distinct populations of olivine are recognised. The first population comprises Mg-rich olivine macrocrysts (Fo89-93), which are interpreted to be xenocrysts derived from disaggregated mantle peridotites. The second population includes Fe-rich olivine macrocrysts (Fo82-89), which are suggested to be the product of metasomatism of mantle wall-rock by precursor lamproite melts. The third population comprises phenocrysts and overgrowth rims (Fo83-92), which are clearly of magmatic origin. The Mn and Al systematics of Mg-rich olivine xenocrysts indicate an origin from diverse mantle lithologies including garnet peridotite, garnet-spinel peridotite and spinel peridotite beneath the WKF, and mostly from garnet peridotite beneath the MKF. Modelling of temperatures calculated using the Al-in-olivine thermometer for olivine xenocrysts indicates a hotter palaeogeotherm of the SCLM beneath the WKF (between 41 and 43?mW/m2) at ~1100?Ma than beneath the MKF (between 38 and 41?mW/m2) at ~65?Ma. Further, a higher degree of metasomatism of the SCLM by precursor lamproite melts has occurred beneath the WKF compared to the MKF based on the extent of CaTi enrichment in Fe-rich olivine macrocrysts. For different lamproite intrusions within a given volcanic field, lower Fo olivine overgrowth rims are correlated with higher phlogopite plus oxide mineral abundances. A comparison of olivine overgrowth rims from the two fields shows that WKF olivines with lower Fo content than MKF olivines are associated with increased XMg in spinel and phlogopite and vice versa. Melt modelling indicates relatively Fe-rich parental melt for WKF intrusions compared to MKF intrusions. The Ni/Mg and Mn/Fe systematics of magmatic olivines indicate derivation of the lamproite melts from mantle source rocks with a higher proportion of phlogopite and/or lower proportion of orthopyroxene for the WKF on the Eastern Dharwar Craton compared to those for the MKF on the Bastar Craton. This study highlights how olivine cores provide important insights into the composition and thermal state of cratonic mantle lithosphere as sampled by lamproites, including clues to elusive precursor metasomatic events. Variable compositions of olivine rims testify to the complex interplay of parental magma composition and localised crystallisation conditions including oxygen fugacity variations, co-crystallisation of groundmass minerals, and assimilation of entrained material.
Abstract: Major, trace element concentrations and Nd, Sr and Ca stable isotopic compositions (?44/40Ca and ?44/42Ca w.r.t. NIST SRM915a) of carbonatites and associated igneous silicate rocks from the ~65?Ma old Ambadongar carbonatite complex and the surrounding Phenai Mata igneous complex of western India are reported. Samples of fluorspar from Ambadongar and the Bagh Limestone and Sandstone, which are part of the country rocks at Ambadongar, have also been analysed. The Ambadongar carbonatites are primarily calcio- and ferro-carbonatites while the silicate rocks from these two complexes are alkaline and tholeiitic in composition. The ?44/40Ca values of the carbonatites (0.58-1.1‰, n?=?7) and the associated igneous silicate rocks (0.50-0.92‰, n?=?14) show a broad range. The low K/Ca values of the carbonatites (<0.2) and silicate rocks (<2) along with their young eruption age (~65 Ma) rule out any effect of radiogenic 40Ca ingrowth due to decay of 40K on the ?44/40Ca values. The lack of correlations between ?44/40Ca and Mg# as well as La/Yb(N) values suggest that the variability in ?44/40Ca is not controlled by the degree of partial melting. The ?44/40Ca values of the carbonatites (0.58-1.1‰) overlap with that of the upper mantle/Bulk Silicate Earth and is mostly higher than the ?44/40Ca value of the Bagh Limestone (0.66‰) suggesting that assimilation of these crustal limestones by the magma is unlikely to have caused the variability in ?44/40Ca of the carbonatites. In plots of ?44/40Ca versus ?Nd(t) and 87Sr/86Sr(t), the igneous silicate rocks from the Ambadongar and Phenai Mata complexes plot on a mixing trend between a primitive (plume) mantle source and the continental crustal basement suggesting the role of continental crustal contamination during eruption of the Reunion plume. While simple binary mixing calculations yield unrealistically high amounts of crustal contamination (40%), assimilation and fractional crystallization (AFC) models suggest up to 20% contribution from a heterogeneous basement for these igneous silicate rocks. The role of continental crustal contamination in the genesis of the igneous silicate rocks is further supported by their unradiogenic ?Nd(t), radiogenic 87Sr/86Sr(t) and low Ce/Pb values. In contrast, carbonatites plot away from the mixing trend between a primitive mantle (plume) source and continental crust in Ca-Sr-Nd isotopic diagrams suggesting that the Ca isotopic variability of carbonatites is not caused by continental crustal contamination. In contrast, the isotopic composition of the carbonatites can be explained by mixing of the plume end-member with up to 20% of ~160?Ma-old recycled carbonates suggesting their derivation from a highly heterogeneous, recycled carbonate-bearing plume mantle source. The composition of one carbonatite sample showing unusually high ?44/40Ca and highly radiogenic 87Sr/86Sr(t) is explained by hydrothermal alteration which is also invoked for the formation of massive fluorspar deposits with high ?44/40Ca (1.44‰) at Ambadongar. In a plot of ?44/40Ca versus K/Rb, the carbonatites plot towards the phlogopite end-member (?44/40Ca?=?1‰, K/Rb?=?40-450) while the igneous silicate rocks plot towards the amphibole end-member (?44/40Ca?=?0.44‰, K/Rb >1000). Phlogopite, especially if F-rich, is stable at greater depths in the mantle compared to amphibole. Hence, the correlated ?44/40Ca and K/Rb values of the carbonatites and associated igneous silicate rocks suggest the derivation of these carbonatites from a relatively deeper mantle source compared to the silicate rocks, both within the Reunion mantle plume. The origin of the carbonatites from the F-rich phlogopite-bearing mantle is also consistent with the occurrence of large fluorspar deposits within the Ambadongar carbonatite complex.
Journal of Metamorphic Geology, Vol. 37, 1, pp. 113-151.
India
geology
Abstract: The Central Indian Tectonic Zone (CITZ) is a Proterozoic suture along which the Northern and Southern Indian Blocks are inferred to have amalgamated forming the Greater Indian Landmass. In this study, we use the metamorphic and geochronological evolution of the Gangpur Schist Belt (GSB) and neighbouring crustal units to constrain crustal accretion processes associated with the amalgamation of the Northern and Southern Indian Blocks. The GSB sandwiched between the Bonai Granite pluton of the Singhbhum craton and granite gneisses of the Chhotanagpur Gneiss Complex (CGC) links the CITZ and the North Singhbhum Mobile Belt. New zircon age data constrain the emplacement of the Bonai Granite at 3,370 ± 10 Ma, while the magmatic protoliths of the Chhotanagpur gneisses were emplaced at c. 1.65 Ga. The sediments in the southern part of the Gangpur basin were derived from the Singhbhum craton, whereas those in the northern part were derived dominantly from the CGC. Sedimentation is estimated to have taken place between c. 1.65 and c. 1.45 Ga. The Upper Bonai/Darjing Group rocks of the basin underwent major metamorphic episodes at c. 1.56 and c. 1.45 Ga, while the Gangpur Group of rocks were metamorphosed at c. 1.45 and c. 0.97 Ga. Based on thermobarometric studies and zircon-monazite geochronology, we infer that the geological history of the GSB is similar to that of the North Singhbhum Mobile Belt with the Upper Bonai/Darjing and the Gangpur Groups being the westward extensions of the southern and northern domains of the North Singhbhum Mobile Belt respectively. We propose a three?stage model of crustal accretion across the Singhbhum craton - GSB/North Singhbhum Mobile Belt - GC contact. The magmatic protoliths of the Chhotanagpur Gneisses were emplaced at c. 1.65 Ga in an arc setting. The earliest accretion event at c. 1.56 Ga involved northward subduction and amalgamation of the Upper Bonai Group with the Singhbhum craton followed by accretion of the Gangpur Group with the Singhbhum craton-Upper Bonai Group composite at c. 1.45 Ga. Finally, continent-continent collision at c. 0.96 Ga led to the accretion of the CGC with the Singhbhum craton-Upper Bonai Group-Gangpur Group crustal units, synchronous with emplacement of pegmatitic granites. The geological events recorded in the GSB and other units of the CITZ only partially overlap with those in the Trans North China Orogen and the Capricorn Orogen of Western Australia, indicating that these suture zones are not correlatable.
Abstract: The abstracts broadly summarises petrological aspects of kimberlite clan rocks so far discovered in the Telangana state in light of recent finds emphasising the context for diamond exploration in the state. This was presented in the '1st Telangana Science Congress (TSSC)-2018' organised by the Telangana Academy of Science, Hyderabad and National institute of Technology, Warangal (22-24, December, 2018).
Srivastava: Dyke Swarms of the World: a Modern Perspective, Springer, researchgate 56p. Pdf
India
dykes
Abstract: We present dyke swarm maps generated using Google Earth™ images, ArcGIS™, field data, and available geochronological ages of Neoarchean-Mesoproterozoic (ranging in age from ~2.80 to ~1.10 Ga) mafic dyke swarms and associated magmatic units of the different Archean cratons of the Indian shield which represent the plumbing system of Large Igneous Provinces (LIPs). The spatial and temporal distributions together with the trends of the dyke swarms provide important informations about geodynamics. Twenty four dyke swarms (17 have been precisely dated), mostly mafic in nature, have been mapped from the different cratons and named/re-named to best reflect their location, trend, distribution and distinction from other swarms. We have identified 14 distinct magmatic events during the Neoarchean-Mesoproterozoic in the Indian shield. These intraplate magmatic events (many of LIP scale) of the Indian shield and their matches with coeval LIPs on other crustal blocks suggest connections of the Indian shield within known supercontinents, such as Kenorland/Superia (~2.75-2.07 Ga), Columbia/Nuna (1.90-1.38 Ga), and Rodinia (1.20-0.72 Ga). However, further detailed U-Pb geochronology and associated paleomagnetism are required to come to any definite constraints on the position of the Indian cratons within these supercontinents.
Abstract: Petrological, geochemical, and zircon U-Pb geochronological features of Archean ultramafic-mafic complexes formed in subduction?related settings provide significant insights into mantle source and geodynamic processes associated with subduction-accretion?collision events in the early Earth. Here, we investigate a suite of serpentinized dunite, dunite, pyroxenite, and clinopyroxenite from an ultramafic complex along the collisional suture between the Western Dharwar Craton (WDC) and the Central Dharwar Craton (CDC) in southern India. We present petrology, mineral chemistry, zircon U-Pb geochronology, rare earth element (REE), Lu-Hf isotopes, and whole?rock geochemistry including major, trace element, and platinum?group element (PGE) data with a view to investigate the magmatic and metasomatic processes in the subduction zone. Mineral chemistry data from chromite associated with the serpentinised ultramafic rocks show distinct characteristics of arc?related melt. Zircon U-Pb data from the ultramafic suite define different age populations, with the oldest ages at 2.9 Ga, and the dominant age population showing a range of 2.8-2.6 Ga. The early Paleoproterozoic (ca. 2.4 Ga) metamorphic age is considered to mark the timing of collision of the two WDC and CDC. Zircon REE patterns suggest the involvement continental crust components in the magma source. Zircon Lu-Hf analysis yields both positive and negative ?Hf(t) values from ?3.9 to 1.5 with Hf?depleted model ages (TDM) of 3,041-3,366 Ma for serpentinised dunite and ?0.2-2.0 and 2,833-2,995 Ma for pyroxenite, suggesting that the magma was sourced from depleted mantle and was contaminated with the ancient continental crust. Geochemical data show low MgO/SiO2 values and elevated Al2O3/TiO2 ratios, implying subduction?related setting. The serpentinized dunites and dunites show mild LREE enrichment over HREE, with relatively higher abundance of LILE (Ba, Sr) and depletion in HFSE (Nb, Zr), suggesting fluid-rock interaction, melt impregnation, and refertilization processes. The PGE data suggest olivine, chromite, and sulphide fractionations associated with subduction processes. Our study on the Mesoarchean to Neoarchean ultramafic complex provides important insights to reconstruct the history of the crust-mantle interaction in an Archean suprasubduction zone mantle wedge.
Abstract: We describe the mineralogy of a lamproite dyke from Gundrapalli village (Nalgonda district), Telangana, India. The dyke consists of a mineral assemblage characteristic of lamproites in terms of the presence of amphiboles (mainly potassic-richterite together with potassic-arfvedsonite, magnesio-riebeckite, Ti-rich potassic-magnesio-arfvedsonite, potassic-magnesio-arfvedsonite, katophorite and potassic-ferri-katophorite), Al-poor pyroxene, phlogopite (Ti-rich, Al-poor), pseudomorphed leucite, spinel (chromite-magnesiochromite), fluoroapatite, barite, titanite, rutile, baritocalcite, calcite, ilmenite, hydro-zircon, baotite, strontianite, allanite, quartz and pyrite. The absence of wadeite and priderite have been compensated for by presence of baotite, rutile, titanite, barite, and hydro-zircons. The presence of the secondary phases: allanite, hydro-zircon, chlorite, quartz, and cryptocrystalline silica, implies that the dyke has undergone deuteric alteration. The Gundrapalli dyke on the basis of its typomorphic mineralogy has been classified as a pseudoleucite-phlogopite-amphibole-lamproite. We report the presence of the rare mineral baotite from this lamproite, the first recognition of baotite from a lamproite in India. The mineralogy of the baotite- bearing Gundrapalli lamproite is analogous to the baotite- bearing Kvaløya lamproite from Troms, Norway. Ultrapotassic magmatism is quite prevalent in the Eastern Dharwar Complex of south India. Numerous new finds of lamproite dykes from and around the Palaeo-Mesoproterozoic Cuddapah basin gives an opportunity to understand the nature of the subcontinental lithospheric mantle in southern India. We present the mineralogy of a newly discovered lamproite dyke at Gundrapalli village (Nalgonda district) northwest of Cuddapah basin (Figure 1 inset). The lamproite dyke at Gundrapalli village, intruded into the Paleoproterozoic biotite granite unit of Peninsular Gneissic Complex form part of eastern Dharwar Complex. (Figure 1; Kumar et al., 2013, Ahmed et al., 2012).
Journal of the Geological Society of India, Vol. 93, 2, pp. 163-170.
India
geophysics - seismic
Abstract: The crustal structures of the Narmada region in Central India bounded by fault system (Narmada- North and South faults : NNF and NSF) has been derived from deep seismic sounding (DSS) studies along the two profiles trending almost north-south direction. The wide-angle phases have been modeled kinematically and dynamically using the 2-D asymptotic ray tracing technique. The combined seismic and gravity modeling reveals a multilayer crust in the region. The crustal wide-angle reflection phases map the Moho discontinuity, where the P-wave velocity jumps from 7.2 km s-1 to 8.0-8.1 km s-1, at depth varying between 38 km and 44 km. A layer with velocity 7.2 km s-1, exists above the Moho in most parts of the profiles and is attributed to the magmatic underplating related to the Deccan volcanism (~65 Ma). The intriguing observation of the study is a zone characterized by anomalous high velocity (6.5-6.6 km s-1) within the upper crust. 2-D gravity modeling demonstrates that this anomalous layer has a density of ~2.9 gm cm-3, which is equivalent to the rocks metamorphosed to granulite/amphibolite facies. This high velocity layer probably represents the granulite enclaves within the Archaean granites/gneiss rocks and was formed during the cratonization of the Achaean crust. Importantly, this high velocity layer shows an average upward displacement of ~8.5 km within the region bounded by NNF and NSF as compared to the regions beyond it. The studies suggest that the observed displacement in the high velocity layer of the upper crust is a result of repeated reactivation of the Narmada fault system.
Abstract: The Indian Shield is cross-cut by a number of distinct Paleoproterozoic mafic dyke swarms. The density of dykes in the Dharwar and Bastar Cratons is amongst the highest on Earth. Globally, boninitic dyke swarms are rare compared to tholeiitic dyke swarms and yet they are common within the Southern Indian Shield. Geochronology and geochemistry are used to constrain the petrogenesis and relationship of the boninitic dykes (SiO2?=?51.5 to 55.7?wt%, MgO?=?5.8 to 18.7?wt%, and TiO2?=?0.30?wt% to 0.77?wt%) from the central Bastar Craton (Bhanupratappur) and the NE Dharwar Craton (Karimnagar). A single U-Pb baddeleyite age from a boninitic dyke near Bhanupratappur yielded a weighted-mean 207Pb/206Pb age of 2365.6?±?0.9?Ma that is within error of boninitic dykes from the Dharwar Craton near Karimnagar (2368.5?±?2.6?Ma) and farther south near Bangalore (2365.4?±?1.0?Ma to 2368.6?±?1.3?Ma). Rhyolite-MELTS modeling indicates that fractional crystallization is the likely cause of major element variability of the boninitic dykes from Bhanupratappur whereas trace element modeling indicates that the primary melt may be derived from a pyroxenite mantle source near the spinel-garnet transition zone. The Nd isotopes (?Nd(t)?=??6.4 to +4.5) of the Bhanupratappur dykes are more variable than the Karimnagar dykes (?Nd(t)?=??0.7 to +0.6) but they overlap. The variability of Sr-Nd isotopes may be related to crustal contamination during emplacement or is indicative of an isotopically heterogeneous mantle source. The chemical and temporal similarities of the Bhanupratappur dykes with the dykes of the Dharwar Craton (Karimnagar, Penukonda, Chennekottapalle) indicate they are members of the same giant radiating dyke swarm. Moreover, our results suggest that the Bastar and Dharwar Cratons were adjacent but likely had a different configuration at 2.37?Ga than the present day. It is possible that the 2.37Ga dyke swarm was related to a mantle plume that assisted in the break-up of an unknown or poorly constrained supercontinent.
Journal of the Geological Society of India, Vol. 93, 2, pp. 157-162.
India
craton
Abstract: The eastern Dharwar craton (EDC) of the southern Indian Shield hosts five geochronologically distinct Paleoproterozoic mafic dyke swarms emplaced at 2.37, 2.21, 2.18, 2.08 and 1.89 Ga. Trace element geochemical data available for these dykes display the ‘arc signals’ viz., negative Nb-Ta anomalies and elevated Zr/Nb, Th/Yb and Th/Ta values, which are conventionally interpreted to represent involvement of subduction in their genesis. It is shown that these ‘arc signals’ resulted from coupled assimilation and fractional crystallization (AFC) processes that modified these mantle-derived melts. Since, mafic dykes under study are highly evolved, an attempt has been made to estimate (using PRIMELTS2.xls software) the composition of the primary magma from the most primitive sample available from the 2.21 and 2.37 Ga swarms. The mantle potential temperature derived from the estimated primary magma compositions revealed anomalously hot mantle source regions compared to the known ambient upper mantle temperatures during Paleoproterozoic, thus implying the possible involvement of thermal plumes in their genesis.
Abstract: The spatial and temporal association between lamprophyres and kimberlites provides unique opportunities to explore their genetic relationships. This paper explores such a relationship by detailing mineralogical and geochemical aspects of Korakkodu lamprophyre dykes located within the well?known Mesoproterozoic diamondiferous Wajrakarur Kimberlite field (WKF), towards the south?western margin of Paleo-Mesoproterozoic Cuddapah Basin, Eastern Dharwar Craton, southern India. Mineralogy reveals that these dykes belong to calc?alkaline variety of lamprophyres, but their geochemistry display mixed signals of both alkaline and calc?alkaline lamprophyres. These lamprophyres are highly potassic, and their high Al2O3 and low?TiO2 content implies a shoshonitic character. Low Mg#, Ni, and Cr concentration highlight their evolved nature. High (La/Yb)N and (Gd/Yb)N content is consistent with their derivation from low degrees of partial melting, whereas highly fractionated nature suggests the presence of garnet in their source. Absence of prominent Nb?Ta anomaly implies to the dilution of lithospheric mantle source by melts rich in HFSEs and low La/Nb ratio compared to those of the calc?alkaline island arc volcanics and suggests an asthenospheric overprint on lithospheric mantle source. Carbonatite metasomatism in the source region of these lamprophyres is apparent from conspicuously high?Zr/Hf ratio, and the HFSE budget of these lamprophyres are principally controlled by the presence of phlogopite veins in their lithospheric source. An extremely heterogeneous and layered lithospheric mantle beneath Eastern Dharwar Craton has been inferred from the divergent genetic history of Mesoproterozoic lamprophyres and kimberlites in the Wajrakarur field.
Abstract: Upper mantle anisotropy investigated using 172 core-refracted (SKS, SKKS) seismic phases along a ~660?km long profile at 10 to 20?km intervals from the west to the east coast of South India reveals significant lateral variations in its magnitude and direction. This profile, with 38 broadband seismic stations, covers mid-Archean Western Dharwar craton (WDC), late-Archean Eastern Dharwar Craton (EDC), Proterozoic Cuddapah Basin (CB) and the passive continental margins along the west and east coast. The observed fast polarization directions (FPDs) show lateral variability: NW50o to NW5o beneath the WDC, NW40o to NE30o beneath the EDC and N5o to N85o beneath the CB and further east. The delay time varies between 0.4 and 2.0?s with an average of 1?s. However, we are unable to fit a two layers anisotropy model for the region due to sparse azimuth coverage. Beneath the WDC, the direction of the fast axis follows trends of shear zones and faults, suggesting “frozen-in” anisotropy in the lithosphere, possibly established during the lithospheric evolution in mid-late Archean. In the EDC, the fast axis does not only follow the plate motion direction but it deviates, manifesting late Archean to Proterozoic deformation may still be present as fossil mantle anisotropy. The splitting trend beneath the CB and Eastern Ghat (EG) follows the strike of the rift along with plate motion direction, indicating anisotropy is influenced by the combination of “frozen” anisotropy due to continental rifting along the eastern margin of Indian plate and active asthenospheric flow.
Journal of the Geological Society of India, Vol. 93, 3, pp. 305-312.
India
carbonatite
Abstract: Inclusions of calcite within large euhedral apatite crystals from the pyroxenite-carbonatite-syenite complex of Sevattur, Tamil Nadu, south India, were identified to represent inclusions of a primary carbonatitic melt (calcite I) from which the apatites have crystallized. The apatites themselves are embedded into a younger batch of calcite-carbonatitic melt (calcite II). Using the synchrotron XRF microprobe at beamline L at HASYLAB/DESY (Hamburg), the concentrations of the trace elements Ba, Sr, Y, Zr, Th, La, Ce, Nd, Sm, Gd, Dy, and Er were determined both in melt inclusions as well as in host apatites and younger carbonatite matrix. Unexpected high REE concentrations were found not only in apatite but also in calcite, especially of the younger matrix phase, in agreement with the whole rock geochemistry. The data reveal an equilibrium distribution between melt inclusions and host apatite that allows the calculation of partition coefficients D = CiAp/CiCc=melt for elements of interest. Assuming 9% crystallization of the melt, which can be calculated from the whole rock analyses, the composition of the primary carbonatite melt prior to apatite crystallization can be determined. This composition is, with the exception of only few elements, nearly equal to that of the younger matrix carbonatite melt (calcite II), and thus gives evidence for the existence of different pulses of carbonatite melt during crystallization and consolidation of the carbonatite body. The results allow new insights into the processes of trace element and REE distribution between the two major igneous components of carbonatites and thus into the question of carbonatitic fractionation processes. The data reveal that mere apatite crystallization and fractionation does not lead to enriched REE compositions during carbonatite evolution but lowers their concentrations in the residual melts. But alternatively, if segregated apatite is collected and incorporated by a new melt batch, the overall REE of this melt will be increased.
Abstract: New in-situ U-Pb ages and Sr-Nd-Hf isotopic data on mineral phases of the Sung Valley and Jasra ultramafic-alkaline-(carbonatite) intrusions (Shillong Plateau, India) shed new light on the petrogenetic processes of volcanism in north-eastern India during the Cretaceous. Perovskites of Sung Valley dunite, ijolite and uncompahgrite yielded U-Pb ages of 109.1?±?1.6, 104.0?±?1.3 and 101.7?±?3.6?Ma, respectively. A U-Pb age of 106.8?±?1.5?Ma was obtained on zircons of a Sung Valley nepheline syenite. Perovskite of a Jasra clinopyroxenite yielded an age of 101.6?±?1.2?Ma, different from the U-Pb age of 106.8?±?0.8?Ma on zircon of Jasra syenites. The variation in Sr-Nd-Hf isotopic compositions [initial 87Sr/86Sr?=?0.70472 to 0.71080; ?Nd i?=??10.85 to +0.86; ?Hf i?=??7.43 to +1.52] matches the bulk-rock isotopic composition of the different rock units of Sung Valley and Jasra. Calcite and apatite in the carbonatites, the perovskite in a dunite, and the bulk-rock carbonatites of Sung Valley intrusion have the lowest initial 87Sr/86Sr and ?Nd, taken to be the best proxies of the mantle source composition, which is dominated by components derived from the lithospheric mantle. The alkaline intrusions of north-eastern India are significantly younger than the Sylhet tholeiitic magmatism. The silicate rocks of both intrusions have isotopic composition trending to that of the underlying Shillong crust, indicating the effects of fractional crystallization and low-pressure crustal contamination during the emplacement of the various intrusive magma pulses.
Abstract: The Dongargarh Supergroup along with the basal Amgaon Gneissic Complex constitutes the northwestern part of the central Indian Bastar craton. In the present study, we report a new finding of a boninite dyke intruded in the Amgaon gneisses of this area. The dyke composed of mainly pyroxenes, amphiboles and subordinate amount of plagioclase. The higher contents of SiO2 (51-54 wt.%), MgO (12-14 wt.%), Ni (375-473 ppm), Cr (1416-1580 ppm) and very low TiO2 (0.2-0.4 wt.%) are consistent with the boninite nature of the dyke as well as the unevolved primary nature of the source magma. The extraordinarily high CaO content (15.97-17.7 wt.%) with higher CaO/Al2O3 (3.13-3.96) ratios classifies it as high-Ca boninite. The trace element ratios including Zr/Ti, Ti/V, Ti/Sc and Ti/Yb further show its geochemical similarity with the Archaean boninite. The dyke also shows negative high-field strength element (Nb, Ta and Ti) anomalies which are the characteristics of the boninite rocks reported elsewhere and along with the enriched light rare earth element pattern, it shows more affinity particularly with the northern Bastar boninite dyke. The mineralogical and geochemical similarities of the boninite dykes from the Bastar craton indicate a widespread boninitic event during the Palaeoproterozoic having a similar origin. These boninite dykes indicate the preservation of subduction-related signatures in the lithospheric mantle beneath the Bastar craton at the time of its evolution or may be during the convergence of the Bastar and Bundelkhand cratons.
Journal of Asian Earth Sciences, Vol. 176, pp. 253-263.
India
geophysics - magnetotellurics
Abstract: The vertical extension and structure of the sub-continental lithospheric mantle beneath the Archean Dharwar craton is the main attraction of the work presented here. To delineate the electrical conductivity structure of the Dharwar craton, a magnetotelluric study is carried out. This study comprises magnetotelluric data at 22 stations along a west-east slanting profile. Inter-station spacing is approximately 15?km. This magnetotelluric study is initiated from Dandeli (in the west) to Sindhanur (in the east side). The preferable geoelectric strike directions for the crust and lithospheric mantle are N3°E and N16°E respectively. A 2-dimensional (2-D) resistivity model derived by using the crustal and lithospheric mantle strike azimuths, identified conductive features in the stable continental Dharwar craton. In the crust, prominent conductors are present in the eastern and western part of the profile. A conducting feature is present in the deeper crust associated with the Chitradurga shear zone (CSZ). The study infers a thick lithosphere beneath Dharwar craton as a preserved cratonic nucleus on the eastern and a few conductive anomalies in the western part of the Dharwar craton. The model shows two separate conductors in the depth range of 110-250?km. This study shows, the possibility of presence of kimberlite melt in the western Dharwar craton in the depth range of 110-150?km.
Abstract: The Neoarchaean era is characterized by rapid crustal growth corresponding to some fundamental global changes in geodynamic processes. However, the nature of crustal growth including the mechanism and tectonic setting of the Neoarchaean are controversial issues. The eastern Dharwar Craton (EDC) exposes widespread Neoarchaean granite?greenstone belts, which provide an opportunity to evaluate the various models proposed for Neoarchaean crustal growth. In this study, we present field, petrographic, and geochemical data and discuss the petrogenesis and significance for crustal evolution for a suite of previously undescribed banded gneisses, TTG (tonalite-trondhjemite-granodiorite), biotite granites, alkali feldspar granite and gabbro. These rocks are associated with Neoarchaean metavolcanic and metapelites rocks of the Tsundupalle greenstone belt, in the eastern fringe of the EDC. Whole?rock major and trace element geochemical data are consistent with diverse sources, including both crust and enriched mantle in an evolving subduction zone. A convergent orogenic setting is proposed for interpreting the association of various granitoids in the Tsundupalle area. Finally, intrusion of crustally derived, highly silicic, alkali?rich granite, and mantle?derived gabbro emplaced in a post?subduction regime is proposed. Lithospheric delamination and attendant mantle melting are suggested as possible mechanisms for generation of these rocks. The understanding of generation of the different granitoid types along with gabbro provides significant insights into the mechanism of Neoarchaean crustal growth.
Abstract: A number of NE-SW to ENE-WSW trending Palaeoproterozoic mafic dykes, intruded within the Archean basement rocks and more conspicuous in the southern parts of the western Dharwar Craton (WDC), was studied for their whole?rock geochemistry to understand their petrogenetic and geodynamic aspects. Observed mineralogical and textural characteristics classify them either as meta?dolerites or dolerites/olivine?dolerites. They show basaltic to basaltic-andesitic compositions and bear sub?alkaline tholeiitic nature. Three geochemically distinct groups of mafic dykes have been identified. Group 1 samples show flat REE patterns (LaN/LuN = ~1), whereas the other two groups have LaN/LuN = ~2-3 (Group 2; enriched LREE and flat HREE patterns) and LaN/LuN = ~4 (Group 3; inclined REE patterns). Chemistry is not straightforward to support any significant role of crustal contamination and probably reflect their source characteristics. However, their derivation from melts originated from a previously modified metasomatized lithospheric mantle due to some ancient subduction event cannot be ignored. Most likely different mantle melts were responsible for derivation of these distinct sets of mafic dykes. The Group 2 dykes are derived from a melt generated within spinel stability field by ~10% batch melting of a lithospheric mantle source, whereas the Group 3 dykes have their derivation from a melt originated within the spinel-garnet transition zone and were fed from slightly higher (~12-15%) batch melting of a similar source. The Group 1 samples were also crystallized from a melt generated at the transition zone of spinel-garnet stability field by higher degrees (~20%) of melting of a primitive mantle source. Geochemistry of the studied samples is typical of Palaeoproterozoic mafic dykes emplaced within the intracratonic setting, reported elsewhere globally as well as neighbouring cratons. Geochemistry of the studied mafic dyke samples is also compared with the mafic dykes of the eastern Dharwar Craton (EDC). Except the Group 3 samples, which have good correlation with the 1.88-1.89 Ga Hampi swarm, no other group shows similarity with the EDC mafic dykes. There is an ample possibility to have some different mafic magmatic events in the WDC, which could be different from the EDC. However, it can only be confirmed after precise age determinations.
Soderlund, U., Bleeker, W., Demirer, K., Srivastava, R.K., Hamilton, M., Nilsson, M., Personen, L.J., Samal, A.K., Jayananda, M., Ernst, R.E., Srinivas, M.
Abstract: Large igneous provinces (LIPs) and especially their dyke swarms are pivotal to reconstruction of ancient supercontinents. The Dharwar craton of southern Peninsular India represents a substantial portion of Archean crust and has been considered to be a principal constituent of Superia, Sclavia, Nuna/Columbia and Rodinia supercontinents. The craton is intruded by numerous regional-scale mafic dyke swarms of which only a few have robustly constrained emplacement ages. Through this study, the LIP record of the Dharwar craton has been improved by U-Pb geochronology of 18 dykes, which together comprise seven generations of Paleoproterozoic dyke swarms with emplacement ages within the 2.37-1.79 Ga age interval. From oldest to youngest, the new ages (integrated with U-Pb ages previously reported for the Hampi swarm) define the following eight swarms with their currently recommended names: NE-SW to ESE-WNW trending ca. 2.37 Ga Bangalore-Karimnagar swarm. N-S to NNE-SSW trending ca. 2.25 Ga Ippaguda-Dhiburahalli swarm. N-S to NNW-SSE trending ca. 2.22 Ga Kandlamadugu swarm. NW-SE to WNW-ESE trending ca. 2.21 Ga Anantapur-Kunigal swarm. NW-SE to WNW-ESE trending ca. 2.18 Ga Mahbubnagar-Dandeli swarm. N-S, NW-SE, and ENE-WSW trending ca. 2.08 Ga Devarabanda swarm. E-W trending 1.88-1.89 Ga Hampi swarm. NW-SE ca. 1.79 Ga Pebbair swarm. Comparison of the arcuate trends of some swarms along with an apparent oroclinal bend of ancient geological features, such as regional Dharwar greenstone belts and the late Archean (ca. 2.5 Ga) Closepet Granite batholith, have led to the hypothesis that the northern Dharwar block has rotated relative to the southern block. By restoring a 30° counter clockwise rotation of the northern Dharwar block relative to the southern block, we show that pre-2.08 Ga arcuate and fanning dyke swarms consistently become approximately linear. Two possible tectonic models for this apparent bending, and concomitant dyke rotations, are discussed. Regardless of which deformation mechanisms applies, these findings reinforce previous suggestions that the radial patterns of the giant ca. 2.37 Ga Bangalore-Karimnagar dyke swarm, and probably also the ca. 2.21 Ga Anantapur-Kunigal swarm, may not be primary features.
Journal of the Geological Society of India, extended abstract of Monthly Scientific Lecture March 12, 1p.
India
carbonatites
Abstract: Carbonatites, defined as carbonate-rich rocks of igneous origin, pose considerable challenges in understanding their genesis and evolution. These mantle-derived, rare, magmatic rocks are enigmatic in many facets compared to their associated co-magmatic rocks. These include: (a) The very-low viscous, water-soluble, Na- and K-carbonate (nyererieite and gregoryite respectively)-bearing lavas with low temperature (500-600°C) of eruption with only one active volcano as an example (e.g. Ol Doinyo Lengai, Tanzania) in contrast to the numerous acid and basic lava eruptive centres that are well-known around the world. (b). Carbonatites show very high solubilities of many elements considered rare in silicate magmas, and they have the highest known melt capacities for dissolving water and other volatile species like halogens at crustal pressures. With such ‘fluxing and fusing’ characters, carbonatite magma, actively reacts and ‘fenitises’ the country rocks through Na and K metasomatism when they get emplaced. Thus the carbonatite magma loses its Na and K, a feature rare to other magmatic rocks. (c) Primary mineralogy is highly variable from simple carbonate species to a variety of silicate, oxide, phosphate, niobates, rare-earth carbonates and others not found in more common igneous rocks. This feature, unlike other magmatic rocks, influences the variety and size of mineral deposits including the formation of ‘super-giant’ resources such as Nb (Araxa, Brazil) and rare-earths (Bayan Obo, China). (d) They can be direct partial melts or comagmatic with a variety of mantle-derived silicate magmas such as nephelinite, melilitite, kimberlite, phonolite, trachyte, basanite, alkali pyroxenite, ijolite and others from which they can form through liquidimmiscibility or through crystal-liquid differentiation. (e) Carbonatites can also be formed as low-temperature, carbo-thermal residual fluids rich in CO2, H2O and fluorine forming calcite-barite-fluorite veins which may lack the higher abundances of some trace elements. Carbonatites of India, found in some twenty four (24) localities, are associated with a variety of rocks as mentioned above and range in age from late Achaean (e.g. Hogenakal and Khambamettu, Tamil Nadu) to late Cretaceous (e.g. Amba Dongar, Gujarat). These are briefly reviewed with regard to their anomalous features.
Abstract: The NNW trending tholeiitic Sonakhan mafic dyke swarm of the Northern Bastar Craton is comprised of basalt to basaltic andesite (SiO2?=?46.3?wt% to 55.3?wt%; Mg#?=?37 to 70) dykes. A single basaltic dyke yielded a weighted-mean 207Pb/206Pb baddeleyite age of 1851.1?±?2.6?Ma. The Sr and Nd isotopes (87Sr/86Sri?=?0.70396 to 0.70855; ?Nd(t)?=??5.7 to +2.0) are variable which is a consequence of crustal contamination. Trace element modeling suggests the dykes were likely derived by partial melting of a spinel-bearing mantle source. The Sonakhan dykes are 30 million years younger than the 1.88?Ga Bastar-Cuddapah dykes (Bastanar-Hampi swarm) of the southern and central Bastar Craton indicating they represent a distinct period of magmatism. However, much like the 1.88?Ga dykes, the Sonakhan dykes appear to be correlative with dykes from the Yilgarn Craton (Yalgoo dyke?=?1854?±?5?Ma) of Western Australia. The temporal and compositional similarity of the Sonakhan dykes with the Yalgoo dyke is evidence that they are petrologically related and may represent different branches of the same dyke swarm. The existence of two distinct Paleoproterozoic dyke swarms in the Bastar Craton that each have a correlative unit in the Yilgarn Craton is supportive of a link between India and Australia before 1.9?Ga. Moreover, it suggests that the break-up of India and Western Australia was protracted and lasted for at least 30 million years.
Abstract: Magmatic pulses in intraplate sedimentary Basins are windows to understand the tectonomagmatic evolution and paleaoposition of the Basin. The present study reports the U-Pb zircon ages of mafic flows from the Cuddapah Basin and link these magmatic events with the Pangean evolution during late Carboniferous-Triassic/Phanerozoic timeframe. Zircon U-Pb geochronology for the basaltic lava flows from Vempalle Formation, Cuddapah Basin suggests two distinct Phanerozoic magmatic events coinciding with the amalgamation and dispersal stages of Pangea at 300 Ma (Late Carboniferous) and 227 Ma (Triassic). Further, these flows are characterized by analogous geochemical and geochronological signatures with Phanerozoic counterparts from Siberian, Panjal Traps, Emeishan and Tarim LIPs possibly suggesting their coeval and cogenetic nature. During the Phanerozoic Eon, the Indian subcontinent including the Cuddapah Basin was juxtaposed with the Pangean LIPs which led to the emplacement of these pulses of magmatism in the Basin coinciding with the assemblage of Pangea and its subsequent breakup between 400 Ma and 200 Ma.
International Geology Review, Vol. 58, 12, pp. 1461-1480.
India
carbonatites
Abstract: The South Indian Granulite Terrane (SGT) is a collage of Archaean to Neoproterozoic age granulite facies blocks that are sutured by an anastomosing network of large-scale shear systems. Besides several Neoproterozoic carbonatite complexes emplaced within the Archaean granulites, there are also smaller Paleoproterozoic (2.4 Ga, Hogenakkal) carbonatite intrusions within two NE-trending pyroxenite dikes. The Hogenakkal carbonatites, further discriminated into sövite and silicate sövite, have high Sr and Ba contents and extreme light rare earth element (LREE) enrichment with steep slopes typical of carbonatites. The C- and O-isotopic ratios [?13CVPDB = ?6.7 to ?5.8‰ and ?18OVSMOW = 7.5-8.7‰ except a single 18O-enriched sample (?18O = 20.0‰)] represent unmodified mantle compositions. The ?Nd values indicate two groupings for the Hogenakkal carbonatites; most samples show positive ?Nd values, close to CHUR (?Nd = ?0.35 to 2.94) and named high-?Nd group while the low-?Nd group samples show negative values (?5.69 to ?8.86), corresponding to depleted and enriched source components, respectively. The 87Sr/86Sri ratios of the two groups also can be distinguished: the high-?Nd ones have low 87Sr/86Sri ratios (0.70161-0.70244) while the low-?Nd group shows higher ratios (0.70247-0.70319). We consider the Nd-Sr ratios as primary and infer derivation from a heterogeneous mantle source. The emplacement of the Hogenakkal carbonatites may be related to Paleoproterozoic plume induced large-scale rifting and fracturing related to initiation of break-up of the Neoarchean supercontinent Kenorland.
International Journal of Trend in Scientific Research and Development, Vol. 2, 2, pp. 964-975. pdf
India
lamproite
Abstract: The pedogenic carbonates, found mainly in arid and semi-arid regions of the world, are commonly referred to as calcretes or caliche or kankar authigenic carbonate products which occur in association with soil, forming the residual regolith. Many rock types can produce calcretes upon weathering and denudation, but calcrete derived from certain rocks like kimberlite/lamproite acts as an exploration guide. Calcrete is a prominent sampling medium in diamond-rich countries like Australia and South Africa whereas it has not received popularity in the Indian context. Kimberlites being ultrapotassic in nature and owing to the enrichment of olivine and serpentine often produce calcrete duricrust as a capping. Recently more than twenty lamproites have been discovered in the Telangana state by the Geological Survey of India. These occurrences unravel a new panorama that the state has a substantial potential for occurrence of more kimberlite/lamproite clan rocks. An attempt has been made here to test the geochemical affinity of calcretes from various locations within Nalgonda district. The geochemical data of calcrete samples of this study has been compared with published geochemical data of lamproites of Ramadugu Field, to understand their geochemical association to kimberlite/lamproite. The calcretes are low in SiO2 (33.92-45.1 wt %), high in K2O (1.07-2.21 wt %) and CaO (0.78 When compared to other major elements, MgO displays low concentration. The trace elements are found to be enriched in some of the samples collected in close vicinity of known lamproite occurrences. The samples show high degree of chemic alteration and compositional variation indices. It is observed that enrichment of elements like Cr, Nb, Ba, Ti, Zr etc. indicates, similar to parent kimberlite/lamproite rock, favourable targets for further ground exploration in virgin areas present study, two samples, towards five kilometers northeast of Vattikodu Lamproite Field, possess higher concentrations of Nb (>25ppm), Ba (>400 ppm), Zr (>650 ppm) and Ti (>3500 ppm) which stand out as plausible explorable targets for further ground investigations. Further investigations on these two locations are suggested to ascertain whether or not these two targets unveil new kimberlites/lamproite occurrences in the area.
Abstract: Madawara ultramafic complex (MUC) in the southern part of Bundelkhand Craton, Central India comprises peridotite, olivine pyroxenite, pyroxenite, gabbro, and diorite. Coarse?grained olivine, clinopyroxene (Cpx), amphibole (Amp), Al?chromite, Fe?chromite, and magnetite with rare orthopyroxene (Opx) are common minerals in peridotite. Chromites are usually coarse?grained euhedral found as disseminated crystals in the olivine matrix showing both homogeneous and zoned texture. Al?chromite, primarily characterizes Cr?spinels and its subsequent fluid activity and alteration can result in the formation of Fe?chromite, chrome magnetite, and magnetite. Mineral chemistry data suggest that Al?chromite is characterized by moderately high Cr2O3 (38.16-51.52 wt.%) and Fe2O3 (3.22-14.51 wt.%) and low Al2O3 (10.63-21.87 wt.%), MgO (1.71-4.92 wt.%), and TiO2 (0.22-0.67 wt.%), whereas the homogeneous Fe?chromite type is characterized by high Fe2O3 (25.54-47.60 wt.%), moderately low Cr2O3 (19.56-37.90 wt.%), and very low Al2O3 (0.06-1.53 wt.%). Subsequent alteration of Al?chromite and Fe?chromite leads to formation of Cr?magnetite and magnetite. The Cr# of Al?chromite varies from 55.12 to 76.48 and ?Fe3+# from 8 to 19, whereas the ferrian chromite has high Cr# varying from 94.27 to 99.53 while its ?Fe3+# varies from 38 to 70. As a whole, the primary Al?chromite shows low Al2O3, TiO2 contents, and high Fe#, Cr# values. Olivines have forsterite ranging from 75.96% to 77.59%. The bulk?rock geochemistry shows continental arc geochemical affinities indicated by the high concentration of large?ion lithophile elements and U, Th relative to the low concentration of high?field strength elements. These petrological and mineralogical as well as primary Al?chromite compositions plotted in different discrimination diagrams suggest an arc environment that is similar to Alaskan?type intrusion.
Abstract: Detailed field, petrography and whole?rock geochemical study was carried out in order to constrain the mantle sources and geodynamic setting of the Madawara Ultramafic Complex (MUC) of the Bundelkhand Craton. Studies reveal that there are two types of ultramafic rocks: (a) high?Mg ultrabasic/basic rocks and (b) undeformed ultramafic-mafic plutonic rocks. The high?Mg ultrabasic/basic rocks have undergone severe low?grade (greenschist) metamorphism and are characterized by stringer and veinlet structures of talc-tremolite-actinolite schists with alternate layers of serpentinites showing comparatively higher SiO2 (46.1-49.4 wt%), lower MgO (24.6-26.2 wt%), and higher Al2O3 (4.58-7.06 wt%) and CaO (2.72-6.77 wt%) compared to the undeformed ultramafic rocks. The undeformed ultramafic rocks (mainly harzburgite, lherzolite, and olivine websterite) are characterized by globular structures and have lower SiO2 (40-44.1 wt%), higher MgO (30.4-38 wt%) and lower Al2O3 (1.84-4.03 wt%) and CaO (0.16-3.14 wt%). The undeformed mafic rocks (mainly gabbro) occur as small pockets within the undeformed ultramafic rocks as well as independent outcrops. Limited variation in Nb/Th against Nb/Yb along with negative Nb?Ti anomalies of all the rock types in the multi?element diagram reveals the significant role of the metasomatized mantle in their genesis. All the rocks show enrichment in light rare earth element and large?ion lithophile elements compared to heavy rare earth elements and high?field strength elements. The geochemical characteristics coupled with Ce/Yb versus Ce variation of the rocks of MUC point towards two different sources for their genesis. The high?Mg ultrabasic/basic rocks are derived from partial melting of metasomatized mantle at shallow depth, while the undeformed ultramafic rocks were formed as a result of asthenospheric upwelling from a greater depth that induced the melting in the overlying lithosphere. Gabbro rocks represent the last and most evolved phase of the complex. Geochemical signatures suggest that the rocks of MUC were formed in a continental arc setting.
Geochemical and Nd-Sr-Ca isotopic compositions of carbonatites and alkaline igneous rocks from the Deccan Igneous Province: role of recycled carbonates, crustal assimilation and plume heterogeneity.
Journal of Petrology , Vol. 60, 6, pp. 1119--1134.
India
carbonatite
Abstract: There are disparate views about the origin of global rift- or plume-related carbonatites. The Amba Dongar carbonatite complex, Gujarat, India, which intruded into the basalts of the Deccan Large Igneous Province (LIP), is a typical example. On the basis of new comprehensive major and trace element and Sr-Nd-Pb isotope data, we propose that low-degree primary carbonated melts from off-center of the Deccan-Réunion mantle plume migrate upwards and metasomatize part of the subcontinental lithospheric mantle (SCLM). Low-degree partial melting (?2%) of this metasomatized SCLM source generates a parental carbonated silicate magma, which becomes contaminated with the local Archean basement during its ascent. Calcite globules in a nephelinite from Amba Dongar provide evidence that the carbonatites originated by liquid immiscibility from a parental carbonated silicate magma. Liquid immiscibility at crustal depths produces two chemically distinct, but isotopically similar magmas: the carbonatites (20% by volume) and nephelinites (80% by volume). Owing to their low heat capacity, the carbonatite melts solidified as thin carbonate veins at crustal depths. Secondary melting of these carbonate-rich veins during subsequent rifting generated the carbonatites and ferrocarbonatites now exposed at Amba Dongar. Carbonatites, if formed by liquid immiscibility from carbonated silicate magmas, can inherit a wide range of isotopic signatures that result from crustal contamination of their parental carbonated silicate magmas. In rift or plume-related settings, they can, therefore, display a much larger range of isotope signatures than their original asthenosphere or mantle plume source.
Journal of Earth Science System, doi:10.1007/s12040- 019-1228-0
India
geochemistry
Abstract: The mafic dyke swarm, newer dolerite dykes (NDDs) intrudes the Archaean Singbhum granite of the Singhbhum craton, eastern India. The present investigation focuses on the petrography and geochemistry of 19 NNE-SSW to NE-SW trending NDDs in two sectors in the northern and south-western part of Bahalda town, Odisha, Singhbhum. Chondrite normalised rare earth element (REE) patterns show light REE (LREE) enrichment among majority of the 13 dykes while the remaining six dykes show a flat REE pattern. Critical analyses of some important trace element ratios like Ba/La, La/Sm, Nb/Y, Ba/Y, Sm/La, Th/La, La/Sm, Nb/Zr, Th/Zr, Hf/Sm, Ta/La and Gd/Yb indicate that the dolerite dykes originated from a heterogeneous spinel peridotite mantle source which was modified by fluids and melts in an arc/back arc setting. REE modelling of these dolerite dykes were attempted on LREE-enriched representative of NDD which shows that these dykes might have been generated by 5-25% partial melting of a modified spinel peridotite source which subsequently suffered around 30% fractional crystallisation of olivine, orthopyroxene and clinopyroxene. The reported age of ~2.75-2.8 Ma seems to be applicable for these dykes and this magmatism appears to be contemporaneous with major scale anorogenic granitic activity in the Singhbhum craton marking a major event of magmatic activity in eastern India.
Journal of the Geological Society of India, Vol. 94, 2, pp. 117-138.
India
carbonatite
Abstract: Based on the field relations, associated rock types and age, the carbonatite-alkaline rock complexes of India, that are spatially related to deep main faults, rifts and shear zones, have been classified in to two major groups, namely: 1. Middle — late Cretaceous, subvolcanic -volcanic complexes (Amba Dongar, Siriwasan, Swangkre, Mer-Mundwara, Sarnu-Dandali-Kamthai) and 2. Paleo-Neoproterozoic plutonic complexes (Newania, Sevathur, Samalpatti, Hogenakal, Kollegal, Pakkanadu, Udaiyapatti, Munnar, and Khambamettu). The middle Cretaceous Sung Valley and Samchampi complexes also belong to this plutonic group. Three minor associations, belonging to these two age groups include, the Neoproterzoic, late stage veins of carbonatites in peralkaline syenite complexes (e.g., Kunavaram, Elchuru), the diamond-bearing carbonatite and kimberlite at Khaderpet and the lamprophyre-lamproite association (e.g., Pachcham Is. Upper Cretaceous, Deccan Volcanic Province, and the Proterozoic Chitrangi Group). Petrological associations include carbonatite-nephelinite-phonolite (e.g. Amba Dongar, Sarnu-Dandali-Kamthai), dunite-peridotite-pyroxenite-ijolitemelilitite (e.g. Sung Valley), miaskitic syenite-pyroxenite ± dunite (e.g. Sevathur, Samalpatti, Pakkanadu), carbonatite alone with fenites (e.g. Newania), besides those minor associations mentioned above. Sovites (calico-carbonatites) occur as the most dominant type in some ten (10) complexes. Beforsite (magnesio-carbonatite) is the dominant type at Newania and ankeritic-sideritic types are mainly found at Amba Dongar, Siriwasan and Newania. The rare benstonite-bearing carbonatites are found at Jokkipatti and Udaiyapatti in Tamil Nadu. Mineralogically and chemically the carbonatites show considerable diversity. Fenitised zones and types of fenites (Na, K and mixed) vary widely since the carbonatites are emplaced in a variety of hostrocks ranging from granitic, mafic, ultramafic, charnockitic types besides basalts and sandstones. Stable (?13C and ?18O) and radiogenic (Sr, Nd and Pb) isotopes clearly indicate their mantle origin and also the diverse types of sources (both depleted HIMU and enriched EM 1 and 2). Petrogenetic considerations reveal three types of carbonatites, namely direct partial melts from metasomatised mantle (e.g. Newania), liquid immiscibility from carbonatite-nephelinite association (e.g. Amba Dongar) and through fractionation of ultra-alkaline ultramafic and mafic association (e.g. Sung Valley). Carbonatites of India that host significant resources include Amba Dongar (Fluorite, REE, Nb, P, Ba, Sr), Kamthai (REE), Sevathur (Nb, P, vermiculite), Beldih (P, Fe), Sung Valley (P, Nb, REE, Fe) and Samchampi (P, Nb, Fe, REE).
Journal of the Geological Society of India, Vol. 94, 2, pp. 188-196.
India
diamond genesis
Abstract: The Bundelkhand craton is surrounded by different mobile belts. The central Indian tectonic zone (CITZ) in the southern part is one of the prominent tectonic zones. CITZ is an important structural controlling factor for the Majhgawan and Hinota Kimberlite pipes. Several dyke swarms and quartz vein fractures are resulted due to volcanic and tectonic activity in the present study area. The objective of the present study is to delineate the subsurface lineaments using different edge enhancement techniques for mineral exploration in the future. Initially, First vertical derivative (FVD), total horizontal derivative (THD), tilt derivative (TDR) and theta (THETA) map have been applied to EIGEN6C4 Bouguer anomaly data. Composite lineament density map has been generated using all enhanced maps to analyze the effect of length of lineaments in the unit area. Upward continuation maps for different height have been generated to distinguish the shallower and deeper body effects. Further, Euler 3D deconvolution technique has been applied to Bouguer anomaly data to calculate the possible depth of associated lineaments. A comparative analysis of upward continuation depth and Euler’s depth has been carried out zone wise.
Abstract: We present petrology, geochemistry and radiogenic isotope (Sr and Nd) data of thirteen post-Deccan lamprophyre dykes in the Narmada rift zone from the Chhotaudepur alkaline province of the Deccan Large Igneous Province (DLIP). Mineralogically, these dykes show affinity towards alkaline (sannaite and camptonite) as well as ultramafic (damtjernite) varieties of lamprophyres. Their major oxides and certain trace element ratios increase with increasing silica content highlighting the strong influence of fractionation processes. Their Nb/U and Ce/Pb ratios are similar to the mantle array defined by MORBs and OIBs and suggests an uncontaminated nature. Major oxide (K2O, Na2O, SiO2 and TiO2) contents show geochemical similarity towards shoshonitic volcanic series, whereas elevated Zr/Hf and Nb/La coupled with suppressed Rb/Nb and Zr/b display their affinity towards HIMU-type intraplate basalts. Their radiogenic initial 87Sr/86Sr (0.706034-0.710582) and sub-chondritic initial ?Nd (?8.6 to 2.1) are akin to those of the (i) ca. 65?Ma Ambadongar carbonatite, NW India, and (ii) ca. 65?Ma orangeites from Bastar Craton, central India, highlighting an enriched lithospheric mantle source. REE inversion modeling suggests ~3% enrichment of an undepleted mantle followed by small degrees of melting of this enriched mantle source are sufficient- as in the case of ocean island basalts (OIB)- to reproduce their observed REE concentrations. Their TDM Nd model ages (564-961?Ma) are consistent with widespread convergent margin-related magmatism during the amalgamation of the Rodinia supercontinent. We propose that enriched lithospheric mantle developed during the Neoproterozoic was metasomatized by small-volume CO2-rich melts imparting a HIMU-type geochemical character during Late Cretaceous, when the mantle plume (viz., Réunion) responsible for the flood basalt eruption, impinged at the base of the NW Indian lithosphere. From the presence of F-rich apatite and high K/Rb in mica, we infer the (i) presence of F-phlogopite in their source regions, and (ii) that the depth of post-Deccan lithosphere-asthenosphere boundary (LAB) beneath NW India was at least ~100?km at ca. 65?Ma.
International Journal of Mining and Geo-Engineering, Vol. 53, 2, pp. 1-11. pdf
India, Andhra Pradesh
deposit - Wajrakarur
Abstract: A plausible case of collective and economical mining of diamondiferous kimberlite deposits of Wajrakarur and adjoining places in Andhra Pradesh, southern India, along with the whole-rock geochemical evidences in support of their diamond potentiality are discussed in this article. The kimberlites/lamproites are mantle-derived ultrabasic rocks which rarely carry diamonds from mantle to the earth's surface through carrot-shaped intrusions referred to as pipes. Even though few hundreds of diamondiferous kimberlite pipes were discovered in India so far, there is no other production unit than Panna diamond mine in the country where primary rock is mined. In ancient India, diamond mining in south India in the Krishna river valley was well-known to the world fascinated by famous gemstones like Koh-i-Noor, Hope, Darya-e-Noor, Noor-ul-ain etc. which were mainly extracted from alluvium or colluvium in Krishna river valley. Having bestowed with more than 45 kimberlite pipes, the Wajrakarur kimberlite field (WKF) forms a favourable region for initiating diamond mining in the country. Geochemically, majority of the WKF show low TiO2 content and considerably high diamond grade (DG) values (>3) except some pipes viz., P-5 (Muligiripalli), P-13 (Tummatapalli) and P-16 (Pennahobilam) are barren due to high TiO2 and ilmenite contents. The TiO2 content (0.66-6.62 wt %) is inversely proportional to the DG (3.33 to 22.13). The DG value of some of the WKF pipes is close to that of Panna (8.36). The cationic weight% values clearly portray the diamondiferous nature of these deposits. The WKF pipes were also proved to be diamondiferous by exploratory drilling and bulk sample processing results by the government and multinational organisations. In southern India, due to several reasons, diamond mining has not seen its initiation and impetus till now although it records a considerable number of fertile kimberlite pipes at Wajrakarur, Lattavaram, Chigicherla, Timmasamudram etc. Though the majority of WKF diamondiferous kimberlite deposits in Wajrakarur are small in their areal extent (0.06-4.48 Ha) some of them are large (>10 Ha up to 120 ha). They occur in close proximity to each other offering feasibility for collective mining and winning the precious stone through a central processing unit by deploying the latest processing technologies. The geographic conditions of this region such as availability of human resources, water resources, vast open lands, wind power generation etc. also support to initiate mining of kimberlite pipes in this area. The availability of rough diamonds produced from local mines will make the polishing industry to meet its business needs during circumstances of the shortage of rough stone influx from foreign. Hence, although it demands liberal investments, reviving diamond mining in southern India can be materialised with a meticulous evaluation of these deposits ascertaining profitability. This will certainly help to restore the past glory of diamond mining in the southern part of the subcontinent.
Abstract: Petrogenesis of a newly discovered diamondiferous kimberlite pipe (3-021) by the Rio Tinto Group, ~2 km east of Dibbasanipalli, Wajrakarur Kimberlite Field, eastern Dharwar craton is attempted. The pipe is located in field based on published literature and consultation with local villagers [1,2]. Local geology is dominated by Archaean basement granitoids and genisses intruded by younger nordmarkites and dolerites. The rock is highly chloritised giving rise to poor preservation of kimberlitic matrix. However, olivine pseudomorphs are distinctly visible in thin sections. The rock possesses crustal xenoliths of granite, syenite, dolerite etc. with petrographic similarities to Khaderet pipe (3-106). Geochemically, the kimberlite is silica undersaturated (SiO2 39.32-45.67 wt%), MgO rich (26.51- 28.82 wt%) with K2O (1-88-2.1 wt%) higher than Na2O (0.29-0.39 wt%), akin to archetypal Group-I variety. High amounts of MgO and Mg# correspond to enrichment of magnesium bearing mineral phases like olivine and to some extent Mg-ilmenite. The higher concentration of elements Ba, Cr, Co, Nb, Ni, V, Zr is attributed to enrichment of mantle originated xenocrysts like chromite, perovskite, pyrope, Crdiopside. Based on trace element content, the kimberlite appears to be of magmatic origin within a stable continental geological set up. The enrichment of LREE over HREE supports inferior degree of partial melting (0.1-2%) indicating a metasomatically enriched phlogopite bearing garnet lherzolite source, inturn indicating a deep seated and depleted mantle origin, within an estimated temperature range of 1150- 1280oC and low viscosity (0.05 Pa s).
Abstract: Kyanite-eclogite xenoliths from Wajrakarur are considered as remnants of subducted ocean-floor crust [1]. Here trace element concentration and isotopic data are presented in garnet (Grt) and kyanite (Ky) from xenoliths KL-2 E1-E4, characterized by [2]). We use the precise 87Sr/86Sr host kimberlite groundmass perovskite ratio (0.70312-0.70333, [3]) as a proxy for the extent of kimberlitic magma infiltration at 1.1 Ga. The xenolithic Grt and Cr-rich (upto 1506 ppm) Ky have more radiogenic 87Sr/86Sr values than kimberlite, at 1.1 Ga, of 0.703829-0.705203 and 0.703811-0.704502, respectively. Furthermore, the Grt and Ky 143Nd/144Nd ratios, at 1.1 Ga, are 0.509321-0.511372 and 0.510951-0.511156, respectively, and are distinctly lower than those of the host kimberlite (0.511870-0.512290, [4]). This indicates that the infiltration of kimberlitic fluid has not altered the 87Sr/86Sr and 143Nd/144Nd ratios in the Grt and Ky, and therefore their isotope compositions must be inherited and predate the kimberlite magma generation event at 1.1 Ga. Trace elements in Grt and Ky indicate extreme metasomatism (Sr in Grt 104-296 ppm, in Ky 672-8713 ppm [limit Sr<2ppm] and Nb in Grt 0.64-1.78 ppm, in Ky 1.7-4.54 ppm [limit Nb<0.5ppm]). The xenoliths underwent at least one major melting event inferred from extreme depletions in Re, Os and 177Os/178Os ratios [5]. Their mantle-like ?18O values (Grt 5.3-5.4‰, Ky 5.3-5.9‰), positive Eu anomalies in both Grt and Ky (similar to Group 1 HREE-depleted garnets of [1]) suggests that the protolith likely was a chromite-bearing leucogabbro, emplaced as a high-pressure cumulate at the crust-mantle boundary, which was later eclogitized due to deep-seated subduction and underwent episodes of extreme melting and metasomatism before 1.1 Ga and at least before 1.7 Ga, as inferred from their youngest Re depletion dates [5].
Earth and Planetary Science letters, Vol. 520, pp. 175-187.
India
carbonatites
Abstract: Carbonatite metasomatism has been widely implicated for worldwide mafic mantle suites but so far, no combined data have been available for highly siderophile element systematics (HSE - Os, Ir, Ru, Pt, Pd, Re) and Re-Os isotopic compositions in carbonatites themselves. We present the first systematic survey of the HSE and Re-Os isotopic compositions in a suite of well-characterized Neoproterozoic carbonatites, silicocarbonatites and associated silicate rocks (pyroxenites, monzogabbros, syenites) from south India in order to place constraints on the HSE systematics in carbonatite magmas, anchoring possible mantle sources of carbonatites and relationship to the ambient crustal lithologies as well as preliminary constraints on carbonatite metasomatism in Earth's mantle. The most plausible explanation for generally low HSE contents in calciocarbonatites from Tamil Nadu (?HSE < 1.22 ppb) involves a low-degree (<1%) partial melting of the mantle source producing sulfur-saturated carbonatitic magmas leaving behind sulfide phases retaining HSE. The new data also indicate a strong FeO control on the distribution of Os and Pt during segregation of carbonatite melt from its enriched mantle source and/or melt differentiation. The combined 187Re/188Os values (from 0.10 to 217), 187Os/188Os ratios (0.186-10.4) and initial ?Os values back-calculated to 800 Ma (from +0.1 to +6052) predict that most Tamil Nadu calciocarbonatites were plausibly derived from a carbonated peridotite source with <10% recycled component. This model would thus provide significant constraints on the origin/source of carbonatites, irrespective of their post-emplacement history. The unusual, volumetrically rare, Mg-Cr-rich silicocarbonatites (?HSE = 14-41 ppb) display almost identical HSE patterns with those of host pyroxenites and predominantly high Pt (up to 38 ppb), the origin of which remains unknown. Positive co-variations between Pt, Pd and Re, and the well-developed positive correlation between Pt and MgO in these Mg-Cr-rich silicocarbonatites argue for a source coming predominantly from the upper mantle. The Re-Os isotopic systematics agree with direct incorporation of enriched mantle-derived material into parental melts but variable incorporation of potassium-rich crustal materials is evidenced by highly positive ?Os800 Ma values for a sub-suite of Mg-Cr-rich silicocarbonatites, indicating intense fenitization. The highly radiogenic Os isotopic compositions of monzogabbros and a syenite argue for their derivation from crustal lithologies with no or only negligible contribution of mantle material. Collectively, low Ir, Ru, Pt and Pd contents found in the Tamil Nadu carbonatites appear to indicate the incapability to significantly modify the total budget of these elements in the Earth's mantle during carbonatite metasomatism. In contrast, very high Re/Os ratios found in some of the analyzed carbonatites, paralleled by extremely radiogenic 187Os/188Os signature, can produce large modification of the Re-Os isotopic composition of mantle peridotites during carbonatite melt percolation when high melt/rock ratios are achieved.
Abstract: Kimberlite field is an example of widespread Mesoproterozoic intracontinental magmatism. Recent studies have identified deep subcontinental lithospheric mantle as a source region of the kimberlite magmatism while timing, origin, and processes responsible for the generation of coeval lamprophyres remain poorly constrained. Here, we present and discuss new petrological and geochemical data for two lamprophyre dykes from the Wajrakarur kimberlite field and assess their petrogenetic relation to the kimberlite occurrences. Based on mineral compositional and whole?rock geochemical characters, it is suggested that lamprophyres are formed through low degrees of partial melting of “enriched” lithospheric mantle that was modified and metasomatized by melts derived from recycled crust. This differs from geochemical imprints found in coeval kimberlites, where a crustal source component appears to be absent and is more consistent with rock derivation from “depleted” lithosphere which has experienced interaction with asthenosphere?derived melts. An apparent lack of garnet in the mantle sources of lamprophyres is suggestive of melting at comparatively shallow depth (~100 km) relative to the kimberlites. Hence, these geochemically contrasting rocks, although have formed at the same time, are derived from vertically heterogeneous lithospheric mantle sources and can be explained through and linked with a thermal anomaly in the underlying convective asthenosphere. We suggest that the deeper mantle source region of the kimberlites was more pristine and devoid of subduction?related signatures, whereas the shallower mantle source region of the lamprophyres seems to have preserved imprints of plate convergence and subduction associated with the evolution of the Dharwar Craton.
Abstract: The spatial and temporal association between lamprophyres and kimberlites provides unique opportunities to explore their genetic relationships. This paper explores such a relationship by detailing mineralogical and geochemical aspects of Korakkodu lamprophyre dykes located within the well?known Mesoproterozoic diamondiferous Wajrakarur Kimberlite field (WKF), towards the south?western margin of Paleo-Mesoproterozoic Cuddapah Basin, Eastern Dharwar Craton, southern India. Mineralogy reveals that these dykes belong to calc?alkaline variety of lamprophyres, but their geochemistry display mixed signals of both alkaline and calc?alkaline lamprophyres. These lamprophyres are highly potassic, and their high Al2O3 and low?TiO2 content implies a shoshonitic character. Low Mg#, Ni, and Cr concentration highlight their evolved nature. High (La/Yb)N and (Gd/Yb)N content is consistent with their derivation from low degrees of partial melting, whereas highly fractionated nature suggests the presence of garnet in their source. Absence of prominent Nb?Ta anomaly implies to the dilution of lithospheric mantle source by melts rich in HFSEs and low La/Nb ratio compared to those of the calc?alkaline island arc volcanics and suggests an asthenospheric overprint on lithospheric mantle source. Carbonatite metasomatism in the source region of these lamprophyres is apparent from conspicuously high?Zr/Hf ratio, and the HFSE budget of these lamprophyres are principally controlled by the presence of phlogopite veins in their lithospheric source. An extremely heterogeneous and layered lithospheric mantle beneath Eastern Dharwar Craton has been inferred from the divergent genetic history of Mesoproterozoic lamprophyres and kimberlites in the Wajrakarur field.
Magna, T., Viladkar, S., Rappirich, V., Pour, O., Cejkova, B.
Nb-V enriched sovites of the northeastern and eastern part of the Amba Dongar carbonatite ring dike, India - a reflection of post emplacement hydrothermal overprint.
Abstract: The Bastar craton in central India, surrounded by cratonic blocks and Paleoproterozoic to Neoproterozoic orogenic belts, is a window to investigate the Archean-Paleoproterozoic crustal evolution and tectonic processes. Here we propose a new tectonic classification of the craton into the Western Bastar Craton (WBC), Eastern Bastar Craton (EBC), and the intervening Central Bastar Orogen (CBO). We present petrologic, geochemical and zircon U-Pb, REE and Lu-Hf data from a suite of rocks from the CBO and along the eastern margin of the WBC Including: (1) volcanic successions comprising meta-andesite and fine-grained amphibolite, representing arc-related volcanics along a convergent margin; (2) ferruginous sandstone, in association with rhyolite, representing a volcano-sedimentary succession, deposited in an active trench; and (3) metamorphosed mafic-ultramafic suite including gabbro, pyroxenite and dunite invaded by trondhjemite representing the section of sub-arc mantle and arc root adjacent to a long-lasting subduction system. Petrologic studies indicate that the mafic-ultramafic suite crystallized from an island arc tholeiitic parental magma in a suprasubduction zone environment. The chondrite-normalized and primitive mantle normalized diagrams of the mafic and ultramafic rocks suggest derivation from MORB magma. The mixed characters from N-MORB to E-MORB of the studied samples are consistent with subduction modification of a MORB related magma, involving partial melting of the metasomatized mantle wedge. Our zircon U-Pb age data suggest that the cratonic nuclei was constructed as early as Paleoarchean. We present evidence for active subduction and arc magmatism through Mesoarchean to Neoarchean and early Paleoproterozoic, with the trench remaining open until at least 2.3 Ga. Two major crust building events are recognized in the Bastar craton: during Mesoarchean (recycled Paleoarchean subduction-related as well as juvenile/depleted mantle components) and Neoarchean (accretion of juvenile oceanic crust, arc magmatism including granite batholiths and related porphyry mineralization). The final cratonization occurred during latest Paleoproterozoic, followed by collisional assembly of the craton and its incorporation within the Peninsular Indian mosaic during Mesoproterozoic. In the global supercontinent context, the craton preserves the history of Ur, the earliest supercontinent, followed by the Paleo-Mesoproterozoic Columbia, as well as minor thermal imprints of the Neoproterozoic Rodinia and associated Grenvillian orogeny.
Neues Jahbuch fur Mineralogy, Vol. 196, p2, pp. 149-177.
India
camptonite
Abstract: We report petrology and geochemistry (including Sr and Nd isotopes) of a fresh lamprophyre at Ankiraopalli area at the north-western margin of Paleo-Mesoproterozoic Cuddapah basin, eastern Dharwar craton, southern India. Ankiraopalli samples possess a typical lamprophyre porphyritic-panidiomorphic texture with phenocrysts of kaersutite and diopside set in a plagioclase dominant groundmass. Combined mineralogy and geochemistry classify it as alkaline lampro- phyre in general and camptonite in particular. Contrary to the calc-alkaline and/or shoshonitic orogenic nature portrayed by lamprophyres occurring towards the western margin of the Cuddapah basin, the Ankiraopalli samples display trace element composition revealing striking similarity with those of ocean island basalts, Italian alkaline lamprophyres and highlights an anorogenic character. However, the87 Sr/86 Srinitial (0.710316 to 0.720016) and ?Ndinitial (- 9.54 to - 9.61) of the Ankiraopalli lamprophyre show derivation from an 'enriched' mantle source showing long term enrichment of incompatible trace elements and contrast from those of (i) OIB, and (ii) nearby Mahbubnagar alkaline mafic dykes of OIB affinity. Combining results of this study and recent advances made, multiple mantle domains are identified in the Eastern Dharwar craton which generated distinct Mesoproterozoic lamprophyre varieties. These include (i) Domain I, involving sub-continental lithospheric mantle source essentially metasomatized by subduction-derived melts/fluids (represented by orogenic calcalkaline and/or shoshonitic lamprophyres at the Mudigubba, the Udiripikonda and the Kadiri); (ii) Domain II, comprising a mixed sub-continental lithospheric and asthenospheric source (represented by orogenic-anorogenic, alkaline to calc-alkaline transitional lamprophyres at the Korakkodu), and (iii) Domain III, representing a sub-continental lithospheric source with a dominant overprint of an asthenospheric (plume) component (represented by essentially alkaline lamprophyres at the Ankiraopalli). Our study highlights the varied mantle source heterogeneities and complexity of geodynamic processes involved in the Neoarchean-Paleo/Mesoproterozoic evolution of the Eastern Dharwar craton.
Abstract: Based on trend, cross-cutting relationships and U-Pb dating, Precambrian mafic dykes in the Singhbhum craton, earlier collectively identified as ‘Newer Dolerite Swarm’ have been separated into seven distinct swarms, which are thought to be the plumbing systems for Large Igneous Provinces (LIPs). These Singhbhum swarms range in age from ?2.80 Ga to ?1.76 Ga, and include the ?2.80 Ga NE-SW trending Keshargaria swarm, ?2.75-2.76 Ga NNE-SSW to NE-SW trending Ghatgaon swarm, the ?2.26 Ga NE-SW to ENE-WSW trending Kaptipada swarm (based on a new U-Pb ID-TIMS age 2256 ± 6 Ma), the ?1.77 Ga WNW-ESE trending Pipilia swarm, the early-Paleoproterozoic E-W to ENE-WSW trending Keonjhar swarm, the middle-Paleoproterozoic NW-SE to NNW-SSE trending Bhagamunda swarm, and the late-Paleoproterozoic N-S to NNE-SSW trending Barigaon swarm. Two of the Singhbhum swarms, the ?2.26 Ga Kaptipada and ?1.77 Ga Pipilia, are closely matched with the ?2.26-2.25 Ga Ippaguda-Dhiburahalli and ?1.79 Ga Pebbair swarms, respectively, of the eastern Dharwar craton. The correlations suggest that the Singhbhum and Dharwar cratons were close enough at these times to share two reconstructed LIPs, a 2.26-2.25 Ga Kaptipada- Ippaguda-Dhiburahalli LIP and a 1.79-1.77 Ga Pipilia-Pebbair LIP, and if so, both swarms must be present in the intervening Bastar craton (candidates are proposed). Also, the 2.76-2.75 Ga Ghatgaon swarm of the Singhbhum craton can be provisionally correlated with ?2.7 Ga Keshkal swarm of the Bastar craton. The 2.26-2.25 Ga Kaptipada-Ippaguda-Dhiburahalli LIP of the Singhbhum-Bastar-Dharwar reconstruction has age matches in the Vestfold Hills of Antarctica (?2.24 Ga dykes), the Kaapvaal craton (the ?2.25-2.23 Ga Hekpoort lavas) and perhaps the Zimbabwe craton (2.26 Ga Chimbadzi troctolite intrusions). The 1.76-1.79 Ga Pipilia-Pebbair LIP of the Singhbhum-Bastar-Dharwar reconstruction has age matches in the North China, Australian Shield, Amazonian, Rio de Plata and Sarmatia cratons. The relevance of these matches for reconstructions will require future testing using paleomagnetic studies. While there are ?2.7-2.8 Ga LIP-type greenstone belts in many crustal blocks, there are no precise matches with the 2.76-2.75 Ga Ghatgaon swarm of the Singhbhum craton. Howe
South African Journal of Geology, Vol. 122, 4, pp. 397-420.
India
Gondwana
Abstract: On the last page of his 1937 book "Our Wandering Continents" Alex Du Toit advised the geological community to develop the field of "comparative geology", which he defined as "the study of continental fragments". This is precisely the theme of this paper, which outlines my research activities for the past 28 years, on the continental fragments of the Indian Ocean. In the early 1990s, my colleagues and I were working in Madagascar, and we recognized the need to appreciate the excellent geological mapping (pioneered in the 1950s by Henri Besairie) in a more modern geodynamic context, by applying new ideas and analytical techniques, to a large and understudied piece of continental crust. One result of this work was the identification of a 700 to 800 Ma belt of plutons and volcanic equivalents, about 450 km long, which we suggested might represent an Andean-type arc, produced by Neoproterozoic subduction. We wondered if similar examples of this magmatic belt might be present elsewhere, and we began working in the Seychelles, where late Precambrian granites are exposed on about 40 of the >100 islands in the archipelago. Based on our new petrological, geochemical and geochronological measurements, we built a case that these ~750 Ma rocks also represent an Andean-type arc, coeval with and equivalent to the one present in Madagascar. By using similar types of approaches, we tracked this arc even further, into the Malani Igneous Province of Rajasthan, in northwest India. Our paleomagnetic data place these three entities adjacent to each other at ~750 Ma, and were positioned at the margins, rather than in the central parts of the Rodinia supercontinent, further supporting their formation in a subduction-related continental arc. A widespread view is that in the Neoproterozoic, Rodinia began to break apart, and the more familiar Gondwana supercontinent was assembled by Pan-African (~500 to 600 Ma) continental collisions, marked by the highly deformed and metamorphosed rocks of the East African Orogen. It was my mentor, Kevin Burke, who suggested that the present-day locations of Alkaline Rocks and Carbonatites (called "ARCs") and their Deformed equivalents (called "DARCs"), might mark the outlines of two well-defined parts of the Wilson cycle. We can be confident that ARCs formed originally in intracontinental rift settings, and we postulated that DARCs represent suture zones, where vanished oceans have closed. We also found that the isotopic record of these events can be preserved in DARC minerals. In a nepheline syenite gneiss from Malawi, the U-Pb age of zircons is 730 Ma (marking the rifting of Rodinia), and that of monazites is 522 Ma (marking the collisional construction of Gondwana). A general outline of how and when Gondwana broke apart into the current configuration of continental entities, starting at about 165 Ma, has been known for some time, because this record is preserved in the magnetic properties of ocean-floor basalts, which can be precisely dated. A current topic of active research is the role that deep mantle plumes may have played in initiating, or assisting, continental fragmentation. I am part of a group of colleagues and students who are applying complementary datasets to understand how the Karoo (182 Ma), Etendeka (132 Ma), Marion (90 Ma) and Réunion (65 Ma) plumes influenced the break-up of Gondwana and the development of the Indian Ocean. Shortly after the impingement of the Karoo plume at 182 Ma, Gondwana fragmentation began as Madagascar + India + Antarctica separated from Africa, and drifted southward. Only after 90 Ma, when Madagascar was blanketed by lavas of the Marion plume, did India begin to rift, and rapidly drifted northward, assisted by the Marion and Deccan (65 Ma) plumes, eventually colliding with Asia to produce the Himalayas. It is interesting that a record of these plate kinematics is preserved in the large Permian - Eocene sedimentary basins of western Madagascar: transtensional pull-apart structures are dextral in Jurassic rocks (recording initial southward drift with respect to Africa), but change to sinistral in the Eocene, recording India’s northward drift. Our latest work has begun to reveal that small continental fragments are present in unexpected places. In the young (max. 9 Ma) plume-related, volcanic island of Mauritius, we found Precambrian zircons with ages between 660 and 3000 Ma, in beach sands and trachytic lavas. This can only mean that a fragment of ancient continent must exist beneath the young volcanoes there, and that the old zircons were picked up by ascending magmas on their way to surface eruption sites. We speculate, based on gravity inversion modelling, that continental fragments may also be present beneath the Nazareth, Saya de Malha and Chagos Banks, as well as the Maldives and Laccadives. These were once joined together in a microcontinent we called “Mauritia”, and became scattered across the Indian Ocean during Gondwana break-up, probably by mid-ocean ridge “jumps”. This work, widely reported in international news media, allows a more refined reconstruction of Gondwana, suggests that continental break-up is far more complex than previously perceived, and has important implications for regional geological correlations and exploration models. Our results, as interesting as they may be, are merely follow-ups that build upon the prescient and pioneering ideas of Alex Du Toit, whose legacy I appreciatively acknowledge.
Abstract: The Taj Mahal evokes an image of a monumental building and reflecting pool—its classic view. But the Taj Mahal complex is much more than that. It is actually a series of beautiful buildings and gardens in Agra, India, built in the seventeenth century in loving memory of Mumtaz Mahal. This name, given by the Mughal emperor Shah Jahan to one of his brides, means “Chosen One of the Palace.” Famed for its architectural magnificence, the landmark holds additional significance for the gemologist. Upon closer investigation, one is impressed with the intricacies of the inlay of numerous gems to create thousands of designs throughout the buildings on the grounds. This article sheds light on the gems used in decorating the Taj Mahal and in the extraordinary jewelry collected by Shah Jahan and other Mughals. These gems often took intricate trade routes to Agra, which are also discussed, along with the craft used to create the inlays and the efforts undertaken to preserve this Wonder of the World.
Abstract: The Bastar craton in central India, surrounded by cratonic blocks and Paleoproterozoic to Neoproterozoic orogenic belts, is a window to investigate the Archean-Paleoproterozoic crustal evolution and tectonic processes. Here we propose a new tectonic classification of the craton into the Western Bastar Craton (WBC), Eastern Bastar Craton (EBC), and the intervening Central Bastar Orogen (CBO). We present petrologic, geochemical and zircon U-Pb, REE and Lu-Hf data from a suite of rocks from the CBO and along the eastern margin of the WBC Including: (1) volcanic successions comprising meta-andesite and fine-grained amphibolite, representing arc-related volcanics along a convergent margin; (2) ferruginous sandstone, in association with rhyolite, representing a volcano-sedimentary succession, deposited in an active trench; and (3) metamorphosed mafic-ultramafic suite including gabbro, pyroxenite and dunite invaded by trondhjemite representing the section of sub-arc mantle and arc root adjacent to a long-lasting subduction system. Petrologic studies indicate that the mafic-ultramafic suite crystallized from an island arc tholeiitic parental magma in a suprasubduction zone environment. The chondrite-normalized and primitive mantle normalized diagrams of the mafic and ultramafic rocks suggest derivation from MORB magma. The mixed characters from N-MORB to E-MORB of the studied samples are consistent with subduction modification of a MORB related magma, involving partial melting of the metasomatized mantle wedge. Our zircon U-Pb age data suggest that the cratonic nuclei was constructed as early as Paleoarchean. We present evidence for active subduction and arc magmatism through Mesoarchean to Neoarchean and early Paleoproterozoic, with the trench remaining open until at least 2.3?Ga. Two major crust building events are recognized in the Bastar craton: during Mesoarchean (recycled Paleoarchean subduction-related as well as juvenile/depleted mantle components) and Neoarchean (accretion of juvenile oceanic crust, arc magmatism including granite batholiths and related porphyry mineralization). The final cratonization occurred during latest Paleoproterozoic, followed by collisional assembly of the craton and its incorporation within the Peninsular Indian mosaic during Mesoproterozoic. In the global supercontinent context, the craton preserves the history of Ur, the earliest supercontinent, followed by the Paleo-Mesoproterozoic Columbia, as well as minor thermal imprints of the Neoproterozoic Rodinia and associated Grenvillian orogeny.
Journal of Earth System Science, Vol. 128, 17p. Pdf
India
boninite
Abstract: The Dongargarh Supergroup along with the basal Amgaon Gneissic Complex constitutes the northwestern part of the central Indian Bastar craton. In the present study, we report a new finding of a boninite dyke intruded in the Amgaon gneisses of this area. The dyke composed of mainly pyroxenes, amphiboles and subordinate amount of plagioclase. The higher contents of SiO2 (51-54 wt.%), MgO (12-14 wt.%), Ni (375-473 ppm), Cr (1416-1580 ppm) and very low TiO2 (0.2-0.4 wt.%) are consistent with the boninite nature of the dyke as well as the unevolved primary nature of the source magma. The extraordinarily high CaO content (15.97-17.7 wt.%) with higher CaO/Al2O3 (3.13-3.96) ratios classifies it as high-Ca boninite. The trace element ratios including Zr/Ti, Ti/V, Ti/Sc and Ti/Yb further show its geochemical similarity with the Archaean boninite. The dyke also shows negative high-field strength element (Nb, Ta and Ti) anomalies which are the characteristics of the boninite rocks reported elsewhere and along with the enriched light rare earth element pattern, it shows more affinity particularly with the northern Bastar boninite dyke. The mineralogical and geochemical similarities of the boninite dykes from the Bastar craton indicate a widespread boninitic event during the Palaeoproterozoic having a similar origin. These boninite dykes indicate the preservation of subduction-related signatures in the lithospheric mantle beneath the Bastar craton at the time of its evolution or may be during the convergence of the Bastar and Bundelkhand cratons.
Abstract: India has a rich tradition of mineral exploration. Innumerable old workings, mine dumps slag heaps, etc. are the tell tale signs of this glorious tradition. The flourishing diamond trade in the Deccan peninsula, mainly in the Golconda kingdom, had attracted world’s attention during historical time. Copper and gold were also used locally since the days of Indus Valley civilizations. East India Company started exploration for coal in the Eighteenth century with setting up of , the premier Earth science organisation and the second oldest survey of the country, in 1851 for the systematic geological survey and prospecting for coal. India was a notable producer of gold in the early part of twentieth century and major exporter of mica, sillimanite, kyanite, magnetite and chromite. Metallurgical industry started with the setting up of steel plants at Burnpur, Jamshedpur and copper smelter at Ghatsila. Second World War created great demand for various minerals and metals including those of strategic importance e.g., tungsten. Industrial policy, formulated after Independence, brought about a radical change in the mining and metallurgical industry. During the post-Independence period, GSI has embarked upon the exploration for minerals, particularly in favourable geological milieu spread over Dharwar, Bastar, Singhbhum and Aravalli cratons. The investigations carried out since 1960s provide us firsthand information of different mineral occurrences as well as their potential. Keeping in tune with the modern trends of mineral exploration, the GSI oriented its programmes through multidisciplinary surveys. From time to time it equipped itself with state-of-the-art laboratories to back up its various exploration programmes. The efforts have led to discovery of several mineral deposits in virgin areas in different parts of the country. A few other central and state government organisations were also involved in mineral exploration now and then, mostly in collaboration with foreign organisations. The liberalisation of India’s National Mineral Policy in 1993 paved the way for the entry of private entrepreneurs, including those from overseas for carrying out mineral exploration. The database developed by GSI has been found very useful for taking investment decisions by the Multi-National Companies.
Mineralogy and Petrology, in press available, 20p. Pdf
India
craton
Abstract: We report mineral compositions and bulk rock geochemistry of mafic dykes intruded in the western part of Bastar craton, comprising of Archaean Amgaon Group and Proterozoic Dongargarh Supergroup of rocks. Field relations show two distinct trends of these dykes which are almost perpendicular to each other but having similar mineralogical and geochemical characteristics. Dykes are mostly composed of pyroxenes, plagioclase and subordinate amount of amphiboles and Fe-Ti oxides (magnetite and ilmenite). These hypersthene normative basaltic dykes show tholeiitic trend and are characterised by narrow compositional variations of MgO (6.067.08 wt%), FeOt (15.0617.78 wt%), TiO2 (1.182.24 wt%), Al2O3 (11.9615.54 wt%) and low Mg# [atomic Mg/(Mg?+?Fe2+)?×?100] values in the range of 3748. Low loss on ignition (LOI) values <2 wt% and significant trends of trace elements (Nb, La, Th, Sr) with Zr indicate insignificant effects of post magmatic processes in these dykes. Smooth correlations between major oxides and MgO, among trace element ratios (Ce/La, Th/Yb, Nb/Yb) and negative Nb-Ta anomalies without positive Zr and Hf anomalies negate the crustal contamination effects. The correlations of compatible (e.g. Cr, Ni) and incompatible (e.g. Ba, Rb) elements show involvement of both fractional crystallisation and partial melting processes in their formation. Flat heavy rare earth element (HREE) pattern with low (Tb/Yb)n values reveal their genesis from a mantle source without involvement of garnet and geochemical models suggested in the present study indicate melting from spinel lherzolite mantle source. Strong geochemical similarities of present dykes with those of earlier reported Lakhna (1.46 Ga) and Bandimal (1.42 Ga) dykes of northern Bastar craton suggest a widespread mafic magmatic event across the Bastar craton during 1.421.46 Ga. Present dykes therefore represent a subduction related outgrowth of Columbia supercontinent due to the accretion of continental margins.
International Journal of Mining and Geo-Engineering, 11p. Pdf
India
deposit - Wajrakarur
Abstract: A plausible case of collective and economical mining of diamondiferous kimberlite deposits of Wajrakarur and adjoining places in Andhra Pradesh, southern India along with the whole-rock geochemical evidences in support of their diamond potentiality are discussed in this article. The kimberlites/lamproites are mantle-derived ultrabasic rocks which rarely carry diamonds from mantle to the earth’s surface through carrot-shaped intrusions referred to as pipes. Even though few hundreds of diamondiferous kimberlite pipes were discovered in India so far, there is no other production unit than Panna diamond mine in the country where primary rock is mined. In ancient India, diamond mining in south India in the Krishna river valley was well-known to the world fascinated by famous gemstones like Koh-I-Noor, Hope, Darya-e-Noor, Noor-ul-ain etc. which were mainly extracted from alluvium or colluvium in Krishna river valley. Having bestowed with more than 45 kimberlite pipes, the Wajrakarur kimberlite field (WKF) forms a favourable region for initiating diamond mining in the country. Geochemically, majority of the WKF show low TiO2 content and considerably high diamond grade (DG) values (>3) except some pipes viz., P-5 (Muligiripalli), P-13 (Tummatapalli) and P-16 (Pennahobilam) are barren due to high TiO2 and ilmenite contents. The TiO2 content (0.66-6.62 wt%) is inversely proportional to the DG (3.33 to 22.13). The DG value of some of the WKF pipes is close to that of Panna (8.36). The cationic weight% values clearly portray the diamondiferous nature of these deposits. The WKF pipes were also proved to be diamondiferous by exploratory drilling and bulk sample processing results by the government organisations. In southern India, due to several reasons, diamond mining has not seen its initiation and impetus till now although it records a considerable number of fertile kimberlite pipes at Wajrakarur, Lattavaram, Chigicherla, Timmasamudram etc. Though the majority of WKF diamondiferous kimberlite deposits in Wajrakarur are small in their areal extent (0.06-4.48 Ha) some of them are large (>10 Ha up to 120 ha). They occur in close proximity to each other offering feasibility for collective mining and winning the precious stone through a central processing unit by deploying the latest processing technologies. The geographic conditions of this region such as availability of human resources, water resources, vast open lands, wind power generation etc. also support to initiate mining of kimberlite pipes in this area. The availability of rough diamonds produced from local mines will make the polishing industry to meet its business needs during circumstances of the shortage of rough stone influx from foreign. Hence, although it demands liberal investments, reviving diamond mining in southern India can be materialised with a meticulous evaluation of these deposits ascertaining profitability. This will certainly help to restore the past glory of diamond mining in the southern part of the subcontinent.
Abstract: The Bastar craton in central India, surrounded by cratonic blocks and Paleoproterozoic to Neoproterozoic orogenic belts, is a window to investigate the Archean-Paleoproterozoic crustal evolution and tectonic processes. Here we propose a new tectonic classification of the craton into the Western Bastar Craton (WBC), Eastern Bastar Craton (EBC), and the intervening Central Bastar Orogen (CBO). We present petrologic, geochemical and zircon U-Pb, REE and Lu-Hf data from a suite of rocks from the CBO and along the eastern margin of the WBC Including: (1) volcanic successions comprising meta-andesite and fine-grained amphibolite, representing arc-related volcanics along a convergent margin; (2) ferruginous sandstone, in association with rhyolite, representing a volcano-sedimentary succession, deposited in an active trench; and (3) metamorphosed mafic-ultramafic suite including gabbro, pyroxenite and dunite invaded by trondhjemite representing the section of sub-arc mantle and arc root adjacent to a long-lasting subduction system. Petrologic studies indicate that the mafic-ultramafic suite crystallized from an island arc tholeiitic parental magma in a suprasubduction zone environment. The chondrite-normalized and primitive mantle normalized diagrams of the mafic and ultramafic rocks suggest derivation from MORB magma. The mixed characters from N-MORB to E-MORB of the studied samples are consistent with subduction modification of a MORB related magma, involving partial melting of the metasomatized mantle wedge. Our zircon U-Pb age data suggest that the cratonic nuclei was constructed as early as Paleoarchean. We present evidence for active subduction and arc magmatism through Mesoarchean to Neoarchean and early Paleoproterozoic, with the trench remaining open until at least 2.3?Ga. Two major crust building events are recognized in the Bastar craton: during Mesoarchean (recycled Paleoarchean subduction-related as well as juvenile/depleted mantle components) and Neoarchean (accretion of juvenile oceanic crust, arc magmatism including granite batholiths and related porphyry mineralization). The final cratonization occurred during latest Paleoproterozoic, followed by collisional assembly of the craton and its incorporation within the Peninsular Indian mosaic during Mesoproterozoic. In the global supercontinent context, the craton preserves the history of Ur, the earliest supercontinent, followed by the Paleo-Mesoproterozoic Columbia, as well as minor thermal imprints of the Neoproterozoic Rodinia and associated Grenvillian orogeny.
Russia, South America, Brazil, Africa, Tanzania, Zimbabwe, India, Asia, Sri Lanka
alexandrite
Abstract: The gem and jewelry trade has come to place increasing importance on the geographic origin of alexandrite, as it can have a significant impact on value. Alexandrites from Russia and Brazil are usually more highly valued than those from other countries. In 2016, GIA began researching geographic origin of alexandrite with the intent of offering origin determination as a laboratory service. Unfortunately, collecting reliable samples with known provenance can be very difficult. Alexandrite is often recovered as a byproduct of mining for other gemstones (e.g., emerald and corundum), so it can be difficult to secure reliable parcels of samples because production is typically erratic and unpredictable. The reference materials studied here were examined thoroughly for their trace element chemistry profiles, characteristic color-change ranges under daylight-equivalent and incandescent illumination, and inclusion scenes. The data obtained so far allow us to accurately determine geographic origin for alexandrites from Russia, Brazil, Sri Lanka, Tanzania, and India. Future work may help to differentiate alexandrites from other localities.
South African Journal of Geology, Vol. 122, 4, pp. 397-420.
India
alkaline, carbonatites
Abstract:
On the last page of his 1937 book “Our Wandering Continents” Alex Du Toit advised the geological community to develop the field of “comparative geology”, which he defined as “the study of continental fragments”. This is precisely the theme of this paper, which outlines my research activities for the past 28 years, on the continental fragments of the Indian Ocean. In the early 1990s, my colleagues and I were working in Madagascar, and we recognized the need to appreciate the excellent geological mapping (pioneered in the 1950s by Henri Besairie) in a more modern geodynamic context, by applying new ideas and analytical techniques, to a large and understudied piece of continental crust. One result of this work was the identification of a 700 to 800 Ma belt of plutons and volcanic equivalents, about 450 km long, which we suggested might represent an Andean-type arc, produced by Neoproterozoic subduction. We wondered if similar examples of this magmatic belt might be present elsewhere, and we began working in the Seychelles, where late Precambrian granites are exposed on about 40 of the >100 islands in the archipelago. Based on our new petrological, geochemical and geochronological measurements, we built a case that these ~750 Ma rocks also represent an Andean-type arc, coeval with and equivalent to the one present in Madagascar. By using similar types of approaches, we tracked this arc even further, into the Malani Igneous Province of Rajasthan, in northwest India. Our paleomagnetic data place these three entities adjacent to each other at ~750 Ma, and were positioned at the margins, rather than in the central parts of the Rodinia supercontinent, further supporting their formation in a subduction-related continental arc. A widespread view is that in the Neoproterozoic, Rodinia began to break apart, and the more familiar Gondwana supercontinent was assembled by Pan-African (~500 to 600 Ma) continental collisions, marked by the highly deformed and metamorphosed rocks of the East African Orogen. It was my mentor, Kevin Burke, who suggested that the present-day locations of Alkaline Rocks and Carbonatites (called “ARCs”) and their Deformed equivalents (called “DARCs”), might mark the outlines of two well-defined parts of the Wilson cycle. We can be confident that ARCs formed originally in intracontinental rift settings, and we postulated that DARCs represent suture zones, where vanished oceans have closed. We also found that the isotopic record of these events can be preserved in DARC minerals. In a nepheline syenite gneiss from Malawi, the U-Pb age of zircons is 730 Ma (marking the rifting of Rodinia), and that of monazites is 522 Ma (marking the collisional construction of Gondwana). A general outline of how and when Gondwana broke apart into the current configuration of continental entities, starting at about 165 Ma, has been known for some time, because this record is preserved in the magnetic properties of ocean-floor basalts, which can be precisely dated. A current topic of active research is the role that deep mantle plumes may have played in initiating, or assisting, continental fragmentation. I am part of a group of colleagues and students who are applying complementary datasets to understand how the Karoo (182 Ma), Etendeka (132 Ma), Marion (90 Ma) and Réunion (65 Ma) plumes influenced the break-up of Gondwana and the development of the Indian Ocean. Shortly after the impingement of the Karoo plume at 182 Ma, Gondwana fragmentation began as Madagascar + India + Antarctica separated from Africa, and drifted southward. Only after 90 Ma, when Madagascar was blanketed by lavas of the Marion plume, did India begin to rift, and rapidly drifted northward, assisted by the Marion and Deccan (65 Ma) plumes, eventually colliding with Asia to produce the Himalayas. It is interesting that a record of these plate kinematics is preserved in the large Permian - Eocene sedimentary basins of western Madagascar: transtensional pull-apart structures are dextral in Jurassic rocks (recording initial southward drift with respect to Africa), but change to sinistral in the Eocene, recording India’s northward drift. Our latest work has begun to reveal that small continental fragments are present in unexpected places. In the young (max. 9 Ma) plume-related, volcanic island of Mauritius, we found Precambrian zircons with ages between 660 and 3000 Ma, in beach sands and trachytic lavas. This can only mean that a fragment of ancient continent must exist beneath the young volcanoes there, and that the old zircons were picked up by ascending magmas on their way to surface eruption sites. We speculate, based on gravity inversion modelling, that continental fragments may also be present beneath the Nazareth, Saya de Malha and Chagos Banks, as well as the Maldives and Laccadives. These were once joined together in a microcontinent we called "Mauritia", and became scattered across the Indian Ocean during Gondwana break-up, probably by mid-ocean ridge "jumps". This work, widely reported in international news media, allows a more refined reconstruction of Gondwana, suggests that continental break-up is far more complex than previously perceived, and has important implications for regional geological correlations and exploration models. Our results, as interesting as they may be, are merely follow-ups that build upon the prescient and pioneering ideas of Alex Du Toit, whose legacy I appreciatively acknowledge.
Geochimica et Cosmochimica Acta, Vol. 274, pp. 118-135.
India
carbonatite
Abstract: Carbonatites crystallise along a wide range of solidus temperatures and are commonly affected by post-magmatic textural re-equilibration and diagenesis. Further insights into the formation and modification of carbonatites are provided using carbon, oxygen and clumped isotope (?47) data of rocks from spatially associated Amba Dongar and Siriwasan alkaline complexes in the north-western Deccan igneous province, India. We derive apparent equilibrium blocking temperatures to help constrain the thermal evolution of the different rock types found within the alkaline complexes in a petrographic context. The apparent temperatures for the carbonatites are significantly low but are consistent with reports on other global carbonatites and model predictions. Rapidly cooled Oldoinyo Lengai natrocarbonatite yielded similar low temperatures, even in the absence of bulk isotopic alteration. The isotopic proxies and petrographic observations favour both isotopic exchange reactions and diagenesis in altering ?47 values in calciocarbonatites. Diagenetic reactions are however strongly favoured, as secondary calcites in nephelinites and ferrocarbonatites record much lower temperatures than in the calciocarbonatites, highlighting the effect of fluids and diagenetic reactions in 13C18O bond ordering in carbonatites. Variations in the CO isotope data reveal the coupling of fractional crystallisation and post-magmatic fluid-rock interactions on bulk rock composition. After emplacement, the resetting of clumped isotope signatures in carbonatites is facilitated by post-magmatic processes in both open and closed systems.
Abstract: An emerging view is that Earth’s geodynamic regime witnessed a fundamental transition towards plate tectonics around 3.0 Ga (billion years). However, the manifestations of this change may have been diachronous and craton-specific. Here, we review geological, geophysical and geochronological data (mainly zircon U-Pb age-Hf isotope compositions) from the Dharwar craton representing over a billion year-long geologic history between ~3.5 and 2.5 Ga. The Archean crust comprises an oblique section of ~12 km from middle to deep crust across low- to mediumgrade granitegreenstone terranes, the Western and Eastern Dharwar Cratons (WDC and EDC), and the highgrade Southern Granulite Terrain (SGT). A segment of the WDC preserving Paleo- to Mesoarchean gneisses and greenstones is characterised by ‘dome and keel’ structural pattern related to vertical (sagduction) tectonics. The geology of the regions with dominantly Neoarchean ages bears evidence for convergent (plate) tectonics. The zircon U-Pb age-Hf isotope data constrain two major episodes of juvenile crust accretion involving depleted mantle sources at 3.45 to 3.17 Ga and 2.7 to 2.5 Ga with crustal recycling dominating the intervening period. The Dharwar craton records clear evidence for the operation of modern style plate tectonics since ~2.7 Ga.
Abstract: The Central Indian Tectonic Zone (CITZ) is a major E-W striking mobile belt dissecting the Indian Craton along which the northern and southern Indian cratonic blocks have joined to make the Greater Indian Landmass (GIL). CITZ has a long evolutionary history spanning over 1000 Myrs (2.1-0.9 Ga), overlapping with the assembly and dispersal of two supercontinents - Columbia and Rodinia. Despite a lot of recent work carried out on the CITZ, several outstanding issues remain, especially on the nature and timing of different orogenic events identified in the southern part of this mobile belt. The present contribution attempts to summarize the major petrological, structural and geochronological studies carried out in the CITZ and reappraise the tectonic models in the context of the current database. It is surmised that, while the northern part of CITZ records Paleoproterozoic (ca. 1.8 Ga) orogenic events, the southern part is dominated by a late Palaeoproterozoic-early Mesoproterozoic (ca.1.6-1.5 Ga) collision, followed by crustal extension, and finally a late Mesoproterozoic to early Neoproterozoic (ca. 1.04-0.93 Ga) collision that led to the final stitching of the North and South Indian cratonic blocks. Tectonic evolution of the CITZ is discussed in the context of the Proterozoic supercontinent cycle.
Abstract: The Singhbhum Craton is built up by successive pulses of discrete granitic magmatism at ~3.52 Ga, ~3.47-3.43 Ga, and ~3.40-3.35 Ga that produced tonalitetrondhjemite-granodiorite (TTG)-type suites and were followed by younger pulses at 3.32-3.35 Ga, and 3.31-3.28 Ga producing voluminous granitic-granodioritic magma. There is enough evidence to indicate that continental crust building activity started in the Hadean time and continued through Eoarchean. But the rocks of this period were fully recycled to generate the Paleoarchean and younger crust. The different pulses of granitic magmatism during the Paleoarchean were interspersed with the formation of supracrustal rocks which are now preserved as supracrustal belts peripheral to the craton or as internal screens within the craton. Halfnium isotopic record suggests that the Hadean and Eoarchean granitoids were sourced in an enriched reservoir, probably some form of early mafic protocrust. From ~3.6-3.5 Ga a shift in the isotopic composition of Hf is noticed, marked by upward excursion of ?Hf(t) plots towards suprachondritic values, signifying that the early mantle reservoir was serially modified by contamination by a juvenile melt derived from a depleted source. This probably signals a change in the geodynamic scenario, major depletion of the mantle and generation of voluminous TTG melts. There are contending hypotheses of plume-driven and subduction-driven mechanisms of continental crust formation. In the Singhbhum Craton during Hadean and Eoarchean times episodic mantle plumes probably operated in a stagnant lid tectonic setting. Repeated plume activities and the formation of oceanic plateaus might have triggered the onset of subduction which at the initial stages might have been of short duration. The transition from plume-driven tectonics to subduction-driven tectonics might have taken place at about 3.5 Ga. The supracrustal belts of the Older Metamorphic Group (OMG) and the Iron Ore Group (IOG) are thought to have formed in supra-subduction settings. Widespread metamorphism and deformation affected the craton during 3.34-3.26 Ga. By 3.1 Ga the Singhbhum Craton had stabilized and emerged as a landmass. Paleosols developed on the surface; rift basins were formed which were receptacles of siliciclastic sediments and mafic volcanics; anorogenic K-feldspar bearing granites were emplaced. Swarms of mafic dykes of Paleo- to Meso-Proterozoic age intruded the craton marking a tensional regime that was probably related to the initial stage of basin formation in the North Singhbhum Mobile Belt.
Precambrian Research, in press available, 17p. Pdf
India
craton
Abstract: The geodynamic evolution of the eastern Dharwar craton, southern India, is widely debated with a number of contrasting models ranging from uniformitarian plate convergence to the mantle plume and their combination. We report here the petrology and geochemistry of two undeformed and unmetamorphosed lamprophyre dykes from the Mudigubba area located immediately towards the western margin of the Paleo-Mesoproterozoic Cuddapah basin from this craton. The Mudigubba lamprophyres are free from crustal xenoliths, and have a typical porphyritic-panidiomorphic texture predominated by phenocrysts of amphibole. Clinopyroxene occurs as microphenocrysts with feldspar essentially confined to the groundmass. F-rich apatite and sphene are the other accessories. Mineral chemistry reveals that the amphiboles are of calcic variety (dominantly magnesio-hornblende), the clinopyroxene to be a diopside (Wo45.01-50.40 En36.74-44.58 Fs6.79-12.73 Ac0.42-2.24) and the albitic (Or1.12 Ab91.17 An7.70) nature of the feldspar. The lower abundance of TiO2 in both the amphibole and clinopyroxene, suggest a calc-alkaline nature of the magma. High Mg# (76.8-79.3), Ni (140-240 ppm) and Cr (380-830 ppm) contents along with (i) depletion in U, and Th, (ii) variable Ba/La and (iii) low Nb/La as well as Th/La strikes out possibility of crustal contamination and supports the primary nature of the lamprophyre magma. The presence of significant Nb-Ta, Zr-Hf and Ti negative anomalies in the primitive mantle normalized multi-element plots and their striking similarity with the global calc-alkaline lamprophyres imply the involvement of subduction-related mantle source modification. Various geochemical ratios (e.g., Hf/Sm, Ta/La, Th/Yb, Nb/Yb, La/Nb, Ba/Nb) demonstrate the source enrichment was caused by a fluid-related, rather than silicate-melt related, subduction metasomatism. Binary-mixing calculations assuming average upper crust and N-MORB as the two end members reveals ?10-30% influx of subducted component in the generation of the Mudigubba lamprophyres. A re-examination of the limited geochemical data available for the co-spatial Paleoproterozoic (2200-1600 Ma) alkaline plutons suggests this Neoarchaean subduction-event in this domain could in fact be a regional feature - all along the western margin of the Cuddapah basin and represents a hitherto unrecognised suture zone in the eastern Dharwar craton with the Paleoproterozoic (?) emplacement of Mudigubba lamprophyres post-dating this collisional event. Our findings provide significant geochemical support to the models invoking convergence towards the evolution of the Eastern Dharwar craton and impose important constraints on the geodynamics of the southern peninsular India.
Abstract: The Indian subcontinent is a repository of Archean cratonic nuclei with plethora of geoscientific data to better understand the early Earth evolution and the operating processes. The Bundelkhand Craton (BuC) in the north-central India is one of the five Archean cratons which preserves signatures of Paleoarchean magmatism, Archean subduction, Neoarchean metamorphism, spectacular craton-scale landforms as a testimony of Paleoproterozoic episodic silico-thermal fluid activity and plume-generated mafic magmatism, and a Paleoproterozoic meteoritic impact event, currently the seventh oldest in the world. Based on available geological and geophysical data, the BuC has been divided into north BuC (NBuC) and south BuC (SBuC) across the Bundelkhand Tectonic Zone (BTZ). The evolution of BuC has many similarities with other Indian cratons and the available geochronological data suggest that it forms a part of the Ur Supercontinent.
Abstract: The Proterozoic cratonic basins of peninsular India preserve records of repeated opening and closing of rifts along the zone of Neoarchean sutures and/or along the weak zones. These sedimentary basins, ranging in age from late Palaeoproterozoic through Neoproterozoic are traditionally referred to as Purana basins in Indian literature. The successions of each of the basins may be represented by successive unconformity-bound sequences, which represent several cycles of fluvialshallow marine to shelf-slope-basin sedimentation punctuated by local hiatuses and/or volcanic upheavals. The advance retreat of ancient seaways and their complex are recorded in the sedimentary successions of Purana basins. Papaghni-Chitravati; Kaladgi-Badami; Lower Vindhyan record the oldest cycle of sedimentation. These basins opened after 2.0 Ga and closed by 1.55 Ga. The Chattisgarh and its satellite basins, namely Indravati; Khariar; Ampani opened after the 1.6 Ga. and closed shortly after the 1000 Ma. Albaka; Mallampalli; Kurnool; Bhima preserve Neoproterozoic sedimentation history. The upper Vindhyan basin likely opened after 1.4 Ga. and continued through the Neoproterozoic. The sequence of events indicates a close relationship of craton interior histories with plate tectonics and variations in the heat flow regime underneath the continental crust. Periods of formation of the cratonic basins are coincident with the amalgamation or fragmentation of supercontinents further indicates genetic linkage between the two processes. Synchronous development of the cratonic basins with closely comparable stratigraphy and basin development events, in different small continents, strengthens the view that basin formation processes operated on a global scale, and stratigraphic basin analysis on a regional scale is a significant tool in evaluating the basins’ history. The available stratigraphic, geochronologic or palaeomagnetic data from India is still inadequate, and further information is required to constrain its definite position in the context of global tectonics.
Abstract: This study reports major, trace, rare earth and platinum group element compositions of lava flows from the Vempalle Formation of Cuddapah Basin through an integrated petrological and geochemical approach to address mantle conditions, magma generation processes and tectonic regimes involved in their formation. Six flows have been identified on the basis of morphological features and systematic three-tier arrangement of vesicular-entablature-colonnade zones. Petrographically, the studied flows are porphyritic basalts with plagioclase and clinopyroxene representing dominant phenocrystal phases. Major and trace element characteristics reflect moderate magmatic differentiation and fractional crystallization of tholeiitic magmas. Chondrite-normalized REE patterns corroborate pronounced LREE/HREE fractionation with LREE enrichment over MREE and HREE. Primitive mantle normalized trace element abundances are marked by LILE-LREE enrichment with relative HFSE depletion collectively conforming to intraplate magmatism with contributions from sub-continental lithospheric mantle (SCLM) and extensive melt-crust interaction. PGE compositions of Vempalle lavas attest to early sulphur-saturated nature of magmas with pronounced sulphide fractionation, while PPGE enrichment over IPGE and higher Pd/Ir ratios accord to the role of a metasomatized lithospheric mantle in the genesis of the lava flows. HFSE-REE-PGE systematics invoke heterogeneous mantle sources comprising depleted asthenospheric MORB type components combined with plume type melts. HFSE-REE variations account for polybaric melting at variable depths ranging from garnet to spinel lherzolite compositional domains of mantle. Intraplate tectonic setting for the Vempalle flows with P-MORB affinity is further substantiated by (i) their origin from a rising mantle plume trapping depleted asthenospheric MORB mantle during ascent, (ii) interaction between plume-derived melts and SCLM, (iii) their rift-controlled intrabasinal emplacement through Archean-Proterozoic cratonic blocks in a subduction-unrelated ocean-continent transition zone (OCTZ). The present study is significant in light of the evolution of Cuddapah basin in the global tectonic framework in terms of its association with Antarctica, plume incubation, lithospheric melting and thinning, asthenospheric infiltration collectively affecting the rifted margin of eastern Dharwar Craton and serving as precursors to supercontinent disintegration.
Journal of the Geological Society of India, Vol. 95, pp. 464-474.
India, global
REE
Abstract: The RM (Li, Be, Ti, Zr, Nb, Ta, Th and U) and REE (Light Rare Earths and Heavy Rare Earths including Yttrium) are strategic and critical for sustaining a variety of industries such as nuclear, defence, information technology (IT) and green energy options (wind, solar, electric vehicles and others). The 2010 ‘Rare Earth’ crisis of the world, following China’s monopoly with over 80% share and export restrictions in the REE market, led to an exploration boom for REE all over the world including India. This led to a substantial increase in REE mineral resources (98 Mt of contained REO in 2015) outside China located in Canada (38 Mt), Greenland (39 Mt) and Africa (10.3 Mt) that represents a fivefold increase in resources (c.f. Paulick and Machacek, 2017). As per the 2019, USGS commodity survey, the world reserves of REE have been estimated at 120 Mt in countries such as China (44Mt), Brazil (22Mt), Vietnam (22 Mt), Russia (12 Mt), India (6.9 Mt) and others (13 Mt). At present world resources of RM and REE are adequate to cater the demands of the different industries. The constraints, however, appear to be not technical but mainly environmental and social issues.
Journal of Earth System Science, Vol. 129, 1, 102 10p. Pdf
India
deposit - Pipe 6
Abstract: The Wajrakarur kimberlite Pipe 6 in Eastern Dharwar Craton, is hardly explored using latest ground-based geophysical techniques. The present study uses the Very Low Frequency Electromagnetic (VLF-EM) method for understanding the aerial extension, depth and geometry of the kimberlite pipe. The VLF-EM data have been analyzed using Fraser filtering of in-phase component, 3D Euler deconvolution of Fraser filtered in-phase data, radially average power spectrum (RAPS) of VLF data (raw data) and 2D inversion of VLF data (raw data). The Fraser filtered in-phase grid anomaly map has witnessed as an effective tool for mapping extension of the kimberlite pipe. The maxima of Fraser filtered in-phase component has been observed as a key parameter to delineate the conducting bodies. The high apparent current density in Karous-Hjelt (K-H) pseudo section locate relatively conducting body possibly associated with kimberlite pipe. Two depth interfaces at about 15 and 32 m have been delineated using RAPS. 3D Euler solution indicate dyke-like structure associated with kimberlite pipe having depth solutions ranging from 6 to 40 m with mode of depth 17 m in the study area. 2D resistivity sections indicate that causative bodies are in the depth range of 15-50 m. The results of VLF-EM study are well validated using geological borehole data over the study area reported by Geological Survey of India.
Abstract: Bunder diamond-bearing lamproite cluster, located in Madhya Pradesh, India, was discovered in 2004. The Precambrian lamproites are intruding Paleoproterozoic and Mesoproterozoic intracratonic sedimentary rocks covering the Archean Bundelkhand craton. The study of Bundelkhand craton through global dVs% TX2011 model (1D and 2D) led us to recognize that it is underlain by Archean lithospheric mantle as is observed in other locations, in mines with medium to very high diamond-grade (greater than 100 cpht). The Bunder Archean lithospheric mantle has 35 mW/m2 surface heat flow, typical of Archons with pipes with a very high degree of diamonds such as the Argyle lamproite and the kimberlites Internationalnaya, Mir, Ekati, among others. In the Bunder lamproite cluster, the Rio Tinto Exploration estimates for the pipes diamond-grade are below 100 cpht. To understand why Bunder lamproite pipes are low grade in diamonds, we combined comparative gravimetric studies to study the structural architecture model of the crystalline basement. In fact, very-rich diamond pipes develop in different crystalline basement architecture when compared to the pipes discovered in the Bunder cluster; for example the pipe Atri. The pipes next to the Argyle lamproite, the kimberlites pipes International, Mir, Diavik and others were located in the most depressed center of graben/micro graben structures; while the pipe Atri would have positioned on the edges of a graben. It is expected that additional exploration focused on the structural configuration of Bundelkhand craton basement may help to discover new lamproite pipes with a much greater diamond degree than the Bunder cluster.
Journal of South American Earth Sciences, Vol. 102, 102659, 12p. Pdf
India
geochronology
Abstract: The Dharwar Craton developed progressively over a billion years, through two main stages of crustal growth separated by a few-hundred million year long period of relative quiescence. The first stage between 3.4 and 3.0 Ga developed a proto-craton, which was considerably amplified during the second main stage between 2.7 and 2.4 Ga, through extensive magmatism, tectonism, and crustal consolidation. This paper reports U-Pb dating results obtained in four specific areas of the craton, with the data encompassing key moments in this long development. Rocks formed during the proto-craton stage include a 3089 Ma augen gneiss and a 2973 Ma evolved granite, the latter of which marks the final cratonization event of the proto-craton. The beginning of the second main stage is recorded in this study by 2650 Ma tonalite and trondhjemite, a 2623 Ma granite dyke cutting augen gneiss, and 2614, 2602 and 2588 Ma volcanic rocks. Titanite responded differently to the long evolution, as a function of location and type of overprint. It preserved an original 2973 Ma magmatic age in the west, but was reset and/or crystallized during secondary events in central domains of the craton, yielding ages between 2590 and 2360 Ma. A diorite stock intruded at 2207 Ma in the consolidated crust. It is correlated with the Anantapur-Kunigal mafic dyke swarm, one of a series of such events in the Dharwar Craton between 2.35 and 1.79 Ma. In terms of its overall evolution the Dharwar Craton has an affinity with the Slave clan, which includes the Wyoming and Zimbabwe cratons. It also matches many features in the evolution of the São Francisco Craton, a probable other member of Sclavia. This is in contrast to the Amazonian Craton, which has more affinity with the Superior clan.
Journal of Earth System Science, Vol. 128, 1, 7p. Pdf
India
minette
Abstract: Lamprophyre dykes within the granitoid and charnockite are reported for the first time from the Western Bastar Craton, Chandrapur district, Maharashtra. It shows porphyritic-panidiomorphic texture under a microscope, characterised by the predominance of biotite phenocrysts with less abundance of amphibole and clinopyroxene microphenocryst. The groundmass is composed more of K-feldspars over plagioclase, amphiboles, clinopyroxene, biotite, chlorite, apatite, sphene and magnetite. The mineral chemistry of biotite and magnesio-hornblende is indicative of minette variety of calc-alkaline lamprophyre (CAL), which is further supported by preliminary major oxides and trace element geochemistry. This unique association of CAL with granitoid provides an opportunity to study the spatio-temporal evolution of the lamprophyric magma in relation to the geodynamic perspective of the Bastar Craton.
Abstract: Kimberlites are volatile-rich deep mantle-derived rocks that often contain diamonds. Numerous Grenvillian (ca. 1.1 Gyr) diamondiferous kimberlites, ultramafic lamprophyres, and lamproites are exposed in the Eastern Dharwar Craton and the Bastar Craton, India, and are aligned almost parallel to the Eastern Ghats (granulite) Mobile Belt (EGMB). The trigger for these kimberlite and related magmatic events still remains an open question. We review the available geochronological and radiogenic isotopic data for the ~1.1 Gyr kimberlites, lamproites, and ultramafic lamprophyres from the Eastern Dharwar Craton and the Bastar Craton of the Indian shield. We show that kimberlites and associated magmas were emplaced for a longer duration (ca. 130 Myr) in the Indian shield during the Mesoproterozoic and sampled distinct mantle source regions. The kimberlites and ultramafic lamprophyre are characterized by slightly depleted to chondritic Nd isotopic ratios revealing their origin at deeper sub-lithospheric regions, whereas the lamproites essentially show an enriched Nd isotopic signature suggesting their derivation from enriched sub-continental lithospheric mantle. We argue that the absence of linear age progression, prolonged magmatic activity compared to the time span of coeval large igneous provinces (the Umkondo, the Keweenawan, and the Warakurna) and a cooler ambient mantle as revealed from the entrained xenoliths, constitute important limitations for a plume model earlier proposed for the genesis of these kimberlites and related magmas. These observations together with a geographical and temporal (Grenvillian) link to the EGMB points towards edge-driven convection as a trigger for kimberlite magmatism- similar to the model proposed for the Mid-Cretaceous kimberlite corridor in North America. However, this model can't explain the coeval formation of sub-continental lithospheric mantle-derived lamproites. As the timing of kimberlite and related magmatism coincides with that of the Grenvillian orogeny and succeeded a magmatic lull of ~360 Myr in the Dharwar Craton during the Mesoproterozoic, we instead, propose that small scale partial melting of heterogeneous mantle caused by plate reorganization during Columbia to Rodinia supercontinent extroversion served as a trigger for this ca. 1.1 Gyr magmatism in the southeastern Indian shield.
Russian Journal of Earth Sciences, Vol. 20, ES3006 14p. Pdf
India, Andhra Pradesh
deposit - Kristipadu
Abstract: This paper addresses geochemical and petrological aspects of two outcropping kimberlites (5023 and 5119) of the Gooty cluster, emplaced in carbonate sediments of Vempalli Formation of lower Cuddapah basin at Krishtipadu, Anantapur district, Andhra Pradesh, southern India. These pipes were discovered by the Rio Tinto Exploration Group in the recent past. The 5023 kimberlite is enriched in olivine and serpentine while the 5119 pipe possesses haematitised olivine pseudomorphs. The field, textural characteristics and whole rock geochemistry qualify both the pipes for hypabyssal kimberlite breccias of Group-I type similar to world’s classical occurrences. The carbon and oxygen stable isotope data, aided with field and petrological studies, indicates existence of possible carbonatite (sovite) phase associated with the 5119 kimberlite. The two kimberlites appear to be originated from a low degree of partial melting ranging from 0.5 to 2.5%. Enrichment of LREE with a high LREE/HREE ratio indicates fractionation at the mantle source region. Whole rock geochemistry supports their diamondiferous nature. Presence of crustal xenoliths post-dates subsequent emplacement of the two pipes to lower Cuddapah sedimentation (2.4 Ga), manifesting kimberlite magmatism. These pipes are the only known Group-I kimberlites from the Proterozoic Cuddapah Basin and therefore warrant detailed investigations. KEYWORDS: Kimberlite; carbonatite; archetypal Group-I; Gooty Kimberlite Cluster; lower Cuddapah basin; stable isotope; Palaeoproterozoic.
Journal of Earth System Science, Vol. 128, 43, 8p. Pdf
India
geophysics, seismics
Abstract: An improved shear wave velocity (Vs) structure of the lithosphere of peninsular India using the surface wave tomography from the ambient noise and earthquake waveforms suggests its bipolar character. While most of the geological domains of India are characterised by a uniform lithospheric mantle of Vs?4.5 km/s, the three cratonic regions, eastern Dharwar, Bastar and Singhbhum, hosting most of the diamondiferous kimberlite fields, show significantly high Vs of 4.7 km/s and above in their lower lithosphere beyond ?90 km depth. The higher velocity could best be explained by the presence of diamond and/or eclogite along with peridotite in mantle. This unique relationship suggests the regional seismic image of lithosphere as a guide for exploration of diamonds.
Proceedings Natural Science Academy, Vol. 86, 1, pp. 301-311.
India
kimberlite, lamproites
Abstract: Highlights of researches on kimberlites, lamproites and lamprophyres (and their entrained xenoliths) during 2016-2019 from the Indian context are presented. A few previously unknown occurrences have been brought to light, and a wealth of petrological, geochemical and isotopic data on these rocks became available. All these studies provided new insights into the nomenclatural as well as geodynamic aspects such as subduction-tectonics, mantle metasomatism, lithospheric thickness, supercontinent amalgamation, and break-up and nature of the sub-continental lithospheric mantle from the Indian shield.
Abstract: P-5 and Kl-4 Mesoproterozoic (ca. 1110 Ma) kimberlites from the Wajrakarur and Kalyandurg clusters, Eastern Dharwar craton (EDC), southern India are intruded into the diamondiferous cratonic roots. The spinel compositions is straddling between magnesian ulvöspinel (Group-1 kimberlite) and titanomagnetite (Group-2 kimberlite), comparable with orangeite and lamproites. These Ti-rich minerals have orangeitic affinity, as in the Kaapvaal craton of South Africa, and reflect the high Ti-, high Ca- and the low Al-bearing nature of the parent magma (Group II kimberlites). Larger chrome spinel macrocrysts/xenocrysts show >500 ?m of size with distinctly high chromium (Cr2O3 up to 59.62 wt%), and TiO2-poor (<1.19 wt%). The high chromium spinel macrocrysts represent fragments of mantle xenocrysts and their composition falls within the diamond stability field. The groundmass spinel has been replaced by Ti- schorlomite. The schorlomite garnet represents solid solution of schorlomite -pyrope -almandine-grossular and Crrich schorlomite -pyrope -almandine- uvarovite solid solution. These associations recommend that the schorlomite formed through the replacement of spinel through interaction of late residual fluids/melts in the final stages of crystallization of the kimberlite magma and enrichment in Fe and Ti in schorlomite suggests the involvement of metasomatized sub-continental lithospheric mantle. Present study may have useful application in diamond prospectivity.
Physics of the Earth and Planetary Interiors, Vol. 306, 106543, 9p. Pdf
India
geophysics -seismic
Abstract: The south Indian shield, primarily consisting of Archean cratons and Cretaceous-Tertiary Deccan Volcanic Province (DVP), has undergone several major tectonic episodes during its evolution. The Deccan volcanism at Cretaceous-Tertiary (~65 Ma) is the last major tectono-thermal event, which influenced a substantial part of the south Indian shield. To understand the influence of the Deccan volcanism on the evolution of the south Indian shield, we study the crustal seismic structure of the ~65 Ma Deccan Volcanic Province and the adjacent ~2.6 Ga Eastern Dharwar Craton (EDC), which forms the basement of the volcanic terrain. We calculate teleseismic receiver functions for 18 broadband seismic stations along a ~1000 km long seismological profile that cut across both the EDC and DVP. The analysis and modelling, using H-Vp/Vs stacking and generalized neighbourhood algorithm inversion of the receiver functions show distinct crustal structure (crustal thickness, average composition, shear wave velocity variation, nature of crust-mantle boundary, etc.) across the EDC and DVP. The results clearly indicate that the crustal structure is heterogeneous beneath the DVP compared to a relatively uniform structure below the EDC. Using results from this study along with earlier results, we infer that the present Eastern Dharwar Craton terrain is not affected by any tectono-thermal event for a long geological time, including the Deccan volcanism. Whereas, the present Deccan Volcanic Province is highly affected by the Reunion mantle plume-crust interaction.
Abstract: The Eastern Dharwar Craton (EDC) of the southern Indian shield host numerous Mesoproterozoic (~1100 Ma) kimberlite fields. Shoshonitic lamprophyre dykes synchronous to these kimberlites are exposed in the diamondiferous Wajrakarur kimberlite field (WKF). These lamprophyre dykes are characterized by the presence of olivine, biotite and zoned clinopyroxene phenocrysts set in a groundmass of feldspar, spinel and apatite. High K2O/Na2O (1.5-3.4) along with elevated Th content (2.2-8.6) in these dykes reveal their shoshonitic affinity. High Th/Yb (1.5-4.1), Nb/Yb (23.8-59.0) and superchondritic Zr/Hf ratios (40-50.4) similar to the oceanic island basalts rules out crustal assimilation and highlights the role of an enriched mantle source in their genesis. Age corrected bulk-rock ?Nd of the shoshonitic lamprophyres vary between -13.73 and -22.90, whereas the initial 87Sr/86Sr ranges between and 0.70533-0.71218 suggesting their derivation from an enriched lithospheric mantle. However, the coeval kimberlites of the WKF have distinct Sr-Nd isotopic composition with bulk-rock ?Nd and initial 87Sr/86Sr varying from 0.44 to 2.75 and 0.70209 to 0.70744, respectively, similar to that of the groundmass perovskite ?Nd (2.08-2.92) and initial 87Sr/86Sr (0.70234- 0.70255). Since, these kimberlites tap an isotopically depleted mantle source, unlike the shoshonitic lamprophyres, the geochemistry of these rocks points to a vertically heterogeneous lithospheric mantle beneath the EDC. We suggest that the Wajrakarur kimberlites generated from a deeper depleted mantle (>150 km) compared to the shallower enriched lithospheric mantle source for the shoshonitic lamprophyres. Neoarchean (2.6-2.8 Ga) Nd depleted mantle model ages for these lamprophyres suggest that the subcontinental lithospheric mantle was enriched during Neoarchean accretion-related evolution of the Dharwar Craton.
Journal of Mineralogical and petrological Sciences, Vol. 115, 2, pp. 202-215. pdf
India
deposit - Wajrakarur
Abstract: Petrology and geochemistry (including Sr and Nd isotopes) of two lamprophyre dykes, intruding the Archaean granitic gneisses at Sivarampeta in the diamondiferous Wajrakarur kimberlite field (WKF), eastern Dharwar craton, southern India, are presented. The Sivarampeta lamprophyres display porphyritic-panidiomorphic texture comprising macrocrysts/phenocrysts of olivine, clinopyroxene (augite), and mica set in a groundmass dominated by feldspar and comprising minor amounts of ilmenite, chlorite, carbonates, epidote, and sulphides. Amphibole (actinolite-tremolite) is essentially secondary in nature and derived from the alteration of clinopyroxene. Mica is compositionally biotite and occurs as a scattered phase throughout. Mineralogy suggests that these lamprophyres belong to calc-alkaline variety whereas their bulk-rock geochemistry portrays mixed signals of both alkaline as well as calc-alkaline (shoshonitic) variety of lamprophyres and suggest their derivation from the recently identified Domain II (orogenic-anorogenic transitional type mantle source) from eastern Dharwar craton. Trace element ratios imply melt-derivation from an essentially the garnet bearing-enriched lithospheric mantle source region; this is further supported by their 87Sr/86Srinitial (0.708213 and 0.708507) and ‘enriched’ ?Ndinitial (?19.1 and ?24.2) values. The calculated TDM ages (2.7-2.9 Ga) implies that such enrichment occurred prior to or during Neoarchean, contrary to that of the co-spatial and co-eval kimberlites which originated from an isotopically depleted mantle source which was metasomatized during Mesoproterozoic. The close association of calc-alkaline shoshonitic lamprophyres, sampling distinct mantle sources, viz., Domain I (e.g., Udiripikonda) and Domain II (Sivarampeta), and kimberlites in the WKF provide further evidence for highly heterogeneous nature of the sub-continental lithospheric mantle beneath the eastern Dharwar craton.
Abstract: Geochemical and petrological characteristics of lamprophyre dykes at Kalagalla intruded into the auriferous schistose rocks of the Ramagiri- Penakacherla Schist Belt, Anantapur district, Andhra Pradesh, India are presented here. The Kalagalla lamprophyre (KGL) is a melanocratic rock exhibiting typical knobby or pustular texture on the surface. The microtextures and mineralogy typical of lamprophyres are obscured by metamorphism; however, it exhibits porphyritic, nemato-granoblastic texture representative of greenschist facies of metamorphism. The rock is sheared and possesses several globules formed by polycrystalline aggregates of calcite rimmed by coronitic subhedral plagioclase and biotite, evidencing its mantle-magmatic origin. The mineral assemblages noticed in thin-sections include amphibole, plagioclase, biotite, phlogopite and calcite ocelli as essential while apatite, zircon, magnetite, ilmenite, Ni-bearing chalcopyrite and pyrite as accessory phases. The SEM-EDS investigation on the accessory minerals revealed accessory sulphide and silicate phases like As-free pyrite, haematitised Ni-bearing chalcopyrite and Ni-As-Co- minerals indicative of sulphidation associated with greenstone auriferous lodes, along with silicates like LREE-bearing titanite partially transformed into leucoxene and oxide phases like magnetite altered to goethite at places. Based on mineral chemistry, whole rock geochemistry, presence of amphibole and dominance of plagioclase, the KGL is classified as a calc-alkaline variety in general and as spessartite in particular possessing shoshonitic affinity. No anomalous chemical composition is noticed in the ocellar calcite. The LREE-bearing titanite appears to be the contributor of LREE enrichment. The high Mg# (77- 79), Ni (153-162 ppm) and Cr (380-470 ppm) support a mantle source. The absence of Eu anomaly reflects lack of plagioclase fractionation. The high Zr/Hf ratio (163-202) indicates absence of crustal contamination and contribution of magmatic carbonate at the source to form ocelli as product of late-stage liquid silicate-carbonate immiscibility of segregation mechanism. The trace and REE patterns (?REE: 326-343 ppm, LREE>HREE) indicate involvement of residual garnet at the source presumably enriched in phlogopite in a ‘subduction-related’ environment.
Abstract: An emerging view is that Earth’s geodynamic regime witnessed a fundamental transition towards plate tectonics around 3.0 Ga (billion years). However, the manifestations of this change may have been diachronous and craton-specific. Here, we review geological, geophysical and geochronological data (mainly zircon U-Pb age-Hf isotope compositions) from the Dharwar craton representing over a billion year-long geologic history between ~3.5 and 2.5 Ga. The Archean crust comprises an oblique section of ~12 km from middle to deep crust across low- to mediumgrade granitegreenstone terranes, the Western and Eastern Dharwar Cratons (WDC and EDC), and the highgrade Southern Granulite Terrain (SGT). A segment of the WDC preserving Paleo- to Mesoarchean gneisses and greenstones is characterised by ‘dome and keel’ structural pattern related to vertical (sagduction) tectonics. The geology of the regions with dominantly Neoarchean ages bears evidence for convergent (plate) tectonics. The zircon U-Pb age-Hf isotope data constrain two major episodes of juvenile crust accretion involving depleted mantle sources at 3.45 to 3.17 Ga and 2.7 to 2.5 Ga with crustal recycling dominating the intervening period. The Dharwar craton records clear evidence for the operation of modern style plate tectonics since ~2.7 Ga.
Journal of Earth System Science *** note date 2006 , Vol. 115, 1, Feb. pp. 161-183. pdf
India
deposit - Panna
Abstract: Amongst all the perceptible igneous manifestations (volcanic tuffs and agglomerates, minor rhyolitic flows and andesites, dolerite dykes and sills near the basin margins, etc.) in the Vindhyan basin, the two Mesoproterozoic diamondiferous ultramafic pipes intruding the Kaimur Group of sediments at Majhgawan and Hinota in the Panna area are not only the most conspicuous but also well-known and have relatively deeper mantle origin. Hence, these pipes constitute the only yet available ‘direct’ mantle samples from this region and their petrology, geochemistry and isotope systematics are of profound significance in understanding the nature of the sub-continental lithospheric mantle beneath the Vindhyan basin. Their emplacement age (? 1100 Ma) also constitutes the only reliable minimum age constrain on the Lower Vindhyan Group of rocks. The Majhgawan and Hinota pipes share the petrological, geochemical and isotope characteristics of kimberlite, orangeite (Group II kimberlite) and lamproite and hence are recognised as belonging to a ‘transitional kimberlite-orangeite-lamproite’ rock type. The namemajhagwanite has been proposed by this author to distinguish them from other primary diamond source rocks. The parent magma of the Majhgawan and Hinota pipes is envisaged to have been derived by very small (<1%) degrees of partial melting of a phlogopite-garnet lherzolite source (rich in titanium and barium) that has been previously subjected to an episode of initial depletion (extensive melting during continent formation) and subsequent metasomatism (enrichment). There is absence of any subduction-related characteristics, such as large negative anomalies at Ta and Nb, and therefore, the source enrichment (metasomatism) of both these pipes is attributed to the volatile- and K-rich, extremely low-viscosity melts that leak continuously to semi-continuously from the asthenosphere and accumulate in the overlying lithosphere. Lithospheric/crustal extension, rather than decompression melting induced by a mantle plume, is favoured as the cause of melting of the source regions of Majhgawan and Hinota pipes. This paper is a review of the critical evaluation of the published work on these pipes based on contemporary knowledge derived from similar occurrences elsewhere.
Abstract: Lamprophyres are some of the oldest recognized alkaline rocks and have been studied for almost the last 150 years. Known for hosting economic minerals such as gold, diamond and base metals, they are also significant in our understanding of the deep-mantle processes (viz., mantle metasomatism and mantleplume-lithosphere interactions) as well as large-scale geodynamic processes (viz., subduction-tectonics, supercontinent amalgamation and break-up). The Indian shield is a collage of distinct cratonic blocks margined by the mobile belts and manifested by large igneous provinces (LIPs) such as the Deccan. A plethora of lamprophyres, varying in age from the Archaean to the Eocene, with diverse mineralogical and geochemical compositions, are recorded from the Indian shield and played a key role in clarifying the tectonic processes, especially during the Paleo- and Mesoproterozoic and the Late Cretaceous. A comprehensive review of the occurrence, petrology, geochemistry and origin of the Indian lamprophyres is provided here highlighting their tectonomagmatic significance. The relationship of the lamprophyres to the Kimberlite clan rocks (KCRs), focusing on the Indian examples, is also critically examined.
Abstract:
Carbonatite melts derived from the mantle are enriched in CO2- and H2O-bearing fluids. This melt can metasomatize the peridotitic lithosphere and liberate a considerable amount of CO2. Experimental studies have also shown that a CO2-H2O-rich fluid can form Fe- and Mg-rich carbonate by reacting with olivine. The Sung Valley carbonatite of NE India is related to the Kerguelen plume and is characterized by rare occurrences of olivine. Our study shows that this olivine is resorbed forsterite of xenocrystic nature. This olivine bears inclusions of Fe-rich magnesite. Accessory apatite in the host carbonatite contains CO2-H2O fluid inclusions. Carbon and oxygen isotopic analyses indicate that the carbonatites are primary igneous carbonatites and are devoid of any alteration or fractionation. We envisage that the forsterite is a part of the lithospheric mantle that was reprecipitated in a carbonatite reservoir through dissolution-precipitation. Carbonation of this forsterite, during interaction between the lithospheric mantle and carbonatite melt, formed Fe-rich magnesite. CO2-H2O-rich fluid derived from the carbonatite magma and detected within accessory apatite caused this carbonation. Our study suggests that a significant amount of CO2 degassed from the mantle by carbonatitic magma can become entrapped in the lithosphere by forming Fe- and Mg-rich carbonates.
Journal of Asian Earth Science, Vol. 200, 104432, 18p. Pdf
India
deposit - Munnar
Abstract: Metacarbonate assemblages in high-grade metamorphic terranes often pose challenges when trying to distinguish between mantle-derived carbonatite and sedimentary carbonate protoliths. We present a study of granulite-facies metacarbonate samples of the putative Munnar carbonatite described as decimeter-thick dikes and veins, and layers of a meter-thick metacarbonate and calc-silicate assemblage, respectively. Thin sections of the metacarbonate dike samples show absence of pyrochlore and ubiquitous scapolite, titanite, wollastonite, and detrital zircons are compatible with impure limestone protoliths. Nd and Sr isotope compositions indicate protoliths with Paleoproterozoic crustal residence times which contrast the mantle sources of Indian and global carbonatites. Trace-element patterns display the characteristics of upper crust, and Ce- and Y-anomalies in a number of samples suggest protolith formation under marine conditions. Carbon and oxygen isotope compositions of the metacarbonate samples interlayered with calc-silicate rocks are similar to those in marine limestone. The metacarbonate dikes, however, show mantle-like compositions which are interpreted as reflecting equilibration with mantle-derived CO2 during granulite-facies metamorphism. The dikes yielded a U-Pb zircon crystallization age of 1020 ± 70 Ma and a cross-cutting quartz syenite, thought to be cogenetic, a magmatic age of 620 ± 35 Ma; the hosting gneiss provided a magmatic age of 2452 ± 14 Ma. We conclude that the layered metacarbonate and calc-silicate rocks represent a former marine limestone and marl sequence and the metacarbonate dikes and veins small-volume melts of crust-derived carbonate-rich sediment.
Abstract: Gravity anomalies across the Indian region depict most of the geological and tectonic domains of the Indian continental lithosphere, which evolved through Archean cratonic nucleation, Proterozoic accretion, Phanerozoic India-Eurasia plate convergence, and modification through many thermal perturbations and rifting. Integrated analysis of gravity and geoid anomalies together with topographic and heat flow data led to deciphering the mechanism of isostatic compensation of topographic and geological loads, lithospheric structure, and composition. This study discusses the nature of gravity (free-air, Bouguer and Isostatic) and geoid anomalies in relation to the topography, geology, and tectonics, and presents a lithospheric density model across the peninsular India and Himalaya. Southern peninsular Indian region shows relatively low Bouguer gravity anomalies compared to the northern region. The mobile belts are generally observed to have relatively higher Bouguer gravity anomalies, e.g., Eastern Ghats Mobile Belt compared to the shield regions. The gravity lows are observed over topographic features like the Western Ghats and Himalaya, while some of the topographic highs like Aravalli show positive gravity anomaly. The Indian Ocean Geoid Low varies from -82 m over Dharwar Craton to -98 m over the Southern Granulite Terrain and finally reaches a significant low of -106 m in the Indian Ocean. Flexural isostatic compensation with variable Effective Elastic Thickness (EET) ~10 km to 50 km prevails over the stable continental region. The lithospheric thickness varies from 80 km along the coastal region to 120-130 km beneath the Saurashtra Plateau, the Southern Granulite Terrain, and the Eastern Indian Shield, and reaches to more than 200 km under the Himalayan orogenic belt in the north. From Dharwar Craton to Bundelkhand Craton in central India, the lithospheric thickness varies between 160 and 180 km.
Abstract: The Singhbhum Craton is built up by successive pulses of discrete granitic magmatism at ~3.52 Ga, ~3.47-3.43 Ga, and ~3.40-3.35 Ga that produced tonalitetrondhjemite-granodiorite (TTG)-type suites and were followed by younger pulses at 3.32-3.35 Ga, and 3.31-3.28 Ga producing voluminous granitic-granodioritic magma. There is enough evidence to indicate that continental crust building activity started in the Hadean time and continued through Eoarchean. But the rocks of this period were fully recycled to generate the Paleoarchean and younger crust. The different pulses of granitic magmatism during the Paleoarchean were interspersed with the formation of supracrustal rocks which are now preserved as supracrustal belts peripheral to the craton or as internal screens within the craton. Halfnium isotopic record suggests that the Hadean and Eoarchean granitoids were sourced in an enriched reservoir, probably some form of early mafic protocrust. From ~3.6-3.5 Ga a shift in the isotopic composition of Hf is noticed, marked by upward excursion of ?Hf(t) plots towards suprachondritic values, signifying that the early mantle reservoir was serially modified by contamination by a juvenile melt derived from a depleted source. This probably signals a change in the geodynamic scenario, major depletion of the mantle and generation of voluminous TTG melts. There are contending hypotheses of plume-driven and subduction-driven mechanisms of continental crust formation. In the Singhbhum Craton during Hadean and Eoarchean times episodic mantle plumes probably operated in a stagnant lid tectonic setting. Repeated plume activities and the formation of oceanic plateaus might have triggered the onset of subduction which at the initial stages might have been of short duration. The transition from plume-driven tectonics to subduction-driven tectonics might have taken place at about 3.5 Ga. The supracrustal belts of the Older Metamorphic Group (OMG) and the Iron Ore Group (IOG) are thought to have formed in supra-subduction settings. Widespread metamorphism and deformation affected the craton during 3.34-3.26 Ga. By 3.1 Ga the Singhbhum Craton had stabilized and emerged as a landmass. Paleosols developed on the surface; rift basins were formed which were receptacles of siliciclastic sediments and mafic volcanics; anorogenic K-feldspar bearing granites were emplaced. Swarms of mafic dykes of Paleo- to Meso-Proterozoic age intruded the craton marking a tensional regime that was probably related to the initial stage of basin formation in the North Singhbhum Mobile Belt.
Abstract: We report petrography, mineralogy, major- and trace-element compositions of a rare selection of spinel- and garnet-bearing peridotite xenoliths and single crystals separated from peridotites hosted in the Mesoproterozoic Wajrakarur kimberlites from the Eastern Dharwar craton (EDC), India. These ultramafic xenoliths consist of olivine (modal 74-82 vol%) with Fo92-93, clinopyroxene, orthopyroxene, spinel, garnet, and/or ilmenite. Calculated equilibrium pressure and temperature conditions are 2.5-5.0?GPa and 710-1179?°C for these peridotites, which suggests residence depths >160?km near the base of the Dharwar cratonic lithospheric mantle. Garnet in these ultramafic rocks [with Mg#?=?molar (Mg/(Mg?+?Fetotal)?×?100 of 80-88] displays either “sinuous” LREE-enriched patterns with depletion in Gd and Er for harzburgites or “normal” LREE-depleted, HREE-enriched patterns for lherzolites. Two groups of clinopyroxenes (group-I and group-II) were also observed with high LREE (LaN?>?10) and low LREE (LaN?
Abstract: The alkaline rocks and carbonatites (ARCs) of the Great Indian Proterozoic belt bear the testimony of tectonic processes operating in the Proterozoic during the continental assembly and breakup of both Columbia and Rodinia. We present a comprehensive review, mainly focused on the petrology, geochemistry, and geochronology of 38 ARCs of Peninsular India, which are mostly concentrated within the Eastern Ghats Mobile Belt and Southern Granulite Terrain. Available geochronologic data reveals three distinct alkaline magmatic phases (2533-2340 Ma, 1510-1242 Ma, 833-572 Ma) and two metamorphic events (950-930 Ma and 570-485 Ma) that correlate with the Grenvillian and Pan-African orogeny events. Whereas clinopyroxene, amphibole, titanite and apatite fractionation seems to have affected the nephelinite, nepheline syenite and syenite, carbonatite is affected by fractionation of calcite, dolomite, ankerite, pyroxene, apatite, magnetite, mica, and pyrochlore. Trace elements and Sr-Nd-Pb-C-O isotopic compositions of these ARCs strongly suggest a subcontinental lithospheric mantle source, that is enriched either by distribution of subducted crustal material or by metasomatism of mantle-derived fluids, for the generation of ARCs. Despite some isotopic variability that can result from crustal contamination, a trend showing enrichment in 87Sr/86Sri (0.702 to 0.708) and depletion in ?Nd(i) (-1.3 to -14.1) over a 2 Gyr duration indicates temporal changes in the lithospheric/asthenospheric source of ARCs, due to periodic enrichment of the source by mantle-derived fluids. ARC generation starts in an intracontinental rift setting (beginning of Wilson cycle). These early-formed ARCs are carriedto 100 km depths during continental collision (termination stage of Wilson cycle) and undergo extensive melting because of renewed rifting along suture zones to form new generation of ARCs.
Journal of Petrology, in press available, 73p. Pdf
India
deposit - Wajrakarur
Abstract: The Wajrakarur Kimberlite Field (WKF) on the Eastern Dharwar Craton in southern India hosts several occurrences of Mesoproterozoic kimberlites, lamproites, and ultramafic lamprophyres, for which mantle-derived xenoliths are rare and only poorly preserved. The general paucity of mantle cargo has hampered the investigation of the nature and evolution of the continental lithospheric mantle (CLM) beneath cratonic southern India. We present a comprehensive study of the major and trace element compositions of clinopyroxene and garnet xenocrysts recovered from heavy mineral concentrates for three ca. 1.1 Ga old WKF kimberlite pipes (P7, P9, P10), with the goal to improve our understanding of the cratonic mantle architecture and its evolution beneath southern India. The pressure-temperature conditions recorded by peridotitic clinopyroxene xenocrysts, estimated using single-pyroxene thermobarometry, suggest a relatively moderate cratonic mantle geotherm of 40?mW/m2 at 1.1 Ga. Reconstruction of the vertical distribution of clinopyroxene and garnet xenocrysts, combined with some rare mantle xenoliths data, reveals a compositionally layered CLM structure. Two main lithological horizons are identified and denoted as layer A (?80-145?km depth) and layer B (?160-190?km depth). Layer A is dominated by depleted lherzolite with subordinate amounts of pyroxenite, whereas layer B comprises mainly refertilised and Ti-metasomatised peridotite. Harzburgite occurs as a minor lithology in both layers. Eclogite stringers occur within the lower portion of layer A and at the bottom of layer B near the lithosphere-asthenosphere boundary at 1.1 Ga. Refertilisation of layer B is marked by garnet compositions with enrichment in Ca, Ti, Fe, Zr and LREE, although Y is depleted compared to garnet in layer A. Garnet trace element systematics such as Zr/Hf and Ti/Eu indicate that both kimberlitic and carbonatitic melts have interacted with and compositionally overprinted layer B. Progressive changes in the REE systematics of garnet grains with depth record an upward percolation of a continuously evolving metasomatic agent. The intervening zone between layers A and B at ?145-160?km depth is characterised by a general paucity of garnet. This ‘garnet-paucity’ zone and an overlying type II clinopyroxene-bearing zone (?115-145?km) appear to be rich in hydrous mineral assemblages of the MARID- or PIC kind. The composite horizon between ?115-160?km depth may represent the product of intensive melt/rock interaction by which former garnet was largely reacted out and new metasomatic phases such as type II clinopyroxene and phlogopite plus amphibole were introduced. By analogy with better-studied cratons, this ‘metasomatic horizon’ may be a petrological manifestation of a former mid-lithospheric discontinuity at 1.1 Ga. Importantly, the depth interval of the present-day lithosphere-asthenosphere boundary beneath Peninsular India as detected in seismic surveys coincides with this heavily overprinted metasomatic horizon, which suggests that post-1.1 Ga delamination of cratonic mantle lithosphere progressed all the way to mid-lithospheric depth. This finding implies that strongly overprinted metasomatic layers, such as the ‘garnet-paucity’ zone beneath the Dharwar craton, present structural zones of weakness that aid lithosphere detachment and foundering in response to plate tectonic stresses.
Abstract: The early Cretaceous (ca. 118-100 Ma) alkaline/ultraalkaline silicate and carbonatite magmatism, exclusively recorded in the Chhotanagpur Gneissic Complex and the Shillong Plateau-Mikir Hills in the eastern/northeastern regions of the Indian Shield, have been reviewed to understand their genetic aspects. These are thought to be associated to the Kerguelen hot spot, active in this region during ca. 118-100 Ma. The existing geochemical, geochronological and isotopic data do not support any definite emplacement order for these diverse groups of magmatic suites. It is likely that they were derived from distinct magma batches with direct or indirect involvement of the Kerguelen plume. The available data suggest their possible derivation from the depleted asthenosphere/lithosphere with negligible contribution from the Kerguelen mantle plume. It is likely that mantle plume provided additional heat necessary to melt the asthenosphere/lithosphere. These data also suggest effects of low-pressure crustal contamination, crystal accumulation and fractional crystallization, rather than mantle-derived heterogeneity. These identified magmatic events together with other known magmatic events such as southeastern Tibet, Abor volcanics, SW Australia and eastern Antarctica during ca. 140-100 Ma could be related to the Kerguelen plume and integral part of the Greater Kerguelen Large Igneous Province, and have possible impact on the breakup of East Gondwanaland.
Abstract: Knowledge of the crust and lithospheric structure of the Indian sub-continent primarily comes from several active and passive seismic experiments. These studies are i) controlled source, ii) surface wave studies, iii) receiver functions and v) tomographic studies. The results from these studies in the Indian shield have emanated several interesting features that were hitherto unknown. The peninsular, central and north-western part of the shields, Himalayan and Andaman-Nicobar regions have shown that continental collision and extension from the Proterozoic to Recent time has played an important role in formation and geodynamics of these features. The granulites, in the southern granulite terrain, are formed primarily due to the release of the carbonic fluids from the supracrustal rocks of the subduction zone and volcanic arc environment. These were later exhumed from the deep crust during the collision process. In the central Indian shield the Narmada-Son lineament and the central Indian suture are the main features of the crust. In the Narmada region, mafic intrusion in the upper crust appears to have played an important role in shaping the present structural trends. The Central Indian suture is a collision zone developed due to the interaction of the Bastar and Bundelkhand cratons. In the northwesternpart of the India, the Aravalli-Delhi trend is the controlling feature for the tectonics of the region. Demarcation of the various boundaries between different crustal units are marked across the trend, by changes in the dip direction and steeply dipping reflections, cutting across the nearly horizontal reflections at various depths in the crust. Plate tectonics appears to be responsible for generation of this belt. In the crustal block between the Delhi-Aravalli system and the Narmada-Son Lineament, which is running to the south of the Saurashtra peninsula the crust up uplifted by as much as 4 to 6 km as compared to the regions outside these trends. Apart from the deep crustal structure, lithospheric and upper mantle studies till 660km depth have also been conducted in the entire Indian plate using seismological tools e.g. P-to-s and S-to-p receiver function, surface waves dispersion and tomographic studies. The Himalayan region shows the architecture of the under thrusting Indian plate beneath the Tibetan plate in the north and north-west, while the subduction beneath the Burmese arc has been mapped in the eastern part. Further, a number of studies have been conducted in the Andaman-Nicobar Islands to image the subduction of Indian oceanic plate in order to understand the genesis of earthquakes in these regions.
Mineralogy and Petrology, doi.org/10.1007/s00710-020-00722-y 26p. Pdf
India
lamproite
Abstract: Mesoproterozoic lamproite dykes occurring in the Banganapalle Lamproite Field of southern India show extensive hydrothermal alteration, but preserve fresh spinel, apatite and rutile in the groundmass. Spinels belong to three genetic populations. Spinels of the first population, which form crystal cores with overgrowth rims of later spinels, are Al-rich chromites derived from disaggregated mantle peridotite. Spinels of the second population include spongy-textured grains and alteration rims of titanian magnesian aluminous chromites that formed by metasomatic interactions between mantle wall-rocks and precursor lamproite melts before their entrainment into the erupting lamproite magma. Spinels that crystallised directly from the lamproite magma constitute the third population and show five distinct compositional subtypes (spinel-IIIa to IIIe), which represent discrete stages of crystal growth. First stage magmatic spinel (spinel-IIIa) includes continuously zoned macrocrysts of magnesian aluminous chromite, which formed together with Al-Cr-rich phlogopite macrocrysts from an earlier pulse of lamproite magma at mantle depth. Crystallisation of spinel during the other four identified stages occurred during magma emplacement at crustal levels. Titanian magnesian chromites (spinel-IIIb) form either discrete crystals or overgrowth rims on spinel-IIIa cores. Further generations of overgrowth rims comprise titanian magnesian aluminous chromite (spinel-IIIc), magnetite with ulvöspinel component (spinel-IIId) and lastly pure magnetite (spinel-IIIe). Abrupt changes of the compositions between successive zones of magmatic spinel indicate either a hiatus in the crystallisation history or co-crystallisation of other groundmass phases, or possibly magma mixing. This study highlights how different textural and compositional populations of spinel provide important insights into the complex evolution of lamproite magmas including clues to elusive precursor metasomatic events that affect cratonic mantle lithosphere.
Journal of Geological Society of India, Vol. 96, 3, pp. 239-249. pdf
India
craton
Abstract: Dharwar craton (DC), by far the largest geological domain in South Indian Shield, occupying about 0.5 million sq. km area, is well-studied terrain both for regional geoscientific aspects and as part of mineral exploration over several important blocks such as the greenstone belts, ultramafic complexes, granite-gneissic terrain and the Proterozoic sediments of Cuddapah basin. The re-look into regional gravity data offers several insights into nature of crust, sub-divisions within the craton, bedrock geology in the covered areas and mineral potentiality of this ancient and stable crust. The regional gravity profiles drawn across the south Indian region mainly suggest that the area can be divided into five domains as Western Dharwar craton (WDC), Central Dharwar craton (CDC), Eastern Dharwar craton (EDC 1), transitory zone of EDC (EDC 2) and Eastern Ghats mobile belt (EGMB) areas. The Bouguer gravity anomaly pattern also questions some of the earlier divisions like eastern margin of Chitradurga schist belt between the WDC and EDC and the boundary of DC with southern granulite terrain (SGT) as they do not restrict at these main boundaries. In this study, mainly four issues are addressed by qualitative and quantitative analysis of regional gravity data and those revealed significant inferences. (1) A distinct gravity character in central part of south Indian shield area occupying about 60, 000 sq. km, suggests that the transitory crustal block, faulted on both sides and uplifted. This area designated as central Dharwar craton (CDC) is characterized with schist belts having characters of both parts of western and eastern Dharwar craton. This inference also opens up the debate about the boundary between western and eastern parts of the craton. Another significant inference is the extension of major schist belts beneath both Deccan volcanic province (DVP) in northwestern part and Cuddapah basin (CB) in southeastern part. (2) Eastern Dharwar craton is reflected as two distinct domains of different gravity characters; one populated with number of circular gravity lows and a few linear gravity high closures indicative of plutonic and volcanic activity and another domain devoid of these intrusive younger granites or schist belts. (3) Large wave length gravity highs occupying thousands of sq.km area and those not relatable to surface geology in eastern Dharwar craton that may have significance for mineral exploration. (4) Gravity data was subjected to further processing like two dimensional modeling which have yielded insights into crustal architecture beneath the Dharwar craton, crustal scale lineaments, craton-mobile belt contact zone and younger intrusives.
Abstract: Kl-4 and P-5 mesoproterozoic kimberlite pipes along with several other well-known diamondiferous (ca. 1110 Ma) kimberlites in the Wajrakarur kimberlite field (WKF) intruded into the cratonic roots of Eastern Dharwar craton (EDC) in southern India. The groundmass minerals of the kimberlites exhibit inequigranular texture contain spinel, Ti-rich schorlomite garnet, two generations of olivine (macrocrysts and groundmass microphenocrysts), phlogopite, perovskite, clinopyroxene (diopside), ilmenite (low Mn) and rare apatite. We identified three distinct spinel associations in Kl-4 and P-5: (i) fine-grained (<50 ?m) microcrysts in the groundmass; (ii) resorbed euhedral atoll spinel, consisting of titanomagnetite (magnesian-ulvospinel-magnetite to titanian-chrome-magnetite) which is isolated from the rim of magnetite by spongy lagoon phase of schorlomite, and (iii) larger chrome spinel macrocrysts/xenocrysts (>500 ?m). The schorlomite garnet in both P-5 and Kl-4 represents solid solution of schorlomite-pyrope-almandine-grossular. Additionally, Kl-4 contains another Cr-rich schorlomite-pyrope-almandine-uvarovite solid solution. Macrocrystic spinel exhibits distinct composition of chromium (Cr2O3 up to 59.62 wt%), and poor in TiO2 (<1.19 wt%). The high chromium spinel macrocrysts from Kl-4 are confirmed to be fragments of mantle xenocrysts and their composition falls within the diamond stability field. Atoll-textured epitaxial mantled resorbed spinel associated with schorlomite suggests that they formed through the replacement of spinel possibly through interaction of late residual fluids/melts in the final stages of crystallization of the kimberlite magma. The significant enrichment of Fe and Ti in schorlomite suggests the involvement of metasomatized sub-continental lithospheric mantle. It is also inferred that spinel immiscibility played an important role in the metasomatic replacement. The Ti-rich minerals have orangeitic affinity, similar to those in the Kaapvaal craton of South Africa, and suggest the high Ti-, high Ca- and the low Al-bearing nature of the parent magma (Group II kimberlites). The groundmass tetraferriphlogopite is Al- and Ba-poor and spinel show compositions straddling between magnesian ulvöspinel (Group I kimberlite) and titanomagnetite (Group II kimberlite) comparable with orangeite and lamproites. The results presented in this study suggest that the P-5 and Kl-4 has orengeitic or lamproitic affinity. Our findings can be useful as an indicator mineral in diamond prospecting.
Abstract: The Ongole Domain in the southern Eastern Ghats Belt of India formed during the final stages of Columbia amalgamation at ca. 1600 Ma. Yet very little is known about the protolith ages, tectonic evolution or geographic affinity of the region. We present new detrital and igneous U-Pb-Hf zircon data and in-situ monazite data to further understand the tectonic evolution of this Columbia-forming orogen. Detrital zircon patterns from the metasedimentary rocks are dominated by major populations of Palaeoproterozoic grains (ca. 2460, 2320, 2260, 2200-2100, 2080-2010, 1980-1920, 1850 and 1750 Ma), and minor Archaean grains (ca. 2850, 2740, 2600 and 2550 Ma). Combined U-Pb ages and Lu-Hf zircon isotopic data suggest that the sedimentary protoliths were not sourced from the adjacent Dharwar Craton. Instead they were likely derived from East Antarctica, possibly the same source as parts of Proterozoic Australia. Magmatism occurred episodically between 1.64 and 1.57 Ga in the Ongole Domain, forming felsic orthopyroxene-bearing granitoids. Isotopically, the granitoids are evolved, producing ?Hf values between ? 2 and ? 12. The magmatism is interpreted to have been derived from the reworking of Archaean crust with only a minor juvenile input. Metamorphism between 1.68 and 1.60 Ga resulted in the partial to complete resetting of detrital zircon grains, as well as the growth of new metamorphic zircon at 1.67 and 1.63 Ga. In-situ monazite geochronology indicates metamorphism occurred between 1.68 and 1.59 Ga. The Ongole Domain is interpreted to represent part of an exotic terrane, which was transferred to proto-India in the late Palaeoproterozoic as part of a linear accretionary orogenic belt that may also have included south-west Baltica and south-eastern Laurentia. Given the isotopic, geological and geochemical similarities, the proposed exotic terrane is interpreted to be an extension of the Napier Complex, Antarctica, and may also have been connected to Proterozoic Australia (North Australian Craton and Gawler Craton).
Abstract: We report a 40Ar/39Ar Mesoproterozoic radiometric age for a calc-alkaline lamprophyre dyke from the Mudigubba area towards the western margin of the Cuddapah Basin, Eastern Dharwar Craton (EDC), Southern India. Amphibole phenocryst separates from this lamprophyre yielded a plateau age of 1169 ± 8 Ma (2? ), which is almost 50 million years older than the majority of radiometric dates available for the Wajrakarur field kimberlites which are proximal to this dyke. Bulk-rock Sr-Nd isotopic analyses of the Mudigubba lamprophyre dykes (?Nd(t) between -13.3 and -12.4) reveal their derivation from an old, enriched, continental lithospheric mantle unlike the kimberlites (bulk-rock and perovskite in situ ?Nd(t) between -0.77 and +7.93), which originated either from a chondritic or depleted mantle source. This study provides further evidence for emplacement of compositionally distinct, mantle-derived Mesoproterozoic alkaline magmas in the EDC and highlights the extremely heterogeneous character of the lithospheric mantle beneath this craton.
First occurrence of melilite, potassic richterite and tetraferriphlogopite in Deccan Trap- related alkaline rocks, and its petrogenetic significance: the Rajpuri ijolitenephlinite intrusion, Murud, Mumbai area, India.
Journal of Mineralogy and Geochemistry, https://doi.org/ 10.1127/njma/2020/0236
Abstract: Hydrogen as an atomic impurity in mantle minerals is recurrently proposed as a key element impacting significantly on many mantle properties and processes such as melting temperature and mechanical strength. Nevertheless, interpretation based on the natural samples remains weak as we do not have yet a robust world-wild database for hydrogen concentrations in mantle minerals and rocks. Here, we report the first hydrogen concentrations in nominally anhydrous minerals from a rare selection of ultramafic rocks and minerals embedded in Mesoproterozoic Wajrakarur kimberlites (Eastern Dharwar craton, India). Based on key chemical elements, we demonstrate that olivine, pyroxenes and garnet from the Dharwar craton are of mantle origin. We quantify the hydrogen concentrations using Fourier transform infrared spectroscopy (FTIR) and mineral-specific FTIR calibrations. Calculated hydrogen concentrations are, in average, 18 ppm wt H2O in olivine, 70 ppm wt H2O in orthopyroxene and 207 ppm wt H2O in clinopyroxene. Garnet has highly variable hydrogen concentration ranging from 0 to 258 ppm wt H2O, probably influenced by nano-scale inclusions. The average of clean garnet spectra yields 14.5 ppm wt H2O. The reconstructed hydrogen bulk concentrations of Dharwar peridotites yields ppm wt H2O. This value is two to five times lower than the estimated hydrogen concentration in the lithospheric mantle, and agree well with the lower range of hydrogen bulk concentration from the current data base for the upper mantle minerals transported by kimberlites from other cratons (e.g., South Africa, Siberia). The low hydrogen concentration in mantle minerals, together with petrological and geochemical evidence of carbonated silicate melt metasomatism in Dharwar cratonic lithospheric mantle, suggest that these xenoliths are possibly related to proto-kimberlite melts with low water activity prior to being transported to the surface by the Mesoproterozoic Wajrakarur kimberlites. These observations, valid to a depth of ~165-km, suggest that cratonic lithosphere beneath the Dharwar craton may not be particularly indicative of an abnormal hydrogen-rich southern Indian lithosphere in the late Archean and that hydroxylic weakening in olivine would induced a negligible effect on the mantle viscosity of Indian subcontinent.
Earth and Planetary Letters, Vol. 550, 116549, 13p.
Global, United States, Wyoming, Canada, Northwest Territories, Europe, Baltic, India
geothermometry
Abstract: Cratonic lithosphere is believed to have been chemically buoyant and mechanically resistant to destruction over billions of years. Yet the absence of cratonic roots at some Archean terrains casts doubt on the craton stability and longevity on a global scale. As unique mantle-derived melts at ancient continents, silica-poor, kimberlitic melts are ideal tools to constrain the temporal variation of lithosphere thickness and the processes affecting the lithosphere root. However, no reliable thermobarometer exists to date for strongly silica-undersaturated, mantle-derived melts. Here we develop a new thermobarometer for silica-poor, CO2-rich melts using high-temperature, high-pressure experimental data. Our barometer is calibrated based on a new observation of pressure-dependent variation of Al2O3 in partial melts saturated with garnet and olivine, while our thermometer is calibrated based on the well-known olivine-melt Mg-exchange. For applications to natural magmas, we also establish a correction scheme to estimate their primary melt compositions. Applying this liquid-based thermobarometer to the estimated primary melt compositions for a global kimberlite dataset, we show that the equilibration depths between primary kimberlite melts and mantle peridotites indicate a decrease of up to ?150 km in cratonic lithosphere thickness globally during the past ?2 Gyr. Together with the temporal coupling between global kimberlite frequency and cold subduction flux since ?2 Gyr ago, our results imply a causal link between lithosphere thinning and supply of CO2-rich melts enhanced by deep subduction of carbonated oceanic crusts. While hibernating at the lithosphere root, these melts chemically metasomatize and rheologically weaken the rigid lithosphere and consequently facilitate destruction through convective removal in the ambient mantle or thermo-magmatic erosion during mantle plume activities.
Abstract: Carbonatite melts derived from the mantle are enriched in CO2- and H2O-bearing fluids. This melt can metasomatize the peridotitic lithosphere and liberate a considerable amount of CO2. Experimental studies have also shown that a CO2-H2O-rich fluid can form Fe- and Mg-rich carbonate by reacting with olivine. The Sung Valley carbonatite of NE India is related to the Kerguelen plume and is characterized by rare occurrences of olivine. Our study shows that this olivine is resorbed forsterite of xenocrystic nature. This olivine bears inclusions of Fe-rich magnesite. Accessory apatite in the host carbonatite contains CO2-H2O fluid inclusions. Carbon and oxygen isotopic analyses indicate that the carbonatites are primary igneous carbonatites and are devoid of any alteration or fractionation. We envisage that the forsterite is a part of the lithospheric mantle that was reprecipitated in a carbonatite reservoir through dissolution-precipitation. Carbonation of this forsterite, during interaction between the lithospheric mantle and carbonatite melt, formed Fe-rich magnesite. CO2-H2O-rich fluid derived from the carbonatite magma and detected within accessory apatite caused this carbonation. Our study suggests that a significant amount of CO2 degassed from the mantle by carbonatitic magma can become entrapped in the lithosphere by forming Fe- and Mg-rich carbonates.
Abstract: The mantle beneath the Western Dharwar Craton of the Indian shield comprises a suite of refractory and fertile peridotites and mafic granulites. Detailed petrographic studies coupled with new mineral analysis and geothermobarometric estimations permit to decipher the thermal architecture and get an insight into the evolution of this ancient craton. The refractory rocks are coarse grained harzburgites/dunites, whereas the more fertile ones are at times, porphyroclastic lherzolites. Both show a similar range of equilibration temperatures and pressures indicating intermixing between the two at various levels. The peridotites contain undeformed interstitial REE-enriched clinopyroxene, phlogopite, apatite and carbonates recording post-kinematic modal and cryptic metasomatic events in the Precambrian cratonic lithosphere. Xenoliths of mafic granulite contain layers of clinopyroxenite which also vein the granulite. The P-T range of the granulites overlaps that of the ultramafic rocks. This study in combination with previous investigations reveals a distinct change in the thermal architecture of the craton from a warm/hot geotherm in the Proterozoic to a highly perturbed, still hotter geotherm of the Palaeocene. The Cenozoic thermotectonic rifting episodes heated, refertilized and thinned the bulk of the cratonic lithosphere beneath the Western Dharwar Craton, which has witnessed the most re-activation among cratons of the Indian shield. The waning of the Deccan Traps volcanism in Palaeocene time saw the reworking of ancient cratonic lithosphere and its replacement by non-cratonic, juvenile mantle and magmatic accretions, indicated by compound xenoliths. Differing petrological and geochemical characteristics of refractory xenoliths and fertile lherzolites serve to constrain the relative timing and composition of non-cratonic lithosphere. By the end of the Palaeocene the Western Dharwar Craton was characterised by a thermal high, an attenuated continental lithosphere (60-80 km), and a thin crust (<10- ~ 21 km), reflecting the decratonization of at least the western part of the Western Dharwar Craton.
Episodes, doi.org/10.18814/ epiiugs/2020 /020092 24p. Pdf
India
Craton - Bundelkhand
Abstract: Spatial association of tonalite trondhjemite granodiorites (TTGs) and high-K granitoids (anatectic and hybrid granites) from the Bundelkhand Craton (BC), Central India, is well known. Geochronological data indicates multiple episodes of formation of these high silica rocks showing a spread of ~1 Ga during Paleo to Neoarchaean. In the present study, we try to understand the evolution of TTGs and high-K granitoids (hybrid granites) from the BC using amphibole composition. The amphibole in both TTGs and high-K granitoids (hybrid granites) from the BC are characterised as magmatic, zoned, and calcic in nature. We find that the amphibole composition of the studied rocks is dominated by magnesiohornblende along with less common occurrence of tschermakite, magnesiohastingsite and edenite. Overall variation in amphibole compositions in terms of exchange vectors show a well defined linear trend (except for a late stage low-grade metamorphic readjustment), which suggests melt control over crystallization and evolution of amphibole chemistry. Moreover, the geothermobarometric analysis points towards higher pressure formation of TTGs in comparison to that of high-K granitoids (hybrid granites), with nearly the same temperature conditions in both the cases. Combining all our findings, we propose the evolution of the two considered rock types through lower crustal melting under varying PH2O conditions at different depths of emplacement.
Geochemistry, Vol. 80, doi.org/10.1016 /j.chemer.2019 .125534 11p. Pdf
India
deposit - Amba Dongar
Abstract: Wakefieldite-(Ce,La) and vanadinite in coarse-grained calciocarbonatites (sovites) are for the first time reported from the northeastern part of the worldwide largest fluorite deposit at the Amba Dongar carbonatite ring dike, India. Sovite in this part of the carbonatite ring dike is rich in pyrochlore, calcite and magnetite. Pyrochlore makes up almost 50% of some sovite samples and shows core-to-rim compositional changes. The core of pyrochlore consists of primary fluorcalciopyrochlore with high F and Na contents while the margins gained elevated amounts of Pb, La and Ce with the associated loss of F and Na due to circulation of hydrothermal solutions. The presence of wakefieldite-(Ce,La) and vanadinite points to an exceptionally high V abundance in hydrothermal solutions formed towards the end of the carbonatite magma activity. This investigation thus opens new promising areas for Nb and REE prospection in the eastern part of the Amba Dongar carbonatite body.
Precambrian Research, Vol. 352, doi.org/1016 /j.precamres .2020.105982 15p. Pdf
India
deposit - Wajrakarur
Abstract: Hydrogen as an atomic impurity in mantle minerals is recurrently proposed as a key element impacting significantly on many mantle properties and processes such as melting temperature and mechanical strength. Nevertheless, interpretation based on the natural samples remains weak as we do not have yet a robust world-wild database for hydrogen concentrations in mantle minerals and rocks. Here, we report the first hydrogen concentrations in nominally anhydrous minerals from a rare selection of ultramafic rocks and minerals embedded in Mesoproterozoic Wajrakarur kimberlites (Eastern Dharwar craton, India). Based on key chemical elements, we demonstrate that olivine, pyroxenes and garnet from the Dharwar craton are of mantle origin. We quantify the hydrogen concentrations using Fourier transform infrared spectroscopy (FTIR) and mineral-specific FTIR calibrations. Calculated hydrogen concentrations are, in average, 18 ppm wt H2O in olivine, 70 ppm wt H2O in orthopyroxene and 207 ppm wt H2O in clinopyroxene. Garnet has highly variable hydrogen concentration ranging from 0 to 258 ppm wt H2O, probably influenced by nano-scale inclusions. The average of clean garnet spectra yields 14.5 ppm wt H2O. The reconstructed hydrogen bulk concentrations of Dharwar peridotites yields ppm wt H2O. This value is two to five times lower than the estimated hydrogen concentration in the lithospheric mantle, and agree well with the lower range of hydrogen bulk concentration from the current data base for the upper mantle minerals transported by kimberlites from other cratons (e.g., South Africa, Siberia). The low hydrogen concentration in mantle minerals, together with petrological and geochemical evidence of carbonated silicate melt metasomatism in Dharwar cratonic lithospheric mantle, suggest that these xenoliths are possibly related to proto-kimberlite melts with low water activity prior to being transported to the surface by the Mesoproterozoic Wajrakarur kimberlites. These observations, valid to a depth of ~165-km, suggest that cratonic lithosphere beneath the Dharwar craton may not be particularly indicative of an abnormal hydrogen-rich southern Indian lithosphere in the late Archean and that hydroxylic weakening in olivine would induced a negligible effect on the mantle viscosity of Indian subcontinent.
Abstract: Carbonatite melts derived from the mantle are enriched in CO2- and H2O-bearing fluids. This melt can metasomatize the peridotitic lithosphere and liberate a considerable amount of CO2. Experimental studies have also shown that a CO2-H2O-rich fluid can form Fe- and Mg-rich carbonate by reacting with olivine. The Sung Valley carbonatite of NE India is related to the Kerguelen plume and is characterized by rare occurrences of olivine. Our study shows that this olivine is resorbed forsterite of xenocrystic nature. This olivine bears inclusions of Fe-rich magnesite. Accessory apatite in the host carbonatite contains CO2-H2O fluid inclusions. Carbon and oxygen isotopic analyses indicate that the carbonatites are primary igneous carbonatites and are devoid of any alteration or fractionation. We envisage that the forsterite is a part of the lithospheric mantle that was reprecipitated in a carbonatite reservoir through dissolution-precipitation. Carbonation of this forsterite, during interaction between the lithospheric mantle and carbonatite melt, formed Fe-rich magnesite. CO2-H2O-rich fluid derived from the carbonatite magma and detected within accessory apatite caused this carbonation. Our study suggests that a significant amount of CO2 degassed from the mantle by carbonatitic magma can become entrapped in the lithosphere by forming Fe- and Mg-rich carbonates.
Geological Society, London, Special Publication , 10.1144/SP513- 2020-247 53p. Pdf
India
lamproites
Abstract: We report Mesoproterozoic 40Ar-39Ar (whole-rock) ages of lamproites from (i) the Ramadugu field (R4 dyke : 1434 ± 19 Ma and R5 dyke: 1334 ± 12 Ma) and the Krishna field (Pochampalle dyke: 1439 ± 3 Ma and Tirumalgiri dyke: 1256 ± 12 Ma) from the Eastern Dharwar Craton (EDC) and (ii) the Garledinne (1433 ± 8 Ma) and the Chelima (1373 ± 6 Ma) dykes from within the Paleo-Mesoproterozoic Cuddapah Basin, southern India. The ages reported for the Ramadugu and Tirumalgiri lamproites constitute their first radiometric dates. Ages of the Pochampalle and the Chelima lamproites from this study are broadly comparable to their previously reported 40Ar-39Ar (phlogopite) ages of c. 1500 Ma and 1418 ± 8 Ma, respectively. The ages of all these lamproites are much older than those of the (i) c. 1.1 Ga kimberlites from the Wajrakarur and Narayanpet fields of the EDC and (ii) c. 1.09 Ga lamproitic dykes at Zangamarajupalle which intrude the Cumbum Formation of the Cuddapah Basin. However, the age of the Tirumalgiri lamproite (c. 1256 Ma) is similar to that of the Ramannapeta lamproite (c. 1224 Ma) within the Krishna field. Our study provides evidence for protracted ultrapotassic (lamproitic) magmatism from c. 1.43 to 1.1 Ga over a widespread area (c. 2500 km2) in and around the Cuddapah Basin and the EDC. Implications of the obtained new ages for the diamond provenance of the Banganapalle Conglomerates, the age of the Kurnool Group and for the timing of break-up of the Paleo-Mesoproterozoic supercontinent of Columbia/Nuna are explored.
Mineralogy and Petrology, Vol. 115, pp. 87-112. pdf
India
lamproite
Abstract: Mesoproterozoic lamproite dykes occurring in the Banganapalle Lamproite Field of southern India show extensive hydrothermal alteration, but preserve fresh spinel, apatite and rutile in the groundmass. Spinels belong to three genetic populations. Spinels of the first population, which form crystal cores with overgrowth rims of later spinels, are Al-rich chromites derived from disaggregated mantle peridotite. Spinels of the second population include spongy-textured grains and alteration rims of titanian magnesian aluminous chromites that formed by metasomatic interactions between mantle wall-rocks and precursor lamproite melts before their entrainment into the erupting lamproite magma. Spinels that crystallised directly from the lamproite magma constitute the third population and show five distinct compositional subtypes (spinel-IIIa to IIIe), which represent discrete stages of crystal growth. First stage magmatic spinel (spinel-IIIa) includes continuously zoned macrocrysts of magnesian aluminous chromite, which formed together with Al-Cr-rich phlogopite macrocrysts from an earlier pulse of lamproite magma at mantle depth. Crystallisation of spinel during the other four identified stages occurred during magma emplacement at crustal levels. Titanian magnesian chromites (spinel-IIIb) form either discrete crystals or overgrowth rims on spinel-IIIa cores. Further generations of overgrowth rims comprise titanian magnesian aluminous chromite (spinel-IIIc), magnetite with ulvöspinel component (spinel-IIId) and lastly pure magnetite (spinel-IIIe). Abrupt changes of the compositions between successive zones of magmatic spinel indicate either a hiatus in the crystallisation history or co-crystallisation of other groundmass phases, or possibly magma mixing. This study highlights how different textural and compositional populations of spinel provide important insights into the complex evolution of lamproite magmas including clues to elusive precursor metasomatic events that affect cratonic mantle lithosphere.
Journal of Petrology, Vol. 61, 9, egaa087 23p. Pdf
India
deposit - Wajrakarur
Abstract: The Wajrakarur Kimberlite Field (WKF) on the Eastern Dharwar Craton in southern India hosts several occurrences of Mesoproterozoic kimberlites, lamproites and ultramafic lamprophyres, for which mantle-derived xenoliths are rare and only poorly preserved. The general paucity of mantle cargo has hampered the investigation of the nature and evolution of the continental lithospheric mantle (CLM) beneath cratonic southern India. We present a comprehensive study of the major and trace element compositions of clinopyroxene and garnet xenocrysts recovered from heavy mineral concentrates for three c.1•1?Ga old WKF kimberlite pipes (P7, P9, P10), with the goal to improve our understanding of the cratonic mantle architecture and its evolution beneath southern India. The pressure-temperature conditions recorded by peridotitic clinopyroxene xenocrysts, estimated using single-pyroxene thermobarometry, suggest a relatively moderate cratonic mantle geotherm of 40 mW/m2 at 1•1?Ga. Reconstruction of the vertical distribution of clinopyroxene and garnet xenocrysts, combined with some rare mantle xenoliths data, reveals a compositionally layered CLM structure. Two main lithological horizons are identified and denoted as layer A (?80-145?km depth) and layer B (?160-190?km depth). Layer A is dominated by depleted lherzolite with subordinate amounts of pyroxenite, whereas layer B comprises mainly refertilised and Ti-metasomatized peridotite. Harzburgite occurs as a minor lithology in both layers. Eclogite stringers occur within the lower portion of layer A and at the bottom of layer B near the lithosphere-asthenosphere boundary at 1•1?Ga. Refertilisation of layer B is marked by garnet compositions with enrichment in Ca, Ti, Fe, Zr and LREE, although Y is depleted compared to garnet in layer A. Garnet trace element systematics such as Zr/Hf and Ti/Eu indicate that both kimberlitic and carbonatitic melts have interacted with and compositionally overprinted layer B. Progressive changes in the REE systematics of garnet grains with depth record an upward percolation of a continuously evolving metasomatic agent. The intervening zone between layers A and B at ?145-160?km depth is characterized by a general paucity of garnet. This ‘garnet-paucity’ zone and an overlying type II clinopyroxene-bearing zone (?115-145?km) appear to be rich in hydrous mineral assemblages of the MARID- or PIC kind. The composite horizon between ?115-160?km depth may represent the product of intensive melt/rock interaction by which former garnet was largely reacted out and new metasomatic phases such as type II clinopyroxene and phlogopite plus amphibole were introduced. By analogy with better-studied cratons, this ‘metasomatic horizon’ may be a petrological manifestation of a former mid-lithospheric discontinuity at 1•1?Ga. Importantly, the depth interval of the present-day lithosphere-asthenosphere boundary beneath Peninsular India as detected in seismic surveys coincides with this heavily overprinted metasomatic horizon, which suggests that post-1•1?Ga delamination of cratonic mantle lithosphere progressed all the way to mid-lithospheric depth. This finding implies that strongly overprinted metasomatic layers, such as the ‘garnet-paucity’ zone beneath the Dharwar craton, present structural zones of weakness that aid lithosphere detachment and foundering in response to plate tectonic stresses.
Abstract: We present new key paleomagnetic pole at 13°S, 152°E (k = 21, A95 = 7.8°) for recently identified 1864.4 ± 2.7 Ma (weighted mean age of four Pbsingle bondPb ages) mafic magmatic event, based on a detailed paleomagnetic study of dolerite dykes and sills intruding Archean basement rocks and Tadipatri formation of the Cuddapah basin, Dharwar craton respectively. The Pbsingle bondPb baddeleyite geochronology yields a crystallisation age of 1867.1 ± 1.0 Ma (MSWD = 1.02) for N77°E trending dyke in the southern region to Cuddapah basin. This new age obtained, confirms the presence of ~1864 Ma magmatic episode with a spatial extent of ~400 km in the Eastern Dharwar craton, within the brief period of ~5 Ma. The paleomagnetic results in these dykes revealed reverse polarity magnetisation direction with mean D = 107°, I = 24° (N = 13 sites, ?95 = 10°). Here, we also update the normal polarity magnetic directions on ~1.89 Ga swarm, and the corresponding paleopole situated at 21°N, 336°E (N = 79 sites, A95 = 3.6°). The paleoposition of India is constrained around the equator during ~1.89-1.86 Ga time. The paleogeographic reconstructions were also been attempted at ~1.89 Ga and ~ 1.86 Ga with available key poles from other cratons, indicates the possibility of single plume acting as a source for two distinguishable radial emplacement of mafic dyke swarms across India (Dharwar and Bastar craton) and Western Australia (Yilgarn craton) within a time span of ~35 Ma. The individual movement of India, Baltica and Siberia with a drift rate of ~5.55 cm/yr towards the south, whereas Amazonia craton has moved rapidly to the north (~24.9 cm/yr), do not suggest the amalgamation of a supercontinent (Columbia/ Nuna) during ~1.88-1.86 Ga time.
Abstract: The Deccan volcanic province (DVP) witnessed a massive outpouring of flood basalts of ?2 million km3 volume, at ?65 Ma, in less than a Myr. The volcanic eruption is concomitant with crustal extension, lithospheric thinning and magma influx beneath the major rift systems namely Cambay, Narmada, and Kutch. In this study, we investigate the anisotropic and isotropic variations within the crust and upper mantle beneath the northwestern DVP by estimating the shear wave velocity (VSV, VSH, and VSoigt) and radial anisotropy (?oigt) models using the Surface Wave Tomography technique. A joint inversion of the regionalized Rayleigh and Love wave group velocities is performed, using the genetic algorithm approach. Our results reveal different intracrustal layers, lid, and a low?velocity zone (LVZ). This LVZ comprises of a uniform asthenospheric low?velocity layer (LVL) of average VSV 4.44 km/s and VSH 4.47 km/s, and another LVL below, of average VSV 4.45 km/s and VSH 4.41 km/s. Furthermore, the LVZ represents a negative anomaly with reference to different global models (AK135, STW105, PREM, and S2.9EA). A negative ?oigt is observed in the LVZ, indicating dominance of vertical flow. This could be related to presence of partials melts, volatile materials and/or a thermal anomaly. We also identified the Moho (?34-40 km) and lithosphere?asthenosphere boundary (?84-123 km). The low VS values, negative ?oigt and a thin lithosphere (?84 km) in the vicinity of Gulf of Cambay affirm the presence of a plume head beneath it, in concurrence with the hypothesis of Indian Plate?Reunion plume interaction.
Contributions to Mineralogy and Petrology, 176, 21p. Pdf
India
alkaline rocks
Abstract: Strongly SiO2-undersaturated alkalic rocks (Mg#?>?50, SiO2???45 wt%, Na2O?+?K2O???3 wt%) occur in three early-stage (Sarnu-Dandali, Mundwara, Bhuj) and one late-stage (Murud-Janjira) rift-associated volcanic complexes in the Cretaceous-Paleogene Deccan Traps flood basalt province of India. Thermobarometry based on clinopyroxene-liquid equilibrium suggests that they mostly crystallized beneath the Moho at?~?15 kbar/1270 °C to?~?11-12 kbar/1115-1156 °C pressures and temperatures. Primary magma compositions in equilibrium with lherzolite were estimated through reverse fractionation calculations by incrementally adding equilibrium phases to the rocks in olivine:clinopyroxene:spinel:phlogopite?=?12:68:20:15 proportions at low temperatures followed by olivine:clinopyroxene:spinel?=?12:68:20 proportions at higher temperatures. A comparison of the primary magmas with experimentally generated melts shows that their compositions are consistent with an origin from garnet lherzolite sources with?
Abstract: Carbonatites and associated alkaline rocks are the primary sources for REE mineralization. The Ambadongar Carbonatite Complex (ADCC) from NW Deccan Volcanic Province (DVP) constitutes the largest Carbonatite Associated REE Deposits (CARD) in India. ADCC belongs to the final stages of the Late Cretaceous alkaline-carbonatite magmatism associated with main Deccan basalt volcanic activity. The ADCC is an envisioned diatreme structure in which four carbonatitic phases are recognized, mainly calcio-carbonatites and ferro-carbonatites. Each successive carbonatite phase shows higher REE enrichment. The primary REE mineralization with bastnäsite as the dominant REE phase is hosted by pervasive hydrothermally altered ferro-carbonatite plugs. The secondary mineralogy formed with barites in the main orebody during late- to post-magmatic hydrothermal fluid alteration is fluorite, quartz, ankerite, and other REE-bearing minerals like bastnäsite, parisite, synchysite, strontianite, florencite, monazite and columbite. Carbonatite samples contain 18.61% to 52.42% of CaO, and the LOI varies from 5.28% to 38.79%. Most can be classified as calcio-carbonatites. Since all the samples also contain an appreciable amount of Fe2O3 (4.13% to 20.20%) and MnO (0.07% to 5.46%), some may be classified as ferro-carbonatites. Total REE content varies from 0.6 to 4%, with a high Ce concentration and LREE/HREE ratio. The highest values for La, Ce, Pr, and Nd are 1.95%, 1.56%, 0.16%, and 0.45%, respectively. Metasomatism of SCLM from asthenospheric melts followed by the low degree partial melting of the SCLM region is responsible for fertile carbonatite generation in ADCC. The multiphase liquid immiscibility of carbonatite melts from carbonate-silicate magma followed by immiscibility of REE rich carbonatite melt and REE deficient fluoride-rich aqueous fluids explain the higher level of REE enrichment in each successive phases of carbonatites in ADCC. The mineralizing fluids were probably the result of residual magmatic volatiles that brought mainly REE and later SiO2 into the overprinted rocks. Ambadongar carbonatites' stable isotopic compositions agree with a magmatic origin (?13C = ?4.1 ± 1.9‰ [PDB] and ?l8O = 10.3 ± 1.7‰ [SMOW]). The C-O stable isotopic modeling indicates re-equilibration under hydrothermal conditions between 180 °C and 70 °C. Significant amounts of REE fluorocarbonate minerals, relatively Sr- and Th-rich, were deposited during re-equilibration. The REE fluorocarbonate bastnäsite-(Ce) occurs as late individual crystals, overgrown on the synchysite and parisite polycrystals. Textural and chemical reactions between the REE fluorocarbonates provide insights into rare-earth elements' mobility during fluid-rock interaction. Early crystallization of synchysite/parisite indicates the high activity of Ca2+, OH?, (SO4)2?, Al and Si in the fluid. Later, the fluid was characterized by increased activity of F?, (SO4)2?, REE and Si, and decreased activity of Ca2+ as reflected in the association of barite, fluorite, quartz, and bastnäsite typical of strongly overprinted ferro-carbonatites. Re-equilibration and recrystallization of the primary minerals in the presence of OH?, (SO4)2?, F?, REE, Al, and Si carried in solution by the hydrothermal fluid is the leading cause behind the refixing of REE in the form of REE fluorocarbonate in REE rich ferro-carbonatites.
Journal of Asian Earth Sciences, Vol.213, 104750, 12p.pdf
India
deposit -P12
Abstract: Petrology, bulk-rock geochemistry, and perovskite U-Pb age for the P-12 kimberlite pipe from the Wajrakarur kimberlite field, Eastern Dharwar craton (EDC) of southern India is reported. Perovskites yielded a high-precision U-Pb age of 1122 ± 7.7 Ma, taken to be an emplacement age of the host P-12 kimberlite pipe. The groundmass of coherent facies P-12 kimberlite contains monticellite, clinopyroxene, andradite, atoll spinel with titanomagnetite trend, and perovskite with an elevated REE contents. Phlogopite shows restricted Al2O3 and TiO2 contents. Furthermore, olivines with a wider and higher range of core compositions (i.e. Mg# = 84-94) and multi-granular nodules are the hallmark features of the P-12 pipe. This assorted primary mineral content and its composition indicates the transitional nature of the P-12 towards the Kaapvaal lamproites. However, concentrations of bulk-rock major and trace elements in the P-12 and other Wajrakarur kimberlites are similar to the global hypabyssal magmatic kimberlites. Large ion lithophile and high field strength elements (e.g. Ba and Nb) and their ratios (e.g. La/Nb and Th/Nb) suggest the presence of a heterogeneous and lithosphere influenced mantle source region which have been severely overprinted by metasomatizing fluids/melts emanating from the deep sourced upwelling mantle. The presence of such mixed and metasomatized mantle source regions likely to be an important factor for the transitional nature of the P-12 and other Mesoproterozoic kimberlites. Based on the availability of the newest emplacement ages, we propose a geodynamic model for the origin of kimberlites in the Indian subcontinent. The U-Pb age of 1122 ± 7.7 Ma for the P-12 pipe shows its close temporal association to the emplacement of the recently proposed 1110 Ma Large Igneous Province (LIP), with plume center beneath the NW part of the Kalahari craton. Emplacement of the P-12 and other contemporaneous Indian kimberlites, therefore, marks the impingement of mantle plume which contributed heat and triggered partial melting of metasomatized lithospheric mantle without melt input. The eruption phase of ~ 100 million years (i.e. 1050-1153 Ma), for the kimberlites and related rocks in the Indian shield, does not appear to be continuous and can be separated into several short-durational magmatic events. For this reason, small-volume, volatile-rich magmatism during the Mesoproterozoic time in India is linked to the presence of a number of LIPs and associated mantle plumes during Columbia to Rodinia supercontinent transition and assembly of cratonic blocks of the latter.
Abstract: High-resolution P-wave velocity and anisotropy structure of the hitherto elusive uppermost mantle beneath the Indian shield and its surrounding regions are presented to unravel the tectonic imprints in the lithosphere. We inverted high quality 19,500 regional Pn phases from 172 seismological stations for 4780 earthquakes at a distance range of 2° to 15° with a mean apparent Pn velocity of 8.22 km/s. The results suggest that the Pn velocity anomalies with fast anisotropic directions are consistent with the collision environments in the Himalaya, Tibetan Plateau, Tarim Basin, and Burmese arc regions. The higher Pn anomalies along the Himalayan arc explicate the subducting cold Indian lithosphere. The cratonic upper mantle of the Indian shield is characterized by Pn velocity of 8.12-8.42 km/s, while the large part of the central Indian shield has higher mantle-lid velocity of ~8.42 km/s with dominant anisotropic value of 0.2-0.3 km/s (~7.5%) suggesting the presence of mafic ‘lava pillow’ related to the Deccan volcanism. The impressions of the rifts and the mobile belts are conspicuous in the velocity anomaly image indicating their deep seated origin. The Pn anisotropy in the Indian shield exhibits a complex pattern and deviates from the absolute plate motion directions derived from the SKS study, demonstrating the presence of frozen anisotropy in the Indian lithospheric uppermost mantle, due to the large scale tectonic deformation after its breakup from the Gondwanaland. Whereas, Pn and SKS anisotropic observations are well consistent in Tarim basin, Tibetan regions, eastern Himalayan syntaxis and the Burmese arc. The modeled anisotropic Pn clearly manifests a lower velocity anomaly bounded by 85°E and 90°E ridges in the southern Bay of Bengal. Further, 85°E ridge spatially separates the BoB lithosphere into faster and slower regions consistent with the body wave tomography and free-air gravity observation.
Physics of the Earth and Planetary Interiors, Vol. 315, 106708, 13p. Pdf
India
kimberlites
Abstract: Complex geological structures and processes that took place in the Dharwar craton formation make it difficult to understand the evolution history. 3-D magnetotelluric inversion is a challenging task for the imaging of sub-surface structures. Data at 40 stations in a gridded fashion are used in this study for inversion. A controversy exists regarding the subduction polarity between the eastern and western Dharwar craton. Based on the conductivity anomalies mapped in the sub-surface, the lithosphere can be divided into the shallower and deeper lithosphere. The study delineated several crustal and lithospheric upper mantle conductors. In the crustal region, several conductive features (~10 ?-m) are imaged in the western part, central, and eastern part of the profile. A new finding of this 3-D study is a conductor in the eastern Dharwar craton in the depth range of 65-140 km. The base of this conductor shows the graphite diamond stability field and is correlated with the kimberlites/lamproites present in the region. An uppermost mantle conductor is present at the depth range of 80-200 km in the central part of the study area. Sulphides and carbon-rich fluids could be one cause of the conductors mapped in the crust. The low electrical resistivity imaged in the deeper lithosphere could be due to the refertilization of the mantle scar in the Cretaceous age by the passage of several hotspots. The lithospheric thickness estimated beneath the Dharwar craton in this study is more than 200 km. This study reveals geophysical evidence for the eastward subduction polarity in the Dharwar craton.
Abstract: The eastern Indian shield consists of Archaean Singhbhum Craton and Proterozoic Chhotanagpur Gneissic Complex sandwiching the Singhbhum Mobile Belt. Since the cratonization of the Singhbhum Craton in Archaean, the growth of the eastern Indian shield took place in time and space through tectono-magmatic processes. The stability of cold and thick lithosphere is fundamental to long-term survival of cratons, whereas the geophysical studies have detected the lithosphere-asthenosphere boundary (LAB) under the eastern Indian shield at depths too shallow to be called stable. We analysed the terrestrial Bouguer gravity anomaly, and satellite-based free-air anomaly, geoid undulation, and elevation data to ascertain the 2D lithospheric density structure across the region. Our density model illustrates that the density inhomogeneity exists in the crust across the three tectonic domains of the eastern Indian shield. The derived crustal model shows an upper and lower crustal density variation from 2740 to 2770 kg/m3, and from 2930 to 2940 kg/m3, respectively, and a reasonably smooth Moho at 37-41 km depth. Towards the north, the Moho undulates from 40 to 43 km under the foreland Ganga basin, whereas in the south, it varies from 38 to 30 km under the Eastern Ghats Mobile Belt and lastly moves to ~20 km in the Bay of Bengal. In the southern part of the Singhbhum Craton, an undissipated lithospheric mantle root is found at a depth of ~150 km. Otherwise, the LAB shallows to ~132 km in the northern Singhbhum Craton and Singhbhum Mobile Belt and then thickens to about 135-140 km depth beneath the Chhotanagpur Gneissic Complex. The foreland Ganga basin toward the extreme north is characterized by a more in-depth LAB lying at a depth of over 200 km. The LAB, in the Bay of Bengal, is at a depth of 112-125 km, except for the Kolkata coast (135 km). Moderate crustal density difference in various crustal domains, as well as an almost smooth crust-mantle boundary at 37-40 km depth, suggests the effect of substantial mafic-ultramafic crustal intrusion and together with the thin (135-140 km) lithosphere reinforces the evidence of thermo-chemical processes that controlled the lithospheric modification in the eastern Indian shield.
The International Achives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol. XLII-5 12p. Pdf
India, Madhya Pradesh
ASTER, lineament
Abstract: In the present study, we have prepared the thematic evidence layers for identifying the potential zones of kimberlite emplacement in parts of Chhatarpur district, Madhya Pradesh. These thematic layers or evidence layers are geological structure, alteration zones, lineament density, surface alteration and geomorphic anomaly and these layers are prepared from the remote sensing data. As orientation of the geological structures (i.e fault system) and their density have the major role in the emplacement of kimberlite; both of these evidence layers are integrated using "AND" Boolean Logical Operator. On the other hand, two evidential layers regarded as the proxy to indicate the "surface expressions on kimberlite (i.e. alteration zones and geomorphic anomaly) are combined using "OR" operator as either of these two surface expression is indicative of kimberlite. Consequently, conjugate evidence layers on the surface expressions of kimberlite are integrated with the causative evidence layers of kimberlite emplacement using "AND" operator to identify the potential zones of diamond occurrences. Potential zones of kimberlite are overlaid on the residual gravity anomaly map derived from space-based gravity model of European Improved Gravity of Earth by New Technique (EIGEN6C4) to relate potential zones of kimberlite with the similar structural alignment (delineated in the residual gravity map) of known occurrence of kimberlite. We also have carried out indicator mineral survey around these potential zones and some of the kimberlite specific indicator minerals are identified in the stream sediments within these potential zones.
Journal of the Geological Society of India, Vol. 97, pp. 567-570.
India
deposit - Kalyandurg
Abstract: The paper pertains to the studies carried out on the eclogitic xenoliths of KL-2 kimberlite of Kalyandurg kimberlite cluster in south India. Petrographic studies revealed bi-mineralic and kyanite-bearing eclogitic xenoliths in KL-2 kimberlite. The bimineralic and kyanite-bearing eclogites of Kalyandurg KL-2 kimberlite pipe show variation in modal proportion of garnet, omphacite, clinopyroxene and kyanite. The paper reports discovery of native gold grains and Au-Pt alloy in the kyanite-bearing eclogite xenoliths of KL-2 kimberlite. The flaky gold grains occurring in the matrix of kyanite-bearing eclogite are homogeneous and two grains of Au-Pt alloy with Au and Pt in the proportion of 9.8:1.2 are also present. This is the first report of gold and gold-platinum alloy specs from eclogitic xenoliths of Indian kimberlites.
Journaof Mineralogical and Petrological Sciences, Vol. 116, pp. 121-128 pdf
India
deposit - Newania
Abstract: This study presents first report of the sulfur isotopic compositions of carbonatites from the Mesoproterozoic Newania complex of India along with their stable C and O isotope ratios. The ?34SV-CDT (?1.4 to 2‰) and ?33S (?0.001 to ?0.13‰) values of these carbonatite samples (n = 7) overlap with the S isotope compositions of Earth’s mantle. Additionally, the ?13CV-PDB and ?18OV-SMOW values of these carbonatites also show overlapping compositions to that of Earth’s mantle. Based on these mantle-like stable isotopic compositions of carbonatites along with their higher crystallization temperature (~ 600 °C) compared to a hydrothermal fluid (<250 °C), we suggest that the sulfide minerals in these carbonatites were formed under a magmatic condition. The mantle like signatures in the ?34S, ?13C- ?18O, and 87Sr/86Sr values of these carbonatites rule out possible crustal contamination. Coexistence of the sulfide phase (pyrrhotite) with magnesite in these carbonatites suggests that the sulfide phase has formed early during the crystallization of carbonatite magmas under reducing conditions. Overall restricted variability in the ?34S values of these samples further rules out any isotopic fractionation due to the change in the redox condition of the magma and reflect the isotopic composition of the parental melts of the Newania carbonatite complex. A compilation of ?34S of carbonatites from Newania and other complexes worldwide indicates limited variability in the isotopic composition for carbonatites older than 400 Ma, which broadly overlaps with Earth’s asthenospheric mantle composition. This contrasts with the larger variability in ?34S observed in carbonatites younger than 400 Ma. Such observation could suggest an overall lower oxidation state of carbonatite magmas emplaced prior to 400 Ma.
Abstract: The Singhbhum craton is among the five Archean cratons of Peninsular India that preserves some of the oldest continental nuclei. In this work, we present a new and complete Bouguer gravity map of this craton with insights into its deep crust-mantle structure, lithospheric thickness and density variations beneath this craton. The conspicuous presence of high-order residual gravity low anomalies, together with low estimated densities, suggests voluminous presence of Singhbhum granitic batholiths that built the dominant crustal architecture. The isolated residual gravity highs correspond to the mafic and ultramafic volcanic suites like, Dhanjori, Simlipal and Dalma, while the relatively low gravity anomalies observed over the western volcanic suites like Malangtoli, Jagannathpur and Ongarbira, indicate their relatively felsic nature. The estimated lithospheric thickness of about ~ 130 km below the granitic batholithic region, and about 112 km beneath the Precambrian volcanic terranes, together with low effective elastic thickness (Te,) of only about 31 km, suggest a thin and weak lithosphere. The craton witnessed extensive lithospheric destruction with the removal of nearly 100-150 km of the cratonic root. The decratonization may be linked to subduction during the Paleo-Mesoproterozoic period, together with mantle plumes at different times, suggesting a combined mechanical, thermal and chemical erosion of the cratonic keel.
Abstract: Rio Tinto had been developing a diamond mining project in Madhya Pradesh for a decade when in 2017 it hastily abandoned the project. We analyse this counterintuitive exit through an ethnographic approach nested within a qualitative case study framework. We argue that the exit was caused by multi-scalar politics. Local protests over livelihood and labour issues -pre-emptively rearticulated by regional civil society groups through an ecological ‘framing’ - led to litigation. The national forest bureaucracy posed regulatory hurdles, and a change in the national political regime in 2014 brought to power a party that leveraged national capital of a certain variety, which weakened Rio Tinto’s political position. Lastly, a slump in the global diamond market created economic uncertainties, finally leading to its exit. It has not, however, deterred the government from facilitating investment by Indian mega-corporate houses in mining diamonds, once again ignoring local dissent. Under the current regime in India, the space for activism is increasingly restricted, and that restriction, we contend, can lead to the disarray in strategising alliances and goals between ecological and social justice concerns.
Abstract: A 150-200 km thick, cold (35-45 mWm?2), melt-depleted lithospheric keel characterised the eastern cratons of the Indian shield at the end of the Precambrian. Differing chemical- and isotopic-characteristics, and ages of the crust and mantle rocks reveal the decoupling of the crust and mantle beneath the cratons, beginning at 2.45 Ga, in the Bastar craton. The Pan-African event was more pervasive and brought about widespread reworking in most of the cratons of the shield. Major-, trace- and rare-earth elements combined with Sr, Nd and Hf isotope data suggest a heterogenous SCLM beneath southern India. The trace element signatures of xenoliths and the presence of majoritic garnet inclusions in diamond suggest that some kimberlites were derived from the mantle transition zone. Mesoproterozoic (1.2-1.4 Ga) modal and cryptic refertilisation by asthenosphere-derived, low-degree carbonated melts led to the generation of the fluids responsible for the metasomatic transformation of the source rocks. The western craton of the shield has witnessed more severe reactivation than the eastern due to the frequent interaction of the Indian plate with mantle plumes. One plume caused major igneous activity during the late Cretaceous, synchronous with crustal attenuation, rifting and the ridge-jump at 66 Ma, in the Indian Ocean. By the end of the Palaeocene the geotherm of the western craton had risen from 50 to 55 mWm?2 in the Proterozoic to a peak 80-90 mWm?2. This increase in heat flow not only modulated the mantle thermal regime, but led to a net loss of more than 100 km of lithosphere and to destabilisation of the craton. After this thermal event, the lithosphere preserves a thickness of barely 60-80 km, and a thin crust (10-21 km) beneath the continental margin in the west. These changes decratonized the western part of the shield and the transitional region further west in the Indian Ocean where the continental ridges are almost devoid of crustal sections and the lithosphere is ~60 km thick. The waning of the Deccan Traps (65 Ma) magmatism was marked by alkaline intrusive activity along the western margin of the shield, probably derived from the SCLM in response to the rise of the mantle plume. Low degree (2-3%) partial melting of a modally and cryptically metasomatized source may have been involved in the generation of alkaline magmas from a depleted mantle source variously contaminated by an enriched endmember.
Abstract: Pyrochlore-group minerals are common in the Neoproterozoic Sevattur carbonatite complex. This complex is composed of dolomite-, calcite-, banded- and blue carbonatite together with pyroxenite, albitite and diverse syenites. This work reports the paragenetic-textural types and compositional variation of pyrochlore hosted by dolomite carbonatite, banded carbonatite and albitite together with that of alteration assemblages containing belkovite and baotite. On the basis of composition, five different types of pyrochlore are recognised and termed Pcl-I through to Pcl-V. The Pb-rich Pcl-I are present exclusively as inclusions in U-rich Pcl-IIa in dolomite carbonatite. The alteration assemblages of Pb-poor Pcl-IIb + Ba-rich or Ba-Si- rich Pcl-IV + belkovite (dolomite carbonatite) and Si-rich Pcl-V + baotite (banded carbonatite) formed after Pcl-IIa differ in these carbonatites. The albitite hosts extremely U-Ti-rich Pcl-III, mantled by Ba-rich potassium feldspar. In common with the banded carbonatite, Pcl-V is formed by alteration of Pcl-III where this mantle is partially, or completely broken. The Ba-Si-enrichment of Pcl-IV and Pcl-V together with the ubiquitous presence of baryte in all Sevattur lithologies suggests late-stage interaction with a Ba-Si-rich acidic hydrothermal fluid. This fluid was responsible for leaching silica from the associated silicates and produced Pcl-V in the silicate-rich lithologies of the banded carbonatite and albitite. The absence of Pcl-V in dolomite carbonatite is a consequence of the low modal abundance of silicates. The complex compositional diversity and lithology specific pyrochlore alteration assemblages suggest that all pyrochlore (Pcl-I to Pcl-IV) were formed initially in an unknown source and transported subsequently in their respective hosts as altered antecrysts.
Abstract: The Neoproterozoic Sevattur complex is composed essentially of calcite and dolomite carbonatites together with pyroxenites and diverse syenites. This work reports the compositions and paragenesis of different pyrochlore generations hosted by albitite veins in this complex. The pyrochlore are distinctive, being exceptionally rich in uranium (26 to 36 wt.% UO2). Five types of pyrochlore (Pcl-I to Pcl-V) are recognised on the basis of composition and texture. With the exception of Pcl-V, the majority of the pyrochlore (Pcl-II to Pcl-IV) are surrounded by a thick orbicular mantle of Ba-rich potassium feldspar. This mantle around Pcl-V is partially-broken. Pcl-I is restricted to the cores of crystals, and associated with Pcl-II and -III and is relatively rich in Nb (0.53-0.62 apfu) together with more A-site vacancies (0.37-0.71 apfu) compared to Pcl-II to Pcl-IV. Other pyrochlore (Pcl-II to Pcl-IV) are characterised by elevated Ca and Ti compared to Pcl-I, which are related to the (3Nb5+ + Na+ ? 3Ti4+ + U4+) and (2Nb5+ ? 2Ti4+ + Ca2+) substitutions, respectively. These substitutions represent replacement of Pcl-II to Pcl-IV. Alteration and Ba-enrichment in all the pyrochlore are marked by interaction with an externally-derived Ba-rich hydrothermal fluid following the (2Nb5+ ? 2Ti4+ + Ba2+) substitution. This substitution, coupled with extensive metamictisation leads to the formation of Ba-rich (15.9-16.3 wt.% BaO) patchy-zoned Pcl-V. The orbicular mantles around Pcl-I to Pcl-IV have prevented extensive metamictisation and extensive secondary alteration compared to Pcl-V, where mantling is partially disrupted. The compositional and textural variation suggests that Pcl-II to Pcl-IV form by nucleation on Pcl-I, and are transported subsequently as antecrysts in the host albitite.
Abstract: Pyrochlore-group minerals are common accessory rare-metal bearing minerals in the calcite and ankerite carbonatites of the Amba Dongar complex (India). Pyrochlore from the Amba Dongar carbonatites differs from that in other Indian complexes in Ta, Zr, Ti, rare earth element (REE) and Pb contents, but is similar with respect to Ca, Ba and Sr abundances. The evolution of pyrochlore composition was studied to understand the alteration processes and the formation of late-stage pyrochlores enriched in REE and Pb. The early magmatic pyrochlore are calcio- and niobium-dominant types and were replaced by secondary cation-deficient varieties as a consequence of the action of hydrothermal fluids and supergene weathering. These processes produce changes mainly at the A site, rarely at the B site, and the original F is replaced by OH- groups. Calcium and Na can be extracted from the structure at the alteration stage and charge balance is achieved by the introduction of REE, Th, U, Ba or Sr. At the latest supergene stages, marginal and fractured zones of pyrochlore grains are altered to Pb-rich, Si-rich and cation-deficient hydrated varieties. The magmatic pyrochlore was crystallised in a highly alkaline environment at a high activity of Ca and at temperatures near 600°C, the alteration of pyrochlore began in a hydrothermal environment at temperatures below 350°C. The major compositional changes that are associated with the alteration are summarised by the following reactions: Ca2+ + Nb5+? REE3+ + Ti4+; Nb5+ + Fe3+ ? Ti4+ + Zr4+; and 2Nb5+ + Ca2+ ? Ti4+ + Si4+ + U4+.
Transactions Indian Institute of Metallurgy, Vol. 74, 8, pp. 1969-1977. 9p. Pdf
India
deposit - Panna
Abstract: During diamond mining at Panna Mines [India], a huge amount of tailing (Kimberlite) is generated. About 6 Million Tonnes is accumulated and stored near mine site and about 0.9 Million Tonnes of tailings are further being generated each year. Till now, no established method is available for utilization of this material, effective utilization is essential for sustainable diamond mining. The Kimberlite contains about 32% [MgO?+?CaO] and 2% Al2O3, and rest are primarily silica, iron oxide [Fe2O3] and LOI. Initially, attempts were made to utilize Kimberlite as flux in iron ore pellet making; however, during green pellet making, it was observed that strength of pellets improved with higher dosages of Kimberlite. Accordingly, further studies were taken to replace conventional binder bentonite with binder prepared from Kimberlite. The iron ore pellet produced with Kimberlite-based binder have exhibited better physical and metallurgical characteristics than pellets produced with conventional binders (Bentonite).
Indian Journal of Science and Technology, 12p. Pdf
India
Craton - Dharwar
Abstract: Objectives: To map the kimberlite pipes emplaced in parts of Anantpur District, India using Landsat-8 OLI multispectral data. Kimberlite are considered as the primary host of natural diamond. Kimberlite pipes have very limited exposure and are altered, therefore the indirect surface indicators associated with kimberlite such as ferric iron bearing minerals (hematite, goethite), hydroxyl (clay) and carbonate (calcrete) minerals, were mapped to trace kimberlite pipe. Methods: Feature based Principal Component Analysis (FPCA) was applied over the OLI bands 2, 4, 5 and 6, and 2, 5, 6 and 7 to generate ferric iron (F image) and hydroxyl/carbonate image (H/C images). The color composite was generated by assigning RGB colours to F, H/C and F+H/C images. Findings: When matched with the pre-explored kimberlite pipe locations, it was observed that the kimberlitic pipes display different colours in the above colour composite. Hence, the Isodata clustering was carried out to segregate the classes, which resulted in 12 unique classes. Of these, the kimberlite pipes fall in 4 classes. However, due to the moderate resolution of OLI, false positive areas were also noted. Further the target area was found to be reduced by incorporating the structural control (lineament) over the emplacement of Kimberlite pipes. Novelty: The present work highlights the usefulness of the moderate resolution multispectral image in mapping the Kimberlite pipes in semiarid region, in absence of a hyperspectral sensor.
Third World Quarterly, Vol. 42, 8, pp. 1770-1787. pdf
India
economics
Abstract: Rio Tinto had been developing a diamond mining project in Madhya Pradesh for a decade when in 2017 it hastily abandoned the project. We analyse this counterintuitive exit through an ethnographic approach nested within a qualitative case study framework. We argue that the exit was caused by multi-scalar politics. Local protests over livelihood and labour issues -pre-emptively rearticulated by regional civil society groups through an ecological ‘framing’ - led to litigation. The national forest bureaucracy posed regulatory hurdles, and a change in the national political regime in 2014 brought to power a party that leveraged national capital of a certain variety, which weakened Rio Tinto’s political position. Lastly, a slump in the global diamond market created economic uncertainties, finally leading to its exit. It has not, however, deterred the government from facilitating investment by Indian mega-corporate houses in mining diamonds, once again ignoring local dissent. Under the current regime in India, the space for activism is increasingly restricted, and that restriction, we contend, can lead to the disarray in strategising alliances and goals between ecological and social justice concerns.
Journal of the Geological Society of India, Vol. 97, 10, pp. 1169-1189.
India
geophysics - seismics
Abstract: The article reviews the history and accomplishments of CSIR-NGRI over the past 60 years, related to elucidating the seismic structure of the crust and lithospheric mantle of the Indian shield. Extensive investigations have been carried out in diverse geological and tectonic provinces of India, employing seismic reflection, refraction/wide-angle reflection and passive seismology to decipher (a) the evolution of the Indian plate through geological time, (b) hazard and its mitigation and (c) accumulation and disposition of natural resources. These endeavours entailed the application and development of state-of-the-art methodologies. Synthesis of the results from active and passive seismology reveals that the thickness of the crust varies between 28 and 65 km in the Kachchh and Aravalli regions respectively, consistent with their evolutionary histories. The thickest crust is observed in the western Dharwar craton (WDC) and the shallowest lies in the west coast. The crust in the shield region is mostly thicker, while it is thin beneath the rift zones. Results from coincident reflection and wide-angle seismic reflection studies broadly suggest a three-layered crust with magmatic underplating. Interestingly, the seismic sections traversing the Aravalli fold belt, central Indian suture zone, Dharwar craton and Southern Granulite Terrain (SGT) depict paleo-collision and subduction environments. The diverse character of the Moho, crustal fabrics and structure in different geological provinces indicate that contrasting tectonic environments might have influenced their evolution and support the hypothesis that plate tectonic processes were operative since Neoarchean. The thickness of the lithosphere estimated from receiver functions varies from 80 to 140 km. An undulation in the Lithosphere Asthenosphere Boundary reveals evidence for a flexure on a regional scale, owing to the continental collision of the Indian and Asian plates. However, the lithospheric thickness derived from surface wave dispersion studies is somewhat larger, ranging from 100 to 250 km, with some body wave tomographic studies suggesting it to be ?400 km, in consonance with the concept of Tectosphere. The thickness values derived from both the methods agree at a few locales such as the Eastern Dharwar Craton, SGT, Cambay, Singhbhum and western DVP. However, a broad disagreement prevails in WDC and northern part of the Indian shield where surface wave tomography reveals the thickness of lithosphere to be 140 to 200 km.
Abstract: The Hope diamond is perhaps the most famous jewel in the world. In addition to its illustrious royal provenance, it is a rare marvel of nature due to its size, blue color and red phosphorescence. Follow Senior Manager of Diamond Identification Dr. Sally Magaña as she delves through both the history and recent scientific discoveries surrounding this gem.
Abstract: Olivine is one of the most abundant phases in kimberlites and cratonic lamproites, where it occurs as mantle-derived xenocrysts and magmatic phenocrysts or rims overgrowing xenocrystic cores, indicating its prevalence throughout most of the crystallisation sequence of these magmas. Thus, olivine can provide valuable insights into kimberlite and lamproite petrogenesis. Here, we present a detailed study of olivine compositional zoning in two lamproites (P2 and P12) of the Mesoproterozoic Wajrakarur kimberlite-lamproite field in southern India and use these data to propose a genetic link between lamproites and kimberlites in the region. Olivine macrocrysts (i.e., anhedral grains >1 mm) from the P2 and P12 intrusions are strongly zoned. Comparisons with olivine from mantle xenoliths worldwide demonstrate that the cores of olivine macrocrysts are xenocrysts derived from disaggregated mantle wall-rocks. The internal zones and overgrowth rims of olivine macrocrysts and the cores of olivine phenocrysts from P2 and P12 contain magmatic Mg-chromite and Ti-magnetite inclusions and hence crystallized from the host lamproite melt. These magmatic olivine zones show increasing Mg# (molar Mg/(Mg + Fe2+)), CaO and MnO contents with decreasing NiO. This reverse differentiation trend appears to be a characteristic feature of olivine in lamproites from the Wajrakarur field. To evaluate potential petrogenetic links between coeval lamproites and kimberlites from Wajrakarur, the composition of olivine xenocrysts (i.e., macrocryst cores) was compared with that of early crystallized olivine in P2, P12 and previously studied kimberlites and lamproites. The average Mg# of olivine macrocryst cores is directly correlated with the average Mg# of magmatic olivine in lamproites and kimberlites from Wajrakarur. Coupled with their indistinguishable Sr-Nd-Hf isotope compositions, these data suggest derivation of the Wajrakarur lamproites and kimberlites from a common source, The more Fe-rich composition of liquidus olivine in the Wajrakarur lamproites compared to coeval kimberlites suggests a higher degree of assimilation of metasomatised Fe-richer lithospheric mantle by the lamproites and provides a plausible explanation for the different petrological features of the Wajrakarur lamproites and kimberlites Our results suggest that cratonic lamproites can have a remarkably similar petrogenetic history to kimberlites.
Raghuvanshi, S., Sharma, A., Talukdar, D., Chalapathi Rao, N.V.
Chrome-diopside xenocrysts entrained in a Neoproterozoic lamprophrye dyke from the Mysuru area: their origin and implications or lithospheric thickness beneath the Western Dharwar craton, southern India.
Journal of Geological Society of India, in press available 12p. Pdf
International Journal of Sustainable Engineering, Vol. 14, 5, pp. 1269-1285. pdf
Global, India
markets
Abstract: Sustainable supply chain management has become one of the significant areas of concern for modern industries. Enterprises are now adopting management that implements viable practices involving environmental protection and financial savings in a combined form. In this aspect, this study focuses on detecting various concerns associated with sustainable supply chain management in the diamond mining industry globally. These parameters are classified based on their dependency and driving power (DP) with the help of fuzzy MICMAC analysis. In addition to this, a structural model of the recognised concerns has been established using the interpretive structural modelling technique. Furthermore, the interdependence among the respective concerns have been identified by utilising the decision-making trial and evaluation laboratory (DEMATEL) approach. Also, an integrated ISM-DEMATEL model has been employed to form an evident understanding of these concerns. The findings of this study illustrate that ‘Awareness Programmes’ and ‘Proper Infrastructure Investment’ should be given due consideration to ensure a sustainable competitive advantage.
Journal of Asian Earth Sciences, in press available 10.1016/j.jseaes.2021.105092 99 p. Pdf
India
geophysics - seismics
Abstract: The Dharwar Craton of southern India is an important stable cratonic province of the world with complex geology and tectonic settings. Extensive studies provide insights of crustal velocity structure for the tectonic and geodynamic evolution of this Archean craton. This region has experienced several tectonically disturbed zones like Chitradurga Shear Zone (CSZ), Bababudan Shear Zone (BSZ) and Closepet Granites (CG). We have developed a comprehensive geologically plausible tectonic model using both P- and S-wave velocity structures to image major structural elements like shear zones and decipher the compositional distinctions of different rock assemblages of Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC) part using 3-C wide-angle seismic data acquired along the 200-km long Perur-Chikmagalur deep seismic profile. The tectonic model show large compositional changes of subsurface rocks with anomalous high , , , Poisson’s ratio () and density () forming a major tectonic divide or suture zone called CSZ between EDC and WDC blocks. Significant crustal thinning (37-41 km) is observed due to Moho upwarping towards the Neo-Archean EDC block mainly composed of felsic granites and granodiorites. The WDC block show relatively thick crust (48-50 km) due to mafic underplating and mantle plume activity below CSZ forming Meso-Archean greenschist-facies-gneisses with dominant mafic/ultra-mafic compositions. Hence, crustal velocity, density, heat-flow, geology and geochronology studies support a plume-arc model with evidence of thick magmatic underplating of the lower-crust, complex subduction and development of highly strained shear zones like CSZ as suture juxtaposing EDC and WDC blocks.
Journal of the Geological Society of India, Vol.98, pp. 35-40.
India
carbonatite
Abstract: A new carbonatite body has been discovered from Gundlupet area, western Dharwar craton, southern India which is located at juncture of major shear zones namely, Kollegal shear zone to the east, Sargur shear zone to the west and Moyar shear zone to the south. The carbonatite and associated syenite have intruded into the peninsular gneissic complex. The southern margin of the syenite has a tectonic contact with the peninsular gneissic complex suggesting their emplacement is related to the splay shear of Moyar shear zone. The Gundlupet carbonatite is dominantly sövite with minor beforsite and iron rich carbonatite which are associated with phenocrystic magnetite, apatite, amphibole, pyroxene and monazite. Fenitisation is observed in local scale along the contact of carbonatite and syenite where metasomatic alterations took place to give rise to alkali amphibole and pyroxene rich rock. Geochemically, the carbonatite is characterised by high CaO content (48.86%-51.80%), P2O5 (0.35%-3.23%) and low SiO2 (3.09%-5.30%). The high Sr (5750-13445 ppm) content and low Ni, Cr, Zn and Cu content indicates that the melt has undergone some degree of fractionation before crystallization. Gundlupet carbonatite is enriched in LREE with values ranging from 5666 ppm to 7530 ppm and average LREE of 6248 ppm.
Geological Society of London Special Publication 513, pp. 179-207.
India
minette
Abstract: The lamproites and kimberlites are well known from the Eastern Bastar Craton, Central India. However, a Proterozoic lamprophyre dyke is discussed here, from the Western Bastar Craton (WBC). The field geology, petrographic, mineralogical and whole-rock and in-situ trace element geochemistry of biotite are described to understand the petrogenesis and lithospheric evolution in the WBC. The Thanewasna lamprophyre (TL) is undeformed and unmetamorphosed, intruded into c. 2.5 Ga charnockite and metagabbro but closely associated with c. 1.62 Ga undeformed Mul granite. The TL has a characteristic porphyritic texture, dominated by phenocrysts of biotite, microphenocryst of amphibole, clinopyroxene and a groundmass controlled by feldspar. Mineral chemistry of biotite and amphibole suggest a calc-alkaline (CAL) type, and pyroxene chemistry reveals an orogenic setting. The TL is characterized by high SiO2 and low TiO2, MgO, Ni and Cr, consistent with its subcontinental lithospheric origin. The presence of crustal xenolith and ocelli texture followed by observed variations in Th/Yb, Hf/Sm, La/Nb, Ta/La, Nb/Yb, Ba/Nb indicate substantial crustal contamination. Whole-rock and in-situ biotite analysis by laser ablation inductively coupled plasma mass spectrometry show low concentrations of Ni (30-50 ppm) and Cr (70-150 ppm), pointing to the parental magma evolved nature. Enrichment in H2O, reflected in magmatic mica dominance, combined with high large ion lithophile element, Th/Yb ratios, and striking negative Nb-Ta anomalies in trace element patterns, is consistent with a source that was metasomatized by hydrous fluids corresponding to those generated by subduction-related processes. Significant Zr-Hf and Ti anomalies in the primitive mantle normalized multi-element plots and the rare earth element pattern of the TL, similar to the global CAL average trend, including Eastern Dharwar Craton lamprophyres. Our findings provide substantial petrological and geochemical constraints on petrogenesis and geodynamics. However, the geodynamic trigger that generated CAL magmatism and its role in Cu-Au metallogeny in the WBC, Central India, is presently indistinct in the absence of isotopic studies. Nevertheless, the lamprophyre dyke is emplaced close to the Cu-(Au) deposit at Thanewasna.
Geological Society of London Special Publication 513, pp. 157-178.
India
lamproites
Abstract: We report Mesoproterozoic 40Ar-39Ar (whole-rock) ages of lamproites from (i) the Ramadugu field (R4 dyke : 1434 ± 19 Ma and R5 dyke: 1334 ± 12 Ma) and the Krishna field (Pochampalle dyke: 1439 ± 3 Ma and Tirumalgiri dyke: 1256 ± 12 Ma) from the Eastern Dharwar Craton (EDC) and (ii) the Garledinne (1433 ± 8 Ma) and the Chelima (1373 ± 6 Ma) dykes from within the Paleo-Mesoproterozoic Cuddapah Basin, southern India. The ages reported for the Ramadugu and Tirumalgiri lamproites constitute their first radiometric dates. Ages of the Pochampalle and the Chelima lamproites from this study are broadly comparable to their previously reported 40Ar-39Ar (phlogopite) ages of c. 1500 Ma and 1418 ± 8 Ma, respectively. The ages of all these lamproites are much older than those of the (i) c. 1.1 Ga kimberlites from the Wajrakarur and Narayanpet fields of the EDC and (ii) c. 1.09 Ga lamproitic dykes at Zangamarajupalle which intrude the Cumbum Formation of the Cuddapah Basin. However, the age of the Tirumalgiri lamproite (c. 1256 Ma) is similar to that of the Ramannapeta lamproite (c. 1224 Ma) within the Krishna field. Our study provides evidence for protracted ultrapotassic (lamproitic) magmatism from c. 1.43 to 1.1 Ga over a widespread area (c. 2500 km2) in and around the Cuddapah Basin and the EDC. Implications of the obtained new ages for the diamond provenance of the Banganapalle Conglomerates, the age of the Kurnool Group and for the timing of break-up of the Paleo-Mesoproterozoic supercontinent of Columbia/Nuna are explored.
Journal of Asian Earth Sciences, in press available 10.1016/j.jseaes.2021.105701 46 p. Pdf
India
Craton
Abstract: The lithosphere-asthenosphere boundary (LAB) is a fundamental element of the plate tectonic hypothesis that accommodates the differential motion of rigid lithosphere over the weaker asthenosphere. In recent times, various usages have been used to define the LAB, depending on the nature of their measurements. Here, we investigate the lithospheric structure beneath the Eastern Dharwar Craton (EDC) of the Indian Shield using geochemical, thermal and seismological data sets. We analysed S-receiver functions from the stations deployed in the EDC along with the surface wave dispersion tomography. We also added thermal measurements from 5 different locations and geochemical data from 34 Kimberlite/Lamproite xenolith samples to constrain the nature of the LAB. The seismological measurements using Rayleigh wave dispersion and receiver function analysis indicate the lithospheric thickness of 98-118 and 94-118 km respectively, with sharp transition across the LAB. The P-T results from xenoliths are interpreted in concurrence with the heat-flow measurements suggesting a thick thermal lithosphere of ?200 km for the normal mantle solidus with cold geotherm. To reconcile our observations, we invoke partial melts or enriched in volatiles, which significantly lowers the viscosity of mantle rocks inducing a zone of weakness between the rigid lithosphere (?125km) and the convective asthenosphere. Further, we favour the view that the thick lithosphere beneath the Indian plate has been thinned by a plume during the Gondwanaland breakup at ?130Ma. The presence of younger kimberlites from the Indian shield support that it is further degenerated by the delamination leading to an uneven topography in the LAB.
Journal of the Geological Society of India, Vol.. 98, 12p. Pdf
India
craton - Dharwar
Abstract: In comparison to the eastern Dharwar Craton, the mantle-derived xenocrysts/xenoliths are extremely rare or even unreported from the western Dharwar Craton, southern India. A Neoproterozoic (ca. 800-900 Ma) lamprophyre cropping out in the Mysuru area of southern India contains chrome-diopside xenocrysts (Cr2O3 content varying from 0.2-1.23 wt%) which provide important evidence about the pressure-temperature conditions and lithospheric thickness beneath the western Dharwar Craton. Studied chrome-diopsides show compositional zoning which is lacking in the liquidus phases (amphiboles and feldspars) of the lamprophyre which additionally favors a non-cognate origin of the former. Based on the compositional zoning, all the chrome-diopside xenocrysts can be divided into three groups: (i) Group I- which are euhedral and show reverse zoning with increasing Cr-content from core to rim; (ii) Group II- which are characterized by fractures and resorption textures, show complex reverse zoning and display up to three distinct compositional layers, and (iii) Group III- which evidence the reaction of chrome-diopsides with lamprophyric melt and are marked by alteration phases, such as actinolite and chlorite, together with relicts of some unaltered xenocrysts. High Cr2O3, moderate MgO and low Al2O3 content of all the three varieties of chrome-diopside suggest them to represent disaggregated xenocrysts of mantle-derived garnet peridotite. Temperature-pressure estimates for chrome-diopside xenocrysts ranges from 895-1026 °C (± 30 °C) and 32-38 kbar respectively and correspond to depth range of 106-127 km. The study reveals that lithospheric thickness during the Neoproterozoic beneath the western Dharwar craton was at least ?115 km and is similar in composition to that of the cratonic lithosphere found in the other cratonic domains.
Moscow University Bulletin, Vol. 76, 5, pp. 471-481.
South America, Brazil, Russia, India, China
legal
Abstract: The mineral resource sectors of BRICS countries complement each other perfectly; one of the possible areas for their cooperation in this field is the expansion of mutual trade in mineral commodities and metals in order to provide continuous supplies and price stability. In 2006-2018, the principal beneficiaries of such cooperation were Republic of South Africa and Brazil, which managed to sharply increase their exports of mineral commodities. At the same time, close cooperation with these countries allowed China to become the largest purchaser of mineral commodities and metals in the global market, to ensure continuous supplies and price stability, and to obtain access to mineral resources of the other countries from the organization. However, the expectations of future cooperation among BRICS countries relating to regulation of the global market of mineral resources were to be too high for a number of reasons.
Abstract: The Amba Dongar carbonatite complex in western India comprises an inner ring of carbonatite breccia surrounded by a sövite ring dike. The various carbonatite units in the body include calcite carbonatite, alvikite, dolomite carbonatite, and ankerite carbonatite. The carbonate phases (calcite and ankerite) occur as phenocrysts, groundmass phases, fresh primary grains, and partially altered grains and/or pseudomorphs when hydrothermally overprinted. Rare earth element (REE) enrichment in the groundmass/altered calcite grains compared to the magmatic ones is ascribed to the presence of micron-sized REE phases. Fluorapatite and pyrochlore constitute important accessory phases that are altered to variable extents. Higher concentrations of Sr, Si, and REEs in fluorapatite are suggestive of a magmatic origin. Fresh pyrochlore preserves its magmatic composition, characterized by low A-site vacancy and high F in the Y-site, which on alteration becomes poorer in Na, Ca, and F and displays an increase in vacancy. The C-O isotope compositions of the carbonates also corroborate the extensive low-temperature hydrothermal alteration of the carbonatites. The REE mineralization is the result of interaction of the carbonatite with a sulfur-bearing, F-rich hydrothermal fluid that exsolved from late-stage carbonatitic magmas. The hydrothermal fluids caused dissolution of the primary carbonates and simultaneous precipitation of REEs and other high field strength element (HFSE)-bearing minerals. Complex spatial associations of the magmatic minerals with the REE fluorocarbonates, [synchysite-(Ce), parisite-(Ce), bastnäsite-(Ce)] and florencite-(Ce) point to the formation of these REE phases as a consequence of postmagmatic hydrothermal dissolution of the REEs from fluorapatite, pyrochlore, and carbonates. Ubiquitous association of fluorite and barite with REE minerals indicates transport of REEs as sulfate complexes in F-rich fluids. Precipitation of REE fluorocarbonates/florencite resulted from fluid-carbonate interaction, concomitant increase in pH, and decrease in temperature. Additionally, REE precipitation was aided and abetted by the removal of sulfur from the fluid by the precipitation of barite, which destabilized the REE sulfate complexes.
Journal of the Geological Society of India, Vol. 98, 12p. Pdf
India
chrome-diopside
Abstract: In comparison to the eastern Dharwar Craton, the mantle-derived xenocrysts/xenoliths are extremely rare or even unreported from the western Dharwar Craton, southern India. A Neoproterozoic (ca. 800-900 Ma) lamprophyre cropping out in the Mysuru area of southern India contains chrome-diopside xenocrysts (Cr2O3 content varying from 0.2-1.23 wt%) which provide important evidence about the pressure-temperature conditions and lithospheric thickness beneath the western Dharwar Craton. Studied chrome-diopsides show compositional zoning which is lacking in the liquidus phases (amphiboles and feldspars) of the lamprophyre which additionally favors a non-cognate origin of the former. Based on the compositional zoning, all the chrome-diopside xenocrysts can be divided into three groups: (i) Group I- which are euhedral and show reverse zoning with increasing Cr-content from core to rim; (ii) Group II- which are characterized by fractures and resorption textures, show complex reverse zoning and display up to three distinct compositional layers, and (iii) Group III- which evidence the reaction of chrome-diopsides with lamprophyric melt and are marked by alteration phases, such as actinolite and chlorite, together with relicts of some unaltered xenocrysts. High Cr2O3, moderate MgO and low Al2O3 content of all the three varieties of chrome-diopside suggest them to represent disaggregated xenocrysts of mantle-derived garnet peridotite. Temperature-pressure estimates for chrome-diopside xenocrysts ranges from 895-1026 °C (± 30 °C) and 32-38 kbar respectively and correspond to depth range of 106-127 km. The study reveals that lithospheric thickness during the Neoproterozoic beneath the western Dharwar craton was at least ?115 km and is similar in composition to that of the cratonic lithosphere found in the other cratonic domains.
Mineralogy and Petrology, 10.1007/s00710-022-00777-z 20p. Pdf
India
deposit - Shilong Plateau
Abstract: A comprehensive mineralogical, geochemical and isotopic review of six ultramafic-alkaline-carbonatite magmatic intrusions of the Shillong Plateau (Sung Valley, Jasra, Swangkre-Rongjeng, and Mawpyut) and Mikir Hills (Samchampi-Samteran and Barpung) is presented here, using the published data. These intrusions emplaced ca. 115-102 Ma ago, thus are significantly younger than the tholeiitic flood basalts erupted in Rajmahal-Sylhet province (ca. 118-115 Ma). The intrusive lithologies vary from ultramafic (dunites, clinopyroxenites, melilitolites) to mafic (ijolites, gabbros sensu lato, shonkinites), to felsic (syenites, nepheline syenites) and carbonatites (mostly calcite-rich varieties). The volcanic-subvolcanic facies (lamprophyres, phonolites) are not abundant. The range of chemical compositions of the magmatic phases in the various assemblages is notable; the intrusive rocks are thus the result of crystallization of magmas from variably evolved, independent liquid-lines-of descent, generally of alkaline/strongly alkaline lineages and sodic-to-potassic in affinity. The large variations of the Sr-Nd isotopic ratios of the silicate intrusive rocks (sensu lato) suggest a role of shallow-level crustal contamination during their formation. The carbonatites of the Sung Valley and Samchampi-Samteran have different isotope ratios than the associated silicate rocks, have some isotopic affinity with the Group I tholeiitic basalts of Rajmahal Traps and have an ultimate genesis in a carbonate-bearing lithospheric mantle.
