Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

S. M. Naqvi , D. Srinivasa Sarma , R. H. Sawkar , M. Ram Mohan , J. G. Rana Prathap , S. Viswanathan ¹

Affiliations
1 Flat B-203, Block-B, United Avenue Apts, South End, 7 1-29, Ameerpet, Hyderabad-500016, IN

Abstract

No Abstract.

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S. M. Naqvi ¹, D. Srinivasa Sarma ¹, R. H. Sawkar ², M. Ram Mohan ¹, J. G. Rana Prathap ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN
2 Mineral Sales Private Limited, Hospet - 583 203, IN

Abstract

Acid volcanics found in auriferous greenstone belts of the Dharwar Craton are rhyolites, adakitic rhyohtes and high silica adakites Adakites are compositionally similar to TTG and characterized by high Na^/K^O, depleted MgO, Cr, Ni, Y and Yb. The adakitic melts were most probably generated by the partial melting of the basaltic slab below a mantle wedge LILE and LREE enriched IAB are the dominant volcanic members of the greenstone belts IAB of the greenstone belts were generated from partial melting of the mantle wedge under the influence of slab derived fluids. The wedge derived IAB and slab derived adakites were deformed and metamorphosed to generate fluids responsible for the gold endowment of these belts. Rhyohtes and possibly rhyohtic adakites were generated by the melting of the sialic continental crust forming top of the descending slab. Identification of adakites in auriferous greenstone belts strengthens the genetic link between adakite magmatism, subduction and Neoarchaean gold endowment.

Keywords

Adakites, Dharwar Craton, Gold Greenstone Belt, Karnataka.

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S. N. Charan ¹, E. V. S. S. K. Babu ¹, S. M. Naqvi ¹, J. G. Rana Prathap ¹, M. Ram Mohan ¹, D. Srinivasa Sarma ¹

Affiliations
1 National Geophysical Research Institute (Council of Scientific and Industrial Research), Hyderabad - 500 606, IN

Abstract

The major part of the Peninsular Gneiss in Dharwar craton is made up of Trondjhemite-Tonalite-Granodiorite (TTG) emplaced at different periods ranging from 3.60 to 2.50 Ga. The sodic-silicic magma precursors of these rocks have geochemical features characteristic of partial melting of hydrated basalt. In these TTGs, enclaves of amphibolites (± garnet) are abundant. These restites are considered to be the residue of a basaltic crust after its partial melting. A detailed study of these (residue) enclaves reveals textures formed due to the process of partial melting. Major, trace and REE analysis of these residue enclaves and the melt TTGs and microprobe analysis of the coexisting minerals show partitioning of REE and HFSE between the precursor melt of TTGs and the upper amphibolite facies residues. Formation of garnetiferous amphibolites with biotite, Cpx and plagioclase consequent to melting, has squeezed the original MORB type of basaltic crust and given rise to the TTGs, depleted in Y, Yb, K₂O, MgO, FeO, TiO₂ and enriched in La, Th, U, Zr and Hf. Coevally during the process of melting, the hydrated basalt was depleted in Na₂O, Al₂O₃, LREE, Th, U and enriched in K₂O, MgO, Nb, Ti, Yb, Y, Sc, Ni, Cr and Co. Mineral chemistry of co-existing garnet-biotite and amphiboleplagioclase in these amphibolitic (restite) enclaves indicates an average temperature of 700 ± 50 °C and pressure of 5 ± 1 Kbar. These data are inferred to indicate that during the garnet stability field metamorphism, effective fractionation of HREE and HFSE has taken place between the restites having Fe-Mg silicates, ilmenites and the extracted melt generated from the MORB type of hydrated basalt. These results are strongly substantiated by the reported melting experiments on hydrated basalts.

Keywords

Fractionation, HFSE, REE, TTG, Restite, Dharwar Craton.

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