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S. N. Charan ¹, S. M. Naqvi ¹, S. L. Ramesh ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN

Abstract

Spinifex-textured peridotitic komatiite occurs on the southeastern margin of Mayasandra schist belt near Nagamangala in Karnataka. Preservation of spinifex-texture made up of pseudomorphically altered phenocrysts of olivine and less commonly clinopyroxene set in a groundrnass of skeletal augite and devitrified glass in the komatiite, is essentially perfect in spite of alteration, deformation and metamorphism. Petrography shows dissimilar shapes and dimensions of the crystals, with more than one pattern of crystal arrangement in the texturally layered komatiite, clearly indicating that conditions and rates of crystallization were not uniform throughout the cooling unit. Compositional variations in terms of major and trace elements appear to be broadly consistent with fractionation of olivine followed by clinopyroxene. The high contents of MgO, Ni, Cr and extremely low percentages of TiO₂, Na₂O, K₂O in the spinifex-textured ultramafic komatHte compare favourably with similar rock types reported from Canada, S. Africa and Australia.

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D. Rameshwar Rao ¹, B. L. Narayana ¹, S. N. Charan ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN

Abstract

Detailed mineralogical studies of amphibolite to granulite facies transition zone rocks and granulite facies rocks in Dharmapuri district, Tamil Nadu, were carried out in order to know the gneiss-charnockite relations, nature of protolith, and metamorphic conditions.

The studies show that the rocks are of magmatic origin. and show a gradual decrease in modal composition of hornblende and biotite and gradual increase of hypersthene and iron oxide in a north to south traverse. Breakdown of hydrous minerals to anhydrous minerals is evident with the release of iron oxide suggesting progressive transformation of gneisses to charnockite, Biotite occurs in two generations, an earlier one is of massive type and breaks down to anhydrous minerals, while the latter one occurs as tabular flakes and likely to have formed during the waning stages of amphibolite to granulite facies metamorphism, and forms a zone of concentration just above the stage or CO₂ migration. The mineralogy of pyroxene, plagioclase, hornblende, biotite and iron oxides also suggests that the pressure-temperature conditions in the transition zone are lower than those of the granulite zone.

Keywords

Petrology, Igneous and Metamorphic, Charnockites, Dharmapuri District, Granulites, Tamil Nadu.

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D. V. Subba Rao ¹, S. N. Charan ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

A lense shaped gabbro-norite pluton emplaced into the Archaean migmatitic Peninsular gneiss and intruded in turn by 2.6 Ga Banavar granite is exposed in the vicinity of Kalyadi, Hassan district, Karnataka. The norite is metamorphosed to upper greenschist-lower amphibolite facies. The parental magma of this suite is noritic (high Mg and Si rich) and may have been derived from an enriched mantIe source at higher degrees of partial melting.

Keywords

Igneous Petrology, Geochemistry, Gabbro-Norites, Karnataka.

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M. Ram Mohan ¹, D. Srinivasa Sarma ¹, S. N. Charan ¹, V. Balaram ¹, V. B. Rajasekhar ¹, Talat Ahmad ²

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad-500007, IN
2 Department of Geology, University of Delhi, Delhi-110007, IN

Abstract

Gadag Greenstone Belt (GGB) is the northern continuation of Chitradurga Greenstone Belt (CGB). It consists of a variety of metavolcanic and metasedimentary rocks. Two types of metavolcanic assemblages are found in this terrane (l) the tholeute-calc-alkaline island arc bimodal assemblage and (u) the tholeute-high-Mg basalt assemblage The tholente-calc-alkaline assemblage is exposed in the central and northern parts, whereas the tholentic-high-Mg basaltic assemblage is found in the southwestern part of the belt. Tholente-high-Mg basalts are represented by the coarse-grained amphibolites formed under lower amphibolite facies conditions REE and HFSE data along with major element compositions confirm that these coarse-grained amphibolites are tholentic basalts derived from an intraoceanic island arc setting. The REE patterns are coherent, flat to slightly LREE depleted (La/Yb_n =0.79 to 1.20, La/Sm_n =0.84-0.97, Gd/Yb_n=1.07-1.50) with no Eu anomaly. Relationship between compatible and incompatible elements suggests least effects of alteration and no crustal contamination or fractional crystallization. The mixing calculations indicate that these rocks are derived by partial melting of a depleted mantle source, with source composition in between that of the N-MORB and high-Mg basalts.

Keywords

Geochemistry, Petrogenesis, Amphibolites, Gadag Greenstone Belt.

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V. Balaram ¹, C. Manikyamba ¹, S. N. Charan ¹, M. satyanarayanan ¹

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad-500007, IN

Abstract

No Abstract.

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D. Srinivasa Sarma ¹, R. H. Sawkar ², S. N. Charan ², D. V. Subba Rao ², S. M. Naqvi ¹

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN
2 292, 10th Main, Jayanagar, Bangalore - 560 041., IN

Abstract

No Abstract.

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V. Balaram ¹, S. N. Charan

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

No Abstract.

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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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