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

A. T. Rao ¹, V. Divakara Rao ², M. Yoshida ³, M. Arima ⁴

Affiliations
1 Department of Geology, Andhra University, Visakhapatnam- 530 003, IN
2 National Geophysical Research Institute, Hyderabad - 500 007, IN
3 Department of Geosciences, Osaka City University, Osaka - 558, JP
4 Department of Geological Science, Yokhoma National University, Yokahoma, JP

Abstract

Oxygen isotope values in metapelites including garnet-sillimanite gneisses (khondalites), quartzo-feldspathic gneisses (leptynites), cordierite-sapphirine gneisses and the associated marbles, calc-silicates, quartzites, basic granulites and charnockites from the Eastern Ghats Granulite Belt are presented and their significance discussed. Metapefites show a wide variation in δ¹⁸O values (4.50 to 13.90‰) that is controlled largely by the mineralogy of the rocks. Coarse grained porphyritic charnockites have higher δ¹⁸O values (8.60-11.20‰) than that of medium grained ones (5.90-6.60‰) Basic granulites have the minimum δ¹⁸O values (∼5.0‰). Oxygen isotope values of different rock units in the granulite belt are constrained mostly by the composition of the source rocks of the sediments and of the protoliths in the case of charnockites. Post-depositional metamorphic and metasomatic fluids had little contribution to the δ¹⁸O variations in these rocks.

Keywords

O-Isotopes, Eastern Ghats Granulite Belt, Khondalites, Charnockites, Andhra Pradesh.

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A. T. Rao ¹, V. Divakara Rao ², M. Yoshida ³, M. Arima ⁴

Affiliations
1 Department of Geology, Andhra University, Visakhapatnam-530 003, IN
2 National Geophysical Research Institute, Hyderabad- 500 007, IN
3 Department of Geosciences, Osaka City University, Osaka 558, JP
4 Department of Geological Sciences, Yokohama National University, Yokohama, JP

Abstract

The whole rock analyses confirm the basic to intermediate nature of the protolith (SiO₂, > 53% - < 67%) with high alumina (highest 22.18%) and high K₂O (∼ 4%). Metamorphic segregation as well as source rock compositions appear to have contributed -to the formation of high alumina granulites. The temperatures of metamorphism estimated using garnet - hypersthene/cordierite pairs range from 680-980°C, The gamet-cordierite reference temperatures with narrow range 700-750°C, indicate a pressure range of 5-7 kbar for the garnet-orthopyroxene-plagioclase-quartz assemblage in the aluminous granulites. Textural relationships of cordierite breakdown and reappearance of cordierite supplement the two generations of cordierite from aluminous granulites in the Eastern Ghats Granulites Belt.

Keywords

Geochemistry, Aluminous Granulites, P-T Estimates, Eastern Ghats Granulite Belt (EGGB), Andhra Pradesh.

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D. Prakash ¹, M. Arima ², A. Mohan ¹

Affiliations
1 Departmeont of Geology, Banaras Hindu University, Varanasi-221005, IN
2 Geological Institute Yokohama National university, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, JP

Abstract

Orthopyroxene-plagioclase symplecti tes have been observed in garnet-bearing mafic granulites from Panrirnalai, South India. Symplectites are arrested textures from which one can study diffusional processes. We have used colour-coded compositional mapping for reaction textures from mafic granulites to understand the element mobility pattern. The variable.composition of anorthite in plagioclase supports the interpretation that the orthopyroxene-plagioclase symplectites adjoining embayed garnet blast were formed by open-system diffusion - controlled reaction in the NCFMAS model system.

Keywords

Mafic granulites, Symplectites, Diffusion, South India.

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