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

C. Sivaprakash ¹

Affiliations
1 Geological Department, The Associated Cement Companies Ltd., 121, Maharshi Karve Road, Bombay 400020, IN

Abstract

Calc-silicate rocks with the assemblage calcite + K feldspar + quartz + wollastonite + diopside_ss + scapolite + sphene ± grossular-andradite ± hornblende ± magnetite ± zoisite occur associated with metasedimentary manganese deposits at Garividi in Vijayanagaram district of Andhra Pradesh. From the mineral assemblages the temperature, pressure and composition of an ideally mixed H₂O-CO₂fluid phase during metamorphism have been derived for the following mineral reactions:

Calcite + Quartz = Wollastonite + CO₂.

Grossular + Quartz = Anorthite + Wollastonite.

Tremolite + 3 Calcite + 2 Quartz = 5 Diopside + 3 CO₂ + H₂O.

For the analysed mineral compositions temperatures of the order of 700 ± 50°C and pressures of 7 ± 1 kb have been estimated. Mole fraction of H₂O dnring metamorphism was at least 0.6. Fluid compositions appear to have been internally buffered during metamorphism.

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C. Sivaprakash ¹

Affiliations
1 Geological Department, The Associated Cement Companies Ltd., Bombay 400020, IN

Abstract

Rocks associated with manganese formations in Supa, Dandeli and Bisgod (North Kanara district), and Kumsi (Shimoga district) areas consist of quartz ± muscovite± biotite ± plagioclase ± chlorite ± calcite ± ankerite ± ilmenite ± pyrite ± pyrrhotite ± K feldspars ± epidote ± rutile ± apatite. Whole-rock compositions correspond to greywacke, clay/shale and suggest deposition in reducing environments. Muscovite is phengitic, Chlorite is mainly ripidolite variety. Plagioclase is dominantly albite. Ilmenite in some rocks is manganoan. Phase relations between coexisting minerals suggest that they have formed with a close aporoach to equilibrium. Compositions of coexisting pyrite and pyrrhotite, and of phengite suggest temperatures of formation of 300-400°C and pressures 2-4 kb corresponding to greenschist facies conditions.

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C. Sivaprakash ¹

Affiliations
1 SGS India Private Limited, Opp Town Hall, Jamnagar 361 001, IN

Abstract

Sillimanite+K feldspar+quartz±garnet±Fe-Ti oxide-beartng rocks occur associated wIth calc-silicate locks and manganese formations near Ganvidi in Vijayanagaram district of Andhra Pradesh The whole-rock chemistry shows varying rock oxidation ratios which are reflected in the mineralogical assemblages Almandine garnet is oxidized to mixtures of hematite, quartz and hydrous alummosilicates in oxidized rocks and is absent in highly oxidized rocks in the latter, ilmenite is oxidized to mixtures of rutile and hematite Such relatively high temperature alteration/oxidation effects could have occurred with the influx of water Garnet compositions, and the presence of sillimanite + K-feldspar associiation rather than muscovite+quartz, suggest minimum pressures of 6 Kb, temperatures around 700° and XH₂O around 0 6 during metamorphism.

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