Petrology, geochemistry and P-T conditions of metamorphism of supracrustal rocks of Nellore Mica Belt of Andhra Pradesh, south India.

The Nellore Mica Belt forming part of the Nellore schist belt is composed of quartzites, metapelites (kyanite/sillimanite + garnet + biotite + muscovite), iron-stones and amphibolites (plagioclase + hornblende + clinopyroxene). These supracrustal rocks are considered to be equiv. to Sargur group in the Dharwar Craton. They are intruded by pink to gray granites as well as ruby and green mica-garnet -tourmaline and beryl - bearing zoned pegmatites. Based on major element geochem., the schists are grouped into two categories viz., group-1, the quartz- garnet- -biotite-muscovite schists which are characterized by low SiO2 and alkalies, enriched in Fe2O3, TiO2, MgO and CaO, group-2, the garnet- kyanite- staurolite- mica schist which are characterized by their higher SiO2 and Al2O3 content and lesser amts. of Fe2O3, MgO, TiO2 and CaO. The second group of schists compare well with the pelitic schists of Holcuarasipur schist belt. Garnetiferous amphibolites contain higher Fe2O3 than those of non-garnetiferous amphibolites. Amphibolites are low- K tholeiites and sub-alkalic in nature and indicate a tectonic setting intermediate between oceanic and island- arc setting, characteristic of back - arc or marginal basins. The compn. of plagioclase in schists and amphibolites vary in its anorthite content from An30 to An40 and amphiboles are alumino-tschermakite in compn. Garnet is almandine-rich variety, and biotite is Fe-rich variety in the schists and amphibolites. The P-T conditions of metamorphism estd. based on the compn. of co-existing garnet-biotite in the pelitic schists and amphibolites yields a pressure est. of 6 to 7 kb and temp. of around 600°C. [on SciFinder(R)]