Petrology of Sillimanite Gneisses from Garividi, Andhra Pradesh

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.