CONDITIONS OF METAMORPHISM OF THE GARNET-CORDIERITE GNEISSES FROM MALAI-MAHADESWARA HILLS, SOUTH-INDIA AND THE ORIGIN OF ZN-CR-RICH SPINEL

A spinel phase with unusually high ZnO (8.8-11.0) and Cr2O3 (13.2-15.8 %) contents is found in garnet-cordierite-sillimanite bearing migmatis ed pelitic bands interbedded with tonalitic charnockites of Malai Maha deswara (MM) Hills, The textural and microprobe data indicate that thi s spinel phase is associated with late iron-rich garnet (rim)-cordieri te-sillimanite assemblage. P-T conditions of cordierite-garnet assembl ages vary from peak metamorphic conditions of greater than 8 kbar and 750 degrees C to 6.7+/-1.4 kbar and 685+/-20 degrees C. The last retro grade phase of metamorphism took place at P of 4.5 kbar and at T of 58 0 degrees C. The formation of Zn and Cr rich spinel phase must have ta ken place during this waning stages of metamorphism by the activity of hydrothermal fluids.