Thermobarometry and Mineral Chemistry of Granulite Facies Rocks from Eastern Ghats, India

Coexisting garnet, biotite, orthopyroxene, ilmenite, K-feldspar, plagioclase, magnetite and perthite from amphibolite facies feldspathic gneiss and granulite facies khondalite, charnockite and porphyritic charnockite from Araku-Padua section of the Eastern Ghats granulite belt have been analysed by EPMA. Garnet-biotite, garnet-orthopyroxene thermometry and garnet-orthopyroxene-plagioclase-quartz barometry have been used to estimate the P-T conditions of metamorphism. l, the individual mineral compositions are broadly similar, although minerals like garnet, biotite and K-feldspar show perceptible, differences especially in their MgO, CaO and iron. Garnets from feldspathic gneiss are rich in MgO, poor in CaO followed by garnet from khondalite, while these minerals from the charnockite suite of rocks have low MgO and high CaO. Biotite has low CaO and high MgO in feldspathic gneiss and khondalite, while those are slightly low in porphyritic charnockite. Ilmenite in feldspathic gneiss has more iron, less TiO2 compared to ilemenite in khondalite-charnockite-porphyritic charnockite. Mineral compositions vary from core to rim and exhibit gradual change. The P-T estimates show increase in temperature and decrease in pressure at rim compared to core. P-T estimates suggest an average temperature of 750° ± 50°C and pressures of the order of 7.5