Pressure-temperature-fluid evolutionary history of orthopyroxene-bearing quartzofeldspathic and mafic granulites from northern parts of the Eastern Ghats Belt, India: implications for Indo-Antarctic correlation

摘要

Mafic and felsic orthogneisses form an integral component of the rock association of the Eastern Ghats Belt (EGB). Such rocks show similar P-T-fluid evolutionary history at two different localities of the EGB from where UHT metamorphism has already been recorded. Both mafic and felsic orthogneisses show contrasting petrological characters and are classified according to structural and mineralogical characteristics. All the varieties of mafic granulite and the gneissic enderbite preserve the strong regional foliation (S_2/S_3). The pegmatoidal enderbite is post deformational and hybrid in nature containing xenocrysts derived from associated rocks. Textural and thermobarometric data reveal different stages of metamorphism. Peak metamorphism (M_1) occurs at 8-9 kbar pressure and temperature in excess of 950 ℃. Emplacement of mafic magma could be the principal cause of this metamorphism. This was followed by retrograde R_1 stage when the rocks suffered a near isobaric Cooling up to 700- 750 ℃ at 7-7.5 kbar. At the terminal retrograde stage (R_2) fluid-rock interaction took place at 6-6.5 kbar, 600-650 ℃. The fluid composition is calculated to be poor in H_2O and CO_2. Textural data predict that K-rich brine could be responsible for such composition. The observed P-T-fluid characteristics show similarity with other terranes belonging to erstwhile Gondwanaland.