Very high density CO₂ associated with ultrahigh-temperature metamorphism in the Eastern Ghats granulite belt, India

Abstract

Spinel-bearing high Mg-Al granulites of the Vizianagram area in the Eastern Ghats granulite belt show textural features clearly establishing that the association spinel_ss + quartz + Fe-Ti oxide solid solution ± sillimanite ± porphyroblastic orthopyroxene was stable during peak metamorphic conditions. Pressure-temperature (P-T) conditions estimated from both mineralogical thermobarometry and phase-equilibrium limitations indicate that the peak metamorphism occurred under ultrahigh-T conditions (>1000°C) at 8-9 kbar pressure. Retrograde P-T conditions of 750-800°C, 6-7 kbar are deduced from the compositions of coronal garnet and orthopyroxene, which have rims of spinel against quartz, indicating significant cooling with slight lowering of pressure. Quartz associated with the ultrahigh-T assemblage at Vizianagram contains ubiquitous single-phase carbonic inclusions as isolated clusters that belong to two categories. Group I shows extremely high density (homogenization temperature: −51±1.8°C; density 1.15 g/cm³) and group II trapped relatively lower density fluids (homogenization temperature: −18.4±2.4°C; density 1.05 g/cm³). The iso chores for group I inclusions pass through the peak metamorphic P-T conditions, whereas those for group II coincide with the P-T conditions of the formation of coronal garnet and orthopyroxene. Our study is the first report of very high density CO₂ associated with the Eastern Ghats granulite belt rocks and provides a strong case for the presence of CO₂-rich fluids during ultrahigh-T metamorphism at lower crustal levels.