CO2-RICH FLUIDS ASSOCIATED WITH THE GRANULITE FACIES METAMORPHISM IN SOUTHERN GRANULITE TERRANE, SOUTHERN INDIA: INSIGHTS FROM FLUID INCLUSION AND PHASE-EQUILIBRIA MODELING (Invited Presentation)

Identification of fluid inclusions in the high-grade metamorphic rocks provides direct evidence for their involvement in high to ultrahigh temperature metamorphic events. Better characterization of such fluids will enhance our understanding about the high-grade metamorphism and crustal anatexis. The Southern Granulite Terrane (SGT) in the southern India is comprised of Paleoarchean to Neoproterozoic island arcs, which are amalgamated along different shear/suture zones. Mafic granulite rocks from the two major suture zones in the SGT, which are Palghat-Cavery Suture Zone (PCSZ) and Bhavani Suture Zone (BSZ) are considered in this study. Phase equilibria modeling of mafic granulites from the PCSZ yielded a peak metamorphic and retrograde conditions of ~940 °C/~11.5 kbar and ~ 840–850 °C/~9.3–10.2 kbar, respectively, with a near isobaric retrograde path. On the other hand, the mafic granulites from the BSZ displayed a peak P–T condition of 770–860°C /7.6–10.1 kbar followed by an isothermal decompression along a clockwise P–T path. In both granulites, the fluid inclusions occur as primary and secondary phases within the porphyroblastic garnet and matrix plagioclase. The melting temperatures of these fluid inclusions are in the range of −57.6 to −56.6 °C, indicating nearly pure CO₂ composition. Extremely high-density CO₂ inclusions of 1.155 g/cm³ and pressure conditions consistent with the phase equilibria modeling were obtained from the inclusions in PCSZ granulites. In contrast, slightly lower density range (0.927–1.027 g/cm³) and pressure conditions were obtained from the primary inclusions in the mafic granulites from the BSZ. Based on these observations, we infer that the primary ultrahigh-density carbonic inclusions that are trapped during the peak metamorphic conditions are retained in the granulites from the PCSZ. This might be due to the isochore-parallel decompressional cooling path after peak metamorphism. On the other hand, the low density and the lower pressure conditions from the fluid inclusions in the BSZ mafic granulite might be due to density loss during the isothermal decompressional retrograde path. Our results suggest that the preservation of the high density fluid inclusions are dependent on the style of P–T paths and post peak metamorphic exhumation history.