Fluid characteristics of high- to ultrahigh-temperature metamorphism in southern India: A quantitative Raman spectroscopic study

We report the first quantitative compositional data on fluid inclusions in Late Neoproterozoic to Cambrian ultrahigh-temperature (UHT) granulites from major crustal blocks and shear zones in southern India. Fluid inclusions in various high-grade minerals such as garnet, orthopyroxene, plagioclase, cordierite, and quartz from these rocks were studied in terms of petrography, microthermometry and laser Raman spectroscopy. Measured melting temperatures of the inclusions indicate that the trapped fluid phase is dominantly carbonic. Raman analyses confirmed a CO2-rich composition with only minor dilutants such as N2 (≤5.0 mol%), H2O (≤0.50 mol%), and CH4 (≤0.47 mol%). The range of most CO2 isochores computed from density measurements in fluid inclusions from the UHT granulites pass below the peak pressure–temperature conditions of metamorphism indicating considerable resetting of fluid densities during decompression. In contrast, garnetiferous granulites from the Salem area along the Palghat-Cauvery Shear Zone System, which underwent high-pressure prograde metamorphism, contain very high-density syn-metamorphic pure CO2 inclusions. Such dense carbonic fluid inclusions in garnet and plagioclase coexist with carbonate minerals (calcite or dolomite), indicating probable derivation from deep-seated primary magmatic sources. Although models on the origin of CO2 are debatable, the ubiquitous association of carbonic fluids in the UHT mineral assemblages is in favour of large-scale infiltration of CO2-rich fluids during extreme crustal metamorphism associated with collisional orogeny and subsequent extensional collapse at the final stage of amalgamation of the Gondwana supercontinent.