Carbonic fluid inclusions in ultrahigh-temperature granulite from Kumiloothu in the northern Madurai Block, southern India

Abstract

We report fluid inclusion data on ultrahigh-temperature (UHT) granulites from Kumiloothu in the northern Madurai Block situated along the Palghat-Cauvery Shear Zone system in southern India. Three categories of fluid inclusions have been observed: primary inclusions in garnet comprising the dominant category, secondary inclusions in garnet, and rare primary inclusions in rutile and apatite enclosed within garnet. The melting temperatures of all the categories of inclusions lie in the narrow range of −57.0 to −56.6 °C, close to the triple point of pure CO2. Most of the fluid inclusions homogenize into the liquid phase between a temperature range of +11.3 to +30.3 °C, corresponding to low-CO2 densities of 0.59-0.85 g/cm3. Slightly higher density of 0.95 g/cm3 was obtained from a primary inclusion in rutile that homogenized at −3.6 °C. The fluid densities, when computed into isochores, indicate lower pressures (∼ 6.6 kbar at 1000 °C) than the peak P-T conditions (T >1000 °C at ∼ 8 kbar) estimated for this region. The results of this study, together with the primary nature of the inclusions trapped in high-grade minerals, indicates density reversal of peak metamorphic fluid, a common feature for many UHT granulites along the Gondwana suture zone as well as other UHT terranes showing clockwise P-T history.