Ultrahigh-temperature mafic granulites from the Madurai Block, southern India: Constraints from conventional thermobarometry, pseudosection analysis, and rare earth element-based thermometry

The determination of metamorphic pressure–temperature (P–T) conditions of granulites provides us a natural window into the composition, structure, and P–T conditions of the lower crust and process of continental growth. Here, we constrain the P–T evolution of a suite of mafic granulite from the eastern part of Madurai Block. Integrated results from mineral reactions, conventional thermobarometry, and pseudosection analysis suggest that the studied mafic granulites, with a peak assemblage of coarse-grained garnet + coarse-grained clinopyroxene + plagioclase + quartz + [rutile], were metamorphosed under granulite facies conditions of 830 ± 50°C and 9.5 ± 1 kbar. Subsequently, the rocks underwent near isothermal decompression leading to the formation of symplectic assemblages of clinopyroxene + orthopyroxene + plagioclase and orthopyroxene + ilmenite, within a P–T range of 9.0–5.5 kbar and 750–800°C. The estimated peak temperature conditions are somewhat lower than those obtained using characteristic ultra-high temperature (UHT) mineral assemblages and non-conventional thermometers (~1,000°C at ~10 kbar) from the co-metamorphosed metapelitic granulites of the Madurai Block. This may be attributed to the high diffusion rate of divalent Fe and Mg, which often results in retrograde re-equilibration leading to considerable underestimation of peak temperatures in mafic granulites. To overcome this problem, we have additionally determined peak metamorphic temperatures of the studied rocks using a recently developed garnet–clinopyroxene rare earth element (REE)-based thermometer that takes advantage of much lower diffusion rate of trivalent REEs. Using the REE thermometer, we have obtained a peak temperature of 1,050 ± 50°C for the studied rocks, which is in reasonable agreement with the peak metamorphic temperature obtained from previous studies. The constrained P–T conditions and the clockwise P–T path suggest that the studied rocks were buried to lower crustal depths at UHT conditions in a convergent margin setting. The near isothermal decompression suggests rapid exhumation, most likely due to an extensional event. The peak UHT metamorphic conditions could have been attained at the core of a long-lived orogenic plateau, as suggested by other studies.