Pages: 285 - 298
Abstract: The Kosavankovilpatti area consists mainly of granulite facies rocks and is part of the Madurai block in the Southern Granulite Terrane, India. The garnet–cordierite gneiss collected at this location exhibits well-preserved metamorphic reaction textures and is used to infer the metamorphic history of the study area. Garnet resorption at the expense of orthopyroxene–cordierite and spinel–cordierite symplectites characterize the retrograde stages of metamorphism. Phase relationships for bulk rock composition in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2 (MnNCKFMASHT) system, as well as the application of multi equilibrium calculations for the peak metamorphic assemblages, yield maximum temperature (T) and pressure (P) conditions of 900 °C and 9 kbar, respectively. These estimated results are lower compared to those reported for the sapphirine-bearing granulites from the Madurai block that characterize UHT metamorphism. The post-peak P–T path, constructed for garnet-cordierite gneiss based on observed microstructural relationships and thermobarometry results, is characterized by a decompressive P–T segment ending at 810 °C and 5.5 kbar. The geochronological results of SHRIMP U–Pb give an age of 521 ± 30 Ma for the decompression metamorphic overprint that was superimposed on the protolith formation of the garnet–cordierite gneiss at 2561 ± 72 Ma. The proposed P–T path implies that the granulites in the Kosavankovilpatti section probably resulted from denudation of the thickened continental crust. In this way, the present study allows us to understand how this part of the lower continental crust could have evolved during prolonged heating without actually attaining UHT conditions.
Keywords: Madurai Block, SHRIMP, Southern Granulite Terrane (SGT), charnockite, Perple_X, garnet–cordierite gneissDownload PDF document