Metamorphism and isotopic evolution of granulites of southern India: Reference to Neoproterozoic crustal evolution

Granulites are important component of high-grade metamorphic rocks reflecti ng intense conditions observed for crustal rocks in terms of temperature an d pressure. This review paper demonstrates how these high-grade granulites are critical to understanding the evolution of the lower continental crust with special reference to southern India. Geothermobarometric traverse acro ss different granulite blocks in southern India shows wide ranging P-T cond itions of metamorphism (700-1000 degrees C and 5-10 kbar). The sapphirine-, orthopyroxene-sillimanite and spinel-bearing quartz-deficient granulites r ecognised from parts of southern granulite terrain (Ganguvarpatti, Kiranur and Palani hill ranges etc.) show oriented sillimanite aggregates pseudomor ph after course twinned kyanite, staurolite + kyanite assemblages and corro ded blebs of gedrite within orthopyroxene, suggesting a prograde stage of a clockwise P-T evolution. Evidence of ITD history comes from the textures i n which an parry Mg-rich garnet (X-Mg similar to 52-60) with orthopyroxene (up to 10% Al2O3) involving sillimanite breakdown forming variety of symple ctites having combinations of orthopyroxene, sapphirine, cordierite and spi nel. These spectacular reaction textures and mineralogic sensors from the h igh-grade rocks establish a prograde clockwise P-T-t path with notable deco mpressive history (ITD) in the southern granulite terrain. The inferred P-T -t paths have been further integrated with the recent geochronological and isotopic data to constrain the timing and duration of metamorphism, emplace ment of the magmatic protolith for characterising the evolution of the gran ulites, and their bearing on the geodynamic implications. Based on the emer ging evidence for Neoproterozoic tectonothermal imprints in the southern gr anulite terrain, history of the assembly of dispersed fragments is also add ressed within the East Gondwana framework.