Geochemistry and P-T pseudosection modelling of two-pyroxene mafic granulite from Sonapahar, Shillong Meghalaya Gneissic Complex (SMGC), India: Implication for tectono-metamorphic evolution

Abstract

Granulites exposed in high-grade regional metamorphic belts and exhumed as xenoliths in basaltic pipes are considered as window into the deep crust thus play a key role in constraining models of crustal processes and evolution. Understanding granulites not only helps us place limits on the nature and composition of the lower crust in the existing continents, but it also sheds light on the crucial tectonic and chemical processes that lead to the accretion and eventual stabilisation of present crust dynamics, which in turn endorse continental growth. The study area, Sonapahar situated in the central portion of Shillong Meghalaya Gneissic Complex (SMGC) contain rich assemblage of different metamorphic rocks ranging from amphibolite to granulites. Unlike pelitic granulite, which receive much attention throughout the world, this work primarily focus in constraining the metamorphic evolution of two-pyroxene mafic granulite, which lack proper documentation hereto. Here we construct for the first time the P-T evolution of two-pyroxene mafic granulite from the Sonapahar, SMGC. Metamorphic P-T conditions estimated for the mafic granulite using conventional thermobarometer and winTWQ shows temperature in excess of 800 OC and pressure of about 9 kbars, stand for high temperature granulite facies metamorphism. The metamorphic evolution path continuing from prograde, peak to post-peak is obtained by constructing P-T pseudosection using Perple_X software in NCKFMASHTO system. Result suggest a clockwise P-T evolution path, thus signify isothermal decompression and indicate rapid upliftment prior to cooling of the tectonically thickened crust. Geochemical study of trace and REE, further suggest protolith is of tholeiite basalt in nature that is derive from hybrid (VAB+MORB) tectonic setting.