Barrovian metamorphism of the metapelites in NE Sikkim (Eastern Himalaya): Constraints from chemographic projection and geothermobarometry

Abstract

The Sikkim Himalaya is mostly constituted of Proterozoic metapelites and metavolcanics of the Daling Group and high-grade gneisses and metasediments of the Darjeeling Group. Metapelitic schists and gneisses of the Sikkim Himalaya show an inverted sequence of metamorphic rocks ranging from lower greenschist facies (chlorite zone) to upper amphibolite facies (sillimanite + K-feldspar zone). The relative XMg in the minerals varies as: garnet > staurolite > biotite > muscovite. The P-T evolution of the study area has been constrained through the use of an internally consistent winTWQ programme and Perple_X software in the MnNCKFMASHTO model system. The combination of these two approaches demonstrates that the schists and gneisses have experienced a range of pressure and temperature with increasing grade. Constraints on the interpretation of mineral reactions and metamorphic history provided by pseudosection modelling for mineral proportions suggest the peak metamorphic temperature as ~750 °C and pressure ~7.5 kbar. The proposed prograde P-T path implies that the rocks from the study area may have resulted from a single metamorphic event with continuous changes in pressure and temperature. The conclusions made on the basis of the study of metamorphic event in the area and the regeneration of the reaction history adheres to classical Barrovian type metamorphism. The tectonic implication of such a metamorphic evolution is also discussed in the present work. Finally, we conclude that the possible explanation for the exhumation in this section of Himalaya may most likely be explained with the help of the channel flow model. © 2021 Elsevier Ltd