Tectonic evolution of the Higher Himalayan Crystalline Sequence, Dhauliganga valley, Garhwal Himalaya: Insights from P–T conditions of metamorphism and partial melting

Abstract

Metamorphism of pelites from the Higher Himalayan Crystalline Sequence (HHCS) in the Dhauliganga valley, Garhwal Himalaya has been studied. Two structurally lower lithounits of the HHCS, namely the Joshimath and Pandukeshwar Formations, contain kyanite-grade mineral assemblages with no evidence of partial melting and yield P–T values in the ranges of 7.8–10.3 kbar and 646–735 °C. The Badrinath Formation, which is the upper lithounit, shows partial melting and increase in P–T conditions from 5.5 to 7.0 kbar and 712–724 °C (sillimanite–K-feldspar grade) in its lower part to 10.5–11.9 kbar and 790–801 °C (kyanite–K-feldspar grade) in the upper part. Corona texture of plagioclase around relict kyanite in the uppermost part of the Badrinath Formation indicates decompression following peak metamorphism. Based on the sharp change in T/depth ratio (geothermal gradient), and diachroneity in peak metamorphism and exhumation from available geochronological data, a metamorphic discontinuity known as the Badrinath Shear Zone (BSZ) is suggested between the lower and upper Badrinath Formation. The upper HHCS, which comprises the upper Badrinath Formation, was tectonically extruded and juxtaposed over the lower HHCS along the BSZ. Decreasing trend of P–T conditions of peak metamorphism and younging of exhumation ages southward from the upper to lower HHCS are compatible with critical taper model for the exhumation of the HHCS.