Metamorphism of the inverted sequence in Himachal Himalaya: A study from the Kullu-rohtang pass-khoksar section

Abstract

An inverted metamorphic gradient has been documented within the rocks of the Kullu-Rohtang Pass-Khoksar region, with increase in grade of metamorphism from biotite through garnet, staurolite and kyanite to sillimanite zones. Microtextures, mineral equilibria and geothermobarometry of mineral assemblages indicate deformation at a minimum temperature of 500°C and 4.7 kbar pressure in the Kullu region to a maximum temperature of 635°C and 6.8 kbar pressure in the Rohtang Pass region of Higher Himalaya. The lower amphibolite facies rocks further north of Rohtang Pass indicate P-T conditions of 4.5 kbar and 450°C respectively. The range of P-T conditions imply tectonic burial of 16 to 23 km and geothermal gradient variation of 31°C/km to 28°C/km. There is a maximum P-T difference of 2100 bars and 135°C upsection between samples near Kullu to samples along the Rohtang Pass, with dP/dT values of around 16 bars/°C. The inverse metamorphism of the region is due to synto post-metamorphic folding of isograds accompanied by thrusting of normal Barrovian metamorphic sequence, that is subsequently followed by a retrogressive cooling path during exhumation.