Metamorphism of the inverted sequence in Himachal Himalaya: A study from the Kullu-Rohtang Pass-Khoksar section

An inverted metamorphic gradient has been documented within the racks of th e 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 assembl ages indicate deformation at a minimum temperature of 500 degreesC and 4.7 kbar pressure in the Kullu region to a maximum temperature of 635 degreesC and 6.8 kbar pressure in the Rohtang Pass region of Higher Himalaya. The lo wer amphibolite facies rocks further north of Rohtang Pass indicate P-T con ditions of 4.5 kbar and 450 degreesC respectively. The range of P-T conditi ons imply tectonic burial of 16 to 23 km and geothermal gradient variation of 31 degreesC/km to 28 degreesC/km. There is a maximum P-T difference of 2 100 bars and 135 degreesC upsection between samples near Kullu to samples a long the Rohtang Pass, with dP/dT values of around 16 bars/degreesC. The in verse metamorphism of the region is due to synto post-metamorphic folding o f isograds accompanied by thrusting of normal Barrovian metamorphic sequenc e, that is subsequently followed by a retrogressive cooling path during exh umation.