Contrasting metamorphic P–T –time paths of the Kohistan granulites and tectonics of the western Himalayas

Abstract

In the Swat-Indus Kohistan area, lower crustal rocks of the Kohistan arc including amphibolites, granulites and ultramafic rocks are widely exposed. Detailed petrological-structural studies reveal that the Chilas-Jijal metamorphic complex comprises essentially intermediate- to high-pressure granulites with a granulite/amphibolite transition controlled by extensive retrograde metamorphism and ductile deformation. Contrasting patterns of compositional zoning in garnet and orthopyroxene suggest complicated P–T paths in the granulites. Geothermobarometry based on garnet-bearing granulites identify two distinct P–T–time paths: a high-pressure anticlockwise path of the Jijal complex and a low-pressure retrograde path of the Chilas complex. These P–T –time paths represent the evolution of middle to lower crustal rocks of the Kohistan arc. The anticlockwise P–T -time path combined with published isotopic ages reveals that: (i) the high pressure granulitefacies metamorphism is related to the Kohistan-Asia collision and (ii) the crust of the Kohistan arc had been thickened at least 55 km before the India-Asia collision.