Abstract

The Jijal ultramafic-mafic sequence is the lowermost exposed part of the Kohistan terrane. Petrography and geothermobarometry show that garnet-bearing assemblages of the Jijal sequence consist of a mosaic of magmatic and metamorphic equilibria. Garnet-bearing magmatic assemblages started their crystallization at a depth >50 km, either at the base of a thickened arc-type crust or within rising magmatic batches in the upper mantle. Quasi-isobaric cooling of magmatic assemblages occurred in the high-P, high-T granulite field without reaching the eclogite field. Subsequently, these assemblages were partly retrograded under amphibolite- to greenschist-facies conditions. This succession of equilibria attests to a switch from a high-P quasi-isobaric cooling regime to a major decompression followed by final cooling at the upper-crustal level. This P-T evolution is best explained by a yet unrecognized tectonic event that consists of partial exhumation of the base of the Kohistan terrane during its accretion to the Asian plate. Final cooling and unloading in turn, could be related to extension following the obduction of the Kohistan terrane onto the Indian plate.

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