Magmatic crystallization, isobaric cooling, and decompression of the garnet-bearing assemblages of the Jijal sequence (Kohistan terrane, western Himalayas)

The Jijal ultramafic-mafic sequence is the lowermost exposed part of the Ko histan terrane, Petrography and geothermobarometry show that garnet-bearing assemblages of the Jijal sequence consist of a mosaic of magmatic and meta morphic equilibria. Garnet-bearing magmatic assemblages started their cryst allization at a depth > 50 km, either at the base of a thickened am-type cr ust or within rising magmatic batches in the upper mantle. Quasi-isobaric c ooling of magmatic assemblages occurred in the high-P, high-T granulite fie ld without reaching the eclogite field. Subsequently, these assemblages wer e partly retrograded under amphibolite- to greenschist-facies conditions. T his succession of equilibria attests to a switch from a high-P quasi-isobar ic 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 unrecog nized tectonic event that consists of partial exhumation of the base of the Kohistan terrane during its accretion to the Asian plate. Final cooling an d unloading in turn, could be related to extension following the obduction of the Kohistan terrane onto the Indian plate.