Timing of magmatic and metamorphic events in the Jijal complex of the Kohistan arc deduced from Sm-Nd dating of mafic granulites

Abstract

AbstractMafic to ultramafic granulites in the northeastern part of the Jijal complex include two-pyroxene granulite, garnet-clinopyroxene granulite and garnet hornblendite. Field and textural relations indicate that two-pyroxene granulite is a relict left after formation of the garnet-clinopyroxene granulite and garnet hornblendite was an originally intrusive rock which dissected the protoliths of mafic granulites. Sm-Nd mineral isochron ages of 118 ± 12 Ma, 94.0 ± 4.7 Ma and 83 ± 10 Ma were determined for two-pyroxene granulite, garnet-clinopyroxene granulite and garnet hornblendite respectively. These ages, together with previously reported chronological data, led to the following tectonic implications: (1) crystallization of the granulite protoliths predates, or is coeval with, the tectonic accretion of the Kohistan arc to the Asian continent; (2) crustal thickening related to the accretion was probably responsible for the high-pressure granulite-facies metamorphism in the Jijal complex; (3) formation of the garnet hornblendite assemblage was probably after crystallization of garnet-clinopyroxene granulite.