The Jijal Complex in the roots of the Kohistan island arc in the northwest Himalaya of Pakistan revisited

Abstract

The Kohistan Magmatic arc started building during the Early Cretaceous as an intra-oceanic island arc thousands of kilometers to the south of its present position. It was welded to the Karakoram plate along the Shyok suture during the midCretaceous, after which it became an Andean-type continental margin. Collision with India along the Indus suture occurred during the Early Eocene. The Kohistan arc exposes a complete section across the crust and consists of a range of variably metamorphosed plutonic, volcanic, and subordinate sedimentary rocks. The lower crust in Kohistan is represented by a series of mafic to ultramafic rocks. These include the granulite facies metamorphosed Jijal complex, which covers 150 sq km area of the southern fringe of the arc along the Indus River. Relics of similar rocks in amphibolites also occur 35 km to the southwest, north of Shangla. The southern (structurally lower) part of the Jijal complex in the hanging wall of the Indus suture comprises chromite-layered dunites, peridotites, and pyroxenites. These pass upwards into garnet-bearing ultramafic and mafic granulites. The transition zone contains pyroxenites (±Grt) or peridotites showing the development of garnet at the interface of plagioclase and opaque oxide /olivine. The main bulk of the granulites is represented by the assemblage Grt+Px+Pl+Qtz+Rt±Hbl. These rocks are mostly homogeneous, but locally well-layered. In addition to the principal assemblage, the layers comprise garnet pyroxenites (Cpx ± Opx ± Hbl), garnetites (±Cpx±Pl±Hbl), and metaanorthosites represented by the assemblages Pl+Grt+Cpx+Scp and (Zo+Grt±Cpx±Hbl±Qtz). The northern part of the complex contains relics of gabbronorites/pyroxene granulites (Pl+Opx+Cpx+Hbl+Ilm+Mag) protolith. The garnet granulites here invade the protolith in networks of veins and patches that appear to have formed along joints and fractures due to release of H2O during compression (increasing load pressure and temperature). Field data, combined with petrographic study and mineral analysis, suggest the following reaction for the transformation: Pl(An45)+Opx(En62)+Cpx(Mg 33.8, Fe 18.6, Ca 47.6; Al2O3 7.2%, Na2O 1.8%)+Prg=Pl(An48)+Grt(Mg 31.7, Fe 44.9, Ca 23.4)+Cpx(Mg 37.7, Fe 13.0, Ca 49.2; Al2O3 5.6%, Na2O 1.6%)+Rt+Qtz. Jan et al. (1997) and Yamamoto and Yoshino (1998) showed that this transformation was isochemical for major elements. Further XRF data (Table 1) show that apart from a loss in soda, the major and trace elements remained immobile during the pyroxene-granulite to garnet granulite transition. When normalized to primordial mantle values, the analyses of the mafic granulites display island arc signatures (Figure 1).