Petrotectonic significance of the chemistry of chromite in the ultramafic–mafic complexes of Pakistan

Abstract

The suture zones, which border the Indian craton in Pakistan are characterized by several large mafic–ultramafic complexes. From south to north these include: (1) Bela, (2) Muslimbagh-Zhob valley, (3) Waziristan, (4) Dargai (Skhakot-Qila), (5) Shangla-Mingora, (6) Jijal, and (7) Sapat. Besides these, there is the huge (300x≤40 km) Chilas mafic–ultramafic complex in the Cretaceous Kohistan magmatic arc. The Sapat, Jijal, Shangla, and Dargai (Skhakot-Qila) complexes are distributed along the Indus suture. The Waziristan, Zhob valley and Bela complexes lie along the suture on the western margin of the Indian plate. All these complexes were emplaced during late Cretaceous–Paleocene time at ca. ∼65 Ma. These complexes largely consist of ultramafic and mafic plutonic rocks (ranging from dunite and peridotite to gabbro). The mafic–ultramafic rocks of the Sapat, Chilas and Jijal complexes are generally well layered and appear to be mainly cumulates. In addition to cumulate plutonic rocks, the remaining five complexes also contain abundant residual peridotites, sheeted dykes, a variety of lava flows (locally pillowed), and associated pelagic sediments. Disseminated and segregated chromite occurs in all the complexes. On the basis of the chromium number [ Cr # = 100 × Cr / ( Cr + Al ) ] of their chromite, the complexes can be divided into three groups: (1) the Sapat and Jijal complexes contain chromites with Cr#>60; (2) the Shangla-Mingora, Dargai (Skhakot-Qila), Waziristan, Muslimbagh-Zhob and Bela rocks contain chromites that display a wide range of composition with Cr# varying from 15 to 90; and (3) rocks of the Chilas complex are characterized by low-Cr chromites (Cr# mostly <60). The chromite compositions appear to indicate different tectonic settings for the origin of the complexes under consideration. Those with high Cr chromite belong to an island arc type tectonic setting. Opinion differs on the origin of the group (2) chromite-bearing complexes; they may well be of complex origin with components, which originated in different tectonic settings. The Chilas complex of group (3) contains low Cr chromite similar to oceanic rocks supporting the idea that it is related to rifting within an island arc setting.