The Cambrian initiation of intra-oceanic subduction in the southern Paleo-Asian Ocean: Further evidence from the Barleik subduction-related metamorphic complex in the West Junggar region, NW China

Abstract

In this study, we present new evidence from the Barleik subduction-related metamorphic complex in the southern West Junggar region, northwestern China, for the Cambrian initiation of intra-oceanic subduction in the southern Paleo-Asian Ocean. The Barleik metamorphic complex is mainly composed of blueschist and amphibolite blocks within an ophiolitic mélange and their protoliths are calc-alkaline andesite and alkali and tholeiitic basalts. The calc-alkaline andesite has a zircon U–Pb age of 502±2Ma, obtained from magmatic cores of zircon grains, and shares geochemical features similar to the 515–485Ma intra-oceanic arc magmatic rocks in the West Junggar region. By contrast, the alkali and tholeiitic basalts have trace element features similar to ocean island and enriched mid-ocean ridge basalts, respectively. Rutile and sodic–calcic amphibole from the amphibolite have a U–Pb age of 502±25Ma and a 40Ar/39Ar age of ∼504Ma, respectively, which are in good agreement within errors with a 40Ar/39Ar age of 492±4Ma for phengite from the blueschist. These metamorphic ages of ∼500Ma are interpreted to represent the timing of Pacific-type subduction-related metamorphism and are also compatible with ages of the oldest supra-subduction zone ophiolites (531–512Ma) and intra-oceanic arc plutons (515–485Ma) in the southern West Junggar region. Being one of the oldest subduction-related metamorphic complexes (509–490Ma) in the southern Central Asian Orogenic Belt, the Barleik metamorphic complex, together with the oldest arc plutons, definitely indicate the initial intra-oceanic subduction in the southern Paleo-Asian Ocean at least in the Early Cambrian.