Geochemistry of mafic rocks and cherts in the Darbut and Karamay ophiolitic mélanges in West Junggar, northwestern China: Evidence for a Late Silurian to Devonian back-arc basin system

Abstract

The Darbut and Karamay ophiolitic mélanges in the West Junggar region of the Chinese Altaids, Central Asia, represent the relict fragments of the Late Paleozoic Junggar ocean basin. The mélanges have typical block-in-matrix structures and contain tectonic blocks of serpentinized perdiotite, gabbro, pillow basalt, chert, and sandstone that are dispersed in a strongly-sheared serpentinized matrix. In this contribution, we present new whole-rock major and element, and Sr and Nd isotope data for mafic rocks and cherts in the Darbut and Karamay ophiolitic mélanges. Compositionally, the mafic rocks are divided into two groups: Group 1 consists of gabbros and tholeiitic basalts displaying both N-MORB- and arc-like geochemical affinities; and Group 2 is characterized by alkaline basalts with OIB-like geochemical characteristics. Geochemical data suggest that Group 1 rocks are likely to have formed in a back-arc ocean basin and were derived from a depleted mantle source that had been metasomatized by slab-derived fluids. The origin of Group 2 rocks is attributed to ocean island basalts derived from a hotspot. The cherts in the mélanges appear to have formed in a restricted oceanic basin, showing a transitional depositional setting between a deep marine pelagic environment and a shallow continental margin environment. Accordingly, we propose a revised geodynamic model for the Late Paleozoic tectonic evolution of West Junggar. We suggest that a back-arc ocean basin opened in response to a northwest-dipping intra-oceanic subduction system during the Late Silurian to Devonian and contemporaneous hotspot-derived OIBs erupted in this basin. The back-arc basin evolved to a relict oceanic basin in response to the northwestward subduction of the oceanic lithosphere of the basin in the Early Carboniferous and was gradually filled up with volcaniclastic sedimentary rocks by the latest Carboniferous. Following the closure of the back-arc ocean basin, West Junggar was transformed into an intra-continental setting in the Early Permian.