Geochemistry and geodynamic implications of the Triassic bimodal magmatism from Western Kunlun Orogen, northwest China

Abstract

The Western Kunlun Orogen occupies a key tectonic position at the junction between the Tarim block and the Tethyan domain. However, the late Paleozoic to early Mesozoic, especially the middle to late Triassic tectonic evolution history of the Western Kunlun Orogen remains controversial. This study reports SHRIMP zircon U–Pb ages and geochemical as well as Sr–Nd–Hf isotopic data for middle to late Triassic Taer pluton in Western Kunlun Orogen, Northwest China. The Taer pluton shows a strong bimodal distribution of compositions, with the felsic rocks dominant and the mafic rocks subordinate. Zircon U–Pb dating reveals that the coexisting mafic and felsic rocks are coeval, both emplacing in a period between 234 and 225 Ma. Most of the studied rocks are potassium rich and can be classified into high-K calc-alkaline to shoshonitic series. They are also strongly enriched in LREE, LILE and depleted in HFSE with strong negative Ti and Nb anomalies, and characterized by enriched Sr–Nd–Hf isotopic signatures. Detailed geochemical and isotopic studies indicate that the Taer pluton was emplaced in a post-collisional extensional setting, with the mafic rocks derived from partial melting of the enriched continental lithospheric mantle in the spinel facies field, and the felsic rocks formed by anatexis of newly underplated basaltic rocks. The existence of middle to late Triassic post-collisional magmas in Western Kunlun region suggests that the final closure of Paleo-Tethys and the initial collision between the Western Kunlun and the Qiangtang terranes may have happened before ~234 Ma, most probably in late Permian, rather than in late Triassic or early Jurassic. In assistance with other geological evidences, such as the presence of early Triassic to late Triassic/early Jurassic S-type magmatism, terrestrial molasse depositions, regional unconformities, and strong deformation, we propose that the Western Kunlun Orogen may have undergone a long post-collisional intracontinental process from early Triassic to late Triassic/early Jurassic.