Anisian granodiorites and mafic microgranular enclaves in the eastern Kunlun Orogen, NW China: Insights into closure of the eastern Paleo-Tethys

Granitoids and their mafic microgranular enclaves (MMEs) provide important insights into magmatic activity and tectonic evolution associated with accretionary orogens. Here we investigate the origin of Middle Triassic granodiorites and MMEs in the central segment of the eastern Kunlun Orogen (EKLO) in NW China, which is an important region to evaluate the history of closure of the Paleo-Tethys, through zircon U-Pb geochronology, major and trace element geochemistry, and Sr-Nd-Hf isotopes. Zircon U-Pb ages indicate the coeval formation of granodiorites and their MMEs during the Anisian (ca. 245 Ma). The granodiorites are calc-alkaline and metaluminous with high SiO2 contents and Mg-# values. They are characterized by enrichment of LREEs and LILEs, and depletion of Nb, Ta, and Ti. Their Sr-Nd -Hf isotopic compositions indicate that the magmas were derived from enriched mantle and Paleoproterozoic basement. The monzodioritic-dioritic MMEs exhibit low SiO2 contents and high Al2O3 and MgO with high Mg-# values, enrichment in LREEs and LILEs, and depletion in HFSEs with negative Nb, Ta, Ti, and P. In contrast, the gabbroic MMEs show higher Mg-# values and Cr contents. Both types of MMEs have Sr-Nd-Hf isotopic compositions similar to the host granodiorites, and originated from mixing of lower crust- and enriched mantle-derived magmas. The mantle-derived sources were metasomatized by slab-derived melts. The Middle Permian to Middle Triassic magmatism, metamorphism, sedimentation, and tectonics in the EKLO suggest northward subduction of the Paleo-Tethys Ocean as represented by the A'nyemaqen Ocean. The magma mixing that generated the Anisian granodiorites and MMEs was likely associated with slab break-off of the Paleo-Tethys Ocean. The formation of the Anisian magmatic rocks in the EKLO suggest that the cessation of subduction of the Paleo-Tethys Ocean and onset of collision took place during the late stage of Middle Triassic.