Silurian intermediate–felsic complex in the Xiangtaohu area of central Qiangtang, northern Tibet: Evidence for southward subduction of the Longmuco–Shuanghu Prototethys oceanic plate

Abstract

Data from ophiolites indicate that the Longmuco–Shuanghu Suture Zone (LSSZ) was connected with the Changning–Menglian Suture Zone (CMSZ) during the Paleozoic. Numerous geological records of late Cambrian–Silurian arc magmatism have been found along the LSSZ and CMSZ. However, all the reported early Paleozoic arc magmatism in the South Qiangtang subterrane (SQT) was based on crust-derived magmatic rocks, which constrained our acknowledge of the complicated geodynamic evolution processes. Here, we present bulk-rock geochemistry, SrNd isotopes, and zircon UPb dating and Hf isotope data for a newly discovered Silurian complex in the Xiangtaohu (XTH) area of the central Qiangtang Terrane, central Qinghai–Tibetan Plateau, to investigate the magmatic and tectonic evolution of the LSSZ and CMSZ. The XTH complex is composed of diorite, monzogranite, and biotite monzogranite. Zircon UPb dating results show that the XTH complex was emplaced at 438–429 Ma. The diorites belong to the calc-alkaline to high-K calc-alkaline series, showing metaluminous to weakly peraluminous features. All of the granites are high K, calc-alkaline, and weakly peraluminous. The diorites have low initial 87Sr/86Sr ratios (0.704526–0.705002) and εNd(t) values (−1.32 to 0.12), with zircon εHf(t) values ranging from −1.2 to 0.4, indicating a metasomatic mantle source with subducted-sediment-derived melts, and low MgO, Mg#, Cr, and Ni values, reflecting crystal fractionation from basaltic magma. The monzogranites and biotite monzogranites show I-type granite geochemical affinity, with slight negative εNd(t) values (−2.03 to −1.67) and low initial 86Sr/86Sr values (0.704056–0.705165), with zircon εHf(t) values ranging from −3.1 to 4.8, suggesting that they were derived mainly from partial melting of juvenile mafic lower crust. The samples are enriched large-ion lithophile elements, depleted high-field-strength elements, and have high U/Yb ratios of zircons, suggesting that the XTH complex was formed in a continental arc setting. The XTH Silurian complex is the youngest magmatic record of the southward subduction of the Longmuco–Shuanghu Prototethys oceanic plate, which represent the melting productions of the mantle wedge and mafic lower crust. It is different from the previously discovered S-type granites in the same area. Combining our new results with comprehensive existed data in the region, we propose that the Changning–Menglian Prototethys oceanic plate subducted beneath the Baoshan block (eastward) and the Yangtze block (westward) during early Paleozoic, while the Longmuco–Shuanghu Prototethys oceanic plate only subducted southward under the southern Qiangtang subterrane.