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

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 bulkrock geochemistry, Sr-Nd isotopes, and zircon U-Pb 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 U-Pb 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 epsilon Nd(t) values (-1.32 to 0.12), with zircon epsilon Hf(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 epsilon Nd(t) values (-2.03 to -1.67) and low initial 86Sr/86Sr values (0.704056-0.705165), with zircon epsilon Hf(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.

Automated summary

The study of a newly discovered Silurian complex in the central Qiangtang Terrane of the Qinghai-Tibetan Plateau reveals the magmatic and tectonic evolution of the area. Through zircon U-Pb dating and Hf isotope analysis, it is found that the magmas originated from partial melting of different crustal and lithospheric sources, and were formed in a continental arc setting.