Petrogenesis of Late Devonian–Early Carboniferous volcanic rocks in northern Tibet: New constraints on the Paleozoic tectonic evolution of the Tethyan Ocean

Abstract

Paleozoic arc-type volcanic rocks in northern Tibet, provide important information on the Paleozoic tectonic evolution of the Longmu Co–Shuanghu Tethyan Ocean (LSTO). In this study, we obtained zircon U–Pb and Hf isotopic data, and major and trace element data for volcanic rocks from the Riwanchaka and Laxiongco areas in northern Tibet. LA-ICP-MS U–Pb analyses of zircons show that volcanic rocks in the Riwanchaka and Laxiongco areas formed at 374–372 and 356–353 Ma, respectively. These calc-alkaline volcanic rocks consist of basalt, andesite, dacite, and rhyolite. The basalt and andesite in the Riwanchaka area exhibit low TiO2 and high MgO and Na2O concentrations, and are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs). The basalt probably originated from partial melting of a lithospheric mantle source, metasomatized by subduction-related fluids, whereas the andesite was probably derived from hydrous basalts after fractional crystallization. The Riwanchaka and Laxiongco rhyolites are characterized by high Na2O/K2O ratios and low Al2O3 concentrations, are weakly metaluminous to peraluminous (A/CNK=0.97–1.11), and exhibit typical arc-like geochemical signatures (e.g., enrichment in LILEs and strong depletion in HFSEs and Sr). In addition, they show high zircon εHf(t) values (+7.61 to +4.45). The geochemical characteristics and zircon εHf(t) compositions of the rhyolites indicate that they were derived from partial melting of juvenile basaltic lower crust. These results, combined with previous data, indicate that the LSTO lithosphere was subducted northward beneath the North Qiangtang subterrane (NQT) during the Late Devonian–Early Carboniferous. We suggest that the NQT represents a slice of an active continental margin developed on the southern margin of the Eurasian continent during the Late Devonian–Early Carboniferous.