Early Cretaceous (ca. 100 Ma) magmatism in the southern Qiangtang subterrane, central Tibet: Product of slab break-off?

Abstract

The lack of Early Cretaceous magmatic records with high-quality geochemical data in the southern Qiangtang subterrane has inhibited a complete understanding of the magmatic processes and geological evolution of central Tibet. In this study, we present zircon U–Pb ages, whole-rock geochemistry, and Sr–Nd–Pb and zircon Hf isotopic data for the newly discovered Moku pluton in the southern Qiangtang subterrane. Zircon U–Pb dating reveals that the Moku granites were emplaced in the Early Cretaceous (ca. 100 Ma) and are coeval with the hosted dioritic enclaves. The granites are slightly peraluminous and high-K calc-alkaline I-type granites and characterized by initial (87Sr/86Sr)i ratios of 0.70605–0.70658, negative e Nd(t) values (−4.44 to −3.35), and Nd isotopic model ages of 1.19–1.29 Ga. The granites have a wide range of zircon e Hf(t) values (−24.4 to 2.6) and concordant ratios of (206Pb/204Pb)t = 18.645–18.711, (207Pb/204Pb)t = 15.656–15.666, and (208Pb/204Pb)t = 38.751–38.836. The coeval dioritic enclaves are medium- to high-K calc-alkaline rocks with zircon e Hf(t) values of −13.3 to +3.6. The geochemical signatures of the host granites and coeval dioritic enclaves indicate that the Moku pluton was most likely generated by partial melting of the ancient lower crust with contributions from mantle-derived melts. Our new data, together with other recently published data, indicate that the ca. 100 Ma magmatic rocks were derived from anatexis of the Qiangtang lower crust that mixed with upwelling asthenosphere materials in response to the slab break-off of the northward subduction of the Bangong–Nujiang oceanic lithosphere.