Petrogenesis of Early Jurassic (ca. 181 Ma) dacitic–rhyolitic volcanic rocks in the Amdo ophiolite mélange, central Tibetan Plateau: Low‐pressure partial melts of Bangong–Nujiang Tethys oceanic crust?

Abstract

The identification of oceanic-crust-derived melts is critical to improving our understanding of oceanic lithosphere subduction in various geodynamic settings. However, low-pressure melts derived from the oceanic crust can be misidentified because their compositions are often indistinguishable from those of typical arc-related rocks. In this study, we report geochronological and geochemical data for the Amdo volcanic rocks in the central Tibetan Plateau. Zircon U-Pb dating indicates that they were generated in the Early Jurassic (ca. 181 Ma), simultaneous with spatially associated MORB-like rocks of the Amdo ophiolite. The Amdo volcanic rocks are characterized by high SiO 2 and Na 2 O concentrations, low MgO, K 2 O, and Fe 2 O 3 T concentrations, and low Mg# values. They are enriched in light rare earth and large-ion lithophile elements, display negative Eu, Nb, and Ta anomalies, and yield low Sr (<360 ppm) and high Y (up to 57.6 ppm) concentrations with low Sr/Y ratios (<15.4). Whole-rock Sr-Nd and zircon Hf isotopic compositions (( 87 Sr/ 86 Sr) i = 0.7040-0.7058, ε Nd (t) = +3.9to +6.8, and zircon ε Hf (t) = +13.9 to +17.9) are different from those of regional continental-crust-derived granitoids but similar to those of contemporary MORBlike rocks that occur in regional ophiolites. Furthermore, zircon from the Amdo volcanic rocks yields U/Yb ratios similar to those of oceanic zircon. These characteristics, combined with regional geological observations, indicate that the Amdo volcanic rocks are a component of the Amdo ophiolite mélange and represent low-pressure partial melts of oceanic crust, generated during incipient northward subduction of the Bangong-Nujiang oceanic lithosphere beneath the Qiangtang terrane. Our study helps to constrain the history of the Bangong-Nujiang Ocean and provides a case study on the low-pressure melting of oceanic crust.