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 SiO2 and Na2O concentrations, low MgO, K2O, and Fe2O3T 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 ((87Sr/86Sr)i = 0.7040–0.7058, εNd(t) = +3.9 to +6.8, and zircon εHf(t) = +13.9 to +17.9) are different from those of regional continental‐crust–derived granitoids but similar to those of contemporary MORB‐like 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.