Search

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. [ABSTRACT FROM AUTHOR]

The time duration and evolution of the major North Tianshan Ocean (also the Junggar Ocean) in the Chinese Tianshan orogenic collage remain poorly constrained. Here we report new geochronological and geochemical data of Carboniferous volcanic rocks documented from the North Tianshan accretionary complex in the Gangou area (Xinjiang, NW China). The studied rocks include basaltic andesites and andesites, with one andesite yielding a zircon U–Pb age of 332 ± 4 Ma. This reflects nearly synchronous eruption of the volcanics with the formation of host Early Carboniferous Yamansu Formation. These rocks are mostly tholeiitic in compositions, with a few samples transitioning to calcalkaline. All the samples show enriched LREE and LILE but depleted HFSE, a ubiquitous subduction signature observed in supra‐subduction zone volcanics. The positive εNd(t) (+5.2 to +6.7) and low initial 87Sr/86Sr (0.7038 to 0.7040) in these rocks suggest derivation from a MORB‐depleted peridotite source. However, the overall low MgO contents (and Mg#) in these rocks compared with typical high‐Mg andesites indicate that they are less likely formed via simple melting of metasomatized mantle wedge peridotites. Instead, partial melting of mafic‐ultramafic metasomes (e.g., pyroxenites) formed by slab melts/fluids‐peridotite interactions may explain these rocks. The studied rocks share many geochemical similarities with those Early Carboniferous volcanic rocks widely documented from the Chinese Western Tianshan and those volcanics from typical continental arcs (e.g., the Gangdese arc). Therefore, we suggest the volcanic rocks, combining with recent results from the North Tianshan ophiolites, tend to support a continued southward subduction of the major North Tianshan Ocean from the Cambrian to the Carboniferous periods. [ABSTRACT FROM AUTHOR]