Petrogenesis of Carboniferous volcanic rocks from the Gangou area, Chinese North Tianshan: Constraints on the evolution of the North Tianshan Ocean

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 epsilon Nd(t) (+5.2 to +6.7) and low initial Sr-87/Sr-86 (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.