Paleozoic subduction of the Paleo–Asian Ocean plate: constraints from Ordovician–Carboniferous volcano–sedimentary rocks in the northern Beishan orogenic collage, NW China

Abstract: The subduction history of the Hongshishan Ocean within the Beishan orogenic collage is crucial to understand the evolutionary mechanism of the Paleo–Asian Ocean. In this study, we describe the petrography, ages and geochemistry of an Ordovician–Carboniferous volcano–sedimentary succession, which provides a record of the subduction history of the Hongshishan Ocean. We obtained early Ordovician zircon U–Pb ages (482 ± 6 Ma and 472 ± 2 Ma) from the Xianshuihu Formation. The youngest weighted mean detrital zircon age from the Yuanbaoshan Formation is 421.0 ± 8.8 Ma. Most detrital zircons have prominent age peaks at 900–840 Ma and 543–462 Ma, suggesting a main source from the southern Mongolian microcontinent. Volcanic rocks from the Baishan Formation yielded weighted mean ages of 299.3–300.3 Ma. During the Ordovician–Devonian, some volcanic arcs formed on the northern side of Hongshishan–Baiheshan–Erlegen Ula (HBEU) ophiolitic belt, indicating that the Hongshishan Ocean had a northward subduction polarity. During the Carboniferous, continental arcs were well developed on both sides of the HBEU belt, suggesting a bidirectional subduction. An archipelago of multiple arcs of the Hongshishan Ocean resembles that of the West Pacific Ocean. The change in subduction polarity can be attributed to characteristics of multi-oceanic accretion.