Timing of the final closure of the middle segment of the Paleo-Asian Ocean: Insights from geochronology and geochemistry of Carboniferous–Triassic volcanosedimentary successions in western Inner Mongolia, China

Abstract

Much controversy exists regarding the timing of the final closure of the Paleo-Asian Ocean and the resultant formation of the Central Asian orogenic belt. The Yagan and Zhusileng-Hangwula arcs in northern China, generally considered as the eastern extension of the Beishan orogenic belt, occupy the key junction between the South Tianshan suture to the west and the Solonker suture to the east, but they nonetheless have received little attention. The well-preserved Carboniferous- Triassic volcanosedimentary successions in the Yagan and Zhusileng-Hangwula arcs provide an ideal location at which to constrain the closure time of the middle segment of the Paleo-Asian Ocean. This forms the focus of this study, in which systematic field-based zircon U-Pb-Hf isotopic and whole-rock elemental analyses were employed. Our results reveal that the Carboniferous strata are likely marine deposits consisting mainly of clastic rocks interfingering with volcanic rocks and minor cherts and limestones. The 330-310 Ma sandstones were dominantly sourced from the local Yagan and Zhusileng- Hangwula arcs, the adjacent Beishan orogenic belt and southern Mongolia, and the eastern Central Asian orogenic belt. The associated 330-311 Ma andesites and rhyolites suggest a continental arc setting, probably owing to northward subduction of the Paleo- Asian Ocean. The subsequent clastic deposition of the Lower Permian section fines from conglomerates and sandstones at the base to interbedded sandstones and siltstones at the top, characteristics of shallow-marine successions. The interlayered 286-283 Ma volcanic interlayers from the Lower Permian indicate the further northward subduction of the Paleo-Asian Ocean. Up section, most of the Upper Permian strata are characterized by thick successions of marine clastic and volcanic rocks. The increasing age abundance in the late Paleozoic and the markedly negative εHf(t) values of the Early Permian detrital zircons in the 276-269 Ma sandstones suggest an augmented supply of detritus from the Alxa terrane to the south of the Paleo-Asian Ocean. This implies that initial closure of the Paleo-Asian Ocean occurred at ca. 276 Ma. Comparatively, in the 266-263 Ma sandstones, the striking increase in early Paleozoic detritus and the considerable spread of negative εHf(t) values for the 1.1-0.8 Ga detrital zircons more likely indicate derivation from the Beishan orogenic belt and/or southern Mongolia or the eastern Central Asian orogenic belt and subsequent transport to the northern Paleo-Asian Ocean. Such a fluctuating detrital supply to the north and south sides of the Paleo-Asian Ocean can be interpreted as a result of the continuous closure of the Paleo-Asian Ocean, involving latest consumption processes between 276 Ma and 263 Ma, as also evidenced by the associated 274-263 Ma volcanic arcs recognized in the Upper Permian. By contrast, the top of the Upper Permian to the Lower Triassic section consists of pebbly conglomerates and coarsegrained sandstones, representing alluvialfluvial sedimentation, in which sandstones with depositional ages between 256-254 Ma and 230 Ma were probably derived from the local areas. Accordingly, a depositional transition from marine to terrestrial and a provenance shift from multiple source regions to a local detrital provenance can be inferred to have occurred in the end-Permian to Early Triassic, reflecting uplift of the Yagan and Zhusileng-Hangwula arcs during the period subsequent to the disappearance of the Paleo- Asian Ocean. Together with other lines of geological evidence in the area, we infer that the final closure of the middle segment of the Paleo-Asian Ocean occurred between the end of the Early Permian and the Early Triassic. In conjunction with available data for the eastern and western segments of the Paleo-Asian Ocean, we proposed a scissorlike closure model for the whole Paleo-Asian Ocean, in which its western, middle, and eastern segments were closed at 320-300 Ma, 280-265 Ma, and 260-245 Ma, respectively. © 2018 Geological Soci