The closing of the southern branch of the Paleo-Asian Ocean: Constraints from sedimentary records in the southern Beishan Region of the Central Asian Orogenic Belt, NW China

The Carboniferous–Permian strata in the southern Beishan Orogenic Collage (BOC) have been regarded as deposits formed either in a branch of the Paleo-Asian Ocean (PAO) or in continental rift basins. This paper presents stratigraphic, sedimentological, and provenance data of a continuous Carboniferous–Permian sequence on the southern margin of the Shuangyingshan Block within the southern BOC in NW China. Carboniferous Baishan and Shibanshan formations record a shallowing-upward change from marine tidal flat to continental braided stream environments. Integrated sandstone modes and U-Pb-Hf data indicate that these formations were fed from an orogenic uplift composed of Precambrian and early Paleozoic rocks of the Shuangyingshan and Dunhuang blocks (ca. 2492, 953, and 439–400 Ma). Latest Carboniferous–Early Permian depositional environments shifted from alluvial fans (Ganquan Formation) to delta-front and prodelta environments in an open marine setting (Shuangbutang and Jushitan formations) and source areas were composed of early Paleozoic granitoids (ca. 433 Ma) and penecontemporaneous volcanoes (ca. 278 Ma). These results indicate that the closure of the branch of the PAO between the Shuangyingshan and Dunhuang blocks occurred before the Visean (ca. 347 Ma), and was followed by the development of the latest Carboniferous–Early Permian marine rift basin on the amalgamated continental block.