A Late Carboniferous-Early Permian back-arc basin in the southeastern Central Asian Orogenic Belt: Constraint from sedimentological and geochronological investigations of the Shoushangou Formation in the central Inner Mongolia

摘要

The Solonker Suture Zone was formed during the closure process of the Paleo-Asian Ocean (PAO). In order to better constrain its tectonic setting and evolution history in the Late Carboniferous-Early Permian, we conducted an integrated study of the Shoushangou Formation in the central Inner Mongolia, involving the sedimentological features, depositional age, and detrital zircon populations. Our results indicate that the Shoushangou Formation was deposited in ca. 320-280 Ma, overlain by the Dashizhai Formation with ages of ca. 280-274 Ma, and the 330- to 280-Ma detrital age data almost serve as the entire age component of this formation. Also, its stratigraphic reconstruction shows a coarsening-upward marine sequence upwards, which is characterized by sandstone turbidites with a Bouma sequence and the numerous tuff involvements in the lower strata, showing a deep marine environment and synchronous volcanism. Integrating previous data and our results, with a gradually deepening water depth and a geographic variation, detrital zircon age spectra from the Late Carboniferous-Early Permian sequence show distinct differences, and a marked shift of multiple resources to a single source occurred in the Late Carboniferous-Early Permian. These clues suggest an active marginal setting and a northward-dipping subduction during this period in the north side of the PAO, and we further attribute such a source variation to the spreading of a marine back-arc basin after the amalgamation of the Uliastai Active Continental Margin with the Northern Accretionary Orogen. The Shoushangou Formation was the direct depositional response to such a back-arc basin.