Sedimentary characteristics of Sanmianjing Formation in Huade area,Inner Mongolia: Constraints on the tectonic evolution of the southern margin of Xing'an-Mongolia orogenic belt

The Huade area in Inner Mongolia is located at the southern extremity of the central segment of Xing'anMongolia Orogenic Belt. The Permian marine strata exposed in this area serve as ideal repositories for studying the tectonic evolution of Paleo-Asian Ocean. In this study, multiple methods including paleontology, zircon U-Pb geochronology and sandstone framework grain analysis were employed to investigate the Sanmianjing Formation in Huade area. The results show that the Sanmianjing Formation represents a transgressive succession deposited in littoral to neritic environments,containing Early-Middle Permian fusulinid fossils. The youngest detrital zircon age from the sandstones is 274±6 Ma,defining the minimum depositional age of Sanmianjing Formation. The detrital zircons exhibit three main age populations:274-338 Ma,1 567-2 193 Ma,and 2 310-2 656 Ma,indicating a mixed provenance of a magmatic arc related to the subduction of Paleo-Asian Ocean and the basement of North China Craton. The framework grain compositions and mineral assemblages of the sandstones further show that the tectonic affinity of the provenance includes both a magmatic arc and a stable craton or uplifted basement. Integrated with regional geological data,these results reveal that the Sanmianjing Formation was deposited during the Early Permian and originated in a back-arc basin,further suggesting that during the Early Permian,the Paleo-Asian Ocean was still subducting southward beneath the North China Block. These findings provide new foundational data for understanding the tectonic evolution of Xing'an-Mongolia orogenic belt.