The Age and Potential Provenance Information of Amushan Formation in Wulanaobao Area, Northwestern of Langshan, Inner Mongolia and Its Geological Significance

Guo Shuo; Teng Xuejian; Liu Yang; Teng Fei; He Peng; Tian Jian; Wang Wenlong; Duan Xiaolong

Abstract：The study of sedimentary basins is important for understanding the tectonic evolution of orogenic belts.The Langshan region of Inner Mongolia is located in the middle central Asian orogenic belt and has a complicated structural evolutionary history.The tectonic setting of Late Paleozoic sedimentary basin is still in controversy.Based on the study of the lithology, sedimentary structure and sedimentary environment of Amushan Formation in Wulanaobao area in the northwestern of Langshan, we divide the Formation into three members.The first member is mainly composed of coarse clastic rocks with cross-bedding developed, and the sedimentary environment is braided river delta.The bottom of the second member is mainly composed of coarse clastic rocks with increasingly finer grain-size in the upper sediments and the bioclastic limestone at the top of second member, and the depositional environment is from braided river delta to littoral circumstances.At the same time, we firstly discovered plant fossils such as Calamites sp., Alethopteris sp., Cordaites principalis Geinfrom the lithic quartz sandstone of the second member.The third member consists of crystalline limestone and bioclastic limestone, and the sedimentary facies type is littoral-neritic facies.The detrital zircon ages of sandstone from the first and second members of Amushan Formation are divided into five groups of about～2.5Ga, ～1.8-1.2Ga, ～826Ma, 461-440Ma and 313-273Ma, respectively.The youngest age of Amushan Formation is 273Ma.Combining the results of plant fossils and the age of overlying volcanic rocks, the age of"Amushan Formation"is limited between 265Ma and 273Ma.The contemporaneous magmatic rocks in the study area were formed in the continental marginal arc.With the research of sedimentary characteristics, sedimentary age and tectonic position of Amushan Formation, we propose the method of stratigraphical divisions and correlation.