The Study On Stectonic And Sedimentary Environment During The Carboniferous-Permian In The Eastern Part Of Inner Mongolia

The study area is in the eastern part of the Inner Mongolia, China, tectonically located between the North China Plate and the Siberian Plate. The area underwent the tectonic overprinting and transformation from the Paleo-Asia ocean domain to the Western Pacific domain, which resulted in a complexity of the tectonic evolution. In recent years, based on the study of the late Paleozoic strata, Wang et al. (2008) proposed that there was a unified block, called Jiamusi-Mongolia Block (Jia-meng Block), during late Paleozoic period, including large areas of the NE China and Inner Mongolia. However, there are many controversies on the geotectonic nature and sedimentary environments about the southern margin of the Jia-meng Block. Therefore, we selected the eastern Inner Mongolia as the research area, located in the southern margin of the Jia-meng Block. Many sheets of 1:200000 scaled geological maps were collected and analyzed for the study of lithofacies, sedimentary environment, in order to reveal the regional tectonic setting and environment changing.The research results show that there were little/no sediments deposited during early Carboniferous, we inferred that the area may ever be an uplifted land. During the Late Carboniferous marine clastic and carbonate sediments were deposited, belonging to Benbatu Formation and Amushan Formation in the southern part of the study area, while there was no sediment in the northern part.During the Early Permian period, the volcanic activity happened frequently, and the volcanic sedimentary sequences were developed thickly, named Dashizai Formation in the southern part and Baoligaomiao Formation in the northern part, respectively. The Middle Permian Zhesi Formation is marine and paralic facies, mainly consists of carbonate, sandstone, mudstone and silty mudstone. In this sequence, researchers found a large number of marine fossils, including the brachiopods and corals and some others. Numbers of sedimentary cycles like shallow-water clastic rocks to carbonate rocks to delta clastic facies were developed, indicating sea level retreating and passive margin setting. The upper Permian Linxi Formation is terrestrial, mainly consists of sandstone, mudstone, silty mudstone, etc. Researchers found a lot of continental plant fossils, and parts of paralic deposition, in which there occurs a small amount of salt water bivalves in Linxi Formation. The sandstone developed parallel bedding and horizontal bedding, indicates that the Linxi Formation should be formed in a stable sedimentary environment. Combined with the previous research works, we conclude that the Paleo-Asian Ocean, between the North China Plate and the Jia-meng Block, had shrunk before the middle period of the Middle-Permian, finished collision in the late period of the Late Permian.For the better constraint to the sedimentary environment during Carboniferous– Permian,we selected a number of sandstone samples, from Zhesi Formation and Linxi Formation in the Suolun town, Linxi town and Hexigten Banner, to determine their tectonic settings by analyzing the geochemical major elements of the sandstones. The samples for analysis are mainly poorly sorted feldspathic lithic sandstone and lithic arkose, angularity of grains varies from subangular to angular with a high proportion of unstable lithic fragments. It shows that the Permian sedimentary basin during Zhesi Formation and Linxi Formation should be close to the hinterland with rapid transportation and deposition.The various projection diagrams for the major elements of the analyzed sandstones indicate that the main sedimentary provenance of the Zhesi Formation and Linxi Formation were probably some acid felsic volcanic material with characters of active continental margin or continental island arc. Considering the regional geological setting, we suggest that these volcanic materials should be from the Sonidzuoqi-Xilinhot-Xi Ujimqin magmatic arc.To summarize the studies above, we conclude that during Carboniferous the Paleo-Asian Ocean subducted under the southern margin of the Jia-meng Block, leading to form the Sonidzuoqi - Xilinhot - Xi Ujimqin arc. In the Early Permian the subduction of Paleo-Asian Ocean rolled back, and formed regionally distributed volcanic rocks, i.e. Dashizai Formation. In the Middle Permian the Paleo-Asian Ocean mainly subducted to south, and began to close in middle of the Middle Permian. Meanwhile, the southern margin of the Jia-meng Block became a passive continental margin, and developed stable marine facies Zhesi Formation. In the Late Permian the Paleo-Asian Ocean had already been closed, there developed of terrestrial facies Linxi Formation in the southern margin of the Jia-meng Block.