Middle-Late Permian Sedimentary-Tectonic Evolution In The Eastern Inner Mongolia

The eastern Central Asian Orogenic Belt(CAOB)encompasses a vast region from northeast China to Inner Mongolia,where there were many Late Paleozoic sedimentary basins that can provide key constraints to tectonic evolution of the eastern CAOB.In order to find out whether there was the Late Paleozoic oceanic accretionary prism recorded by continuous subduction of the PAO or inland sea and extensional basin superimposed on the pre-existing orogenic lithosphere,we selected the Linxi basin in the eastern Inner Mongolia as the study area and two sections of the Zhesi Formation in the basin have been measured.Combined with previous studies in the Linxi basin,a comprehensive study has been carried out based on sedimentology and detrital zircon chronology in order to deduce the tectonic evolution of the basins and ranges developed in the intracontinental extension in the eastern Inner Mongolia.The sedimentary strata of the Permian Zhesi Formation in the Linxi basin are mainly characterized by thick-bedded conglomerate and the sedimentary cycles of thick-bedded sandstone and siltstone,interbedded with limestone and mudstone.Multiple small beddings,like cross bedding,horizontal bedding,graded bedding and current bedding and syndepositional deformation structures are widely developed in sandstone and siltstone,indicating that the depositional environment of the Zhesi Formation is alternating with delta and littoral facies.Specifically,the delta facies is characterized by irregular current bedding,horizontal bedding and small cross bedding in sandstone and siltstone,and the gravels with poor sorting and complex composition in conglomerate,which shows an unstable water environment.The littoral facies is mainly characterized by thick-bedded siltstone and sandstone interbedded with limestone,which shows a high-energy sedimentary environment.The zircon spectrogram of six sandstone samples from two sections of the Zhesi Formation in this study shows that the youngest peak ages are 274 Ma,273Ma,264 Ma,270Ma,268 Ma and 271 Ma,respectively.According to the zircon U-Pb ages of 258 Ma of andesite in the Zhesi Formation of Hexigtenqi,the maximum depositional age of the sedimentary strata can be constrained by the volcanic rocks formed in magmatic activities during the sedimentary period.Therefore,the Zhesi Formation in Linxi basin was formed in the Middle-Late Permian.Combined with previously published zircon data by other scholars in the Linxi basin,two different zircon age distributions from the southern and northern Linxi basin have been recognized,which are similiar to the North China Craton in the south and Airgin SumXilinhot uplift in the north,respectively.This implies that there were southern and northern provenances during the sedimentary evolution of the Linxi basin,which is consistent with the suggestion that there were two detrital supply systems in the Linxi basin obtained by the method of paleocurrent analysis.The analysis of sedimentary sequence and sedimentary environment of two measured sections and other studies in the Linxi basin indicates that there were shallow marine clasticcarbonate sedimentary facies during the Late Carboniferous,delta and littoral facies during the Early to Middle Permian and terrestrial facies during the Late Permian in the Linxi basin,and no oceanic sedimentary facies was found.This sedimentary sequence from occurrence of initial clastic-carbonate rocks to formation of basalts and siliceous siltstones reveals a transgression process during the Early Permian while littoral detrital and carbonate rocks with conglomerates and plant fossils represent a regression process during the Middle-Late Permian.Combined with previous studies on the West Ujimqin basin,the Late Carboniferous to Permian tectonic frame of the eastern Inner Mongolia expresses extensional tectonics characterized by alternating combination of basins and uplifts superimposed on a pre-existing Early-Middle Paleozoic orogenic lithosphere and Precambrian basement,rather than a single ocean basin of the PAO,which provides constraints for understanding sedimentary and tectonic evolution of the eastern CAOB during the Late Paleozoic.