Sedimentary And Provenance Characteristics And Tectonic Significance Of The Linxi Formation In Linxi Area,Inner Mongolia

The tectonic evolution history of Xing'an-Mongolia Orogenic Belt in eastern Central Asian Orogenic Belt is still unclear,one of the important questions is whether the Xing'an-Mongolia Orogenic Belt was in continuous subduction or in post-orogenic extension during the Late Paleozoic.To solve the problem,we select the clastic sedimentary rocks of the Upper Permian Linxi Formation as the research object,which are widely distributed in Guandi area,Linxi Country,Inner Mongolia,detailed sedimentary section measurement and sedimentary environment and sedimentary facies analyses on the Linxi Formation have been carried out in this paper,together with the zircon U-Pb geochronological studies to research the sedimentary environment and provenance and structure significance,and further to discuss the paleogeographic pattern and the structure evolution of Xing'an-Mongolia Orogenic Belt in the late Paleozoic.Analysis of sedimentary facies and sedimentary environment of Linxi Formation shows that,the lithologic assemblage of Linxi Formation mainly consists of gray-black siltstone,argillaceous siltstone and gray-black,yellow-brown fine sandstone,locally intercalated gray black thin limestone or limestone lens.The horizontal bedding in the lower sandstone reflects low energy sedimentary environment,and the sedimentary structures such as oblique bedding and ripple mark in the upper sandstone reflect strong hydrodynamic conditions.Combined with the previous fossil and geochemical research results,it is generally believed that the sedimentary environment of Linxi Formation is mainly continental shore-shallow lake facies and deep lake facies,with short coastal facies.The restoration of paleocurrent indicates that the provenance is mainly from the North West.Zircon U-Pb Geochronology Studies show that the concordant age of the youngest zircons from the sandstone sample 190811-2 from the Linxi Formation in Guandi area is256±3Ma;the concordant age of the youngest zircons from the lithic sandstone 190812-13 is 264±10Ma.Combined with previous studies on zircon chronology,fossils and stratigraphic correlation,the formation age of Linxi Formation is determined to be Late Permian.The age-frequency distribution histograms of fine sandstone sample(190811-2)and lithic sandstone sample(190812-13)show that they have similar peak ages of about 265 Ma,300Ma and 450 Ma,and a certain number of Precambrian ancient zircons.Zircons with peak ages of 265 Ma represent the Late Permian magmatic activity developed in the basin,while zircons with peak ages of 300 Ma and 450 Ma represent the denudation process of early magmatic rocks in the western north of the basin,and older Precambrian zircons are from the basement of the Xing'an-Ailigemiao block.Overall,the external provenance mainly comes from the basement of the Xing'an-Ailigemiao block,Xilinguole Complex and magmatic rocks in the south of Suzuoqi-Xilinhot-Xiwuqi.This is consistent with the paleocurrent analysis.Based on the current and previous studies on detrital zircons in the whole Linxi Basin,the zircon collection area is divided into the western North and the eastern South,and the frequency spectra are plotted respectively.The results show that the main peak ages of zircons in the western North are 261 Ma and 445 Ma,and contain a large number of ancient zircons of 0.6-1.8 billion years(795 Ma,956 Ma,1371Ma and 1571Ma);the zircon peak ages of the eastern South are 271 Ma and 429 Ma,with two peaks of 1.8-1.9 billion years and about 2.5 billion years.Combined with the distribution of uplift geological bodies around the basin,the provenance area in the western north of the basin should be Xing'an-Ailigemiao block,and the provenance area in the eastern south of the basin should be the North China Craton,which indicates that the two-way material supply of the Early Paleozoic southern and northern orogenic belts and the basement of the ancient continental block has been accepted in the Linxi Basin in the Late Permian.This two-way provenance feature shows that the Linxi Basin is limited by the paleo-uplifts on both sides and jointly forms basin-ridge structure,and its geodynamic background is related to extension.Therefore,Xing'an-Mongolia Orogenic Belt should be in post-orogenic extension stage in the Late Paleozoic.