Study On Permian Volcanic Sedimentary Stratigraphy In Suolun Basin,Inner Mongolia And Its Tectonic Significance

The research area,Keyouqianqi area in the northeast of Inner Mongolia,and the geotectonic location is located in the east of Xingmeng orogenic belt and the east of Xing’an Ailigemiao massif.The predecessors have done researches on the magmatism and formation age of the Dashizhai Formation volcanic rocks in the Keyouqianqi area,but the research on the sedimentary rocks is still scarcely.Therefore,this paper selects the original early Permian Dashizhai Formation volcanic rocks and sedimentary rocks in the study area as the research object,conducts systematic petrography,sedimentology,chronology,Hf isotope,whole rock geochemistry,and determines the sedimentary facies.The sedimentary environment and the rock assemblage,tectonic setting of volcanic rocks in different stages and the trend of crustal thickness changes time further restrict the evolution process of the crust by combining the stratigraphic framework with magmatism.Through the zircon Hf isotope,the Permian crustal accretion attribute and the crustal evolution process in the eastern Xing’an Ailigemiao block were revealed.The main achievements of the paper are as follows:First of all,zircon U-Pb dating results show that the volcanic rocks in this set of volcanic sedimentary strata were formed in two periods of~272 Ma and~282 Ma,suggesting that the volcanic rocks of Dashizhai Formation were the products of the volcanic eruption stage in the middle Permian under the extensional background,and the youngest zircons of the sandstone of the lower Dashizhai Formation and the siltstone and mudstone of Zhesi Formation were obtained respectively at 277 Ma and261 Ma,It shows that the volcanic sedimentary strata were formed in the middle late Permian.Through the comparative study of the age spectrum of detrital zircons,it is believed that the Permian sediments in the study area are mainly supplied by the near source plates,and only a small part of the provenance comes from the distant North China Craton and Siberian plates.This two-way provenance suggests that the Sauron Basin is a basin range structure,which is related to the extensional environment in geodynamics.In addition,the strata that have been divided by predecessors are redefined as the Carboniferous Wujiatun Formation,the upper and lower sections of the Lower Permian Dashizhai Formation and the Middle Permian Zhesi Formation.Secondly,the research results of sedimentary facies,tectonic background and the evolution of Sauron Basin show that the strata can be divided into two cycle sequences from bottom to top,corresponding to the two cycles of extension cracking of Sauron Basin,which can be divided into two types of sedimentary facies: pre delta facies and delta front facies,with the characteristics of upward grain size thinning,indicating that the study area belongs to the sedimentary environment of rapid supply of delta facies.The geochemical ratios of zircon and whole rock record the changes of the crustal thickness with the age: the crustal thickness decreases from 300 Ma to260Ma,and the Hf isotope is depleted,suggesting that there is a period of thinning of the crustal thickness.The crustal thickness of acidic volcanic rocks of the same age is calculated based on the whole rock Sr/Y ratio,and the trend of change is basically consistent.In conclusion,combined with the characteristics of sedimentary rocks such as large thickness,slumping accumulation,rapid accumulation,and bimodal volcanic rock association composed of dacite and rhyolite of volcanic rocks,the study area has been in an intracontinental extension environment in the early Permian,and further revealed the extension through the fitting trend of crustal thickness,combined with previous studies on volcanism,sedimentation,metamorphism,etc,This paper suggests that the extensional environment recorded in the Permian in the study area may correspond to the extensional environment of the Paleo Asian Ocean closed before Devonian.