The Magmatic Activity Characteristics Of Dashizhai Formation In Xi Ujimqin Area,Inner Mongolia And Its Tectonic Evolution Significance

The study area is located in Xi Ujimqin,central Inner Mongolia,and the tectonic position is in the middle of the Xing’an-Airgin Sum Block.The magmatic rocks of the Dashizhai Formation are widely exposed in the study area.The predecessors have carried out a series of chronological and geochemical studies on the magmatic rocks in the Xi Ujimqin,and basically determined that the formation age of the magmatic rocks in this area is the early Permian Although the tectonic setting of its formation has been debated on subduction and post-collision extension.Therefore,on the basis of collecting the previous research data,this paper carried out systematic field geological survey,petrology and LAICP-MS zircon U-Pb chronology of the middle-acid volcanic rocks of the Dashizhai Formation in the south of Xi Ujimqin County,Inner Mongolia.Research,obtain the age of its formation,identify the volcanic rock sequence,divide the volcanic rock lithofacies,and determine the characteristics of the volcanic eruption.In addition,the trace elements in magmatic zircon and clastic zircon can effectively reflect the temperature and starting depth of their crystallization from magma.The Zr saturation temperature and thickness changes recorded by the late Carboniferous to Early Permian magmatic rocks and zircon trace elements of Carboniferous clastic rocks and the whole rock records of Late Carboniferous to Early Permian acid rocks in this area are combined,and Combined with regional research results,we will jointly reveal the characteristics of magmatic activity from Carboniferous to Early Permian and the evolution of crustal thickness in this area,so as to provide new evidence for the late Paleozoic tectonic evolution pattern of the Xing-Meng Orogenic Belt.According to the research results,the main conclusions of this paper are as follows:The LA-ICP-MS zircon U-Pb dating results show that the formation age of the Dashizhai Formation rhyolite in this area is 278.4±1.7 Ma,which is consistent with the previous dating results,and is the product of the Early Permian magmatic activity.According to field profile characteristics and indoor petrographic analysis,the volcanic rocks of the Dashizhai Formation in the study area are mainly divided into two types of volcanic facies and four volcanic subfacies,which are eruption facies and eruption facies,as well as lava flow subfacies,empty subfacies,heat base wave subfacies and thermal debris flow subfacies.Among them,the first section of the Dashizhai Formation is dominated by extrusive facies,and the second section of the Dashizhai Formation is dominated by eruptive facies and eruptions alternately.The Zr saturation temperature and zircon trace elements recorded in this paper from the Late Carboniferous to Early Permian magmatic rocks,the Carboniferous to Early Permian clastic rock zircon trace elements,and the late Carboniferous to Early Permian acid rock whole-rock records.The thickness shows regular changes with the new time:1)350～335Ma shows relatively low zircon crystallization temperature and increased zircon Eu/Eu*and crustal thickness,showing that the crustal thickness has increased This process can be coupled with the shallow process of regional uplift and denudation extensive loss of Early Carboniferous sedimentary strata due to the opening and continuous expansion of the Erenhot-Hegenshan ophiolite belt;2)325270 Ma zircon The crystallization temperature rises as a whole,and the zircon Eu/Eu*and crustal thickness continue to decrease.In addition,the zircon saturation temperature and the whole-rock Sr/Y value thickness of the acid rock at the same age record the same evolutionary trend.This crustal thickness continues The thinning process is basically consistent with the regional extension process caused by the large-scale magmatic activity in the Late Carboniferous Daqing Ranch and the Early Permian Dashizhai.This paper calculates that the overall zirconium saturation temperature of the Early Permian acid rock in the study area is between 790 and 1200℃,and the average temperature is 942℃.The author infers that the study area and the adjacent area were due to the upwelling bottom of the mantle material in the Early Permian.Continued melting of the invading crust resulted in regional lithospheric stretching and lower crustal thickness.Therefore,based on the above research and combined with previous research results,the author believes that the early Permian region has entered a post-orogenic extension system.