Zircon U-Pb Ages Of Late Paleozoic Granites In The Yakeshi-A'ershan Distric,and Its Geological Significance

The study area is located in Yakeshi-A'ershan area,which is in the east of Inner Mongolia autonomous region,the tectonic units belong to Ergun block and Xing' an block and their collage zone.The late Paleozoic granitoid rocks of this area can be mainly divided into three stages:the Early Carboniferous,the Late Carboniferous and the Early Permian Epoch.Rock types of the Early Carboniferous granitoid rocks mainly include diorite,quartzdiorite,granodiorite,tonalite and biotite granite,which distributed in the northwest side of the collage zone-Toudaoqiao-Wunuer fault zone in belts.Rock types of the Late Carboniferous mainly are monzogranite and syenogranite,which also mainly distributed in the northwest side of Toudaoqiao-Wunuer fault zone.Type of granitoid rocks of the Early Permian Epoch is mainly granites sensu stricto which distributed in both sides of the fault zone but appearred mainly in southeast side.The Early Carboniferous granitoids is a set of calc-alkaline rock series in general,with basic characteristics of I-type granite,and the zircon U-Pb age is 320Ma-328Ma.The Late Carboniferous granitoids with the biotite granite being the most typical show transitional character of I-type granite to S-type granite,and the zircon U-Pb age is 315Ma~304Ma.Granites sensu stricto of the Early Permian Epoch is S-type granite,belonging to calc-alkaline rock series with basic characteristics of collisional granites,and the zircon U-Pb age is 293Ma~275Ma.The above-mentioned characteristics reflected the feature of the granitoids in the study area with continuous evolution from neutral rock to acidite,along with tectonic environment by island arc changes to syn-collision,from the Early Carboniferous to the Early Permian Epoch.The study shows that,the Carboniferous Toudaoqiao-Wunuer fault controlled the geological development process of the study area.There is the northwest subduction existing along Toudaoqiao-Wunuer area in the Early Carboniferous,then the subduction-related I-type granites formed in the northwest of fault zone,and an extensional back-arc basin formed in Yakeshi region.Late Carboniferous,when the tectonic environment gradually changed to collision,forming granites with transitional characteristic of I-S type.To the Early Permian Epoch,tectonic environment turned into syn-collision,forming a set of light-colored S-type peraluminous collision granite.