Stucture Analysis And Geochronology In Tongcheng-Qianshan Area In The East Of The Dabie Orogenic Belt

The Dabie orogenic belt(DOB)is a typical area for the study of the exhumation of high-ultrahigh pressure rocks.Lots of works had been done for exhumation of the DOB during the past 20 years,but the exhumation process of the DOB still exists disputes.The Early Cretaceous dome uplifting rebuilds structural information of the exhumation and makes the work more difficult,and almost exhumation models had not considered the effect of the uplifting.There are high pressure rocks outcroping in the Tongcheng-Qianshan area in the east of the Dabie orogenic belt,which located on the east side of the Yuexi dome.This area was effected only by Yuexi dome,which would help to recover structural information of exhumation.So,stuctural and geochronological studies were taken in this area can provide useful evdiences for the exhumation of the DOB.Detailed field study show that there are many E-W striking ductile shear zones in the Tongcheng-Qianshan area.The attitude of these shear zones are different from the NE-NEE striking ductile shear zones of the Tanlu fault zone.The E-W striking ductile shear zones disappears when extending to the west into the North Dabie unit,which indicates that these shear zones formed earlier than the Early Cretaceous dome and may be related to exhumation of the DOB.Therefore,this paper takes some works on the E-W striking ductile shear zone,such as thin-section identification,quartz c-axis fabrics analysis,zircon geochronology and structure recovery.Detailed microscopic observation shows that the mylonite mainly charactered with micro-fracture of feldspar and quartz SR dynamic recrystallization from E-W striking ductile shear zones,indicates that the deformation temperature is about 400?.Moreover,the quartz c-axis fabrics also records the information of the deformation temperature of 400 ?.Zircon CL images show that most of the zircons in this area have core-rim structure.The core of the zircons show oscillatory zoning and Th/U value is equal to about 1 and ages of 730-770 Ma were got.The rim of the zircons show the feature of metamorphic zircon and Th/U value is less than 0.1 and ages of about 220 Ma were got.After structure recovery,the E-W striking ductile shear zones have the same structural feature,such as steep foliation,E-W striking,sinistral shear sense and 20°W-lineation.Combined with previous studies,this study can draw a conclusion that the early exhumation of the high-pressure rocks occurred 220 Ma ago in this area.The shear activity of the E-W striking ductile shear zones occurred after 220 Ma and before 190 Ma,and is accompanied by the late-stage exhumation of the DOB.The E-W striking ductile shear zones formed when the south side was faster than the north side during eastward exhumation of the high-pressure rocks in the DOB.