大别山商麻断裂韧-脆性变形转换的时间约束及其构造意义

Zircon U-Pb Geochronology of the Shangma Fault,Dabie:Constraints on the Transformation from Ductile to Brittle Deformation,and Its Significance to the Tectonic Evolution of Dabie Orogenic Belt

通过对商麻断裂的几何学和运动学的详细构造解析,鉴别出呈先后依次发育层次由深到浅的3种不同构造层次的伸展性断裂构造活动,从早到晚分别为:①低角度向北西西方向滑脱的韧性剪切;②西倾的低角度脆性、脆-韧性滑脱正断层;③西倾的高角度脆性正断层.在构造年代学的基础上,选择①末期与围岩同构造并顺围岩面理侵入结晶的花岗质伟晶岩脉和后期的花岗岩侵入体进行了锆石U-Pb年龄测试,花岗质伟晶岩脉中13颗岩浆锆石给出了125.9±4.2Ma的年龄数据,花岗岩中10颗岩浆锆石给出了118.8±4.1Ma年龄数据,从而可以将商麻断裂带从伸展滑脱型韧性变形向脆性滑脱断层转换的时间很好的约束在126～119Ma之间.通过与前人研究结果的对比,笔者将罗田穹窿形成的时间限定在了150～126Ma之间,为研究整个大别山构造演化提供了新的年代学约束.

According to the particular investigations on the geometry and kinematics of the Shangma fault, three different crust-levels of extension movements have been differentiated in sequence of development from the deep to the shallow： ① ductile shearing with top to the NWW in low dip angle;② brittle or brittle-ductile transition normal fault with top to the NWW in low dip angle;③ brittle normal fault with top to the NWW in high dip angle. Two samples were chosen for zircon U-Pb age dating to test the constraints on the activity age of the Shangma fault. A bedding intrusive granitoid pegmatite vein which was parallel to the foliation of the ductile shear zone of the country rock which exhibits a boudinage deformation, showing syn-tetonic emplacing at the end of ductile deformation period and deformation in the brittle-ductile transition domain. The zircon U-Ph age dating of the granitoid pegmatite vein gives a result of 125.9±4.2 Ma, expressing the extension age in the brittle-ductile transition domain of the Shangma fault. Another sample, which was collected from a granite pluton cutting the foliation of the extension ductile shear zone, gives a zircon U-Ph age of 118.8±4. 1 Ma, constraining the end of the ductile shearing. Then the transformation time from ductile to brittle deformation can he constrained at 126-119 Ma.Compared with the former researches, the formation of the Luotian dome, which is located to the east of the Shangma fault can be constrained at 150-126 Ma. This study offers a new time constraint to the evolution of the Dabie orogenic belt.