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
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摘要: 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.Abstract: 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.
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图 1 商麻断裂带区域地质构造略图（据刘早学和李金平，1993；周汉文等，1991修改）
1.白垩—第四系；2.古元古代变质岩系；3.太古宙片麻岩（大别杂岩）；4.英云闪长质片麻岩；5.花岗质片麻岩；6.混合花岗片麻岩；7. 花岗岩；8.脆性断裂；9.韧性剪切带；10.地质界线；11.定向标本；12.锆石U —Pb年龄样品；SMF.商麻断裂；JSF.金寨—舒城断裂；MXF.磨子潭—晓天断裂；TLF.郯庐断裂；XGF.襄樊—广济断裂
Fig. 1. Sketch map of regional geology in the area of the Shangma fault
图 2 麻城市冲口一带地质剖面图
1.砂砾岩；2.构造碎裂岩；3.碎裂的花岗质糜棱岩；4.花岗质糜棱岩—构造片麻岩；5.二长花岗岩；6.花岗质片麻岩；7.脆性正断层
Fig. 2. Structural section of the Shangma fault in the Chongkou area, Macheng
图 3 商麻断裂带不同构造层次伸展型断裂现象
A.桃林河低角度韧性滑脱中长石的拖尾构造；B.明山水库顺面理的藕节状香肠化的花岗伟晶岩脉；C.新洲区低角度韧性滑脱引起的面理拖曳；D.冲口高角度脆性断层破碎带
Fig. 3. Field images of ductile deformation from Shangma fault
图 4 样品D5-2和D11-3中的锆石阴极发光照片
Fig. 4. CL images of dating zircons from sample D5-2and D11-3
图 5 样品D5-2花岗质伟晶岩的错石U-Pb 年龄Tera-Wasserburg 图解
Fig. 5. Concordia U-Pb plot of LA-ICP-MS analyses ofsample D5-2 from Shangma fault
图 6 样品D11-3二长花岗岩的错石U-Pb 年龄Tera-Wasserburg 图解
Fig. 6. Concordia U-Pb plot of LA-ICP-MS analyses ofsample D11-3 from Shangma fault
图 7 商麻断裂带及东大别地区区域构造略图
1.上白垩统；2.新元古界随县群；3.太古宇—下元古界大别杂岩；4.燕山期花岗岩；5.断层
Fig. 7. Schematic map of regional structure geology in the area of the East Dabie and Shangma fault
表 1 样品D5-2和D11-3的错石LA-ICP-MS U-Pb年龄测试结果
Table 1. Results of the zircon U-Pb age dating by LA-ICP-MS for sample D5-2 and D11-3 in Shangma fault, Dabieshan
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