桂东北地区鹰扬关韧性剪切带的厘定及其构造意义

Discovery of Yingyangguan Ductile Shear Zone in Northeastern Guangxi and Its Tectonic Significances

论文通过宏-微观构造、磁组构、热液锆石和石英EBSD组构等,厘定鹰扬关韧性剪切带并讨论其构造意义。鹰扬关韧性剪切带具有宏-微观韧性变形组构,发育糜棱岩、拉伸线理、S-C组构、旋转碎斑系、书斜构造、压力影和石英的动态重结晶等。磁组构和宏-微观构造表明,鹰扬关韧性剪切带呈NNE向延伸超过40 km,宽2.5~8 km。糜棱C面理的极密点产状127°∠50°;磁面理的极密点产状107°∠83°。宏-微观构造研究表明,鹰扬关韧性剪切带具有早期左旋逆冲剪切,晚期右旋正滑剪切的运动学性质。石英EBSD组构表明,鹰扬关韧性剪切带具有晚期中低温变形(400~550℃)叠加于早期中高温变形(550~650℃)的特征。年代学研究表明,鹰扬关韧性剪切带早期左旋逆冲剪切的时代为(441.1±2.3)Ma,晚期右旋正滑剪切的时代应晚于420 Ma,区域构造应力由挤压转为伸展的时限为420 Ma。在磁组构、石英EBSD组构和热液锆石定年的基础上,结合区域地质资料,认为鹰扬关韧性剪切带形成于华夏陆块自SE向扬子陆块造山挤压的构造背景。早期造山挤压,产生压扁型应变和中高温左旋逆冲剪切;晚期造山后伸展,产生拉伸型应变和中低温右旋正滑剪切。鹰扬关韧性剪切带的上述认识暗示其形成与早古生代扬子和华夏陆块的造山作用有关,而非构造混杂作用的产物。

Based on macro-micro structure,magnetic fabric,hydrothermal zircon and quartz EBSD fabric,the Yingyangguan ductile shear zone was determined and its tectonic significance is discussed.The Yingyangguan ductile shear zone has typical macro-micro structural characteristics,such as mylonite,stretching lineation,S-C fabric,rotating porphyry system,domino structures,pressure shadow structure and dynamic recrystallization of quartz.Magnetic fabric measurement and field geological survey showed that the Yingyangguan ductile shear zone extends over 40 km in NNE direction and is 2.5-8 km wide.The extremely dense point occurrence of the C folication of the mylonite is 127°∠50°,and the occurrence of extremely dense points of magnetic foliation is 107°∠83°.Analysis of the macro-micro structure showed that the Yingyangguan ductile shear zone has kinematic properties of early sinistral thrust shear and late dextral normal slip shear.Quartz EBSD fabric analysis revealed that the Yingyangguan ductile shear zone is characterized by late mid-low temperature deformation(400-550℃)superimposed on early mid-high temperature deformation(550-650℃).The geochronology showed that the age of the early sinistral thrust shear is(441.1±2.3)Ma,and the age of late dextral normal slip shear is later than 420 Ma.The time limit of regional tectonic stress from compression to extension is 420 Ma.Analysis of the magnetic fabric,quartz EBSD fabric,and hydrothermal zircon dating,combined with regional geological data,revealed that the Yingyangguan ductile shear zone formed in the tectonic background of the collision of the Cathaysia block from the SE to the Yangtze block.In the early stage of extrusion and collision,flattening strain was generated and mid-high temperature sinistral thrust shear was formed.After the late orogenic extension,tensile strain was generated,and dextral normal slip shear was formed at mid-low temperature.The strain characteristics,deformation temperature,deformation age,and dynamic setting of the Yingyangguan ductile shear zone provide new data that improve the understanding of Caledonian tectonic movement in South China.The above understanding of the Yingyangguan ductile shear zone may be related to the orogeny of the Yangtze and Cathaysian blocks in the Early Paleozoic,rather than being the product of tectonic mélange.