哀牢山-红河剪切带中段多阶段新生代花岗岩脉:同位素年代学及对于剪切应变型式转变时间的约束

Multiple stages of granitic dykes along the Ailao Shan-Red River shear zone: Constraints on timing of switch of shear strain types

哀牢山-红河剪切带是新生代印度板块与欧亚板块碰撞过程中发育的大规模走滑型剪切带,其发育对于碰撞过程中印支地块的南东向逃逸以及藏东南地区构造格局的形成具有重要的贡献。与剪切带演化相关,伴随发育多阶段花岗岩脉就位,它们为限定剪切变形时限、阐明剪切作用属性提供了重要证据。本文在野外观察基础上,应用显微构造和EBSD石英c-轴组构分析查明花岗岩脉的构造特点与应变型式,同时采用锆石LA-ICP-MS测年方法获得岩脉侵位与结晶年龄。年龄分析结果表明,岩脉年龄分别为27.09±0.48Ma、25.17±0.23Ma和25.16±0.50Ma,其中年龄为27.09±0.48Ma的花岗岩脉具有糜棱岩化现象,其变形特征体现为中温变形后叠加低温变形,且剪切变形形式由一般剪切转换为简单剪切;年龄为25.17±0.23Ma的花岗岩脉表现出同剪切晚期构造特征,且具有较低温度简单剪切变形特点;25.16±0.50Ma的切穿糜棱叶理,矿物未见变形,可能代表剪切期后岩脉。结合区域构造,推测剪切方式由纯剪为主的剪切向由单剪为主的剪切转换发生在27Ma和25Ma之间,哀牢山-红河剪切带中段在约25Ma走滑运动结束。

The Ailao Shan-Red River shear zone is a large scale strike slip fault resulted from collision between the Indian and EuroAsia plates in Cenozoic. The formation and evolution contributed to the generation of the present structural framework and the southeastward extrusion of the Indochina block during the collision. Multiple stages of granitic dykes were emplaced in association with shearing along the shear zone. They provide evidence constraining the timing and nature of shearing. Based on field observations,microstructural and EBSD quartz c-axis analysis were conducted to clarify structural properties and strain patterns of the granitic intrusions. LA-ICP-MS zircon U-Pb dating results of the granitic emplacement and crystallization suggest that the three dated dykes were emplaced at 27. 09 ± 0. 48 Ma,25. 17 ± 0. 23 Ma and 25. 16 ± 0. 50 Ma,respectively. The dyke with age of 27. 09 ± 0. 48 Ma possesses evidence for intensive low temperature mylonitization superimposed on an earlier middle temperature deformation. Fabric and microstructural analysis reveals an obvious change from pure-shear dominated to simple shear-dominated strains. The dyke with an age of 25. 17 ± 0. 23 Ma exhibits structures and fabrics characteristics of late syn-shearing dykes and of simple shearing at low temperatures.The dyke with age of 25. 16 ± 0. 50 Ma is a post-shearing dyke that cuts across the mylonitic foliations of the host rocks and show very weak mineral deformation. Combining with regional tectonic analysis,it is expected that shearing along the Ailao Shan-Red River shear zone transformed from pure shear to simple shear between 27 Ma and 25 Ma,and the cessation of strike-slip shearing along the middle segment of the Ailao Shan-Red River shear zone occurred at ca. 25 Ma.