高喜马拉雅普兰地区东西向韧性拆离作用及其构造意义

East-west-trending ductile detachment in the Burang area, Greater Himalaya, and its tectonic significance

高喜马拉雅地体西部普兰地区东西向面状韧性拆离作用发育,韧性拆离带内的岩石都经历了不同程度的塑性变形,糜棱岩化作用普遍存在,面理向东缓倾,拉伸线理向东倾伏,倾伏角5-18°。不对称旋转应变构造指示韧性拆离带具有早期自西向东的运动性质,及后期又经历了自东向西脆韧性变形的特点。EBSD组构测定结果反映糜棱岩中变形石英的晶格优选方位具有自西向东的高温柱面{10 10}<a>、中温菱面{101 1}<a>和自东往西的中温菱面 {101 1)<a>、低温底面{0001)<a>两组滑移系,结合糜棱岩中普遍存在的不对称旋转构造特点分析说明构造带早期自西向东韧性拆离作用形成于中高温环境,后期自东向西的脆韧性变形形成于中低温环境。锆石sHRIMP U-Pb 定年结果表明普兰地区高喜马拉雅变质基底原岩形成于1863Ma。普兰东西向韧性拆离作用形成于中新世,与高喜马拉雅地体南北两侧的MCT和STDS形成时代相吻合,是印度板块向北俯冲产生的挤压应力作用于弧形喜马拉雅造山带时,分解出平行于造山带方向的剪切应力促使造山带内中下地壳的物质沿平行于造山带方向发生塑性流动形成的,是与MCT和STDS在同一构造事件中同一构造应力场内形成的,是青藏高原大规模南北向缩短、隆升和一部分物质向东逃逸的产物。

All the rocks in the east-west planar ductile detachment zone in the Burang area in the western part of the Greater Himalaya terrane have suffered from plastic deformation to different degrees and mylonitization. The foliation of the mylonites dips gently to the east and the stretching lineation plunges to the east at 5°-18°. Asymmetrical rotational strain structure indicates that the ductile detachment had the nature of east-directed movement m the early stage and the features of west-directed brittle-ductile deformation. Electron backscatter diffraction （EBSD） fabric analysis shows that the preferred orientation of deformed quartz in mylonites has two slip systems ：the east-directed high-temperature prism{ 10（1）0} 〈a〉 and intermediate-temperature rhomb glide{ 10（1） （1）} 〈a〉 system and the west-directed intermediate-temperature rhomb glide { 10（1） （1）} 〈a〉and low-temperature basal glide{0001 }〈a〉 system. This, combined with the characteristics of the asymmetric rotational structure prevalent in mylonites, suggests that the east-directed ductile detachment in the early stage formed in an intermediate- to high-temperature environment and that the west-directed brittle-ductile in the late stage formed in an intermediate- to low-temperature environment. Zircon SHRIMP U-Pb dating shows that the protolith of the metamorphic basement in the Burang area of the Greater Himalaya terrane formed at 1863 Ma. The isotopic dating of mylonitic granite and undeformed leucogranite show that the east-west-trending Burang planar ductile detachment took place in the early Miocene, which coincides with the ages of the MCT and STDS on the northern sides of the Greater Himalaya terrane. When the compressional stress produced by northward subduction of the Indian plate acted on the arcuate Himalayan orogenic belt, it was resolved into two components, one being orogen-parallel and the other orogen-perpendicular. The former caused plastic flow of the material of the middle and lower crustal below the orogenic belt, thus forming the Burang planar ductile detachment zone, while the latter resulted in the formation of the MCT and STDS. They are different tectonic styles formed in the same tectonic stress field and same tectonic event. The detachment zone is the product of large-scale north-south shortening and uplift and eastward escape of a part of material.