藏南冈底斯岩浆带中段曲水韧性剪切带的变形特征及其年代学约束

Deformational Characteristics and Geochronological Constraints of Quxu Ductile Shear Zone in Middle Gangdese Magmatic Belt,South Tibet

藏南曲水地区鸡公-色甫韧性剪切带系统的运动学和动力学研究,不仅对造山带构造研究具有重要理论意义,而且对青藏高原中-南部区域地质研究具有推进作用.作为藏南冈底斯岩浆带曲水岩基中的重要断裂构造,曲水韧性剪切带经历了新生代以来的大规模构造变形,其中走滑剪切作用最为显著,整体表现为右行走滑为主.通过对曲水剪切带中的构造片岩、初糜棱岩、糜棱岩以及长英质脉体等野外观测并结合室内镜下薄片以及石英EBSD（electron backscatter diffraction）组构分析,认为曲水剪切带主要经历了一期韧性变形事件.根据石英-长石变形矿物对并结合石英EBSD组构分析得出,构造变形发生的温度大约为500~550℃,高绿片岩相到角闪岩相.剪切带内普遍发育有不对称的褶皱、构造透镜体、σ碎斑、S-C组构和石香肠等变形组构,显示出右行走滑的特征.对研究区34组糜棱面理和9组拉伸线理进行极射赤平投影,糜棱面理的优选方位约为355°∠70°,拉伸线理产状约为95°∠8°.此外,在糜棱岩中发育两类长英质脉体,根据野外分布特征、显微组构、锆石成因学以及岩石地球化学特征综合研究认为,两类长英质脉体为同构造剪切脉体,其年代学可以对剪切带的形成时限起到很好的制约.通过对两套长英质脉体分别进行LA-ICP-MS锆石U-Pb定年,得到了38.67±0.88Ma和35.05±0.29Ma两组加权平均年龄,表明了曲水剪切带韧性变形发生于35~38Ma的始新世末期（普利亚本期）.这一年龄值处于印度-亚洲大陆晚碰撞期,因此曲水韧性剪切带右行走滑事件可能是印度板块持续向北俯冲,构造应力在欧亚板块边缘释放引起的陆内构造响应.

The systematic kinematic and dynamic researches of Jigong-Sefu ductile shear zone not only have theoretical significance for the orogenic tectonics,but also advance geological studies of the middle-southern segments of the Tibetan plateau.Quxu ductile shear zone,a vital part of geological structures of Quxu batholith in Gangdese magmatic belt,has experienced large-scale structural deformations featuring with ductile strike-slip shearing,predominantly of dextral shearing since Cenozoic（top to the east）.Based on outcrop observations of structural schist,protomylonite,mylonite and felsic veins and thin section analyses as well as quartz EBSD fabric testing,one stage of ductile deformation was identified in the Quxu shear zone.It isfound by mineral deformation geothermometer of quartz-feldspar and quartz EBSD fabric analyses that the shear zone is characterized by middle temperature deformations with 500-550℃（from high greenschist facies to amphibolite facies）.Structural deformation of the Quxu shear zone observed in South Tibet reveals that the asymmetric folds,structural lenses,σporphyroclast,S-C fabrics and boudinages are good indicators,which suggest a dextral strike-slip shearing.Average values of 34 mylonitic foliations and 9stretching lineation are 355°∠70°and 95°∠8°.Moreover,two kinds of granitic veins were identified in the Quxu shear zone.Combined with field occurrence,microstructures,zircon CL features and geochemistry of the felsic veins,we interpret that the two kinds of granitic veins are syn-tectonic shearing intrusions,whose ages can make an accurate geochronological constraint on the formation of the Quxu shear zone.LA-ICP-MS zircon dating of the granitic veins yields ages of 38.67±0.88 Ma and 35.05±0.29 Ma,respectively,further indicating that the Quxu shear zone started at 35-38Ma（Priabonian stage of Eocene）.The shearing time is consistent with that of the late stage of the Indian-Asian plate collision.Therefore,it is concluded that the Quxu shear zone might be related to continuous northward subduction of the Indian plate,corresponding to tectonic stress relation and an intra-continental deformation event.