【作者】 董汉文；

【导师】 许志琴；

【作者基本信息】 中国地质科学院 ， 构造地质学， 2014， 博士

【摘要】 本文通过对东喜马拉雅构造结墨脱剪切带进行了详细的构造变形、变质岩石学、岩石组构学及同位素年代学研究,确定了该断裂带的几何学和运动学性质、活动时限,查明区内高级变质岩的P-T轨迹,同时对构造结东、西两侧剪切带进行了对比,初步认识了新生代以来东喜马拉雅构造的构造演化过程。本研究主要获得如下几点认识：1、通过野外详细的构造解析,初步确定墨脱剪切带与东久-米林剪切带(西侧)具有类似的特征,不同部位的变形性质具有逐渐演化的特征。从南至北分为三段：阿尼桥-希让段、旁辛-达木段及甘登-加拉萨段。几何学和运动学研究结果显示其分别表现为右行兼斜向正滑的运动学特征、右行走滑运动学特征及右行兼斜向逆冲的运动学特征；2、通过对同构造岩脉进行锆石U-Pb定年,及二云母片岩、石英云母片岩等岩石中白云母的40Ar-39Ar定年分析,共同限定了墨脱剪切带的活动时限。年代学结果表明：自印度-亚洲板块碰撞以来,墨脱剪切带经过多期多阶段的演化过程,起始活动时限不晚于～28.6Ma；3、在墨脱剪切带附近出露大量高级变质岩(如含蓝晶石-石榴石云母片岩、角闪岩类等),温度和压力计算结果表明研究区内变质岩变质作用经历了顺时针的P-T轨迹和近等温降压(ITD)的退变质演化过程,并且其峰期变质阶段属于高压变质相系；4、通过对东构造结东、西边界(墨脱剪切带和东久-米林剪切带)进行几何学和运动学及活动时限的对比,我们认为：(1)东、西两侧断裂带不同地段具有相对应的几何学和运动学特征,如：南段均具有右/左行兼下滑的运动学特征；中段则均以走滑性质为主,其中东侧为右行走滑运动,西侧为左行走滑运动；北段均具有右/左行兼逆冲的运动学特征；即南迦巴瓦变质体受制于这两条断裂带相对向北推移,并深深楔入拉萨地体之下；(2)两侧剪切带的活动时限具有一致性,并且与南迦巴瓦变质体的隆升过程具有同步性；5、综合已有研究结果,我们提出自印度-亚洲板块碰撞以来东喜马拉雅构造结的构造演化过程,主要经历了俯冲-碰撞阶段(从65Ma至40Ma左右)、折返阶段(从37Ma至23Ma左右)及快速隆升阶段(从22Ma至今)三个阶段。更多还原

【Abstract】 In this thesis, I present tectonic deformation, metamorphic petrology, rock fabric and isotope chronology for the Medog shear zone, with the aim of constraining the geometry, kinematics and initiation time of the shear zone, as well as the P-T paths of high-grade metamorphic rock in the study area. Then I will compare the Medog shear zone with the Milin-Dongjiu shear zone, which is the western boundary of the eastern Himalyan Syntaxis. Finally the tectonic evolution of the eastern Himalayan Syntaxis will be discussed. The main results are as follows:1. Through the detailed field observations, we find out that the Medog shear zone is complex and shows variations from place to place, depending mainly on its orientation. From south to north, the Medog shear zone can be divided into three segments:Aniqiao-Xirang, Pangxin-Damu and Gandeng-Jialasa. Our research demonstrates that they are a normal shear zone with oblique stretching lineation, a dextral ductile strike-slip shear zone and a right lateral thrust shear zone, respectively.2. Based on the new LA-ICP-MS zircon U-Pb dating for syn-kinematic leucocratic intrusions and40Ar/3Ar ages of muscovite mostly from two micas schists and quartz-mica schists,the initiation time of the Medog shear zone can be well constrained. The geochronology analysis reveals that the Medog shear zone has undergone multistage evolution since India-Asia collision, and its right lateral shearing initiatied no later than～28.6Ma.3. Large amount of high grade metamorphic rocks, e.g. mica schists bearing Ky-Grt and amphibolites, are widely exposed along the Medog shear zone. Thermobaromertric computation shows that these rocks have undergone metamorphism characterized by clockwise P-T paths and nearly isothermal decompression (ITD) with the peak high-presure conditions.4. According to comparisons (geometrics, kinematics and the initiation time) of the two boundaries of the syntaxis, i.e. the Medog shear zone and Dongjiu-Milin shear zone, it suggests that the two shear zones have similar geometry and kinematics: normal slip dominates in the south section, strike slip for the central parts where sinistral in western and dextral in eastern segment, respevtively, and finally thrust slip controls the north section. It indicates that the Namche Barwa metamorphic terrane is controlled by the two shear zones, which moved northward and subducted deeply beneath the Lhasa terrane. On the other hand, the two shear zones started at the same time, and are coeval as the uplifting of Namche Barwa metamorphic terrane.5. Combining with the published data, a three-stage model has been established to explain the formation and evolution of the eastern Himalayan Syntaxis:the subduction-collision stage (65-40Ma), exhumation stage (37-23Ma) and rapid uplift stage (22Ma-present).更多还原