【作者】 叶培盛；

【导师】 吴珍汉； 胡道功； 江万；

【作者基本信息】 中国地质科学院 ， 构造地质学， 2004， 博士

【摘要】 蛇绿岩与逆冲推覆构造是造山带构造演化的2种不同地质记录，是现代大陆动力学研究的2个重要内容，在区域构造和造山带研究中占据重要学术地位。论文选择拉萨地块中部蛇绿岩和逆冲推覆构造为主攻方向，以西藏当雄幅（H46C002001）1：25万区域地质调查、构造专题研究及邻区对比观测为基础，通过野外地质测量、岩石地球化学测试、古构造分析和多种方法同位素测年，研究拉萨地块中部纳木错西岸蛇绿岩、永珠藏布—果忙错蛇绿岩与拉萨地块北侧班公—怒江缝合带安多—东巧蛇绿岩、拉萨地块南侧雅鲁藏布江缝合带泽当—罗布莎蛇绿岩及纳木错逆冲推覆构造、旁多逆冲推覆构造，分析早中侏罗世、晚侏罗—早白垩世、晚白垩世区域古构造环境及变迁过程，探讨不同时期区域构造样式、逆冲推覆构造运动历史和区域构造发展演化规律；深化了区域构造研究程度和认识深度，取得若干创新性研究成果。1、典型蛇绿岩地球化学特征及形成环境分析：对纳木错西岸蛇绿岩、永珠藏布—果忙错蛇绿岩、安多—东巧蛇绿岩和泽当—罗布莎蛇绿岩进行了系统的剖面观测、岩石薄片鉴定、岩石化学分析及稀土元素地球化学、微量元素地球化学对比分析。在纳木错西岸蛇绿岩片带发现枕状熔岩和放射虫硅质岩。揭示纳木错西岸、永珠藏布—果忙错、安多—东巧蛇绿岩带玄武岩具有相似的稀土元素配分模式，表现为LREE亏损的平坦型，无负Eu异常，类似于大洋中脊玄武岩；泽当—罗布莎蛇绿岩带玄武岩呈现平缓向右倾斜的稀土元素配分模式，属轻稀土富集、重稀土亏损型。测出纳木错西岸蛇纹石化橄榄岩全岩Rb-Sr等时线年龄为166±26Ma，变质辉长岩全岩和单矿物Rb-Sr等时线年龄为173±10Ma，确定纳木错西岸蛇绿岩形成时代为166-173Ma。根据岩石地球化学参数和产出构造部位，推断永珠藏布—果忙错—纳木错蛇绿岩形成于弧间盆地古海底扩张环境，东巧—安多蛇绿岩带形成于规模比较小的古大洋环境，泽当—罗布莎蛇绿岩形成于比较慢速的海底扩张环境。2、纳木错逆冲推覆构造研究：发现纳木错西岸嘎龙垭韧性变形带、饿弄怒布舍糜棱岩带和生觉—各昌茶玉韧性剪切带，详细观测典型逆冲断层的地质特征、变形特点、运动方向和序次关系，分析逆冲断层、褶皱构造、韧性剪切带的几何配置关系和动力学成因联系；分析纳木错逆冲推覆构造的空间影响范围，向东延伸与嘉黎逆冲断裂相连接，向西延伸与果忙错—永珠藏布构造带相归并，与纳木错西岸蛇绿岩、永珠藏布—果忙错蛇绿岩共同组成拉萨地块中部最重要的区域构造，命名为纳木错—永珠缝合带（NYS）。通过生觉糜棱岩的角闪石-斜长石矿物对地质温度压力计，测定变形变质温度为490-625℃，平均为547℃；形成压力为0.20-0.76GPa，平均为0.42Gpa；估算生觉—各昌茶玉韧性剪切带形成深度为12-15km。测定糜棱岩角闪石的³⁹Ar-⁴⁰Ar坪年龄为173.97±0.50Ma、斜长石的³⁹Ar-⁴⁰Ar坪年龄为109.45±0.50Ma，磷灰石裂变径迹年龄为43.6±3.8Ma，良好地揭示出纳木错西岸3期逆冲推覆构造运动发生时代。3、旁多逆冲推覆构造研究：厘定旁多逆冲推覆构造，自北向南划分为4个构造带：南部挤压滑脱构造带、中部叠瓦状逆冲断层带、北部高角度逆冲断层带及后缘逆冲推覆构造带，分别伴生紧闭倒转褶皱、斜歪倒转褶皱、直立—斜歪褶皱，组成白上而下倾角趋于变缓、自前缘向后缘产状逐步变陡、自北向南逆冲的铲型推覆构造。详细分析哈母前锋逆冲断层、下拉逆冲推覆断层、日阿—领布冲韧性剪切带及邦中逆冲推覆断层的地质特征、变形特点和运动方向，发现日阿—领布冲构造带自西向东变形深度逐步增大，表现为西段逆冲推覆、东段韧性剪切变形的显著分段活动特征，尚具有一定的右旋走滑分量。通过测定逆冲断层控制、切割侵入岩的同位素年龄及晚期吞噬、阻断逆冲断层的地质体时代分析，确定旁多逆冲推覆构造主要形成时代为古近纪中晚期（49-18.3Ma）。4、中生代区域古构造分析：通过分析典型蛇绿岩形成环境及相关沉积建造、火山活动、岩浆热事件、变质记录、构造变形，再造早中侏罗世、晚侏罗—早白垩世、晚白垩世区域古构造环境及变迁过程。论证拉萨地块中部早中侏罗世—早白垩世存在纳木错—永珠弧间古洋盆，提出中生代拉萨地块发育班公—怒江古大洋、班戈—桑雄古岛弧、纳木错—永珠弧间古洋盆、冈底斯岩浆弧和雅鲁藏布江古大洋，组成复杂的沟—弧—盆系统，构成古亚洲大陆板块南缘比较典型的活动大陆边缘。认为班公—怒江古大洋在早中侏罗世扩张增生的同时，开始向南俯冲消减，导致宁中花岗岩侵位和纳木错西岸生觉—各昌茶玉韧性剪切变形；至早白垩世完全闭合，形成班公—怒江缝合带；纳木错—永珠弧间古洋盆于早中侏罗世开始扩张增生，晚侏罗—早白垩世开始向北俯冲于班戈—桑雄岛弧带之下，早白垩世末期最终闭合，形成纳木错—永珠缝合带；雅鲁藏布江古洋盆自三叠纪开始扩张，早中侏罗世继续增生，晚侏罗—早白垩世在大规模扩张增生的同时，开始向北俯冲消减，于古近纪早期封闭，形成雅鲁藏布江缝合带（YZS）。探索性恢复侏罗—白垩纪古亚洲大陆南缘拉萨地块不同时期的沟—弧—盆体系。5、逆冲推覆构造组合与演化过程分析：发现北侧东巧—伦坡拉逆冲断裂（DLT）、中部纳木错逆冲推覆构造（WNT）、当雄南侧旁多逆冲推覆构造（SDT）具有相似的结构组成和相同的运动方向，存在有序的空间配置关系及动力学成因联系；主要逆冲断层均向北倾斜，呈叠瓦状叠覆关系，向地壳深部延伸会聚于中上地壳≥12-15km深度的缓倾斜构造滑脱面，构成拉萨地块北部影响宽度超过250km的大型逆冲推覆构造系统（NLT）。通过WNT糜棱岩的矿物对热年代学分析，发现纳木错西岸存在191-174Ma、174-144Ma、109.4Ma和44Ma四期区域性构造热事件，其中191-174Ma、174-144Ma、44Ma三期逆冲推覆运动导致的平均构造隆升速率分别为0.4mm／a、0.22mm／a、0.2mm／a。认为侏罗纪早中期（173-174Ma）、晚白垩世（约109Ma）、古近纪早期（约44Ma）WNT的逆冲推覆构造运动分别与班公—怒江古大洋板块南向俯冲、纳木错—永珠残余洋盆的萎缩和雅鲁藏布江古大洋板块的北向俯冲、新特提斯残余古大洋板块北向俯冲闭合及随后印度—欧亚陆—陆碰撞存在动力学成因联系。始新世中晚期—渐新世（≤18.3Ma），由于印度大陆快速北向俯冲，在中上地壳发生大规模拆离滑脱运动（NLT），在WNT南缘形成新的逆冲推覆构造（SDT），导致地壳大幅度挤压增厚和快速隆升。更多还原

【Abstract】 Ophiolite and thrust system are two types of geological record in orogenic belt andimportant contents of tectonics and continental geodynamics. The west Namco ophiolite slices andYongzhuzangbu-Guomanco ophiolites in central Lhasa block, Dongqiao-Ando ophiolites in northLhasa block, Zedang-Luobusha ophiolites in south Lhasa block and west Namco thrust, southDamxung thrust of central Lhasa block are studied in detail by field profiling, geochemical testing,isotopic dating and palaeo-structure analyses based on geological survey at scale 1:250000 forreconstruction of paleo-tectonics in Early-Meso-Jurassic, Late Jurassic-Early Cretaceous, LateCretaceous and understanding tectonic evolution of the Tibetan Plateau, which lead toaccumulation of new data and several key progresses.1. Analyses on geochemistry and tectonic setting of ophiolitesGeological profiling, microscope observation, geochemical testing and isotopic dating havefinished systematically for studying ophiolites of west Namco Lake, Yongzhuzangbu-Guomanco,Dongqiao-Ando and Zedang-Luobusha. Pillow lava and cherts were discovered in west Namcoophiolite mélange. Lava and serpentinite peridote from ophiolite mélanges of west Namco Lake,Yongzhuzangbu-Guomancuo and Dongqiao-Ando in central and north Lhasa block exist similargeochemical features of MORB characterized by rich LREE and slightly right-dipping pattern ofrare-earth elements without Eu anomaly. Rb-Sr isochronic ages of serpentinite peridote andmetamorphic gabbra of west Namco ophiolite mélange are 166±26Ma and 173±10Ma respectively,giving time constraints for ophiolites of central Lhasa block. Ophiolites in west Namco Lake andYongzhuzangbu-Guomanco of central Lhasa block formed in spreading back- or intra-island arcbasin, and ophiolites in Dongqiao-Ando of north Lhasa block and Zedang-Luobusha of southLhasa block formed in smaller ocean and slow-spreading ocean respectively according topetrological geochemistry.2. Study on west Namco thrustThree ductile shear zones were discovered in Gayalong, Elongnubu andShenjue-Gechangchayu, and geological features, deformation, motion direction and structuralpattern are studied in detail for major thrust faults, ductile shear zones and folds in west bank ofNamco Lake, leading to identification of west Namco thrust （WNT）. And west Namco thrust andophiolite mélanges outcropped from west Namco Lake to Yongzhuzangbu coexist together,indicating another tectonic suture in the Tibetan Plateau termed as the Namco-Yongzhu suture orNYS with its east extension to Jiali fault. Milonite from ductile Shenjue-Gechangchayu ductileshear zone formed in 490-625℃averaged 547℃and 0.20-0.76GPa averaged 0.42GPa,corresponding to 12-15km deep in crust. And ³⁹Ar-⁴⁰Ar dating of hornblende and plagioclase ofmilonite gave plateau ages of 173.97±0.50Ma and 109.45±0.50Ma respectively, indicatingthrusting of WNT in Early-Meso Jurassic and Late Cretaceous. While fission track dating of apatefrom milonite yielded an age of 43.6±3.8Ma, corresponding to Eocene thrusting of WNT.3. Study on south Damxung thrustThe first discovered south Damxung thrust （SDT） in east the Nyainqentanglha Mts. aredivided into 4 sections vertical to its extension including south section of detachment, middlesection of pilled thrust, north section of high-angle fault and back section of thrust slices accompanied by tight folding in south, overturn folding in middle and open wide folds in north.Major faults seems to merge into a deep detachment extended from the south frontier of SDT. Andgeological features, deformation, movement history of the Hamu frontier thrust fault, the Xialathrust fault, the Ria-Linbuchong tectonic belt consisted by high-angle thrust in west and ductileshear zone in east and the Bangzhong thrust slices were studied systematically by field profiling.And The Ria-Linbuchong thrust shows dextral strike-slipping evidences in many localities. TheSDT is inferred to be formed in 49-18.3Ma after isotopic dating of faulted complex and graniteemplaced after thrusting.4. Analyses on Jurassic-Cretaceous palaeo-tectonicsPaleo-tectonics in Early-Meso Jurassic, Late Jurassic-Early Cretaceous and Late Cretaceousare analyzed based on data of petrologic geochemistry of ophiolite mélange, sedimentaryformation, volcanic activity, magma emplacement, metamorphism and deformation. It is inferredthat an intra-island oceanic basin termed as the Namco-Yongzhu Ocean existed inEarly-Meso-Jurassic until Early Cretaceous in central Lhasa block. And Bangoin-Nujiang Ocean,Bange-Sangxiong Island-Arc, Namco-Yongzhu intra-island Ocean, Gangdese Island-Arc andYaluzangbu Ocean formed in Jurassic-Cretaceous in the Lhasa block, formed active continentalmargin in south of the Asia Plate. The Bangoin-Nujiang Oceanic Plate began southwardsubduction while spreading in Early-Meso Jurassic and closed in Early Cretaceous, lead toformation of the Bangoin-Nujiang suture （BNS）. The Namco-Yongzhu intra-island Ocean openedand widened in Early-Meso Jurassic by spreading, subducted northward beneath theBange-Sangxiong Island-Arc in Late Jurassic-Early Cretaceous and closed at the end of LateCretaceous, lead to formation of the Namco-Yongzhu suture （NYS）. The Yaluzangbu Ocean beganspreading since Triassic via Early-Meso Jurassic until Late-Jurassic-Early Cretaceous whennorthward subduction began, and closed in Paleocene-Early Eocene, lead to formation of theYaluzangbu suture （YZS）.5. Analyses of thrust association and tectonic evolution of north Lhasa blockThe Dongqiao-Lunpola thrust （DLT） in north Lhasa block, west Namco thrust （WNT） incentral Lhasa block and south Damxung thrust （SDT） in south Lhasa block show same thrustingdirection, similar composition and texture, ordered spatial pattern and related geodynamic cause.Major thrust faults dip to north, pilled in cross section and merge into a detachment in deep crust（≥12-15km）, consisting a large-scale thrusting system in north Lhasa block termed as north Lhasathrust （NLT） covering 250km range vertical to extension. Thermo-chronological diagram ofminerals from milonite shows 4 phases of thrusting in 191-174Ma, 174-144Ma, 109Ma and 44Maof WNT, lead to 0.4mm/a, 0.22mm/a and 0.2mm/a uplifting respectively. It is concluded thatthrusting in 174-173Ma, 109Ma and 44Ma of WNT were caused by southward subduction ofBangoin-Nujiang Oceanic Plate in Early-Meso Jurassic, northward subductions ofNamco-Yongzhu intra-island arc Oceanic Plate and Yaluzangbu Oceanic Plate in Late cretaceousand close of Yaluzangbu Ocean and its following continental collision in Early Eocenerespectively. And Eocene-Oligocene northward subduction of India continent caused southDamxung thrusting and detachment of NLT and huge shortening and thickening of crust and rapiduplift of the Tibetan Plateau before 18.3-13.5Ma.更多还原