【作者】 徐梦婧；

【导师】 张兴洲； 李才；

【作者基本信息】 吉林大学 ， 构造地质学， 2014， 博士

【摘要】 狮泉河–永珠–嘉黎蛇绿混杂岩带是青藏高原一条重要的构造带，它与班公湖–怒江缝合带密切相关，均是青藏高原中部新特提斯洋演化的产物。目前，狮泉河–永珠–嘉黎蛇绿混杂岩带研究还很薄弱，在构造属性，演化时限和东段延伸等问题上存在很大争议，这直接制约了对青藏高原中部新特提斯洋演化历史的认识。本文选择了狮泉河–永珠–嘉黎蛇绿混杂岩带中西段发育较好的果芒错蛇绿混杂岩和中仓蛇绿混杂岩为重点研究对象，开展了详细的野外地质填图；对蛇绿混杂岩各端元岩石进行了岩相学、矿物化学、地球化学以及锆石LA–ICP–MS U–Pb定年和Lu–Hf同位素的研究；总结了狮泉河–永珠–嘉黎蛇绿混杂岩带和班公湖–怒江缝合带在蛇绿混杂岩方面已取得的研究成果，对它们的特征进行了对比分析；结合区域地层和岩浆岩等相关资料，探讨了狮泉河–永珠–嘉黎蛇绿混杂岩带的构造演化。果芒错蛇绿混杂岩发育完整的彭罗斯型蛇绿岩岩石组合，包括变质橄榄岩、镁铁质–超镁铁质堆晶岩、基性岩墙、枕状玄武岩、硅质岩以及少量块状铬铁矿。矿物化学和地球化学分析显示，果芒错蛇绿混杂岩形成于靠近大陆边缘具有强烈MORB特征的弧后盆地之中。部分基性岩墙具有典型E–MORB特征，可能是弧后盆地演化后期，板片回转引起地幔楔中形成新的地幔对流，导致受到俯冲板片熔体混染的地幔发生部分熔融形成E–MORB。单颗粒锆石定年结果初步限定，果芒错蛇绿混杂岩的形成时代为早侏罗世到早白垩世。果芒错蛇绿混杂岩的伴生岩石索尔碎屑岩，主要由蛇纹质碎屑和碳酸盐基质所组成，其角度不整合覆盖在果芒错变质橄榄岩之上。索尔碎屑岩的野外地质特征和岩相学特征显示，它可能是地幔橄榄岩发生破碎后受到碳酸盐热液流体交代所形成的。中仓蛇绿混杂岩是在达查沟地区进行1:5万区域地质调查过程中新发现的，主要由变质橄榄岩、堆晶辉长岩、辉长岩墙以及少量玄武岩和硅质岩组成。堆晶辉长岩和辉长岩墙同时具有MORB和IAB的特征，可能形成于洋内弧后盆地环境。地球化学和锆石Lu–Hf同位素特征显示，辉长岩可能是尖晶石二辉橄榄岩经过30%部分熔融形成的，并受到了俯冲流体和熔体的影响。堆晶辉长岩锆石U–Pb定年为114.3±1.4Ma和116.1±1.8Ma，辉长岩墙定年结果为113.4±1.7Ma。中仓蛇绿混杂岩可能形成于早白垩世洋内弧后盆地。前人在狮泉河–永珠–嘉黎蛇绿混杂岩中厘定的年龄为晚三叠世到早白垩世，镁铁质岩石主要以MORB和岛弧玄武岩特征为主，地幔源区为DDM和EM2；而班公湖–怒江缝合带中蛇绿混杂岩的形成时代为晚二叠世到早白垩世晚期，镁铁质岩石按照地球化学特征可分为MORB、岛弧玄武岩、碱性洋岛玄武岩和玻安岩，地幔源区为DDM、EM1和EM2。结合区域地层和岩浆岩等资料，班公湖–怒江缝合带可能代表了青藏高原中部新特提斯洋的主洋盆，从晚二叠世开始形成，并持续演化到早白垩世，狮泉河–永珠–嘉黎蛇绿混杂岩带为班公湖–怒江洋内弧后盆地的产物。狮泉河–永珠–嘉黎蛇绿混杂岩带在嘉黎以东汇入班公湖–怒江缝合带，早侏罗世晚期东段最终闭合。嘉黎以西的狮泉河–永珠–嘉黎蛇绿混杂岩带是班公湖–怒江洋南向俯冲形成的弧后盆地，规模有限、演化时限较短，在嘉黎–永珠段靠近大陆边缘，向西逐渐远离。狮泉河–永珠–嘉黎弧后盆地打开时间从晚三叠世到中侏罗世，具有东早西晚的穿时现象，晚侏罗世开始南向俯冲，至早白垩世最终闭合。更多还原

【Abstract】 Shiquanhe–Yongzhu–Jiali ophiolitic mélange belt (SYJMB) is an important tectoniczone in Tibetan plateau and the relict of Neo–Tethys Ocean in central Tibet as well asBangong–Nujiang suture zone (BNSZ). The study of SYJMB is so limited that its attribute,evolutional time and eastward extend are still controversial, which has constituted an obstaclefor understanding the evolution of Neo–Tethys Ocean in central Tibet.In the thesis, the Guomangco and Zhongcang ophiolitic mélanges in middle and westernparts of SYJMB were investigated by fieldwork, petrology, mineral chemistry, geochemistry,zircon LA–ICP–MS U–Pb geochronology and Lu–Hf isotope. Based on the previous studieson ophiolitic mélanges, strata and magmatic rocks, the relation between SYJMB and BNSZand evolution of SYJMB were investigated.Guomangco ophiolitic mélange develops all subunits of a typical Penrose–type ophiolitepseudostratigraphy, including metamorphic peridotite, mafic–ultramafic cumulate, mafic dike,pillow basalt, siliceous and minor chromitite. Mineral chemical and geochemical analysesshow that Guomangco ophiolitic mélange are probably originated in a back–arc basin withstrong MORB–related characteristics near continental margin. The mafic dikes mostly haveE–MORB major and trace element signatures, and formed in late stage of evolution ofback–arc basin. With the evolution of back–arc basin, subducted slab gradually retreated, andnew mantle convection occurred in mantle wedge. The recycling may make themetasomatized mantle undergo high degrees of partial melting and generate E–MORBs.According to the single zircon U–Pb dating, the evolutional time of Guomangco ophioliticmélange is between Early Jurassic and Early Cretaceous. Suoer clastic rock covers theperidotite of Guomangco ophiolitic mélange in angular unconformity, and is mainly composed of clastic and groundmass. Based on the field and petrologic characteristics, anorigin of Suoer clastic rock is considered: Guomangco peridotites break and peridotitecalstics were metasomated by hydrothermal fluid of carbonate.Zhongcang ophiolitic mélange is new discovered during regional geological surveying inDachagou area. The ophiolitic sequence mainly consists of metamorphic peridotite, cumulategabbro and gabbro dike with minor basalt and siliceous. The cumulate gabbros and gabbrodikes have island arc and mid–ocean ridge basalt affinities, suggesting that they wereoriginated in an oceanic back arc basin. Whole rock geochemistry and high positive εNd(t)values show that these gabbros were derived from~30%partial melting of a spinel lherzolitemantle, which was enriched by interaction with slab–derived fluids and melt from sediment.U–Pb analyses of zircons from cumulate gabbros and gabbro dike yield weighted mean agesof114.3±1.4Ma,116.1±1.8Ma and113.4±1.7Ma, respectively. Zhongcang ophiolitic mélangewere probably formed in an Early Cretaceous oceanic back arc basin.The ages of SYJMB ophiolitic mélanges vary from Late Triassic to Early Cretaceous,and geochemistry and stratigraphic log could be connected to short–lived intra–oceanicback–arc basin origin. The ages from BNSZ ophiolitic mélanges are between Late Permianand Early Cretaceous. The BNSZ mafic rocks show variable mixing of components fromMORB–type to IAT and to OIB end–members, the mantle source of BNSZ is probablymainly composed of DDM and EM1with minor EM2. The BNSZ probably represents themain oceanic basin of Neo–Tethys ocean in central Tibet, which underwent a complexevolution.The SYJMB joins into BNSZ in the east of Jiali, which was closed in late Early Jurassic.The middle and western parts of SYJMB represent a short–lived back–arc basin caused bysouthward subduction of BNSZ, and the back–arc basin is near continental margin from Jialito Yongzhu. The back–arc basin opened earlier in the east and propagated westwards frommiddle Jurassic to early Cretaceous. In Late Jurassic, the back–arc basin subductedsouthward and finally closed in Early Cretaceous.更多还原