【作者】 刘银；

【导师】 李荣社；

【作者基本信息】 中国地质大学（北京） ， 构造地质学， 2011， 硕士

【摘要】 金沙江蛇绿混杂带是青藏高原二十条蛇绿混杂带的一分子,其物质组成、结构构造及俯冲闭合时限、极性等方面的研究长期以来为地学界所关注。治多地区的多彩沟蛇绿构造混杂岩带是其西段,本文通过构造剖面测制,蛇绿岩和岛弧带相关样品的采集,构造样式的调研,几何学、运动学标志的收集,构造分期和配套关系的建立,剖面内部物质组成的厘定以及同位素地质年代学测试等工作,结合前人已有研究成果,对金沙江蛇绿混杂带治多段开展了相关研究,取得了如下认识与进展:1.通过野外和室内对不同岩片内部构造样式的解析,认为多彩蛇绿混杂岩经历了四期变形特征:①以复理石碎屑岩内部发育的无根勾状褶皱为特征,属于地壳深部构造层次变形,指示NNW向左行韧性剪切(枢纽Lb:330-350°∠60-73°),可能代表了最早期的俯冲变形;②以碎屑岩中透入性片理构成的小型顶厚-相似褶皱、紧闭褶皱为代表,运动学特征指示其为近NE向的挤压和NW向的左行剪切作用(枢纽Lb:125-140°∠55-70°),属于主俯冲期的构造群落;③以NE-SW向的中浅构造层次的逆冲型韧性剪切变形为特征,切割第二期的顶厚褶皱为特点,指示NNW向左行韧性剪切(S剪:50-65°∠55-65°),代表了碰撞期的构造群落;④以碎屑岩中发育的枢纽倾角变陡的大尺度倾竖褶皱(Lb:310-330°∠70-85°)和拉伸线理(La:130-150°∠0-10°)为代表,均反映了近NW向左行走滑剪切的运动学指向,代表后期隆升期构造特征。2.多彩蛇绿岩主要由辉长岩、堆晶辉长岩和玄武岩(基性熔岩)组成。其中,基性熔岩可分为板内洋岛玄武岩(OIB)和过渡型(MORB-IAT)两种系列。系列一属于碱性系列,来源于富集地幔的Em2端元,不属于蛇绿岩成分,与地幔柱或热点作用有关;系列二属于拉斑玄武系列,可分为洋脊型(MORB)和岛弧型(IAT)两种类型,并且具有过渡的特点,反映了其构造位置上处于洋中脊向海沟的偏海沟区域。辉长岩地球化学特征与系列二中的IAT组分相似,来源于原始地幔与亏损地幔之间的过渡带,为二者具有岛弧带特征提供了佐证。3.多彩蛇绿混杂岩带中出露三种不同类型的熔岩(OIB-MORB-IAT),前两者的出现可能暗示了金沙江洋经历了板内裂谷向大洋的转化,而IAT的出现则说明与板块消减作用相关。4.蛇绿混杂岩剖面中以出现绿泥石(Chl)+绿帘石(Ep)+阳起石(Act)组合为主,属于绿片岩相变质。除了受后期热液接触变质而在石英脉体中出现矽线石(Sil)外,在岩石本身并未发现相关的高温高压变质矿物。5.当江火山岩岩性组合以流纹岩-英安岩-安山岩为主,是一套典型的高Si、低Ti、富Na的钙碱性岩石组合,富集大离子亲石元素(Rb、Ba等),亏损高场强元素(Nb、Ta等)和P等元素,轻稀土元素富集,轻重稀土分馏较强以及轻度的Eu负异常等特点,指示其形成于与活动大陆边缘有关的岛弧构造环境。6.堆晶辉长岩LA-ICP-MS单颗粒锆石U-Pb测年为252.5±0.58(MSWD =0.95),指示多彩蛇绿岩形成于晚二叠世末期,可能代表了金沙江洋在多彩一带于晚二叠世存在与俯冲相关的弧前扩张事件年龄。当江岛弧带流纹斑岩LA-ICP-MS单颗粒锆石U-Pb年龄为237.6±1.7Ma(MSWD=2.3),代表了当江火山岩中流纹斑岩的结晶时代。说明金沙江洋俯冲作用在晚二叠世仍在继续,可能持续到后继中三叠纪的弧火山岩的出现。7.多彩蛇绿混杂岩基性熔岩的地球化学特征和判别图解显示:其为一套与俯冲相关的岛弧型蛇绿岩。而当江火山岩与多彩蛇绿岩在时空和成因上存在同源性和成对性,不但进一步证实了蛇绿岩为SSZ型,也指示了其由北东向南西的俯冲极性。8.依据多彩蛇绿混杂岩岩片变形、变质特征,接触关系等特征以及对比分析蛇绿岩三种主要的就位机制类型,认为多彩蛇绿岩的就位机制属于俯冲拼贴刮削式。更多还原

【Abstract】 Abstract:As one of the twenty opholite mélange zones in Qinghai-Tibet Plateau, the material composition, structure and tectonics, subduction and closure age and subduction polarity of Jinsha ophiolite mélange zone had attracted more and more attention in the long term. Based on the research of the anterior research fruit of other people, through measureing and making tectonic cross sections, collecting related samples of Duocai ophiolite and Dangjiang volcanic rocks(gabbro, basalt, greenschist, rhyolite, dacite, andesite, etc.), investigating and researching of tectonic patterns, gathering geometry and kinematics characteristics, building tectonic stage and matching relationship, identificating material composition and analyzing isotopic geochronology, etc. of Duocai ophiolite mélange in Zhiduo area , west part of Jinsha ophiolite méglange belt, the following thoughts and conclusions were obtained:1. By the field and interior research in the tectonic patterns of different sliver, the deformations of Duocai ophiolite mélange can be divided into four stages:①The first stage was characterized by the rootless hook-shaped folds in the flysch clastic which represent the early subduction stage, belonged to the deep tectonic arrangement and indicated the NNW-trending ductile shearing.②The second stage was characterized by the pint-sized similar fold and top-thickness fold developed from the penetrational schistosity which present the mainly subduction stage, indicated the NE-trending extrusion and NW-trending laevogyrate shearing.③The third stage was characterized by the trust ductile shearing from NE-SW which incised the top-thickness fold of the second stage and presented the collisional stage, belong to middle-shallow arrangement, suggested the NNW-trending ductile shearing.④The fourth stage was characterized by the large-scale steep plunging vertical fold which presented the late uplift stage, indicated NW-trending laevogyrate strike-slip ductile shearing in accordance with the NW-trending stretching lineation in the first sliver of the section.2. The Duocai ophiolites mainly consist of gabbro, cumulate gabbro and basalt(basic lavas), and the basic lavas can be divided into OIB and MORB-IAT two series: seriesⅠwere alkalescent, related with the mantle plume or hotspot, derived from the Em2 of enriched mantle, didn’t belong to the member of ophiolite. seriesⅡare tholeiitic and also can be divided into MORB and IAT two transitional series. The geochemistry of gabbros were similar to the basalt of seriesⅡand derived from the transitional zone between the prime mantle and depleted mantle which provide the evidence of both have the characteristic of island arc.3. There were three basic lavas of different component in Duocai ophiolites(OIB-MORB-IAT), the combination of OIB and MORB indicated that the Jinsha Ocean may have experienced the transitional process from the continental rift to ocean, while the exit of IAT suggested that the opiolites were related to the subduction of plate.4. The metamorphic minerals of ophiolite mélange were mainly composed of chlorite, epidote and actinolite and have no mineral of high temperature and high pressure expect there existed some sillimanites in quartz veins by the later hydrothermal contact metamorphism.5. The combination of rhyolite, dacite and andesite in Dangjiang volcanic rocks were rich in SiO2 and Na2O and poor in TiO2, belonging to calc-alkaline type rocks. The chondrite-normalized REE-patterns were characterized by LREE enrichment(Rb,Ba)with variable degree of negative Eu anomalous values and the primary mantle-normalized trace elements-parterns by HFSE depleted as well as P element, indicating that it formed in the island arc environment related to active continental margin.6. The single zircon U-Pb age of cumulate gabbro was 252.5±0.58Ma (MSWD=0.95) which showed that Duocai ophiolites formed in the end of late permian, may present the age of for-arc spreading related to the subduction in Zhiduo area of the Jinsha Ocean. The single zircon U-Pb age of rhyolite porphyry of Dangjiang volcanic rocks was 237.6±1.7Ma(MSWD=2.3)which presented the crystallization age. The two group of ages suggested that the subduction was continuing in the end of Permian and lasted to the occurrence of arc volcanic rocks in middle Triassic.7. The geochemistry characteristic and the discrimination diagrams of basic lavas showed that the ophiolites were formed in the environment of island-arc. The well matching in time, space and causes which not only indicated the ophiolites belong to SSZ type, but also suggested the subduction polarity were from northeast to southwest.8. By systematical analyzing the characteristics of deformation, metamorphosis, contact relationship in ophiolite méglanges and throuth the contrastive analysis of the mainly three emplacement mechanism types related with continental movement, we concluded that the emplacement mechanism of Duocai ophiolite méglange belonged to subduction scrape adhesion type.更多还原