【作者】 朱弟成；

【导师】 潘桂棠； 莫宣学；

【作者基本信息】 中国地质科学院 ， 构造地质学与地球动力学， 2003， 博士

【摘要】 位于青藏高原南部高喜马拉雅和北部印度河-雅鲁藏布缝合带之间的特提斯喜马拉雅带是研究冈瓦纳大陆北缘拉张破裂、板块漂移和大陆拼合以及洋盆形成、演化和消亡等重要地质过程的理想场所，长期以来倍受国内外地学界的关注。前期研究主要是从沉积学、古生物学、构造地质学等方面对冈瓦纳大陆北缘的拉张破裂和特提斯的构造演化进行讨论，对赋存于特提斯喜马拉雅带的大量火山岩所开展的研究工作非常有限，主要原因，一是以往地质调查程度不够，对这些火山岩的分布情况不了解；二是从根本上对这些火山岩缺乏足够的重视，以致忽略了从火山活动记录的角度探讨特提斯构造演化的一些基本问题。火山活动作为壳-幔、岩石圈-软流圈相互作用的信息载体、岩石探针和重要地质过程的重要指示剂，在岩石学和大地构造研究领域一直占有非常重要的地位。2000年开始进行的国土资源大调查和我们开始于同期的野外调查工作，为从火山岩角度研究冈瓦纳大陆北缘的拉张破裂和特提斯的构造演化提供了机会和条件。本论文以特提斯喜马拉雅带中段晚古生代以来不同时代的火山岩为对象，采用区域构造演化与火山事件相结合的思路，在回顾了特提斯喜马拉雅带地质背景的基础上，重点对该带中段晚古生代以来不同时代火山岩的特点、时空演化、岩浆成因及其在冈瓦纳大陆北缘的拉张破裂和特提斯构造演化中的意义进行了详细分析和讨论，取得以下初步认识：（1）根据前人研究成果和最新资料，指出特提斯喜马拉雅带的构造演化与雅鲁藏布弧后洋盆的发生和发展息息相关，二者均受到由班公湖-怒江缝合带再造的特提斯大洋壳南向俯冲的约束。（2）特提斯喜马拉雅带中段晚古生代以来的地层系统中，从二叠纪→三叠纪→侏罗纪→白垩纪，共有11个层位含规模不等的火山岩，它们以透镜体、薄的夹层或以块状玄武岩或玄武质安山岩等形式产出于不同地层系统中，火山活动较为频繁但规模不大，并且从二叠纪→早中三叠世→晚三叠世→侏罗纪和白垩纪，火山活动具有由西向东、从南→北→南→北的迁移规律。（3）通过系统的地球化学分析，首次查明了不同时代火山岩的地球化学特征及其演化特点。结果表明，无论是常量元素、稀土元素、微量元素丰度及相关参数，还是Pb同位素组成，在中晚二叠世与早三叠世之间、中三叠世与晚三叠世之间，均发生了明显改变。常量元素特点：二叠纪玄武岩以高镁低钾为特点，具原生岩浆性质；早、中三叠世玄武岩以低镁高钾、TiO₂高含量为特征，演化程度增大，从相似于MORB转变为与OIB相似；晚三叠世玄武岩又转变为高镁低钾特点，演化程度相对较低；中侏罗世和侏罗纪末期桑秀组玄武岩具低镁特点，演化程度增高，TiO₂含量与OIB接近；早白垩世玄武岩TiO₂含量低于OIB，K₂O含量明显降低。从二叠纪到中三叠世、晚三叠世到早白垩世常量元素演化显示出两个旋回的特点。稀土元素特点：二叠纪玄武岩以较低的稀土元素总量和轻重稀土分异程度为特征，中晚二叠世微弱负Eu异常；早、中三叠世稀土元素总量增高，与社会群岛OIB接近，分异程度也增大，早三叠世不具Eu异常，而中三叠世显示特征的正Eu异常；晚三叠世稀土元素总量和分异程度显著降低，与P-MORB接近；中侏罗世、侏罗纪末期玄武岩稀土元素总量和分异程度明显增高，无Eu异常，与早、中三叠世一样，接近于OIB；桑秀组玄武岩的稀土元素总量和轻重稀土分异程度明显低于流纹英安岩，并且后者具明显负Eu异常（δEu平均0.56）；与早期侏罗纪玄武岩相比，早白垩世玄武岩稀土元素总量和轻重稀土分异程度都有所降低。微量元素特点：二叠纪玄武岩以高含量的相容元素和低丰度的高场强元素为特征；而早、中三叠世相容元素丰度明显降低而高场强元素丰度显著增高，正Nb异常也越明显；晚三叠世玄武岩较高的相容元素和非常低的高场强元素，明显负Nb异常，中侏罗世和侏罗纪末期玄武岩再次转变为以低相容元素和高高场强元素丰度为特征，正Nb异常非常显著，侏罗纪末期流纹英安岩以具有较高的Nb含量而相似于早期玄武岩；早白垩世玄武岩以低丰度的Cr和高丰度的Nb相似于早期侏罗纪玄武岩，而以低丰度Zr有别于早期玄武岩。Pb同位素特点：早二叠世玄武岩具有较高的Pb同位素组成，早、中三叠世玄武岩的Pb同位素组成明显降低，到了晚三叠世又明显增大，中侏罗世早期玄武岩保持了晚三叠世玄武岩的特点，而晚期玄武岩与侏罗纪末期桑秀组玄武岩类似，侏罗纪末期桑秀组流纹英安岩具有非常高的Pb同位素组成。（4）通过采用不活动的高场强元素丰度、元素对比值及微量元素蛛网图等多元判别方法，初步查明了晚古生代以来不同时代火山岩的性质和产出的构造环境。二叠纪玄武岩为产于大陆板内拉张环境的亚碱性拉斑玄武岩；晚三叠世为大洋板内玄武岩；其它时代玄武岩主要为陆缘裂谷区的过渡型玄武岩，桑秀组流纹英安岩产出背景为大陆边缘伸展盆地的亚碱性酸性火山岩。（5）在系统的地球化学和同位素对比分析基础上，对特提斯喜马拉雅带中段晚古生代以来不同时代火山岩的岩浆源区性质、岩浆过程以及演化特点进行了详细深入研究，取得以下初步认识：岩浆源区由3部分组成，即岩石圈地幔、亏损软流圈、富集地幔热柱物质，不同时代火山岩的岩浆源区正是由这三种组分以不同比例混合而成，岩浆过程以部分熔融作用为主，还有少量的分离结晶作用。早二叠世玄武岩和中晚二叠世玄武岩具有相同或相似的岩浆源区，具大陆OIB型岩浆的特点，由来源于亏损的软流圈地幔物质和源于富集岩石圈地幔物质混合形成的混合岩浆，并可能受到了再循环物质成分的影响，以部分熔融为主，同时还经历了有限的分离结晶作用的方式，没有受到明显的地壳混染。早、中三叠世深部地幔热柱物质活动性增强，玄武岩浆源区由深部地幔热柱物质活动诱导的大陆OIB型岩浆与亏损MORB型岩浆的混合体，晚三叠世岩浆源区为EMII+MORB的混合体。中侏罗世玄武岩浆源区继承了晚三叠世的一些特点，但深部地幔热柱物质的贡献增大，岩浆源区主要与N-MORB+OIB组分混合有关。侏罗纪末期桑秀组玄武岩浆源区为岩石圈地幔与OIB型岩浆不同比例混合而成，后期地幔热柱物质贡献增大，岩浆上升过程中地壳混染不明显，主要通过部分熔融作用以及有限的分离结晶作用的方式快速喷溢形成；桑秀组流纹英安岩是在伸展减薄的大陆边缘环境，由早期同源的玄武岩浆分异演化而来的，岩浆上升过程中遭受了浅部地壳物质的同化混染。早白垩世，岩石圈地幔对玄武岩浆源区贡献增强，深部地幔热柱物质贡献减弱甚至消失。（6）在综合分析岩石学和地球化学特征的基础上，重点对二叠纪火山岩和侏罗纪末期桑秀组火山岩的岩石成因进行了详细研究：根据二叠纪玄武岩的地球化学特征并结合野外产状，初步认为这些火山岩是由于色龙地区深大断裂的存在和岩石圈拉张减薄的程度不同，使源于亏损软流圈的地幔物质与岩石圈地幔物质发生混合并快速上升，在滨浅海环境形成一系列小规模的、断续的、具有大陆OIB型特征的、缺乏明显地壳混染的岩浆喷溢事件。根据桑秀组流纹英安岩和玄武岩、安山质岩石之间的关系及地球化学特征，提出流纹英安岩的三阶段成因模式：第一阶段：玄武岩由起源于岩石圈地幔的岩浆与不同比例的上升的OIB型玄武质岩浆发生混合形成混合岩浆，这些混合岩浆发生低度部分熔融形成碱性玄武岩浆，在上升过程中发生一定程度的单斜辉石和斜方辉石的分离结晶，形成具有演化岩浆特征的碱性玄武岩浆的喷溢，在此次喷溢事件之后，地温梯度降低，软流圈-岩石圈界面下沉，岩石圈处于拉张峰期之后；第二阶段：紧接着岩石圈拉张峰期之后，玄武岩浆在较深部层次继续发生分离结晶作用形成安山质岩浆，同时受到中下部地壳物质的混染，但由于此时岩石圈拉张峰期刚刚过去，岩浆在中下部地壳停留时间较短，结晶分异不充分，在还没有遭受到强烈地壳混染的条件下发生小规模安山质岩浆的喷溢；第三阶段：随着时间的推移，岩石圈拉张程度相对减小（总体仍然处于拉张状态），在这种总体拉张背景下，岩浆向上运移，一方面发生强烈结晶分异，另一方面又遭受更浅部地壳（小于16.5km）物质的同化混染，最后发生明显受到上部地壳物质混染的流纹英安岩浆喷溢事件。（7）在详细的地球化学研究基础上，对特提斯喜马拉雅带晚古生代以来火山岩的指示意义进行了详细分析，取得的初步认识如下：①通过与特提斯喜马拉雅带西段、东段二叠纪火山岩的详细对比研究，并结合最新的古地磁结果，初步认为晚古生代时冈瓦纳大陆北缘岩石圈具有不同规模的拉张程度，以特提斯喜马拉雅带西段岩石圈拉张程度最大为特征，这可能指示雅鲁藏布洋盆的开启具有西早东晚的特点。②晚古生代以来，特提斯喜马拉雅带中段火山岩的岩浆源区深度表现出两个由深到浅的旋回性变化，这种现象可能指示冈瓦纳大陆北缘岩石圈发生了不同程度的拉张减薄，在这种拉张减薄背景下区内之所以没有发生大规模的岩浆喷溢活动，是因为裂谷作用持续时间长，裂谷带之下上涌的地幔物质由于对流传导作用而变冷了。③根据二叠纪火山岩既不具有MORB的微量元素组成，又不具备MORB的Pb同位素组成等地球化学特征，以及火山岩产于滨浅海环境的事实，提出二叠纪火山岩只是冈瓦纳大陆北缘晚古生代末期响应岩石圈拉张减薄的一系列火山岩浆事件之一，并不能指示雅鲁藏布洋盆已经开启。④根据早、中三叠世玄武岩的地球化学特征和Pb同位素具有MORB组成的特点，以及各种地球化学参数和岩石圈拉张减薄程度在二叠纪末期与早三叠世之间的突变，并结合火山岩产出的空间位置，提出雅鲁藏布洋盆的初始开启发生于二叠纪末期和早三叠世之间的某个时期，早、中三叠世时该洋盆只具初始洋盆性质，规模很小。⑤根据中三叠世和晚三叠世玄武岩的地球化学特征及Pb同位素组成以及各种地球化学参数的突变，并结合火山岩产出空间位置的变化和古地磁成果，认为雅鲁藏布洋盆在中、晚三叠世之间发生了第一次快速扩张，其结果是导致该洋盆完全开启，并达到一定规模，研究区转换为成熟的被动大陆边缘。⑥根据区域地质背景、玄武岩的地球化学特征及其在剖面上的演化关系，并结合古地磁成果，认为侏罗纪末期桑秀组中上部玄武岩代表了雅鲁藏布洋盆的第二次快速扩张，而桑秀组上部流纹英安岩则指示喜马拉雅地块运动方向发生了由南向北的转换，雅鲁藏布洋盆达到最大宽度，随后进入慢速扩张甚至萎缩阶段。强调桑秀组火山岩记录了雅鲁藏布洋盆发生第二次快速扩张的全过程，在研究该洋盆的构造演化方面具有重要的“指示剂”作用。（8）在分析了几种主要裂谷作用特点的基础上，结合研究区晚古生代以来火山活动的岩浆源区的性质和演化特征，对引起这些火山活动的动力学机制进行了探讨。初步认为与这些火山活动有关的火山岩浆事件的动力学机制是受到地幔热柱物质上涌而强化了的被动裂谷作用，其动力源自地幔对流，提出由于班公湖-怒江洋壳岩石圈晚古生代以来的南向俯冲消减诱发和强化了雅鲁藏布弧后洋盆下面的地幔物质的对流和深部地幔热柱物质的活动，进而引起区内发生岩浆喷溢事件。（9）在综合地质背景和火山活动基本特征的基础上，对区内晚古生代以来所发生的一系列火山裂陷事件进行了总结。初步认为区内晚古生代以来不同规模的火山岩浆活动和长期裂陷的特点表明，与传统意义上的被动大陆边缘还有差异，更可能是一种相对活动的火山裂陷型被动大陆边缘，在晚三叠世到早白垩世时间跨度长达100Ma的地质历史时期内，研究区位于陆壳和洋壳过渡区，处于一种裂而不谷的状态，属于被动大陆边缘发展演化的裂陷阶段。更多还原

【Abstract】 Studies of Tethyan Himalayas offer the opportunity to reveal the important geologicalprocesses that are related to the continental breakup of northern margin of Gondwana, platedrifting, continent converging and the formation, development, closure of Tethyan ocean basin.More attentions have been focused on this area for a long time. However, the previous studies aremainly focused on the sedimentology, paleontology and tectonics/structural geology to infer theimportant geological processes that mentioned above. There is the fact that a poorly known for thevolcanic rocks in the Tethyan Himalayas, these would be ascribed to the inadequate geologicalinvestigation and the neglect of importance of the volcanic rocks in Tethyan Himalayas. As acarder of information for the interaction of crust-mantle, lithosphere-asthenosphere, petroprobeand indicator for most important geological processes, volcanic activity has been playing animportant role in the study of petrology and tectonics. Fortunately, here is a possibility to study thecontinental breakup of northern margin of Gondwana and the tectonic evolution of Tethys inferredfrom the volcanic rocks, which have been found in the process of our fieldworks and othergeological investigations.As viewed from the main idea of regional tectonic evolution-volcanic activity event, theprimary purposes of the present thesis are to study the features, spatial-temporal evolution,petrogenesis of the volcanic rocks in Tethyan Himalayas since the Late Paleozoic, and to probeinto their implications in the process of continental breakup of northern margin of Gondwana andthe tectonic evolution of Tethys. Some preliminary results have been inferred from these volcanicrocks by the present thesis as following.(1)Based on the re-appraisement of the studies by previous workers and recent researchers,the present thesis points out that the tectonic evolution of Tethyan Himalayas is related toformation and development of the Yarlung Zangbo back-arc basin which is constrained by thesubduction southward of Tethyan oceanic crust that is reconstituted by Bangong Co-Nujiangsuture zone.(2)11 volcanic rocks horizons-bearing, which are characterized by lens, thinned interlayersand massive basalts, have been found in the central belt of Tethyan Himalayas from Permian toCretaceous, the fact indicates that there are frequent volcanic activities with limited scales sincethe Late Paleozoic. From the Permian→the Early/Middle Triassic→the Late Triassic→the Jurassicto the Cretaceous, the volcanic activities move from west to east, and from south to north, to southand to north again.(3)The geochemical and evolutional characteristics of these volcanic rocks have been firstreported in the present thesis. According to the detailed results of geochemical studies, whether thecontent of major elements, REE, trace elements and related parameters, or Pb isotopiccompositions have been changed between the Middle-Late Permian and the Early Triassic, andalso between the Middle Triassic and the Late Triassic, some particular tectonic implications could be inferred from these two considerable distinctions.(4)The nature and the tectonic settings of volcanic rocks since the Late Paleozoic have beenwell known by using the abundances and the ratios of high field strength elements, and the spiderdiagrams in the present thesis.The Permian continental tholeiites are formed in the continental extensional zone or initialrift in the within-plate, the Late Triassic tholeiite plots in the normal mid-ocean-ridge basalt field,the rhyodacites in Sangxiu Formation are formed in the stretched basin of continental margin.Apart from mentioned above, the Early Triassic basalts, the Middle Triassic basalts, and theMiddle Jurassic basalts, as well as the Late Jurassic basalts (Sangxiu Formation) and the EarlyCretaceous basalts are all characterized by transitional alkalic-tholeiitic characteristics, which plotin the rift field of continental margin.(5)The nature of magmatic source, the processes of magma and the evolutional characteristicsof volcanic rocks in the central belt of Tethyan Himalayas since the Late Paleozoic have beenrevealed preliminarily by systematically comparative analysis of geochemistry and Pb isotopiccomposition. The results show that three types of magmatic sources, which consist of lithosphericmantle, depleted asthenospheric mantle and enriched mantle plume, have been identified by thepresent study of geochemistry in detail. The sources of magma of volcanic rocks in different agesare the mixture of these three types of magma in different ratios, which mainly underwent partialmelting, as well as limited evolution by fractional crystallization.(6)Based on the analysis of petrology and geochemistry, petrogenesis of the volcanic rocks inSelong and Sangxiu Mountain has been studied in detail by the present thesis.Based on the study of geochemistry and field attitude of the Permian basalts, the presentthesis proposes that the magma that derived from the depleted asthenospheric mantle can tap theenriched lithospheric mantle due to the faults that cut through the stretched lithosphere, the finite,intermittent effusions of magma that has not experienced continental crustal contamination and ischaracterized by continental OIB-type could be occurred in the shore-shallow setting.According to the relationship and the geochemistry between rhyodacites and basalts,andesitic rocks in Sangxiu Formation, the present thesis puts forward a three-stage model ofpetrogenesis to interpret these volcanic activities: the first stage is the forming of alkaline basalticmagma, which is originated from the mixture of magma that experienced a lower degree of partialmelting in different ratios, the mixed magma derived from lithospheric mantle and from theOIB-type basaltic magma that induced by the activity of mantle plume. Subsequently, a decreasedgeothermal gradient and a subsidised interface between asthenosphere and lithosphere as aconsequence of this effusion of alkaline basaltic magma would led to the depth of magmaticsource become deeper than that of before. The second stage is the time that the forming of limitedandesitic magma, which originated from the early basaltic magma by fractional crystallization.The third stage is the time that the forming of rhyodacitic magma, which strongly differentiatedfrom the early basaltic and andesitic magma and experienced strikingly assimilation andcontamination by the upper crust in the process of ascend of magma.(7)Based on the detailed geochemical study of the volcanic rocks in the central belt ofTethyan Himalayas, the present thesis probes into the implications for these volcanic rocks sincethe Late Paleozoic. The main preliminarily conclusions as following: a. According to systematically comparative study of the Permian basalt in the western and theeastern belt of Tethyan Himalayas, as well as the newly paleomagnetic results by other researchers,the present thesis suggests that the initial opening of Yarlung Zangbo back-arc oceanic basin maybe happened early in the western and late in the eastern belt of Tethyan Himalayas.b. The depth of magmatic source of volcanic rocks is characterized by two cyclic changesfrom deep to low since the Late Paleozoic, this fact indicate that the lithosphere of northernmargin of Gondwana has been thinned strikingly by stretching from the Permian to the Cretaceous.The frequent, limited volcanic activities in the central belt of Tethyan Himalayas would attributeto the cooling of mantle materials beneath the stretched lithosphere by convecting.c. The Permian basaltic magmatic activity, which has neither trace element nor Pb isotopiccomposition relative to the MORB, is only one of the volcanic events that are response to thestretched lithosphere of northern margin of Gondwana at the end of the Late Paleozoic, the initialopening of Yarlung Zangbo back-arc oceanic basin could not inferred from this basaltic volcanicactivity.d. The present thesis proposes that the initial opening of Yarlung Zangbo back-arc oceanicbasin may be happened at the transition from the end of Permian to the Early Triassic, which couldbe inferred from the Pb isotopic compositions of the Early-Middle Triassic basalts are similar tothe MORB, and from the abruptly changes of geochemical parameters, and also from the stretcheddegree of lithosphere between the end of Permian and the Early Triassic. Yarlung Zangbo back-arcoceanic basin may be as a juvenile ocean at the Early-Middle Triassic deduced from the nature andthe spatial location of the Early-Middle Triassic basaltic activities.e. According to the distinct changes of geochemistry, Pb isotopic composition, and thegeochemical parameter, as well as the spatial location of volcanic activity, the paleomagneticresults of Himalayan block form the Middle Triassic to the Late Triassic, the present thesisindicates that the first rapid spreading of Yarlung Zangbo back-arc oceanic basin has beenoccurred at the transition of the Middle Triassic and the Late Triassic, and as a consequence of therapid spreading event, this oceanic basin opened completely and up to a certain scale, the studiedarea turns into a matured passive continental margin.f. The present thesis, which based on the regional geological setting, the geochemistry, therelationship in field section of the volcanic rocks, as well as the paleomagnetic results ofHimalayan block, points out that the middle-upper basalt in Sangxiu Formation may be as an"indicator" to indicates the second rapid spreading of Yarlung Zangbo back-arc oceanic basin,whereas the rhyodacites in the upper parts of Sangxiu Formation would indicate that the kineticdirection of Himalayan block has been transformed from south to north, and the Yarlung Zangboback-arc oceanic basin reaches to its greatest width and subsequently enters into slowly spreadingand closing stage. It is emphasizes that the volcanic rocks in Sangxiu Formation have recorded theprocess of the second rapid spreading of Yarlung Zangbo back-arc oceanic basin, and these rocksmay be as the indicators to reveal the information of tectonic evolution for this oceanic basin.(8)Based on the studies of the nature and the evolutional characteristics of magmatic sources,the dynamic mechanism of the volcanic activities in the central belt of Tethyan Himalayas sincethe Late Paleozoic has been discussed by the present thesis. It is suggests that the dynamicmechanism of these volcanic activities is induced by the passive rifting that may be enhanced dueto the upwelling of a deep mantle plume from a major thermal boundary layer within the deep mantle (660km discontinuity), the force source of passive rifting is from the mantle convection,which induced by the subduction southward of Bangong Co-Nujiang oceanic crust.(9)At the end of the present thesis, the author points out that the studied area may berepresents a relatively active volcanic rifted-type passive continental margin, which is differ fromthe traditionally stable passive continental margin. The studied area, which locates in thetransitional region between continental crust and oceanic crust, is in a rifted stage in the process ofevolution of the passive continental margin that is characterized by rifting, rift valley has not beenformed from the Late Triassic to the Early Cretaceous.更多还原