【作者】 耿全如；

【导师】 潘桂棠； 金振民；

【作者基本信息】 中国地质大学 ， 构造地质学， 2007， 博士

【摘要】 冈底斯作为重要的中、新生代岛弧岩浆岩带，历来是青藏高原最热门的地质研究领域。中、新生代岩浆岩在冈底斯带出露最多，研究程度相对较高。近年来的地质大调查发现，冈底斯带在石炭—二叠纪也有大量火山岩喷发，但对这套火山岩的研究却很少，对冈底斯带晚古生代的构造意义缺乏研究。作者在地质调查资料的基础上，对冈底斯晚古生代火山岩的重点区域西藏八宿县然乌乡、波密县、林周县的石炭、二叠纪火山—沉积岩系进行了详细地质剖面测量、系统采样和观察。对火山岩样品采用ICP-MS、ICP-AES（TJA IRIS）等离子质谱，MAT-262热电离质谱等先进分析仪器和手段，获取岩石的常量元素、微量元素、稀土元素、Sr-Nd-Pb同位素等地球化学数据。通过对各类资料的综合对比分析，对冈底斯石炭、二叠纪火山岩时空分布、岩石地球化学及其形成的大地构造背景、动力学机制等问题进行了探讨，取得以下有意义的重要成果。（1）本文首次对冈底斯带石炭纪火山岩的区域展布、地球化学和形成环境进行了系统研究。诺错组（C₁）、来故组（C₂）和拉嘎组（C₂）火山岩作为夹层，夹在深海—浅海相碎屑岩中。石炭纪火山岩主要为浅变质安山玄武岩、玄武安山岩和英安岩、流纹岩等，具有双峰式火山岩特点。常量元素地球化学研究表明，石炭纪火山岩具有拉斑玄武岩特点，与典型MORB和岛弧玄武岩相比，具有MgO含量低，TiO₂、Al₂O₃、P₂O₅等含量高等特点，稀土和微量元素为LREE和LILE富集型分配模式，与岛弧玄武岩存在差异，与大陆拉斑玄武岩有相似之处。石炭纪酸性火山岩属于亚碱性系列，稀土和微量元素地球化学特征与陆内流纹岩相似。岩石地球化学和共生的沉积岩系特征表明，石炭纪火山岩形成于伸展背景下的大陆边缘裂陷环境，为陆内裂谷型双峰式火山岩组合。冈瓦纳大陆北缘在早石炭世已发生局部裂谷作用。石炭纪火山活动的特点是断续出现、持续时间长、火山岩喷溢的厚度小，火山作用的强度远小于典型的火山裂谷型大陆边缘。初步研究表明，冈底斯地区石炭纪并非典型的火山裂谷边缘，应属被动裂谷，为不完整的裂谷演化过程。（2）同位素和微量元素地球化学特征表明，石炭纪火山岩源区位于60—100 km的石榴石橄榄岩稳定区和尖晶石橄榄岩稳定区。然乌诺错组（C₁）玄武岩的地壳物质混染程度较大，波密地区诺错组和西部拉嘎组（C₂）玄武岩受地壳混染相对较低。源区发生过复杂的混合作用，然乌一带涉及到原始地幔、富集地幔EMⅡ和冈瓦纳大陆北缘下地壳的混合；波密地区的源区混合主要涉及原始地幔和冈瓦纳北缘上地壳。石炭纪火山岩的源区地幔具有典型Dupal异常，可能是由于冈瓦纳古陆的壳源物质俯冲、再循环进入古老地幔所致。（3）本文对冈底斯带二叠纪火山岩的地层学特征、地球化学和形成环境进行了系统研究。二叠纪火山岩在冈底斯东段属于洛巴堆组，以玄武岩为主；在冈底斯西段属于中、上二叠统下拉组，岩石类型有玄武岩、英安岩、安山岩等。玄武岩常量元素地球化学的显著特点是Al₂O₃含量较高、MgO含量低，与岛弧高铝玄武岩相似。火山岩均表现为LREE和LILE富集型的微量元素和稀土元素分配曲线，Nb、Ta负异常显著，与岛弧火山岩相似。岩石地球化学研究表明，二叠纪火山岩形成于岛弧环境中，为古特提斯洋壳俯冲、消减的产物。冈底斯在二叠纪进入岛弧演化阶段。早二叠世由于古特提斯洋壳向南的俯冲、消减作用，冈底斯带演化为洋内弧环境，形成拉斑玄武岩。中晚二叠世演化为陆缘岛弧—安底斯型活动大陆边缘环境，形成钙碱性系列火山岩。二叠纪末期和早、中三叠世，冈底斯带已隆升成陆相剥蚀区。（4）Sr、Nd、Pb同位素地球化学特征表明，洛巴堆组玄武岩形成于不均匀地幔源区，都存在Dupal异常，但林周—勒青拉玄武岩Dupal异常更明显。唐家乡洛巴堆组玄武岩源区发生过略亏损地幔和下地壳成分的混合过程，为混合源区，源区为深度50 km左右的上地幔。林周—勒青拉剖面洛巴堆组玄武岩和措勤来故组玄武岩源区深度为50～100 km，为EMⅡ型富集地幔，源区未发生混合作用，岩浆上升和演化过程中可能存在地壳混染作用。（5）本文对石炭—二叠纪冈底斯带的构造—岩浆演化作了深入总结。在时间演化坐标上，各类成分特征指数演化曲线在早／中石炭世、石炭纪／二叠纪和早／中二叠世界面上发生显著变化，分别与三次构造活动有关。早石炭世为被动大陆边缘初始裂陷阶段；晚石炭世为大陆边缘裂谷活动强烈阶段，壳—幔物质发生强烈交换；冈底斯带在二叠纪为俯冲消减带，进入岛弧演化阶段。（6）通过对古特提斯构造域火山—沉积建造进行了区域对比，指出古特提斯东段和西段在晚古生代具有不同的构造演化过程。古特提斯西段石炭纪发生南、北大陆碰撞，形成海西期Variscan造山带，晚石炭世之后逐渐转入陆内演化阶段。东段在石炭纪仍为伸展阶段，处于浅海环境。东段未出现海西造山带，直到中晚二叠世由于岛弧造山而局部隆起。早中石炭世，劳亚大陆与冈瓦纳大陆发生初始碰撞。碰撞首先开始于冈瓦纳非洲西北部（摩洛哥）和劳亚大陆东南部的Armorican。石炭纪强烈的造山作用形成欧洲中南部的海西期Variscan造山带，东西向的挤压作用在美洲、非洲—阿拉伯，甚至澳大利亚东部均受到广泛影响，造成了隆升、逆冲和沉积盆地转化的过程。晚石炭世，劳亚大陆与冈瓦纳大陆发生全面的右旋剪切碰撞，沿Variscan造山带聚合成一个超级大陆Pangea。碰撞从西向东进行，石炭纪中期在北美为残留洋，东特提斯仍为开阔海域。晚石炭世在欧洲为同造山和晚造山期花岗岩侵入和流纹岩喷发。在冈底斯和喜马拉雅地区，石炭纪为伸展背景下的陆缘裂谷盆地，局部伴随火山岩喷发。特提斯东段的印度北缘和泛华夏大陆群之间未发生石炭纪的碰撞，不存在类似于欧洲的海西期造山带。石炭—二叠纪期间，泛华夏大陆群是古特提斯东部位于劳亚大陆和冈瓦纳大陆之外的独立地块。本文研究区位于冈瓦纳北缘的外喜马拉雅带（特提斯），晚古生代主要受古特提斯洋拉张、消减过程的控制，在早石炭世至中二叠世均为浅海相沉积，直到晚二叠世才发生隆升，未发生明显的海西期造山。北喜马拉雅带和冈底斯带在早石炭世及其以前的沉积和构造环境相似。早石炭世以后，冈底斯带逐渐向活动大陆边缘和岛弧演化，大部分地区海水逐渐变浅，到晚二叠世成为滨浅海相或陆相；北喜马拉雅带则随着新特提斯洋盆的扩张，在二叠纪逐渐演化为离散型被动大陆开阔陆棚—深海盆地相沉积。两者在生物面貌和区系上也逐渐分异。冈底斯带在晚古生代作为冈瓦纳大陆北缘的一部分，经历了从稳定的被动边缘到陆缘弧的演化过程，其中石炭、二叠纪火山岩记录了这一构造演化过程的重要信息。本文的研究弥补了冈底斯带地质研究的不足，有重要的意义。更多还原

【Abstract】 As an important Mesozoic and Cenozoic magmatic arc, the Gangdise has attracted attentionsof many geologists.The widely- exposed Mesozoic and Cenozoic magmatic rocks have been wellstudied. Geologic survey and investigations in recent years have confirmed that there were volcaniceruptions in Carboniferous and Permian times in the Gangdise region. The late Paleozoic volcanicrocks however, still lack for petrological study. The author, funded by a geologic survey project anda "973" project, carded out investigations on the late Paleozoic rocks in the Gangdise zone. Basedon geologic, data of geologic mapping at 1:250,000 and 1:200,000 and preyiously published articles,the author measured the volcanic-sedimentary geologic sections in Ranwu of Basu County, Bomi,and Linzhou for Carboniferous and Permian volcanic rocks. After systemic sampling anddescription, the author did geochemical analysis with advanced apparatuses such asICP-MS, ICP-AES（TJA IRIS）, MAT-262 to obtain major element, trace element, REE andSr-Nd-Pb isotopic components. By comprehensive geochemical study and comparison with thevolcanic and sedimentary sequences around the realm of paleo-Tethys, the author obtained primaryimprovements for the Permo-Carbiniferons volcanic rock distributions, petrochemistry and theirtectonic settings.（1） This paper described for the ftrst time the regional distribution geochemistry and tectonicsetting for Gangdise Carboniferous volcanic rocks. The Nuocuo Fm （C₁）, Laigu Fm （C₂） and LagaFm （C₂） have volcanic rock interbeddings in marine clastic rocks. Major rock types aremeta-basalts, dacite and rhyolite, as a bimodal series. Major element geochemical study suggestthat the Carboniferous volcanic rocks are characterized by tholeiite with lower MgO and higherTiO₂、Al₂O₃、P₂O₅ contents compared with MORB. These are different from the arc basalts andsimilar to continental basalts. The Carboniferous silicic volcanic rocks belongs to sub-alcali serieswith similar geochemistry to continental rhyolite. Petrochemistry of volcanic rocks and paleo-faciescharacteristics of sedimentary rocks suggest that Carboniferous volcanic rocks erupted inextensional environment at marginal riffs. As parts of northern Gondwanaland, the Gangdise and northem Himalayas have similar extensional situation. Carboniferous rifting in Gangdise.Carboniferous volcanic activities lasted for long period but formed only a small quantity of lavacompared with those in volcanic rifted margins. Therefore, the gangdise zone did not form avolcanic rifted margin in Carboniferous but just a passive rifted margin. The Carboniferous riftingincluded double phases with hi-model volcanic activities. Post-rift thermal uplift appear as in mostcases of volcanic rifted margins. It was an incomplete or immature process of rift evolution. Theregion of Gangdise was affected by both the intensional Variscan orogenic regime and theextensional regime situated within the passive margin of the paleo-Tethys. This could be the reasonfor the typical marginal rifting process in the Gangdise.（2） Sr, Nd and Pb isotopic geochemical studies suggested that the source region situated inthe gamet zone and spinal zone of upper mantle with 60-100 km depth. The mantle source of thebasalts in the Nuocuo Formation in Ranwu might have experienced crustal assimilation beforemelting. The basalts Nuocuo Formation in Bomi and that in Laga Formation in Cuoqin haverelatively lower degrees of crust involvements in source regions. Source regions have experiencedcomplex mixing process. The primary mantle, EMⅡand lower crust have been involved in themixing process in Ranwu, while in Bomi, primary mantle and upper crust were involved. TypicalDupal has been recognized that could be resulted by crust recycling into the old mantle.（3） The Gangdise Permian volcanic rocks, stratigraphy geochemistry and tectonic setting aresystematically studied in this paper. The Permian volcanic rocks in the eastem Gangdise zone occurmainly in the Luobadui Formation of lower to middle Permian. To the western part of Gangdise,the middle and upper Permian volcanic rocks Occur in the Xiala Formation, with basalt, dacite andandesite, he volcanic rocks in the Luobadui Formation are basalt belonging to tholeiite series. Thebasalt in the Leqingia section has higher content of Al₂O₃ than those of continental flood basalt（CFB） and close to arc basalt and lower MgO content than that of CFB but close to archigh-aluminum basalt. The Permian volcanic rocks have enriched LREE and LILE contents andprofound Nb, Ta negative anormaly on spider diagrams. Geochemical studies suggest that the lowerand middle Permian Luobadui basalts and upper Permian silicic rocks generated in volcanic arcsituation which were related to the paleo-Tethys southward subduction. Reactivated subductionhappened in early Permian to form a oceanic arc and Tangjiaxiang arc basalt. It evolved tocontinental island arc and Andean type continental in middle and late Permian. The Gangdiseregion evolved to a strip of erosional land in latest Permian and early Triassic. The rifting innorthern Himalayas continued in Permian and the Cimmeride and Sibumasu blocks driftednorthward in middle and late Permian to form the neo-Tethys along northern Gondwanaland.（4） Sr, Nd, and Pb isotopic geochemistry reveal that the Luobadui basalts formed in differentupper mantle regions with Dupal anormaly. A mixing process happened between depleted mantleand lower crust for the Tangjiaxiang basalt at depth of 50 km±. The Leqingla and Cuoqin basaltwere formed by EMⅡmantle sources at depth of 50-100 km without evidence for mixing process.（5） This paper summarized the Permo-Carboniferous tectono-magmatic evolution in theGangdise and compared with other parts of the paleo-Tethys realm. Typical features display sharpcontacts at the boundaries of C₂/C₁, P/C and P₂₃/P₁, which correlated with three tectonic events. The northem passive margin of Gondwanaland began initial rifting in early Carboniferous andevolved to strong rifting stage in late Carboniferous with speeding up crust-mantle exchange. ThePermian volcanic rocks with imprints of subduction zone and crustal involvements imply the thirdphase of tectonic evolution in the Gangdise zone that became a volcanic arc.The wastern Tethysclosed in Carboniferous and formed Hercynian Variscan orogeny. It evolved into continental in lateCarboniferous. The eastern Tethys kept as extensional shallow marine environments inCarboniferous time. Paleo-Tethys subduction began in Permian to the north of Gangdise, while theHimalaya kept as extensional rifting period.（6） This paper analysed and compared volcanic-sedimentary sequences around paleo-Tethy.We suggest that the eastern and western parts of paleo-Tethys are different in geologic evolution inlate Paleozioc. In the early Carboniferous, the Laurussia and Gondwanaland began initial contactwhich merged in the composite called Pangea in mid-Carboniferous. The Variscan collisional stressand subsequent uplift felt as far as America, Africa-Arobia and eastern Australia. EarlyCarboniferous magmatism occurred as bi-model volcanic activities in south Europe, north Americaand the region of Gangdise and as alkali intrusions in northwest Hinalaya. These imply a transitionprocess of passive margin to rifting margin. In late Carboniferous, syn-orogenic and post-orogenicgranite intrusions and rhyolite eruption in Europe represents the formation of Pangea continent.Meanwhile, the eastern Tethys still remained an open seaway, where the northern Gondwanalandkeep rifting. The Hercynian continental collide and orogenesis did not happen between northernIndia and Cathaysia continents. In Carboniferous and Permian, the Cathaysia remained asindependent continents to the east of Gondwanaland and Laurussia. The Gangdise as the northernmargin of Gondwanaland occurred as shallow marine environment until middle Permian. It evolvedinto a subduction and continental arc in late Permian. The north Himalaya and Gangdise havesimilar_tectonic setting until Carboniferous. The Gangdise evolved to island arc and Andean typecontinental margin since middle Carboniferous. The north Himalaya however, became aextensional passive margin while the neo-Tethys incipient opening and widening since middlePermian.As part of northern Gandwana, in late Paleozoic time, the Gangdise evolved from passivemargin to continental arc. The Permo-Carboniferous volcanic rocks recorded the process.更多还原