【作者】 张玉修；

【导师】 张开均；

【作者基本信息】 中国科学院研究生院（广州地球化学研究所） ， 构造地质学， 2007， 博士

【摘要】 班公湖—怒江缝合带是拉萨地体和羌塘地体的分界线，对班公湖—怒江洋盆的性质、演化模式、俯冲极性和闭合机制等关键的大地构造问题都存在着激烈争论，对班公湖—怒江蛇绿岩带的地幔源区动力学和缝合带中东段聂荣—嘉玉桥变质微陆块的演化地位还缺乏高精度的数据积累，这直接影响了对特提斯域构造和青藏高原形成演化的理解，以及对青藏高原中部缝合带两侧中—新生代含油气盆地的评价。本文选择班公湖—怒江缝合带研究最薄弱的中西段改则地区3条重要的蛇绿岩剖面（改则南拉果错，缝合带内洞错和改则北查尔康错），开展详细的野外地质填图；对蛇绿岩单元和岛弧类岩石进行系统的岩石学、岩相学、矿物化学（尖晶石）、地球化学、Sr-Nd-Pb同位素地质学及锆石SHRIMP U-Pb年代学研究；对拉萨地体和羌塘地体邻近缝合带的构造推覆体进行野外地质学、矿物化学和同构造新生云母⁴⁰Ar-³⁹Ar年代学研究；综合分析上述研究资料，建立班公湖—怒江缝合带中西段演化模式，并在前人研究基础上，探讨班公湖—怒江缝合带的构造—沉积演化。拉果错、洞错和查尔康错蛇绿岩的岩石组合、岩相学、矿物学和矿物化学（尖晶石）、Sr-Nd-Pb同位素特征，以及变质橄榄岩、堆积岩、玄武岩和辉绿岩地球化学特征（如TNT（Ti-Nb-Ta）负异常）均显示为SSZ型蛇绿岩，但具体构造环境各异。拉果错蛇绿岩的地球化学特征兼备MORB和IAT型，蛇绿岩形成于洋内俯冲带之上洋内岛弧的弧间盆地环境；洞错蛇绿岩中的尖晶石矿物化学显示MORB和IAT的特征，堆积岩的地球化学特征显示为MORB型，蛇绿岩形成于洋内俯冲带之上不成熟的弧后盆地环境；查尔康错蛇绿岩兼有MORB和IAT的特征，形成于洋内俯冲带之上的岛弧环境。详细研究拉果错斜长花岗岩的成因及其锆石SHRIMP U-Pb年代学，拉果错斜长花岗岩可能为剪切带中含水条件下辉长岩剪切深熔作用形成的，其锆石SHRIMP U-Pb年龄为166.6±2.5 Ma，证明在班公湖—怒江中西段存在中侏罗世洋盆。首次在羌塘地体南缘的查尔康错蛇绿岩北侧发现岛弧类岩石，岩石组合为流纹岩、安山岩和闪长岩等，闪长岩的锆石SHRIMP U-Pb年龄为166.6±2.5 Ma，表明班公湖—怒江洋盆于中侏罗世向羌塘地体下北向俯冲消减。拉果错和查尔康错蛇绿岩是分别被构造仰冲推覆于拉萨地体和羌塘地体的前中侏罗世陆棚相沉积地层之上的，其底部均为厚～20 m的韧性断层带，断层带内构造片岩的同构造新生云母矿物的⁴⁰Ar-³⁹Ar年龄为151～153 Ma。同时的双向仰冲表明班公湖—怒江洋盆于晚侏罗世早期闭合，这种快速闭合双向仰冲模式是雅鲁藏布江新特提斯洋的低角度俯冲，致使拉萨地体的快速北移与羌塘地体强烈碰撞的结果，并造成了班公湖—怒江蛇绿岩带面状分布，形成班公湖—怒江缝合带两侧晚侏罗世的构造推覆体。班公湖—怒江洋盆的裂开、MOR型洋盆阶段、SSZ型洋盆阶段和洋壳俯冲闭合均存在东早西晚的穿时现象，班公湖—怒江洋盆可能只发育了约130 Ma，并以聂荣—嘉玉桥变质微陆块为轴作剪刀状的开裂和闭合，存在二叠纪（或石炭纪）—晚三叠世和早—中侏罗世（西段延至晚侏罗世）东西两期残留海盆地。班公湖—怒江缝合带演化过程制约着拉萨地体晚侏罗世周缘前陆盆地和羌塘地体早—中侏罗世弧后盆地的沉积面貌。在改则周边地区甄别出一套早白垩世(长石⁴⁰Ar-³⁹Ar年龄为122.9 Ma)OIB型火山岩，证明由于雅鲁藏布江洋壳低角度俯冲的回返（roll-back）在该区产生强烈的裂谷作用，在青藏高原中部形成了一套巨厚的早白垩世—晚白垩世早期冈底斯弧弧后盆地沉积以及班公湖—怒江带内及其拉萨和羌塘陆块周边的一套早白垩世OIB型火山岩，冈底斯弧弧后裂谷作用的影响范围可能越过班公湖—怒江缝合带达到羌塘地体，青藏高原的隆升不应早于晚白垩世。改则地区蛇绿岩的源区显示为印度洋型的MORB特征，存在Dupal同位素异常，洞错和拉果错蛇绿岩具有亏损地幔（DDM）和富集地幔Ⅱ（EMII）混合的特征，而查尔康错地幔源区是亏损地幔（DMM）和富集地幔Ⅰ（EMI）混合的结果。Dupal同位素异常源于早二叠世（～280 Ma）携带Dupal物质组分的地幔柱，该地幔柱导致冈瓦纳大陆北缘的裂解，是古、中、新特提斯洋形成的源动力。更多还原

【Abstract】 The ophiolite-bearing Bangong-Nujiang suture zone, traversing central Tibet from east to west, separates the Qiangtang block to the north from the Lhasa block to the south. Some key aspects about the Bangong-Nujiang suture zone, such as its nature of the ocean basin the suture represents, its evolution, subduction polarity, and closure mechanism, are still open to intense debate. The mantle geodynamics of the Bangong-Nujiang ophiolites, along with the role of the Nierong-Jiayuqiao metamorphic microcontinents in the central-eastern Bangong-Nujiang suture, also remain obscure. This has constituted a obstacle for understanding the evolution of the Tethys and the development of the Tibetan Plateau. This scientific issue is also critical to correctly assessing the structure and evolution of the Mesozoic-Cenozoic petroliferous basins on the suture zone.Intensive studies, including fieldwork, petrology, petrography, mineral geochemistry, geochemistry, Sr-Nd-Pb isotopes and zircon SHRIMP U-Pb and ⁴⁰Ar-³⁹Ar geochronology, were conducted on three key ophiolite profiles in the Bangong-Nujiang suture across Gaize, a famous town in western Tibet, where the ophiolites are well cropped out but little work has been covered so far. The rock assemblages, geochemistry, Sr-Nd-Pb isotopes, petrography, and spinel mineral chemistry of the Laguo Co, Dong Co and Chaerkang Co ophiolites profiles indicate that they were produced under environments of a supra-subduction zone. The metamorphic peridotites, cumulates, basalts and diabases from these three profiles all display negative TNT （Ti-Nb-Ta） anomalies, even though they belong to different tectonic units.The Laguo Co ophiolite in the northern margin of the Lhasa block geochemically show a distinct affinity with middle oceanic ridge basalts （MORB） and island arc tholeiites （IAT） and could possibly have been produced in an inter-arc basin above an intra-oceanic subduction zone. The mineral chemistry of spinels indicates that the Dong Co ophiolite within the suture exhibits MORB and IAT affinity, and the geochemistry of the cumulates shows that they could have formed under a middle oceanic ridge environments. Therefore, the Dong Co ophiolite perhaps was generated in an immature back-arc basin above the intra-oceanic subduction zone. In contrast, the Chaerkang Co ophiolite in the southern margin of the Qiangtang block geochemically displays characteristic features of MORB and IAT and could likely have been formed in an arc environment above the intra-oceanic subduction zone.The plagiogranites from the Laguo Co ophiolite were carefully studied for their genesis and geochronology. Our results indicate that these rocks could possibly have been derived from anatexis of hydrated gabbros by ductile shearing during transport of the oceanic crust. These plagiogranites have an age of 166±2.5 Ma by zircon SHRIMP U-Pb measuring. This age may represent the formation age of the ophiolite. indicative of that an oceanic basin could have existed in the western part of the Bangong-Nujiang zone in the Middle Jurassic.The arc-related rocks were identified the first time in this study in the southern margin of the Qiangtang block and on the north of the Chaerkang Co ophiolite. These magmatic rocks include rhyolites, andesites, and diorites, and have an age of 157.5±2.2 Ma based on zircon SHRIMP U-Pb method. Clearly they were a result of the northward subduction of the Bangong-Nujiang ocean under the southern Qiangtang block during the middle Jurassic.The Laguo Co and Chaerkang Co ophiolites are thrust over the Pre-Upper Jurassic shelf-facies sedimentary cover rocks, which are marked by -20 m-thick ductile faults filled by syn-tectonic felsic schists. Three samples of mica grains from these schists produce ages of 151-153 Ma by ⁴⁰Ar-³⁹Ar geochronology. Such bilaterally simultaneous obduction indicates that the Bangong-Nujiang oceanic basin must have closed just at this time. The closure is suggested to be a result of the low-angel subduction of the Neo-Tethys Ocean on the south of the Lhasa block, which drove the Lhasa block moved northward quickly and the Bangong-Nujiang ocean was closed.The rifting of the Bangong-Nujiang oceanic basin, the development of the MOR and SSZ oceanic basin, and the subduction of the oceanic crust developed earlier in the east and propagated westwards, showing a scissors-like opening and closure during a stage of -130 My. Two-stage remnant oceanic basins, present in eastern and western segments, respectively, were existed during the Permian （or Carboniferous） to late Triassic and the early to middle Jurassic （extended to late Jurassic in the western segment）.The evolution of the Bangong-Nujiang suture zone, strictly speaking, controlled the sedimentation of the Late Jurassic peripheral foreland basin in the Lhasa block and the Early-Middle Jurassic back-arc basin in the Qiangtang block. A suite of Lower Cretaceous OIB-type volcanic rocks, with a ⁴⁰Ar-³⁹Ar age of 122.9 Ma, were recognized across Gaize and provided strong evidence that the thick Lower to Middle Cretaceous sedimentary rocks in the back-arc basin of the Gandese arc in central Tibetan Plateau should have been formed under an environment of rifting possibly owing to the roll-back of the Neo-Tethys oceanic crust after the low-angle subduction. The back-arc rifting of the Gandese arc could probably have extended to the Qiangtang block, and the uplifting time of the Tibetan plateau should not be earlier than the late Cretaceous.As the origin is concerned, the ophiolites across Gaize are characterized by the Indian Ocean-type MORB with Dupal Sr-Nd-Pb isotope anomalies. The Dong Co and Laguo Co ophiolites could geochemically be a result of mixture of DDM-type and EM II-type magmas. However, the Chaerkang Co ophiolite could geochemically have been generated from mixture of DMM-type and EM I-type magmas. These Dupal isotope anomalies are considered to have resulted from the early Permian （-280 Ma） mantle plume upwelling with Dupal isotopic features. This event possibly resulted in the rifting of the north margin of the Gondwana and initiated the Paleo-, Meso- and Neo-Tethyan Oceans.更多还原