【作者】 王冉；

【导师】 夏斌；

【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】 雅鲁藏布缝合带将印度-欧业板块分为南北两块，乃新特提斯缝合带在西藏的延伸，并以该带中段日喀则蛇绿岩及相关岩石单元代表的构造带最为典型。日喀则特提斯构造演化存在单一俯冲和两期俯冲的争论，关键问题在于缺乏洋内岛弧和大洋盆印记。为解决此关键问题，选择日喀则相对完整的吉定蛇绿岩剖面和仁布蛇绿混杂带进行详细解剖，并在前人基础上提出了藏南新特提斯弧盆构造演化的新模式。对日喀则西侧吉定蛇绿岩辉长岩中锆石的SHRIMP U-Pb定年确定其形成于128±2Ma。仁布蛇绿岩地幔橄榄岩和堆积岩全岩地球化学特征（如Ta、Nb和／或Ti（TNT）负异常，轻稀土亏损）表明蛇绿岩形成于SSZ构造环境，也说明日喀则蛇绿岩洋壳和洋下地幔是熔体和残留体的成因关系。仁布地区新厘定出一套洋岛碱性玄武岩和辉绿岩，Ti、P含量较高，轻稀土强烈富集，大离子亲石元素和高场强元素也明显富集，蛛网图显示随元素不相容性降低，其原始地幔标准化值逐渐降低，具较高的⁸⁷Sr／⁸⁶Sr比值，εNd（t）平均为5.4，显示DM向EMⅡ的演化趋势；这些特征表明辉绿岩少有地壳混染，可能代表了特提斯洋内热点，并类似Kerguelen洋岛碱性玄武岩（印度洋Ninety East Ridge洋岛链），暗示印度洋可能继承了特提斯洋的物质组成。仁布洋岛火山岩可能是日喀则特提斯已俯冲殆尽的老洋盆印记。首次发现侵入于仁布蛇绿岩地幔橄榄岩和堆积岩中的英云闪长岩具有高镁和Mg^#、Sr和Sr／Y比值，强烈亏损HREE和Y，非常类似于埃达克岩或埃达克岩交代的弧火山岩系列，可能是特提斯俯冲板片与弧下地幔相互反应的产物，标志着一次洋内俯冲事件。基于前人基础，本研究认为藏南日喀则特提斯洋存在向北消减的弧-盆体系。日喀则特提斯可能于中侏罗世已存在俯冲并形成了拉萨地块南缘的包括正常岛弧安山岩-英安岩-流纹岩组合的叶巴弧；在晚侏罗世，印度板块继续向北漂移，虽然受到拉萨地块的阻碍，但新特提斯洋沿着拉萨地块南缘的俯冲仍然继续，形成了叶巴弧南部的桑日岛弧。同时，特提斯中段洋内发生局部拉张，此时诱发洋内俯冲，新特提斯双俯冲的构造格局形成。于120～128Ma，一些规模较小的边缘洋盆形成，卷入陆缘碎屑和洋壳沉积的地幔楔发生熔融产生岩浆，沿着破裂的扩张脊上涌产生了具有TNT负异常的SSZ型新洋壳，期间产生了俯冲洋壳重熔作用，标志着年轻的热的洋壳俯冲形成的埃达克岩侵入于仁布SSZ蛇绿岩地幔橄榄岩中。印度板块继续向北漂移导致该洋内俯冲系统逐渐消亡，而沿拉萨地块南缘的俯冲并未停止，形成林子宗正常岛弧火山岩组合，日喀则蛇绿岩最终于80～90Ma左右构造侵位于拉萨地块的南缘。约45～50Ma，伴随着印度—欧亚板块碰撞，在大陆边缘产生了冈底斯花岗岩基。在雅鲁藏布蛇绿混杂带和冈底斯岩浆弧的前缘之间形成了山前磨拉石建造，缝合带及相邻构造单元被后期冲断层和伸展作用改造成现在的构造面貌。更多还原

【Abstract】 The Yarlung Zangbo suture zone, separating the Lhasa block to the north and Indian sub-continent to the south, is marked by ophiolites, subduction zone complex, and volcanic arc terranes. Preveous modle for Xigze Tethyan evolution have controversial problemes on whether the intra-arc and the information about Sunducted Ocean preserved in Xigaze Tethys sutre zone. Detailed studies for them based on field structural geology, petrology and geochemistry have allowed us to characterize the Xigaze segment of YZS and understand its implications for tectonic history before India-Asian collision. SHRIMP zircon U-Pb dating for the west end of Tiding ophiolite yields a 128±2 Ma age for gabbros. Geochemical features of ophiolites (i.e. the Ta, Nb and/or Ti (TNT) negative anomaly, depletion of LREE) infer their supra-subduction-zone (SSZ) environment.The oceanic volcanic basalts and diabase associated with the Rinbung ophiolite, east end of the segment, are similar to oceanic island alkalic basalt series (OIAB) with respect to many aspects such as enrichment of Ti, P and total alkalis, the enrichment distribution patterns of LREE; and a decreasing trend of the normative value of incompatible elements to primitive mantle with their incompatibility in the spider diagram. Significantly, Sr-Nd isotopic ratios of the Rinbung OIAB display an evolution trend from DM to EMⅡwith no crustal contamination and and similar to the OIAB of Keguelen(the NinetyEast Ridge hotspots of Indian ocean). Therefore, the Rinbung OIAB probably represents the product from the hotspots in Tethys, it is believed that Indian mantle have inherited some characteristics of Tethyan ocean.The adakites are also fistly found in this study, and they are a suite of High magnesium tonalites intruding the Rinbung mantle peridotites with higher MgO and Mg#, Sr or Sr/Y, strongly depletion of HREE and Y, probably produced by the reaction between Tethyan subduction slab melting and sub-arc mantle and mark an intra-oceanic subduction.Based on the previous study, an intra-oceanic subduction with a subduction polarity directed to the north does exist in Xigaze Tethyan Ocean. The subduction of Tethys Ocean would like to subduct at least from the middle Jurassic so that they can produce Yeba-arc along the southernmost margin of Lhasa continental block. During the J3-K1 period, the subduction along the southern margin of Lhasa block continued to form a suite of ADRs (include rare adakites) in Sangri group. And then a local extension took place in the region of intra-Eastern Tethys when the India plate drifted to the north but suffered the resistance from the Lhasa block, contemporarily, the intra-oceanic subduction are triggered, subsequently, magmas derived from a partial melting of mantle wedge which suffered from the modification by subducted continental marginal clastics and oceanic sediments upwelled along the oceanic ridges, forming several small marginal basins during 120～128 Ma, and thus lead to the so-called SSZ-type oceanic crust. Adakties, the typical fingerprint of intra-oceanic subduction, intruded the Rinbung mantle peridotites.With the continued subduction of the India plate to the north along the southern margin of Lhasa block, forming Linzizong volcanic rocks, the intra-oceanic subduction system was gradually extinct and the ophiolite was tectonically emplaced onto the southern margin of the Lhasa block at～80～90 Ma. During～45～50Ma, the Gangdese granitic batholiths were emplaced into the southern margin of the Eurasia continent and mark the collision of India-Asia plate. In the Tertiary, mountain frontal molasses deposited between the Yarlung Zangbo ophiolitic melanges and the Gangdese granitic batholiths, and the suture zone together with adjacent tectonic units were modified by thrusting and extension to the present tectonic configuration.更多还原