藏北蛇绿岩地球化学特征及其地质意义

发布时间：2018-03-31 18:46

本文选题：蛇绿岩　切入点：双湖　出处：《中国地质大学(北京)》2017年硕士论文

【摘要】：青藏高原是多地体北向造山拼贴的产物,发育有多条近东西向的蛇绿岩带。蛇绿岩研究一直是青藏高原的研究热点和前沿课题。本文结合藏北龙木错-双湖-澜沧江蛇绿混杂岩带、班公湖-怒江蛇绿岩带及狮泉河-纳木错-波密蛇绿岩带中双湖、赞宗错以及果芒错蛇绿岩的岩石学、地球化学等特征,分析研究了其形成的大地构造环境及其源区性质,进而探讨各蛇绿岩带的形成演化历史。双湖蛇绿岩带内蛇绿岩各组成单元均有出露,均被后期构造运动改造呈混杂片岩产出。其中产自双湖西江爱达日那-嘎错一带蛇绿混杂岩中的变质辉绿岩、辉长岩基性岩墙群,其地球化学特征表明其来源于富集型地幔源区,其环境判别显示兼具洋中脊玄武岩和洋岛玄武岩特征,为准洋中脊环境,是地幔柱热点与洋中脊相互作用的结果。赞宗错蛇绿混杂岩自下而上具有较完整的蛇绿岩层序组合,但受后期构造作用的影响,层序被严重破坏。岩性以地幔橄榄岩组分为主。赞宗错蛇绿混杂岩地球化学性质显示蛇绿岩具有陆缘弧后盆地及陆内裂谷两种不同性质的地球化学组成。赞宗错第一组基性岩样品来源于亏损地幔的部分熔融,不同程度的叠加了少量的俯冲流体、沉积物等物质的影响。赞宗错第二组基性岩样品应源于软流圈上地幔,且叠加有少量的陆壳组分。果芒错蛇绿岩是狮泉河-纳木错-波密蛇绿岩带上出露面积较大的一处蛇绿岩,各层序均有出露。超铁镁质堆晶杂岩其中堆晶杂岩显示明显的堆晶韵律,显示了明显的分离结晶特征,并指示其源自一个被富集组分交代混染的地幔。基性岩具有高Al特征,稀土元素和微量元素具E-MORB型配分模式。基性岩地球化学环境判别显示果芒错蛇绿岩可能形成于扩张的弧后盆地环境中,结合区域构造特征,认为狮泉河-纳木错-波密蛇绿岩带是班怒带南向俯冲形成的弧后小洋盆,蛇绿岩各单元块体是古洋壳俯冲消减、碰撞挤压和构造抬升而形成现今的构造岩片,并非是逆冲推覆异地搬运而来。本文通过野外地质调查、蛇绿岩岩石学及地球化学特征分析研究,结合前人蛇绿岩研究相关成果,构建了藏北蛇绿岩的形成演化历史:双湖蛇绿岩作为龙木错-双湖-澜沧江蛇绿混杂岩带的一部分伴随着古特提斯洋在晚二叠世的闭合侵位,经过随后多期的构造变质变形作用层序被打乱,呈变质混杂岩片分布;班-怒特提斯洋于二叠纪之前裂开,并随着逐渐的裂开,开始了在拉萨地块北缘和羌塘地块南缘的双向俯冲,并在晚三叠世诱发了狮泉河-纳木错-波密洋东段的裂开;之后,与俯冲相关的东巧蛇绿岩在早侏罗世构造侵位;而狮泉河-纳木错-波密洋盆持续西向裂开,形成了具有弧后盆地背景的果芒错蛇绿岩;伴随洋壳俯冲消减、洋壳闭合、蛇绿岩构造侵位,形成了班公-怒江缝合带和狮泉河-纳木错-波密缝合带。
[Abstract]:The Qinghai-Xizang Plateau is the product of the orogenic collage in the north direction of the multi-terrane. There are several near east-west ophiolite belts. Ophiolite study has always been a hot research topic in the Qinghai-Xizang Plateau. This paper combines the Longmao-Shuanghu Lancangjiang ophiolite melange belt in northern Tibet. The petrology and geochemistry of the Shuanghu, Zanzomuo and Gomangao ophiolite in the Bangonghu Nujiang ophiolite belt and the Shiquanhe-Namuco -Beaumie ophiolite belt are analyzed and studied. Furthermore, the formation and evolution history of each ophiolite belt is discussed. The ophiolite elements in Shuanghu ophiolite belt are exposed. All of them were produced as melange schist by late tectonic transformation, including metamorphic diabase and gabbro basic dyke group in the Aidana-Gacao area of Shuanghu, Xijiang River, Shuanghu. Its geochemical characteristics indicate that it originated from the enriched mantle source region, and its environmental discrimination shows that it has the characteristics of both mid-ocean ridge basalt and oceanic island basalt, and quasi-oceanic mid-ridge environment. It is the result of the interaction between mantle plume hot spots and mid-ocean ridges. The Zhanzongcao ophiolite melange has a relatively complete ophiolite sequence assemblage from the bottom to the top, but is affected by late tectonic processes. The lithology is mainly composed of mantle peridotite. The geochemical properties of Zhanzongcao ophiolitic melange show that the ophiolite has two different geochemical compositions: the continental margin back-arc basin and the intracontinental rift valley. The first group of basic rock samples came from partial melting of the depleted mantle. A small amount of subduction fluid, sediment and other materials were superimposed to different degrees. The second group of basic rock samples from Zanzongqiao should be derived from the upper mantle of the asthenosphere. And there are a few continental crust components superimposed. The Gomangcao ophiolite is a ophiolite with relatively large exposed area along the Shiquanhe-Namuco-Beaumie ophiolite belt. Each sequence is exposed. The superferromagnesite heap crystal complex shows obvious stacking rhythm, which shows obvious characteristics of separation and crystallization, and indicates that it originated from a mantle mixed with metasomatism of an enriched component. The basic rock is characterized by high Al content. The REE and trace elements have a E-MORB pattern. The geochemical environmental discrimination of the basic rocks indicates that the gomangcao ophiolite may have been formed in the extensional back-arc basin environment, and combined with the regional tectonic characteristics. It is considered that the Shiquanhe-Namuco-Bomi ophiolite belt is a small back-arc ocean basin formed by the southern subduction of the Banrar belt, and the blocks of the ophiolite units are paleo-oceanic crust subduction, collision and compression, and tectonic uplift to form the present tectonic lithotectonics. Based on the field geological survey, the petrology and geochemistry of ophiolite, combined with the related achievements of the former ophiolite study, The formation and evolution history of ophiolite in northern Tibet has been constructed: Shuanghu ophiolite, as a part of the Longmu-Shuanghu-Lancangjiang ophiolite melange belt, was accompanied by the closed emplacement of the PaleoTethys ocean in the late Permian. The sequence of tectonic metamorphism and deformation was disrupted and distributed as a metamorphic melange, and the Ban-Nutthys ocean was split before Permian and gradually split. The bidirectional subduction in the northern margin of the Lhasa block and the southern margin of the Qiangtang block was initiated, and the rift of the Shiquanhe-Namuco-Pomi eastern segment was induced in the late Triassic, and then the subduction-related Dongqiao ophiolite emplaced in the early Jurassic tectonic emplacement. However, the Shiquanhe-Namuco-Bomi ocean basin continued to split westward, forming the Gomangao ophiolite with the background of the back-arc basin, accompanied by the subduction of the oceanic crust, the closure of the oceanic crust, and the emplacement of the ophiolite structure. The Bangong-Nujiang suture zone and the Shiquanhe-Namuco-Bomi suture zone were formed.
【学位授予单位】：中国地质大学(北京)
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P584

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