藏南雅鲁藏布江蛇绿岩带中西段蛇绿岩及其就位机制研究

发布时间：2018-02-21 19:57

本文关键词： 雅鲁藏布江蛇绿岩带橄榄石组构就位机制岩石地球化学年代学　出处：《中国地质大学(北京)》2016年博士论文　论文类型：学位论文

【摘要】：本论文选取雅鲁藏布江蛇绿岩带中西段日喀则—桑桑—萨嘎—仲巴一带为研究靶区,重点研究区为桑桑蛇绿岩区、萨嘎蛇绿岩区和仲巴蛇绿岩区,研究对象为重点研究区内蛇绿岩,重点关注研究区内蛇绿岩的岩石地球化学、年代学、显微构造和地幔流变学研究,从而探究雅江蛇绿岩带中西段的就位机制,并提出具有建设性意义的合理模型。雅鲁藏布江蛇绿岩带自桑桑蛇绿岩往西分成南北两个亚带。桑桑蛇绿岩序列中地幔橄榄岩、玄武岩和辉长岩均贫HFSE,玄武岩和辉长岩属钙碱性系列,发育碱性玄武岩,属MORB型,桑桑所测基性岩锆石U-Pb年龄为125±1.2Ma。萨嘎蛇绿岩南亚带未发现蛇绿岩层序地层,北亚带出露蛇绿岩层序地层,其中基性岩属钙碱性—低钾系列,玄武岩普遍发育,并呈现边缘亚碱性中间碱性的分布特征,发育贫硅富碱的富碱矿物,辉绿岩锆石U-Pb年龄为125.6±2.7Ma,推测为洋内俯冲带之上(SSZ)的弧前-弧-弧后环境。仲巴蛇绿岩未发现地幔橄榄岩,其边缘分布钙碱性玄武岩,中部分布碱性玄武岩,具有典型洋岛玄武岩组合特征,仲巴日玛玄武岩锆石U-Pb年龄为129.3±1.0Ma,推测形成于弧前拉张环境。日喀则白朗石榴辉石岩属钙碱性基性岩,年龄为149.0±3.1Ma,源于地幔,标志着新特提斯洋脊已俯冲消亡。选取萨嘎地幔橄榄岩进行橄榄石组构分析,其EBSD解析结果是该橄榄石组构属“B”型橄榄石组构,主导滑移系为(010)[001],产生于高压高应力环境,与俯冲带之上(SSZ)关系密切,指示向南逆冲推覆,其结果与野外实际观察的现象一致,橄榄岩中斜方辉石和单斜辉石也具有同样的EBSD解析结果。根据研究资料,提出可能的运动学模型“俯冲面不均论”运动模型,通过研究区内边界处的线理和面理可以证实该模型有一定合理性。通过研究区内玄武岩的研究,对研究内蛇绿岩的构造环境和动力学机制进行探究,推演出地幔流变学机制。大洋中脊是蛇绿岩形成的原始动力,由于地幔柱和热点的推动力使得蛇绿岩中玄武岩复杂化,从而表现为经历了俯冲带多阶段多源区的改造,同时受到板内裂谷作用的影响,形成具有复杂源区和复合地球化学特征的玄武岩组合。根据本文研究,提出了较为合理的适用于中西部蛇绿岩的“五段式”就位模型。
[Abstract]:This paper selects the Yarlung Zangbo River ophiolite zone in Western Shigatse - Saga - Sangsang - Zhongba area as the research target, the key research area for Sangsang ophiolite zone, Saga ophiolite zone and Zhongba ophiolite zone, the object of the study is the key research area within the ophiolite, geochemistry, focus in the research area of ophiolite Study on geochronology, microstructure and rheology of the mantle in place, so as to explore the mechanism of Yajiang ophiolite belt in the west, and put forward the reasonable constructive model. The Yarlung Zangbo River ophiolite zone is divided into West Sangsang ophiolite from north to South and two sub belts. The mantle peridotite Sangsang ophiolite sequence, basalt and gabbro are poor HFSE, basalt and gabbro of calc alkaline series, the development of alkaline basalts belong to MORB type, SinSin measured by mafic zircon U-Pb age of 125 + 1.2Ma. ophiolite belt in South Asia was not found in the saga ophiolite strata Sequence stratigraphy, northern belt exposed ophiolite sequence stratigraphy, the mafic rocks belong to the calc alkaline series, low potassium, widely developed basalt, distribution characteristics and presents the edge sub alkaline intermediate alkaline and alkali rich mineral development and poor silicon and alkali, diabase zircon U-Pb age of 125.6 2.7Ma, presumably above the ocean subduction zone (SSZ) the forearc island arc environment. Zhongba ophiolite was found in mantle peridotite, the marginal distribution of calc alkaline basalts, the central distribution of alkali basalts, with typical oceanic island basalt assemblage, zircon U-Pb age Rima basalt in Zhongba was 129.3 + 1.0Ma, presumably formed in the fore arc extensional environment. Shigatse white pomegranate pyroxenite calc alkaline rocks, age 149 + 3.1Ma, derived from the mantle, marking the new Tethys ocean ridge subduction and extinction. The saga of selected mantle peridotite olivine fabric analysis, the analytical result is the olivine EBSD Fabric is a "B" type of olivine fabric, the dominant slip system for [001] (010), resulting in high stress environment, and above the subduction zone (SSZ) closely, indicating southward thrusting, and the results of field observation of the phenomenon, peridotite in orthopyroxene and clinopyroxene also EBSD has the same analytical results. According to the research data, the kinematics model of "possible subduction of the uneven surface of the motion model of the study area within the boundary of the lineation and foliation can confirm that the model is reasonable. Through the study of basalt in the area, the tectonic environment of ophiolite and dynamics research to explore the mechanism, deduce the mantle rheology mechanism. The mid ocean ridge is the original power of the ophiolite formation, due to plume and hot force makes the ophiolite complex in basalt, which manifested as experienced multi stage multi subduction zone The transformation of the area is also influenced by the intraplate rift, forming a basalt combination with complex source area and complex geochemical characteristics. Based on this study, a more reasonable "five stage" positioning model suitable for Middle West ophiolite is proposed.

【学位授予单位】：中国地质大学(北京)
【学位级别】：博士
【学位授予年份】：2016
【分类号】：P588

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