【作者】 李文霞；

【导师】 赵志丹；

【作者基本信息】 中国地质大学（北京） ， 地球化学， 2013， 博士

【摘要】 雅鲁藏布蛇绿岩带是当今世界上著名的蛇绿岩带之一,代表了中生代以来新特提斯洋岩石圈的残片,对其开展详细的野外地质、岩石学和地球化学研究,有助于揭示新特提斯洋演化、发展历史和构造属性。出露在雅鲁藏布蛇绿岩带中段的日喀则蛇绿岩发育完好,本文对其中的夏鲁、群让和白朗三个蛇绿岩剖面的基性岩石(玄武岩、辉绿岩和辉长岩)开展了野外工作、室内主量、微量元素和Sr-Nd-Os-O同位素地球化学的系统研究工作,揭示了基性岩石的地球化学特征、形成的构造环境和地幔域的地球化学特征。论文获得如下结论：1.日喀则蛇绿岩的基性岩为典型的玄武质岩石,具有富Mg、高Al,高Ca,富Na、低Ti、低K的特征,岩石属于亚碱性玄武质系列,绝大多数为低钾(拉斑)系列。岩石烧失量较高,说明其普遍遭受到蚀变作用。2.岩石稀土元素和微量元素特征与正常的大洋中脊玄武岩(N-MORB)相似,但富集大离子亲石元素,亏损高场强元素,岩石具有从洋脊玄武岩向具有俯冲组分加入的岛弧玄武岩过渡的特征。3.岩石具有亏损地幔的Nd-Sr同位素特征(εNd(t)为+6.2至+8.7,87Sr/86Sr为0.70313-0.70546)。Sr同位素向着高值方向漂移显示了海水蚀变作用,岩石Re、Os含量较低,Os同位素具有火山弧玄武岩的特征,并受到海水蚀变的影响,岩石δ18O高达10.3～12.8‰,同样经受了海水蚀变和俯冲的影响,但其源区并未受到大陆地壳物质的混染。Sr-Re-Os-O同位素由于受到海水蚀变及俯冲的影响,其组成已经无法指示源区的特征。4.揭示了日喀则蛇绿岩的构造环境和地幔域特征。属于非典型的洋中脊成因,应为SSZ型蛇绿岩,可能形成于与洋内俯冲有关的SSZ型弧后或者弧前盆地中；雅鲁藏布蛇绿岩与现今印度洋地幔域特征吻合,表明现今印度洋地幔继承了特提斯洋的地球化学特征。5.将本文结果与已有研究结合,识别出东西延伸超过1500km的雅鲁藏布蛇绿岩至少存在大陆岛弧型、地幔柱—洋内热点型、洋中脊—大洋岛弧型和典型大洋岛弧型等四种类型的洋壳,揭示了特提斯洋存在多种构造环境的特征,有利于恢复和还原一个长期演化的、丰富的特提斯洋。更多还原

【Abstract】 The Yarlung Zangpo ophiolite is one of the best preserved ophiolites in the world. They are remnants of the Neo-Tethys ocean-floor, marking a suture within Gondwanan in Mesozoic era. The study of petrology and geochemistry may reveal the evolution history and tectonic features. The Xigaze ophiolite in the center of the Yarlung Zangpo ophiolite is intact and has important implication on the research of evolution of Tethyan. In this paper, major and trace elements, as well as Sr-Nd, Re-Os and O isotopic compositions for the basic rocks (basalts, diabases and gabbros) from three sections of ophiolites in Xialu, Qunrang and Bailang of Xigaze area were studied.These rocks are charactered by low Ti, K, high Mg, Al, Ca, Na and LOI, implying the metasomatic process after the rocks formed in the ocean; the rocks are sub-alkaline basaltic rocks. Most of them is low-K tholeiitic, few of them is calc alkali basaltic rock.Their REE and trace elements are very similar to the N-MORB, but with enrichment of large ion lithophile elements (LILE) and depletion of high field strength elements (HFSE). The rocks show the characteristics from middle ocean ridge basalts to island arc basalts.These rocks are characterized by deleted mantle Sr-Nd isotopes (εNd (t):+6.2～+8.7,87Sr/86S:0.70313～0.70546). Strontium isotopic value toward to the high direction, lower rhenium and osmium content, volcanic arc basalt-like osmium isotope and high δ18O value (10.3～12.8‰) indicated that these rocks had undergone the seawater alteration, but without the contamination of continental crust. Sr-Re-Os-O isotopic compositions can’t reveal the Characteristic of the source region because of the seawater alteration and oceanic slab subduction..The Xigaze ophiolite is Supra Subduction Zone (SSZ) type oceanic crust, instead of the typical N-MOR type ophiolite. It may have occurred in back-arc or fore-arc tectonic settings in an oceanic slab subduction-related system.There are at least four different types of tectonic environment within the1500km-long Yalung Zangpo ophiolite. The four-type basalts are (1) continental arc,(2) mantle plume or hot spot,(3) MORB and Oceanic arc, and (4) typical oceanic arc. The presence of multiple tectonic settings rusulting from geochemical signatures which associated with the basaltic rocks within the Yalung Zangpo suture zone reconstructs a long-evolutional history of the Tethyan Ocean.更多还原