【作者】 张斌辉；

【导师】 刘勇胜；

【作者基本信息】 中国地质大学 ， 地球化学， 2006， 硕士

【摘要】 中国中东部的秦岭-大别造山带是中国中央造山带的重要组成部分，是华北-扬子两大板块俯冲-碰撞的产物，是世界上规模最大的超高压变质带。对于榴辉岩俯冲折返过程中的壳-幔相互作用及地球化学循环是中国大陆科学钻探工程(CCSD)研究的重点之一。本文通过研究榴辉岩退变质后的元素变化特征以及榴辉岩中微量元素的赋存矿物，探讨了榴辉岩退变质过程中微量元素的活动性，揭示了榴辉岩退变质过程中微量元素的变化特征、控制因素及其来源和迁移形式等。同时，对许沟蛇纹石化橄榄岩成因和演化的地球化学记录进行了初步探讨。 本次研究主要取得了以下几个方面的认识 一、榴辉岩中的微量元素除重稀土元素在石榴石中富集以外，大多集中在副矿物中，主要矿物中含量相对较低。Cs、Rb、Ba等主要集中在云母中，轻稀土元素主要集中在磷灰石、绿帘石、榍石中，Nb、Zr等主要集中在金红石、榍石中。由于这些矿物在榴辉岩退变质过程中稳定性的差异，决定了微量元素也必然表现出不同的变化特征。 二、对具有退变质程度过渡关系的榴辉岩样品不同部分的对比研究表明，在退变质过程中随着矿物相发生变化大离子亲石元素和轻稀土元素表现出了较大的活动性，而高场强元素Nb、Ta与重稀土元素则变化较小。该结果表明作为元素迁移载体的外来流体可能富集大离子亲石元素和轻稀土元素，但并不富集高场强元素Nb、Ta和重稀土元素，但可能富集了部分Zr、Hf。这暗示外来流体中缺乏络阴离子，与前人通过流体包裹体研究获得的角闪岩相退变质流体以低盐含水流体为主的认识一致。 三、副矿物和退变质流体是控制退变质过程中微量元素行为的主要因素。副矿物的形成和分解以及退变质流体中是否富集络阴离子等决定了微量元素的溶解和迁移特性及其含量变化。 四、许沟橄榄岩已经高度蛇纹石化，详细的研究表明这些蛇纹石化橄榄岩独特的微量元素组成是部分熔融、地幔交代和蛇纹石化共同作用的结果。蛇纹石化作用主要导致了Pb和Sr的高度富集，Pb含量与蛇纹石化程度具有良好的正相关性。蛇纹石化橄榄岩的Pb／Ce比值远高于岛弧玄武岩和大陆地壳。同时可能造成了Zr、Hf的富集。更多还原

【Abstract】 The Qingling-Dabie-Sulu orogenic belt resulted from collision between of North China Craton and Yantze Craton is the largest UHP metamorphic belt in the world. One main target of the CCSD project is to reveal the mantle-crust interaction and geochemical recycling during continental subduction and eclogite exhumation. Trace elemental compositions and variations of the eclogites and single minerals were measured to study the trace element mobility during retrograde metamorphism. Geochemical records of the origin and evolution of serpentinized peridotite from Xougou were also studied. Main conclusions made in this work are following:(1) Except for heavy rare earth elements in garnet, trace elements in eclogite are strongly distributed in favor of minor/accessory minerals compared with major minerals. For example, Cs, Rb and Ba are largely enriched in mica, light rare earth elements in apatite, epidote and sphene, and Nb and Zr in rutile and sphene. Thus, stability of primary minerals and formations of secondary minerals affect significantly the behavior of trace elements during retrograde metamorphism, and resulted in fractionations of trace element during retrograde.(2) Studies on samples suffered from transitional retrograde metamorphism demonstrate that large ion lithophile elements and light rare earth elements show significant variations, whereas high field strength elements and heavy rare earth elements show little variations. These observations suggest that fluids associated with retrograde metamorphism were enriched in large ion lithophile elements and light rare earth elements, not enriched in high field strength elements Nb, Ta and heavy rare earth elements. Zr and Hf maybe enriched during retrograde metamorphism Depletions of high field strength elements and heavy rare earth elements in the fluids imply that the external fluids were poor in anionic complex.(3) Accessory minerals and retrograde fluids are main factors controlling the trace elemental behavior during retrograde metamorphism. Formations and/or decompositions of minor/accessory minerals and chemical compositions (e.g., contents of anionic complex) of the fluids affect significantly the behavior of trace elements during retrograde metamorphism.(4) Serpentinized peridotites from Xougou were studied. Based on bulk rock chemical compositions, we suggest that the trace elements of the serpentinized peridotes were resulted from melt extraction, mantle metasomatism and serpentinization near surface. Serpentinization caused enrichments of Pb and Sr. Pb contents are positively correlated with serpentinization degree. Also Pb/Ce ratio is higher than island arc basalt and continental crust. Zr、 Hf maybe enriched in serpentinization fluid.更多还原