祁连造山带拉脊山-盐池湾蛇绿增生杂岩带及其与西秦岭造山带构造关系研究

This project will focus on studies of ophiolites and the accompanied arc volcanic complex outcropped as a zone along the Lajishan-Yanchiwan accretionary belt in the Qilian-Qaidam compound orogen. Through detailed investigations of petrology, elemental and isotopic geochemistry, isotopic geochronology, by using various techniques including in situ element and isotopic analysis for whole-rock and minerals, SIMS and LA-ICP-MS U-Pb isotopic dating for zircon, we will try to unveil rock assemblages, spatial distribution, forming environment and ages of the ophiolite and arc-volcanic rocks in this belt, and to reveal the tectonic relations with the North Qilian and Western Qinling accretionary belts. Through comparison of Sr-Nd-Pb-Hf isotopic compositions with world-wide ophiolites of Early Paleozoic ages, we try to determine the properties of mantle reservoirs and their tectonic regime in the Earth’s history. Not only can this study determine the tectonic evolution of the Qilian-Qaidam compound orogen, but also plays important role in understanding characteristics of mantle-plume activity and oceanic spreading and subduction, and in understanding the position and function of the Qinling-Qilian-Kunlun giant orogen during the breakup of Rodinia and the formation of Pangea supercontinents.

本立项将以祁连-柴北缘复合造山带拉脊山-盐池湾增生杂岩带蛇绿岩和岛弧火山岩为研究对象，通过岩石学、常量元素、微量元素和同位素地球化学、同位素年代学等方面研究，利用元素、同位素分析技术以及锆石等矿物原位定年等手段，查明拉脊山-永靖-盐池湾蛇绿岩和岛弧火山岩的岩石类型、空间分布、形成环境、形成时代，揭示与北祁连增生杂岩带和西秦岭增生杂岩带的关系；进行全球早古生代蛇绿岩Sr-Nd-Pb-Hf同位素组成的对比，确定其地幔源区特征和全球构造分区。该研究不仅能够揭示祁连山-柴北缘复合造山带的形成和演化历史，而且对于了解早古生代地幔柱活动特征、与Rodinia超大陆的裂解和Pangea超大陆的汇聚过程, 以及秦-祁-昆巨型造山带在全球超大陆形成过程的地位和作用等方面都有重要意义。

本项目对祁连-柴北缘-东昆仑复合造山带及西秦岭增生杂岩带蛇绿岩、岛弧火山岩及高压-超高压变质带的形成过程和机制，以及祁连山和西秦岭之间的构造关系进行了深入研究。该研究的成果不仅对于了解地幔柱有关的洋底高原和洋内弧岩浆岩特征、大洋俯冲带的形成起始和演化、洋底高原与俯冲带相互作用、祁连造山带与秦岭造山带的构造关系，而且对于揭示俯冲带岩浆形成机制、壳幔相互作用以及大陆地壳的增生过程等方面都具有重要的理论意义。在国内外学术期刊上发表标注本项目的论文22篇，其中SCI检索期刊论文20篇，包括国际著名的期刊，如 Journal of Petrology, Lithos, Geology, Nature Geoscience 等，论文被其它SCI论文引用130余次。.本项目取得了以下进展和成果：(1) 查明了拉脊山-永靖蛇绿岩的分布、岩石类型、地球化学特征，确定了其形成环境为洋底高原，形成时代为寒武纪 (525 Ma); (2) 识别出一条沿南祁连-西秦岭分布的/延伸达1000公里长的IBM型洋内弧的火山岩带，其形成时代为奥陶纪 (470-440 Ma)；(3) 揭示了洋内弧火山岩与洋底高原蛇绿岩的时空关系及其形成过程和机制，完善了俯冲带形成和发展了理论；(4) 建立了连接祁连和秦岭、总长度约2000 公里的“祁秦增生杂岩带”，剖析了祁连和秦岭的构造关系；(5) 识别出东昆仑造山带的超高压变质带及其形成时代，并确立了原特提斯洋最终闭合的时限为410Ma。