新疆西准噶尔古生代陆壳生长

The Central Asian Orogenic Belt is the largest region of the Phanerzoic continental crustal growth around the world. North Xinjiang in the southwestern part of the Central Asian Orogenic Belt is characterized by the major crustal growth during the Paleozoic. Recent researches have revealed a complete suite of Cambrian - Permian plutons in the West Junggar and they formed in relation to an orogenic process of oceanic subduction, isalnd-arc amalgamation and collision. Thus the West Junggar is one of the regions suitable for exploring the Paleozoic crustal growth in the Central Asian Orogenic Belt. It is known that orogens may record two ways of crustal growth: one is a laterial process, which is thought to occur mainly in active margins and to be related to oceanic subduction, and the other is a vertical process in a post-collitonal setting, resulting from addition of mantle-derived magma to the crust caused by removal of lithospheric mantle and/or lower crust and upwelling of the asthenospheric mantle. Previous studies indicate that Late Carboniferous - Permian post-collitional plutons with positive and high ε(t)-values are typical of vertical growth of the crust, whereas the Cambrian - Early Carboniferous plutons geochamically show the arc affinity and may be representative of lateral growth of the crust. As continuing work of the last project supported by the National Natural Science Foundation of China (grant 41072168,2011-2013), this project is aimed at exploring laterial and vertical processes of the crustal growth in the West Junggar during the Paleozoic and estimating the contributions of each process to the crustal growth by means of Sr-Nd-Pb isotopic geochemistry, in accordance with the framework of zircon U-Pb chronology and tectonic stages of the West Junggar.

中亚造山带是全球最大的显生宙地壳生长区，位于中亚造山带西南部的新疆北部就是以发生了重要的古生代地壳生长为特征。近年的研究已经揭示新疆的西准噶尔发育了从寒武纪-二叠纪的一套完整的侵入岩序列，它们的形成与洋壳俯冲、岛弧拼贴和碰撞造山过程密切相关，因而西准噶尔是中亚造山带中研究古生代地壳生长的理想区域之一。造山带记录的地壳生长可以有两种不同的过程：与俯冲过程相关的侧向生长和与后碰撞深部过程相关的垂向生长。以往研究表明，西准噶尔乃至新疆北部晚石炭世-二叠纪后碰撞侵入岩浆活动记录了地壳垂向生长，而寒武纪-早石炭世侵入岩则可能记录了地壳侧向生长。本项目将在前一个基金项目研究的基础上，通过建立西准噶尔构造岩浆活动的年代学格架，重点根据古生代侵入岩的全岩Sr-Nd-Pb同位素地球化学特征，按照区域构造演化的阶段性，深入探讨西准噶尔古生代地壳生长问题，并估算两种方式对地壳生长的贡献。

中亚造山带在增生造山过程发生了显著的地壳生长。在执行前一个国家自然科学基金项目期间，除已经报道的石炭纪—二叠纪侵入岩外，我们在西准噶尔地区发现并初步研究了寒武纪—泥盆纪岩浆活动的记录，提出西准噶尔地区存在着寒武纪-早石炭世与俯冲过程相关的地壳侧向生长和晚石炭世-二叠纪与后碰撞深部过程相关的地壳垂向生长。因此西准噶尔地区是研究中亚造山带显生宙地壳生长问题的理想地区之一。.本项目的主要研究内容包括：（1）花岗质侵入岩全岩Sr-Nd-Pb同位素地球化学特征；（2）同一构造背景下同时代岩体的成因和岩石组合特征；（3）未知时代岩体的锆石U-Pb年龄测定；（4）造山带构造演化阶段与地壳生长方式的关系。根据以往的研究情况，选择了巴尔鲁克山北坡开展火山沉积地层的岩石组合特征及形成环境研究，在南部地区重点研究拉巴等地的增生杂岩，并适当补充面上工作。.获得的重要结果和关键数据：（1）巴尔鲁克山北坡区分出早石炭世（324–320Ma）俯冲相关的和晚石炭世—二叠纪（314-259Ma）后碰撞岩浆岩；沉积环境自晚石炭世初期已从浅海相转变为河流相；从俯冲到后碰撞的构造转换发生在早石炭世末，对应于准噶尔-巴尔喀什洋的闭合。（2）西准噶尔南部的拉巴单元是伊犁地块上大陆弧的近源堆积，科克萨依单元形成于洋内弧背景，不是北哈萨克斯坦山弯构造的延伸。西准噶尔南部地体先是独立演化的，在晚志留世与伊犁地块碰撞，形成哈萨克斯坦大陆。（3）西准噶尔地区可区分出新元古代末期–早奥陶世（572 ~ 485 Ma）洋内岛弧、早志留世–早泥盆世（433 ~ 405 Ma）洋内岛弧、早石炭世（345 ~ 324 Ma）洋内岛弧、早-晚石炭世碰撞（324-318 Ma）和晚石炭世–二叠纪（318 ~ 259 Ma）后碰撞等五个构造岩浆演化阶段。前三个阶段对应于自南向北在不同岛弧背景下发生的地壳侧向生长，对应于多岛洋的特点，而晚石炭世—早二叠世则是在全区处于后碰撞背景下的地壳垂向生长。