新疆西准与伊犁北缘之间中古生代俯冲带迁移过程及其大地构造意义探讨

A key issue of the Central Asian Orogenic belt (CAOB) is continental lateral accretion. Following this scientific problem, we will systematically anatomy the Middle Paleozoic subduction migration between Western Junggar and northern margin of Yili block, NW Xinjiang, which are the critical areas to constrain the subduction-accretion process of CAOB, build the architecture along active continental margin and fully understand the geodynamic process of accretionary orogen. Therefore, we will complete these issues from following points. Firstly, we will combine with field mapping, petrology, geochemistry and geochronology to define the provenance of terrigenous clastic rocks and tectonic environment and source property of ophiolite and volcanic rocks to constrain their affinity and tectonic setting between the two belts during Early Paleozoic. Secondly, we will reveal ages and tectonic setting of Manasi ophiolite, northern margin of Yili by geological mapping, geochemical and geochronological analysis, and combine coeval ophiolites to constrain their tectonic setting between the two belts during Middle-Late Paleozoic. Thirdly, by investigating the deformation styles and spatial arrangement of structural elements, we uncover their paleo-stress field and tectonic system, which contributes to rifting process of back-arc basin. Finally, combining with tectonic properties of these key units and the tectonic system, we reveal the processes of subdution migration between these two belts during Middle Paleozoic and their significance to transition of tectonic framework, which further implicates for the manner of continental lateral growth.

侧向增生方式是中亚造山带研究的争论焦点。选择构造线斜交的新疆西准噶尔和伊犁北缘，以解剖研究薄弱且处于地壳增生拐点的中古生代的俯冲带迁移为切入点，构建中亚造山带南缘洋-陆转换过程的精细结构，揭示地壳增生方式及动力学过程。开展中古生代之前的陆缘碎屑岩和蛇绿混杂带的野外走廊式填图、室内岩石学、地球化学和年代学分析，查明碎屑岩的物源性质、蛇绿岩的构造环境及源区特征，厘定两构造带的亲缘关系及大地构造属性。通过地质填图、地球化学和年代学分析，查明玛纳斯蛇绿岩的形成时代和构造环境；结合同时代蛇绿岩的构造背景，厘定中古生代构造带之间的大地构造属性。查明中古生代构造岩的变形特征和构造要素空间展布，厘定古构造应力场和构造体系，反演弧后盆地的裂解过程。综合上述地质体的构造属性及所属构造体系，厘定中古生代构造带之间的俯冲带迁移过程，揭示两构造带间构造体制变化的过程与机理，探讨其对中亚造山带地壳侧向增生方式的启示。

西准噶尔和中天山是北疆多条构造带的衔接部位，二者的时空联系影响到中亚造山带西部造山带结构及造山过程的解读，如单一主俯冲带上不同位置的两构造带拼贴、或多岛洋模型所示的多块体拼贴。本研究通过蛇绿混杂带、海沟浊积岩及侵入岩体的岩性-构造填图和年代学-地球化学分析，结合已发表资料，查清岩石间的接触关系、各岩石组份的时代和构造属性，厘定了西准噶尔南部发育早古生代增生楔、中天山发育476 Ma以来的大陆边缘弧岩浆。西准噶尔南部志留纪-泥盆纪海沟沉积杂砂岩和拉巴河石英岩中含有与中天山石英岩类似的前寒武纪(600, 900-1000, 1500-1700, 2400-2500 Ma)碎屑锆石、以及西准噶尔无岩浆活动期（446-473 Ma）的碎屑锆石，表明西准噶尔在早古生代与中天山相接触。结合两地大地构造配置的互补性，建立了西准噶尔和中天山发育早古生代俯冲带的构造模型。446-414 Ma时期，西准噶尔增生楔中沿洪古勒楞-谢米斯台-沙尔布尔提山-玛依勒一线侵入辉长岩-玄武岩-安山岩-英安岩岩浆岩带；与中天山北缘一线同时代发育的岛弧岩浆带构成了近平行展布的双岩浆带。我们将这种造山带结构解释为近平行于海沟的洋中脊俯冲的岩浆活动行为。当浮力较大的俯冲洋中脊折返至地表时，它撕裂上盘的增生楔，造成西准噶尔早古生代增生楔的北半部分脱离出俯冲带，发展成被动陆缘，停止380-349 Ma的岩浆活动，沉降泥盆纪浅海相杂砂岩和砾岩等。折返的洋中脊进一步扩张，形成325-414 Ma蛇绿岩所代表的洋盆，具有OIB和MORB特征，略为亏损Nb，类似于现今加州湾新形成的洋壳。该构造模型的提出不仅将西准噶尔和天山从早古生代到晚古生代的构造演化贯穿起来，为北疆演化的解读注入新的思想；而且指出洋中脊在大洋俯冲过程中可以有多幕次的俯冲，对于大洋俯冲的认识有更深入的理解。