新疆西准噶尔晚古生代蛇绿混杂岩带的构造变形特征及其就位机制研究

The Darbut-Baijiantan region in West Junggar, Xinjiang, located in the easternmost inner end of the Kazakhstan orocline, is characterized by the occurrence of two subparallel ophiolitic belts—Darbut and Baijiantan ophiolitic belts and has been considered to be part of the triple junction where the Siberian, Tarim and Kazakhstan plates were sutured in the Central Asian Orogenic Belt (CAOB). Therefore, this region is known as a key study area for better understanding of the tectonic mechanism of the CAOB. Different tectonic models have been proposed for the Late Paleozoic reconstruction of West Junggar, such as two northwestward subduction model along the Darbut and Baijiantan ophiolitic belts, double subduction and accretionary model along the Darbut ophiolitic belt, a model of one single accretionary ophiolitic belt displaced by transcurrent Darbut strike-slip fault, and a model of the Darbut and Baijiantan ophiolitic mélanges emplaced in crustal-scale fault zones within a continuous Late Paleozoic remnant ocean basin, but no consensus has yet been reached. According to different tectonic models, different characteristic deformational features, component composition and spatial configuration of the Darbut and Baijiantan ophiolitic belts should be expected. Therefore, further investigation on these two ophiolitic belts is urgent to be done in order to resolve the enigma of Late Paleozoic tectonic framework and evolution in West Junggar. Based on the anatomy of the Darbut and Baijiantan ophiolitic belts in terms of the composition and tectonic fabrics, the detailed description of the structural deformation, and the extensive investigation of their contact relationships with surrounding stratigraphic units and Darbut Fault, the objective of this research is to clarify the deformational characteristics and tectonic attribute of the Darbut and Baijiantan ophiolitic belts. With the aid of geochronological study for the age constraint on the deformation and comparative study with classic geological cases, the overall goal is to understand the emplacement mechanism of these two ophiolitic belts and reconstruct Late Paleozoic tectonic framework and evolution of West Junggar and the CAOB.

西准噶尔达拉布特—白碱滩地区位于中亚造山带的东、西构造单元的结合部位，是解剖中亚造山带大陆动力学过程的关键所在。新的研究进展提出不同的构造模型来解释该地区晚古生代的构造演化，如北西向的洋脊俯冲模式、双向俯冲增生模型、统一增生楔后期错断改造模型、以及残余洋盆充填并蛇绿岩沿洋内断裂带就位模型。然而，不同的模型之间又存在很大争议。对于不同可能的构造模型，达拉布特和白碱滩蛇绿混杂岩带应表现出不同特征性的构造变形特征、物质组成以及空间配置关系。故针对这两条蛇绿岩带急需进一步的详细工作来验证。本项目计划在剖析达拉布特和白碱滩蛇绿混杂岩带带内的整体结构和物质组成、构造变形的性质和运动学特征的基础上，明确蛇绿混杂岩带与周边地层和达拉布特断层的接触关系、构造变形特征的配置关系，最后配套构造变形的时限研究，对照不同的经典构造模式，系统厘定混杂岩带的构造属性和就位机制，恢复晚古生代西准噶尔地区的古构造格局和演化

西准噶尔地区最基本的构造-岩石组合是：两条北东向平行延伸的古生代蛇绿岩带被同一套以下石炭统为主的岩石组合单元所围限，并且同被晚石炭世的未变形的花岗岩和中基性岩墙侵入或穿切。对白碱滩和达拉布特蛇绿岩带进行详细填图和构造解析研究，发现二者在构造特征，物质组成等方面非常相似。二者平行并相距40Km，呈走向50°延伸，混杂带内部各类岩石混杂堆叠，分布于几条强变形带内部，在两侧均与下石炭统呈高角度断层接触 (倾角＞70°)，带中超镁铁岩大多蛇纹石化作为混杂带基质，强能干性的岩块混杂其中。白碱滩和达拉布特蛇绿混杂岩带的岩石组成划分为上地幔–洋壳物质和洋壳之上沉积物两种类别。碎屑锆石的年代学数据表明混杂带中的岩块都来源于原地的残余洋盆层系，与被俯冲洋壳传输的外来岩块明显不同，区别于传统意义上的代表洋壳俯冲缝合标志的蛇绿岩带。从构造特征来看，超镁铁岩基质中的面理近直立 (倾角＞75°)，走向与蛇绿混杂带走向基本一致约50°，线理以水平为主(倾角＜20°)；从镜下尺度到露头尺度，一致的运动学指向标志显示构造运动主体为走向近50°的左行水平剪切，整体呈现左行走滑剪切带的特征，而穿切和侵入到混杂带中的“钉合岩体”限定混杂带侵位时代的上限为~310 Ma。西准噶尔地区在晚古生代发育一个残余洋盆，而准噶尔地块和西准噶尔残余洋盆之间的相对运动，在西准噶尔残余洋盆中诱发了一系列北东向的左行走滑深断裂，最终在晚石炭世形成白碱滩和达拉布特走滑断裂带型蛇绿混杂岩带的侵位标志着西准噶尔地区古生代洋盆演化的终结。