青藏高原拉萨地体松多缝合带厘定与印支造山作用研究

Qinghai-Tibet (Q-T) Plateau is regognized as the natural laboratory for geodynamics research. As known, the Q-T Plateau is the assembly of several blocks. Since suture zone is the front end of plate collision, it is important to determine the suture zone in the Q-T Plateau. This project focused on the tectonic evolution and intra-continental orogeny in the Lasha Terrain since Indosinain. As planned, the following research work would be done in the following four years. 1) Based on the comparison of the isotopic ratio and trace elements composition, we hope to clarfiy the relation between the Songduo ecologite and its surrounding rocks. Mineral separation would be done to search for the Ultra-high pressure (UHP) minerals, such as coesite in the Songduo ecologite, which is the direct evidence of the UHP metamorphism. We will calculate the pressure and the temperature of the ecologites, and radioactive dating the metamorphism. These work is aimed to construct the Pressure－Temperature－time evolutional path of the Songduo ecologites. 2) It is necessary to find much evidences to surpport the existence of the ophiolite suite in the Lasha Terrain. Therefore, the rock type, occurrence, forming age and their tectonic environment of the ophiolite suite would be studied. 3) The island arc volcanics occure in the north Gangdise. They might be related with plate subduction. With dating the volcanics using the isotopic method, we will study the genesis of this volcanics. 4) We will study the tectonic framework, the orogenic polarity, the spatial distribution, and the evolution history of Indosinian orogenic belt in the Lasha Terrain. The results of the study of the Songduo ecologites, the ophiolite suite and the island arc volcanics would be synthesized to determine the Songduo suture zone. It is possible for us to discuss the Indosinian orogeny and its effect in the tectonic framework of Q-T Plateau. In addition to that, the role of Indosinian orogeny in the uplift and evolution of Q-T Plateau would be discussed. As the major contribution of this project, it is possible to determine a new suture zone in the Lasha Terrain. This suture zone might be formed by the subduction of the Paleo-Tethys Ocean and in the destructive plate. Our research will contribute to understand the plate margin, the terrain division, and the formation and evolution of Q-T Plateau.

青藏高原是研究地球动力学的天然实验室，作为多地块拼合的高原，确定板块碰撞作用前端的各类缝合带具有重要意义。本课题瞄准青藏高原自古特提斯至新特提斯以来的构造演化和陆内汇聚作用，主要研究：1）松多榴辉岩与围岩之间的关系，寻找榴辉岩的超高压变质证据，建立榴辉岩的P－T－t演化轨迹；2）拉萨地块蛇绿岩的主要岩石类型、产出、时限及其构造环境，以寻找更多的蛇绿岩证据；3）冈底斯北部岛弧火山岩的同位素年代学，进一步探讨这套火山岩的成因机制和俯冲的动力学机制；4）拉萨地体内印支期造山带的构造格架、造山极性、发展演化和展布规模。结合拉萨地块松多榴辉岩带、蛇绿岩以及该区域广泛存在的印支期构造－岩浆事件，探讨印支造山作用对青藏高原构造格架的影响及其在高原隆升与演化中的地位。本课题有可能在拉萨地块中厘定出一条新的、与古特提斯洋俯冲、消减有关的板块缝合带，深化对青藏高原板块边界确定、地体划分、演化历史的认识。

青藏高原是研究地球动力学的天然实验室，作为多地块拼合的高原，确定板块碰撞作用前端的各类缝合带具有重要意义。本项目主要研究青藏高原拉萨地块榴辉岩及其他相关片岩、片麻岩，开展了野外地表地质调查、常规矿物学、岩石学、同位素地球化学和年代学等研究。.（1）利用Thermocalc变质相平衡模拟软件，结合详细岩相学观察，在NCKMnFMASHTO体系下，模拟松多含多硅白云母榴辉岩的变质演化过程。其中，榴辉岩峰期矿物组合为g+o+law+phn+ru，石榴石核部最大镁铝榴石值和石榴子石包体中，多硅白云母最大Si值确定的榴辉岩峰期温压条件，约为620 ℃，32×10^5 Pa，榴辉岩经历了近等温降压的退变质过程。相平衡模拟结果表明，拉萨地块松多榴辉岩经历了超高压变质作用过程，并经历了相对快速的折返过程到中部地壳层次。.（2）利用Thermocalc变质相平衡计算软件，在MnNCKFMASHO体系下计算出含石榴石云母石英片岩的P-T、P-M (H2O)视剖面图。结合石榴石镁铝榴石等值线、钙铝榴石等值线及饱和水含量等值线，限定出含石榴石云母石英片岩的峰期变质条件，约为27×10^5 kPa，523/580 ℃，对应的峰期矿物组合为g-Jd-Cr-Law (+Phn+q/Coe+H2O)。对比含石榴石云母石英片岩与榴辉岩的P-T轨迹，峰期变质温压及变质演化特征，提出含石榴石云母石英片岩曾经历过高压变质。结合野外相互伴生的地质关系，认为该片岩与榴辉岩经历了相同或相似的俯冲折返过程。.（3）拉萨地体榴辉岩中石榴石和绿辉石的3He/4He值是0.27-0.60 Ra，高于地壳源区的3He/4He，但低于地幔的3He/4He比值。这表明，拉萨地体榴辉岩中的He保留了地幔He的信息，具壳幔两端元混合的特点。拉萨地体榴辉岩相比中国大别地区的榴辉岩，具有更高的3He/4He比值，这种3He/4He比值特征，说明拉萨地体榴辉岩的成因可能与深部地幔物质有关。