羌塘中部片石山和果干加年山一带低温（超）高压变质作用研究

The central Qiangtang Basin records the important information of the early growth of the Qinghai-Tibet Plateau and the evolution of the Tethys ocean. The study on the LT-(U)HP metamorphic belt in central Qiangtang is very significant for constraining the formation of this basin and the evolution of the Tethys ocean. In this project, we will focus on LT-(U)HP metamorphism of the eclogites, blueschists and the surrounding metasedimentary rocks in the Pianshishan and Guoganjianianshan areas of the central Qiangtang metamorphic belt. Based on detailed field work, the distribution and the field occurrences of the three types of (U)HP rocks mentioned above as well as the relationship among them will be confirmed. The main purposes of this project is, on the basis of detailed petrographical study, to check whether there are any index minerals (e.g. coesite) or mineral assemblages of UHP metamorphism in eclogites and their surrounding rocks aiming to determine whether they have undergone UHP metamorphism. This project plans to perform phase equilibria modeling on (U)HP rocks in these two areas and to determine peak metamorphic conditions and P-T paths of these rocks, in order to test whether they have experienced lawsonite eclogite facies and lawsonite blueschist facies metamorphism as proposed previously and to constrain their metamorphic history. We will then further discuss the formation and evolution of this metamorphic belts, the subduction of oceanic rocks and the mechanism of uplift and exhumation for these (U)HP rocks.

羌塘盆地记录了高原早期生长和特提斯洋演化的重要信息。羌塘中部低温（超）高压变质带的研究对于约束该盆地的形成和特提斯洋的演化具有重要意义。本申请拟以羌塘中部片石山和果干加年山一带低温榴辉岩、蓝片岩及围岩变沉积岩为主要研究对象，依据详细的野外勘查工作，查明三类岩石的野外分布特征及产状，重点开展榴辉岩及其围岩的变质作用研究。通过岩相学研究检验榴辉岩或其围岩中是否存在柯石英等超高压特征矿物或矿物组合；运用相平衡模拟，结合岩相学和矿物学等研究方法，获得（超）高压岩石的峰期变质条件及P-T轨迹，检验它们是否经历过硬柱石榴辉岩相变质作用，深入揭示其变质演化历史。查明羌塘中部变质带低温（超）高压变质作用的机制，进而为理解该变质带的形成与演化、洋壳岩石的俯冲过程和（超）高压岩石的抬升和折返机制提供岩石学证据。

结合近年来野外考察所获得的野外地质资料，通过对羌塘中部片石山及其邻近地区低温高压变质岩的岩相学、矿物学和热力学相平衡模拟研究，取得了以下几项阶段性成果：1. 通过野外实地勘察查明了片石山地区榴辉岩的野外产状及其岩相学特征；2. 对片石山榴辉岩进行了相平衡模拟研究，获得榴辉岩峰期温压条件为560 °C和>24 kbar，并获得其进变升温升压和退变等温降压的P-T轨迹；3. 首次在片石山榴辉岩的围岩石榴石云母片岩中发现了蓝晶石、硬绿泥石等矿物，这些矿物的发现指示片石山石榴石云母片岩与典型的低温高压变质岩差异较大，相平衡模拟结果显示这些高压变质岩峰期温压条件达~560 °C和~24 kbar，与岩相学观察相吻合，表明榴辉岩和其围岩经历了相似的变质作用；4. 对冈塘错地区首次发现的榴辉岩进行的相平衡模拟研究显示，显示其经历了进变质升温升压变质作用演化，峰期温压条件达~470 °C和~23 kbar；5.对蓝岭含硬绿泥石蓝片岩和含硬绿泥石硬柱石石榴石云母片岩进行了相平衡模拟研究，其结果显示蓝片岩压力峰期温压条件可达490 °C和24 kbar，温度峰期条件为530 °C和22 kbar，而石榴石多硅白云母片岩峰期温压条件为530 °C和22 kbar，两者与冈塘措榴辉岩温压条件相似；6. 对羌塘中部高压变质带北侧俄久卖地区低压高温片麻岩进行了岩石学研究，获得其变质作用峰期温压条件为5-6 kbar and 750-760 °C。上述成果对认识羌塘中部低温高压变质带的变质作用演化特征及造山带演化具有重要科学意义。