哀牢山杂岩带南段巴罗型递增变质系列P-T-t-D轨迹研究

The Ailao shan (ALS) complex belt separated the Indochina Block to the west and the South China Block to the east, southeastern Tibetan Plateau. Barrovian sequence was preserved in the south segment of the ALS complex belt. The construction of the P-T-t-D trajectory of this sequence is crucial for understanding the convergent process between the Indian plate and Euro-Asian plate. Therefore, the Barrovian sequence, in Yuanjiang and Yuanyang area, is treated as the main study object. On the basis of microstructures and possible metamorphic reaction analyses, phase equilibria modeling as well as traditional thermobarometers are applied to precisely constrain the P-T conditions of the multi-stage metamorphic mineral assemblages and subsequently construct their P-T paths. Based on cathodoluminescence and backscattered image, monazite, zircon, xenotime U-Th-Pb dating, as well as Ar-Ar dating of mica are used to restrict the timing of multi-stage metamorphism and therefore construct the P-T-t paths. Detailed geological mapping of outcrops at a large scale and profile surveying are used for structural analysis in the field. Micro focus CT analysis for oriented specimen is devote to observe the foliation and porphyroblast inclusion trials in three-dimensional. Then metamorphism and deformation information is combined to construct the P-T-t-D path. The study shows scientific significance of depicting the metamorphic evolution of ALS complex reconstructing the regional geodynamic process of southwestern Tibetan Plateau.

我国青藏高原东南缘的哀牢山杂岩带位于印支板块与华南板块之间，其南段具有典型巴罗型递增变质岩系特征，建立其变质演化P-T-t-D轨迹对解析印度-亚欧板块的汇聚过程极为重要。因此，本项目拟以分带明显的元江-元阳地区巴罗型递增变质岩系为重点研究对象，在详细的岩相学观察基础上，应用相平衡模拟配合传统地质温压计，精确限定各变质阶段矿物组合特征及P-T条件。以背散射、阴极发光图像分析为基础，对独居石、锆石、磷钇矿进行原位和靶内U-Th-Pb定年配合云母Ar-Ar定年，精确构筑递增变质系列的年代格架。结合小范围大比例尺填图和祥测剖面进行野外构造解析，应用微焦点工业CT三维扫描分析定向样品面理的三维几何学形态及变斑晶内包体径迹匹配相应的变质阶段，最终构筑哀牢山南段递增变质系列的P-T-t-D轨迹。该项研究对精细刻画青藏高原东南缘哀牢山杂岩带变质演化历史及重塑造山带侧向动力学过程都具有重要的科学意义。

青藏高原东南缘哀牢山变质杂岩带南段发育典型的巴罗式递增变质带，其形成时代与构造背景并不明确，递增变质带的空间展布。本项目通过独居石、锆石U-Pb测年工作揭示其变质时代为新生代始新世并受到渐新世红河-哀牢山韧性剪切作用的改造。递增变质带内夕线石带研究首次在青藏高原东南缘发现高压泥质麻粒岩透镜体，相平衡模拟计算揭示其峰期温度、压力条件为810-820 ℃、13.5-14.0 kbar，变质锆石增生边揭示其变质峰期的时代为早始新世（约49-28 Ma）揭示青藏高原东南缘哀牢山地区在始新世地壳即已加厚至50km。锆石测年发现递增变质带的原岩时代为早古生代而非早期认为的中元古代，为限定递增变质带变形时代确定了下限。夕线石带内含蓝晶石石榴石夕线片麻岩研究确定流体作用下石榴石溶解-沉淀蠕变新的变形机制。对哀牢山南延进入越南的大象山部分进行地质调查，综合研究揭示出递增变质带经历三期褶皱变形。