青藏高原东南缘鲜水河断裂带古近纪构造变形及动力变质作用研究

SHRIMP U-Pb date reveal a magma event about 35Ma and a metamorphic event about 27Ma in east Tibet along the Xianshuihe fault zone, proved the Paleogene deformation in the Xianshuihe fault which always be known as a sinistral strike slip fault after 12Ma. The tested sample gathered form a migmatite zone which elongates about 80km in length and 3-5km in width along the XSH fault. However, we don't know the geometric and kinematic features, and deformation mechanism of the early deformation in the XSH fault zone. Weather the deformation indicates the extrusion and uplift of the Chuan-Dian block in Paleogene, or indicates the strike-slip faulting after the early deformation? What the kinematic background of the deformation and how to illustrate it? These scientific questions above all need much intensive study on the geometry, kinematics, and dynamic metamorphism of the fault. Therefore, the project have set several research contents, for example, the structural geology, the microstructure geology and the metamorphic petrology. The project will take different research methods to solve these questions, such as the local large-scale mapping, the strain measurement, the structural analysis, the geothermometer-barometer,the isotopic dating, and so on. The study will contribute to reconstruct the geometry model of the Chuan-Dian block in Paleogene, and will uncover the dynamic background of the deformation.

普遍认为鲜水河断裂带左行走滑变形始于新近纪12Ma左右，之前的历史并不清楚。对鲜水河断裂带东南段伴生的一套混合岩的锆石SHRIMP U-Pb测年揭露了35Ma的岩浆事件和27Ma左右的动力变质事件，指示了鲜水河断裂带古近纪存在变形。目前缺乏这期变形详细的几何学、运动学资料，以及伴生动力变质过程研究。这期变形指示了川滇地块古近纪怎样的运动方式:是挤出隆升还是先隆升后期挤出？反应了怎样的动力学背景呢？该问题的核心需要对这期变形的几何学、运动学、动力变质过程进行深入研究。项目设定了断裂带及伴生动力变质岩的几何学、运动学、同位素年代学等研究内容，采取了局部大比例尺填图、应变测量统计、温压计测试、同位素测年等不同的研究方法。该项研究将有助于认识古近纪青藏高原东南缘块体的运动方式。

印度-欧亚大陆碰撞导致青藏高原隆升、以及物质发生大规模的、沿着大型断裂带向东挤出。在青藏高原东南缘鲜水河断裂带内发育长约120km、宽约15-20km的贡嘎山杂岩体。早期研究认为这是鲜水河断裂带同构造侵位花岗岩体。最新的资料将其分为五个单元，分别为1）晚三叠系216-204Ma侵位单位；2）中侏罗世~170Ma的侵入体；3）中晚侏罗世变沉积岩；4）渐新世-中新世32-20Ma的混合岩；5）~20Ma以来的花岗岩。216-204Ma的岩浆侵位与松潘甘孜造山有关，其主要分布在该杂岩体的南段。~170Ma的侵位体可进而可以两套。一套是~170Ma与断裂带同剪切侵位的花岗岩，其最年轻的锆石U-Pb年龄限定了鲜水河断裂带在中生代171Ma发生过左行剪切。另一套是出露在杂岩体内部的淡色花岗岩。中侏罗世的夕线石-石榴石变沉积岩出露在贡嘎山杂岩体的东侧，其和淡色花岗岩一同指示了沿着鲜水河断裂带的块体汇聚或者陆内俯冲的地质背景。. 32-20Ma混合岩带长约120km、宽约1-3km。这套混合岩两期形成。第一期形成于32~27 Ma，可能与鲜水河断裂带的挤压变形相关。第二期形成于~25-20 Ma，以发育左行走滑矿物生长线理为特征，反映了鲜水河断裂带在27-25 Ma已经启动左行走滑。构造隆升史分析表明32-20Ma鲜水河断裂带及邻区处于快速的构造隆升期。区域资料显示，这期隆升事件同时存在于龙门山、东昆仑-西秦岭一带，反映了青藏高原东缘渐新世-早中新世的整体隆升。这不支持挤出模型所设想的向北有序扩展、逐渐隆升的观念。这期渐新世至早中新世的隆升、剥露事件之后，沿鲜水河断裂带形成了20-12Ma花岗岩，反映了深部物质的减压熔融。中新世约~13Ma以后，青藏高原物质发生大规模向东挤出，使得鲜水河断裂带发生强烈左行走滑，形成沿鲜水河断裂带分布的糜棱岩带。此后，鲜水河断裂连通安宁河-则木河-小江断裂，形成了清晰的向东南迁移的运动轨迹。