哀牢山—红河剪切带中流体记录及地质意义

摘要

As a high-strain localization zone in continental lithosphere, shear zone is an important place for metamorphism-deformation and fluid activities of rocks.The Diancang Shan metamorphic complex, exposed along the Ailao Shan-Red River strike-slip fault zone, preserved an abundance of information of thermal and tectonic evolution since the Cenozoic.Different types of quartz veins were developed in the Diancang Shan metamorphic complex.Based on the geological field observation and microstructure analysis, we carried out a detail study of deformed quartz veins during shearing by using Fourier Transformation Infrared Spectroscopy (FTIR) , Laser Raman Spectroscopy (LRM) and fluid inclusion (FI) techniques.The results showed: (1) Quartz veins displayed different characteristics of ductile microstructural deformation under the microscope.In some samples, intense fine-grained and dynamic recrystallized quartz grains had CO2-H2 O or pure CO2 fluid inclusions that mainly distributed at the grain boundaries.Other samples contained stripped elongated porphyroclast or fine grained quartz grains showing typical core-mantle structures.In the mainly fine-grained quartz grains we did not find fluid inclusions, but the elongated stripped quartz porphyroclasts had abundant watery inclusions with no distinctive distribution characteristics. (2) According to inclusion types, the estimated capture temperatures and pressures of the fluid inclusions ranged in 500-550 ℃ and260-290 MPa, respectively.The capture depth of the fluid inclusions was about 10 km, with fluid origin mainly of metamorphic fluid.The occurrence of CO2-H2 O or aqueous inclusions may be mainly due to water loss caused by phase separation and deformation of fluids, owing to rapid decompression during the exhumation process of the Diancang Shan metamorphic complex. (3) The quartz grains in quartz vein and surrounding rocks contained a certain amount of inclusion water and a small amount of structurally defective water.The water distribution characteristics and content varied with fluid inclusions.Water distribution in the quartz grain was inhomogeneous, showing higher water content at the edge than at the core. (4) Inclusions in the recrystallized quartz grains were often enriched at the grain boundaries.Therefore we believe that, local fluid pressure at the boundaries and permeation/release of fluid weakened the crystal plane and promoted lattice dislocation slip and dissolution creep.%剪切带作为大陆岩石圈中的局部高应变带, 是岩石变质-变形作用和流体活动的重要场所.沿着哀牢山—红河走滑剪切带内发育的点苍山深变质杂岩, 保存了新生代以来极其丰富的构造-热演化信息.在点苍山杂岩体内出露有一系列具有不同构造特征的石英脉, 我们选取了遭受剪切变形的石英脉作为主要研究对象, 在野外构造观测和室内显微构造特征分析的基础上, 利用傅立叶变换红外光谱 (FTIR) 、激光拉曼显微光谱 (LRM) 以及流体包裹体技术 (FI) 对其开展了进一步的精细研究.研究结果表明: (1) 变形石英脉在显微镜下具有不同类型的塑性变形特征, 其中以强烈细粒化和动态重结晶为主要特征的石英颗粒中包裹体类型主要为CO2-H2O包裹体和纯CO2流体包裹体, 包裹体多沿颗粒边界分布;在以条带状和细粒化为主要特征且具有典型核-幔构造的石英颗粒中, 重结晶颗粒基本不含流体包裹体, 石英条带中含有大量水溶液包裹体;在定向拉长的粗颗粒石英中其变形行为主要表现为波状消光和局部应变导致的塑性流动构造, 在该类型石英颗粒中只含有水溶液包裹体且无明显分布特征. (2) 根据各类流体包裹体测温数据估算出包裹体捕获温压为500～550℃、260～290MPa, 深度约为10km, 其流体来源主要为变质流体, 剥露过程中的快速降压使得流体发生相分离及变形作用并导致的水丢失, 这是CO2-H2O包裹体与水溶液包裹体出现的主要原因. (3) 石英脉中的石英颗粒除含有一定量的包裹体水之外还含有少量的结构水, 水在颗粒内部的分布为非均匀状态, 通常边部水含量高于核部, 同时水的分布特征和含量可随包裹体的变化而改变. (4) 重结晶石英颗粒中的包裹体常在颗粒边界富集, 认为其边界局部流体压力和流体的渗透或释放能弱化晶体平面, 促进晶格位错滑移和溶解蠕变.