Deformation And Geotectonic Significance Of Quartz Veins In Gangmacuo Area, Southern Qiangtang Of Tibet

The study area is located in the northern Tibet autonomous region, which also inside the Qinghai-Tibet plateau hinterland. Tectonic environment belongs to the central uplift zone of the north of south Qiangtang. Accretionary complex distributed in Shuanghu to Gangmacuo area, and this paper mainly studies the accretionary complex of Gangmacuo area. Overall,the study areaconsist of matrix-block structure,blockcompositions are mainly the quartz sandstone, diabase, conglomerate, graywacke and high-pressure metamorphic rocks. Only the matrix of high pressure metamorphic rock is garnet mica quartz schist, other matrix are metamorphic mudstone. Accretionary complex is covered with the wedge-top basin. Study area mainly experienced two structural deformation, and structural transpositionis very strong, only in the weak deformation block exists a small amount of cross-bedding.The first phase of deformation formed shear foliation during subduction period, relying mainly on shear deformation; The second phase of the deformation based the deformation surface which was formed by first phase of deformation, and developed the bulking fold, superimposing and transforming the first phase of structural deformation.The accretionary complex matrix of study area widely has developed the syntectonic quartz vein. The quartz vein took part in the structural deformation during the later phase, and which has transformed as quartz mortar, soft flow fold,etc. This paperpicked quartz vein respectively andsystemicly, in the matrix of accretionary complex which in different deformation and metamorphic degree. For quartz vein was formed during the first phase shear deformation, the study of fluid inclusions can reflect the first phase temperature and pressure condition of the matrix. Therefore, the condition of temperature and pressure can be used to discuss subduction deformation environment and regional tectonic implications.The microscopic structure of quartz veins shows that the dynamic recrystallization mechanism is bulging recrystallization, which represents that low temperature metamorphism; The temperature of fluid inclusions for homogenization temperature is 100-200 ℃, that consistent withlow temperature conditions of petrographic analysis;Salinity is calculated from the temperature measurement experiment, combined with the formation of homogenization temperature to estimate the formation pressure of fluid pressure is 418-601 MPa.That indicates the first phase of deformation formed in the low temperature–high pressure environment.Combining the geothermal gradient and formation pressure, I calculated that the quartz veins deformed between 14.2-25 km, which belongs to the middle crust. Through the subduction exhumation process, quartz veins were taken out and disorderly deposits on the crust surface, that become part of the accretionary complex system. And considering the isotherm distribution, I find that the substrate which include the first phase deformation quartz veins belongs to the low temperature-high press metamorphism, don’t experience the high-temperature metamorphism during the subduction process.