HR: 14:55h
AN: V53E-06    [Abstracts]
TI: Timing of Fluid Flow During Exhumation of Deeply Subducted Continent
AU: * Zheng, Y
EM: yfzheng@ustc.edu.cn
AF: University of Science and Technology of China, School of Earth and Space Sciences, Hefei, 230026 China
AU: Gao, T
EM: tsgao@ustc.edu.cn
AF: University of Science and Technology of China, School of Earth and Space Sciences, Hefei, 230026 China
AU: Wu, Y
EM: ybwu@ustc.edu.cn
AF: University of Science and Technology of China, School of Earth and Space Sciences, Hefei, 230026 China
AU: Gong, B
EM: gongb@ustc.edu.cn
AF: University of Science and Technology of China, School of Earth and Space Sciences, Hefei, 230026 China
AB: Quartz veins are common within UHP eclogites in the Dabie-Sulu orogenic belt of China. While their formation has been linked to dehydration reactions, time of veining has been uncertain during either subduction or exhumation. SHRIMP U-Pb dating for zircons from kyanite-quartz vein and its host eclogite in the Dabie orogen yields two groups of age at 212±7 Ma and 181±13 Ma, respectively. They are significant younger not only than SHRIMP zircon U-Pb ages of 243±4 and 224±3 Ma for host eclogite, but also than known UHP metamorphic ages of 234±4 to 227±2 Ma as dated by the SHRIMP U-Pb technique for coesite-bearing domains of zircon. The U-Pb age of 224±3 Ma for the eclogite dates zircon growth at the onset of HP eclogite-facies recrystallization during exhumation. Corresponding temperatures may be about 670°C as estimated for both eclogite-facies recrystallization and veining from a petrological study. The second group of zircon U-Pb age at 181±13 Ma is much later than the HP-UHP-HP metamorphic events during the orogenic cycle and thus may not be relevant to post-collisional exhumation. Therefore, the two groups of vein age date the two episodes of fluid flow, respectively, due to decompression dehydration during exhumation and heating dehydration in response to breakup of supercontinent Pangea. This provides for the first time the radiometric dates for timing of fracture fluid flow that transports both mass and heat during plate collision. Laser fluorination O isotope analyses show the almost same δ¹⁸O values for the minerals of both vein and eclogite, indicating the same origin of fluid and material for them and thus internally derived fluid for veining. High O isotope temperatures of 695±20 to 715±35°C are obtained for Qz-Ky and Qz-Gt pairs, suggesting the attainment of O isotope reequilibration during the HP eclogite-facies recrystallization. On the other hand, low O isotope temperatures of 490±10 to 510±15°C occur in Qz-Rt and Qz-Zo pairs, suggesting that the ca.180 Ma thermal perturbation may just take place at the temperatures about 500°C. In either case, the O isotope temperatures may represent the temperature of fluid loss by the different types of hydration from the eclogite because they are similar to the closure temperatures of O diffusion in these minerals. The rapid loss of aqueous fluid from intragrains and intergrains would terminate O isotope exchange by diffusion between coexisting phases just as effectively as quenching from high temperature. Synchronous cooling between them is suggested by the almost same magnitudes of quartz-mineral fractionations with regularly decreased temperatures that are consistent with rates of O diffusion in the minerals. While the major veining by decompression dehydration would take place at 700 to 650°C in a transition from the HP eclogite-facies to amphibolite-facies retrogression, the minor veining by heating dehydration occurred at 500 to 600°C without corresponding metamorphism. The very low U contents occur in the ca.180 Ma zircons, indicating that the heating dehydration may be associated more with the exsolution of structural hydroxyl than with the decomposition of hydrous minerals. Therefore, the kyanite-quartz vein formed later than the peak UHP metamorphic event at the Middle Triassic, pointing to veining during exhumation rather than subduction. This demonstrates that the significant amounts of metamorphic fluid were focused as fracture flow to transport both mass and heat during exhumation and thus the fluid flux occurs in the direction of decreasing pressure.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1115 Radioisotope geochronology
DE: 3653 Fluid flow
DE: 3654 Ultra-high pressure metamorphism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005