HR: 14:25h
AN: V53E-04    [Abstracts]
TI: Fluid/melt activities during the exhumation of subducted continental crust in the Sulu UHP metamorphic terrane, China
AU: * Li, H
EM: hyli@public3.bta.net.cn
AF: Institute of Mineral Resources, Chinese Academy of Geological Sciences, 26 Baiwanzhuan Road, Beijing, 100037 China
AU: Jahn, B
EM: jahn@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55, Nankang, Taipei, 11529 Taiwan
AB: The widespread occurrence of coesite in the UHPM rocks in the Sulu terrane suggests a rapid exhumation process. The extremely low oxygen isotopic compositions of eclogites imply that the protoliths of eclogites resided at mantle depths for a short time and did not undergo significant water-rock interaction on a regional scale. However, we report here field observations and zircon SHRIMP and LA-ICP/MS dating results demonstrating important fluid/melt activities, at least on a local scale, during the exhumation of UHPM rocks in the Sulu terrane. In the northern Sulu terrane, widespread granitic layers or veins in centimeter to several meter scale can be observed in felsic gneisses, especially in biotite-rich felsic gneisses. The layers/veins are strongly deformed and occasionally show foliation in concordance with the surrounding gneisses. Zircons from a granitic gneiss in Weihai have a core-mantle-rim structure. Oscillatory zoning is well preserved in cores and rims, and irregular patch structure is present in mantle domains. The cores and rims yielded ages of 772 +/- 7 Ma and 195 +/- 4 Ma, respectively, and the mantles show a large age span. Zircons from a pegmatite within the gneiss gave two ages of 221 +/- 7 Ma and 199 +/- 3 Ma. We interpret the age of 221 Ma as the time of pegmatite crystallization and 199 Ma as a post-magmatic thermal event. In the southern Sulu terrane, muscovite-bearing quartz veins and zoisite-kyanite-paragonite quartz veins or nodules occur in eclogites and country gneisses. They have mica-WR Rb-Sr isochron ages of 226 to 215 Ma. Zircons from a granitic gneiss from Qinglongshan show a very complicated internal structure. This is probably due to complete metamorphic recrystallization in a condition of strong fluid activity. U-Pb isotope analyses of these zircons only revealed a unique metamorphic age of 218 +/- 5 Ma. This is in strong contrast with the published zircon U-Pb ages of the Dabie-Sulu UHP metamorphic rocks in which protolith ages of 600 to 800 Ma are commonly recorded. The age of 218 Ma probably corresponds to the time of a complete Pb loss, produced by the metamorphic fluid event.
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 1115 Radioisotope geochronology
DE: 3654 Ultra-high pressure metamorphism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005