HR: 0800h
AN: V11A-1409    [Abstracts]
TI: SHRIMP U-Pb Dating of Zircon From the Xugou UHP Eclogite, Sulu Terrane, Eastern China
AU: * Zhao, R
EM: rxzhao@pangea.stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, Bldg 320, Stanford University, Stanford, CA 94305 United States
AU: Liou, J G
EM: liou@pangea.stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, Bldg 320, Stanford University, Stanford, CA 94305 United States
AU: Zhang, R Y
EM: zhang@pangea.stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, Bldg 320, Stanford University, Stanford, CA 94305 United States
AU: Wooden, J L
EM: jwooden@usgs.gov
AF: U.S. Geological Survey, 345 Middlefiedl Rd. , Menlo Park, CA 94025 United States
AB: Eclogites, together with garnet clinopyroxenites, occur as lenses within the Xugou garnet peridotite body from the southern Sulu ultrahigh-pressure (UHP) terrane. Combined cathodoluminescence (CL) imaging and SHRIMP U-Pb dating of zircon from two Xugou eclogites provide added constraints on the timing of UHP metamorphism in this area. Zircon grains from both mafic eclogite samples show subrounded to rounded shapes, ranging from 50 to 250 |?m in diameter. CL images reveal that zircons from both samples are not oscillatory zoned, and have no obvious cores of magmatic or xenocrystic origin. Most zircons show a homogeneous CL image, in a few cases with a weak cloudy or patchy pattern. Th-U chemistry of studied zircons is characterized by variable but low U and Th contents (U: 4-423 ppm; Th: 0.2-67 ppm) and variable Th/U ratios, ranging from 0.01 to 1.78 for sample XG-3C and 0.06 to 0.50 for sample XJ-1E. There is also a weak linear relation between Th/U ratios and 206Pb/238U age, as well as U and Th, indicating that these eclogites might have undergone systematic variation during UHP metamorphism. All these features suggest that zircons from our samples are metamorphic. The analyses of both samples fall into one population on a Tera-Wasserburg (TW) diagram and a histogram. U-Pb data of both samples yield apparent 206Pb/238U ages ranging from 214 to 280 Ma with a weighted mean age of 237+/-8 Ma (n=30). This age is interpreted as the time of peak UHP metamorphism of the Xugou eclogites and is consistent with previous reported UHP metamorphic ages from eclogite pods and country rock gneisses in the Sulu terrane. The Xugou mafic lenses may have formed by partial melting of the enclosing peridotites in the mantle before subduction (Zhang et al., 2003); then these eclogites, together with host peridotites, were emplaced into the subduction zone and subjected to UHP metamorphism at 237+/-8 Ma.
DE: 1749 Volcanology, geochemistry, and petrology
DE: 1035 Geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting