HR: 0800h
AN: T21A-0506    [Abstracts]
TI: Geochronology and Mineral Inclusions of Eclogite, Ortho-, and Paragneiss Zircon, North Qaidam UHP Terrane, Northwest China
AU: * Mattinson, C G
EM: cgm@pangea.stanford.edu
AF: Dept. of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115 United States
AU: Liou, J G
EM: liou@pangea.stanford.edu
AF: Dept. of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115 United States
AU: Bird, D K
EM: bird@pangea.stanford.edu
AF: Dept. of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115 United States
AU: Wooden, J L
EM: jwooden@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Wu, C
EM: wucailai@ccsd.org.cn
AF: Chinese Academy of Geological Sciences, Institute of Geology, Beijing, 100037 China
AU: Yang, J
EM: yangjsui@ccsd.org.cn
AF: Chinese Academy of Geological Sciences, Institute of Geology, Beijing, 100037 China
AB: Amphibolite-facies felsic gneisses of the North Qaidam Mountains enclose minor ($<$10 vol%) eclogite, peridotite and pyroxenite ($\pm$garnet) which record ultra-high pressure (UHP) metamorphism. Field relations, and the presence of coesite inclusions in zircons from paragneiss suggest felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. Cathodoluminescence (CL) and SHRIMP-RG U/Pb and REE analyses of zircons from two granitic orthogneisses reveal oscillatory zoning, moderate to high Th/U, U, and REE concentrations, and large negative Eu anomalies, consistent with magmatic crystallization at ca.~936 Ma (all ages are $^{238}$U/$^{206}$Pb, $^{207}$Pb corrected for common Pb, except as noted). Inclusions identified by EMP include biotite, muscovite, plagioclase, and SiO$_2$. Zircon cores from two paragneisses contain high Th/U, moderate to high U and REE concentrations, prominent Eu anomalies, and yield discordant Early Proterozoic $^{207}$Pb/$^{206}$Pb ages, and are probably of detrital origin, but zircon rims contain garnet, rutile, phengite, and SiO$_2$ inclusions, very low Th/U, high U, low REE abundances, and Eu anomalies are small or absent, suggesting eclogite-facies growth. Weighted mean ages of the rims are 418$\pm$3 Ma and 411$\pm$3 Ma. Two fresh eclogites contain zircon with inclusion-rich CL dark cores, and rounded, inclusion-poor, CL grey overgrowths. Analyses of cores (partial overlap on rims) yield moderate to high Th/U and U, and ages up to 475 Ma, which places a minimum age on eclogite protolith crystallization; albite and Th + REE-rich epidote inclusions suggest greenschist- or epidote-amphibolite-facies growth. The rims contain garnet, omphacite, rutile, and phengite, yield low Th/U, moderate to high U, and weighted mean ages of 449$\pm$3 Ma and 440$\pm$4 Ma, reflecting eclogite-facies growth. Two retrogressed eclogites contain inclusion-poor zircon with CL bright, mottled cores ($\pm$faint oscillatory zoning; garnet, omphacite, and rutile inclusions) surrounded by irregular, dark rims characterized by low Th/U, and low to very low U, which yield weighted mean ages of 421$\pm$5 Ma and 415$\pm$12 Ma. Amphibole and plagioclase inclusions in the rims suggest the measured ages reflect retrogression. The spread in ages (449--411 Ma) is probably too large to be explained by a single metamorphic event, and suggests that mafic enclaves record polymetamorphic/tectonic histories prior to their incorporation in the surrounding gneisses. Juxtaposition of the different rock types probably occurred during exhumation from UHP conditions.
DE: 9320 Asia
DE: 3660 Metamorphic petrology
DE: 1035 Geochronology
DE: 1065 Trace elements (3670)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting