HR: 15:12h
AN: V13C-08    [Abstracts]
TI: Length-Scales of Oxygen, Hydrogen, and Argon Isotope Exchange During UHP Metamorphism: an Example From the Qinglongshan UHP Eclogites (Sulu Terrain, China)
AU: * Cosca, M A
EM: mcosca@img.unil.ch
AF: Institute of Mineralogy and Geochemistry, University of Lausanne, Lausanne, 1015 Switzerland
AU: Giorgis, D
EM: david.giorgis@sit.vd.ch
AF: Institute of Mineralogy and Geochemistry, University of Lausanne, Lausanne, 1015 Switzerland
AU: Rumble, D
AF: Geophysical Laboratory, 5251 Broad Branch Rd., NW, Washington, DC 20015-1305 United States
AU: Liou, J G
AF: Dept. of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 United States
AB: New and previously published stable isotope (oxygen and hydrogen) and $^{40}$Ar/$^{39}$Ar data are compared from selected outcrops of the Sulu UHP terrain near Qinglongshan (Jiangsu Province, China). These rocks retain unusually low and heterogeneous $\delta$$^{18}$O and $\delta$D values acquired in a Neoproterozoic geothermal system despite undergoing Triassic (220-240 Ma) UHP metamorphism. Incremental heating $^{40}$Ar/$^{39}$Ar analyses of muscovite, biotite, and K-feldspar from metagranite, quartzite, and gneiss (all metamorphosed to the coesite-eclogite facies) yield cooling ages between 190 and 204 Ma. In contrast, phengite $^{40}$Ar/$^{39}$Ar data from eclogite, quartzite, and gneiss contain variable amounts of extraneous argon, consistent with inheritance from a Neoproterozoic protolith. Plots comparing phengite $\delta$$^{18}$O, $\delta$D, and $^{40}$Ar/$^{39}$Ar total gas ages from from different lithologies within individual Qinglongshan outcrops only meters apart highlight significant inter-outcrop isotopic heterogeneities ($\Delta$D$_{phengite}$ = 26$\permil$; $\Delta$$^{18}O$_{phengite}$ = 8.8$\permil$; $\Delta$$^{40}$Ar/$^{39}$Ar$_{phengite}$ = 664 Ma), however maximum intra-outcrop isotopic variations between lithologies are limited ($\Delta$D$_{phengite}$ = 5$\permil$; $\Delta$$^{18}O$_{phengite}$ = 1.8$\permil$). The oxygen and hydrogen isotopic variations are interpreted to reflect primary isotopic heterogeneities acquired during Neoproterozoic hydrothermal fluid circulation with cold-climate meteoric waters. The limited intra-outcrop isotopic variations suggest extensive isotopic exchange occurred during UHP metamorphism within discrete outcrops, irrespective of lithology. Likewise, extraneous argon within the phengites reflects inheritance from a Neoproterozoic protolith, and the age variations between outcrops is probably due to differential argon loss during thermal and baric equilibration accompanying differential exhumation following UHP metamorphism. The retentivity of extraneous argon in phengite is partially controlled by the host rock lithology; for example the armoring effects of basaltic eclogite are greater than quartzite or gneiss. Collectively these data indicate that, while extensive, isotopic exchange during subduction and UHP metamorphism was limited to contiguous blocks of coherent subducted crust with each block preserving small isotopic heterogeneities acquired from their protoliths roughly 0.5 Ga previously. Such length-scales of isotopic exchange, defined by the size of a contiguous outcrop, underscore the closed system isotopic behavior that can occur during continental subduction, collision, and uplift from UHP conditions.
DE: 3660 Metamorphic petrology
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting