HR: 14:00h
AN: V13C-02    [Abstracts]
TI: UHP Metamorphism and Continent Subduction/Collision - An Overview
AU: * Liou, J
EM: liou@pangea.stanford.edu
AF: Stanford University, Dept. of Geological and Environmental Sciences, Stanford, CA 94305 United States
AU: Zhang, R
EM: zhang@pangea.stanford.edu
AF: Stanford University, Dept. of Geological and Environmental Sciences, Stanford, CA 94305 United States
AU: Maruyama, S
EM: smaruyam@geo.titech.ac.jp
AF: Tokyo Institute of Technology, Dept. of Space and Earth Sciences, Tokyo, 152 Japan
AB: Ultrahigh-pressure (UHP) metamorphism refers to mineralogical and structural modification of predominantly continental and minor oceanic crustal protoliths and associated mafic-ultramafic rocks at P greater than $\sim$ 2.7 GPa ($>$ 80-90 km). Typical products include eclogite, garnet peridotite and associated metapelite, quartzite, marble, and gneisses. For appropriate bulk compositions, subduction-zone metamorphism at great depths produces coesite, microdiamond and other characteristic UHP minerals. Thus far, at least 15 coesite-bearing eclogitic and 7 diamond-bearing UHP belts have been documented in the world; most lie within major continental collision zone in Eurasia. UHP metamorphic assemblages occur in long, discontinuous belts which may extend several hundred km or more and 10 to 50 km wide, and are associated with HP blueschist belts. Recent petrological data suggest that contrary to the long-held orthodoxy, low temperatures at very high pressures (the so-called 'forbidden zone') are realized in the Earth. A considerable number of collisional orogens also contain relics of coesite-grade metamorphism. They may have been overlooked due pervasive late-stage overprinting and Barrovian-type recrystallization during uplift. In such cases inclusions in zircon offer the best means to unravel the early UHP history. Majoritic garnet and HP clinoenstatite occur in garnet peridotites of some UHP terranes; supracrustal rocks together with the enclosing mafic-ultramafic rocks have experienced subduction zone UHP metamorphism. The discovery of these ultramafic denizens of the forbidden zone confirms cold subduction-zone models. These cold subduction zones are the sites of major recycling of water into the mantle, for HP experiments reveal that numerous, important hydrous phases are stable in the P-T forbidden zone. Research in the past 20 years on UHP rocks containing coesite and diamond has produced a dramatic restructuring of our understanding of plate tectonics. The discovery of garnet peridotites from the forbidden zone provides a revolutionary new window into the subduction of continental margins, the thermal structure of subduction zones, and the recycling of volatiles into the mantle.
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting