International Geologiical Congress - Oslo 2008


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MPI-02 Integrated perspectives on the accretion of oceanic crust


Metamorphosed shallow mantle wedge witnessed from the Paleozoic Ust'-Belaya ophiolite in northern Koryak Mts., Chukotka, NE Russia: A preliminary report


Akira Ishiwatari, Tohoku University (Japan)
Sergey D. Sokolov, Geological Institute, Russian Academy of Sciences (Russian Federation)
Yasutaka Hayasaka, Hiroshima University, Faculty of Science (Japan)
Galina, V. Ledneva, Geological Institute, Russian Academy of Sciences (Russian Federation)
Boris, A. Bazylev, Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences (Russian Federation)
Suren, A. Palandzhyan, Geological Institute, Russian Academy of Sciences (Russian Federation)
Sumiaki Machi, Kanazawa University, School of Natural Science and Technology (Japan)
Artem, V. Moiseev, Geological Institute, Russian Academy of Sciences (Russian Federation)


The Ust'-Belaya(-Otrozhnaya-Eldenyr) ophiolite is one of the largest lherzolite-type ophiolites in the world, distributed in a NE-trending 80 x 40 km area around 65°N, 173°E (Sokolov et al. 2003; Geol. Soc. London, Spec. Publ., 218, 619-). The associated fossiliferous limestone suggests Devonian or older age of this ophiolite, which thrust onto the Utyosiki mélange with younger(?) ophiolitic fragments. The mélange in turn tectonically overlies the Jurassic-Cretaceous volcano-sedimentary rocks of the Algan terrane.
Following the 2006 field research of the garnet metagabbro-ultramafic complexes of the late Paleozoic-early Mesozoic age in the Pekulney Range, 50 km to the east of Ust'-Belaya Village (Ishiwatari et al. 2007; Island Arc, 16, 1-), we carried out a JSPS/RFBR-supported, 40-days field research of the Ust'-Belaya ophiolite in 2007 to clarify igneous-metamorphic evolution of the Paleozoic oceanic lithosphere preserved in the circum-Pacific orogenic belt.
We find that the "lherzolite" dominant in the Ust'-Belaya ultramafic massif (25 x 15 km) shows fresh (only slightly serpentinized) appearance and typical coarse-grained mantle fabric ("spinel-pyroxene" foliation) in the field, but shows mostly metamorphic features under the microscope. The meta-lherzolite consists of olivine-antigorite-chlorite-acicular clinopyroxene and olivine-antigorite-chlorite-tremolite assemblages, which are almost devoid of relict "mantle minerals". The olivine frequently exhibits distinct cleavages developed in mutually perpendicular three directions ("cleavable olivine"). Mafic dikes cutting the meta-lherzolite are commonly metamorphosed into garnet amphibolite. However, the eastern and some other parts of this massif are composed of the harzburgite-dunite-chromitite-spinel websterite suite characterized by granular texture free from metamorphism. Although the geological relationship between the two units is not clear, the granular harzburgite can not be protolith of the meta-lherzolite. Gabbroic rocks in the Ust'-Belaya, Otrozhnaya and Utyosiki nappes are mostly hornblende metagabbro, but a large quantity of fresh troctolite is also associated.
The regional distribution of the low-temperature (<=500°C, acicular clinopyroxene-bearing) meta-lherzolite is a unique feature of this ophiolite, and it may witness the metamorphic evolution of the shallow part of supra subduction-zone (SSZ) mantle wedge. Rare mantle xenoliths from the island arc volcanoes in Kamchatka and Philippines bear high-temperature-metamorphosed, fine-grained meta-peridotites. The mantle peridotites dredged and drilled from the Izu-Bonin-Mariana forearc also exhibit various, moderate-temperature metamorphism with diagnostic minerals such as talc, Mg-cummingtonite, and metamorphic orthopyroxene. It is likely that the Ust'-Belaya massif represents a composite, shallow SSZ mantle with metamorphosed old lherzolite and new igneous product of oceanic arc magmatism (harzburgite-dunite suite).


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