Intra-oceanic island arc origin for Iratsu eclogites of the Sanbagawa belt,central Shikoku,southwest Japan |
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| Authors: | Atsushi Utsunomiya Bor-ming Jahn Kazuaki Okamoto Tsutomu Ota Hironao Shinjoe |
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| Institution: | 1. Faculty of Earth Science and Mineral Resources, China University of Geosciences, Beijing 100083, China;2. MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China;3. No. 916 Geological Team, Jiangxi Bureau of Geology, Mineral Resources, Exploration and Development, Jiujiang 332000, China;4. Northwestern Geological Team, Jiangxi Bureau of Geology, Mineral Resources, Exploration and Development, Shahe 332100, China;1. Department of Earth and Environmental Sciences and GeoBio-Center, LMU Munich, Theresienstr. 41, 80333, Munich, Germany;2. GeoZentrum Nordbayern, Universität Erlangen-Nürnberg, Schlossgarten 5a, 91054, Erlangen, Germany;3. Institut für Geologie, Universität Freiberg, Bernhard-von-Cotta Str. 2, 09599, Freiberg, Germany;4. Mineralogische Staatssammlung München, Theresienstr. 41, 80333, Munich, Germany;5. Sankt-Petersburg State University, University Embankment 7/9, 199034 Sankt-Petersburg, Russia;1. Division of Earth Sciences, Faculty of Social and Cultural Studies, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan;2. Vietnam Institute of Geosciences and Mineral Resources, Thanh Xuan, Hanoi, Viet Nam;3. General Department of Geology and Minerals of Vietnam, 6 Pham Ngu Lao Street, Hanoi, Viet Nam |
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| Abstract: | New geochemical and Sr–Nd isotopic data for the Iratsu eclogite and surrounding metamorphic rocks of the Sanbagawa belt, Japan, show that, while the protoliths of the metamorphic rocks formed in a variety of tectonic settings, the Iratsu body represents a deeply subducted and accreted island arc. The igneous protoliths of eclogites and garnet amphibolites were probably generated from a mantle source that had components of both a depleted mantle modified by slab-released fluid (as seen in a negative Nb anomaly) and an enriched mantle, similar to that of ocean island basalts (OIB). Fractional crystallization modeling indicates that the protoliths of some garnet clinopyroxenites from the Iratsu body are cumulates from a basaltic magma that crystallized under high O2 and H2O fugacities in the middle to lower crust. The source characteristics and crystallization conditions suggest that the protoliths of the Iratsu rocks formed in an oceanic island arc. Quartz eclogites from the marginal zone of the Iratsu body have geochemical signatures similar to turbidites from the Izu–Bonin island arc (as seen in a negative Nb anomaly and a concave REE pattern). The protoliths might be volcaniclastic turbidites that formed in a setting proximal to the oceanic island arc. Geochemical and isotopic signatures of the surrounding mafic schists are similar to normal (N-) and enriched (E-) mid-ocean-ridge basalt (MORB), and distinct from the rocks from the Iratsu body. The protoliths of the mafic schists likely formed in a plume-influenced mid-ocean ridge or back-arc basin. Pelitic schists from the surrounding rocks and pelitic gneisses from the marginal zone of the Iratsu body have evolved, continental geochemical signatures (as seen in a negative εNd(t) value (~?5)), consistent with their origin as continent-derived trench-fill turbidites. |
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