Magma mixing and quartz-bearing boninites from Chichi-jima and Ani-shima, Bonin Islands

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  • KURODA,Naoshi
    Department of Geosciences, Faculty of Science, Shizuoka University
  • SHIRAKI,Kei-ichi
    Department of Mineralogical Sciences and Geology, Faculty of Science, Yamaguchi University
  • URAN0,Hayaomi
    Department of Earth Sciences, Aichi University of Education

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  • マグマの混合と父島・兄島の石英含有無人岩

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Chichi-jima consists of dominant latest Eocene submarine boninite, accompanied by andesite and dacite, and subordinate sedimentary rocks of late Oligocene-early Miocene age. In general boninites of Chichi-jima were erupted prior and posterior to those dacites which can be classified into quartz phenocryst-bearing and-free types. Quartz-bearing boninite occurs as dikes and pil lows at some places on both islands of Chichi-jima and Ani-shima. The boninite includes a small amount of reversely zoned orthopyroxene with rounded cores of lower opitic axial angles and honey-combed crystals of orthopyroxene, sometimes of augite and plagioclase, and corrowded quartz attaining up to 6 model %. Judging from the texture and optics, these inclusions are xenocrysts, leading to quartz phenocryst-bearing dacite. No xenoliths have yet been found to occur in boninite laves and dikes. The linear relation of oxides to SiO_2 is obviously found among a quartz-bearing boninite as dike rim, a more magnesian boninite as dike rim posterior to dacite dike and adacite lava with quartz phenocrysts. Considering this assemblage in the field and the petrological evidence, quartz-bearing boninite was produced by mixing of boninite-forming magma and a part of quartz-bearing dacite-forming magma in the same magma chamber after the eruption of dacite carrying quartz phenocrysts.

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