小笠原諸島, 父島北部の石英含有無人岩 : 無人岩と石英デイサイトのマグマの混合

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Submarine boninite series volcanic rocks with quartz dacite and subordinate submarine sedimentary rocks, belonging to middle Eocene to late Oligocene in age, are widespread on Chichi-jima. Quartz-bearing boninite is found to occur as the chilled rims of dikes, east of Kiyose and southwest of Tsuri-hama. On Miyano-hama near Kiyose, boninite pillows are intruded by a multiple dike of dacite and boninite. The multiple dike suggests the existence of a density-stratified chamber capped by dacite. The quartz-bearing boninite includes about 4 mm across corroded quartz up to several modal%, with reversely zoned orthopyroxenes carrying pale brown rounded cores and sieved orthopyroxenes. Based on the textures, these inclusions are xenocrysts, derived from quartz dacite. The chemical composition of the dike rims from Kiyose is nearly constant according to the whole rock analyses. Glasses in the same dike rims, with related ones of the Miyano-hama multiple dike, were analysed by electron probe to compare in genesis the compositions. In the oxides (e.g., MgO)-SiO_2,relationship for the boninite series volcanic rocks of Chichi-jima, the linear relationship of MgO to SiO_2,is obvious in a quartz-bearing boninite as the dike rims from Kiyose, in a more magnesian boninite as the chilled rim of the Miyano-hama multiple dike and a quartz dacite lava. Inferring from the field and the petrological evidence, quartz-bearing boninite was formed by rapid mixing of boninitic magma and a small amount of quartz dacite magma in a similar chamber to the possible stratified chamber beneath Miyano-hama.
静岡大学の論文

小笠原諸島, 父島北部の石英含有無人岩 : 無人岩と石英デイサイトのマグマの混合

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