小笠原父島無人岩の記載岩石学的研究 : 無人岩枕状溶岩の垂直変化

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Chichijima, Ogasawara Islands is the place where boninite has been exposed most voluminously all over the world. Moreover, it is “type locality” of boninite. Boninite is high-magnesian andesite (HMA) which is a primary andesite basis of its high Mg, Ni and Cr content, directly derived from mantle. We did detailed research and sampling of boninite based on geological map of Chichijima (Umino and Nakano, 2007) and clarified and classified petrographic characteristics of boninite. Uminio (1986) classified aphyric boninite from Chichijima into five types: (I) olivine (ol) + clinoenstatite (cen) + bronzaite (brz); (II) ol + brz; (III) brz; (IV) brz + pigeonite (pig) + augite (aug); (V) cen + brz. We can classified the type (II) of Umino (1986) divided into four types by phenocryst and micro-phenocryst assemblage. They are Type AI (phenocryst: (ol + serpentine (ser)) / brz ≧1 and micro-phenocryst: brz / clinopyroxene (cpx) ≧1), Type AII (phenocryst: (ol + serpentine (ser)) / brz ≧1 and micro-phenocryst: brz / clinopyroxene (cpx) < 1), Type BI (phenocryst: (ol + serpentine (ser)) / brz < 1 and micro-phenocryst: brz / clinopyroxene (cpx) ≧1 ) and Type BII (phenocryst: (ol + serpentine (ser)) / brz <1 and micro-phenocryst: brz / clinopyroxene (cpx) <1 ). It has been understood that the above four types rock appears in respective height in the pillow lava bed.