Two types of chemical trends found in low-Ca boninite suite from Chichijima, Ogasawara (Bonin) arc
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- YAJIMA Kazuhito
- Institute of Mineralogy, Petrology and Economic Geology, Graduate School of Science, Tohoku University Present address: Nichias Corporation, Tsurumi Laboratory
Bibliographic Information
- Other Title
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- 小笠原群島父島に産する低Caボニナイト系列岩の2つの組成変化トレンド
- オガサワラ グントウ チチジマ ニ サンスル テイ Ca ボニナイト ケイレツガン ノ 2ツ ノ ソセイ ヘンカ トレンド
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Abstract
Chichijima, located in Ogasawara (Bonin) arc, Japan has been known as the type locality of boninite. Meanwhile, extremely evolved volcanics (SiO2>70 wt%) also occur together with the high-magnesian andesites. The evolved rocks have been considered as the products of differentiation from the boninitic parental magma by fractional crystallization processes. The differentiation processes are considered to produce low-Ca boninite through bronzite-andesite, dacite, quartz-dacite and quartz-rhyolite. Some of the quartz-dacites, however, would not be necessarily regarded as differentiates from the dacite magma. Quartz-dacites are enriched in SiO2 for the fractional crystallization products from dacite. Some quartz-dacites show mineralogical evidences of magma mixing, while such features can not be observed in the dacites. Quartz phenocrysts in the silica rich quartz-rhyolite exhibit euhedral form, but quartz-dacites have corroded quartz phenocrysts. The compositions of mafic pyroxene phenocrysts in quartz-dacites are similar to those in bronzite-andesites. Groundmass pyroxenes in some quartz-dacites include orthopyroxene and augite. In contrast, pigeonite and augite are ubiquitous groundmass pyroxenes in the quartz-free dacites. These chemical and petrographical features imply that the quartz-dacites have calc-alkalic affinities, whereas the dacites have tholeiitic affinities. The tholeiitic trend of the dacites might be formed by fractional crystallization under low fO2 conditions, judging from the crystallization of Fe-rich pigeonite phenocrysts, and the quartz-rhyolite would be produced from the dacite magma by fractional crystallization in the tholeiite suite. Mixing calculations between bronzite-andesite and quartz-rhyolite reproduce the observed chemical variations of quartz-dacites. Therefore, calc-alkalic trend of the quartz-dacites might be formed by internal magma mixing between the bronzite-andesite magma and the quartz-rhyolite magma.
Journal
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- Japanese Magazine of Mineralogical and Petrological Sciences
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Japanese Magazine of Mineralogical and Petrological Sciences 32 (2), 51-67, 2003
Japan Association of Mineralogical Sciences
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Details 詳細情報について
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- CRID
- 1390001206525741312
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- NII Article ID
- 130000253540
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- NII Book ID
- AA1146088X
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- COI
- 1:CAS:528:DC%2BD3sXlvFels74%3D
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- ISSN
- 13497979
- 1345630X
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- NDL BIB ID
- 6630389
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- Text Lang
- ja
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- Data Source
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- JaLC
- NDL
- Crossref
- CiNii Articles
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- Abstract License Flag
- Disallowed