阿蘇カルデラ形成後に活動した多様なマグマとそれらの成因関係について

概要

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Between 270 and 90ka, Aso volcano produced four large caldera-forming pyroclastic flows, all of which probably derived from a single zoned magma chamber, as suggested by previous geological and geochemical studies. On the other hand, after 90ka, for the post-caldera activities, it is difficult to create a wide compositional range of post-caldera magmas from a single magma chamber model, when the following observations are considered. (1) Seven distinct magma groups with distinct petrographic and compositional characteristics are found; I. 2px-rhyolite, II. bt-rhyolite, III. hb-dacite, IV. 2px-dacite, V. aphyric andesite, VI. porphyritic andesite, VII. basalt-basaltic andesite. (2) Incompatible trace element plots show that the seven magma groups were formed by at least four end-member magmas that do not show parent-daughter relationships. (3) Vents extruding basaltic magmas are centered in the caldera, whereas those extruding andesitic, dacitic, and rhyolitic magmas are distributed around them. These observations indicate that several (at least four) magma chambers are developed during the post-caldera activities, and that the compositional diversity of the magmas can be explained by magma mixing among the end-member magmas in addition to fractional crystallization.
2005-10-31