森吉火山の岩石学 カルデラ形成後に主として活動した混合マグマ

Abstract

Petrography and geochemistry of the volcanic rocks of Moriyoshi volcano, northeastern Japan, reveals the mechanism which had produced the different chemical trends of the rocks between the earlier stage (pre-caldera activity) and the later one (post-caldera activity). In the later stage, phenocryst assemblage of basalt and andesite (SiO2=53-66%) is complex (olivine+orthopyroxene+hornblende+clinopyroxene+Fe-Ti oxide+ quartz+ plagioclase). The compositions of phenocrystic minerals, clinopyroxene (Wo45 En40), orthopyroxene (Wo2 En60) and plagioclase (An 35-55), are nearly constant in spite of the wide variation of whole-rock chemistry. Magnesian olivine phenocrysts (Fo 78-71) are in disequilibrium with quartz, iron-rich orthopyroxene and clinopyroxene. Compositional zonations of phenocrystic minerals are normal in olivine and reverse in clinopyroxene, orthopyroxene and plagioclase. These features cannot be explained in terms of simple crystallization differentiation model. The linear trends on variation diagrams indicate that two magmas, with different temperature and compositions, were mixed. Low temperature magma (SiO2=66%) would contain plagioclase, quartz, orthopyroxene, clinopyroxene and hornblende phenocrysts. This dacitic magma was mixed with basaltic magma, containing olivine phenocryst, to produce the calc-alkaline trend (low iron-enrichment on the SiO2-FeOt/ MgO diagram) in the later stage. In the earlier stage, the rocks consist of augite-hypersthene andesite to dacite, which are accompanied by a small amount of olivine-bearing augite-hypersthene andesite. The compositions of phenocrystic minerals gradually change according to whole-rock chemistry. The distinct disequilibrium petrographic features are not found in olivine free rocks. On the contrary, in olivine-bearing rocks, normally zoned olivine phenocryst (Fo87-73) is in disequilibrium with reversely zoned orthopyroxene (mg value 68-62) and clinopyroxene (72-67). The olivine-bearing rocks would be produced by magma mixing process, between basaltic magma and andesitic magma. But magma mixing process at the earlier stage is negligible, because the proportion of mixed magma to eruptives of the earlier stage is estimated to be less than 10%. Therefore, the tholeiitic trend (strong iron-enrichment on the SiO2-FeOt/MgO diagram) of these rocks can be explained in terms of fractional crystallization mechanism. Basaltic magma would not have crystallized calcic plagioclase with olivine phenocryst (Fo87-71). This delay of crystallization of plagioclase does not depend on H20 content judging from F content. Crystallization of plagioclase is supressed by crystallization of basaltic magma at high pressure. So basaltic magma of Moriyoshi volcano had stopped at the deeper level (〉25km) and crystallized olivine phenocryst. This fractionated basaltic magma had rised rapidly to shallower level magma chamber to mix with andesitic or dacitic magma. Mixed magma effused mainly after the formation of caldera. In the earlier stage, which is the stage of constructing a stratovolcano, if magma mixing process had occurred, mixed magma could not rise easily under the regional compressional stress field on the northeastern Japan. Hence evidence of magma mixing would not be clear. In the later stage, mixed magma would have rised easily through the crack formed by the caldera collapse.