Petrology of the Higashi-Izu Monogenetic Volcano Group
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概要
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The Higashi-Izu monogenetic volcano group is situated in the northeastern part of the Izu Peninsula and is composed of more than 70 monogenetic volcanoes scattered in the area 350km2 wide. Tephrochronological studies indicate that the volcanism of the Higashi-Izu monogenetic volcano group started 30,000 years ago and continued up until less than 3,000 years before present. Actually the activity possibly began much earlier than 30,000 years ago. The Si02 contents of the rocks from this volcano group show a bimodal distribution : i.e., one group in the range from 49 to 60 wt.% (basalts-andesites) and the other in the range from 68 to 74 wt.% (dacites). Phenocryst assemblages of the basalt-andesite group are olivine+plagioclase, olivine+plagioclase+clinopyroxene, and olivine+plagioclase+clinopyroxene+orthopyroxene. Andesites contain granitic xenocrysts (quartz, plagioclase, hornblende and opaques). The phenocryst assemblage of dacite is plagioclase+orthopyroxene+hornblende+magnetite+ilmenite. Dacite dome lavas contain basaltic xenocrysts (magnesian olivine, Ca-rich plagioclase and clinopyroxene). The rocks of the olivine+plagioclase assemblage are mostly confined to the northern part of the area and those of the olivine+plagioclase+clinopyroxene assemblage to the southern part of the area. The rocks of the olivine+plagioclase+clinopyroxene+orthopyroxene assemblage are concentrated in the eastern part of the latter. The rocks containing xenocrysts are mostly confined to the northeastern part of the area. Most of the rocks without xenocrysts of this volcano group fall within the high-almina basalt series field and those with phenocrysts fall in the tholeiitic rock series field in the Si02 versus total alkali diagram. The anhydrous melting expriments of basalts suggest that the unknown but limited amount of water enough to affect the phase relations was dissolved in the basaltic magmas before eruption. Most of the andesite magmas of the Higashi-Izu monogenetic volcano group were produced by the contamination of the basaltic magma with the granitic rocks constituting the crust under the Izu Peninsula. This is supported by the positive corelation between the amount of xenocrysts and the Si02 and Al2O3 contents and by rare-earth element data. The rocks showing evidence of such contamination are distributed mostly in the northeastern part of the region. Groundmass pyroxene assemblage changes from augite+pigeonite or augite+pigeonite+hypersthene in basalts to augite+pigeonite+hypersthene or augite+hypersthene in andesites. This change is related to the degree of contamination, the hypersthene-bearing assemblages being the product of a more advanced stage of contamination. Dacite magma was probably produced by the partial melting of the granitic rock constituting the crust by the heat of the basaltic magma. This is supported by the close occurrence of the basic and acidic magmas in time and place, the existence of basaltic xenocrysts in dacites and the existence of the granitic xenoliths and xenocrysts in basic magmas. Also this is suggested by the major element and rare-earth element data.
- 東京大学地震研究所,Earthquake Research Institute, University of Tokyo,東京大学地震研究所の論文
- 1986-02-07
東京大学地震研究所,Earthquake Research Institute, University of Tokyo,東京大学地震研究所 | 論文
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