著者

佐藤 鋭一 和田 恵治 野口 昌宏

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.67, no.3, pp.255-271, 2022-09-30 (Released:2022-10-27)

参考文献数

37

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Kurodake volcano in the Taisetsu volcano group was formed approximately 0.2 Ma, producing andesitic lava flows and a dome. The lavas contain numerous mafic inclusions (<20 vol.%) ranging from approximately 1 cm to about 30 cm in diameter. The mafic inclusions exhibit typically rounded to ellipsoidal shapes and have smooth contacts with the host lavas. The mafic inclusions are classified into two types, fine and coarse, based on the size of the groundmass crystals. The groundmass crystals of the fine-type inclusions are composed of acicular minerals (0.1-0.3 mm in length). On the other hand, the groundmass of the coarse-type inclusions is primarily composed of tabular minerals (>85 vol.% and 0.2-0.5 mm in length). The plagioclase core compositions of the host lavas and two types of mafic inclusions vary substantially from An38 to An90. The plagioclase phenocrysts are classified into three groups based on their core compositions: An-rich (type A: An>80), An-poor (type B: An<60), and intermediate (type C: 60<An<80). Type A and type B plagioclases were derived from mafic and silicic magmas, respectively, and type C was derived from a hybrid magma formed by the mixing of the mafic and silicic magmas. The host lavas predominantly contain type B plagioclase phenocrysts, with infrequent types A and C, and most of the plagioclase microphenocrysts and groundmass crystals are type C. In the fine-type inclusions, type A and type B plagioclase phenocrysts coexist, and most of plagioclase microphenocrysts and groundmass crystals are classified into the type C, similar to the host lavas. In the coarse-type inclusions, most of the plagioclase phenocrysts, microphenocrysts, and groundmass crystals are classified as type B. These assemblages in the host lavas and fine-type inclusions can be explained by the mixing of the magmas, whereas the coarse-type inclusions were formed in the silicic end-member magma. Initially, mafic magma containing type A plagioclase was injected into bottom of the silicic magma chamber containing type B. A small amount of mafic magma was mixed with silicic magma to form the host magma. Subsequently, mixing occurred near the boundary between the mafic and silicic magmas, producing the fine-type inclusion forming magma. We presume that the margin of the silicic magma chamber was highly crystalline and the coarse-type inclusions were derived from the margin of the silicic magma chamber.

著者

野口 昌宏

出版者

大東文化大学

雑誌

大東法学 (ISSN:02870940)

巻号頁・発行日

vol.4, pp.一九一-二〇六, 1977-03-30