2025 年 54 巻 1 号 論文ID: 250714
“Sasamoriyama Volcanic Rocks” are distributed around the south-western part of Fukushima city in the volcanic front of Northeast Japan arc. The volcanic rocks consist of four geological members: Sasamoriyama Andesite Member, Chusakuyama Andesite Member, Ishiyama Andesite Member, and Kongosan Andesite Member. Lower units of Sasamoriyama Andesite Member and Chusakuyama Andesite Member are correlative to each other, and they were derived from a dissected volcanic edifice. Middle and upper units of Sasamoriyama Andesite Member are composed of anorthite megacryst-bearing andesite lavas and dacite lavas. Sasamoriyama lower units are composed of Icelandite-like tholeiite, whereas middle and upper units have the composition of island-arc tholeiite. Middle and upper units of Chusakuyama Andesite Member are composed of calc-alkaline rocks associated with pigeonitic and hypersthene rock series. Ishiyama Andesite Member is composed of calc-alkaline rocks with pigeonitic rock series, which can be interpreted as a lava dome effused from a subaerial vent. Kongosan Andesite Member is composed of calc-alkaline rocks with hypersthene rock series.
Above all, Icelandite-like tholeiitic lavas in lower units of Sasamoriyama Andesite Member and Chusakuyama Andesite Member contain few opaque minerals as phenocrysts, whereas lavas in middle and upper units of Sasamoriyama Andesite Member contain abundant phenocrysts of opaque minerals as glomerocrysts with other minerals. The 87Sr/86Sr and 143Nd/144Nd ratios of the island-arc tholeiitic lavas increase and decrease, respectively, with the whole-rock SiO2 contents, whereas these ratios of Icelandite-like tholeiitic lavas do not show systematic variation with the SiO2 contents. This observation suggests that the Icelandite-like and island-arc tholeiitic magmas were derived from distinct magma plumbing systems, and they evolved in closed and open systems, respectively. We also suggest that the Icelandite-like tholeiitic magmas had higher temperature, lower water content, and lower oxygen fugacity than those of the island-arc tholeiitic magmas that were similar to those of Quaternary volcanic front tholeiitic magmas. Finally, calc-alkaline magmas occurred in the western part of volcanic front, not only Fukushima area but also some other areas in Northeast Japan arc.
