福島県東部野手上山高マグネシア玄武岩

抄録

High-magnesian tholeiitic basalts of early Miocene age are distributed on Nodegamiyama which is located in the eastern part of Fukushima Prefecture, Northeast Japan. These basalts (Nodegamiyama basalts) occur as a dome, with a diameter of about 300 meters, which intrude into Cretaceous granitic rocks occupying in Abukuma mountainland.
Nodegamiyama basalts are sparsely phyric (phenocrysts of olivine with chrome-spinel inclusions, clinopyroxene, orthopyroxene and plagioclase 10 vol%>). These basalts are characterized by relatively high SiO₂ (52.0-52.7 wt%), high MgO (8.0-9.3 wt%), relatively low CaO (8.7-9.1 wt%), high Ni (167-220ppm) and high Cr (540-690ppm). The clinopyroxene phenocrysts contain considerable amounts of Al₂O₃ (3.8-8.0 wt%), and the calculated Ca-Tschermak's component ranges from 7.5 to 13.6 mol per cent, which suggest that these clinopyroxene phenocrysts are thought to have crystallized from the basalt magma at a depth of about 30km in the lower crust in this region.
Differences in bulk chemical compositions between Nodegamiyama basalts and Ryozen basalts may be ascribed to differences in the degree of partial melting between these two primary magmas and also in water content in the source mantle peridotites of these two primary magmas.