Primitive olivine tholeiite from the Ryozen district, northeastern part of Fukushima Prefecture, Northeast Japan.

High magnesian olivine basalts of lower Miocene age are distributed in the Ryozen district of the northeastern part of Fukushima Prefecture, Northeast Japan. Olivine basalts have low FeO*/MgO ratios (0.68-1.17) and high Cr contents (323-1193ppm). The olivine basalt R09 is sparsely phyric (phenocrysts of olivine and chromite 10〈vol%), and the olivine phenocrysts have narrow compositional range (Fo=85-89) and high NiO contents (0.24-0.32%). The olivine phenocrysts of the olivine basalt R09 contain chromite only as inclusion. Fe-Mg partitioning of the olivine basalt R09 suggests that phenocryst of magnesian olivine (Fo=87) is in equilibrium with coexisting liquid (=groundmass). These features strongly indicate that the olivine basalt R09 has primitive nature in chemical composition and may represent the primary olivine tholeiitic magma generated in the upper mantle. Differentiated basalts having relatively high FeO*/MgO ratios may be produced from the primary magma by the fractionation of magnesian olivine, chromite and clinopyroxene.