The Nanamagari Formation of the Miocene in the back-arc side of central Japan is composed mainly of silicic pyroclastic rocks associated with basaltic pyroclastic rocks and dikes. New and published K-Ar ages and biostratigraphic ages indicate that the basaltic volcanism occurred during 16-12 Ma. This volcanism may have associated with opening of the Japan Sea back-arc basin. By comparison with experimental data, we estimated that the possible primary magmas of these basalts were segregated from the upper mantle at about 1.4 GPa, 1330°C. The basaltic rocks show a trace element abundance pattern typical of island arc low-K basalts. The incompatible element ratios suggest that these basalts were derived from the mantle source similar to the magma source of basalts from the Quaternary volcanic front of northeast Japan.
