40Ar- 39Ar and Sr isotope analyses were performed on basalts and andesites dredged from the Yamato Seamount chain in the Japan Sea. The 40Ar- 39Ar plateau ages range from about 11 to 17 Ma, though most samples show ages between 10 and 14 Ma. The seamounts seem to have formed within a period of a few million years, although some of them might have formed earlier. Based on the present results together with previously reported radiometric age data, it is thought that the Yamato Basin formed during some period prior to 17 Ma and probably later than around 25 Ma. Taking into account the radiometric age data on rocks from the Japan Basin, it is conjectured that the opening of the Japan Sea might have started almost at this time or a little earlier. The observed 87Sr 86Sr ratios range from 0.70357 to 0.70388, suggesting incorporation of some time-integrated components enriched in incompatible elements such as continental crustal materials. This may indicate that in the Japan Sea area, at least the Yamato Basin had not developed enough to show the characteristics of typical N-type MORB source materials without being affected by pre-existing continental crustal materials.