ABSTRACT

Based on the radiometric age (mostly K-Ar and ⁴⁰Ar-³⁹Ar ages) and isotopic ratio data for volcanic rocks recovered from the Japan Sea floor, the formation age and the characteristics of magma sources for these volcanic rocks are discussed. Most volcanic rocks from the Yamato and Japan Basins, where oceanic crusts are presumed to exist, show radiometric ages of less than 20Ma. Igneous rocks dredged from such areas as continental remains are conjectured show radiometric ages of more than 100Ma. Volcanic activity in the Yamato Bank area along the borderland of the Yamato Basin seems to have ceased after 20Ma, which might have related to the formation of the Yamato Basin. Combining such data with the detailed information on the obtained ages, the formation of the Yamato Basin has been inferred to have started prior to 17Ma and not later than around 25Ma. The Japan Basin probably formed almost at the same period or slightly earlier. ⁸⁷Sr/⁸⁶Sr ratios for volcanic rocks are systematically different between the Yamato Basin and the Yamato Bank together with continental shelf areas: the former indicate values of 0.7035-0.7040, whereas the latter of higher than 0.7045. This might reflect the difference in the degree of incorporation of continental crustal materials.