ABSTRACT

⁴⁰Ar-³⁹Ar analyses were performed on five igneous rocks dredged from the continental shelves and slopes around the Yamato Basin in the central part of the Japan Sea during the GH78-2 Cruise of the Geological Survey of Japan. Three igneous rocks dredged from the slopes of the Japanese Islands show ⁴⁰Ar-³⁹Ar plateau ages of 23-24 Ma. One andesite from the Yamato Bank gives a total ⁴⁰Ar-³⁹Ar age of about 26 Ma, whereas one basalt from a height of the northern part of the Yamato Basin shows a scattered age spectrum, indicating the existence of excess ⁴⁰Ar. Except for this sample, the other four rocks seem to be related to volcanic activity which occurred before or during the formation of the Yamato Basin. Based on the present data together with radiometric ages previously reported for the rocks from the Yamato Basin area, the Yamato Basin is estimated to have formed during a period prior to 17 Ma and probably later than about 25 Ma.