ABSTRACT

Pleistocene volcanic rocks of the Yatsugatake volcanic chain were dated by the K-Ar method in order to study the geochronological significance of the Quaternary volcanic activities along the Fossa Magna, the most remarkable tectonic structure in central Japan. One of the porphyrites of Mt. Nakadake, which are regarded to have formed the root of the Yatsugatake volcanoes shows a K-Ar age of about 1.3 Ma. An andesite of Kasuga volcanic rocks from the northern area of the Yatsugatake volcanic chain indicates a K-Ar age of about 10Ma. K-Ar ages for seven andesites from four different lava units of the southern area range from 0.23 to 0.38Ma. These results are concordant with those estimated stratigraphically. Present results suggest that the volcanic activities to form the present Yatsugatake volcanic chain probably started about 1∼1.5Ma ago. This might have been related to some tectonic effects which triggered volcanic activities around the area like Kirigamine Volcano in central Japan. Furthermore, available radiometric age data together with present results suggest that some volcanic activities already occurred about 1.5∼2Ma ago along the present volcanic front, at least in the Fossa Magna region.