K-Ar dating of Mutsu-Hiuchidake, Osoreyama, Nanashigure, and Aoso volcanoes of the Aoso-Osore volcanic zone

New K-Ar ages of four volcanoes; Mutsu-Hiuchidake, Osoreyama, Nanashigure, and Aoso volcanoes are presented. All of which are located in the Aoso-Osore volcanic zone along the Quaternary volcanic front of the Northeast Japan arc, and characterized by relatively low-K contents. The obtained ages are as follows: 0.73±0.05 Ma for Mutsu-Hiuchidake, 0.74+0.06, 0.81±0.05 Ma for Osoreyama, 0.96±0.09, 1.06±0.07 Ma for Nanashigure, and 0.38±0.04, 0.40±0.02, 0.38±0.03, 0.38±0.04, 0.38±0.05 Ma for Aoso volcano. Lateral and temporal variation of silica-normalized potassium contents (SiO2=57.5%) of the rocks from volcanoes, which are distributed in the volcanic front side during the past 4 m. y., is investigated. Lavas with relatively low potassium content have erupted continuously during past 4 m.y. in the whole area, whereas lavas with relatively high potassium content have erupted from volcanoes of the Sekiryo zone with ages younger than 0.7-1.0 Ma which is the formation age of the present volcanic zonation.