K-AR GEOCHRONOLOGY OF A QUATERNARY MONOGENETIC VOLCANO GROUP IN OJIKAJIMA DISTRICT, SOUTHWEST JAPAN

An unspiked K-Ar dating method using a mass-fractionation correction p rocedure was applied to a Quaternary independent group of monogenetic volcanoes, Ojikajima Volcano Group, located in northwestern Kyushu in Southwest Japan, in order to clarify in detail secular variations in e ruptive volume, locations of eruptive vents, and magma compositions in a single monogenetic volcano group. The major results were as follows : (a) K-Ar ages of monogenetic volcanoes distribute from 1.08 to 0.30 Ma, with voluminous peaks at approximately 1.0 and 0.6 Ma. (b) The vol canic activity commenced in the central part of the field, expanded to the whole field at approximately 0.6 Ma, and then shrank to the centr al area. (3) Concentrations of incompatible elements, such as Ba, K, a nd Nb, increase with decreasing age, whereas P, Y, and Zr concentratio ns remain constant. These concentrations suggest gradual decrease in t he degree of partial melting from an identical mantle source with resi dual garnet.