Transition of eruptive style in an arc-arc collision zone: K-Ar dating of Quaternary monogenetic and polygenetic volcanoes in the Higashi-Izu region,Izu peninsula, Japan

K-Ar ages were measured on Quaternary polygenetic and monogenetic volcanoes in the Higashi-Izu region, Izu peninsula, central Japan, using the unspike d sensitivity method with mass-fractionation correction procedure to invest igate when eruptive style changed, whether a hiatus existed between the two types of eruptive activity, and the effect of tectonics on the change in e ruptive style. The K-Ar ages range from 0.3-0.08 Ma for monogenetic volcano es and from 1.8-0.2 Ma for polygenetic volcanoes; thus, no volcanic hiatus was found between the two types of eruptive styles. The transition from pol ygenetic to monogenetic volcanism occurred during a time of overlap between 0.3 and 0.2 Ma, after collision of the Izu block (the future Izu peninsula ) with central Japan, estimated as 1.0-0.8 Ma by previous researchers. Base d on the review of several tectonic models of the area, the measured age of transition in eruptive style is interpreted to correspond to the change in the stress field of the Higashi-Izu region.