青森県南部，碇ヶ関カルデラ主要構成噴出物（虹貝凝灰岩）のジルコンLA-ICP-MS U-Pb年代

抄録

The Nijikai Tuff was ejected in the late Cenozoic from the Ikarigaseki Caldera, located in the southwestern part of the Hakkoda-Towada caldera cluster. New U-Pb ages of zircons from the Nijigai Tuff reveal a bimodal age distribution with two peaks at 2.019±0.098 and 4.203±0.083 Ma, based on an Unmix Ages function. Considering the closure temperature of zircon, the younger peak age corresponds to the eruption of the Nijikai Tuff, whereas the older peak age may correspond to zircons inherited from massive assimilation by the Nijikai Tuff of underlying volcanic ejecta, likely the Obirakiyama Tuff. In this study the eruption age of the Nijikai Tuff is established with much higher precision than previously determined.