Publication

The Neogene volcanic rocks are widely distributed along the southwestern coast of Tsugaru Province, Aomori Prefecture. This study focused on the Middle Miocene Juniko Tuff, and designated the area known as Nihon Canyon in Aomori Prefecture as a type locality. We re-evaluated the chronostratigraphy based on a geological field surveys and zircon U–Pb and/or FT/U–Pb double dating. The Juniko Tuff having a maximum 300 m thickness, consists of dominantly felsic hyaloclastites with small amount of subaqueous felsic lavas and feeder dykes. It was formed within a short period from 12.4 to 12.1 Ma, and is thought to be contemporaneous heterotopic facies with the uppermost part of Odoji Formation and lowermost part of Akaishi Formation, respectively, which is consistent with previously suggested chronostratigraphy and lithostratigraphy. The characteristics of the major element compositions for the Juniko Tuff showed two types of felsic magmas: high-Mg rhyolite (HMR) and island-arc rhyolite (IAR). In the early stage, HMR occurred as small-scale volcanic activity and next, shifted to IAR as a majority of the volcanic activity. Subsequently small-scale volcanic activities of the HMR occurred again, and a series of the felsic volcanic activities have ended at 12.1 Ma. The subaqueous felsic volcanic activity that formed the Juniko Tuff occurred in the bathyal zone over a short period (~0.3 My).