K-Ar ages of an andesitic parallel dike swarm in the Takane area, Takayama City, Gifu Prefecture, Central Japan

Niwa, Masakazu   ; Ueki, Tadamasa*; Hoshi, Hiroyuki*; Sugisaki, Yuichi*; Yagi, Koshi*; Tozawa, Terumasa

A large number of paleomagnetic and chronological studies have contributed to the elucidation of the early Miocene clockwise rotation of Southwest Japan associated with the major opening of back-arc basins of the Japan Sea. Paleomagnetic data from an andesitic parallel dike swarm in the Takane area, Takayama City, Gifu Prefecture, Central Japan, were previously reported, showing the easterly deflection of the observed magnetization direction compared to an early Miocene expected paleomagnetic direction. This must represent the clockwise rotation during the opening of the Japan Sea, if the dike swarm was intruded before the opening event. Here, we report new K-Ar hornblende ages from an andesite dike of the dike swarm. These ages are statistically indistinguishable, indicating emplacement of the dike swarm before the clockwise rotation.