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

Numerous paleomagnetic and chronological studies have contributed to the elucidation of the early Miocene (18-16 Ma) clockwise rotation of Southwest Japan associated with the major opening of back-arc basins in the Japan Sea. Paleomagnetic data have been reported previously from an andesitic parallel dike swarm in the Takane area, Takayama City, Gifu Prefecture, Central Japan, showing easterly deflection of the observed magnetization direction, in contrast to the expected paleomagnetic direction for the early Miocene. This must therefore represent clockwise rotation during the opening of the Japan Sea, providing the dike swarm was emplaced prior to its opening. Here we report new K-Ar hornblende ages from an andesite dike from the swarm. Two K-Ar ages of 22.1±1.2 and 21.2±1.2 Ma are statistically indistinguishable, and clearly pre-date the clockwise rotation.