Age estimation of a high-magnesian andesite from the Kii Peninsula, southwest Japan: possible interpretation of anomalous magnetic direction.

Middle Miocene volcanic rocks are sporadically distributed along a narrow inland archipelago, the Seto Inland Sea. The volcanic chain is regarded as the Middle Miocene island arc volcanism and called the Setouchi volcanic belt, which is characterized by an occurrence of high-magnesian andesite. Radiometric ages of those high- magnesian rocks range from 13 Ma to 11 Ma. Recently a high-magnesian andesite was found as a single dike in the trench-ward region relative to the Setouchi volcanic belt at the W coast of the Kii Peninsula. This dike rock is situated about 80km trench-ward from the Setouchi volcanic belt, and vertically intrudes into Early to Middle Miocene marine sediments. In an attempt to define the Middle Miocene volcanic front of SW Japan, it is necessary to reveal whether or not this dike was coeval with the high-magnesian andesite magmatism of the Setouchi volcanic belt. Describes details of magnetic measurements and to estimate an age of this high-magnesian andesite by focusing on its clockwise deflected magnetic direction. -from Authors