K-Ar age and paleomagnetic study of volcanic rocks from the Hohi geothermal area, Kyushu, Japan

Paleomagnetic study and K-Ar age determinations were carried out in the Hohi geothermal area for those poorly correlated volcanics that had been erupted after the Ohno volcanics (ca. 13 Ma) and before the Aso, Kuju and other youngest volcanics. The age data indicate that the volkanic activity in this area took place almost continuously at least during the past four million years and that there was a period of quiescence of volvanic activity between 13 Ma and 5 Ma. The volcanic rocks in the central part are younger than 1 Ma with normal magnetic polarity. On the other hand, those in the western and eastern parts are somewhat older with normal and reversed magnetic polarities, being 1-3 Ma and 1-2 Ma, respectively; the oldest units (3-5 Ma) among the studied volcanics are distributed in the northern part. There is no systematic variation with time in the chemistry or mode of eruption of the volcanics. (author)