Thermochronology of non-volcanic hydrothermal activity in the Kii Peninsula, Southwest Japan; Evidence from fission track dating and helium isotopes in paleo-hydrothermal fluids

Umeda, Koji; Hanamuro, Takahiro  ; Yamada, Kunimi; Negishi, Yoshimitsu*; Iwano, Hideki*; Danhara, Toru*

Fission track dating of zircon and apatite samples from the altered host rocks around three hot springs were carried out in order to elucidate the thermochronology of the hydrothermal system in this non-volcanic region. The apatite FT ages from the altered host rocks around the hot springs ranged from 2.7 to 5.6 Ma, which are significantly younger than the 12 Ma age of the non-altered host rocks far from the hot springs. In the southern Kii Peninsula, therefore, it is obvious that the non-volcanic hydrothermal activity already has been ongoing since around 6 Ma.