Abstract
Obirakiyama Tuff (210 km3 DRE), erupted from Yunosawa Caldera in southern part of Hakkoda-Towada caldera cluster, consists of subaerial welded tuff and subaquatic unwelded tuff facies. This study reports zircon U-Pb and fission-track (FT) double dating for five samples collected from the two facies to reconstruct the eruptive ages of each facies. The resulting U-Pb ages for three pumices from the unwelded tuff facies were concentrated at 3.9 Ma, whereas the U-Pb ages of both the welded tuff exposed at the mid-point of Mt. Obiraki and obsidian in Daibutsu Park yielded U-Pb ages of 8.72±0.08 Ma and 5.31±0.05 Ma, respectively. The resultant U-Pb ages corresponded to the double-dated FT ages within the range of the analytical uncertainties, indicating that we succeeded to obtain highly precise and accurate eruptive ages. Thus, this study determined the unwelded tuff facies of the Obirakiyama Tuff were ejected at 3.9 Ma. In strike contrast, remarkably older ages were obtained from two samples of the welded tuff facies. We suggest that 5.3 Ma obtained from the rhyolite (welded tuff facies) may also indicate the eruptive age of the Obirakiyama Tuff but cannot rule out the possibility that there are some welded tuff facies erupted at 3.9 Ma. Moreover, the welded tuff mainly composing Mt. Obiraki (8.7 Ma) can be rather correlated to late Miocene formation. Thus, reexaminations of the eruptive age of the welded tuff facies and their distributions are further required.
