研究業績

Nodai caldera, which is located in the western part of the Shimoki ta Caldera Cluster (a group of five late Pliocene to early Pleistocene calderas), ejected pyroclastic rocks and deposited the welded tuff of the Hantarozawa Formation. We determined double zircon fission track and U–Pb ages for samples collected from the deposits and ob tained U–Pb ages of 3.22–3.21 Ma and fission-track ages of 3.1–3.0 Ma. The U–Pb ages, obtained using high-precision laser ablation–in ductively coupled plasma–mass spectrometry, have a bimodal distri bution with peaks at 3.37±0.03 Ma and 3.08±0.03 Ma (2σ), estimated using the unmix age function. This suggests that zircon crystallized at least twice from the silicic magma of the Nodai caldera, at 3.37 and 3.08 Ma, and the eruption occurred immediately after the second crystallization event. The presence of welded tuff implies subaerial deposition, and hence that the western Shimokita Peninsula was al ready a terrestrial environment during the late Pliocene.