Zircon U-Pb age of a lapilli tuff bed in the lower Miocene Yoka Formation, northern Hyogo, SW Japan

Abstract

The lower Miocene Yoka Formation in southwest Japan is composed of terrestrial volcaniclastic rocks, and contains no index fossils. The basal part of the formation has yielded zircon U-Pb and fission-track ages of ~21.5 Ma. The upper part of the Toyooka Formation, which unconformably overlies the Yoka Formation, correlated with 17-16.5 Ma by index fossils and zircon ages. In this 5 Myr. long age gap, marine transgression and paleomagnetic rotation occurred in this area. We collected a felsic lapilli tuff sample from the middle part of the Yoka Formation, and the U-Pb ages of zircons were measured using a laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The weighted average of concordant 238U/206Pb age of 19.38±0.23 Ma（95% conf., MSWD = 9.4）was obtained from the zircons excluding an outlier age from one grain. This zircon U-Pb age indicates that the transgression and paleomagnetic rotation occurred rapidly within ~2.5 Myr in this area.