Zircon U-Pb and Fission-track ages for the Ohta Tephra in the Pliocene Tokai Group, Central Japan

Abstract

Zircon U-Pb and fission track (FT) single-grain double dating was conducted on two samples from the Nakatsugawa I and II volcanic ash layers (Tono area, Gifu Prefecture). The U-Pb results, with the exception of discordant ages and ages obtained from heterogeneous grains, yield a weighted average age of 3.94±0.07 Ma. The weighted average FT age from the same grains is 3.97±0.39 Ma. The similar age (within error) for the U-Pb system (higher closure temperature) and FT system (lower closure temperature) indicates that zircon crystallization occurred at high temperature and under quenching conditions. The age is therefore interpreted as an eruption age. The results are consistent with the age of the Ohta Tephra in the Tokai Group, which is one of the most widespread Pliocene tephras in Central Japan, thus contributing to regional correlations of Pliocene strata in Japan.