Abstract
A detailed fission-track (FT) dating study was conducted on zircon crystals to elucidate the age of Miocene volcanic units exposed in the southern margin of the Asahi Mountains. Three samples of pyroclastic rock were collected from the Kitaoguni Formation (KF), Bentozawabashi Formation (BF), and Myozawagawa Tuff Member (MTM) of the Myozawabashi Formation. Dating was carried out by the external detector method, which was applied to the internal (ED1) and external surfaces (ED2) of the zircons. Grouping of zircon crystals was made not only on morphological features including color, shape, elongation, sharpness of edges, and flatness of faces, but also on the density and length distribution of FTs on external surfaces. For the KF and BF samples, FT ages of essential zircon grains were consistent between the ED1 and ED2 methods. Zircons of the MTM sample comprised four groups. ED1 ages showed a single age population, but ED 2 ages varied for each group, and were older than the ED1 ages. Based on FT thermochronologic interpretations and thermal annealing experiments, we conclude that the ED1 ages of detrital grains were completely reset at the time of the MTM eruption. Furthermore, the data suggest that the inconsistency between ED1 and ED2 ages is caused by natural etching of FTs on external surfaces of detrital zircon, as well as by the external effect. Our conclusion is that the KF, BF, and MTM were erupted around 22Ma, 16Ma, and 16 15Ma, respectively.
