2015 Volume 80 Issue 3 Pages 195-200
Fission track dating was carried out on a pyroclastic deposit that formed during the opening of the Japan Sea. We found a welded dacitic pyroclastic flow deposit in the lower part of the Miocene Isomatsu Formation on the Tsugaru Peninsula, Aomori Prefecture. Fission track ages determined for the internal and external surfaces of zircon (about 14 and 16 Ma, respectively) are both younger than expected from published radiometric and paleontological data. Track length measurements suggest either that the fission track ages represent the initial cooling (= depositional) age of the pyroclastic rock or that they are a totally reset age at a reheating event that occurred at about 14 Ma. We prefer the latter possibility and infer that the reheating could result from intrusion of andesitic dikes near the sampling site.
