Abstract
The Isomatsu Formation in northeastern Japan comprises Miocene terrestrial to shallow-marine deposits displaying a typical lithofacies of so-called ‘green tuff’. Zircon fission track ages of the formation (~14-16 Ma) are significantly younger than those expected from stratigraphic inferences. Here, we present the results of U-Pb geochronological analysis using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), performed on the same suite of zircon grains that yielded the young fission track age. The LA-ICP-MS analysis yielded a mean 238U-206Pb age of ~17.7 Ma for a dacitic pyroclastic deposit in a relatively low stratigraphic horizon of the Isomatsu Formation, suggesting deposition at or later than this time. This result is used to revise the chronostratigraphy of the lower Miocene sequence in the study area, and it constrains the depositional age of the Isomatsu Formation to the period from ~17.7 Ma to ~16 Ma. These ages further constrain the timing of: (1) felsic magmatic activity in and adjacent to the study area, (2) the marine transgression that caused the deposition of the Isomatsu Formation, and (3) the possible formation of a half-graben that may also have influenced deposition.
