Zircon U-Pb and fission-track ages of the Cretaceous Sennan Rhyolites in Awaji Island, southwest Japan

Abstract

The Sennan Rhyolites, located in the southern margin of the Late Cretaceous volcanic area of the Inner Zone of southwest Japan, are distributed in a small area along the Median Tectonic Line. They are intruded by Cretaceous granitic rocks and unconformably overlain by the Upper Cretaceous Izumi Group. We present the results of laser-ablation-inductively coupled plasma-mass spectrometry zircon U-Pb dating and fission-track (FT) dating of rhyolitic welded tuff from the Sennan Rhyolites on Awaji Island. The analyses yielded a weighted mean ²³⁸U-²⁰⁶Pb age of 94.0±1.1 Ma (2σ) and a FT age of 72.3±2.6 Ma (1σ), respectively. The U-Pb age, which is consistent with existing geochronological data and observed intrusive relationships between the Sennan Rhyolites and granitic rocks, constrains the eruption age of the Sennan Rhyolites. The FT age is significantly younger than the U-Pb age and indicates thermal resetting by granitic rocks.