2022 Volume 128 Issue 1 Pages 229-237
The depositional age of the E-W-trending Ryujin Complex in the Shimanto Accretionary Prism on the Kii Peninsula has been studied mainly in the western and central parts of the complex, and no geochronological data have been reported from the eastern part. Here we present U-Pb and fission-track (FT) ages of zircon grains from felsic tuff in the Sohgawa Unit in the eastern part of the complex. The weighted mean U-Pb age is 68.1±0.4(2σ) Ma, which indicates deposition during the late Maastrichtian and is similar to ages obtained for the western and central parts of the complex using radiometric data and radiolarian fossils. However, the tuff yields a zircon FT age of 13.3±1.6(2σ) Ma, which is similar to the radiometric ages of the Middle Miocene igneous rocks exposed around the study area, suggesting that the FT age has been reset by heat from the igneous rocks.
