Zircon U–Pb–Hf Isotopes and Whole-rock Geochemistry of Rhyolite and Tuff from the Harachiyama Formation, North Kitakami Mountains, NE Japan

Abstract

The Harachiyama Formation comprises Lower Cretaceous volcanic rocks, which are distributed in the eastern margin of the Kitakami Mountains in northeastern Japan. Whole-rock chemical analysis, U–Pb zircon dating and Hf isotope analysis of the Harachiyama Formation were performed to constrain the formation age and the magmatic origin was discussed. The lava and tuff samples of the Harachiyama Formation from the Omoto and Tsukue areas supported island-arc rhyolitic rocks (SiO2 content > 70%), and yielded the weighted mean U–Pb ages of 127.8 ± 3.4 Ma and 129.2 ± 2.6 Ma (2σ), respectively. Eighteen zircon grains from two samples, dated between 141.6 Ma and 123.9 Ma, yielded positive εHf(t) values between +5.0 and +8.7. The U–Pb ages and εHf(t) values are consistent with those of the Kitakami Granitoid reported in previous studies. Therefore, it is suggested that the Early Cretaceous volcanic rocks in northeastern Japan, such as the Harachiyama Formation, have the same magmatic origin as the Kitakami Granitoid.