2016 年 45 巻 2 号 p. 53-61
The Ieshima Islands, in the eastern Seto Inland Sea of southwest Japan, are dominated by sedimentary complex in the Tamba Belt, Late Cretaceous volcanic and plutonic rocks, and Quaternary sediments. The Late Cretaceous volcanic rocks consist mainly of dacite pumiceous lapilli tuff, rhyolite welded crystal tuff, and rhyolite tuff breccia. Here, they are described for the first time as the Ieshima Formation. Rhyolite welded crystal tuff overlies the dacite pumiceous lapilli tuff, and both are intruded by plutonic rocks. I present laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb and fission-track (FT) ages of the rhyolite welded crystal tuff. U-Pb ages of individual zircons range from 86.0 to 104.5 Ma, and show a bimodal distribution with peaks at ca. 90 and 100 Ma. The younger cluster (89.8 ± 0.9 Ma, n = 19) corresponds the eruption age of the felsic volcanic rocks of the Ieshima Formation, while the older grains may be inherited from the basement rocks. The FT age (76.1 ± 2.5 Ma) is significantly younger than the U-Pb dating age, and represents thermal resetting by concealed plutonic rocks.
