Abstract
Four samples of the Setouchi volcanic rocks from two different Miocene formations on Shodo-Shima Island, SW Japan, were dated multi-chronologically using whole-rock and mineral K-Ar and Ar-Ar, zircon fission-track, and zircon U-Pb techniques, in order to better estimate their eruption ages. Statistical and chronological analyses of the results suggest that the lower and upper formations formed at 14.3-14.4 Ma and 13.1-13.4 Ma, respectively, which is consistent with geological relationships. U-Pb dating tends to show a reliable eruption age for analyses of magmatic zircon, and fission-track dating is suitable for determining the eruption age if the sample is largely free of thermal alteration. A careful examination of the contribution of old basement rocks and alteration is required to constrain the eruption age based on Ar-Ar plateau and K-Ar age data. The ubiquitous occurrence of xenocrystic zircon, even in the least-differentiated high-Mg andesite, indicates the involvement of pre-existing crust in the magma. The U-Pb ages of these zircons reveal the involvement of Late Cretaceous granitic basement, as well as older components.
