Zircon U-Pb age of the acidic tuff from the Miocene Kuma Group, western Shikoku

Abstract

The Miocene Kuma Group, which is mainly distributed south of the Median Tectonic Line (MTL) in western Shikoku, has a stratigraphic record of internal deformation during the opening of the Japan Sea that has attracted much attention in discussions of movement along the MTL. The upper part of the Kuma Group has been dated as lower-middle Miocene on the basis of radiometric ages of the acidic tuffs in the Myojin Formation. However, previously reported ages are considerably dispersed. In this study, U-Pb ages were determined by laser ablation inductively coupled mass spectrometry of zircons from an acidic tuff, yielding a weighted average ²³⁸U-²⁰⁶Pb age of 15.19±0.15 Ma. This dating suggests that the Myojin Formation was probably deposited during the middle Miocene, shortly before the eruption forming the igneous rocks of the Ishizuchi Group, which is constrained by zircon U-Pb ages at 14.2-14.8 Ma. This is consistent with the occurrence of peperite in the Kuma Group, which shows that the time gap between deposition of the Kuma Group and emplacement of the Ishizuchi Group is quite small. Since the U-Pb age of the acidic tuff overlaps with the age range of the Outer Zone Granites, the tuff may be derived from effusive phases of the Outer Zone Granites.