(a) Geological sketch of the quadrilateral area marked in Fig. 4a, modified from S. Suzuki’s sketch4. (b) Sketch showing the occurrence of the tuff in the quadrilateral area (b) marked in (a). (c) Photo of an outcrop consisting of the coarse-grained (greyish white) and the fine-grained (brownish white) tuffs. (d) Photo showing the tuff sample collected from the tuff layer intercalated in the conglomerate for LA-ICP-MS U-Pb zircon analyses. The black and white elements of the scale are 1 cm, respectively. (e) Photomicrograph of the tuff sample consisting mainly of the brownish volcanic glasses. Hbl: hornblende, Pl: plagioclase, Kfs: K-feldspar, Qz: quartz, Bt: biotite.