Fission track ages of the tertiary kori and Yui formations in Dogo, Oki islands, west Japan.

Geochronology of alkali volcanism in Oki-Dogo Island, Southwest Japan: Geochemical evolution of basalts related to the opening of the Japan Sea.

“…These ages are considered to represent the ages of eruption even though these rocks suf fer a little to moderate degrees of alterations, be cause three rocks give identical ages regardless of the considerable variation in K20 contents (1.74 2.65 wt%). These ages are older than the fission track age of 13.7 ± 0.7 Ma (Yamazaki and Ganzawa, 1989), but are concordant to the bios tratigraphic estimate (Yamazaki et al, 1990Yamazaki, 1991).…”

“…Even though a fis sion-track zircon age of 13.7 ± 0.7 Ma is obtained for a tuff intercalated in trachybasalts (Yamazaki and Ganzawa, 1989), Yamazaki et al (1990Yamazaki et al ( , 1991 and Yamazaki (1991) assigned these alkali basal tic rocks to the lower Miocene according to the diatom biostratigraphy. Chemistry of these volca nic rocks is quite different from the younger alkali volcanics, and Xu (1988) called these rocks as shoshonite or banakite.…”

Paleomagnetism and fission-track ages of Oki-Dogo Island in Southwest Japan

“…The zircon F.T. ages of the Koji Tuff Member were reported as 21.2 ± 1.0 Ma by Yamasaki and Ganzawa (1989) and as 19.9 ± 1.4, 21.3 ± 1.3, 21.9 ± 1.3 Ma by Yoshitani et al (1995). Geological age of the Kori Formation is thought to range from the early Middle Miocene to the early Late Miocene based on the Daijima type flora and the F.T.…”

“…(14.6 ± 0.7 Ma: Yamasaki and Ganzawa, 1989). The Oki Trachyte-Rhyolite is widely distributed in the western part of the Oki-Dogo Island.…”

“…ages of the Koji Tuff Member and F.T. age (14.6 ± 0.7 Ma; Yamasaki and Ganzawa, 1989) of the Kumi Formation. The Kumi Formation is divided into five members of the Hei Tuff and Siltstone Alternation, Kumi Sandstone, Utagi Mudstone, Igo Mudstone, and Iinoyama Diatomite Members (Yamasaki, 1991).…”