Isotopic ages of the Shinzan Rhyolites, Oga Peninsula, NE Japan

Abstract

The Shinzan Rhyolites are the uppermost part of the Monzen Formation and discordantly cover the Kamo Lavas of the same formation. The volcanic succession from the Kamo Lavas and correlative Kuguriiwa Lavas to the Shinzan Rhyolites has been accepted to range in isotopic age from 34 Ma to 24 Ma with a large time gap of 2 or 3 million years between the Shinzan Rhyolites and the underlying two units. This paper reports new isotopic ages of the Shinzan Rhyolites, ca. 34 Ma by Ar-Ar and K-Ar dating of biotite and ca. 37 Ma by fi ssion-track dating of zircon. A new fi ssion-track date of the welded felsic tuff from the Kamo Lavas is ca. 36 Ma. These new age data are mutually consistent and much more reliable than the ever-reported isotopic age, and obviously indicate that 1 the Monzen Formation is to be Late Eocene to Early Oligocene in age and 2 the time gap between the Shinzan Rhyolites and the Kamo Lavas should be much smaller than or negligible relative to the time span of the entire volcanic succession. The main part of the Shinzan Rhyolites would be slightly younger than the shallow marine Shiose-nomisaki Sandstone and Conglomerate 34-36 Ma and perhaps demonstrates the Late Eocene to Early Oligocene rifting precedent to the rapid opening of the Japan Sea.

Chemical and Isotopic Characteristics of the Kuroko‐Forming Volcanism

“…Continental margin volcanism first occurred at c. 35 Ma in the western continental rift zone (Kano et al, 2007). Volcanism during this stage was dominated by terrestrial andesite activity.…”

Late Eocene shoreline volcanism along the continental margin:the volcanic succession at Kabuki Iwa, Oga Peninsula, NE Japan

“…ArAr dates of the biotite phenocrysts from the discordantly overlying Shinzan Rhyolites of the same Formation are, however, c. 34 Ma (Kano et al, 2007b). Kabuki Iwa is, therefore, most likely to be Late Eocene in geologic age.…”

“…2). Radiometric ages reported for this volcanic succession have been diverse but could span a few million years of Late Eocene to Early Oligocene time (Kano et al, 2007b. There are many contemporaneous dikes extending mostly in a NE-SW direction, as shown by the geological maps (Huzioka, 1959(Huzioka, , 1973, perhaps reflecting an NW-SE extensional stress field of that time (Yamamoto, 1991 ─ 256 ─ the rapid opening of the Japan Sea (Kano et al, 2007a).…”

Cenozoic molluscan faunas and climatic changes in the northern Pacific related to Pacific gateways: review and perspective

“…Kano et al (2006) proposed new data on the geological age of the pyroclastic rocks related to the tectonic history of the Japan Sea. In addition, Sato et al (2005) and Kano et al (2007) redefined the geological age of the Cenozoic strata of pre-Miocene Nishikurosawa stages of the Oga Peninsula with some new magnetostratigraphic data.…”