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 fission-track dating of zircon. A new fission-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-no-misaki 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.
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