K-Ar ages of Sangun metamorphic rocks in Yamaguchi Prefecture and their geologic significance

Abstract

K-Ar ages have been determined on white micas from four Sangun pelitic schists in the Nishiki-cho and Toyoga-dake areas, Yamaguchi Prefecture. The dates from the Nishiki-cho area are 172 and 175 Ma, while those from the Toyoga-dake area are 264 and 274 Ma. They represent a wide range of time, 172 to 274 Ma, which is nearly identical with that of eight ages, 165 to 264 Ma, reported previously from the Sangun metamorphic rocks. The twelve available dates on the Sangun metamorphic rocks can be divided into two age clusters, an older age cluster around 260 Ma (middle Permian) and a younger cluster around 170 Ma (middle Jurassic).
It is suggested that the two age clusters may coincide with the two stages of uplift for the Sangun metamorphic rocks. Namely, the age cluster around 260 Ma may indicate the event in which the Sangun metamorphic rocks were emplaced as nappes from the deep tectonic position of glaucophanitic metamorphism into a shallow tectonic position and the metamorphism had ceased. On the other hand, the age cluster around 170 Ma may represent an event related to the beginning of the formation of the pile nappe structure in which the Sangun metamorphic rocks tectonically overlie the Jurassic non-metamorphic equivalents of the Tamba Group.