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

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 ages from the Nishiki-cho area are 172 and 175 m.y., while those from the Toyoga-dake area are 264 and 274 m.y. The 12 available measurements on the Sangun metamorphic rocks can be divided into two age clusters, an older cluster around 260 m.y. (Middle Permian) and a younger cluster around 170 m.y. (Middle Jurassic). It is suggested that the two age clusters coincide with the two stages of uplift for the Sangun metamorphic rocks. Thus the older age cluster 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 when metamorphism had ceased. On the other hand, the younger age cluster may represent an event related to the beginning of the formation of a pile nappe structure in which the Sangun metamorphic rocks tectonically overlie the Jurassic non-metamorphic equivalents of the Tamba group.-M.Ak.