中国地方における低度変成岩のK-Ar全岩年代

K-Ar whole-rock ages are obtained from the low-grade metamorphic rocks of the Misumi Group, the Jurassic Chizu Formation, the Jurassic Tsunotani Formation and the Permian Maizuru Group. The low-grade metamorphic rocks are distributed adjacent to the high P/T Sangun metamorphic rocks in the Chugoku district. There are quite differences in small-scale structures between the low-grade metamorphic rocks and the Sangun metamorphic rocks. In the low-grade metamorphic rocks, two generations of penetrative foliation and associated structures can be recognized: slaty cleavage and crenulation cleavage. On the other hand, three generations of foliation are developed in the Sangun metamorphic rocks: schistosity and two stages of crenulation cleavage. Obtained K-Ar whole-rock ages are nearly 170-190 Ma. This indicates that slaty cleavage in the low-grade metamorphic rocks have been formed in Middle Jurassic time, and that the Sangun memtamorphic rock has been formed before 170-190 Ma.