島根県西部,津和野地域の結晶片岩類のK-Ar年代  [in Japanese] K-Ar ages of crystalline schists in the Tsuwano area, western Shimane Prefecture, Southwest Japan  [in Japanese]

K-Ar ages are determined on white micas from the crystalline schists, which were recently found in the Tsuwano area, Shimane Prefecture, Southwest Japan. The obtained K-Ar ages are fairly well concentrated in a range of 180-175 Ma (Middle Jurassic). On the basis of the K-Ar dates and lithologic assemblage, these crystalline schists are correlated to the Chizu Metamorphic Rocks (NISHIMURA, 1989) of the "Sangun Metamorphic Rocks" in the Inner Zone of Southwest Japan. The Chizu Metamorphic Rocks are widely distributed in the eastern Chugoku region, and the crystalline schists of the study area represent the westernmost outcrop of the Chizu Metamorphic Rocks. The crystalline schists of the Tsuwano area, as well as the Suo Metamorphic Rocks with 230-210 Ma (another constituent unit of the "Sangun Metamorphic Rocks"), are tectonically intervened between the Permian subduction complex of the Akiyoshi or Maizuru Terrane and the Jurassic subduction complex of the Tamba Terrane. With reference to the pile nappe structure of the pre-Cretaceous rocks in the Inner Zone of Southwest Japan, geological significance of the 180-175 Ma crystalline schists in the Tsuwano area is briefly discussed.