八重山変成岩類の変成分帯とK-Ar年代

概要

論文の詳細を見る
The Yaeyama metamorphic rocks crop out on the islands of Ishigaki, Taketomi, Kayama, Kohama and Iriomote, southern Ryukyu. The metamorphic terrain is divided into four zones on the basis of mineral paragenesis. In zone I, fine-grained white mica and chlorite are scatterered in the matrix of pelitic and psammitic rocks. Zone II is characterizied by the pumpellyite-glaucophane assemblage in basic schists and metagabbros. Lawsonite and stilpnomelane are also stable in the zone. Zone III is marked by the disappearance of pumpellyite and lawsonite, the glaucophane-epidote assemblage becoming diagnostic. Zone IV is characterized by the appearance of subcalcic amphiboles in basic schists and of garnet in pelitic schists. Glaucophane and actinolite are still stable in the basic schists of zone IV. Zone I coincides with the whole of the Fusaki Formation, and zones II, III and IV are comprised in the Tomuru Formation. Zone I is in fault contact with zone II, while the boundary surfaces between zones II, III and IV are nearly parallel to the bedding schistosity. The grade of metamorphism increases from the lower toward the upper stratigraphic horizon in the Tomuru Formation. Thus, it is evident that the assemblages recognized in the Tomuru Formation has been due to a progressive metamorphism and that this corresponds to the high-pressure type. However, it has not been setteled whether zone I represens the lowest grade part of the progressive metamorphism, K-Ar age determination has been carried out on four white micas separated from the pelitic and basic schists of zones I I, III and IV in Ishigaki-jima. The ages obtained range from 159 to 175 Ma, which are similar to the previously reported data (K-Ar=177 Ma and Rb-Sr=202 Ma in the same specimen). The age cluster of the Yaeyama metamorphic rocks is older than that of the Sanbagawa, the Nagasaki and the Yuli metamorphic rocks, but is identical with the younger ages obtained from the Sangun metamorphic rocks. Several amphiboles are also analyzed and preliminarily discussed.
日本地質学会の論文
1983-03-30

八重山変成岩類の変成分帯とK-Ar年代

スポンサーリンク

概要

著者

関連論文

スポンサーリンク