野母半島長崎変成岩類の岩石学的研究 : とくに黒雲母帯について  [in Japanese] Petrological study of the Nagasaki metamorphic rocks in the Nomo Peninsula : with special reference to biotite zone  [in Japanese]

The Nagasaki metamorphic rocks belong to high-pressure intermediate facies group in the sense of MIYASHIRO (1961). They are distributed in three discrete regions such as Nishisonogi, Nomo and Amakusa. The metamorphic rocks in the Nomo Peninsula consist of three rock units: the crystalline schists (70-177 Ma), the serpentinite complex (91 Ma) and the older metagabbro complex (452-592 Ma). These rock units are all in fault contact with one another. The analysis of the mineral assemblages of the pelitic schists defines three mineral zones: chlorite zone, garnet zone and biotite zone. They represent the metamorphic facies series in the crystalline schists from the pumpellyite-actinolite facies through the greenschist facies to the epidote-amphibolite facies. The biotite isograd is defined by the appearance of biotite in the pelitic schists containing garnet and chlorite with X_<Mg>=0.50. The isograd is nearly horizontal and slightly oblique to the schistosity, concordant with the original bedding, of the crystalline schists. The metamorphic grade increases downward in the apparent stratigraphic sequence. Alkali amphiboles (magnesio-riebeckite) and alkali pyroxenes (chloromelanite and aegirine-augite) occur only in metabasites in the serpentinite complex, which underlie the granet zone or biotite zone crystalline schists with a low-angle fault. The older metagabbro complex have suffered the low-grade metamorphism of the pumpellyite-actinolite facies, comparable with that of chlorite zone in the crystalline schists.