熊本県間の谷変成岩とその帰属

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The Manotani metamorphic belt, about 25 km south east of Kumamoto City, appears to pass into the Higo metamorphic belt southward, and it is in fault contact with the Upper Permian Mizukoshi Formation on the north side. The eastern part of the metamorphic belt (2×4 km), the vicinity of Mount Manotani, is composed of predominant basic schists with sub-odinate interlayers of pelitic schists. The majority of basic schists contain the assemblage: actinolite + epidote + chlorite + albite + quartz + stilpnomelane±muscovite, and some of them include pumpellyite and/or alkali amphibole (crossite and riebeckite) besides those minerals. Pelitic schists contain the assemblage: quartz + albite + muscovite + chlorite + lawsonite + pumpellyite±actinolite + stilpnomelane±graphite±titanite. Lawsonite is rare and restricted to this rock type. The association of crossite or riebeckite and pumpellyite and the occurrence of lawsonite show that the Manotani belt is similar in metamorphic condition to the pumpellyite-actinolite zone of the high-pressure intermediate Sangun metamorphic terrane. b_0 value measurement of K-white mica in pelitic schists shows that there are two types of white mica in the Manotani area. The majority have the b_0 values ranging from 9.025Å to 9.032Å, indicating relatively higher celadonitic contents and therefore higher pressure conditions (Fig. 12). The b_0 value from another sample (9.013Å) is close to the b_0 value measured in a pelitic sample of the Sangun metamorphic rocks shbjected to low grade contact metamorphism (aroud the biotite isograd) by a granite pluton in the Sasaguri area, Fukuoka Prefecture. This implies that portion of the Manotani schists, if not all, are polymetamorphic rocks, which were undergone a higher P/T metamorphism where by the larger b_0 value-white micas were formed, followed by an incipient lower P/T metamorphism whereby the smaller b_0 value white micas were formed. Crossites or riebeckites occur as small, irregular cores surrounded by actinolites. Two types of actinolite are distinguished by means of the EPMA compositional images: (1) the first actinolite commonly occurring as a thick rim surrounding alkali amphibole cores; (2) the second actinolite distinctly replacing the first actinolite and alkali amphibole or partly growing over the first actinolite (Fig. 9). The second actinolite contains relatively less Na_B (Fig. 7) and alkalies (Fig. 11) and more Ca (Fig. 8) in comparison with the first actinolite. These and geological observations suggest that the formation of the second actinolite is related to a separate recrystallization at very low-grade during a later lower P/T metamorphism, the Higo metamorphism of Cretaceous age. Thus, the Higo metamorphic rocks, at least in part, are thought to be a thermally up graded equivalents of the low-grade Manotani schists belonging to the Sangun metamorphic terrane.
日本地質学会の論文
1989-04-26

熊本県間の谷変成岩とその帰属

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