変成作用と造岩鉱物 (<特集>造岩鉱物研究の現状と展望-地団研第33回総会鉱物学ミニシンポジウム-)

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抄録

First, a brief review of the studies on staurolite as a metamorphic mineral is given, and the localities and modes of occurrence of staurolite in Japanese Islands are reviewed. And then the problems on the ZnO content in staurolite are discussed, especially dealing with the Mozambique metamorphic rocks in the Machakos area, central Kenya and with the Ryoke metamorphic rocks in the Hazu-Gamagori-Hongusan area, central Japan. (1) In the Machakos area, a characteristic belt consisting of the association of pelitic gneiss and amphibolite occurs continuously more than 20 km, and this pelitic gneiss is composed of kyanite, staurolite, almandine, biotite, muscovite, plagioclase and quartz. The ZnO content of staurolite of this pelitic gneiss reaches 3.79 % in maximum. Its content is, however, considerably variable even in the same horizon. The lower the content of FeO of staurolite is, the higher the content of ZnO becomes. Even in a same rock specimen, its content is remarkably variable. Staurolite inclusion in almandine porphyroblast is poor in ZnO; staurolite inclusion in muscovite crystal is rich in ZnO; poikiloblastic staurolite exhibits intermediate properties. There are several stages of staurolite formation during metamorphism. This fact suggests that the geochemical behaviour of Zn during metamorphism is governed by the petrochemical and petrological environment. (2) In the Hazu-Gamagori-Hongusan area, three regional metamorphic zones, characterized by the presence of andalusite, sillimanite and sillimanite-K-feldspar for each, are recognized, and an additional contact metamorphic zone is also developed around the granite intrusives. Staurolite schists have been found in all the zones except the sillimanite-K-feldspar zone. Staurolite occurs in the four modes. Despite the various modes of occurrence, staurolites in the schists with muscovite have higher Zn/(Fe+Mg+Zn) than those in the schists without muscovite. The difference of Zn content could be ascribed to the difference in advancing degree of the continuous staurolite-breakdown reaction during the progressive metamorphism.