High-temperature metamorphism and the formation of migmatites

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Under the title "High-temperature metamorphism and the formation of migmatites", our interest was chiefly concerned with various petrological aspects such as mineral equilibria, the occurrence and genesses of migmatites, pressure-temperature-time evolution, crustal anatexis and the origin of S-type granitoid magmas in regional metamorphic belts of Japan (e.g., the Hidaka, Abukuma, Ryoke, Hida and Higo metamorphic belts). A part of our results were already reported (e.g., twenty-one papers in Nos. 5 and 11 of "Monthly Chikyu", 1992), but many articles will be published successively. Because of space limitation, only a few important results are described as follows:1. Anatectic migmatites melted in situ are common in the high temperature parts of high-T and low-P regional metamorphic belts (e.g., Hidaka and Higo). Pelitic and mafic anatexites are characterized by the presence of idiomorphic plagioclase with oscillatory zonal structure and of other textures attributed to segregation of the partial melt (e.g., Hidaka).2. In the highest temperature part, some amphibolites contain a network of veins in which the bulk chemistry of veins varies from gabbro through tonalite to trondhjemite, suggesting that these veins were segregated from their host amphibolites and represented a differentiation process of the partial melt (e.g., Hidaka and Abukuma).3. S-type tonalitic magmas were produced by the extensive partial melting of metapelites in the lower crust during the peak of metamorphism, leaving pelitic granulites as a restite. Some S-type tonalites, however, were admixtures of restite with the partial melt and were products of interaction between the two (e.g., Hidaka and Higo).