Polymetamorphism in the Hida Metamorphic Belt, Central Japan

On the western part of the Hida metamorphic belt, central Japan, there develop quartzo-feldspathic gneiss, basic gneiss and calcareous gneiss, associated with lesser amounts of pelitic gneiss. Various kinds of granite are distributed. Judged from the petrological and mineralogical characters of such meta-morphic minerals as pyroxene, amphibole, garnet, feldspar and mica, the metamorphic facies of the area is, in general, the amphibolite facies or lower.
It must be noted, however, that there occur some interesting metamorphites with critical mineral associations suggesting the metamorphic facies of higher grade, probably the granulite facies. It is represented by such rocks as K feldspar-corundum gneiss (K feldspar-corundum-plagioclase-biotite-rutile), eclogitic rock (clinopyroxene-garnet-quartz) and two pyroxene gneiss (orthopyroxene-clinopyroxene-hornblende-garnet-K feldspar-plagioclase-quartz). The so-called "syenitic rock" of the Inishi type, which consists of wollastonite, clinopyroxene and plagioclase, would suggest the same metamorphic condition as that of the granulite facies rocks. Besides those critical mineral assemblages mentioned above, some minerals show the characteristics in chemical composition, which reflect the condition of higher grade metamorphism. Chemical characteristics of hornblende, garnet, plagioclase, scapolite, etc. suggest the granulite facies metamorphism in the area in question. Products of the granulite facies metamorphism are now preserved in the core parts of zoned crystals or zoned aggregates.
Time and spatial relationship between the granulite facies metamorphism and the before-mentioned amphibolite facies metamorphism has been clarified in terms of polymetamorphism. In this concern, special attentions have been paid to the occurrence and mineralogy of a metamorphosed basic dyke, intruded obliquely into the surrounding gneisses.
The role of granites in relation to the process of metamorphism has been summarized. Importance of the so-called "gray-granite" was especially emphasized. It is pointed out that the "gray-granite", at least a part of it, would have been formed by the process of partial melting under the condition of the granulite facies metamorphism. Meanwhile, the Funatsu granite, which had been regarded to be syntectonic and responsible for the granitization and migmatization on a large scale, has been clarified to be the post-kinematic one, playing a role of lubricunt against the blocks of metamorphites.
Furthermore, uniqueness of the original rocks of the Hida metamorphites has been discussed, compared with those of the other high T — low P type metamorphic belts in Japan.
Lastly, chronological analysis of the Hida Plateau revealed the presence of the older metamorphism of the granulite facies, the age of which goes back to about 500 m. y.