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Polymetamorphism in the Hida Metamorphic Belt, Central Japan
On the western part of the Hida metamorphxc belt, central Japan, there develop quartzo-feldspathic gneiss, basic gneiss and calcareous gneiss, aasociated with lesser amounts of pelitic gneiss. Various kinds of granite are distributed. Judged from the petrological and mineralogical characters of such metamorphic minerals as pyroxene, amphibole, garnet, feldspar and mica, the metamorphic facies of the area is, in general, the amphibolit-e 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 ), eelogitic rock ( clinopyroxene-garnet-quartz ) and two pyroxene gneiss ( ortho-pyroxene- clinopyroxene-hornblende-garnet-K feldspar-plagxoclase -quartz ). The so-called "syenitic rock" of the Inishi type, which consisits of woliastonite, clinopyroxene and plagxoclase. would suggst the same metamorphic condition as that of the granulite facies rocks. Besides those critical mineral assermblages mentioned above, some minerals show the chracteristics In chemical composxtxon, 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, xntruded obliquely into the surrouding gnexsses.
The role of granites in relation to the process of metamorphism has been summarxzed. Importance of the so-called "gray-granite" was especxally 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 meta-morphism. Meanwhile, the Funatsu granite; which had been regarded to be syntectonic and responsible for the granitization and migmatizatxon 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 orxginal rocks of the Hida metarnorphites has been discussed, compared with those of the other high T - low P type metamorphic belts in Japan.
Lastly, chronological analysxs 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. .
Jour, of Sci., Hiroshima Univ., Series C, Vol. 7, No. 4, 1976
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