Fundamental study on behavior of fluids during high-temperature metamorphisms

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Following results have been obtained by the three-year research (1990, 1991 and 1992) supported by this grant.1. The Hidaka metamorphic rocks were formed by progressive metamorphism under the conditions of the greenschist facies to the granulite faces. The P-T conditions are estimated to have been 450ﾟ,2 kbar to 900ﾟC,5 kbar by means of several kinds of geothermometor and geobarometor. The age of the metamorphism are estimated to have been 56-40 Ma (Eocene) on the basis of by K-Ar dating.2. The metamorphic rocks were obviously suffered retrograde metamorphism. The metamorphism can be divided into two stages:the stage of mylonitization (ductile stage) and vein-foming stage (brittle stage). The metamorphic conditions of the former stage correspond to the amphibolite facies and greenschist facies. These of the latter are to have been the greenschit facies, through the prehnite-actinolite faceis and prehnite-pupmpellyite facies to the zeolite facies. The P-T conditions of the former are to have been 600-400ﾟC,2 kbar, the latter to have been 450-150ﾟC,2-1.5 kbar. The ages of the metamorphisms are to have been 40-18 Ma(Eocene to Miocene) of the former and 18-10 Ma(Miocene) of the latter.3. Compositions and their changes of metamorphic fluid during the vein-forming stage can be estimated by means of analysis of the mineral assemblages and mineral compositions of the veins and observation of fluid inclusions of the minerals. These revealed that the compositions of fluids had became Ca- and Mg-poorer and Na- richer.We can envisage the paths of pressure- temperature- time- deformation- compositional changes of metamorphic fluids during the metamorphism of the Hidaka belt.