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Lawsonite, pumpellyite and epidote coexist in the chlorite zone pelitic schists of the Sanbagawa metamorphic belt. As these pelitic schists contain calcite ubiquitously, the phase relations of hydrous Ca-Al silicate minerals are examined by mixed volatile equilibria in the K2O-Na2O-CaO-MgO-Al2O3-SiO2-CO2-H2O system with excess quartz, calcite and a fluid phase. XCO2 of the stability field of pumpellyite is lower than that of lawsonite. An idealized geometry of an isobaric T-XCO2 diagram including mixed volatile reactions shows that the stable hydrous Ca-Al mineral changes from lawsonite through pumpellyite to epidote for pelitic schists in the chlorite zone with progressive increase in both temperature and XCO2 of the fluid phase, though ferric iron expands the stability of epidote throughout the chlorite zone. The fluid phase released by the metamorphic reactions in the chlorite zone is high in H2O content even in the presence of CaCO3. The proposed mixed volatile reactions, instead of the pure dehydration reactions, are applicable directly to the phase relations of CaCO3-bearing Sanbagawa basic schists and to the interpretation of the mineral sequence of the hydrous Ca-Al silicates of other high P/T metamorphic belts.

The frequency of occurrence of minerals in 1876 samples of Sanbagawa pelitic schist in central Shikoku is summarized on the basis of microscopic observation accompanied, in part, by use of an electron microprobe. All samples contain quartz, plagioclase, phengite, chlorite and graphite. More than 90% of samples contain clinozoisite, titanite and apatite. Garnet is present in 95% of samples from the garnet zone, and biotite is present in 64% of samples from the albite-biotite zone. Calcite is found in about 40% of samples of the pelitic schist collected from outcrop, but occurs in 95% of the pelitic schist from drill cores. Calcite was apparently ubiquitous in the pelitic schist during the Sanbagawa metamorphism, but must have been dissolved recently by the action of surface or ground water. The mineral assemblages of the Sanbagawa pelitic schist have to be analyzed in the system with excess calcite, quartz, albite (or oligoclase), clinozoisite, graphite and fluid that is composed mainly of H2O, CO2 and CH4. In the presence of calcite, reactions that produce garnet, rutile, oligoclase, biotite and hornblende, some of which define isograds of the metamorphic belt, should be written as mixed volatile equilibria that tend to take place at lower temperature than the dehydration reactions that have been proposed. The presence of calcite in pelitic schist suggests that fluid composition is a variable as important in determining mineral assemblages as pressure and temperature. Thus Ca-bearing phases must be taken into account to analyze the phase relations of calcite-bearing pelitic schist, even if CaO content of Sanbagawa pelitic schist is low. As calcite is a common phase, the mineral assemblages of the biotite zone pelitic schist may contravene the mineralogical phase rule and warrant further study.

In the northeastern part of the Kanto Mountains, central Japan, in basic schists, green biotite widely occurs in a lower-grade part where garnet is common in pelitic and psammitic schists, while brown biotite is present in a higher-grade part where brown biotite commonly occurs in them. Comparing with brown biotite from higher-grade part of the Sanbagawa metamorphic terrain in this area and central Shioku, green biotite in lower-grade part of this area contains lower Al2O3 and higher Fe2O3, and slightly lower TiO2. On the basis of the paragenetic relation of the green biotite-bearing assemblages, the occurrence of green biotite is explained by high Fe2O3, (FeO+MgO), and low Al2O3 contents of the host rocks. Brown biotite in basic schists in a higher-grade part is formed by the following reaction; green biotite+ (Fe, Mg-rich) chlorite+epidote+albite+quartz=brown bitite+hornblende+(Al-rich) chlorite+magnetite+H2O. This reaction enlarges the stability field of biotite, and in a higher-grade part, brown biotite-bearing assemblages widely occurs in the common basic schists.

Epidote, which contains significant amounts of rare earth elements and thorium, and allanite occur in the Sanbagawa pelitic schists of central Shikoku at all the metamorphic grades. Petrographical and chemical studies suggest a metamorphic origin of the REE-bearing epidote and a detrital origin of the allanite. The detrital allanite is the major source of REE and Th in the metamorphic epidote.

Most garnets in a small volume of the pelitic Sanbagawa schists nucleated simultaneously, grew, and then ceased with their growth, maintaining near-equilibrium with coexisting minerals such as chlorite and biotite during the prograde metamorphism. Chemical trends of zoned garnets systematically shift from the pyrope-poor to pyrope-rich portions in a ternary MnFeMg system in order of ascending metamorphic grade. A ternary (MnFeMg) model system suggests that the higher-grade garnets started to grow at lower pressures and higher temperatures than the lower-grade ones during prograde metamorphism. It follows that high-grade metamorphism took place at lower pressures in the garnet to oligoclase-biotite zones of the Sanbagawa metamorphic terrain in central Shikoku.

During the prograde metamorphism of the Sanbagawa metamorphic terrain in central Shikoku, garnet has grown in the pelitic schists whilst maintaining surface equilibrium with coexisting minerals such as chlorite and epidote, etc. In the quartz-albite layer, it has grown euhedrally. In the micaceous layer, its crystal morphology varies according to the different minerals in contact and their different orientations. Garnet enclosed in or sandwiched between micas is always ellipsoidal, extended parallel to the basal planes of the micas. However, when the edge of a garnet has intruded at an acute angle into the micas, euhedral garnet has grown. It is concluded that the growth of garnet at the interface with micas ceased when the {110} surface of garnet came into contact with the basal plane of the micas in the tectonic c -axis direction. Other models, such as chloritization-resorption, pressure-solution, and mechanical-erosion or grinding, do not adequately explain the origin of the flattened garnet. During retrograde metamorphism, chloritization and resorption of garnets took place in the matrix, resulting in the rough and irregular surface of garnet. Garnets included in albite porphyroblasts have been largely unaffected by the retrograde metamorphism.