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Epidote, which contains significant amounts of rare earth elements and thorium, and allanite occur in the Sanbagawa pelitic schists Qf 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. KOIDE, 1937;HORIKOSHI, 1938;HIDE, 1961; Figure 1 shows the mineral zones of the HIGASHINO, 1975). Recently, it has been found Sanbagawa metamorphic terrain in central that some epidotes in Sanbagawa pelitic schists Shikoku, and the route along which the samples of central Shikoku also contain significant were collected. The grade of metamorphism amounts of REE (rare earth elements), totaling increases from the chlorite through garnet and up to 15 wt. per cent as oxides and of ThO2 albite-biotite to oligoclase-biotite zones with up to 1.0 wt. per cent. In central Shikoku, reference to the pelitic schists (BANNO, 1964; REE and Th-bearing epidotes are always asso HIGASHINO, 1975;BANNO et al., 1978; ENAMI, ciated with REE and Th-free epidotes or zoisite, 1981, 1982, 1983. The pelitic schists always and are sometimes accompanied by allanite. contain quartz, albite, muscovite, epidote and Petrographical and chemical studies suggest a graphite (only poorly crystallized in the chlorite metamorphic origin of REE-bearing epidote*, zone (ITAYA, 1981)), associated with various and a detrital origin of the allanite. This study combinations of chlorite, garnet, biotite, is aimed to understand the behavior of REE oligoclase and hornblende. during the Sanbagawa metamorphism and Two hundred and fifty one samples of regional metamorphism, on the basis of petro pelitic schists from various mineral zones were graphical and chemical studies of REE-bearing examined under the microscope. REE-bearing minerals. In this paper, the mode of occurrence epidote is very fine-grained, very low in modal and chemistry of the REE-bearing epidote and amount, and colorless, and hence optically it is allanite will be described, and then their geneses not distinguished from REE-free epidote and *DEER et al . (1962) called all cerium epidotes allanites (the amount of Ce203 is more than 7.44 wt % in their table). The epidotes under consideration are herein termed REE-bearing epidotes, because the amount of REE2O3 + Th02 of the epidotes is less than that-,of allanites reported by HASEGAWA (1960) andDEER et al. (1962). 45

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¯uid that is composed mainly of H 2 O, CO 2 and CH 4 . In the presence of calcite, reactions that produce garnet, rutile, oligoclase, biotite and hornblende, some of which de®ne 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¯uid 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.

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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 K 2 O -Na 2 O -CaO -MgO -Al 2 O 3 -SiO 2 -CO 2 -H 2 O system with excess quartz, calcite and a fluid phase. XCO 2 of the stability field of pumpellyite is lower than that of lawsonite. An idealized geometry of an isobaric T -XCO 2 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 XCO 2 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 H 2 O content even in the presence of CaCO 3 . The proposed mixed volatile reactions, instead of the pure dehydration reactions, are applicable directly to the phase relations of CaCO 3 -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.

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