The higher grade metamorphic zonation of the Sambagawa ( = Sanbagawa) belt is established for the first time for the whole area of central Shikoku. As discontinuous reactions to define the isograd are absent, the metamorphic grade is primarily determined by the Mg-Fe partitioning between garnet and chlorite along representative traverses. However, for regional mapping, mineralogical features of the pelitic schists, such as using mineral assemblages of more than divariant equilibrium, the modal garnet to chlorite ratio, and the optical properties of chlorite, are employed as auxiliary criteria.The presence of the highest grade mineral zone in the middle of the structural level is confirmed, but its spatial distribution is far more complex than hitherto accepted. Thermal axes are now confirmed at three different structural levels. A model is presented in which the stacking of thrust sheets of different grade took place while metamorphic reactions were in progress. Thermal readjustment brought a continuous metamorphic temperature gradient acros and within the thrust sheets. Tectonic blocks of metagabbro and ultramafic rock were emplaced synchronously with thinning and subsequently also re-equilibrated. Local anomalies of metamorphic grade, represented by mixing of schists of different metamorphic grade, exist, but they are due to a later stage event.

No abstract

No abstract

No abstract

No abstract

No abstract

No abstract

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