Abstract: The western Iratsu mass, the largest tectonic body in the Sambagawa metamorphic belt, central Shikoku, is mainly composed of epidote amphibolite with minor amounts of eclogite. Systematically, a majority of garnets show bell-shaped chemical zoning of pyrope contents and Mg/(Mg+Fe²⁺) monotonously increasing outward. The grossular component in zonal garnet increases outwards, maximizes at an intermediate part, and then decreases towards the outermost rim, reflecting a process from increasing to decreasing pressure conditions during the prograde metamorphism. Jadeite contents of omphacite range from 25~20 mole% within the cores to 15~10 mole% at the rims, implying a pressure-decreasing process (from 11×10⁵ Pa to 8×10⁵ Pa). The peak pressure-temperature (P-T) condition of 630~680 °C and ca. 15×10⁵ Pa in the western Iratsu mass is much higher than that of (610±25) °C and (10±1)×10⁵ Pa of the Sambagawa oligoclase-biotite zone schists. The authors suggest a clockwise P-T-t path for the western Iratsu mass.

Key words: Petrology, P-T-t path, Western Iratsu mass, Sambagawa belt

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