In central Shikoku, SW Japan, eclogitic rock masses expose themselves among the sedimentary schists of the Sambagawa metamorphic belt. When and how they were juxtaposed might reveal the nature of subduction zone tectonics. Garnet is known to have conspicuous zoning in the Sambagawa metamorphic belt. Apart from grains having a normal zoning with a bell-shaped Mn profile, grains with a composite chemical zoning pattern are found exclusively in the schists close to the eastern Iratsu metagabbro

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... s in the Besshi district. It is reasonable to consider that composite zoning formed as a result of an event related to the juxtaposition. P-T paths of the characteristic zoning in garnet were inversely calculated using differential thermodynamic method (Gibbs' method) , to extract the influence of the emplacement of the tectonic block. The derived P-T paths of composite zoning, compared to the prograde P-T path of the Sambagawa metamorphism, showed temperature decreases during their growth interval. It is suggested that the growth interval of garnet corresponds to the juxtaposition of the eclogitic rock masses and the pelitic Sambagawa schists, and that the juxtaposition occurred during subduction.