High-Mg garnets from pelitic schists adjacent to the Sebadani eclogitic metagabbro mass, Sambagawa metamorphic belt, central Shikoku, Japan

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Pelitic schists adjacent to the Sebadani metagabbro mass in the Sambagawa metamorphic belt of central Shikoku, Japan consist mainly of garnet, phengite, albite and quartz, and small or scarce amounts of Na-Ca and Ca-amphiboles, epidote, omphacite, kyanite, rutile and carbonaceous matter. Garnets are zoned from pale reddish brown cores to pale yellowish-green or colorless rims. The cores of the garnets are Mg- and Ca-rich (MgO 3.50-11.30 wt%; CaO 7.27-12.98 wt%), and are overgrown by Fe-rich inner and outer rims. Kyanite occurs as inclusions in the high-Mg cores of the garnets. Inclusion assemblages of the high-Mg core of the garnets indicate relatively high-T conditions such as the kyanite eclogite facies (821 ± 32 °C and 19.4 ± 1.6 kbar). The cores of the garnets are anhedral with irregular embayments, and the outlines of the inner rims also show similar texture. The cores are also intensely fractured and filled by Fe-rich garnet with the same composition as the inner rims. The texture and chemical compositions of the high-Mg cores in the garnets are similar to those of garnets in the eclogites in the Sebadani metagabbro mass. The possible origin of the high-Mg cores are (i) in situ high-T regional metamorphism, (ii) detrital origin, and (iii) mechanical mixing of eclogites in the Sebadani metagabbro and surrounding pelitic schists. It is probable that high-Mg garnets were stripped from the Sebadani metagabbro mass and mechanically introduced into the pelitic schists when the Sebadani metagabbro mass was incorporated into the pelitic schists.