Garnets from the Sebadani metagabbro mass in the Sambagawa metamorphic belt, central Shikoku, Japan

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The Sebadani area is located in the central part of the Besshi district; it is mainly composed of the eclogite-bearing Sebadani metagabbro mass and surrounding Seba basic schists (Seba eclogitic basic schists) with intercalation of pelitic and siliceous schists. Eclogites in the Sebadani metagabbro mass consist mainly of garnet, omphacite, amphibole (sodic, sodic-calcic and calcic) and epidote along with small amounts of rutile and titanite. Phengite, paragonite, albite, quartz and opaque minerals occur as accessories. Garnets exhibit two modes of occurrence and a wide range of chemical compositions suggesting a variety of equilibrium metamorphic P-T conditions. Garnet 1 (Grt 1) occurs as porphyroblast and garnet 2 (Grt 2) is found as fine grain in the matrix. The texture of the minerals and their chemical compositions suggest the Seba metagabbro mass is characterized by two different metamorphic events according to the zoning of the garnets, i.e. a first high-pressure and high-temperature (HP-HT) metamorphic event and a second high-pressure metamorphic event. Garnet (Grt 1) is rich in almandine molecule and shows a prograde zoning. Chemical compositions of the fine grained garnet (Grt 2) are similar to the mantle to the rim of the porphyroblastic garnet (Grt 1). The outermost rim of the porphyroblastic garnet and fine-grained garnet shows another prograde growth zoning, and these outermost rims probably equilibrate in another eclogitic metamorphism.

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