Omphacite-bearing metapelite from the Besshi region, Sambagawa metamorphic belt, Japan: Prograde eclogite facies metamorphism recorded in metasediment

Abstract

Omphacite-bearing metapelite was found from the Seba area of the Besshi region, Sambagawa metamorphic belt, central Shikoku, Japan. Omphacite occurs as inclusions in garnet together with quartz, sodic amphibole, phengite, and paragonite. The major matrix phases are quartz, albite, phengite, chlorite, subcalcic amphibole, calcite, dolomite, and graphite. Garnet shows a prograde zoning and comprises three segments in order from the crystal center to the margin: the core, inner mantle, and outer mantle. The garnet core shows monotonous decrease of MnO content outward, and includes sodic phases: paragonite and glaucophane. The garnet mantle is substantially homogeneous in composition, and poorer in MnO and richer in MgO than the core. The inner mantle of garnet includes omphacite, paragonite and glaucophane. The outer mantle includes omphacite, but paragonite and glaucophane grains are absent. The jadeite content of omphacite inclusions (XJd) increases slightly from 0.55 in grains included in the inner mantle to 0.62 in the outer mantle of garnet. The systematic distribution of sodic minerals in garnet documents a prograde evolution of metamorphism from the blueschist to eclogite facies conditions. The occurrence of omphacite-bearing metapelite in the Seba area of the Besshi region is direct evidence of: (1) at least some of the Sambagawa metapelites in the Besshi region certainly experienced eclogite facies metamorphism, and (2) eclogite facies metamorphism extends beyond the previously assumed eclogite facies area in the Sambagawa belt.