Coexistence of jadeite and quartz in garnet of the Sanbagawa metapelite from the Asemi–gawa region, central Shikoku, Japan

抄録

Coexistence of jadeite (Jd₉₂) and quartz was newly found in an inner segment of a composite–zoned garnet from a metapelite in the southern albite–biotite zone of the Asemi–gawa region, Sanbagawa belt in central Shikoku, Japan. The assemblage of jadeite + quartz in garnet gives a minimum metamorphic pressure of 1.4–1.9 GPa at 500–700 °C, which was significantly higher than that previously proposed for epidote–amphibolite facies conditions in the albite–biotite zone. Garnet includes quartz retaining high residual pressure (Δω₁ values up to 11.7 cm⁻¹), which is comparable to the residual pressure reported in eclogite and metapelite within the eclogite unit in central Shikoku. These results imply that (1) the inner segment of the composite–zoned garnet records prograde recrystallization under high–pressure blueschist to eclogite facies conditions and (2) at least a part of the southern albite–biotite zone of the Asemi–gawa region recrystallized under higher pressure conditions prior to the regional metamorphism from the greenschist to epidote–amphibolite facies that formed the regional thermal structure of the Sanbagawa belt of central Shikoku.