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

摘要

Coexistence of jadeite (Jd_(92)) 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 ℃, 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 (Δω_1 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 pro-grade 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.