Abstract
Mg-rich chloritoid with the ratio Mg/(Mg + Fe2+) = 0.37 to 0.49 was found in a corundum-bearing zoisite rock from the Besshi area in the Sanbagawa metamorphic belt, Japan. The zoisite rock consists mainly of zoisite, corundum, garnet, and amphibole. Corundum was retrogressively replaced by secondary chloritoid, zoisite, clinozoisite, chlorite, and paragonite. Staurolite rarely occurs in aggregates of the secondary phases (chloritoid, chlorite, and paragonite), which occur along the cracks in corundum, and is surrounded by chloritoid. This fact indicates that chloritoid was formed by the following retrogressive reaction: 7chlorite + 8corundum + 4staruolite + 15H2O →51chloritoid. The estimated temperature for the above univariant reaction in the MASH system is 637 ± 30 °C at 10 kbar and 744 ± 34 °C at 20 kbar. The textural evidence and the estimated temperature suggest that chloritoid formed during the exhumation stage from eclogite to epidote-amphibolite facies conditions.
