Mg-rich staurolite from the Iratsu epidote amphibolite body, central Shikoku

Abstract

Mg-rich staurolites are found in corundum-bearing rocks from the Iratsu epidote amphibolite body in the highest grade metamorphic area of the Sanbagawa metamorphic belt. The staurolites have high Mg/(Mg+Fe) ratios up to 0.6, and are mostly represented by a chemical formula, (Mg+Fe)3+1.5XAl18-XSi8O46, calculated as Li2Oand H2O-free. It was formed with garnet, zoisite and amphibole at the highest metamorphic conditions of the Sanbagawa metamorphism, and subsequently replaced by an aggregate consisting of chlorite, paragonite and margarite during a retrograde metamorphism represented by a pressure decrease. Although Fe-rich staurolite has been treated as a typical mineral in the intermediate-pressure metamorphic belt, Mg-rich staurolite is present in the high-pressure metamorphic belt.