Mineral reactions in a granulite facies calc-silicate rock from Scourie

Abstract

Synopsis A calc-gneiss from the type area of the granulite facies Scourian complex contains the mineral assemblage plagioclase (An 43–46 ), scapolite (Me 63–65 , SO 4 26–30 ), clinopyroxene, ilmenite-magnetite, rutile and minor amounts of pyrite, chalcopyrite, carbonate, uralite, sericite, quartz and apatite. Mineral reactions can be written which define the maximum pressure and minimum temperature of the granulite facies metamorphism and stages in the cooling history of the rock. The distribution of Na and Ca between plagioclase and scapolite, calibrated by Goldsmith and Newton (1977) yields excessively high temperatures (c. 1200°C). This is revised using thermodynamic data from their experimental work and suggests a minimum temperature of 915°C for the granulite facies metamorphism; a maximum pressure of 11 kb for the metamorphism is obtained from the stability of plagioclase with respect to clinopyroxene and quartz. Ilmenite-magnetite pairs equilibrated between 512° and 430°C and oxygen fugacity conditions were probably externally controlled. Ilmenite and clinopyroxene may have exchanged Fe and Mg to temperatures as low as 540°C, but at lower temperatures Mn replaces Mg in ilmenite.