Charnockite geobarometers based on coexisting garnet—pyroxene—plagioclase—quartz

Abstract

Numerous geothermometers based on compositions of coexisting minerals have been calibrated experimentally or theoretically during the past 10 yr. In contrast, few continuous-reaction geobarometers exist which have had wide application. One, based on the reaction of cordierite to garnet, sillimanite and quartz, is limited by a relatively narrow pressure range of application and also by conflicting calibrations^1,2 because the mixing relations of Mg and Fe and the role of H₂O in cordierite³ are not understood. Another successful barometer uses the assemblage garnet, plagioclase, Al₂SiO₅ and quartz^4,5. Continuous geobarometers based on the reaction of orthofer-rosilite to fayalite plus quartz⁶ and on the aluminium content of enstatite in equilibrium with garnet⁷ have been used to a lesser extent. The former barometer is limited by rarity of the requisite assemblage, and the latter by uncertain experimental calibration for crustal rocks. We calibrate here two mineralogical geobarometers from measured thermodynamic data. The participating minerals, pyroxenes, garnet, plagioclase and quartz, are frequently associated in high-grade metamorphic rocks. Calculation of pressures for several terrains show that the geobarometers yield reasonable and consistent results for the entire range of crustal pressures.