A comparative study of geothermometers and geobarometers in pelitic schists from South-central Maine

Pelitic schists from a Buchan-type metamorphic terrain contain eight independent geothermometer/geobarometers. Three pairs of fluid-independent geothermometer/geobarometers record Uthostatic pressures consistent with 3500±200 bars. Fluid-independent geothermometers are mutually consistent and record a N–S temperature gradient in the area ranging from a mean of 460°C in the garnet zone to a mean of 550°C in the sillimanite zone. Fluid-dependent geothermometers record temperatures that are systematically higher than those recorded by fluid-independent geothermometers. The discrepancy is due to a combination of imprecise laboratory calibration of mineral equilibria and to conditions of p_H2O < p_total during metamorphism. The mole fraction of H₂O in the metamorphic fluid was variable, and generally ranged between values of ~0.5 and 1. Temperature and pressure estimates based on mineral equilibria in pelitic schists are in good agreement with temperature and pressure estimates based on mineral equilibria in metamorphosed carbonate rocks from the same area.