Two new geobarometers for garnet amphibolites, with applications to southeastern Vermont

Abstract

Two new geobarometers for the assemblage garnet + hornblende + plagioclase + quartz have been calibrated on the basis of the equilibrium 6 Anorthite + 3 Tremolite = 2 Grossular + 1 Pyrope + 3 Tschermakite + 6 Quartz and its Fe end-member equivalent. Data representing 37 natural samples that equilibrated at conditions of 2.5 to 13 kbar and 500 to 800°C were fitted to the general equation -RT In Keq= A - BT + (P - I)C by using a weighted least-squares method. Multiple correlation coefficients are high (0.98 and 0.97). Lines of constant Keqhave extremely shallow slopes in P- T space (- 2 to + 8 bars per degree), suggesting that pressures may be deduced precisely, even where temperatures are only poorly constrained. Typical analytical errors and temperature imprecisions propagate to very small errors in pressure (about ::!:500 bars). Caution is advised in applying the barometers outside the range of calibrant-mineral compositions. Application of the calibrations to samples from southeastern Vermont near the Strafford, Chester, and Athens domes documents relatively high pressure metamorphism (7 to 10 kbar) for these structures.