Recalibration of the Garnet–Muscovite (GM) Geothermometer and the Garnet–Muscovite–Plagioclase–Quartz (GMPQ) Geobarometer for Metapelitic Assemblages

Abstract

The garnet–muscovite (GM) geothermometer and the garnet– muscovite–plagioclase–quartz (GMPQ) geobarometer have been simultaneously calibrated under conditions of T 1⁄4 450–760 C and P 1⁄4 0.8–11.1 kbar, using a large number of metapelitic samples in the compositional ranges X grt Fe 1⁄4 0.53–0.81, X grt Mg 1⁄4 0.05–0.24, X grt Ca 1⁄4 0.03–0.23 in garnet, X pl Ca 1⁄4 0.17–0.74 in plagioclase, and Fe 1⁄4 0.04–0.16, Mg 1⁄4 0.04–0.13, Al 1⁄4 1.74–1.96 in muscovite on the basis of 11 oxygens. The resulting GM thermometer yielded similar temperature estimates (mostly within ±50 C) to that of the garnet–biotite thermometer, and successfully discerned the expected systematic temperature change of prograde sequences, thermal contact zones and an inverted metamorphic zone. The resulting GMPQ barometer yielded similar pressure estimates (mostly within ±1.0 kbar) to the garnet– aluminum silicate–plagioclase–quartz (GASP) barometer and placed the aluminosilicate-bearing samples in the appropriate aluminosilicate stability fields. Application of the GMPQ barometer to thermal contact aureoles or rocks within limited geographical areas confirmed the expected constant pressures that should exist in these settings. The random errors of the GM thermometer and the GMPQ barometer are estimated to be ±16 C and ±1.5 kbar, respectively. When biotite or aluminosilicate is absent in metapelites, metamorphic P–T conditions may be determined by simultaneously applying the GM thermometer and the GMPQ barometer.