Empirical garnet-muscovite-plagioclase-quartz geobarometry in medium- to high-grade metapelites

Based on the net transfer reactions among garnet, muscovite, plagioclase, and quartz (for both Mg and Fe end-member models), the garnet-muscovite-plagioclase-quartz (GMPQ) geobarometry was empirically calibrated under the physical conditions of P=1.0-11.4 kbar and T=505-745degreesC for 128 natural metapelitic rock samples collected from the literature. The input temperatures and pressures were simultaneously determined by the gamet-biotite thermometer and the garnet-alumino silicate-plagioclase-quartz (GASP) barometer. The GMPQ calibrations adopted the same asymmetric quaternary solid solution model of garnet and the same Al-avoidance asymmetric ternary model of plagioclase as the calibrations of the garnet-biotite geothermometer and the GASP geobarometer. A symmetric Fe-Mg-Al-Vl ternary solid solution model of muscovite was adopted, and the Margules parameters of muscovite were obtained through regression. The Mg and Fe model reactions, along with the assumption of whether the ferric iron content in muscovite is 0% or 50%, resulted in four GMPQ barometry formulae. The GMPQ barometry formulae reproduce the input GASP pressures well within +/- 1.0 kbar (mostly within +/- 0.5 kbar). For both alummosilicate-bearing and alummosilicate-absent samples, the GMPQ barometry formulae yield identical pressures for every sample, whether the sample was included or not in calibrating the barometers. For each of the Mg or Fe model reaction, the formulae gave identical pressures within 40 bars. The random error of the GMPQ barometry may be expected as +/- 1.4 kbar. The dP/dT slopes of these GMPQ formulae are close to that of the GASP barometer in the P-Tspace. Applications of the GMPQ barometry to aluminosilicate-absent metapelites within a limited geographic area without postmetamorphic structural discontinuity generally show no pressure difference. It may be concluded that the GMPQ barometry formulae derived in this work may be used as practical tools for metamorphic pelites under the conditions of 505-745 degreesC and 1-11.4 kbar, in the composition range of X-gros>3% in garnet and X-An>17% in plagioclase. (C) 2004 Elsevier B.V. All rights reserved.