Calibration of the garnet-biotite-Al2SiO5-quartz geobarometer for metapelites

A garnet-biotite-Al2SiO5-quartz (GBAQ) geobarometer was empirically calibrated using more than 700 natural metapelites with a broad compositional range of garnet and biotite under P-T conditions of 450-950 degrees C and 1-17kbar. In the calibration, activity models of garnet and biotite identical to those in the garnet-biotite (GB) geothermometer of Holdaway [American Mineralogist 2000, 85:881-892] were used. Therefore, the GBAQ geobarometer and the GB geothermometer can be simultaneously applied to iteratively estimate metamorphic P-T conditions. Successful applications of the GBAQ geobarometer to natural metapelites certify its validity. Most importantly, when plagioclase is absent or CaO components in garnet and/or plagioclase are deficient, this geobarometer may prove useful for estimating metamorphic pressures. The random error of the present GBAQ geobarometer is inferred to be around +/- 1.8kbar. An electronic spreadsheet is available as TableS4 to apply the GBAQ geobarometer in combination with the GB geothermometer.