Revised empirical garnet-biotite-muscovite-plagioclase geobarometer in metapelites

The garnet-biotite-muscovite-plagioclase (GBMP) barometer was empirically revised for P-T conditions of 1-14kbar and 450-840 degrees C, using 263 metapelitic rock samples from all over the world. This barometer is based on activity models for garnet, biotite and plagioclase identical to those of the well-calibrated garnet-biotite thermometer and the garnet-aluminosilicate-plagioclase-quartz (GASP) barometer. The GBMP barometer is less temperature dependent than the GASP barometer and can be applied to either Al2SiO5-absent or Al2SiO5-bearing metapelites. The total error of the GBMP barometer is estimated to be about +/- 1.2kbar on considering input temperature error and analytical errors of chemical compositions of the phases involved. The random error of the GBMP barometer is evenly distributed with respect to pressure, temperature and mineral composition. Simultaneous application of the GBMP barometer and the garnet-biotite thermometer identifies the correct stability field for Al2SiO5-bearing metapelites. Application of the GBMP barometer to metapelitic rocks within the same geological terranes or thermal contact aureoles yielded similar pressures within error. A spreadsheet for implementing the proposed GBMP geobarometer is supplied on the journal's website.