Calibration of the biotite-muscovite geobarometer for metapelitic assemblages devoid of garnet or plagioclase

Abstract

Determination of metamorphic pressure (P) – temperature (T) conditions of metapelitic assemblages sometimes suffers from low-CaO content of garnet and/or plagioclase, thus the garnet-plagioclase-related geobarometers lose their usage under such circumstances. Therefore, calibration of garnet-plagioclase-absent geobarometers is very useful. In this paper, the biotite-muscovite geobarometer was empirically calibrated under P-T conditions of 1–12 kbar and 470–760 °C using 270 natural metapelite samples. About 86.5% of the total calibrant samples yielded similar biotite-muscovite pressure estimates to the well-calibrated garnet-Al2SiO5-plagioclase-quartz (GASP) geobarometer within error of ±1.5 kbar. The total error of the biotite-muscovite geobarometer is estimated to be less than ~±1.6 kbar. The applicability of the biotite-muscovite geobarometer has been certified through application to natural metapelite samples coupled with the Ti-in-biotite geothermometer. When garnet and/or plagioclase are absent in metapelitic assemblages such as two-mica schist / gneiss, this geobarometer may play its unique and irreplaceable role. An electronic spreadsheet (Table S3) is deposited at the journal's website for iterative application of the biotite-muscovite geobarometer in combination with the Ti-in-biotite geothermometer.