Minor element- and carbonaceous material thermometry of high-grade metapelites from the Sauwald Zone, Southern Bohemian Massif (Upper Austria)

The Sauwald area is located at the southern rim of the Bohemian Massif and contains migmatites and high-grade metapelitic and granitic gneisses. These rocks were metamorphosed during the post-collisional high-T/low-P stage of the Variscan metamorphic event (~330 Ma). Metapelitic samples were taken from two localities near Kössldorf and Pyret in Upper Austria and the investigated samples contain the mineral assemblage garnet + cordierite + spinel + sillimanite + K-feldspar + quartz + biotite + muscovite + magnetite + graphite. An important aspect of this study is the evaluation of previously published P-T estimates for these high-grade metapelites (Knop et al. 1995; Tropper et al. 2006) involving Ti-in-biotite, Na-in-cordierite thermometry and the micro-Raman thermometer based upon the degree of crystallization of carbonaceous material. In the two samples studied three texturally and chemically different biotites are distinguished. Biotite inclusions in garnet have the highest Ti contents of 5–6 wt.% TiO₂. Matrix biotites contain 2–4 wt.% TiO₂ and biotites from late-stage muscovite-biotite symplectites contain <2 wt.% TiO₂. This corresponds to temperatures of 730–760°C (stage 1), 600–700°C (stage 2), and 550–610°C (stage 3). Since the Ti-in-biotite thermometer strongly depends on X _Mg of biotite, which is susceptible to changes during retrogression the calculated temperatures for stage 1 are interpreted as minimum temperatures of the peak metamorphic stage. The Na contents of the studied cordierites vary from 0.1 to 0.2 wt.% Na₂O. Application of the Na-in-cordierite thermometer yields temperatures in the range of 770–900°C; they are strongly dependent on the bulk Na₂O content of the samples. The micro-Raman geothermometer of graphite was applied to carbonaceous material, which occurs as inclusions in garnet and cordierite. It yielded a maximum temperature >650°C, i.e. above the calibration limit of this method. This study shows that the obtained temperature estimates agree well with the P-T estimates based on phase equilibrium thermobarometry (Knop et al. 1995; Tropper et al. 2006), thus illustrating the validity of these thermometers. Nevertheless in order to more precisely constrain the metamorphic evolution of these high-grade rocks, better constrained experimental calibrations of, for instance the Na-in-cordierite thermometer, are clearly needed.