How well do pseudosection calculations reproduce simple experiments using natural rocks: an example from high-P high-T granulites of the Bohemian Massif

Abstract

This study represents a comparison between thermodynamic calculations (pseudosections) and the results of the experimental investigation of high-P/high-T granulites from the Bohemian Massif by Tropper et al. (2005). The experiments were conducted at 7501000°C at 1.6 GPa, 950°C, 1.4 GPa and 800-900°C at 1.2 GPa in order to model the high-P/high-T evolution of the south Bohemian granulites. Tropper et al. (2005) used granitic gneiss as starting material whose chemical composition almost perfectly matched the main granulite type of the Southern Bohemian Massif. To test the validity of the experiments pseudosection calculations using the programs THERIAK-DOMINO and PERPLEX were performed in the chemical system K O-Na O-CaO-FeO-MgO-MnO-Al O -TiO -SiO 2 2 2 3 2 2 H O (KNCFMMnTiASH). The calculations revealed that the experimentally obtained phase assemblages could very well be reprodu2 ced despite the use of different activity models for solid solutions, compositional trends of garnet and feldspars good to satisfactorily. The calculations revealed two major inconsistencies: 1.) the stability of biotite is underestimated in the experiments by ca. 50-100°C and 2.) the amount of melt generated in the experiments is much larger than in the calculations. The reasons for these inconsistencies can be explained by: 1.) electron microprobe analysis of micas from the run products which revealed high F-contents not analyzed previously by Tropper et al. (2005), which is not accounted for in relevant biotite activity models, and by: 2.) the nature of the experimental set-up (lack of eqilibrium in the subsolidus region and fast reaction overstep) which is responsible for generation of abnormously high amounts of melts. Nonetheless the combination of experimental and thermodynamic investigations and its comparison to natural rocks allows gathering deeper insights into the metamorphic evolution of these high-P/high-T granulites.