Phase relations, singularities and thermobarometry of metamorphic assemblages containing phengite, chlorite, biotite, K-feldspar, quartz and H2O

Abstract

The phase relations of divariant and trivariant assemblages involving combinations of phengite, chlorite, biotite, K-feldspar, quartz and H2O in the KFASH, KMASH and KFMASH systems were calculated using a single thermodynamic data set (Holland and Powell 1998). The stability fields of the various equilibria are represented in P-T projections by contouring sets of compositional isopleths for the Tschermak (Al2(Fe,Mg)−1Si−1) and FeMg−1 exchanges controlled by the coexisting phases. Five multivariant continuous equilibria, which occur in different regions of P-T-X space, are calibrated as thermobarometers in metamorphic rocks of pelitic to quartzofeldspathic composition. More subtle P-T information, relating to the trajectories (dT/dz) along which reacting rocks have been buried or exhumed, can be extracted from the continuous reactions by investigating the recorded compositional trends in the Al2(Fe,Mg)−1Si−1 and FeMg−1 solutions. Singularities in P-T space are associated with some of these reactions and may result in unusual mineral textures and compositional trends. A fluid-absent singularity has particular petrological significance because it marks the transition between hydration and dehydration along a single reaction with increasing pressure and temperature. This behaviour causes the sequence of reactions among these minerals observed during metamorphism to be critically dependent on the P-T trajectory. Thermobarometric calculations show good agreement with respect to experimental and field-based data for phengite compositions less than about 50 mol% celadonite (<∼3.5 Si p.f.u. phengite).