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

The phase relations of divariant and trivariant assemblages involving combi nations of phengite, chlorite, biotite, K-feldspar, quartz and H2O in the K FASH, KMASH and KFMASH systems were calculated using a single thermodynamic data set (Holland and Powell 1998). The stability fields of the various eq uilibria are represented in P-T projections by contouring sets of compositi onal isopleths for the Tschermak (Al-2(Fe,Mg)(-1)Si-1) and FeMg-1 exchanges controlled by the coexisting phases. Five multivariant continuous equilibr ia, which occur in different regions of P-T-X space, are calibrated as ther mobarometers in metamorphic rocks of pelitic to quartzofeldspathic composit ion. More subtle P-T information, relating to the trajectories (dT/dz) alon g which reacting rocks have been buried or exhumed, can be extracted from t he continuous reactions by investigating the recorded compositional trends in the Al-2(Fe,Mg)(-1)Si-1 and FeMg-1 solutions. Singularities in P-T space are associated with some of these reactions and may result in unusual mine ral textures and compositional trends. A fluid-absent singularity has parti cular petrological significance because it marks the transition between hyd ration and dehydration along a single reaction with increasing pressure and temperature. This behaviour causes the sequence of reactions among these m inerals 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 ab out 50 mol% celadonite (<similar to 3.5 Si p.f.u. phengite).