INTERPRETING METAPELITES: RECENT ADVANCES AND FUTURE CHALLENGES

Charlie Guidotti was always a pelite man. He liked metapelitic mineral assemblages and thinking about the metamorphic processes that led to their textures. This talk in his honour is one person's view of the recent advances and future challenges in interpreting metapelitic mineral assemblages. Leaving aside advances in microstructural analysis and geochronology of metapelites, the calculation of bulk-composition-dependent phase diagrams has been perhaps the biggest conceptual advance in recent years. By contrast, ‘black-box' thermobarometry without careful consideration of mineral assemblages and textures, something that Charlie always decried, has fallen into disfavour. There remain some key problems in interpreting natural mineral assemblages with respect to thermodynamically modeled phase equilibria. Increasingly, the problems relate less to thermodynamic data (although cordierite appears to remain a problem) and more to issues such as choice of appropriate chemical system, variations in effective bulk composition, fractional vs equilibrium behaviour and, more recently, evidence that contrasting reaction kinetics can lead to some reactions being significantly overstepped, or not proceeding at all, or occurring in a different order than predicted from equilibrium modeling - all of this for ostensibly ‘simple' thermally-activated metapelitic dehydration reactions. Examples of each topic will be given. The upshot is a renewed awareness of the care required in applying phase equilibrium modeling to natural rocks, and in using natural mineral assemblages as the litmus test of phase equilibrium modeling. The way forward will likely involve a blend of observationally-intensive field-based studies combined with bulk-composition-specific modeling that takes account of the above complications – the sort of detailed petrology that Charlie Guidotti espoused throughout his career.