CONTACT-METAMORPHOSED TO REGIONALLY-METAMORPHOSED PELITES: THE NATURAL RECORD

The geoscience community relies on metamorphic petrology/petrologists to determine the formation conditions of metamorphic rocks (how hot and how deep). Thermodynamically calculated phase diagrams underpin our ability to make these determinations. A minimum requirement of such modelling is the reproduction of repeated patterns of natural mineral assemblages and mineral compositions. Current thermodynamic models (meaning, end-member thermodynamic data combined with a-X models) reproduce some but not all of these, both in metapelites and metabasites. This presentation focuses on a substantial natural database of metapelites metamorphosed over a range of grade from contact-metamorphosed to regionally-metamorphosed pelites (333 localities; 3,138 samples; 11,707 mineral analyses). First, consistent prograde mineral assemblage sequences as a function of pressure are identified. Second, mineral compositional variations (intra-mineral and inter-mineral) in these sequences are examined. Third, the results of different generations of thermodynamic models are assessed against the natural record. Finally, some suggestions are made for the next iteration of thermodynamic model development, in which as many reliable data as possible, both natural and experimental, are brought to bear.