Contact-Metamorphosed to Regionally-Metamorphosed Pelites: The Natural Record

Abstract A large database of natural metapelites documenting patterns of mineral assemblages and mineral compositions as a function of bulk composition and relative depth (pressure) and temperature is presented. The settings range from low-pressure contact aureoles to intermediate/high-pressure (broadly ‘Barrovian’) sequences developed in continental collisional settings. The data are organized according to repeatedly observed prograde mineral assemblage sequences (MASs) that, for metapelites of average composition, vary consistently with pressure. The database comprises 365 metamorphic settings from 44 countries that span the Neoarchean to the Pliocene. Eleven MASs have been identified and further grouped into five broader categories. Of these, staurolite–kyanite (‘Barrovian’) sequences are especially difficult to classify, interpreted to be due in part to non-equilibrium processes involved in their formation. 140 settings (38%) have mineral compositions tied to mineral assemblages (3293 samples; 12 429 mineral analyses). Mineral compositions vary systematically as a function of bulk composition, grade and pressure within the MASs. Estimates of pressure and temperature from geothermobarometry are assessed against the relative constraints provided by the natural data and show broadly similar trends but with large uncertainties. Predicted phase equilibria and mineral compositions from thermodynamic modelling are assessed against the natural data. These reproduce some but not all of the natural patterns in mineral assemblages and intra- and inter-mineral compositions. The data in this paper provide constraints for future development of thermodynamic models. The paper concludes with suggestions for future research in metamorphic petrology to address some of the main problems that have been identified.