Reactions between olivine and plagioclase, and between pyroxenes and plagioclase, commonly produce corona assemblages of lower volume and entropy. These coronas imply that assemblages representative of intermediate-and high-pressure granulite facies, and of eclogite facies (emphacite + garnet), can be produced by the cooling of dry olivine + plagioclase and pyroxene + plagioclase assemblages from igneous temperatures within the continental crust. They further imply that eclogite is a stable assemblage within the deeper parts of the continental crust; this in turn requires that dP/dT > 0 for the equilibrium curves for the relevant reactions. The general relations between T and rates of nucleation, growth and diffusion suggest that these coronas will only be formed in relatively deep-seated rocks (P > 6 kb ?). The formation of such coronasby regional or contact metamorphism, or by metasomatism, is unlikely; they are best regarded as retrograde features.