Metamorphic facies: A review and some suggestions for changes

The concept of metamorphic facies has been used as a tool in the interpretation of metamorphic rocks for almost 100 years. The preferred definition is a set of mineral assemblages which are repeatedly associated in space and time. Equilibrium or physical conditions (pressure-temperature) should not be part of the definition. The emphasis has always been on identification of the minerals with the petrographic microscope. Chemical analyses of the minerals using the electron microprobe is not necessary. The original definition of metamorphic facies used the metamorphic mineralogy of metabasic rocks. This bulk composition is not useful for the definition of all metamorphic facies. Several critical minerals in metabasic rocks cannot be readily identified with a petrographic microscope (albite versus oligoclase and actinolite versus hornblende). A revised set of metamorphic facies is proposed and mineral assemblages in both metabasic and pelitic rocks are outlined to provide definitions of the individual facies. Metamorphic facies should not be used to give quantitative estimates of P-T conditions. Only relative P-T can be estimated. The interpretation of “equilibrium” in metamorphic facies can be modeled using the Gibbs phase rule and simple assumptions about phases and components. This leads to an interpretation that metamorphic facies could represent divariant or higher variance equilibrium.