METAMORPHIC FACIES-A REVIEW AND SOME SUGGESTIONS

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 is repeatedly associated in space and time. Equilibrium or physical conditions should not be part of the definition. It is true that metamorphic facies can be used to set broad limits on pressure-temperature conditions but variable activities of H₂O are not usually considered. The use of isochemical phase diagrams (also known as pseudosections) can be used to show which mineral assemblages are stable for different bulk compositions. The original definition of metamorphic facies used the metamorphic mineralogy of mafic rocks. This bulk composition is not useful for the definition of all metamorphic facies. It should be emphasized that identification of the minerals by the petrographic microscope is one of the themes of this paper. Several critical minerals in mafic rocks cannot be unambiguously identified with a petrographic microscope. A revised set of metamorphic facies is proposed and mineral assemblages in both mafic and pelitic rocks are outlined to provide definitions of the individual facies. Not every metamorphic sample can be unambiguously assigned to a single metamorphic facies. Metamorphic facies should not be used to give quantitative estimates of P-T conditions. 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.