On the Facies Series in Various Types of Metamorphism

In order to point out some useful aspects of the facies series in various types of metamorphism, the possible stability fields of the facies described from areas of progressive metamorphism are shown in a hypothetical PT diagram. Examples of nine common types are given. These seem to occur at different pressure levels and are distinguished by different series of mineral assemblages. Division into subfacies in the field of normal regional metamorphism is based on assemblages in aluminum-rich pelitic schists. The invariant points of intersection of the equilibrium boundaries of andalusite, kyanite, and sillimanite with the lower stability boundaries of biotite, garnet, staurolite, orthoclase, and hypersthene can serve as relative geobarometers because a different series of subfacies occurs on the higher and lower pressure side of each such intersection. The stability field of cordierite was placed so as to satisfy the zone of entrance of this mineral in various types of metamorphism. Verification of any of the relations shown in the tentative pressure-temperature diagram, and evaluation of mineralogic zonations as observed in the field must await further field and laboratory studies. Recognition of the metamorphic type through its facies series makes the studies of the equilibrium assemblages and the compositional variation of the coexisting minerals meaningful. If such studies are extended systematically over the whole temperature range in each known metamorphic type, we should have information necessary to determine the physical conditions of metamorphism also in areas where no zones are present.