The facies groups and facies series of metamorphism, and their relation to physical conditions in the earth's crust

Abstract

The notions of metamorphic facies, metamorphic grade, facies groups and facies series are reviewed. Some boundary reactions between groups and between series are discussed and the positions of their equilibrium curves in PT-graphs are defined as far as experimental data permit. The role of tectonic overpressure and magmatic superheat in orogenic PT-regimes is assessed on the strength of structural relations in the high PT-facies series of Cabo Ortegal, and of recently reported high-temperature minerals (wollastonite, sapphirine, sanidine, etc.) from granulite facies terrains in the vicinity of charnockitic and other relatively dry magmatic intrusives. Tectonic overpressure may be a requisite for eclogite facies metamorphism when set in dry anatexitic gneisses and granulites, and that the drier its environment, the sooner magmatic superheat may be expected to steepen the geothermal gradient of orogenic metamorphism.