Facies Boundaries in Pelites at the Middle Grades of Regional Metamorphism

Abstract

AbstractAn examination of existing definitions of facies and subfacies boundaries in pelitic rocks suggests that some alteration is necessary to emphasize two important reactions:3 Staurolite + 2 Quartz ⇌ Almandine + 5 Al2SiO5 + 3 Water, andMuscovite + Quartz ⇌ Potash feldspar + Al2SiO5 + Water A generalized plot shows possible relations between the curves of these two reactions and the inversion, curves of the aluminosilicates within the P-T field of metamorphism. The curves of further reactions commonly used to define facies and subfacies boundaries have been schematically added to this plot, and trends of metamorphism in various areas are indicated thereon. P-T curves of dehydration reactions such as the above will vary according to the chemical potential of the water vapour in each particular case. Observations suggest, however, that the curves are not very differently placed from one regional metamorphic terrain to another. That is, the rate of escape of volatiles is broadly similar in the metamorphism of many of these terrains.