Classic views on the scope and subdivision of the field of rock metamorphism are reviewed and summarized, and the general basis for classification of metamorphic rocks is outlined.
The zonal distribution of rocks showing different degrees of metamorphism in those metamorphic terranes that have been studied in detail is attributed to progressive variation in physical conditions controlling development of a state of internal equilibrium in rocks of varying composition in different chemical environments. The resulting equilibrium assemblages of minerals can be interpreted broadly in terms of the phase rule. This leads to the concept of metamorphic facies as a basis for classifying and interpreting metamorphic rocks. Eskola’s scheme of facies is modified and subdivided insofar as this is at present justified by the data of metamorphic petrography. While metamorphism in some cases involves only insignificant change in total chemical composition of the rocks affected, in others important change results either from metasomatism or from metamorphic differentiation.
The fabric of metamorphic rocks has developed by growth and/or deformation of crystalline minerals in a solid medium. Its characters therefore differ strikingly from those of typical igneous and sedimentary fabrics. The essential features of metamorphic fabrics are reviewed, and interpretation of the fabric of deformed rocks is considered in some detail.
The memoir concludes with a discussion of some of the problems specially connected with regional metamorphism, namely, the causes of regional metamorphism, the phenomenon of retrogressive metamorphism, the development of migmatites, and the relation of regional metamorphism to plutonic igneous intrusion.