Metamorphic mineral facies

Abstract

Mineral faeies classification was briefly discussed in Chapter 4. We begin this chapter by adopting Ramberg’s (1952) definition of metamorphic mineral facies: “Rocks formed or recrystallized within a certain P, T-field, limited by the stability of certain critical minerals of defined composition, belong to the same mineral facies.” Thus in Figure 4.3 we labeled various P-T areas by different names, such as greenschist facies, granulite facies, etc. The facies concept still provides us with the most convenient framework for the discussion of metamorphic rocks. However, we must not allow its artificiality to bind us rigidly, nor must we lose sight of its highly qualitative and provisional character. At present there is a growing feeling among various workers that the number of facies should be kept to a minimum and the use of subfacies is quite unrealistic. Winkler (1974) has proposed a fourfold classification of metamorphism—very low grade, low grade, medium grade, and high grade—without the use of facies names.