Metamorphic Grade

The intensity of metamorphism and the vigor of metamorphic transformation are expressed by the term metamorphic grade. If, for instance, a granitic pluton intrudes a pile of sediments, the resulting contact metamorphic aureole will contain rocks of higher grade close to the contact of the pluton (heat source) and rocks of low grade at greater distance from the pluton where the sediments were only moderately heated. Similarly, in a mountain chain, a metamorphic terrain may have developed and is presently at the erosion surface where rock samples can be collected. They contain mineral assemblages that represent the least hydrated state attained during a metamorphic cycle. The assemblages define a metamorphic field gradient (see Chap. 3). Rocks on this P-T array are said to be of low grade at low P-T and of successively higher grade with higher P-T conditions. The term metamorphic grade is a qualitative indicator of the physical conditions that have been operating on the rocks. Increasing P-T conditions produce metamorphic rocks of increasingly higher grade. It is a useful term for comparison of rocks within a single prograde metamorphic area. When applied to rocks in different regions, its meaning is not always clear, however, and its exact nature should be clearly specified.