The order of metamorphic zones on maps for metamorphic rocks with the same bulk composition (e.g. metamorphosed shales) has been interpreted as an increasing intensity of metamorphism (metamorphic grade). Rocks that have not changed much from their protolith are said to have experienced a low grade of metamorphism. Rocks that are significantly changed from their protolith are said to have achieved a high grade of metamorphism. In contact metamorphic zones adjacent to igneous intrusions, the most changed (high grade) metamorphic rocks are observed closest to the intrusion, whereas the least changed (low grade) metamorphic rocks are found farthest from the intrusion. Because the rocks closest to an intrusion (the high grade rocks) will have been heated to the highest temperatures, metamorphic grade is believed to reflect the temperature of metamorphism. Although pressure also plays an important role in metamorphism, modern usage is that metamorphic grade refers to the relative temperature of metamorphism so that higher-grade metamorphic rocks been heated to higher temperatures than lower-grade metamorphic rocks.
Figure 3.06. Zones and Minerals for Metamorphosed Shales. Click on the image to see a larger, zoomable version.
Figure 3.06 lists metamorphic zones for metamorphosed shales, in order, from low grade to high grade. Also shown on the list are the possible occurrences of common minerals in metamorphosed shales at various metamorphic grades. The boundaries of metamorphic zones on geologic maps, as noted in the Vermont map of Figure 3.05, are the first occurrences of the minerals when moving up grade. Zone boundaries are known as isograds because they identify locations that exhibit the same grade of metamorphism.
The repetition of mineral assemblages and the similarity of metamorphic zones in different metamorphic belts supports the conclusion that the mineralogy of a metamorphic rock reflects the conditions of metamorphism. The mineralogy of a metamorphic rock must, therefore, have approached chemical equilibrium at a particular temperature and pressure. And changes in mineralogy must be caused by chemical reactions. "Ideally, the changes marked by an isograd are produced as a result of a specific reaction, ... not changes in rock chemical compositions" (IUGS, Fettes and Desmons, 2011).
