Explain how index minerals are used to establish the metamorphic grade of a rock body.
In areas affected by metamorphism, geologists can observe the usually systematic variations in the mineralogy and texture of the altered rocks. These differences are clearly related to variations in the degree of metamorphism that takes place in each metamorphic zone.
Across areas where regional metamorphism has occurred, rock textures vary based on the intensity of metamorphism. If we begin with a clay-rich sedimentary rock such as shale or mudstone, a gradual increase in metamorphic intensity from low grade to high grade is accompanied by a general coarsening of the grain size. Figure 1 illustrates that as metamorphic intensity increases, shale changes to a fine-grained slate, which then forms phyllite, which, through continued recrystallization, generates a medium-grained schist. Under more intense conditions, a gneissic texture that exhibits layers of dark and light minerals may develop. This systematic transition in metamorphic textures can be observed as we approach the Appalachian Mountains from the west.
Beds of shale, which once extended over large areas of the eastern United States, still occur as nearly flat-lying strata in Ohio. However, in the broadly folded Appalachians of central Pennsylvania, the rocks that once formed flat-lying beds are folded and display a preferred orientation of platy mineral grains, as exhibited by welldeveloped slaty cleavage. As we move further east toward the intensely deformed crystalline Appalachians, we find large exposures of schists. Some of the most intense zones of metamorphism are found in Vermont and New Hampshire, where gneissic rocks are exposed at the surface.
Index Minerals and Metamorphic Grade
In addition to textural changes, we encounter corresponding changes in mineralogy as we shift from areas of low-grade metamorphism to those of high-grade metamorphism. An idealized transition in mineralogy shale.
Garnet that results from the regional metamorphism of shale is shown in Figure 2. The first new mineral to form as shale changes to slate is chlorite. At higher temperatures, flakes of muscovite and biotite begin to dominate.
Under more extreme conditions, metamorphic rocks may contain garnet and staurolite crystals (Figure 3). At temperatures approaching the melting point of rock, sillimanite forms. Sillimanite is a high-temperature metamorphic mineral used to make porcelains used in extreme environments, such as for spark plugs.
Through the study of metamorphic rocks in their natural settings (called field studies) and through experimental studies, researchers have learned that certain minerals, such as those listed in Figure 2, are good indicators of the metamorphic environment in which they formed. Using these index minerals, geologists distinguish among different zones of regional metamorphism.
For example, the mineral chlorite begins to form when temperatures are relatively low—less than 200°C (400°F; see Figure 2). Thus, rocks containing chlorite (usually slates) are categorized as low grade. By contrast, the mineral sillimanite forms only in extreme environments where temperatures exceed 600°C (1100°F), and rocks containing it are considered high grade. By mapping the occurrences of index minerals, geologists can identify zones of varying metamorphic grades (Figure 4).
Migmatites In the most extreme environments, even the highest-grade metamorphic rocks undergo change.
For example, gneissic rocks may be heated sufficiently to trigger melting. However, minerals melt at different temperatures.
The light-colored silicates, usually quartz and potassium feldspar, have the lowest melting temperatures and begin to melt first, whereas the mafic silicates, such as amphibole and biotite, remain solid. When the partially melted rock cools, the light bands will be composed of igneous or igneous appearing components, while the dark bands will consist of unmelted metamorphic material.
Rocks of this type are called migmatites (migma = mixture, ite = a stone) (Figure 5). The bands in migmatites often form intricate folds and may contain tabular inclusions of the dark components. Migmatites serve to illustrate the fact that some rocks are considered transitional and do not fit neatly into any of the three basic rock groups.