Interpreting Metamorphic Environments
Describe the temperature and pressure conditions associated with the following metamorphic facies: blueschist facies, hornfels facies, and zeolite facies.
Geologists realized that groups of associated minerals could be used to determine the pressures and temperatures at which rocks undergo metamorphism.
This discovery led Finnish geologist Pentti Eskola to propose the concept of metamorphic facies.
Simply, metamorphic rocks containing the same assemblage of minerals belong to the same metamorphic facies— implying that they formed in very similar metamorphic environments.
Using metamorphic facies to determine a metamorphic environment is analogous to using a group of plants to define a climatic zone.
Areas that experience similar precipitation and temperature conditions.
For instance, sparsely vegetated regions dominated by cacti identify the desert climate zone, characterized by low precipitation and high temperatures.
Common Metamorphic Facies
The common metamorphic facies are shown in Figure 1. These include the hornfels, zeolite, greenschist, amphibolite, granulite, blueschist, and eclogite facies.
Facies names are based on the minerals that define them. For example, rocks of the amphibolite facies are characterized by hornblende (a common amphibole); the greenschist facies consists of schists in which the green minerals chlorite, epidote, and serpentine are prominent.
Similar groups of minerals are found in rocks of all ages and in all parts of the world. Thus, the concept of metamorphic facies is useful in interpreting Earth’s history. Rocks belonging to the same metamorphic facies all formed under the same conditions of temperature and pressure, and therefore in similar tectonic settings, regardless of their location or age.
It should be noted that the name for each metamorphic facies refers to a metamorphic rock derived specifically from a basaltic parent.
This is because Pentti Eskola concentrated his work on the metamorphism of basalts, and his basic terminology, although now slightly modified, remains. The names of Eskola’s facies serve as convenient labels for particular combinations of temperatures and pressures, no matter what the mineral composition.
In other words, even if a nonbasaltic parent rock produces different indicator minerals under a given set of metamorphic conditions, the facies names shown in Figure 1 are used to denote the temperature and pressure ranges embodied by that metamorphic rock.
For example, mica schist belongs to the amphibolite facies, despite the fact that mica schist was derived from shale—a nonbasaltic parent rock.
Metamorphic Facies and Plate Tectonics
Figure 2 shows how the concept of facies fits into the context of plate tectonics. Near deep-ocean trenches, slabs of relatively cool oceanic lithosphere and the overlying crust are subducted.
As the lithosphere descends, sediments and crustal rocks are subjected to steadily increasing temperatures and pressures (Figure 2A). However, temperatures in the slab remain cooler than the surrounding mantle because rock is a poor conductor of heat and therefore warms slowly.
The metamorphic facies associated with this type of high-pressure, low temperature environment is called the blueschist facies because of the presence of the blue-colored variety of amphibole called glaucophane (Figure 3A).
The rocks of the Coast Range of California belong to the blueschist facies;
these highly deformed rocks were once deeply buried but have been uplifted because of a change in the plate boundary.
In some areas, subduction carries rocks to even greater depths, producing the eclogite facies that is diagnostic of very high temperatures and pressures (Figure 3B).
Along some convergent zones, continental plates collide to form extensive mountain belts (see Figure 2B). This activity results in large areas of regional metamorphism that often include zones of contact and hydrothermal metamorphism. The increasing temperatures and pressures associated with regional metamorphism are recorded by the zeolite greenschist– amphibolite–granulite facies sequence shown in Figure 1.
Mineral Stability and Metamorphic Environments
In most tectonic environments, such as along subduction zones, rocks experience an increase in both pressure and temperature simultaneously. An increase in pressure causes minerals to contract, which favors the formation of high-density minerals.
However, increased temperature results in expansion, so mineral phases that occupy greater volume (are less dense) tend to be more stable at high temperatures. Thus, determining the conditions of temperature and pressure at which a mineral is stable (does not change) is not an easy task. To help in this endeavor, researchers have turned to the laboratory.
Here materials of various compositions are heated and placed under pressures that approximate conditions at various depths within Earth. From such experiments, we can determine which minerals are likely to form in various metamorphic environments.
Some minerals, such as quartz, are stable over a wide range of metamorphic settings and therefore are not useful in determining metamorphic environments. Fortunately, other groups of related minerals do provide useful estimates of conditions during metamorphism.
One of the most important of these groups includes the minerals kyanite, andalusite, and sillimanite. These three minerals have identical chemical compositions (Al2SiO5) but different crystalline structures, which makes them polymorphs (see Chapter 3).
Diagram ( Figure 4 ) that shows the specific range of pressures and temperatures at which each of these aluminum-rich silicates is stable.
Because shales and mudstones, which are very common, contain the elements found in these minerals, metamorphic products of shale (slate, phyllite, schist, and gneiss) often contain varying amounts of kyanite, andalusite, or sillimanite.
For example, if shale were buried to a depth of about 35 kilometers (10 kilobars), at a temperature of 550°C, the mineral kyanite would form.
In general, andalusite is produced by contact metamorphism in near surface environments where temperatures are high but pressures are relatively low.
Kyanite is the high-pressure polymorph that forms during the subduction and deep burial associated with mountain building.
Sillimanite, on the other hand, forms only at high temperatures, as a result of contact with a very hot magma body and/or very deep burial.
Knowing the ranges of temperatures and pressures a rock experienced during metamorphism provides geologists with valuable data needed to interpret past tectonic environments.