Amphibolite facies

Definition

Amphibolite facies refers to a specific set of metamorphic conditions characterized by moderate to high temperatures and pressures, typically ranging from about 500 to 700 degrees Celsius and pressures from 4 to 8 kilobars. This facies is primarily associated with the presence of amphibole minerals, particularly hornblende, and is indicative of a metamorphic environment where the original rock, or protolith, was subjected to significant alteration without melting.

5 Must Know Facts For Your Next Test

1. Amphibolite facies typically forms in tectonically active regions where rocks are subjected to increased heat and pressure due to continental collision or subduction processes.
2. The primary minerals found in amphibolite facies include hornblende, plagioclase feldspar, and garnet, which indicate the specific metamorphic conditions present.
3. Rocks that undergo metamorphism into amphibolite facies often retain their foliation or lineation, giving them a distinct banded appearance due to the alignment of amphibole crystals.
4. Amphibolite facies is crucial for understanding the tectonic history of a region as it provides insights into the pressures and temperatures experienced during mountain-building events.
5. This facies is commonly found in areas such as orogenic belts where high-grade metamorphic rocks are prevalent, serving as an important marker for geological studies.

Review Questions

• How does the amphibolite facies fit within the broader classification of metamorphic rocks and their formation processes?
  • Amphibolite facies is categorized within high-grade metamorphic rocks and represents a specific set of temperature and pressure conditions that indicate significant tectonic activity. It results from the transformation of specific protoliths under conditions that promote the growth of amphibole minerals like hornblende. Understanding this facies helps geologists classify metamorphic environments and reconstruct the geological history of regions that have undergone intense metamorphism.
• What are the key mineralogical characteristics of amphibolite facies, and how do they reflect its formation conditions?
  • Amphibolite facies is primarily characterized by minerals such as hornblende and plagioclase feldspar, along with garnet in some instances. These minerals indicate the high temperatures and moderate pressures typical of this metamorphic environment. The presence of these specific minerals reflects the chemistry of the original rock and the conditions under which it underwent metamorphism, providing insights into both the original protolith and the tectonic setting during its formation.
• Evaluate the significance of amphibolite facies in understanding regional geology and tectonic processes.
  • The study of amphibolite facies is essential for understanding regional geology because it provides evidence for past tectonic events such as continental collision or subduction. By analyzing the mineral composition and texture of rocks within this facies, geologists can infer the thermal and pressure conditions that existed during their formation. This knowledge contributes to a broader understanding of mountain-building processes, plate tectonics, and the evolution of Earth's crust, offering vital clues about how regions have changed over geological time.

Related terms

Metamorphism: The process by which existing rocks are transformed into new types of rocks through changes in temperature, pressure, and chemically active fluids.

Protolith: The original rock from which a metamorphic rock forms, serving as the starting material that undergoes metamorphism.

Greenschist facies: A lower-grade metamorphic facies that is characterized by lower temperatures and pressures compared to amphibolite facies, often containing chlorite and biotite.