Amphibolite facies

Definition

Amphibolite facies is a metamorphic grade characterized by the presence of amphibole minerals, particularly hornblende, in metamorphosed basalt or other mafic rocks. This facies represents moderate to high temperatures and pressures typically found in regional metamorphism, suggesting conditions conducive to the formation of coarser-grained, high-grade metamorphic rocks.

5 Must Know Facts For Your Next Test

1. Amphibolite facies typically forms at temperatures ranging from about 500 to 750 degrees Celsius and pressures of 4 to 12 kilobars.
2. Common minerals found in amphibolite facies include hornblende, plagioclase feldspar, and garnet, which are indicative of the metamorphic conditions present.
3. Rocks within this facies often exhibit a characteristic banding or layering due to the alignment of amphibole crystals under directed pressure.
4. Amphibolite facies can be associated with continental collision zones where tectonic forces generate the necessary heat and pressure for high-grade metamorphism.
5. The study of amphibolite facies is crucial for understanding tectonic processes and the thermal history of regions undergoing mountain-building events.

Review Questions

• How does the formation of amphibolite facies relate to the processes of regional metamorphism?
  • Amphibolite facies is primarily formed during regional metamorphism, which occurs when large areas of crust are subjected to elevated temperatures and pressures over extended periods. In this environment, mafic rocks like basalt undergo metamorphic changes that result in the growth of amphibole minerals, particularly hornblende. The specific conditions of temperature and pressure create a distinct mineral assemblage that characterizes the amphibolite facies, thus linking it directly to the dynamic processes at play during regional metamorphism.
• Discuss the significance of amphibolite facies in understanding tectonic environments, particularly in relation to mountain-building.
  • Amphibolite facies serves as an important indicator of tectonic environments where significant pressure and temperature conditions prevail, such as at convergent plate boundaries. In these areas, continental collision leads to the formation of high-grade metamorphic rocks, including those within the amphibolite facies. By studying these rocks, geologists can gain insights into the thermal history and tectonic forces that shape mountain ranges, making amphibolite facies a key component in reconstructing past geological events.
• Evaluate the role of mineral composition in amphibolite facies and how it influences our understanding of metamorphic conditions.
  • The mineral composition found within amphibolite facies is crucial for interpreting the metamorphic conditions under which these rocks formed. Minerals such as hornblende and plagioclase not only indicate specific temperature and pressure ranges but also reflect the chemical environment during metamorphism. By analyzing these minerals and their ratios, scientists can reconstruct the geothermal gradients and tectonic settings involved in their formation. This evaluation helps establish a clearer picture of geological processes and informs models related to crustal evolution and dynamics.

Related terms

Metamorphism: The process by which rocks are altered in mineralogy, texture, and chemical composition due to changes in temperature and pressure.

Gneiss: A high-grade metamorphic rock that typically exhibits foliation and is often formed from the alteration of granite or sedimentary rocks under conditions of high temperature and pressure.

Foliation: The repetitive layering that can occur in metamorphic rocks, often as a result of differential stress during the metamorphic process.