Amphibolite facies

Definition

Amphibolite facies is a metamorphic grade characterized by the presence of amphibole minerals, typically hornblende, alongside plagioclase feldspar. This facies indicates conditions of moderate to high temperatures and pressures, generally between 500°C to 800°C and 2 to 10 kilobars, which leads to significant changes in the mineralogy and texture of the parent rock. The formation of amphibolite facies reflects a specific range of metamorphic conditions that result in distinctive mineral associations and textures.

5 Must Know Facts For Your Next Test

1. Amphibolite facies typically forms from the metamorphism of basalt or other mafic igneous rocks under specific pressure and temperature conditions.
2. In amphibolite facies rocks, the predominant mineral assemblage usually consists of hornblende and plagioclase feldspar, with potential minor amounts of garnet or biotite.
3. The textures found in amphibolite facies can include a schistose or gneissic texture due to the alignment of amphibole and feldspar minerals under directional pressure.
4. Amphibolite facies is an important indicator of tectonic settings such as subduction zones, where oceanic crust is subjected to high pressures and temperatures.
5. The study of amphibolite facies helps geologists understand the metamorphic history and tectonic evolution of mountain belts and regions affected by regional metamorphism.

Review Questions

• How do the mineral compositions in amphibolite facies help determine the metamorphic conditions under which they formed?
  • The mineral compositions found in amphibolite facies, primarily hornblende and plagioclase feldspar, are key indicators of the metamorphic conditions. These minerals typically form at moderate to high temperatures and pressures. By studying these minerals' stability ranges, geologists can infer the specific temperature and pressure conditions that existed during metamorphism, which can reveal insights into tectonic processes affecting the region.
• Compare amphibolite facies with granulite facies regarding their formation conditions and mineral assemblages.
  • Amphibolite facies forms under moderate to high temperatures and pressures, typically from 500°C to 800°C and 2 to 10 kilobars, with a mineral assemblage that includes hornblende and plagioclase. In contrast, granulite facies represents an even higher metamorphic grade with temperatures exceeding 800°C and greater pressure conditions. The mineral assemblages in granulite facies often include anhydrous minerals like pyroxene and garnet, reflecting the more extreme conditions of formation compared to amphibolite.
• Evaluate the significance of amphibolite facies in understanding tectonic processes within subduction zones.
  • Amphibolite facies plays a crucial role in understanding tectonic processes within subduction zones because its formation indicates the transformation of oceanic crust as it descends into the mantle under high pressure and temperature. The presence of amphibole minerals signifies that water is released from subducting slabs, influencing magma generation and plate dynamics. By studying rocks formed under these conditions, geologists can reconstruct past tectonic events, aiding our understanding of mountain-building processes and the evolution of continental crust.

Related terms

metamorphism: The process by which existing rocks undergo transformation due to changes in temperature, pressure, and chemical environment, resulting in new mineral assemblages.

hornblende: A complex inosilicate mineral that is a common component of amphibolite facies rocks, often contributing to their characteristic dark coloration.

granulite facies: A higher metamorphic grade than amphibolite facies, granulite facies is characterized by very high temperatures and pressures, leading to different mineral assemblages and textures.