The assemblage of hornblende and plagioclase would most likely indicate which metamorphic facies?

The assemblage of hornblende and plagioclase would most likely indicate which metamorphic facies?

• A. Greenschist facies
• B. Amphibolite facies
• C. Blueschist facies
• D. Granulite facies

Answer: (B)

Mineral assemblages in metamorphic rocks reflect the pressure–temperature conditions under which they formed. Hornblende is stable at mid- to high-grade conditions and pairs commonly with plagioclase in rocks produced during mid-grade metamorphism. This combination is the hallmarks of the amphibolite facies, which represents moderate to high temperatures and pressures typical of that grade of metamorphism. In greenschist facies you’d expect minerals like chlorite, epidote, and actinolite rather than abundant hornblende. Blueschist facies feature blue amphiboles such as glaucophane and lawsonite, indicating a different, typically low-temperature/high-pressure subduction environment. Granulite facies involves very high temperatures with minerals like pyroxenes and garnet and little hydration minerals. So the hornblende–plagioclase assemblage points to amphibolite facies, indicating mid-grade metamorphism.

Mineral assemblages in metamorphic rocks reflect the pressure–temperature conditions under which they formed. Hornblende is stable at mid- to high-grade conditions and pairs commonly with plagioclase in rocks produced during mid-grade metamorphism. This combination is the hallmarks of the amphibolite facies, which represents moderate to high temperatures and pressures typical of that grade of metamorphism.

In greenschist facies you’d expect minerals like chlorite, epidote, and actinolite rather than abundant hornblende. Blueschist facies feature blue amphiboles such as glaucophane and lawsonite, indicating a different, typically low-temperature/high-pressure subduction environment. Granulite facies involves very high temperatures with minerals like pyroxenes and garnet and little hydration minerals.

So the hornblende–plagioclase assemblage points to amphibolite facies, indicating mid-grade metamorphism.