What is the amphibolite facies in geology ?

Amphibolite - Metamorphism

Metamorphic facies diagram
The amphibolite facies designates a metamorphic facies of average temperature and pressure (500 to 700°C and from 0.2 to 1 GPa), which frequently results in the appearance of amphibole rocks (hornblende).

Amphibolites are rocks resulting from regional metamorphism, consisting essentially of amphiboles (most often hornblende) and plagioclases, they most often belong to the amphibolite facies.