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Metamorphic facies are groups of rock types with the same mineral assemblages in equilibrium over a range of PT conditions. Systematic facies classification (zeolite, prehnite-pumpellyite, greenschist, amphibolite, granulite, blueschist, eclogite) reflects different geothermal gradients and tectonic regimes; facies associations reveal the thermal and pressure structure of orogens.
From your study of metamorphic grade and pressure-temperature paths, you know that increasing temperature and pressure transform minerals into new assemblages that are stable under the new conditions. The metamorphic facies concept organizes this complexity into a practical classification: rocks that equilibrated under similar PT conditions share the same mineral assemblage, regardless of where on Earth they formed. If you find a metabasalt containing chlorite, actinolite, and albite, it belongs to the greenschist facies — and you immediately know it formed at roughly 300-500°C and moderate pressures, whether it comes from the Scottish Highlands or the Japanese Alps.
The facies are arranged in PT space, and each one occupies a distinct region. At the lowest grades, burial of sedimentary and volcanic rocks produces the zeolite and prehnite-pumpellyite facies, characterized by hydrous minerals stable at temperatures below about 300°C. As temperature increases along a normal continental geothermal gradient, rocks pass through greenschist facies (named for the green minerals chlorite, epidote, and actinolite that dominate altered mafic rocks), then into amphibolite facies (where hornblende and plagioclase are the stable mafic assemblage), and finally into granulite facies at the highest temperatures (>700°C), where even hydrous minerals break down and anhydrous phases like pyroxene and garnet dominate.
The most tectonically diagnostic facies are those that form under anomalous geothermal gradients. Blueschist facies rocks contain the striking blue amphibole glaucophane and the pink mineral lawsonite, and they form under conditions of high pressure but relatively low temperature — the signature of rapid burial without much heating. This is exactly what happens in subduction zones, where cold oceanic crust is dragged to great depths faster than it can thermally equilibrate. Eclogite facies represents even more extreme conditions: pressures so high that plagioclase is no longer stable and is replaced by the dense assemblage of garnet plus omphacite (a sodium-rich pyroxene). Finding eclogite in an ancient mountain belt is strong evidence that rocks were subducted to depths exceeding 45 km and then exhumed.
The concept of facies series — the sequence of facies a rock encounters along its PT path — connects individual facies to tectonic setting. A normal continental collision zone produces a facies series progressing from greenschist through amphibolite to granulite (a medium-pressure series). A subduction zone produces a high-pressure series from prehnite-pumpellyite through blueschist to eclogite. A volcanic arc with high heat flow produces a low-pressure series that may jump from greenschist directly to granulite. By mapping which facies appear across an ancient orogen and in what spatial arrangement, you reconstruct the thermal and pressure structure of a mountain belt that may have eroded away hundreds of millions of years ago. This is why facies associations are among the most powerful tools in metamorphic petrology — they translate mineral assemblages into tectonic history.