As metamorphic rocks change under heat and pressure, their ingredients recombine into new minerals that are suited to the conditions. The concept of metamorphic facies is a systematic way to look at the mineral assemblages in rocks and determine a potential range of pressure and temperature (P/T) conditions that were present when they formed.
It should be noted that metamorphic facies are different than sedimentary facies, which include the environmental conditions present during deposition.
Sedimentary facies can be further divided into lithofacies, which focus on a rock’s physical characteristics, and biofacies, which focus on the paleontological attributes (fossils).
Seven Metamorphic Facies
There are seven widely recognized metamorphic facies, ranging from the zeolite facies at low P and T to eclogite at very high P and T. Geologists determine a facies in the lab after examining many specimens under the microscope and doing bulk chemistry analyses. Metamorphic facies is not obvious in a given field specimen. To sum up, a metamorphic facies is the set of minerals found in a rock of a given composition. That mineral suite is taken as a sign of the pressure and temperature that made it.
Here are the typical minerals in rocks that are derived from sediments. That is, these will be found in slate, schist and gneiss. The minerals shown in parentheses are “optional” and don’t always appear, but they can be essential for identifying a facies.
- Zeolite facies: illite/phengite + chlorite + quartz (kaolinite, paragonite)
- Prehnite-pumpellyite facies: phengite + chlorite + quartz (pyrophyllite, paragonite, alkali feldspar, stilpnomelane, lawsonite)
- Greenschist facies: muscovite + chlorite + quartz (biotite, alkali feldspar, chloritoid, paragonite, albite, spessartine)
- Amphibolite facies: muscovite + biotite + quartz (garnet, staurolite, kyanite, sillimanite, andalusite, cordierite, chlorite, plagioclase, alkali feldspar)
- Granulite facies: alkali feldspar + plagioclase + sillimanite + quartz (biotite, garnet, kyanite, cordierite, orthopyroxene, spinel, corundum, sapphirine)
- Blueschist facies: phengite + chlorite + quartz (albite, jadeite, lawsonite, garnet, chloritoid, paragonite)
- Eclogite facies: phengite + garnet + quartz
Mafic rocks (basalt, gabbro, diorite, tonalite etc.) yield a different set of minerals at the same P/T conditions, as follows:
- Zeolite facies: zeolite + chlorite + albite + quartz (prehnite, analcime, pumpellyite)
- Prehnite-pumpellyite facies: prehnite + pumpellyite + chlorite + albite + quartz (actinolite, stilpnomelane, lawsonite)
- Greenschist facies: chlorite + epidote + albite (actinolite, biotite)
- Amphibolite facies: plagioclase + hornblende (epidote, garnet, orthoamphibole, cummingtonite)
- Granulite facies: orthopyroxene + plagioclase (clinopyroxene, hornblende, garnet)
- Blueschist facies: glaucophane/crossite + lawsonite/epidote (pumpellyite, chlorite, garnet, albite, aragonite, phengite, chloritoid, paragonite)
- Eclogite facies: omphacite + garnet + rutile
Ultramafic rocks (pyroxenite, peridotite etc.) have their own version of these facies:
- Zeolite facies: lizardite/chrysotile + brucite + magnetite (chlorite, carbonate)
- Prehnite-pumpellyite facies: lizardite/chrysotile + brucite + magnetite (antigorite, chlorite, carbonate, talc, diopside)
- Greenschist facies: antigorite + diopside + magnetite (chlorite, brucite, olivine, talc, carbonate)
- Amphibolite facies: olivine + tremolite (antigorite, talc, anthopyllite, cummingtonite, enstatite)
- Granulite facies: olivine + diopside + enstatite (spinel, plagioclase)
- Blueschist facies: antigorite + olivine + magnetite (chlorite, brucite, talc, diopside)
- Eclogite facies: olivine