Drombovski: Metamorphic Facies

Metamorphic facies is a fundamental notion in metamorphic petrology. The concept of metamorphic facies replaced that of depth zones, that is, epi-, meso- and catazone (Grubenmann & Niggli 1924), when it became obvious that metamorphic grade is not necessarily correlated with depth.

The concept of metamorphic facies was first proposed by Eskola (1915) who later (Eskola, 1920) gave the following definition: A metamorphic facies is "a group of rocks characterised by a definite set of minerals which, under the conditions obtaining during their formation, were at perfect equilibrium with each other. The quantitative and qualitative mineral composition in the rocks of a given facies varies gradually in correspondence with variation in the chemical bulk composition of the rocks". In the same paper he defined also mineral facies as a more general term applicable to both igneous and metamorphic rocks. A mineral facies "comprises all the rocks that have originated under temperature and pressure conditions so similar that a definite chemical composition has resulted in the same set of minerals ...". Subsequently Eskola (1939) wrote (translated from German by Fyfe et al. 1958) "In a definite facies are united rocks which for identical bulk composition exhibit an identical mineral composition, but whose mineral composition for varying bulk composition varies according to definite laws".

The Subcommission proposes the following definition of facies, which follows Eskola's writings and the commentaries of other workers (in particular Turner 1981).

A metamorphic facies is a set of metamorphic mineral assemblages, repeatedly associated in time and space and showing a regular relationship between mineral composition and bulk chemical composition, such that different metamorphic facies (sets of mineral assemblages) appear to be related to different metamorphic conditions, in particular temperature and pressure, although other variables, such as PH2O may also be important.

It is one of the strengths of the metamorphic facies classification that it identifies the regularities and consistencies in mineral assemblage development, which may be related to P-T conditions, but does not attempt to define actual pressures and temperatures.

In the broad sense, considering the exceptionally wide range of chemical compositions of rocks, and narrow ranges of P-T conditions over which mineral assemblages may change, it is theoretically possible to define a very large number of facies. In practice it has been found most convenient to define a reasonably small number of facies, which cover the broad range of crustal P-T conditions . These have been based principally on major changes in the mineral assemblages of rocks of basaltic composition, because such rock types are widespread and they show changes in mineral assemblages that are both distinct and reasonably limited in number, as realised by Eskola himself.

Within such major and broad facies, subunits or subfacies have been defined showing, for example, more detailed changes in pelitic assemblages. However no widely used scheme of subfacies exists, and we make no attempt to define such here, since they may be defined for specific circumstances when necessary.

Eskola (1920, 1939) distinguished eight facies, namely: greenschist facies (f.), epidote-amphibolite f., amphibolite f., pyroxene-hornfels f., sanidinite f., granulite f., glaucophane-schist f. and eclogite facies. Coombs et al. (1959), building on a suggestion of Eskola's, added a zeolite facies and a prehnite-pumpellyite zone, which Turner (1968) called prehnite-pumpellyite metagraywacke facies. Miyashiro (1973) used the above ten facies renaming the last one as the prehnite-pumpellyite facies. More recently various authors have recognised distinctions in the assemblages containing prehnite and pumpellyite, and erected three facies or subfacies based on the assemblages prehnite-pumpellyite, prehnite-actinolite and pumpellyite-actinolite (Árkai et al., this vol.). These facies or subfacies, involving prehnite and pumpellyite, may be collectively referred to as the subgreenschist facies (e.g. Bucher & Frey 1994, Merriman & Frey 1999) and this term has accordingly been provisionally accepted by the SCMR as a general term covering a range of very low-grade metamorphism (Árkai et al., this vol., Fig. 5.1).

The merits of recognising such a group of facies are evident from their extensive use over many years, and the SCMR recommends that these ten facies be adopted as the major facies for general use. Note, however that blueschist facies is commonly used as a synonym for the glaucophane-schist facies and that the epidote-amphibolite facies is sometimes considered as part of the greenschist facies (on the basis of the coexistence of epidote with sodic plagioclase, i.e.<An17).

Eskola (1915) developed the concept of metamorphic facies: "In any rock or metamorphic formation which has arrived at a chemical equilibrium through metamorphism at constant temperature and pressure qgppp conditions, the mineral composition is controlled only by the chemical composition. We are led to a general conception which the writer proposes to call metamorphic facies."

Mineral changes and associations that develop with increasing metamorphic grade

- Hydrous minerals are not common in high-T igneous mafic protolith, so hydration is a prerequisite for development of metamorphic mineral assemblages that characterize most facies.

- Coarse-grained intrusives are least permeable, and thus most likely to resist metamorphic changes, while tuffs and graywackes are most susceptible.

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