| Figure 1. Diagram showing metamorphic facies inpressure-temperature space. The domain of the graph corresponds to circumstances within the Earth's crust andupper mantle. |
Ametamorphic facies is a set ofmineral assemblages inmetamorphic rocks formed under similarpressures andtemperatures.[1] The assemblage is typical of what is formed in conditions corresponding to an area on the two dimensional graph of temperature vs. pressure (See diagram in Figure 1).[1] Rocks which contain certain minerals can therefore be linked to certaintectonic settings, times and places in the geological history of the area.[1] The boundaries between facies (and corresponding areas on the temperature v. pressure graph) are wide because they are gradational and approximate.[1] The area on the graph corresponding to rock formation at the lowest values of temperature and pressure is the range of formation ofsedimentary rocks, as opposed to metamorphic rocks, in a process calleddiagenesis.[1]
The namefacies was first used for specificsedimentary environments insedimentary rocks bySwiss geologistAmanz Gressly in 1838. Analogous with thesesedimentary facies a number of metamorphic facies were proposed in 1920 byFinnish petrologistPentti Eskola. Eskola's classification was refined by New-Zealand geologistFrancis John Turner throughout his career. A classic work of Turner's was the book he published in 1948 titledMineralogical and Structural Evolution of Metamorphic Rocks.[2] Turner continued to work in the field, refining the metamorphic facies classifications through the end of his career in the early 1970s.
The different metamorphic facies are defined by the mineralogical composition of a rock. When the temperature or pressure in a rock body change, the rock can cross into a different facies and some minerals becomestable while others become unstable ormetastable. Whether minerals really react depends on thereaction kinetics, theactivation energy of the reaction and how muchfluid is present in the rock.
The minerals in a metamorphic rock and their age relations can be studied byoptical microscopy orscanning electron microscopy ofthin sections of the rock. Apart from the metamorphic facies of a rock, a wholeterrane can be described by the abbreviations LT, MT, HT, LP, MP, HP (from low, medium or high; pressure or temperature). Since the 1980s the term UHP (ultra high pressure) has been used for rocks that experienced extreme pressures.
Which minerals grow in a rock is also dependent upon the original composition of theprotolith (the original rock before metamorphosis).Carbonate rocks have a different composition than abasaltlava, the minerals that can grow in them are different too. Therefore, a metapsammite and a metapelite will have different mineralogical compositions even though they are in the same metamorphic facies.
Every metamorphic facies has some index minerals by which it can be recognized. That does not mean these minerals will necessarily be visible with the naked eye, or even exist in the rock; if the rock does not have the right chemical composition, they will not crystallize.
Very typical index minerals are thepolymorphs ofaluminosilicate (Al2SiO5, all arenesosilicates).Andalusite is stable at low pressure,kyanite is stable at high pressure but relatively low temperature andsillimanite is stable at high temperature.
The zeolite facies is the metamorphic facies with the lowestmetamorphic grade. At lower temperature and pressure processes in the rock are calleddiagenesis. The facies is named forzeolites, stronglyhydratedtectosilicates. It can have the following mineral assemblages:
In meta-igneous rocks andgreywackes:
In metapelites:
The prehnite-pumpellyite facies is a little higher in pressure and temperature than the zeolite facies. It is named for the mineralsprehnite (aCa-Al-phyllosilicate) andpumpellyite (asorosilicate). The prehnite-pumpellyite is characterized by the mineral assemblages:
In meta-igneous rocks andgreywackes:
In metapelites:
The greenschist facies is at low pressure and temperature. The facies is named for the typicalschistosetexture of the rocks and green colour of the mineralschlorite,epidote andactinolite. Characteristic mineral assemblages are:
In metabasites:
In metagreywackes:
In metapelites:
Theamphibolite facies is a facies of medium pressure and average to high temperature. It is named afteramphiboles that form under such circumstances. It has the following mineral assemblages:
In metabasites:
In metapelites:
In Si-dolostones:
Thegranulite facies is the highest grade of metamorphism at medium pressure. The depth at which it occurs is not constant. A characteristic mineral for this facies and the pyroxene-hornblende facies isorthopyroxene. The granulite facies is characterized by the following mineral assemblages:
In metabasites:
In metapelites:
The blueschist facies is at relatively low temperature but high pressure, such as occurs in rocks in asubduction zone. The facies is named after the schistose character of the rocks and the blue mineralsglaucophane andlawsonite. The blueschist facies forms the following mineral assemblages:
In metabasites:
In metagreywackes:
In metapelites:
In carbonate-rocks (marbles):
The eclogite facies is the facies at the highest pressure and high temperature. It is named for the metabasic rockeclogite. The eclogite facies has the mineral assemblages:
In metabasites:
In metagranodiorite:
In metapelites:
The albite-epidote-hornfels facies is a facies at low pressure and relatively low temperatures. It is named for the two mineralsalbite andepidote, though they are also stable in other facies.Hornfels is a rock formed bycontact metamorphism, a process that characteristically involves high temperatures but low pressures/depths. This facies is characterized by the following minerals:
In metabasites:
In metapelites:
In calcaerous assemblage:Calcite + talc + quartz
The hornblende-hornfels facies is a facies with the same low pressures but slightly higher temperatures as the albite-epidote facies. Though it is named for the mineral hornblende, the appearance of that mineral is not constrained to this facies. The hornblende-hornfels facies has the following mineral assemblages:
In metabasites:
In metapelites:
In K2O-poor sediments or meta-igneous rocks:
In Si-rich dolostones:
The pyroxene-hornfels facies is the contact-metamorphic facies with the highest temperatures and is, like the granulite facies, characterized by the mineral orthopyroxene. It is characterized by the following mineral assemblages:
In metabasites:
In metapelites:
(If the temperature is below 750 °C there will be andalusite instead of sillimanite)
In carbonate rocks:
The sanidinite facies is a rare facies of extremely high temperatures and low pressure. It can only be reached under certain contact-metamorphic circumstances. Due to the high temperature the rock experiencespartial melting and glass is formed. This facies is named for the mineralsanidine. It is characterized by the following mineral assemblages:
In metapelites:
In carbonates:
Ecologites and blueschists are associated withsubduction zones. Granulites are associated with volcanic arcs.
