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METAMORPHIC FACIES
Diagram showing metamorphic facies in pressure- temperature space. The domain of
thegraph corresponds to circumstances within the Earth's crust and upper mantle.
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Metamorphic facies 1-Eclogite 2-Blueschist 3-Greenschist 4-Prehnite- 5-Pumpellyite 6-Zeolite 7-Granulite 8-Amphibolite 9-Hornfels 10-Sanidinite
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Metamorphic facies
The metamorphic facies are groups of mineral compositions in metamorphic rocks, that are typical for a certain field in pressure-temperature space. Rocks which contain certain minerals can therefore be linked to certain tectonic settings.
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Historic definition
The metamorphic facies are groups of mineral compositions in metamorphic rocks, that are typical for a certain field in pressure-temperature space
The name facies was first used for specific sedimentary environments in sedimentary rocks by Swiss geologist Amanz Gressly in 1838.
Analogous with these sedimentary facies a number of metamorphic facies were proposed in 1920 by Finnish petrologist Pentti Eskola.
Eskola's classification was refined in the 1970s by New-Zealand geologist Francis John Turner.
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Underlying principles
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 become stable while others become unstable or metastable
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Index mineralsEvery metamorphic facies has some index minerals by
which it can be recognized.
Very typical index minerals are the polymorphs of aluminosilicate (Al2SiO5, all are nesosilicates).
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Metamorphic facies and their mineral assemblages
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Zeolite facies (LP/LT)
The zeolite facies is the metamorphic facies with the lowest metamorphic grade.
The facies is named for zeolites, strongly hydrated tectosilicates
It can have the following mineral assemblages:
heulandite + analcime + quartz ± clay minerals
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PREHNITE-PUMPELLYITE-FACIES (LP/LT)
The prehnite-pumpellyite facies is a little higher in pressure and temperature than the zeolite facies
. The prehnite-pumpellyite is characterized by the mineral assemblages
prehnite + pumpellyite + chlorite + albite + quartz
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GREENSCHIST FACIES (MP/MT)
The greenschist facies is at medium pressure and temperature.
The facies is named for the typical schistose texture of the rocks and green colour of the minerals chlorite, epidote and actinolite.
Characteristic mineral assemblages are:
chlorite + albite + epidote ± actinolite, quartz
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Amphibolite-facies (MP/MT-HT)
The amphibolite facies is a facies of medium pressure and average to high temperature.
It is named after amphiboles that form under such circumstances.
It has the following mineral assemblages:
hornblende + plagioclase ± epidote, garnet,
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Granulite facies (MP/HT)
The granulite 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 is orthopyroxene.
orthopyroxene + clinopyroxene + hornblende
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The blueschist facies is at relatively low temperature but high pressure, such as occurs in rocks in a subduction zone.
The facies is named after the schistose character of the rocks and the blue minerals glaucophane and lawsonite. mineral assemblages:
glaucophane + lawsonite + chlorite + sphene ± epidote
Blueschist facies (MP-HP/LT)
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Eclogite facies (HP/HT)
The eclogite facies is the facies at the highest pressure and high temperature.
It is named for the metabasic rock eclogite.
The mineral assemblages:
omphacite + garnet ± kyanite, quartz,
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ALBITE-EPIDOTE-HORNFELS FACIES (LP/LT-MT)
The albite-epidote-hornfels facies is a facies at low pressure and relatively low temperatures.
It is named for the two minerals albite and epidote, though they are stable in more facies.
Hornfels is a rock formed in contact metamorphism, a process that characteristically involves high temperatures but low pressures/depths.
albite + epidote + actinolite + chlorite + quartz
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Hornblende-hornfels facies (LP/MT)
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 hornblende-hornfels facies has the following mineral assemblages:
hornblende + plagioclase ± diopside,
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Pyroxene-hornfels facies (LP/MT-HT)
The pyroxene-hornfels facies is the contact-metamorphic facies with the highest temperatures and is, like the granulite facies
Characterized by the mineral orthopyroxene.
Mineral assemblages: orthopyroxene + clinopyroxene + plagioclase
± olivine or quartz
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Sanidinite facies (LP/HT)
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 experiences partial melting and glass is formed.
This facies is named for the mineral sanidine.
cordierite + mullite + sanidine + tridymite (often altered to quartz) + glass
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References
Eskola, Pentti Eelis, 1920: "The mineral facies of rocks"
Phillpots, Anthony R., 1990: Principles of Igneous and Metamorphic Petrology
Duff, P. McL. D., 1996; Holmes' Principles of Physical Geology
Visser, W.A., 1980; Geological NomenclatureMetamorphic facies by Dave Waters
