Metamorphic facies

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The metamorphic facies is the classification of metamorphic rocks according to the minerals that can form under the pressure - temperature conditions prevailing during metamorphosis . Rocks with certain minerals can be assigned to special tectonic environments via these conditions .

history

The name Fazies was first coined by the Swiss geologist Amanz Gressly in 1838 for the totality of the deposition conditions during the formation of sedimentary rocks . According to these sedimentary facies, the Finnish petrologist Pentti Eskola defined a series of metamorphic facies in 1921 after several relevant publications between 1910 and 1920. Its classification was further refined in the 1970s by the New Zealand geologist Francis John Turner .

definition

A group of metamorphic rocks, the occurrence of which is characterized by typical mineral associations, which are crystallized with different protolith compositions under almost the same physical-chemical conditions, is called "metamorphic facies". “Almost the same physico-chemical conditions” are understood to mean, in particular, “almost the same thermodynamic conditions” (pressure, temperature, cooling rate), but also the same degree of solubility or the absence of reactive gases in the melt (if any) as well as the same metamorphosis type. (See below for details.)

Basic principles

The various metamorphic facies are defined by the mineralogical composition of a rock. If the pressure or temperature in a rock body changes, it can change to another metamorphic facies, as some mineral paragenesis (= socialization) become unstable or metastable and new minerals are formed. The conversion depends on various circumstances, such as the reaction kinetics , the activation energy of the reaction and the amount of fluids present in the rock.

The minerals in a metamorphic rock and their age relationships can be examined on thin sections of rock in the optical microscope or in the scanning electron microscope . In addition to the facies designation of a rock body, the English abbreviations for the prevailing pressure and temperature conditions have been used to describe a metamorphic rock or an entire terrane . Depending on the strength of these conditions, the abbreviations LT, MT, HT, LP, MP, HP and combinations thereof are used (composition of L ow , M edium or H igh with P ressure or T emperature ). Since the 1980s, the abbreviations UHP (additionally be U ltra H igh P ressure ) and UHT ( U ltra H igh t emperature ) for rocks used, the extreme pressure or temperature conditions were suspended.

Which minerals are formed under given pressure and temperature conditions depends on the original composition of the protolith, i.e. on the original rock before the metamorphosis. For example, have carbonate -Gesteine a different mineral composition than about Basalt - Lava and a Metapsammit different from a Metapelit . With the same metamorphic conditions, other minerals are formed.

The metamorphic facies described below can be further subdivided according to the occurring minerals. The resulting metamorphic subfacies are named like the supergroups after the characteristic minerals. For example, the green schist facies can be further subdivided into the quartz-albite-muscovite-chlorite sub-facies, the quartz-albite-epidote-biotite sub-facies or the quartz-albite-epidote-almandine sub-facies.

Mineral paragenesis (= mineral socialization)

In contrast to the concept of metamorphic zones (Barrow 1893), in which each zone is defined by characteristic minerals (index minerals), the concept of metamorphic facies is based on typical mineral parageneses, the occurrence of which determines the facies.

Examples of minerals are the index Al 2 SiO 5 - polymorphs . With a corresponding overall chemistry of the rock (Al- and Si-rich), depending on the pressure and temperature conditions, either andalusite, kyanite (thistle) or sillimanite are present. Andalusite is stable at low pressure, kyanite is stable at high pressure and relatively low temperature, and sillimanite is stable at high temperature.

In contrast to index minerals, mineral parageneses have smaller areas of stability. For example, the paragenesis jadeite (NaAlSi 2 O 6 ) + quartz shows a high pressure, while jadeite without quartz is stable even at low pressures. The exact definition of the metamorphic facies is made more difficult by the fact that the index minerals are not always visible to the naked eye, they can even be absent if the rock does not have the chemical-mineralogical composition necessary for their formation.

Metamorphic facies and their minerals

Metamorphic facies blanc.svg

eclogite -Fazies
Blue slate facies
Greenschist -
facies
not
realized
Pumpellyite -
facies
Zeolite facies
Granulite -
facies
Amphibolite -
facies
Hornfels facies
Sanidinite facies
T ( ° C )
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Diagram of the various metamorphic facies in the pressure - temperature space.
The diagram shows the conditions prevailing in the earth's crust and in the upper mantle .

Zeolite facies (LP / LT)

The zeolite facies is the metamorphic facies with the lowest degree of metamorphosis . It connects to the pressure and temperature field of diagenesis and is named after the group of zeolites , which is made up of strongly hydrated tectosilicates . The following minerals are characteristic of the zeolite facies:

In metamorphic igneous rocks and greywacke :

In metapelites :

Prehnite Pumpellyite Facies (LP / LT)

The prehnite-pumpellyite facies show slightly higher pressures and temperatures than the zeolite facies. It is named after the minerals prehnite (a Ca - Al - phyllosilicate ) and pumpellyite (a sorosilicate ). The following minerals are characteristic of the prehnite-pumpellyite facies:

In metamorphic igneous rocks and greywacke:

  • Prehnite + pumpellyite + chlorite + albite + quartz
  • Pumpellyite + chlorite + epidote + albite + quartz
  • Pumpellyite + epidote + stilpnomelane + muscovite + albite + quartz

In metapelites:

  • Muscovite + chlorite + albite + quartz

Greenschist facies (MP / MT)

The Greenschist facies are the facies of medium pressures and temperatures. Their name goes back to the typical slate texture of the rocks and the green color of the minerals chlorite , epidote and actinolite . The following minerals are characteristic of the green schist facies:

In meta bases :

  • Chlorite + albite + epidote ± actinolite, quartz, titanite

In Metagrauwacken:

  • Albite + quartz + epidote + muscovite ± stilpnomelane

In metapelites:

In the Si- rich Dolomites :

Amphibolite facies (MP / MT-HT)

The amphibolite facies are medium pressure and medium to high temperature facies. The name goes back to the mineral amphibole and the amphibolite rock , which is largely composed of it and which forms under these conditions. The following minerals are characteristic of the amphibolite facies:

In metabasites:

In metapelites:

In Si-rich Dolomites:

Granulite facies (MP / HT)

The granulite facies are the facies with the highest degree of metamorphosis in the field of medium pressures. The depth of their occurrence under the earth's surface is not constant. A characteristic mineral of these facies and of the subsequent pyroxene-hornblende facies is orthopyroxene . The following minerals are typical of the granulite facies:

In metabasites:

  • Orthopyroxene + clinopyroxene + hornblende + plagioclase ± biotite
  • Orthopyroxene + clinopyroxene + plagioclase ± quartz
  • Clinopyroxene + Plagioclase + Garnet ± Orthopyroxene (higher pressure)

In metapelites:

Blue schist facies (MP-HP / LT)

The blue schist facies show relatively low temperatures but high pressure. These conditions are mainly realized in subduction zones . The facies are named after the schisty character of the rock and the blue index minerals glaucophane and lawsonite . The following minerals are characteristic of the blue schist facies:

In metabasites:

In Metagrauwacken:

  • Quartz + jadeite + lawsonite ± phengite, glaucophane, chlorite

In metapelites:

  • Phengite + Paragonite + Carpholite + Chlorite + Quartz

In carbonate rocks ( marble ):

Eclogite facies (HP / HT / UHP)

The eclogite facies are the facies of the highest pressures and high temperatures (> 450 ° C). The name goes back to the rock eclogite , which is formed under such conditions. The following minerals are characteristic of the eclogite facies:

In metabasites:

  • Omphacite + garnet ± kyanite, quartz, hornblende, zoisite

In meta granodiorites :

  • Quartz + phengite + jadeite / omphacite + garnet

In metapelites:

  • Phengite + garnet + kyanite + chloritoid (Mg-rich) + quartz
  • Phengite + kyanite + talc + quartz ± jadeite

Ultra-high pressure metamorphosis is given when Coesite is present instead of quartz .

Albite-Epidote-Hornfels-Facies (LP / LT-MT)

The Albite-Epidote-Hornfels Facies are facies at low pressure and relatively low temperature. It is named after albite and epidote , although these minerals are also stable in other metamorphic facies. Hornfels is a rock of contact metamorphism , a process that is characteristically high in temperature at low pressure at shallow depths. The following minerals are characteristic of the albite-epidote-Hornfels facies:

In metabasites:

  • Albite + epidote + actinolite + chlorite + quartz

In metapelites:

  • Muscovite + biotite + chlorite + quartz

Hornblende-Hornfels facies (LP / MT)

The Hornblende-Hornfels facies have the same low pressures as the albite-epidote-Hornfels facies, but show slightly higher temperatures. Although the facies were named after the mineral hornblende, this is not limited to the Hornblende-Hornfels facies. The following minerals are characteristic of the Hornblende-Hornfels facies:

In metabasites:

  • Hornblende + plagioclase ± diopside, anthophyllite / cummingtonite, quartz

In metapelites:

  • Muscovite + biotite + andalusite + cordierite + quartz + plagioclase

In K 2 O-poor metasediments or metamorphic igneous rocks:

  • Cordierite + anthophyllite + biotite + plagioclase + quartz

In Si-rich Dolomites:

  • Dolomite + calcite + tremolite ± talc

Pyroxene Hornfels Facies (LP / MT-HT)

The Pyroxen-Hornfels Facies is also a facies of contact metamorphosis and shows the highest temperatures among the representatives of this type of metamorphosis together with the sanidinite facies. Like the granulite facies, it is characterized by the mineral orthopyroxene. The following minerals are characteristic of the pyroxene-Hornfels facies:

In metabasites:

  • Orthopyroxene + clinopyroxene + plagioclase ± olivine or quartz

In metapelites:

  • Cordierite + quartz + sillimanite + K-feldspar ( orthoclase ) ± biotite
  • Cordierite + orthopyroxene + plagioclase ± garnet, spinel

In carbonate rocks:

Sanidinite facies (LP / HT)

The sanidinite facies are rare facies of extremely high temperatures and low pressures that only occur under very special conditions during contact metamorphosis. Due to the high temperatures, the rock partially melts and glass forms . The facies are named after the mineral sanidin , which is characteristic of the rock named after it, sanidinite. The following minerals are characteristic of the sanidinite facies:

In metapelites:

In carbonates:

See also

literature

  • M. Okrusch, S. Matthes: Mineralogy. 8th, completely revised and updated edition. Springer-Verlag, Berlin et al. 2009, ISBN 978-3-540-78200-1 .
  • Anthony R. Phillpots: Principles of Igneous and Metamorphic Petrology. Prentice Hall, 1990, ISBN 0-13-691361-X .
  • P. McL. D. Duff: Holmes' Principles of Physical Geology. Cheltenham 1998, ISBN 0-7487-4381-2 .
  • W. A Visser: Geological Nomenclature. Nijhoff, 1980, ISBN 90-313-0407-7 .
  • Hans Murawski, Wilhelm Meyer: Geological dictionary . 11th edition. Elsevier / Spektrum, Heidelberg 2004, ISBN 3-8274-1445-8 , pp. 140 .

Web links

Remarks

  1. ↑ The prefix meta- denotes a transformed rock in petrology: a metapelite is a metamorphic pelite (claystone)