Metamorphic rocks

Metamorphic rocks are pre-existing rocks whose mineralogy and/or texture has been changed by processes within the Earth. Metamorphic rocks form because of changes in temperature and depth of burial within the Earth without actual melting. This means that the changes that affect these rocks occur in the solid state.

Texture

The first thing that identifies metamorphic rocks is their texture. They can be divided into:

non-foliated (or massive) rocks. These are called:

  • hornfels
  • marble
  • quartzite
  • skarn
  • granulite

and

foliated (leaf-like layers) rocks. These are called:

  • slate (having a cleavage (split along a plane))
  • phyllite (having a cleavage and micaceous sheen)
  • schist (having a parallel arrangement of platy (flat thin planes) minerals)
  • knotted schist (similar to schist but with distinctive large non-platy minerals surrounded by the platy minerals)
  • gneiss (alternating layers of different composition, usually on the scale of a few millimetres to a centimetre)

Mineralogy

The second thing that helps to identify a metamorphic rock is its mineralogy - the minerals that make it up.

Naming

To name a metamorphic rock, the mineralogy is listed (in decreasing order of abundance) and then the rock is given a suffix relating to its general texture. For example:

  • A rock composed of 60% muscovite and 40% biotite, both occurring as interlocking platy grains - a schistose (schist-like; schists are a group of medium-grade metamorphic rocks) texture - is called a muscovite-biotite schist.
  • A massive rock composed of 50% grossular garnet, 40% epidote and 10% wollastonite is called a grossular-epidote-wollastonite skarn (skarns are metamorphically altered limestones).

The Al2SiO5 polymorphs

The minerals andalusite, kyanite and sillimanite are collectively known as the Al2SiO5 polymorphs. They occur in metamorphosed rocks but have different stability fields with respect to temperature and pressure. These minerals are used to determine the grade and pressure of the metamorphic rocks that they occur in.

Compositional groups

Metamorphic rocks can also be classified according to the chemical composition of the original source rock (protolith). Using this system, there are five main groupings:

  1. Pelitic: formed from aluminous sedimentary rocks, commonly shales and mudstones. These contain abundant micas. Common metamorphic minerals in these rocks include andalusite, kyanite, sillimanite, cordierite and staurolite.
  2. Quartzo-feldspathic: formed from quartz-rich sandstones or felsic igneous rocks. These are characterised by high silicon and low iron and magnesium. They usually contain quartz, feldspars and micas - much like their original protoliths.
  3. Calcareous: formed from limestones and dolomites. These are composed of a large variety of calc-silicate minerals such as grossular garnet, wollastonite, tremolite, forsterite and calcite.
  4. Basic: formed from basaltic and andesitic igneous rocks, and impure marly sediments with significant calcium, aluminium, magnesium and iron. Chlorite, actinolite and epidote are common at low grades whereas hornblende, diopside, Ca-plagioclase and almandine garnet are common at medium to high grades.
  5. Magnesian: formed from peridotites, serpentinites and impure dolomites. Talc, olivine, chlorite, tremolite and brucite are the most common minerals produced.

Pelitic rocks

As pelitic rocks (e.g. shales and mudstones) contain many different minerals and elements, many new minerals can form when they are metamorphosed. New minerals grow at progressively higher temperatures, so pelitic rocks are the most useful in determining the metamorphic zones in contact metamorphic assemblages:

  • 'Spotted rocks' are common in the outer-most metamorphic zone around intrusions and resemble the surrounding country rocks except for the presence of patches of iron oxide and/or graphite.
  • In the middle zone, porphyroblasts (that is, coarse-grained crystals) of the metamorphic minerals andalusite and cordierite appear.
  • The inner or hornfels zone is composed of a hard, splintery fine to medium-grained rock with the constituent minerals forming an interlocking mosaic which is termed 'granoblastic'.

Common metamorphic minerals produced from pelitic rocks include:

  • almandine garnet
  • andalusite
  • biotite
  • chlorite
  • chloritoid
  • cordierite
  • corundum
  • microcline
  • muscovite
  • sillimanite
  • spinel

Carbonate rocks

For carbonate rocks, pure limestones simply undergo recrystallisation of the constituent calcite forming marble. Impure carbonate rocks contain many more elements and therefore, many new metamorphic minerals can form. Metamorphosed impure carbonate rocks are commonly termed skarns. With impure carbonate rocks, the carbon dioxide produced by the breakdown of calcite is removed from the system and calc-silicate rocks are formed. In some cases, the metamorphic fluid contains other volatiles such as boron and fluorine, resulting in the formation of different mineral assemblages.

Common metamorphic minerals produced from carbonate rocks include:

  • diopside
  • epidote
  • grossular garnet
  • hornblende
  • scapolite
  • tremolite
  • vesuvianite
  • wollastonite

Basic rocks

In basic rocks, initially chlorite, albite and epidote are produced at relatively low temperatures. With increasing temperature, amphiboles and then pyroxenes form, sometimes with olivine and spinel.


Ondine Evans , Web Researcher/Editor
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