Metamorphism and metamorphic rocks

Introduction

The word metamorphism comes from Greek and means ‘change of form’. Thus, 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 of the rocks taking place. The changes that affect these rocks occur in the solid state.

The cratonic regions of all continents are made up almost exclusively of metamorphic rocks and most of the oldest rocks on Earth are therefore metamorphic. Fold mountain belts such as the Alps, Himalayas and Andes contain large amounts of metamorphic rocks which were deformed by folding, faulting and thrusting and intrusion of granitic magmas. The fact that most metamorphic rocks are deformed indicates that metamorphism and tectonism (e.g. deformation) occur together.

Within New South Wales, most metamorphic rocks occur west of the Sydney Basin, from Oberon outwards to Broken Hill. The Lachlan Fold Belt of New South Wales comprises many metamorphic sequences, along with numerous granitic intrusions. The Broken Hill region of far western New South Wales is composed almost exclusively of metamorphic rocks. The oldest regions in Australia (e.g. the Pilbara and Yilgarn cratons of Western Australia) are likewise composed almost exclusively of high temperature metamorphic rocks and metamorphosed granites.

Metamorphism is also strongly associated with many ore deposits. This is because metallic elements (such as lead, zinc, copper) are particularly mobile during metamorphism, especially when fluid is involved. This mobility is both physical (where the ore minerals actually flow) and chemical (where the original ore minerals breakdown into their components and then move in solution in the metamorphic fluid and precipitate elsewhere). Examples of this include the famous Broken Hill deposit of far western New South Wales.

 Metamorphic processes

 Metamorphism is an internal process of the Earth and occurs as a result of changes in temperature and/or pressure. This is because most minerals are only stable at particular temperatures and pressures, so changes in these result in the formation of new minerals (those stable at the new temperatures and pressures). Other than water, few other components (these include fluorine, boron) enter or leave during metamorphism. No overall chemical changes occur, the original chemical constituents are just re-adjusted during recrystallisation and the texture of the rock changes, usually becoming coarser-grained.

The important factors that produce metamorphic changes are

  • pressure deep within the crust
  • temperature
  • strain (shape and volume changes as a result of stress during deformation)
  • fluid activity (pressure due to fluids in pore spaces within the original rocks)

All of these factors vary widely and result in different types of metamorphism depending upon which factor is dominant. The three main types of metamorphism are:

  • Contact (also known as thermal) metamorphism: this is produced by high temperature, low pressure, low strain and variable fluid pressure. It is normally produced by igneous rocks such as granites intruding into older colder rock sequences within a few kilometres of the Earth’s surface.
  • Regional metamorphism: this is produced by variable temperature, high pressure, variable strain and variable fluid flow and is normally created by tectonic processes within the Earth at variable depths.
  • Dynamic metamorphism: this is produced by variable strain, variable pressure, variable temperature and high fluid pressure and normally occurs in active fault zones.

Contact and dynamic metamorphism are usually restricted to localised areas whereas regional metamorphism affects large areas of the crust, sometimes over tens of thousands of square kilometres. Contact metamorphism occurs as zones a few hundred metres wide around large igneous intrusions while dynamic metamorphism is restricted to fault and thrust zones only a few tens of metres thick. All three types of metamorphism can overlap.

 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:

  • 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.
  • 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.
  • 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.
  • 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.
  • Magnesian: formed from peridotites, serpentinites and impure dolomites. Talc, olivine, chlorite, tremolite and brucite are the most common minerals produced.

Metamorphic grades, zones and facies

The term metamorphic grade is used to give a relative measure of the intensity of metamorphism in a particular area. A high grade rock is one that has been formed at relatively high temperature and/or pressure and a low grade rock at relatively low temperature and/or pressure. Many low-grade metamorphic rocks that were originally derived from mudstones and shales (initially wet, fine-grained sediments) contain hydrated minerals such as micas and chlorites. With increasing metamorphic grade, water is driven-off and all minerals that were initially hydrated become anhydrous phases. All high grade rocks are composed solely of anhydrous minerals.

A progressive metamorphic sequence in an area or region may be subdivided in the field into metamorphic zones with each zone representing a different metamorphic grade. A metamorphic zone is characterised by the appearance of a distinctive index mineral.

Index minerals are those that are stable under the temperature/pressure (often referred to as P-T) conditions of a particular metamorphic grade. For example, in low-grade rocks, the mineral chlorite is the index mineral for the chlorite zone. With increasing metamorphic grade, biotite starts to form and is the index mineral for the biotite zone. Lines on a map that separate different metamorphic zones are termed isograds, meaning lines of equal grade. However, there are exceptions as metamorphic zones are characterised by particular metamorphic minerals and this is dependent on the original protolith. For example, when a quartzose sandstone (essentially composed of 100% quartz) undergoes metamorphism, no new minerals can form and the quartz grains just recrystallise. This also applies to pure limestones, where the calcite just recrystallises into coarser grains and the rock becomes a marble.

The term metamorphic facies is defined as a set of metamorphic mineral assemblages where there is a constant relationship between the mineral assemblage and rock composition. The term is a mineralogical one, incorporating several mineral assemblages or rock types formed under the same broad P-T conditions. That is, the rocks of the same chemical composition have the same mineral assemblage if they belong to the same metamorphic facies. This concept is used to give a broad classification of the P-T conditions of metamorphism.

Contact metamorphism

At shallow depths within the crust (usually less than 6 km) the heat sources responsible for contact metamorphism are bodies of hot magma (e.g. igneous intrusions) which raise the temperature of the surrounding rocks. These thermal affects are usually restricted to the contact zones of the intrusions, hence the term contact metamorphism. However, sometimes hot fluids are released from the intrusions and penetrate the enclosing rocks along fractures and produce contact metamorphic zones. Determining factors governing the extent of contact metamorphism are the size of the intrusion and its temperature. Basic magmas are much hotter than acid magmas and hence will have a greater thermal effect. Also, a large intrusion contains much more heat than a small dyke-like body and its effect on the surrounding country rocks will be much greater and more widespread.

Country rocks surrounding large, hot bodies of magma are heated, initiating mineral reactions and forming new minerals. Rocks adjacent to thin dykes and sills are simply baked and hardened and do not experience any great mineralogical and/or textural changes. Large plutons give rise to contact aureole zones within which the country rocks are thermally metamorphosed, with those closest to the plutons experiencing more heat than those further away (hence they have a higher metamorphic grade). As large plutons take millions of years to cool down, the surrounding country rocks also stay hot for tens of thousands of years allowing chemical reactions to continue to completion.

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 and thus, 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

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

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.

The metamorphic facies produced by contact metamorphism in order of increasing grade are as follows:

  • Albite epidote hornfels
  • Hornblende hornfels
  • Pyroxene hornfels
  • Sanidinite

Dynamic metamorphism

Dynamic metamorphic rocks are restricted to narrow zones adjacent to faults or thrusts. The high shear stresses associated with faults and thrusts crush the adjacent rocks. The rise in temperature is produced by frictional heat generated within the fault zone. The high shear stresses may be short-lived or long-lived depending on the activity of the fault or thrust. Dynamic metamorphism involves high shear stress, high pressure, high strain, high fluid partial pressure and variable temperature. In many instances, water plays a fundamental role.

Crushed rocks in fault zones are known as fault breccias which consist of angular fragments of the country rock in a matrix of crushed or powdered rock, cemented by quartz and/or calcite. Fluids move easily along fault zones between grain boundaries and through cracks and fissures. These fluids are able to transport large amounts of silica, carbonates and other minerals in solution.

Pseudotachylite is a fault-zone rock which is black and glassy. It usually occurs as narrow dykes and veins and forms by frictional melting of the country rock. Mylonites are partially recrystallised rocks with a pronounced foliation that are produced by intense shearing during large-scale movements along faults and thrusts. The different rock types produced by dynamic metamorphism vary with depth from the surface as, with increasing depth, both the surrounding pressure and temperature increase.

Regional metamorphism

Most metamorphic rocks occur in fold mountain belts or cratonic areas. Such rocks cover large areas of the Earth’s crust and are therefore termed regional metamorphic rocks. They arise by the combined action of heat, burial pressure, differential stress, strain and fluids on pre-existing rocks. The resulting rocks are always deformed (as a result of the differential stress) and commonly exhibit folds, fractures and cleavages. Large amounts of granitic intrusions are also associated with regional metamorphic rocks. The most common regional metamorphic rocks are slates, schists and gneisses. Regional metamorphism covers a wide range of temperature and pressure conditions from 200° C – 750° C and 2 kbar – 10 kbar (or 5 km – 35 km depth).

There are three metamorphic facies within regional metamorphosed rocks, which from lowest to highest grade are:
  • Greenschist: can be further divided into chlorite and biotite zones. The term greenschist gets its name from the rocks themselves as many rocks of this facies are grey-green in colour and have a schistose (parallel arrangement of platy minerals) texture.
  • Amphibolite: can be further divided into the garnet and staurolite zones. The term amphibolite gets its name from the most common constituent minerals of this facies, minerals of the amphibole group.
  • Granulite: can be further divided into the kyanite and sillimanite zones. The term granulite reflects the most common texture of these rocks – granular.

High Pressure regional metamorphism

In some parts of the world, geologically young (Cenozoic and Mesozoic) fold mountain belts contain sequences of metamorphosed fine-grained sedimentary rocks and basic volcanic rocks that contain unusual blue amphiboles. These rocks are commonly schistose, may have a characteristic blue colour, and are termed blueschists. These form at low temperature but high pressure conditions in the collision zones of subducting slabs.

When subducting oceanic slabs are dragged down to depths exceeding 50 kilometres, the basalt is metamorphosed at very high pressures to form a dense rock with the same bulk chemical composition but different mineralogy (dominantly pyroxene and garnet) and texture. These rocks are called eclogites.

Retrogressive metamorphism

Many metamorphic rocks contain evidence of retrograde mineral changes, that is, alteration of higher grade minerals into lower grade ones. Many of these changes involve hydration and are the result of a decrease in temperature and increase in the activity of water. Retrograde metamorphism is normally produced by repeated regional metamorphism where a lower grade episode is superimposed on a higher grade one. Most retrogressive events are probably just a consequence of the metamorphic system cooling down after peak metamorphism has been reached (i.e. the system has to cool down with time and as the region undergoes uplift with time, both pressure and temperature are dramatically reduced). The secondary minerals produced during retrogressive metamorphism generally occur as fibrous fringes around, inclusions within, and pseudomorphous grains after, the higher grade metamorphic minerals. A good example of retrogressive metamorphism is the occurrence of serpentinites. These form by generally low temperature hydration of ultramafic rock (containing minerals composed chiefly of magnesium and iron), commonly at subduction zones.

Metamorphic belts

Regional metamorphism occurs over wide areas of the Earth’s crust. The most common metamorphic sequences in relatively young rocks (e.g. younger than 450 Ma (million years old)) occur in fold mountain belts which are produced by tectonic processes associated with the development of these belts. We call such regions metamorphic belts. Within these sequences, the higher grade regional metamorphic rocks generally occur within the lower crust regions and the lower grade ones occur in the upper crust. Older cratonic regions (also termed shields) also contain numerous regional metamorphic sequences. Both older and younger regions contain abundant exposures of granitic rocks whose formation is strongly linked to the metamorphism.

The geology of eastern Australia is dominated by a number of these fold mountain belts. The largest are the Lachlan and New England fold belts. Both of these fold belts contain relatively low-grade regional metamorphic rocks, along with numerous granitic intrusions. The New England Fold Belt contains small amounts of blueschists and eclogites that formed in the collision zones of subducting slabs. These fold belts have formed over hundreds of millions of years by plate tectonic processes.

The Al2SiO5 polymorphs

These are the minerals andalusite, kyanite and sillimanite which occur in metamorphosed pelitic rocks but have different stability fields with respect to temperature and pressure. Because of this, these minerals are used to determine the grade and pressure of the metamorphic rocks that they occur in.

Identification of metamorphic rocks

The first thing that we look at in identifying metamorphic rocks is their texture. A first subdivision can be made between:
  • non-foliated (or massive) rocks. These are termed:
    • hornfels
    • marble
    • quartzite
    • skarn
    • granulite
  • foliated (a planar penetrative feature). These are termed:
    • slate (those with a cleavage)
    • phyllite (those with a cleavage and micaceous sheen)
    • schist (parallel arrangement of platy 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 mm to a cm)

The second thing that we look at to identify a metamorphic rock is its mineralogy. After this has been determined, to name the rock we simply list the mineralogy (in decreasing order of abundance) and then give the rock a general textural suffix. For example:

  • A rock composed of 60% muscovite and 40% biotite, both occurring as interlocking platy grains (i.e. a schistose 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.

The Rock Cycle PDF (30 kb) GIF (31 kb)

source:http://www.austmus.gov.au

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About Rashid Faridi

I am Rashid Aziz Faridi ,Writer, Teacher and a Voracious Reader.
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