Metamorphism

The word metamorphism comes from Greek and means 'change of form'. Metamorphic rocks are pre-existing rocks whose mineral composition and/or texture has been changed by processes within the Earth.

Metamorphism around the world

The cratonic regions (old and stable parts) of all continents are made up almost exclusively of metamorphic rocks meaning that most of the oldest rocks on Earth are 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.

Metamorphic regions in Australia

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 similarly composed almost exclusively of high temperature metamorphic rocks and metamorphosed granites.

Metamorphism and ore deposits

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 grades, zones and facies

Grades

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.

Zones

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

Index minerals are those that are stable under the pressure and temperature (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.

Facies

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 (pressure and temperature) 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.


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