Gneiss

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Gneiss
Metamorphic rock
Gneiss.jpg
Sample of gneiss exhibiting "gneissic banding".

Gneiss ( /ˈns/ ) is a common and widely distributed type of metamorphic rock. Gneiss is formed by high temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Orthogneiss is gneiss derived from igneous rock (such as granite). Paragneiss is gneiss derived from sedimentary rock (such as sandstone). Gneiss forms at higher temperatures and pressures than schist. Gneiss nearly always shows a banded texture characterized by alternating darker and lighter colored bands and without a distinct foliation.

Contents

Etymology

The word gneiss has been used in English since at least 1757. It is borrowed from the German word Gneis, formerly also spelled Gneiss, which is probably derived from the Middle High German noun gneist "spark" (so called because the rock glitters). [1]

Formation

Gneiss is formed from sedimentary or igneous rock exposed to temperatures greater than 320 °C and relatively high pressure.

Composition

Gneissic rocks are usually medium- to coarse-foliated; they are largely recrystallized but do not carry large quantities of micas, chlorite or other platy minerals. Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, etc. Gneiss rocks may also be named after a characteristic component such as garnet gneiss, biotite gneiss, albite gneiss, etc. Orthogneiss designates a gneiss derived from an igneous rock, and paragneiss is one from a sedimentary rock.

Orthogneiss from the Czech Republic Orthogneiss Geopark.jpg
Orthogneiss from the Czech Republic

Gneissose rocks have properties similar to gneiss.

Gneissic banding

Gneiss appears to be striped in bands like parallel lines in shape, called gneissic banding. [2] The banding is developed under high temperature and pressure conditions.

The minerals are arranged into layers that appear as bands in cross section. [2] The appearance of layers, called 'compositional banding', occurs because the layers, or bands, are of different composition. The darker bands have relatively more mafic minerals (those containing more magnesium and iron). The lighter bands contain relatively more felsic minerals (silicate minerals, containing more of the lighter elements, such as silicon, oxygen, aluminium, sodium, and potassium).

A common cause of the banding is the subjection of the protolith (the original rock material that undergoes metamorphism) to extreme shearing force, a sliding force similar to the pushing of the top of a deck of cards in one direction, and the bottom of the deck in the other direction. [2] These forces stretch out the rock like a plastic, and the original material is spread out into sheets.

Some banding is formed from original rock material (protolith) that is subjected to extreme temperature and pressure and is composed of alternating layers of sandstone (lighter) and shale (darker), which is metamorphosed into bands of quartzite and mica. [2]

Another cause of banding is "metamorphic differentiation", which separates different materials into different layers through chemical reactions, a process not fully understood. [2]

Not all gneiss rocks have detectable banding. In kyanite gneiss, crystals of kyanite appear as random clumps in what is mainly a plagioclase (albite) matrix.

Types

Augen gneiss

Henderson augen gneiss Henderson Augen Gneiss.jpg
Henderson augen gneiss

Augen gneiss, from the German : Augen [ˈaʊɡən] , meaning "eyes", is a coarse-grained gneiss resulting from metamorphism of granite, which contains characteristic elliptic or lenticular shear-bound feldspar porphyroclasts, normally microcline, within the layering of the quartz, biotite and magnetite bands.

Henderson gneiss

Henderson gneiss is found in North Carolina and South Carolina, US, east of the Brevard Shear Zone. It has deformed into two sequential forms. The second, more warped, form is associated with the Brevard Fault, and the first deformation results from displacement to the southwest. [3]

Lewisian gneiss

Dark dikes (now foliated amphibolites) cutting light grey Lewisian gneiss of the Scourie complex, both deformed and cut by later (unfoliated) pink granite dikes Road Cutting - geograph.org.uk - 820828.jpg
Dark dikes (now foliated amphibolites) cutting light grey Lewisian gneiss of the Scourie complex, both deformed and cut by later (unfoliated) pink granite dikes
Contact between a dark-colored diabase dike (about 1100 million years old) and light-colored migmatitic paragneiss in the Kosterhavet National Park in the Koster Islands off the western coast of Sweden. Yttre Ursholmen Kontakt Kosterdiabas i Nebulitisk-migmatitisk sedimentgnejs.jpg
Contact between a dark-colored diabase dike (about 1100 million years old) and light-colored migmatitic paragneiss in the Kosterhavet National Park in the Koster Islands off the western coast of Sweden.

Most of the Outer Hebrides of Scotland have a bedrock formed from Lewisian gneiss. In addition to the Outer Hebrides, they form basement deposits on the Scottish mainland west of the Moine Thrust and on the islands of Coll and Tiree. [5] These rocks are largely igneous in origin, mixed with metamorphosed marble, quartzite and mica schist with later intrusions of basaltic dikes and granite magma. [6]

Sample of Sete Voltas gneiss from Bahia in Brazil, the oldest rock outcropping in the crust of South America, c. 3.4 billion years old (Archean) Gnaisse Sete Voltas - Bahia - Brasil.jpg
Sample of Sete Voltas gneiss from Bahia in Brazil, the oldest rock outcropping in the crust of South America, c. 3.4 billion years old (Archean)

Archean and Proterozoic gneiss

Gneisses of Archean and Proterozoic age occur in the Baltic Shield.

See also

Related Research Articles

Schist Medium grade metamorphic rock with lamellar grain

Schist is a medium-grade metamorphic rock formed from mudstone or shale. Schist has medium to large, flat, sheet-like grains in a preferred orientation. It is defined by having more than 50% platy and elongated minerals, often finely interleaved with quartz and feldspar. These lamellar minerals include micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced. Schist is often garnetiferous. Schist forms at a higher temperature and has larger grains than phyllite. Geological foliation with medium to large grained flakes in a preferred sheetlike orientation is called schistosity.

Metamorphic rock Rock which was subjected to heat and pressure causing profound physical or chemical change

Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". The original rock (protolith) is subjected to heat and pressure, causing profound physical or chemical change. The protolith may be a sedimentary, igneous, or existing metamorphic rock.

Rock (geology) Naturally occurring mineral aggregate

A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its chemical composition and the way in which it is formed. Rocks are usually grouped into three main groups: igneous rocks, metamorphic rocks and sedimentary rocks. Rocks form the Earth's outer solid layer, the crust.

Metamorphism The change of minerals in pre-existing rocks without melting into liquid magma

Metamorphism is the change of minerals or geologic texture in pre-existing rocks (protoliths), without the protolith melting into liquid magma. The change occurs primarily due to heat, pressure, and the introduction of chemically active fluids. The chemical components and crystal structures of the minerals making up the rock may change even though the rock remains a solid. Changes at or just beneath Earth's surface due to weathering or diagenesis are not classified as metamorphism. Metamorphism typically occurs between diagenesis, and melting (~850°C).

Migmatite A mixture of metamorphic rock and igneous rock

Migmatite is a composite rock found in medium and high-grade metamorphic environments. It consists of two or more constituents often layered repetitively; one layer was formerly paleosome, a metamorphic rock that was reconstituted subsequently by partial melting; the alternate layer has a pegmatitic, aplitic, granitic or generally plutonic appearance. Commonly, migmatites occur below deformed metamorphic rocks that represent the base of eroded mountain chains, commonly within Precambrian cratonic blocks,

Amphibolite A metamorphic rock containing mainly amphibole and plagioclase

Amphibolite is a metamorphic rock that contains amphibole, especially the species hornblende and actinolite, as well as plagioclase.

Lithology science of rocks

The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock. Lithology is the basis of subdividing rock sequences into individual lithostratigraphic units for the purposes of mapping and correlation between areas. In certain applications, such as site investigations, lithology is described using a standard terminology such as in the European geotechnical standard Eurocode 7.

Granulite A class of high-grade medium to coarse grained metamorphic rocks

Granulites are a class of high-grade metamorphic rocks of the granulite facies that have experienced high-temperature and moderate-pressure metamorphism. They are medium to coarse–grained and mainly composed of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure. They are of particular interest to geologists because many granulites represent samples of the deep continental crust. Some granulites experienced decompression from deep in the Earth to shallower crustal levels at high temperature; others cooled while remaining at depth in the Earth.

Blueschist A metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition

Blueschist, also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures, approximately corresponding to a depth of 15 to 30 kilometers. The blue color of the rock comes from the presence of the predominant minerals glaucophane and lawsonite.

Charnockite A type of granite containing orthopyroxene

Charnockite is applied to any orthopyroxene-bearing quartz-feldspar rock, formed at high temperature and pressure, commonly found in granulite facies metamorphic regions, as an end-member of the charnockite series.

Greenschist metamorphic rock formed in temperatures of 300–450 °C and pressures of 2–10 hPa, with an abundance of green minerals (e.g. chlorite, serpentine, epidote) and platy minerals (muscovite, serpentine)

Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically 300–450 °C (570–840 °F) and 2–10 kilobars (14,500–58,000 psi). Greenschists commonly have an abundance of green minerals such as chlorite, serpentine, and epidote, and platy minerals such as muscovite and platy serpentine. The platiness causes the tendency to split, or have schistosity. Other common minerals include quartz, orthoclase, talc, carbonate minerals and amphibole (actinolite).

Narryer Gneiss Terrane A geological complex of ancient rocks in Western Australia

The Narryer Gneiss Terrane is a geological complex in Western Australia that is composed of a tectonically interleaved and polydeformed mixture of granite, mafic intrusions and metasedimentary rocks in excess of 3.3 billion years old, with the majority of the Narryer Gneiss Terrane in excess of 3.6 billion years old. The rocks have experienced multiple metamorphic events at amphibolite or granulite conditions, resulting in often complete destruction of original igneous or sedimentary (protolith) textures. Importantly, it contains the oldest known samples of the Earth's crust: samples of zircon from the Jack Hills portion of the Narryer Gneiss have been radiometrically dated at 4.4 billion years old, although the majority of zircon crystals are about 3.6-3.8 billion years old.

Rock microstructure includes the texture of a rock and the small scale rock structures. The words "texture" and "microstructure" are interchangeable, with the latter preferred in modern geological literature. However, texture is still acceptable because it is a useful means of identifying the origin of rocks, how they formed, and their appearance.

Foliation (geology) repetitive layering in metamorphic rocks

Foliation in geology refers to repetitive layering in metamorphic rocks. Each layer can be as thin as a sheet of paper, or over a meter in thickness. The word comes from the Latin folium, meaning "leaf", and refers to the sheet-like planar structure. It is caused by shearing forces, or differential pressure. The layers form parallel to the direction of the shear, or perpendicular to the direction of higher pressure. Nonfoliated metamorphic rocks are typically formed in the absence of significant differential pressure or shear. Foliation is common in rocks affected by the regional metamorphic compression typical of areas of mountain belt formation.

Gascoyne Complex A terrane of Proterozoic granite and metamorphic rock in Western Australia

The Gascoyne Complex is a terrane of Proterozoic granite and metamorphic rock in the central-western part of Western Australia. The complex outcrops at the exposed western end of the Capricorn Orogen, a 1,000 km-long arcuate belt of folded, faulted and metamorphosed rocks between two Archean cratons; the Pilbara craton to the north and the Yilgarn craton to the south. The Gascoyne Complex is thought to record the collision of these two different Archean continental fragments during the Capricorn Orogeny at 1830–1780 Ma.

An index mineral is used in geology to determine the degree of metamorphism a rock has experienced. Depending on the original composition of and the pressure and temperature experienced by the protolith, chemical reactions between minerals in the solid state produce new minerals. When an index mineral is found in a metamorphosed rock, it indicates the minimum pressure and temperature the protolith must have achieved in order for that mineral to form. The higher the pressure and temperature in which the rock formed, the higher the grade of the rock.

Metamorphic facies

A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures. The assemblage is typical of what is formed in conditions corresponding to an area on the two dimensional graph of temperature vs. pressure. Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in the geological history of the area. The boundaries between facies are wide because they are gradational and approximate. The area on the graph corresponding to rock formation at the lowest values of temperature and pressure is the range of formation of sedimentary rocks, as opposed to metamorphic rocks, in a process called diagenesis.

The Thiviers-Payzac Unit is a metasedimentary succession of late Neoproterozoic and Cambrian age outcropping in the southern Limousin in France. The unit geologically forms part of the Variscan basement of the northwestern Massif Central.

Geology of North Macedonia

The geology of North Macedonia includes the study of rocks dating to the Precambrian and a wide array of volcanic, sedimentary and metamorphic rocks formed in the last 541 million years.

References

Citations

  1. Harper, Online Etym. Dict., "gneiss"
  2. 1 2 3 4 5 Marshak 2013 , pp. 194–95; Figs. 7.6a–c
  3. Sacks & Secor (1990).
  4. Bjørn Hageskov (1985): Constrictional deformation of the Koster dyke swarm in a ductile sinistral shear zone, Koster islands, SW Sweden. Bulletin of the Geological Society of Denmark 34(3–4): 151–97
  5. Gillen (2003), p. 44.
  6. McKirdy et al. (2007), p. 95.

Bibliography