Ultramafic to mafic layered intrusions
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A layered intrusion is a large
sill Sill may refer to: * Sill (dock), a weir at the low water mark retaining water within a dock * Sill (geology), a subhorizontal sheet intrusion of molten or solidified magma * Sill (geostatistics) * Sill (river), a river in Austria * Sill plate, a ...
-like body of igneous rock which exhibits vertical layering or differences in composition and texture. These intrusions can be many kilometres in area covering from around to over and several hundred metres to over in thickness.Blatt, Harvey and Tracy, Robert J. (1996) ''Petrology: Igneous, Sedimentary and Metamorphic,'' 2nd ed., pp. 123–132 & 194–197, Freeman, While most layered intrusions are Archean to
Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...
in age (for example, the Paleoproterozoic
Bushveld complex The Bushveld Igneous Complex (BIC) is the largest layered igneous intrusion within the Earth's crust. It has been tilted and eroded forming the outcrops around what appears to be the edge of a great geological basin: the Transvaal Basin. It i ...
), they may be any age such as the
Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...
Skaergaard intrusion The Skaergaard intrusion is a layered igneous intrusion in the Kangerlussuaq area, East Greenland. It comprises various rock types including gabbro, ferro diorite, anorthosite and granophyre. Discovered by Lawrence Wager in 1931 during the Br ...
of east Greenland or the
Rum layered intrusion The Rum layered intrusion is located in Scotland, on the island of Rùm ( Inner Hebrides). It is a mass of intrusive rock, of mafic-ultramafic composition, the remains of the eroded, solidified magma chamber of an extinct volcano that was active ...
in Scotland. Although most are ultramafic to mafic in composition, the Ilimaussaq intrusive complex of Greenland is an alkalic intrusion. Layered intrusions are typically found in ancient cratons and are rare but worldwide in distribution. The intrusive complexes exhibit evidence of
fractional crystallization Fractional crystallization may refer to: * Fractional crystallization (chemistry), a process to separate different solutes from a solution * Fractional crystallization (geology) Fractional crystallization, or crystal fractionation, is one of the ...
and crystal segregation by settling or floating of minerals from a melt. Ideally, the stratigraphic sequence of an ultramafic-mafic intrusive complex consists of ultramafic
peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high prop ...
s and pyroxenites with associated
chromitite Chromitite is an igneous cumulate rock composed mostly of the mineral chromite. It is found in layered intrusions such as the Bushveld Igneous Complex in South Africa, the Stillwater igneous complex in Montana and the Ring of Fire discovery in O ...
layers toward the base with more mafic norites, gabbros and anorthosites in the upper layers. Some include diorite, and granophyre near the top of the bodies. Orebodies of platinum group elements,
chromite Chromite is a crystalline mineral composed primarily of iron(II) oxide and chromium(III) oxide compounds. It can be represented by the chemical formula of FeCr2O4. It is an oxide mineral belonging to the spinel group. The element magnesium can s ...
, magnetite, and ilmenite are often associated with these rare intrusions.


Intrusive behaviour and setting

Mafic-ultramafic layered
intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
s occur at all levels within the crust, from depths in excess of to depths of as little as . The depth at which an intrusion is formed is dependent on several factors: * Density of the melt. Magmas with high magnesium and iron contents are denser and are therefore less likely to be able to reach the surface. * Interfaces within the crust. Typically, a horizontal detachment zone, a dense, impermeable layer or even a lithological interface may provide a horizontal plane of weakness which the ascending magma will exploit, forming a
sill Sill may refer to: * Sill (dock), a weir at the low water mark retaining water within a dock * Sill (geology), a subhorizontal sheet intrusion of molten or solidified magma * Sill (geostatistics) * Sill (river), a river in Austria * Sill plate, a ...
or lopolith. * Temperature and viscosity. As an ascending magma rises and cools, it becomes thicker and more viscous. This then restricts the magma from rising further because more energy is required to push it upwards. Conversely, thicker magma is also more efficient at forcing apart the wall rocks, creating volume which the magma may fill.


Intrusive mechanisms

It is difficult to precisely determine what causes large ultramafic – mafic intrusives to be emplaced within the crust, but there are two main hypotheses: ''plume magmatism'' and ''rift upwelling''.


Plume magmatism

The plume magmatism theory is based on observations that most large igneous provinces include both
hypabyssal A subvolcanic rock, also known as a hypabyssal rock, is an intrusive igneous rock that is emplaced at depths less than within the crust, and has intermediate grain size and often porphyritic texture between that of volcanic rocks and plutonic roc ...
and surficial manifestations of voluminous mafic magmatism within the same temporal period. For instance, in most Archaean cratons, greenstone belts correlate with voluminous dike injections as well as usually some form of larger intrusive episodes into the crust. This is particularly true of a series of ultramafic-mafic layered intrusions in the Yilgarn Craton of ~2.8 Ga and associated
komatiite Komatiite () is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava of at least 18 wt% MgO. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Komatiite wa ...
volcanism and widespread tholeiitic volcanism. Plume magmatism is an effective mechanism for explaining the large volumes of magmatism required to inflate an intrusion to several kilometres thickness (up to and greater than ). Plumes also tend to create warping of the crust, weaken it thermally so that it is easier to intrude magma and create space to host the intrusions. Geochemical evidence supports the hypothesis that some intrusions result from plume magmatism. In particular, the Noril'sk-Talnakh intrusions are considered to be created by plume magmatism, and other large intrusions have been suggested as created by mantle plumes. However, the story is not so simple, because most ultramafic-mafic layered intrusions also correlate with craton margins, perhaps because they are exhumed more efficiently in cratonic margins because of faulting and subsequent orogeny.


Rift magmatism

Some large layered complexes are not related to mantle plumes, for example, the
Skaergaard intrusion The Skaergaard intrusion is a layered igneous intrusion in the Kangerlussuaq area, East Greenland. It comprises various rock types including gabbro, ferro diorite, anorthosite and granophyre. Discovered by Lawrence Wager in 1931 during the Br ...
in Greenland. Here, the large magma volumes which are created by mid-ocean ridge spreading allow the accumulation of large volumes of
cumulate rock Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group o ...
s. The problem of creating space for such intrusions is easily explained by the
extensional tectonics Extensional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the stretching of a planetary body's crust or lithosphere. Deformation styles The types of structure and the geometries formed depend on ...
in operation; extensional or listric faults operating at depth can provide a triangular space for keel-shaped or boat-shaped intrusions such as the Great Dyke of Zimbabwe, or the Narndee-Windimurra Complex of Western Australia. It is also possible that what we see as a cratonic margin today were created by the action of a plume event initiating a
continental rift Continental may refer to: Places * Continent, the major landmasses of Earth * Continental, Arizona, a small community in Pima County, Arizona, US * Continental, Ohio, a small town in Putnam County, US Arts and entertainment * ''Continental'' (al ...
ing episode; therefore the tectonic setting of most large layered complexes must be carefully weighed in terms of geochemistry and the nature of the host sequence, and in some cases a mixed mechanism cause is possible.


Causes of layering

The causes of layering in large ultramafic intrusions include convection, thermal diffusion, settling of phenocrysts, assimilation of wall rocks and fractional crystallization. The primary mechanism for forming cumulate layers is, of course, the accumulation of layers of mineral crystals on the floor or roof of the intrusion. Rarely, plagioclase is found in
cumulate Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group o ...
layers at the top of intrusions, having floated to the top of a much denser magma. Here it can form anorthosite layers. Accumulation occurs as crystals are formed by fractional crystallisation and, if they are dense enough, precipitate out from the magma. In large, hot magma chambers having vigorous convection and settling, pseudo-sedimentary structures such as flow banding,
graded bedding In geology, a graded bed is one characterized by a systematic change in grain or clast size from one side of the bed to the other. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into pro ...
, scour channels, and foreset beds, can be created. The
Skaergaard intrusion The Skaergaard intrusion is a layered igneous intrusion in the Kangerlussuaq area, East Greenland. It comprises various rock types including gabbro, ferro diorite, anorthosite and granophyre. Discovered by Lawrence Wager in 1931 during the Br ...
in Greenland is a prime example of these quasi-sedimentary structures. Whilst the dominant process of layering is fractional crystallisation, layering can also result in a magma body through assimilation of the wall rocks. This will tend to increase the silica content of the melt, which will eventually prompt a mineral to reach the liquidus for that magma composition. Assimilation of wall rocks takes considerable thermal energy, so this process goes hand in hand with the natural cooling of the magma body. Often, assimilation can only be proven by detailed geochemistry. Often, cumulate layers are polyminerallic, forming gabbro, norite and other rock types. The terminology of cumulate rocks, however, is usually used to describe the individual layers as, for instance, ''pyroxene-plagioclase cumulates''. Monomineralic cumulate layers are common. These may be economically important, for instance magnetite and ilmenite layers are known to form titanium,
vanadium Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an oxide layer ( pas ...
deposits such as at Windimurra intrusion and hard-rock iron deposits (such as at
Savage River, Tasmania Savage River is a small Australian mining village located on the west coast of Tasmania. History Government surveyor Charles Sprent discovered Savage River's iron ore deposits in 1877, however the minerals were left untouched for nearly a cent ...
).
Chromite Chromite is a crystalline mineral composed primarily of iron(II) oxide and chromium(III) oxide compounds. It can be represented by the chemical formula of FeCr2O4. It is an oxide mineral belonging to the spinel group. The element magnesium can s ...
layers are associated with platinum- palladium group element ( PGE) deposits, the most famous of these being the Merensky Reef in the Bushveld Igneous Complex. The central section or upper sections of many large ultramafic intrusions are poorly layered, massive gabbro. This is because as the magma differentiates it reaches a composition favouring crystallisation of only two or three minerals; the magma may also have cooled by this stage sufficiently for the increasing viscosity of the magma to halt effective convection, or convection may stop or break up into inefficient small cells because the reservoir becomes too thin and flat. Crystal accumulation and layering can expel interstitial melt that migrates through the cumulate pile, reacting with it.


Examples

* Bushveld igneous complex, South Africa *
Dufek intrusion The Dufek Intrusion is a mafic layered intrusion that was emplaced into present-day Antarctica approximately 183 million years ago. It comprises two outcropping sections called the Dufek Massif and the Forrestal Range that are thought to be connecte ...
, Antarctica * Duluth Complex, northeastern Minnesota, United States *
Giles complex Giles may refer to: People * Giles (given name), male given name (Latin: ''Aegidius'') * Giles (surname), family name * Saint Giles (650–710), 7th–8th-century Christian hermit saint * Giles of Assisi, Aegidius of Assisi, 13th-century compa ...
intrusions, central
Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
* Great Dyke, Zimbabwe *
Kanichee layered intrusive complex The Kanichee layered intrusive complex, also called the Kanichee intrusion and Ajax intrusion, is a layered intrusion in Northeastern Ontario, Canada, located in the central portion of Strathy Township about northwest of the town of Temagami. It co ...
, Ontario, Canada * Kiglapait intrusion, Labrador, Canada *
Lac des Îles igneous complex The Lac des Îles igneous complex of northwestern Ontario, Canada is a layered gabbroic intrusion which is the host for the largest palladium orebody in Canada. The orebody is currently being mined as a combined open pit and underground operation ...
, Ontario, Canada *
Muskox intrusion The Muskox intrusion is a layered intrusion in Nunavut, Canada. It is located northeast of Great Bear Lake and south of Kugluktuk on Coronation Gulf. It was formed during a large magmatic event during the Proterozoic by hotspot or mantle plume ...
,
Northwest Territories The Northwest Territories (abbreviated ''NT'' or ''NWT''; french: Territoires du Nord-Ouest, formerly ''North-Western Territory'' and ''North-West Territories'' and namely shortened as ''Northwest Territory'') is a federal territory of Canada. ...
, Canada * Rum intrusion, Scotland *
Skaergaard intrusion The Skaergaard intrusion is a layered igneous intrusion in the Kangerlussuaq area, East Greenland. It comprises various rock types including gabbro, ferro diorite, anorthosite and granophyre. Discovered by Lawrence Wager in 1931 during the Br ...
of east Greenland * Stillwater igneous complex, southwestern Montana, United States *
Trompsburg complex Trompsburg is a town in the Free State province of South Africa off the N1 highway, the major road connection between Johannesburg and Cape Town. Background The town is 120 km south-west of Bloemfontein and 56 km north-east of Philip ...
, South Africa * Windimurra intrusion, Western Australia * Chimalpahad layered complex, Khammam, Telangana, India


See also

*
List of rock types The following is a list of rock types recognized by geologists. There is no agreed number of specific types of rocks. Any unique combination of chemical composition, mineralogy, grain size, texture, or other distinguishing characteristics can desc ...
*
Igneous differentiation In geology, igneous differentiation, or magmatic differentiation, is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement, or eruption. The sequence of (u ...


References

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External links


Overview of Finnish Layered Intrusions
Economic geology Igneous rocks Igneous petrology