Ultramafic to mafic layered intrusions

Ultramafic to mafic layered intrusions are found in typically ancient cratons and are rare but worldwide in distribution. The intrusive complexes exhibit evidence of fractional crystallization and crystal segregation by settling or floating of minerals from a melt.

Ideally the stratigraphic sequence of an ultramafic-mafic intrusive complexe consists of ultramafic peridotites and pyroxenites with associated chromitite 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, magnetite and ilmenite are often associated with these rare intrusions.

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Intrusive behaviour and setting

Mafic-ultramafic layered intrusions occur at all levels within the crust, from depths in excess of 50km to depths of as little as 1.5 to 5km. 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 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 ultramaficmafic 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 and surficial manifestations of voluminous mafic magmatism within the same temporal period. For instance, in most Archaean cratons, greenstone belts correlate with voluminous dyke injection 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 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 13 kilometres). 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.

Plume magmatism is supported in some intrusions by geochemistry. In particular the Noril's-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

The presence of large layered complexes in Greenland such as the Skaergaard Intrusion which are not related to mantle plumes indicated other processes can form these intrusions. Here, the large magma volumes which are created by mid-ocean ridge spreading allow the accumlation of large volumes of cumulate rocks. Also, the problem of creating space for the intrusion is easily explained by the extensional tectonics in operation.

Causes of layering

The causes of layering in large ultramafic intrusions include convection, thermal diffusion, 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 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 enough and hot enough magma chambers, where vigorous convection currents form, pseudo-sedimentary structures such as flow banding, graded bedding, scour channels, foreset beds and other usually sedimentary features can be created by convection and settling processes. The Skaergaard Intrusion in Greenland is a prime example of these quasi-sedimentary structures.

Whilst fractional crystallisation is the dominant process, it can be triggered in the magma body by 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. Note also that 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.

Monominerallic cumulate layers are common. These may be economically important, for instance magnetite and ilmenite layers are known to form titanium, vanadium deposits such as at Windimurra intrusion and hard-rock iron deposits (such as at Savage River, Tasmania). Chromite 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 revervoir becomes too thin and flat.

Examples

• Bushveld igneous complex, South Africa
• Dufek intrusion, Antarctica
• Giles complex intrusions central Australia
• Great Dyke, Zimbabwe
• Kiglapait intrusion, Labrador
• Muskox intrusion, Canada
• Skaergaard intrusion of east Greenland
• Stillwater igneous complex, southwestern Montana
• Windimurra intrusion, West Australia

See also

• Definition of ultramafic
• List of rock types
• Igneous rocks
• Fractional crystallization

References

• Blatt, Harvey and Robert J. Tracy, 1996, Petrology: Igneous, Sedimentary and Metamorphic, 2nd ed., pp. 123-132 & 194-197, Freeman, ISBN 0-7167-2438-3
• Ballhaus, C.G. & Glikson, A.Y., 1995, Petrology of layered mafic-ultramafic intrusions of the Giles Complex, western Musgrave Block, central Australia. AGSO Journal, 16/1&2: 69-90.