A layered intrusion is a large sill-like body of igneous rock which exhibits vertical layering or differences in composition and texture. These intrusions typically are many kilometres in area covering from around 100 km2 to over 50,000 km2 and several hundred metres to over a kilometre in thickness. While most are Archean to Proterozoic in age (for example, the Paleoproterozoic Bushveld complex) they may be any age such as the Cenozoic Skaergaard intrusion of east Greenland or the Rum layered intrusion in Scotland. Although most are ultramafic to mafic in composition, the Ilimaussaq complex of Greenland is an alkalic intrusion.
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 complex 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.
Intrusive behaviour and setting
Mafic-ultramafic layered intrusions occur at all levels within the crust, from depths in excess of 50 km to depths of as little as 1.5 to 5 km. 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.
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.
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.
The presence of large layered complexes in Greenland such as the Skaergaard intrusion which is not related to mantle plumes indicate other processes can form these intrusions. Here, the large magma volumes which are created by mid-ocean ridge spreading allow the accumulation of large volumes of cumulate rocks. The problem of creating space for the intrusion is easily explained by the extensional tectonics 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.
Plume magmatism is supported in some intrusions by geochemistry. 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.
It is also quite likely that what we see as a cratonic margin today may have been created by the action of a plume event initiating a continental rifting 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 can be 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 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.