A layered intrusion is a large sill-like body of igneous rock which exhibits horizontal layering or differences in composition and texture. These intrusions can be many kilometres in area covering from around 100 km2 (39 sq mi) to over 50,000 km2 (19,000 sq mi) and several hundred metres to over one kilometre (3,300 ft) in thickness. While most layered intrusions 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 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 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 Nickel-Copper-Platinum group elements (Ni-Cu-PGE), chromite, magnetite, and ilmenite are often associated with base metal Sulfide mineral assemblages within these rare intrusions. Often overlooked is that economically significant Ni-Cu-PGE deposits can occur in the country rock spatially associated with the layered intrusion.
Intrusive behaviour and setting
Mafic-ultramafic layered intrusions occur at all levels within the crust, from depths in excess of 50 km (160,000 ft) to depths of as little as 1.5–5 km (5,000–16,000 ft). 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. But 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 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 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 km or 43,000 ft). 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 in Greenland. 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 such intrusions 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. 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 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 cause is possible.
Causes of layering
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