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Concept
Layered intrusion
Summary
A layered intrusion is a large sill-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. 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. Additionally, an often overlooked topic regarding economically significant Ni-Cu-PGE deposits can occur within the country rock spatially associated with the layered intrusion.
Mafic-ultramafic layered intrusions 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 or lopolith.
Official source
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