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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.〔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〕 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.〔〔Hamilton MA, Pearson DG, Thompson RN, Kelly SP, Emeleus CH Rapid eruption of Skye lavas inferred from precise U-Pb and Ar–Ar dating of the Rum and Cuillin plutonic complexes. Nature 394:260–263, 1998〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「layered intrusion」の詳細全文を読む スポンサード リンク
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