High Quality Content by WIKIPEDIA articles! 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 kilometers in areal extent covering areas from around 100 km2 to over 50,000 km2 and several hundred meters to over a kilometer in thickness. While most are Archean to Proterozoic in age, for example the Archean Bushveld complex, they may be any age such as the Cenozoic Skaergaard intrusion of east Greenland. 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.
Redaktor Frederic P. Miller
Redaktor Agnes F. Vandome
Redaktor John McBrewster
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