What is "layered intrusion"

n. (context geology English) a large body of igneous rock within which distinct stratification continues over many miles

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 km to over 50,000 km 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.