Anorthosite (/əˈnɔːrθəsaɪt/) is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%).

[1] The areal extent of anorthosite batholiths ranges from relatively small (dozens or hundreds of square kilometers) to nearly 20,000 km2 (7,700 sq mi), in the instance of the Nain Plutonic Suite or Mistastin crater[6] in northern Labrador, Canada.

Mapped onto the Pangaean continental configuration of that eon, these occurrences are all contained in a single straight belt, and must all have been emplaced intracratonally.

Many Proterozoic anorthosites occur in spatial association with other highly distinctive, contemporaneous rock types: the so-called 'anorthosite suite' or 'anorthosite-mangerite-charnockite-granite (AMCG) complex'.

Individual plagioclase crystals may be black, white, blue, or grey, and may exhibit an iridescence known as labradorescence on fresh surfaces.

Mineralogically, labradorite is a compositional term for any calcium-rich plagioclase feldspar containing 50–70 molecular percent anorthite (An 50–70), regardless of whether it shows labradorescence.

Most anorthosite plutons are very coarse grained; that is, the individual plagioclase crystals and the accompanying mafic mineral are more than a few centimetres long.

[citation needed] The trace-element chemistry of Proterozoic anorthosites, and the associated rock types, has been examined in some detail by researchers with the aim of arriving at a plausible genetic theory.

[9][10] HAOM are distinctive because 1) they contain higher amounts of Al than typically seen in orthopyroxenes; 2) they are cut by numerous thin lathes of plagioclase, which may represent exsolution lamellae;[11] and 3) they appear to be older than the anorthosites in which they are found.

One possible model[10] suggests that, during anorthosite formation, a mantle-derived melt (or partially-crystalline mush) was injected into the lower crust and began crystallizing.

Other researchers consider the chemical compositions of the HAOM to be the product of rapid crystallization at moderate or low pressures,[13] eliminating the need for a lower-crustal origin altogether.

However, the solidus of an anorthositic magma is too high for it to exist as a liquid for very long at normal ambient crustal temperatures, so this appears to be unlikely.

Many researchers have argued that anorthosites are the products of basaltic magma, and that mechanical removal of mafic minerals has occurred.

A typical theory is as follows: partial melting of the mantle generates a basaltic magma, which does not immediately ascend into the crust.

This theory has many appealing features, of which one is the capacity to explain the chemical composition of high-alumina orthopyroxene megacrysts (HAOM).

[18] However, debate exists on the magma ocean fractionation complicated by surface impact mixing with evidence potentially indicating MAN being older and more primitive.

Their most characteristic feature is the presence of equant, euhedral megacrysts (up to 30 cm) of plagioclase surrounded by a fine-grained mafic groundmass.

However, some Proterozoic anorthosite bodies have large amounts of labradorite, which is quarried for its value as both a gemstone and a building material.

Anorthosite was prominently represented in rock samples brought back from the Moon, and is important in investigations of Mars, Venus, and meteorites.

In the Adirondack Mountains, soils on anorthositic rock tend to be stony loamy sand with classic podzol profile development usually evident.