anorthosites

Anorthosite is a coarse-grained, intrusive igneous rock characterized by a dominance of plagioclase feldspar, typically calcium-rich varieties that crystallize as anorthite. In hand specimen it is pale, often white to light gray, with large, conspicuous feldspar crystals forming a holocrystalline texture. The modal mineralogy is usually dominated by plagioclase (often more than 90%), with small amounts of mafic minerals such as pyroxene or olivine and accessory minerals.

Texture and subtypes: Anorthosites commonly form as cumulates in crustal magma chambers, where early-crystallizing plagioclase separates from the melt and concentrates, creating rock dominated by large plagioclase crystals. In many complexes the anorthosite occurs with related cumulate rocks such as troctolite and gabbro.

Formation and significance: Terrestrial formation is linked to fractional crystallization and cumulate processes within thick basaltic crust; the lunar highlands, in contrast, are composed largely of plagioclase and are thought to represent a flotation cumulate crust formed from a global magma ocean early in Moon history. These rocks provide important evidence for crustal differentiation and planetary differentiation.

Occurrence: In Earth's crust, notable occurrences include the Adirondack anorthosite complex in New York, the Massif des Anorthosites in southern Quebec, and various units within the Grenville Province of eastern Canada; the Leka anorthosite complex in Norway is another well-studied example.