Anorthositic

Anorthosite

Igneous rock
Anorthosite from Salem district, Tamil Nadu
Composition
Primary	Plagioclase
Secondary	Mafic minerals

Anorthosite ( /ænˈɔːrθəsaɪt/) is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most commonly present.

Anorthosites are of enormous geologic interest, because we still don't fully understand how they form. Most models involve separating plagioclase crystals based on their density. Plagioclase crystals are usually less dense than magma; so, as plagioclase crystallizes in a magma chamber, the plagioclase crystals float to the top, concentrating there.

Anorthosite on Earth can be divided into five types:

Of these, the first two are the most common. These two types have different modes of occurrence, appear to be restricted to different periods in Earth's history, and are thought to have had different origins.

Lunar anorthosites constitute the light-coloured areas of the Moon's surface and have been the subject of much research.

Proterozoic anorthosites were emplaced during the Proterozoic Eon (ca. 2,500–542 Ma), though most were emplaced between 1,800 to 1,000 Ma.

Proterozoic anorthosites typically occur as extensive or batholiths (hence the term "massif," a French term broadly applied to mountain peaks). 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 in northern Labrador, Canada.