thumb|250px|right|[[Moon|Lunar Ferroan Anorthosite #60025 (Lunar Highlands near Descartes Crater. This sample is currently on display at the National Museum of Natural History in Washington, DC.]]
'''Anorthosite''' () is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar (90-100%), and a minimal mafic component (0-10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most commonly present. 

Anorthosite on Earth can be divided into two types: Earth's history, and are thought to have had different origins. 

Lunar anorthosites [http://www.psrd.hawaii.edu/April04/lunarAnorthosites.html] constitute the light-coloured areas of the Moon's surface and have been the subject of much research.

== Proterozoic anorthosite ==
===Age===
Although a few Proterozoic anorthosite bodies were emplaced either late in the Archean Ma).

===Mode of occurrence===
Anorthosite plutons occur in a wide range of sizes. Some smaller plutons, exemplified by many anorthosite bodies in the U.S. and Harris in Scotland, cover only a few dozen square kilometres. Larger plutons, like the Mt. Lister Anorthosite, in northern Labrador, Canada, cover several thousands of square kilometres.

Many Proterozoic anorthosites occur in spatial association with other highly distinctive, contemporaneous rock types (the so-called 'anorthosite suite' or 'anorthosite-mangerite-charnockite complex'). These rock types include iron-rich diorite, gabbro, and norite; leucocratic mafic rocks such as leucotroctolite and leuconorite; and iron-rich felsic rocks, including monzonite and rapakivi granite. Importantly, large volumes of ultramafic rocks are not found in association with Proterozoic anorthosites. 

Occurrences of Proterozoic anorthosites are commonly referred to as 'massifs'. However, there is some question as to what name would best describe any occurrence of anorthosite together with the rock types mentioned above. Early works used the term 'complex' The term 'plutonic suite' has been applied to some large occurrences in northern Labrador, Canada; however, it has been suggested (in 2004-2005) that 'batholith' would be a better term. 'Batholith' is used to describe such occurrences for the remainder of this article. 

The areal extent of anorthosite batholiths ranges from relatively small (dozens or hundreds of square kilometres) to nearly 20,000 km², in the instance of the Nain Plutonic Suite in northern Labrador, Canada.

Major occurrences of Proterozoic anorthosite are found in the southwest U.S., the intracratonally. The conditions and constraints of this pattern of origin and distribution are not clear. However, see the Origins section below.

===Physical characteristics===  
Since they are primarily composed of plagioclase feldspar, most of Proterozoic anorthosites appear, in labradorescence on fresh surfaces. The feldspar variety labradorite is commonly present in anorthosites. Mineralogically, labradorite is a compositional term for any calcium-rich plagioclase feldspar containing between 50-70 molecular percent anorthite (An 50-70), regardless of whether it shows labradorescence.  The mafic mineral in Proterozoic anorthosite may be clinopyroxene, orthopyroxene, olivine, or, more rarely, amphibole. Oxides, such as magnetite or ilmenite, are also common.

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. Less commonly, plagioclase crystals are megacrystic, or larger than one metre long. However, most Proterozoic anorthosites are deformed, and such large plagioclase crystals have recrystallized to form smaller crystals, leaving only the outline of the larger crystals behind. 

While many Proterozoic anorthosite plutons appear to have no large-scale relict igneous structures (having instead post-emplacement deformational structures), some do have rheologically liquid-state magma.

===Chemical and isotopic characteristics===

The composition of plagioclase feldspar in Proterozoic anorthosites is most commonly between An<sub>40</sub> and An<sub>60</sub> (40-60% anorthite). This compositional range is intermediate, and is one of the characteristics which distinguish Proterozoic anorthosites from Archean anorthosites. Mafic minerals in Proterozoic anorthosites have a wide range of composition, but are not generally highly magnesian.

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. However, there is still little agreement on just what the results mean for anorthosite genesis; see the 'Origins' section below. A very short list of results, including results for rocks thought to be related to Proterozoic anorthosites: Bédard (2001); Emslie et al. (1994); Xue and Morse (1994); Emslie and Stirling (1993); and Xue and Morse (1993).

Some research has focused on isotopic determinations for anorthosites, particularly for anorthosites of the Nain Plutonic Suite (NPS). Such isotopic determinations are of use in gauging the viability of prospective sources for magmas that gave rise to anorthosites. Some results are detailed below in the 'Origins' section.

===Origins of Proterozoic anorthosites===    
The origins of Proterozoic anorthosites have been a subject of theoretical debate for many decades. A brief synopsis of this problem is as follows. The problem begins with the generation of magma, the necessary precursor of any igneous rock.

Magma generated by small amounts of partial melting of the mantle is generally of basaltic composition. Under normal conditions, the composition of basaltic magma requires it to crystallize between 50 and 70% plagioclase, with the bulk of the remainder of the magma crystallizing as mafic minerals. However, anorthosites are defined by a high plagioclase content (90-100% plagioclase), and are not found in association with contemporaneous ultramafic rocks. This is now known as 'the anorthosite problem'. Proposed solutions to the anorthosite problem have been diverse, with many of the proposals drawing on different geological subdisciplines. 
      
It was suggested early in the history of anorthosite debate that a special type of magma, anorthositic magma, had been generated at depth, and emplaced into the crust. 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. The presence of water vapour has been shown to lower the solidus temperature of anorthositic magma to more reasonable values, but most anorthosites are relatively dry. It may be postulated, then, that water vapour be driven off by subsequent metamorphism of the anorthosite, but some anorthosites are undeformed, thereby invalidating the suggestion. 

The discovery, in the late 1970s, of anorthositic dykes in the Nain Plutonic Suite, suggested that the possibility of anorthositic magmas existing at crustal temperatures needed to be reëxamined. However, the dykes were later shown to be more complex than was originally thought. In summary, though liquid-state processes clearly operate in some anorthosite plutons, the plutons are probably not derived from anorthositic magmas.

Many researchers have argued that anorthosites are the products of basaltic magma, and that mechanical removal of mafic minerals has occurred. Since the mafic minerals are not found with the anorthosites, these minerals must have been left at either a deeper level or the base of the crust. A typical theory is as follows: partial melting of the mantle generates a  basaltic magma, which does not immediately ascend into the crust. Instead, the basaltic magma forms a large magma chamber at the base of the crust and fractionates large amounts of mafic minerals, which sink to the bottom of the chamber. The cocrystallizing plagioclase crystals float, and eventually are emplaced into the crust as anorthosite plutons. Most of the sinking mafic minerals form ultramafic cumulates which stay at the base of the crust. 

thumb|200px|right|Anorthosite lunaire, Musée national d'histoire naturelle, [[Washington, DC.]]
L''''Anorthosite''' est une roche phanéritique intrusive et ignée, composée pour la plus grande partie de feldspath plagioclase (90-100%), et d'une petite quantité (0-10%) de composants mafiques tels que le pyroxène, l'ilménite, la magnétite, et l' olivine. 

On distingue deux types d''''anorthosite terrestre''' : l'arnorthosite archéenne. Ces deux types d'anorthosite apparaissent dans des contextes différents, apparemment liés à périodes précises de l'histoire de la Terre et semblent avoir des origines distinctes. 

L''''anorthosite lunaire''', formant les parties les plus claires de la surface lunaire,  a fait l'objet de nombreux travaux d'étude<ref Name="Norman04">M. Norman, ''The Oldest Moon Rocks'', Planetary Science Research Discoveries, 2004. [http://www.psrd.hawaii.edu/April04/lunarAnorthosites.html www.psrd.hawaii.edu].</ref>. 

== L’anorthosite protérozoïque==
===Datation===
Bien que quelques ensembles d'anorthosite se soient formés soit à la fin de l'Ma).

===Composants===
Les intrusions de plutons d'anorthosite possèdent des tailles très variées. Les plus réduits, dont on trouve un grand nombre d'exemples aux États-Unis, ne font pas plus de quelques douzaines de kilomètres carrés.  D'autres, comme l'anorthosite du Mont Lister, au nord du Labrador (Canada), font plusieurs milliers de kilomètres carrés. 

Beaucoup d'anorthosites protérozoïques sont associées à la présence d'autres types de roches qui leur sont contemporaines et sont très distinctives (on parle de « suite anorthosite » ou de « complexe anorthosite-mangerite-charnockite »). Ces roches sont entre autres la felsiques riches en fer dont la roches ultramafiques ne sont pas associées à la présence d'anorthosites protérozoïques.

Les formations d'anorthosite protérozoïque sont communément désignées  sous le terme de ''massif''. Mais on discute encore pour savoir quel terme serait le plus propre à décrire les formations d'anorthosite combinée aux roches mentionnées ci-dessus. Les articles les plus anciens utilisent le terme de ''complexe''. L'expression ''suite plutonique'' a servi à décrire la présence de grosses concentrations d'anorthosite dans le nord du Labrador. En 2004-2005, on a proposé le terme de batholite comme étant le plus approprié.

Les batholites d'anorthosite peuvent occuper des aires qui varient entre quelques douzaines de km² à plus de 20&nbsp;000&nbsp;km² dans le cas de la suite plutonique de ''Nain'' dans le nord du Labrador.

Les exemples les plus remarquables se trouvent dans le sud ouest des États-Unis (Pangée en simulant la configuration géologique de cette ère, on observe que ces origines ci-dessous).

===Caractéristiques physiques=== 
Comme elle se compose majoritairement de feldspath plagioclase, la plupart des affleurements d'anorthosite ont une teinte grise ou bleutée. Les cristaux de plagioclase peuvent être noirs, blanc, bleus ou gris, et présentent parfois une belle adularescence, connue sous le nom de ''labradorescence''. On appelle d'ailleurs ce type d'anorthosite de la labradorite. Le minerai mafique contenu dans l'anorthosite peut être du clinopyroxène, de l'orthopyroxène, de l'olivine, ou, plus rarement, de l'amphibole. On y trouve aussi communément des oxydes tels que la magnétite ou l'ilménite.

Les plutons d'anorthosites sont des roches phanéritiques avec une structure grenue assez grossière, c'est-à-dire qu'ils contiennent des cristaux de plagioclase et des minéraux mafiques de plusieurs centimètres de long. On trouve plus rarement des mégacristaux de plagioclase de plus d'un métamorphisme de contact au cours duquel les cristaux géants se sont recristallisés pour donner des cristaux plus petits, ne laissant que la trace du méga-cristal originel.

Si la plupart des plutons d'anorthosite protérozoïque semblent ne pas présenter de traces importantes de structures ignées (on n'observe plus que des traces de métamorphisme qui signalent leur ancienne présence), certains en revanche possèdent une couche ignée qui se distingue par la taille de ses cristaux, son contenu mafique ou ses caractéristiques chimiques. Ce type de couche est due au comportement rhéologique du magma.

===Caractéristiques chimiques et isotopiques===

Les éléments de feldspath plagioclase des anorthosites protérozoïques se situent généralement entre An et An<ref>Les plagioclases, qui par définition contiennent plus de 90% d'anorthosite, sont des solutions solides totales d'un système de deux minéraux : l'anorthite et l'albite. L'anorthite est 100% calcique (Ca) alors que l'albite est sodique (Na). Il existe toute une série de plagioclases que l'on distingue en fonction de leur teneur plus ou moins élevée en anorthite ou en albite par l'indication An (anorthite) suivie d'un chiffre. ''An'' désigne un plagioclase contenant 40&thinsp;% d''''an'''orthite et 60&thinsp;% d'albite sur la base du pourcentage de cations. Voir aussi [http://www.univ-lille1.fr/geosciences/cours/petro_endogene/cours_6.pdf#search=%22anorthite%20albite%20An%2040%25%22]</ref>. Cette fourchette est de caractère intermédiaire et constitue l'une des caractéristiques permettant de distinguer les anorthosites protérozoïques des anorthosites archéennes. Les minéraux mafiques de l'anorthosite protérozoïque varient beaucoup en terme de composition, mais sont généralement peu riches en magnésium.

La origines'' ci-dessous). On trouvera dans la courte liste ci-dessous les références aux résultats de ces travaux, y compris les résultats concernant les roches supposées apparentées à l'anorthosite protérozoïque : Bédard (2001)<ref Name="Bédard01">J.H. Bédard, ''Parental magmas of the Nain Plutonic Suite anorthosites and mafic cumulates: a trace element modelling approach'', Journal of Petrology 141, 747-771, 2001.</ref> ; Emslie ''et al.'' (1994)<ref Name="Emslie94">R.F. Emslie, M.A. Hamilton, R.J. Theriault, ''Petrogenesis of a Mid-Proterozoic Anorthosite-Mangerite-Charnockite-Granite (AMCG) Complex: Isotopic and Chemical Evidence from the Nain Plutonic Suite'', Journal of  Geology 102, 539-558, 1994.</ref> ; Xue et Morse (1994)<ref Name="Xue94">S. Xue, S.A. Morse, ''Chemical characteristics of plagioclase and pyroxene megacrysts and their significance to the petrogenesis of the Nain Anorthosites'', Geochimica et Cosmochimica Acta 58, 4317-4331, 1994</ref>; Emslie et Stirling (1993)<ref Name="Emslie93">R.F. Emslie, J.A.R. Stirling, ''Rapakivi and related granitoids of the Nain Plutonic Suite: geochemistry, mineral assemblages and fluid equilibria'', Canadian Mineralogist 31, 821-847, 1993.</ref>; Xue et Morse (1993)<ref Name="Xue93">S. Xue, S.A. Morse, ''Geochemistry of the Nain massif anorthosite, Labrador: Magma diversity in five intrusions'', Geochimica et Cosmochimica Acta 57, 3925-3948, 1993.</ref>.

Certains travaux ont tenté d'établir l'identité des anorthosites à partir des isotopes du origines''.

===Origines des anorthosites protérozoïques===
Les origines des anorthosites protérozoïques sont l'objet d'un débat théorique qui dure depuis plusieurs magma, précurseur indispensable de toute roche ignée. 