anorthosite
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Anorthosite is a
phaneritic A phanerite is an igneous rock Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rom ...
,
intrusive Intrusive may refer to: * Intrusiveness, a typically unwelcome behavior, interrupting and disturbing to others * Intrusive rock; intrusion of molten magma leaving behind igneous rock * Saltwater intrusion, the movement of saline water into freshwa ...
igneous Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the p ...
rock characterized by its composition: mostly
plagioclase . (unknown scale) Plagioclase is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagiocla ...
feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, containing sodium, calcium, potassium or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldspars and the ''alkali'' (potas ...
(90–100%), with a minimal
mafic A mafic mineral or rock is a silicate mineral Silicate minerals are rock-forming mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science c ...
component (0–10%).
Pyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approx ...
,
ilmenite Crystal structure of ilmenite Ilmenite, also known as manaccanite, is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. From a commercial perspective, ilmenite is the most important o ...
,
magnetite Magnetite is a mineral and one of the main iron ore Iron ores are rocks and minerals from which metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, po ...

magnetite
, and
olivine The mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Ra ...

olivine
are the mafic
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (20 ...

mineral
s most commonly present. Anorthosites are of enormous geologic interest, because it is still not fully understood 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: #
Archean The Archean Eon ( , also spelled Archaean or Archæan) is one of the four geologic Geology (from the Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient w ...

Archean
-age anorthosites #
Proterozoic The Proterozoic () is a geological eon spanning the time from the appearance of oxygen in Earth's atmosphere to just before the proliferation of complex life (such as trilobites or corals) on the Earth. The name Proterozoic combines the two form ...
anorthosite (also known as massif or massif-type anorthosite) – the most abundant type of anorthosite on Earth # Layers within Layered Intrusions (e.g.,
Bushveld The Bushveld (from af, bosveld, af, bos 'bush' and af, veld) is a sub-tropical woodland ecoregion An ecoregion (ecological region) or ecozone (ecological zone) is an ecology, ecologically and geographically defined area that is smaller ...

Bushveld
and Stillwater intrusions) #
Mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics File:Earth cutaway schematic-en.svg, upright=1.35, Diagram of the internal layering of Earth showing the lithosphere above the asthenosphere (not to scale) Plate ...
and
transform fault A transform fault or transform boundary, sometimes called a strike-slip boundary, is a fault Fault commonly refers to: *Fault (geology), planar rock fractures showing evidence of relative movement *Fault (law), blameworthiness or responsibility ...

transform fault
anorthosites # Anorthosite
xenolith Sierra_Nevada_(U.S.).html"_;"title="Gabbroic_xenolith_in_granite_in_Rock_Creek_Canyon,_eastern_Sierra_Nevada_(U.S.)">Sierra_Nevada,_California A_xenolith_("foreign_rock")_is_a_rock_(geology).html" ;"title="Sierra Nevada (U.S.)">Sierra Nevada, Cal ...
s in other rocks (often granites, kimberlites, or basalts) 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 Lunar most commonly means "of or relating to the Moon". Lunar may also refer to: Arts and entertainment * Lunar (series), ''Lunar'' (series), a series of video games * Lunar (song), "Lunar" (song), by David Guetta * "Lunar", a song by Priestess fr ...
anorthosites constitute the light-coloured areas of the Moon's surface and have been the subject of much research.


Proterozoic anorthosite massifs


Age

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


Occurrence

Proterozoic anorthosites typically occur as extensive
stocks Stocks are restraining devices that were used as a form of corporal punishment and public humiliation. Form and application The stocks, pillory, and pranger each consist of large wooden boards with hinges; however, the stocks are distinguished ...
or
batholith Image:Yosemite 20 bg 090404.jpg, upright=1.3, Half Dome, a granite monolith in Yosemite National Park and part of the Sierra Nevada Batholith A batholith (from Greek ''bathos'', depth + ''lithos'', rock) is a large mass of Intrusive rock, intrusiv ...
s. The areal extent of anorthosite batholiths ranges from relatively small (dozens or hundreds of square kilometers) to nearly , in the instance of the
Nain Plutonic Suite Nain may refer to: Places * Nain, Iran, a city in Iran * Nain County, an administrative subdivision of Iran * Nain, Israel, a village in Galilee, mentioned in the New Testament (miraculous raising of the son of the widow of Nain) * Nain, Jamaica, ...
in northern Labrador, Canada. Major occurrences of Proterozoic anorthosite are found in the southwest U.S., the
Appalachian Mountains The Appalachian Mountains, often called the Appalachians, are a system of mountains in eastern North America North America is a continent entirely within the Northern Hemisphere and almost all within the Western Hemisphere. It can als ...

Appalachian Mountains
(e.g., the Honeybrook Upland of eastern Pennsylvania), eastern
Canada Canada is a country in the northern part of North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean, Atlantic to the Pacific Ocean, Pacific and northward into the Arctic Ocea ...

Canada
(e.g., the Grenville Province), across southern
Scandinavia Scandinavia; Sami languages, Sami: ''Skadesi-suolu''/''Skađsuâl''. ( ) is a Subregion#Europe, subregion in Northern Europe, with strong historical, cultural, and linguistic ties. In English usage, ''Scandinavia'' can refer to Denmark, Norw ...

Scandinavia
and eastern
Europe Europe is a continent A continent is one of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven regions are commonly regarded as continents. Ordered from largest ...

Europe
. Mapped onto the
Pangaea Pangaea or Pangea () was a supercontinent In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology) ...

Pangaea
n continental configuration of that eon, these occurrences are all contained in a single straight belt, and must all have been emplaced intracratonally. The conditions and constraints of this pattern of origin and distribution are not clear. However, see the Origins section below.


Related rocks

Many Proterozoic anorthosites occur in spatial association with other highly distinctive, contemporaneous rock types: the so-called 'anorthosite suite' or 'anorthosite-
mangerite Mangerite is a pluton#REDIRECT Intrusive rock , an igneous ''intrusion'' exposed when the surrounding softer rock eroded away Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form '' intru ...
-
charnockite 300px, Late-stage charnockite dykes cutting anorthosite, Rogaland, Norway">Rogaland.html" ;"title="anorthosite, Rogaland">anorthosite, Rogaland, Norway Charnockite () is any orthopyroxene-bearing quartz-feldspar rock formed at high temperature ...
-granite (AMCG) complex'. These rock types can include: *
Mangerite Mangerite is a pluton#REDIRECT Intrusive rock , an igneous ''intrusion'' exposed when the surrounding softer rock eroded away Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form '' intru ...
: a pyroxene-bearing monzonite intrusive igneous rock *
Charnockite 300px, Late-stage charnockite dykes cutting anorthosite, Rogaland, Norway">Rogaland.html" ;"title="anorthosite, Rogaland">anorthosite, Rogaland, Norway Charnockite () is any orthopyroxene-bearing quartz-feldspar rock formed at high temperature ...
: an orthopyroxene-bearing quartz-feldspar rock, once thought to be intrusive igneous, now recognized as metamorphic * Iron-rich
felsic In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the processes ...
rocks, including
monzonite Monzonite is an igneous Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, know ...

monzonite
and
rapakivi granite Image:Rapakiwi-Geschiebe.jpg, Rapakivi from a moraine in Northern Germany. Rapakivi granite is a hornblende-biotite granite containing large round crystals of orthoclase each with a rim of oligoclase (a variety of plagioclase). The name has come to ...

rapakivi granite
* Iron-rich
diorite Orbicular diorite from Corsica (corsite) Diorite ( ) is an intrusive rock, intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metal ...

diorite
,
gabbro Gabbro () is a phaneritic Close-up of phaneritic granite exposed in Chennai, India A phanerite is an igneous rock whose Rock microstructure, microstructure is made up of crystals large enough to be distinguished with the unaided human eye, eye ...

gabbro
, and
norite Norite is a mafic Intrusive rock, intrusive igneous rock composed largely of the calcium-rich plagioclase labradorite, orthopyroxene, and olivine. The name ''norite'' is derived from ''Norge'', the Norwegian name for Norway. Norite also known as ...

norite
* Leucocratic mafic rocks such as leucotroctolite and leuconorite Though Coeval, co-eval, these rocks likely represent chemically-independent magmas, likely produced by melting of Country rock (geology), country rock into which the anorthosites intruded. Importantly, large volumes of ultramafic rocks are not found in association with Proterozoic anorthosites.


Physical characteristics

Since they are primarily composed of plagioclase feldspar, most of Proterozoic anorthosites appear, in outcrop, to be grey or bluish. Individual plagioclase crystals may be black, white, blue, or grey, and may exhibit an iridescence known as labradorite, 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 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 The mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Ra ...

olivine
, or, more rarely, amphibole. Oxides, such as
magnetite Magnetite is a mineral and one of the main iron ore Iron ores are rocks and minerals from which metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, po ...

magnetite
or
ilmenite Crystal structure of ilmenite Ilmenite, also known as manaccanite, is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. From a commercial perspective, ilmenite is the most important o ...
, are also common. Most anorthosite plutons are very Phaneritic, 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 Deformation (engineering), deformed, and such large plagioclase crystals have Dynamic recrystallization, 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 igneous layering, which may be defined by crystal size, mafic content, or chemical characteristics. Such layering clearly has origins with a Rheology, rheologically liquid-state magma.


Chemical and isotopic characteristics

Proterozoic anorthosites are typically >90% plagioclase, and the plagioclase composition is commonly between An40 and An60 (40–60% anorthite). This compositional range is intermediate, and is one of the characteristics which distinguish Proterozoic anorthosites from Archean anorthosites (which are typically >An80). Proterozoic anorthosites often have significant
mafic A mafic mineral or rock is a silicate mineral Silicate minerals are rock-forming mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science c ...
components in addition to plagioclase. These phases can include olivine, pyroxene, Fe-Ti oxides, and/or apatite. 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, Some research has focused on neodymium (Nd) and strontium (Sr) isotope, 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.


High-alumina orthopyroxene megacrysts (HAOMs)

Many Proterozoic-age anorthosites contain large crystals of orthopyroxene with distinctive compositions. These are the so-called high-alumina orthopyroxene megacrysts (HAOM). 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; and 3) they appear to be older than the anorthosites in which they are found. The origins of HAOMs are debated. One possible model suggests that, during anorthosite formation, a mantle-derived melt (or partially-crystalline mush) was injected into the lower crust and began crystallizing. HAOMs would have crystallized out during this time, perhaps as long as 80–120 million years. The HAOM-bearing melt could then have risen to the upper crust. This model is supported by the fact that aluminum is more soluble in orthopyroxene at high pressure. In this model, the HAOM represent lower-crustal cumulates that are related to the anorthosite source-magma. One problem with this model is that it requires the anorthosite source-magma to sit in the low crust for a considerable time. To solve this, some authors suggest that the HAOMs may have formed in the lower crust independent of the anorthosite source-magma. Later, the anorthosite source-magma may have entrained pieces of the HAOM-bearing lower crust on its way upward. Other researchers consider the chemical compositions of the HAOM to be the product of rapid crystallization at moderate or low pressures, eliminating the need for a lower-crustal origin altogether.


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 Earth's mantle, 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 (chemistry), 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 vapor 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 vapor 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 dike (geology), dykes in the Nain Plutonic Suite, suggested that the possibility of anorthositic magmas existing at crustal temperatures needed to be reexamined. 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 Fractional crystallization (geology), fractionates large amounts of mafic minerals, which sink to the bottom of the chamber. The co-crystallizing plagioclase crystals float, and eventually are emplaced into the crust as anorthosite plutons. Most of the sinking mafic minerals form cumulate rocks, ultramafic cumulates which stay at the base of 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). This is detailed below in the section devoted to the HAOM. However, on its own, this hypothesis cannot coherently explain the origins of anorthosites, because it does not fit with, among other things, some important isotopic measurements made on anorthositic rocks in the Nain Plutonic Suite. The Nd and Sr isotopic data show the magma which produced the anorthosites cannot have been derived only from the mantle. Instead, the magma that gave rise to the Nain Plutonic Suite anorthosites must have had a significant crustal component. This discovery led to a slightly more complicated version of the previous hypothesis: Large amounts of basaltic magma form a magma chamber at the base of the crust, and, while crystallizing, assimilating large amounts of crust. This small addendum explains both the isotopic characteristics and certain other chemical niceties of Proterozoic anorthosite. However, at least one researcher has cogently argued, on the basis of geochemical data, that the mantle's role in production of anorthosites must actually be very limited: the mantle provides only the impetus (heat) for crustal melting, and a small amount of partial melt in the form of basaltic magma. Thus anorthosites are, in this view, derived almost entirely from lower crustal melts.


Archaean anorthosites

Archean anorthosites represent the second largest anorthosite deposits on Earth. Most have been dated between 3,200 and 2,800 Ma, and commonly associated with basalts and/or greenstone belts. Archean anorthosites are distinct texturally and mineralogically from Proterozoic anorthosite bodies. Their most characteristic feature is the presence of equant, euhedral megacrysts (up to 30 cm) of plagioclase surrounded by a fine-grained mafic groundmass. The plagioclase in these anorthosites is commonly An80-90.


Economic value of anorthosite

The primary economic value of anorthosite bodies is the titanium-bearing oxide
ilmenite Crystal structure of ilmenite Ilmenite, also known as manaccanite, is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. From a commercial perspective, ilmenite is the most important o ...
. However, some
Proterozoic The Proterozoic () is a geological eon spanning the time from the appearance of oxygen in Earth's atmosphere to just before the proliferation of complex life (such as trilobites or corals) on the Earth. The name Proterozoic combines the two form ...
anorthosite bodies have large amounts of labradorite, which is quarried for its value as both a gemstone and a building material. Archean anorthosites, because they are aluminium-rich, have large amounts of aluminium substituting for silicon; a few of these bodies are mined as ores of aluminium. Anorthosite was prominently represented in rock samples brought back from the Moon, and is important in investigations of Mars, Venus, and meteorites.


Soil development on anorthosite

In the Adirondack Mountains, soils on anorthositic rock tend to be stony loamy sand with classic podzol profile development usually evident. In the San Gabriel Mountains, soils on anorthosite have a dominance of 1:1 clay minerals (kaolinite and halloysite) in contrast to more mafic rock over which 2:1 clays develop. File:Anorthosit fin.jpg, Anorthosite from southern Finland Image:Labradoryt 1.JPG, Anorthosite from Poland File:Apollo_15_Genesis_Rock.jpg, Anorthosite from the Moon, Apollo 15 "Genesis Rock"


See also

*


References


Bibliography

* * * * * * * * *{{cite journal , last=Xue , first=S. , author2=Morse, S. A. , year=1994 , title=Chemical characteristics of plagioclase and pyroxene megacrysts and their significance to the petrogenesis of the Nain Anorthosites , journal=Geochimica et Cosmochimica Acta , volume=58 , issue=20 , pages=4317–4331 , doi=10.1016/0016-7037(94)90336-0 , bibcode = 1994GeCoA..58.4317X


External links


Anorthosite Complexes (web archive)
*[https://web.archive.org/web/20031012172703/http://www.nasm.si.edu/galleries/attm/wl.an.2.html Anorthosite – Lunar Highland Rock]
Lunar Anorthosite Specimen 60025 Photomicrographs
Plutonic rocks Archean Proterozoic