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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. Pluton redirects here. ...
Igneous rocks are formed when molten rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. ...
Plagioclase feldspar Plagioclase feldspars are a very important group of igneous rock forming tectosilicate minerals. ...
Feldspar is the name of an important group of rock-forming minerals which make up perhaps as much as 60% of the Earths crust. ...
In geology, mafic minerals are silicate minerals, magmas, and volcanic and intrusive igneous rocks that have relatively high concentrations of the heavier elements. ...
The pyroxenes are a group of important rock-forming silicate minerals found in many igneous and metamorphic rocks. ...
Ilmenite (FeTiO3) is a weakly magnetic iron-black or steel-gray mineral found in metamorphic and plutonic igneous rocks. ...
Magnetite is a ferromagnetic mineral form of iron(II,III) oxide, with chemical formula , one of several iron oxides and a member of the spinel group. ...
Olivine The mineral olivine is a magnesium iron silicate with the formula (Mg,Fe)2SiO4 in which the ratio of magnesium and iron varies between the two endmembers of the series: forsterite (Mg-rich) and fayalite (Fe-rich). ...
Anorthosite on earth can be divided into two types: Proterozoic anorthosite (also known as massif or massif-type anorthosite) and Archean anorthosite. These two types of anorthosite have different modes of occurrence, appear to be restricted to different periods in earth's history, and are thought to have had different origins. In geology, the Proterozoic is an eon prior to the first abundant complex life on earth. ...
The Archean is a geologic eon; it is a somewhat antiquated term for the time span between 2500 million years before the present and 3800 million years before the present. ...
Lunar anorthosites constitute the light-coloured areas of the Moon's surface and have been the subject of much research.
Proterozoic Anorthosite
Age Although a few anorthosite bodies were emplaced either late in the Archean Eon, or early in the Phanerozoic Eon, the vast majority of Proterozoic anorthosites were emplaced, as their name suggests, during the Proterozoic Eon (ca. 2500-542 Ma). The Phanerozoic (occasionally Phaenerozoic) Eon is the period of geologic time during which abundant animal life has existed. ...
Mode of occurrence Anorthosite plutons occur in a wide range of sizes. Some smaller plutons, exemplified by many anorthosite bodies in the U.S., cover only a few dozen square kilometres. Larger plutons, like the Mt. Lister Anorthosite, in northern Labrador, Canada, cover several thousands of square kilometres. Devils Tower, an igneous intrusion exposed when the surrounding softer rock eroded away. ...
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 Fe-rich diorite, gabbro, and norite; leucocratic mafic rocks such as leucotroctolite and leuconorite; and Fe-rich felsic rocks, including monzonite and rapakivi granite. Importantly, large volumes of ultramafic rocks are not found in association with Proterozoic anorthosites (see the 'Origins' section). Gabbro Gabbro is a dark, coarse-grained, intrusive igneous rock that is chemichly equivalent to basalt. ...
Norite is a mafic intrusive igneous rock composed largely of the calcium rich plagioclase labradorite and hypersthene with olivine. ...
Monzonite is a felsic to intermediate igneous intrusive rock composed of approximately equal amounts of sodic to intermediate plagioclase and orthoclase feldspars with minor amounts of hornblende, biotite and other minerals. ...
Rapakivi granite is a hornblende-biotite granite containing large rounded crystals of orthoclase mantled with oligoclase. ...
Ultramafic rocks are igneous rocks with very low silica content (less than 45%) and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content). ...
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 Appalachian Mountains, eastern Canada, across southern Scandinavia and eastern Europe. Mapped onto the Pangaean continental configuration of that era, 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. A rainy day in the Great Smoky Mountains, Western North Carolina The Appalachian Mountains are a vast system of North American mountains, partly in Canada, but mostly in the United States, extending as a zone, from 100 to 300 miles wide, running from Newfoundland and Labrador, Canada, 1500 miles south...
Scandinavia, Fennoscandia, and the Kola Peninsula. ...
A satellite composite image of Europe Europe is the worlds second-smallest continent in terms of area, with an area of 10,600,000 km² (4,140,625 square miles), making it larger than Australia only. ...
Map of Pangaea Pangaea or Pangea (derived from Παγγαία, Greek for all earth) is the name given to the supercontinent that existed during the Paleozoic and Mesozoic eras, before the process of plate tectonics separated the component continents into their current configuration. ...
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 gray, and may have a beautiful iridescence known as labradorescence. This kind of anorthosite is known as labradorite. The mafic mineral in Proterozoic anorthosite may be clinopyroxene, orthopyroxene, olivine, or, more rarely, amphibole. Oxides, such as magnetite or ilmenite, are also common. Feldspar (from the German Feld, field, and Spat, a rock that does not contain ore) is the name of an important group of rock-forming minerals which make up perhaps as much as 60% of the Earths crust. ...
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 have recrystallized to form smaller crystals, leaving only the outline of the larger crystals behind. Granularity is the extent to which a system contains discrete components of ever-smaller size. ...
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 rheologically liquid-state magma. Magma is molten rock often located inside a magma chamber beneath the surface of the Earth. ...
Chemical and Isotopic Characteristics The composition of plagioclase feldspar in Proterozoic anorthosites is most commonly between An40 and An60. 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 neodymium (Nd) and strontium (Sr) 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. General Name, Symbol, Number neodymium, Nd, 60 Chemical series lanthanides Group, Period, Block ?, 6, f Appearance silvery white, yellowish tinge Atomic mass 144. ...
General Name, Symbol, Number strontium, Sr, 38 Chemical series alkaline earth metals Group, Period, Block 2, 5, s Appearance silvery white metallic Atomic mass 87. ...
Isotopes are forms of an element whose nuclei have the same atomic number–-the number of protons in the nucleus--but different atomic masses because they contain different numbers of neutrons. ...
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 known as 'the anorthosite problem'. Proposed solutions to the anorthosite problem have been diverse, with many of the proposals drawing on different geological subdisciplines. A Mantle is a piece of clothing, similar to a robe but open on the front side and often sleeveless. ...
Basalt Basalt is an extrusive igneous rock, sometimes porphyritic, and is often both fine-grained and dense. ...
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. A dike in geology refers to a tabular intrusive igneous body. ...
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 basltic 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.
This theory has many appealing features, of which one is the capacity to explain the chemical composition of high-alimuna 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 shows 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 (Emslie et al., 1994).
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 (Bédard, 2001).
High-Alumina Orthopyroxene Megacrysts The HAOM have, like Proterozoic anorthosites, been the subject of great debate, although a tentative consensus about their origin appears to have emerged. The peculiar characteristic worthy of such debate is reflected in their name. Normal orthopyroxene has chemical composition (Fe,Mg)Si2O6, whereas the HAOM have anomalously large amounts of aluminium (Al) (up to about 9%) in their atomic structure.
Because the solubility of Al in orthopyroxene increases with increasing pressure, many researchers (Longhi et al., 1993; Emslie (1975)), have suggested that the HAOM crystallized at depth, near the base of the earth's crust. The maximum amounts of Al correspond to a 30-35 km depth.
Other researchers (e.g. Xue and Morse, 1994) consider the chemical compositions of the HAOM to be the product of rapid crystallization at moderate or low pressures.
Archean Anorthosite This section has not yet been created.
Economic Value of Anorthosite The primary economic value of anorthosite bodies is the titanium-bearing oxide ilmenite. However, some Proterozoic 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 Ca-rich, have large amounts of Al substituting for Si; a few of these bodies are mined as a source of aluminium. General Name, Symbol, Number titanium, Ti, 22 Chemical series transition metals Group, Period, Block 4, 4, d Appearance silvery metallic Atomic mass 47. ...
Anorthosite was prominently represented in rock samples brought back from the Moon, and is important in investigations of Mars, Venus and meteorites. Crust composition Oxygen 43% Silicon 21% Aluminium 10% Calcium 9% Iron 9% Magnesium 5% Titanium 2% Nickel 0. ...
Mars is the fourth planet from the Sun in the solar system, named after the Roman god of war (the counterpart of the Greek Ares), on account of its blood red color as viewed in the night sky. ...
(*min temperature refers to cloud tops only) Atmospheric characteristics Atmospheric pressure 9. ...
A meteorite is a small extraterrestrial body that reaches the Earths surface. ...
References Bédard, J.H., 2001. Parental magmas of the Nain Plutonic Suite anorthosites and mafic cumulates: a trace element modelling approach. Journal of Petrology 141, 747-771.
Emslie, R.F., 1975. Pyroxene megacrysts from anorthositic rocks: new clues to the sources and evolution of the parent magmas. Canadian Mineralalogist 13, 138-145.
Emslie, R.F., Hamilton, M.A., Theriault, R.J., 1994. 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.
Emslie, R.F., Stirling, J.A.R., 1993. Rapakivi and related granitoids of the Nain Plutonic Suite: geochemistry, mineral assemblages and fluid equilibria. Canadian Mineralogist 31, 821-847.
Longhi, J., Fram, M.S., Vander Auwera, J., Montieth, J.N., 1993. Pressure effects, kinetics, and rheology of anorthositic and related magmas. American Mineralogist 78, 1016-1030.
Xue, S., Morse, S.A., 1993. Geochemistry of the Nain massif anorthosite, Labrador: Magma diversity in five intrusions. Geochimica et Cosmochimica Acta 57, 3925-3948.
Xue, S., Morse, S.A., 1994. Chemical characteristics of plagioclase and pyroxene megacrysts and their significance to the petrogenesis of the Nain Anorthosites. Geochimica et Cosmochimica Acta 58, 4317-4331.
See also Banded gneiss with dike of granite orthogneiss This page is intended as a list of all rock types. ...
External links - Anorthosite Complexes
- How does anorthosite crystallize?
- History of the Emplacement and Deformation of Anorthosite Bodies in the Eastern Marcy Massif, Adirondacks Mountains, New York
- Anorthosite - Lunar Highland Rock
- Lunar Anorthosite Specimen 60025 Photomicrographs
- Mercury - Evidence for Anorthosite and Basalt from mid-Infrared Spectroscopy
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