Other Definitions
anorthosite (dict)
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AnorthositeAnorthosite 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: Proterozoic anorthosite (also known as massif 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. Lunar anorthosites, which constitute the light-coloured areas of the Moon's surface, have been the subject of much research. Proterozoic Anorthosite Age Although a few anorthosite bodies may have been 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). Mode of occurrence Most Proterozoic anorthosites occur in batholiths with other highly distinctive, contemporaneous rock types (the so-called 'anorthosite suite'). 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 (see the 'Origins' section), large volumes of ultramafic rocks are not found in association with Proterozoic anorthosites. The areal extent of anorthosite plutons themselves range from small (dozens of square kilometres) to huge (up to a few thousands of square kilometres). The areal extent of anorthosite batholiths likewise range 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. Examples of other major occurrences are in the American Southwest, Appalachian Mountains, southeast Canada, across southern Scandinavia and into Eastern Europe. Mapped onto the Pangean continental configuration of that era, these occurrences are all contained in a single straight belt. The conditions and constraints of this pattern of origin and distribution are not clear. However, see the Origins section below. Most Proterozoic anorthosites were emplaced intracratonally. Many, but not all, Proterozoic anorthosites are found associated with granulite-facies metamorphic rocks, which has led to speculation about a possible genetic relationship between them. Physical and Chemical 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. 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. 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. Some of this layering clearly has origins with a rheologically liquid-state magma. Some layering in Proterozoic anorthosites has been attributed to emplacement mechanisms, such as diapirism. The composition of plagioclase feldspar in Proterozoic anorthosites generally ranges from An40 to An60. This compositional range is intermediate, and is one of the characteristics which distinguish Proterozoic anorthosites from Archean anorthosites. The mafic minerals have a wider range of composition, but are not generally highly magnesian. The geochemistry of Proterozoic anorthosites, and the associated rock types, has been examined in some detail by researchers; however, there is still little agreement on just what the results mean for anorthosite genesis. See the 'Origins' section below. 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. 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. 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 rexamined. 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 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. 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. 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; the HAOM have anomalously large amounts of Al in their atomic structure. The solubility of Al in orthopyroxene increases with increasing pressure. The amounts of Al observed could be explained if the HAOM crystallized at depth: for instance, at the base of the crust. This is one reason why Proterozoic anorthosites are thought to have crystallized, in large part, at the base of the crust. It should be noted, however, that some researchers 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; it will be created soon. 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 aluminum. Anorthosite was prominently represented in rock samples brought back from the Moon, and is important in investigations of Mars, Venus and meteorites. See also External links
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