Anorthosite

Protérozoïque anorthosite

Dating

Although some whole of anorthosite was formed either at the end of the Archéen, or at the beginning of the era Phanérozoïque, the great majority of protérozoïques anorthosites date, as their name indicates it, of the era Protérozoïque (between 2500 and 542 My).

Components

The intrusions of let us plutons of anorthosite have very varied sizes. Most reduced, one finds a great number of examples to the the United States, do not make more few dozen of square kilometers. Others, like the anorthosite of the Mount To list, in the north of the Labrador (Canada), make several thousands of square kilometers.

Many protérozoïques anorthosites are associated with the presence of other types of rocks which are contemporary and are very distinctive for them (one speaks about “continuation anorthosite” or “complex anorthosite-mangerite-charnockite”). These rocks are inter alia the Diorite rich in iron, the Gabbro and the Norite; mafic rocks Leucocratique S such as the Leucotroctolite and the Leuconorite; and of the rocks felsic S rich person out of iron of which the Monzonite and the Granite rapakivi. It is important to note that great quantities of rocks ultramafic S are not associated with the presence of protérozoïques anorthosites.

The formations of protérozoïque anorthosite are commonly indicated under the massive term of . But one still discusses to know which term would be most specific to describe the formations of anorthosite combined with the rocks mentioned above. The oldest articles use the complex term of . The plutonic expression continuation was used to describe the presence of large concentrations of anorthosite in the north of Labrador. In 2004-2005, one proposed the term of Batholite as being most suitable.

The batholites of anorthosite can occupy of the surfaces which vary between a few dozen km ² to more 20 000 km ² in the case of the plutonic continuation of Dwarf in the north of the Labrador.

The most remarkable examples are in the western south of the United States (the Appalachian Mountains), in the east of the Canada, the south of the Scandinavia and in Europe of the East. When one defers the above mentioned occurrences on a chart of the Pangée by simulating the geological configuration of this era, one observes that these Batholite S is found aligned after from/to each other, which lets suppose that they were formed in same the Craton. One does not know large thing of the conditions nor of the constraints which governed their formation (see the section origins below).

Physical characteristics

As it is composed mainly of Feldspath plagioclase, the majority of the outcrops of anorthosite have a color Gris E or Bleu tee. The crystals of Plagioclase can be Noir S, Blanc, blue or gray, and present sometimes beautiful a Adularescence, known under the name of labradorescence . Besides one calls this type of anorthosite of the Labradorite. The ore mafic contained in anorthosite can be clino Pyroxène, ortho Pyroxène, Olivine, or, more rarely, Amphibole. One finds there also commonly oxides such as the Magnétite or the Ilménite.

Let us plutons of anorthosites are phaneritic rocks with a rather coarse grained structure, i.e. they contain crystals of plagioclase and minerals mafic several centimetres length. One more rarely finds of plagioclase megacrystals of more than Mètre length. However, the majority of protérozoïques anorthosites were subjected to a metamorphism of contact during which the giant crystals were recristallized to give smaller crystals, leaving only the trace of original megacrystal.

If the majority of plutons of protérozoïque anorthosite seem not to present important traces of igneous structures (one observes nothing any more but traces of metamorphism which announces their old presence), some on the other hand have an igneous layer which is characterized by the size from its crystals, its contents mafic or its characteristics chemical. This type of layer is due to the rheological behavior magma.

Chemical and isotopic characteristics

The feldspar plagioclase elements of protérozoïques anorthosites are generally located between An and Year. This fork is of intermediate nature and constitutes one of the characteristics making it possible to distinguish anorthosites Protérozoïque S from anorthosites Archéen born. The minerals mafic of protérozoïque anorthosite vary much in term of composition, but are generally not very rich in Magnésium.

The Chemistry of the Trace element S contained in protérozoïques anorthosites and the other types of Roche which are associated for them were the subject of studies detailed on behalf of researchers whose objective was to formulate a plausible theory of their Genèse. However there does not exist yet of consensus on the interpretation of the results and thus on the genesis of anorthosites (See the section origins below). One will find in the short list below the references to the results of this work, including the results concerning the rocks presumedly related with protérozoïque anorthosite: Bédard (2001); Emslie and Al (1994); Xue and Morse (1994); Emslie and Stirling (1993); Xue and Morse (1993).

Certain work tried to prove the identity of anorthosites starting from the isotopes of the Néodyme (Nd) and of the Strontium (Sr), by studying more particularly anorthosites of the plutonic continuation of Dwarf (NPS). This isotopic discrimination is used to evaluate the validity of the potential sources of the magmas which gave rise to anorthosites. One will find certain results detailed in the section origins .

Origins of protérozoïques anorthosites

The origins of protérozoïques anorthosites are the object of a theoretical debate which lasts since several Décennie S. the first question which installation is that of the generation of the magma, essential precursor of all igneous Roche.

The magma generated by the fusion of minor amounts of the coat is generally composed of Basalte. Under the normal conditions of Temperature and Pressure, the composition of the magma require that it Cristal dye stick starting from a content of 50 with 70 % in Plagioclase, the remainder of the rock giving mineral crystals mainly mafic. But anorthosites have a strong content of plagioclase, about 90 with 100 %, and they are never found associated with the ultramafic rocks which are contemporary for them. From where the problem of anorthosite . Various solutions were proposed to solve this problem, whose majority are based on various disciplines subordinate to geology.

An assumption which dates from the beginnings of the controversy suggests the existence of a magma of the particular type, the magma anorthositic, which would have been formed in-depth and solidified later in the earth's crust. But the solidus of a magma anorthosic is too high so that there remain very a long time Liquide under the conditions of normal temperature of the earth's crust, which in fact a not very probable assumption. One then could show that the presence of Steam could lower the solidus of a magma anorthosic to reach more reasonable values. But as the majority of anorthosites are relatively low in water, one supposed that the steam could be driven out by a later Métamorphisme of anorthosite. However there exist anorthosites which did not undergo any deformation, which makes this assumption null and void.

The discovery, at the end of the years 1970, the anorthosic Dyke S in the plutonic continuation of Dwarf let think that it had perhaps again to be considered the possibility that the magmas anorthosic survive the conditions of temperature reigning in the earth's crust. But it proved that these dykes were of nature more complex than it had not appeared to with it at the beginning. In short, even if the processes characteristic of the liquid state clearly were with work in some plutons of anorthosite, it is not very probable that those come from magmas anorthosic.

A great number of researchers defend the idea according to which anorthosites would be the product of a basaltic magma and which the absence of minerals mafic would be due to a process of mechanical elimination. Since one does not find minerals mafic associated with anorthosites, it is that they have to settle either with more a great depth, or at the base of the earth's crust. The most current theory is the following one: fusion partial of the coat would produce basaltic magma which, instead of rising immediately in the earth's crust, would form a vast magmatic room with the foot of this crust, causing the split crystallization of a large quantity of minerals mafic which would settle on the bed of the magmatic room. The plagioclase crystals which are formed simultaneously would remain on the surface and would be introduced thereafter into the earth's crust in the form of plutons of anorthosite. The majority of the minerals mafic which settle would form Cumulat S ultramafic which remain with the foot of the crust.

This theory has more than one advantage, in particular that to explain the chemical composition of megacrystals of orthopyroxene rich in aluminum (in English: H igh has luminium O rthopyroxene M egacrysts , shortened in HAOM ). One will find further details on this point in the left which to them is devoted. It does not remain about it less than for it only this theory does not suffice to explain in a coherent way the origin of anorthosites, and this because, inter alia, it contradicts certain important isotopic measurements taken on samples of anorthosite coming from the Plutonic Continuation of Dwarf. Information coming from the Isotope S from Nd and Sr shows that the magma which produced anorthosites cannot come only from the coat, but which it comes on the contrary mainly from the Earth's crust. This discovery obliged the researchers to work out an assumption more complex than the preceding one: a large quantity of basaltic magma would form a room magma tick with the foot of the crust, integral of large quantities of this crust during the process of crystallization (Emslie; Emslie) make the assumption that the méga crystals of pyroxene rich in aluminum ( H igh has luminium O rthopyroxene M egacrysts = HAOM in English) crystallized with one great depth, with the foot of the Earth's crust. The maximum quantities of aluminum correspond to a depth of 30 with 35 km.

Other researchers (cf Xue and Morse. Anorthosite appears in good place among the samples of rocks brought back of the the Moon and plays a big role in the study of the planet Mars, Venus and the Météorite S.

See too

Feldspar
Plagioclase
Anorthite
White feldspar

Random links:

_de_Cimbergo