**4. Petrological specifications of Anorthosites**

Plagioclase is a major constituent mineral which is really important to identify the composition, origin and evolution of igneous rocks and it is observed at different parts of the Earth and even other parts of world such as the highlands of the Moon or the Martian crust. Johann F. C. Hessel showed at 1826 that plagioclase feldspars are solid solutions of albite (the sodium (Na+ ) endmember of the plagioclase solid solution series) and anorthite (the calcium (Ca2+) endmember of the plagioclase feldspar mineral series) that the formula of pure albite and pure albite endmembers are NaAlSi3O8 and CaAl2Si2O8, in turn.

General speaking, Anorthosites are dominantly contained Plagioclase minerals that the first researchers such as Thomas Sterry Hunt at 19 centuries understood this fact. These pioneer geologists tried to explain the origin of Anorthosites like the granite. For instance, there is a lot of anorthosite lenses within Archean gneisses around diverse parts of world such as Africa, Greenland or even the Scotland. So, it is supposed that they get formed like Gneiss. Gneisses are metamorphic rock with foliated texture that they get made by heating and squeezing previous igneous and sedimentary rocks during mineral recrystallizations and The Lewisian complex with the Archean age at the north of Scotland is a typical example and these granitic gneisses form the basement of later deposited sediments. Therefore, these pioneers tried to illustrate the origin of huge massif anorthosites basis on this natural procedure.

A lot of Archean anorthosites are lenses within Archean gneisses. Moreover, it is clear that the ambient temperature of the Archaean mantle (1500–1600°C) was 200–300°C higher than today's mantle (1300–1400°C) [15, 16], so, the higher mantle temperatures under the Archaean oceanic spreading means more 7–10% partial melting than present-day and it was caused thicker oceanic crust was made in comparison to modern ocean ridges. In a simple language, there was more voluminous basaltic magma with generating large, shallow magma chambers. In fact, higher geothermal gradients in the Archaean oceanic crust let shallow magma chambers cool slowly and it was caused more differentiation and stratification to produce cumulates of dunite, peridotite, chromitite, pyroxenite, gabbro and anorthosite (See **Figure 8**).

As far as, lithological and geochemical characteristics are concerned, it seems that these Anorthosite-bearing layered intrusions in the southern West Greenland and Canada are subject Archaean subduction-related ophiolites. The changes of anorthosites can reflect the thermal evolution of the Earth. Furthermore, the presence of water at these magma zones had played great role in the formation of plagioclase mega crystals due to rising cooling time.

In one hand, the major difference between the Earth and Moon and their influence on the formation of anorthosite is the presence water, and in first glance it seems that anorthosite could not form on the Earth if the terrestrial magma ocean was saturated with the water. By considering lunar anorthosites, it can be interpreted that the early earth - terrestrial magma was not saturated with water as the moon [17], so, it is possible that these plagioclases crystallized from the dry terrestrial magma ocean, so the Earth and the Moon were maybe both dry just after the formation.

#### **Figure 8.**

*Simplified geodynamic model for the origin of Archaean anorthosite-bearing layered intrusions. (c) shows an example of mineralogically stratified sill, representing a small version of Archaean magma chambers [16].*

It should be mentioned that the origin of the water on the Earth is still debated, one of the scenarios is the "late-veneer" hypothesis7 in which the Earth and the Moon were dry just after the formation of the Moon, and late-accretion of volatilerich carbonaceous chondrites beyond the asteroid belt supplied water to the "dry" Earth. In this case, it is possible that plagioclase crystallized from the "dry" terrestrial magma ocean and anorthosite formed on the Hadean Earth [17].

Geological records of anorthosite crusts of the Hadean8 have been erased by tectonic erosion on the Earth, or reprocessed by impacts. Once the anorthosite is subducted to a depth of 30 km, the plagioclase changes to garnet due to the phase transition, and the density of the garnet-composing "meta-anorthosite" becomes higher than the pyrolite. The result suggests that the meta-anorthosite could easily be transported into the mantle due to the density difference. Future works should be focusing on the detection of the geophysical evidences of meta-anorthosite buried in the deep interior of the Earth [17].

On the other hand, at the end of Hadean around 4 billion years ago, Earth changed from having a hot, molten surface and atmosphere full of carbon dioxide to being very much like it is today. Furthermore, the abundance of large anorthosite massifs in the Proterozoic<sup>9</sup> (2.5 billion years ago up to 500 million years ago) are good indicators of other the early Earth history and inherits and

<sup>8</sup> It began with the formation of the Earth about 4.6 billion years ago and ended at 4 billion years ago.

<sup>7</sup> Basis on late-veneer hypothesis, it seems nearly all earth water were formed by impacts with icy comets, meteorites and other passing objects at the Late accretion (about 4.5 billion to 3.8 billion years ago).

<sup>9</sup> It began from the appearance of oxygen in Earth's atmosphere (2.5 billion years ago) to just before the proliferation of complex life at 500 million years ago.

it point outs a specific condition of earth-crust evolution and the Proterozoic magmatism.

It seems the Archaean—Proterozoic boundary represents a transitional period during which the Archaean-thickened continental crust was uplifted and eroded to give rise to abundant clastic debris [18]. A Continental rift is a too much deformed continental lithosphere which can lead to form new ocean basins that the East African Rift System is a typical example of this type of tectonic valley. Even, it is possible that, some parts of this old crustal component may be derived either by direct erosion of Archean rocks. It is clear that the Earth's heat flow had been nearly three times as high as it is today in the beginning of the Archean and it was still twice the current level at the transition from the Archean to the Proterozoic (2,5 billion years ago).

It owns to say that, magmatic differentiation (Igneous differentiation) owns important role in making these old anorthosite rocks. It should be reminded that is an umbrella term to describe the natural processes such as partial melting process, cooling, emplacement, or even eruption in which magmas undergo bulk chemical changing. The composition of magma (Parental-primitive magma) can be different into diverse composition magma due to these factors. For example, basis on diverse cooling rate, the various crystallize minerals get created from the melt or liquid portion of the magma. Also, contamination is another cause of magma differentiation which made by mixing other wall rocks of magma chambers.

The anorthosites are even observed at younger the Phanerozoic rocks as xenolith or Layers within Layered Intrusions as in Africa and Nigeria. Finally, it seems anorthosites can be caused from each basaltic magma mass as if the opportunity of accumulation and sorting of plagioclase crystals get provided, as if it was occurred at the lunar surface or early earth crustal evolution time.
