**2. What are Anorthosites?**

Anorthosites are a fascinating type of igneous rocks which are composed predominantly of calcium-rich plagioclase feldspar and mostly formed during Precambrian times. It has to be reminded that all found anorthosites on Earth are contained coarse crystals (See **Figures 1** and **2**) but moon anorthosite samples are finely crystalline which were collected from The Moon highlands (See **Figures 3**–**5**).

Lunar anorthosites within the light-colored areas of the Moon's surface are subject of investigations to understand the history of moon evolution. It seems the light coloured lunar highland crust was formed by the crystallization and floatation of plagioclase from a global magma ocean, although the actual generation mechanisms are still debated [5].

Additionally, Lunar anorthosites are known for displaying a limited range of plagioclase (∼94 to 98) and basis nearly recent studies at 2019 [6, 7], the plagioclase trace-element variations from Apollo ferroan anorthosites (FAN) samples (collected by the Apollo 15 and 16 missions) display more significant chemical heterogeneity and it can give us a better illustration about the timing and formation mechanisms of the Moon's crust. This study clarified a chemical heterogeneity within the LMO1 as a global moon feature in the crustal formation. It seems that maybe during mantle overturn, the act of exhuming deep maficrich cumulates to the base of the lunar crust would have triggered decompression

<sup>1</sup> The lunar magma ocean (LMO).

**Figure 1.** *Large outcrop of anorthosite in the Helleren massif, Jibbeheia, Rogaland-Norway [1].*

#### **Figure 2.**

*Anorthosite from the Pare Mountains with mafic lense and Grt (spell out) corona [2].*

melting. These are likely to be small degree (<10%) partial melts2 , which are typically enriched in incompatible elements3 . It is not contradict with the idea of

<sup>2</sup> Partial melting is the transformation of some mass solid rock fraction into a liquid as a result of heat input, decompression or flux addition. This resulting liquid is nominated either magma or lava as if eruption from a volcano.

<sup>3</sup> Incompatible elements own difficulty in entering cation sites of the minerals, so they get concentrated in the melt phase of magma.

**Figure 3.** *Black and White Photograph of Apollo 15 Sample (s) 15415-Nasa - Lunar Samples [3].*

#### **Figure 4.**

*Ferroan Anorthosite (1836 grams)- 60025. NASA #S72–41586. Cube and scale are 1 cm. Note the thick black glass coating and numerous micrometeorite pits [4].*

#### **Figure 5.**

*Plane-polarized and cross-polarized photos of thin Section 60025,21. Field of view is 2.5 mm. S79–27300 and 301 [4].*

local LMO magmas infiltration4 by more evolved liquids through metasomatism<sup>5</sup> process, too.

The terrestrail anorthosites can be divided into five types: Archean-age anorthosites, Mid-ocean ridge and transform fault anorthosites, Anorthosite xenoliths in other rocks, Layers within Layered Intrusions and Proterozoic anorthosite as the most abundant anorthosite kind. Lunar geological studies also clarified that the light areas of the moon also consist of very old anorthosite. The dark areas of the moon, many of which are circular, consist of the black volcanic rock basalt. In fact, the primary pre-Nectarian lunar highlands are contained light-grey anorthosites which are caused at the early moon crust formation.

## **3. Anorthosites- geologic timescale**

The geological history of Moon is categorized into six major epochs which is called the lunar geologic timescale. The boundaries of this time scale are subject to

<sup>4</sup> Infiltration is defined as the flow of liquid such as water from aboveground into the subsurface.

<sup>5</sup> Metasomatism is replacing one mineral with another that mineral dissolution and minerals deposition are occurred at the same time.

large impact events which affected the lunar surface through time as crater formation and their absolute ages got determined by radiometric dating of obtained lunar samples. The lunar surface is generally modified by impact cratering and volcanism, so, it is possible to define the lunar geological events in time basis on stratigraphic principles [8–10].

In the moon history, the Nectaris Basin and other major moon basins were formed by large impact events at the Nectarian Period (from 3920 million years ago to 3850 million years ago). Also, the pre-Nectarian period of the lunar geologic timescale is between 4.53 billion years ago to 3.92 billion years ago. It is extended between the initial Moon formation time to Nectaris Basin by a large impact. During this time, these light anorthosites came from the cooling of the surface lunar magma ocean (LMO). The impact event is subject the collision of astronomical objects like asteroids, comets or meteoroids on the surface of planets and moons, and they have a significant role in the evolution of the Solar System like the Earth and the Moon. For instance, approximately 4.5 billion years ago the Moon created from the ejecta of a collision between the proto-Earth and a Mars-sized planetesimal and it seems a giant hit causing from this hit at both parent bodies (I, e the earth and moon) [11].

The pre-Nectarian system was characterized by the formation of an anorthositic crust and its subsequent brecciation by large basin- forming impact events [12]. In fact, it contains the minerals that were emplaced at the lunar surface over the period extending from the moon formation to the excavation of the Nectaris impact- basin. According to studies, the lunar highlands being compose of anorthositic gabbros and gabbroic anorthosites, but the pre-Nectarian terrain are now probably breccias (rather than pristine igneous rocks) produced during the excavation of the basin cavities.

During the Nectarian Period (*from 3920 million to 3850 million years ago*), the Nectaris Basin and other major lunar mare or sea basins (Volcanic-basaltic lava as the dark spots of moon) were formed by large impact events that Ejecta from Nectaris forms the upper part of the densely cratered terrain found in lunar highlands. In the following of Nectarian Period, along with the Late Heavy Bombardment of the Inner Solar System, the huge Mare Imbrium basin was formed at 3850 to 3800 million years ago (The Early Imbrian epoch), then, this basin and other formed basins get filled with basalt mostly during the subsequent Late Imbrian epoch (3800 to about 3200 million years ago) (See **Figure 6**).

The lunar geological timescale went away on with *Eratosthenian* (from 3,200 million to 1,100 million years ago) and the massive basaltic volcanism of the Imbrian period tapered off and ceased during this long span of lunar time. In the following, impact craters lost their bright ray systems (thrown out radial streaks of fine ejecta at the formation of impact crater) due to space weathering processes<sup>6</sup> and It ends up to *The Copernican period* (From 1.1 billion years ago to the present day) in which light freshly excavated lunar surface has been growing to become darker over time due to space weathering processes.

This lunar geologic time scale is based on the recognition of few geomorphological markers, but it is practical to correlate the geological events at the solar system, in specific too much far distant early Earth time. The Hadean eon of Earth between 4.6 to 4 billion years ago which it begun with the Earth formation and ended up before the earliest-known rocks on Earth. According to the Lunar geologic timescale, this eon can be subdivided to *Pre-Nectarian* (the formation of the Moon's crust, 4533 to 3920 million years ago) and Nectarian (the Late Heavy Bombardment, from 3920 to 3850 million years ago). Some Hadean rocks as the oldest Earthen rocks

<sup>6</sup> Including, bombardment surface planetary bodies like the Moon or the Mercury by different sizes of meteorites, coalition of solar cosmic rays, solar wind and galactic cosmic rays.

**Figure 6.** *The location of lunar Mare Imbrium [13].*

were found out at western Greenland, northwestern Canada, and Western Australia with the oldest dated zircon crystals in a metamorphosed sandstone conglomerate in the Jack Hills of Western Australia which these xenocryst zircons formed after 200 million years after Earth formation.

The Earth history can be subdivided to few Eons which are the Hadean, the Archean, the Proterozoic and the Phanerozoic (See **Figure 7**). Each eon can be divided into eras, which are in turn divided into periods, epochs and ages. The Phanerozoic (541 million years ago to the present) is the current geologic eon with abundant animal and plant life and it begun at the Cambrian Period when the first hard shells of animals developed and have been preserved as fossil record. The time before the Phanerozoic Eon which is the Hadean, Archaean and Proterozoic eons are called the Precambrian, too.

During the Archean eon (between 4,000 to 2,500 million years ago), the Earth's crust had cooled enough in forming continents and also the emergence of life. At the early Archean, the Earth's heat flow was nearly three times as high as it is today and also it was still twice the current level at the transition from the Archean to the Proterozoic (2.5 billion years ago) due to the heat remnant of planetary formation and the decay of radioactive elements.

Therefore, it can be assumed that these volcanic activities at the Precambrian had great role in The Cambrian explosion at 541 million years ago to rapid emergence life diversification at a short period time between 13 to 25 million years and this trend has being continued. It owns to say the life was mostly simple unicell organisms at the Precambrian, but it got suddenly was turned into colonies as multi-cellular life after the snow earth period.

There is a lot of exposed rocks relevant to this eon which can be observed around the globe such as Greenland, Siberia, the Canadian Shield, the Baltic Shield, India,

**Figure 7.** *Earth geologic time scale- Scale: millions of years [14].*

#### *A View to Anorthosites DOI: http://dx.doi.org/10.5772/intechopen.97787*

Brazil, western Australia, and southern Africa. For example, granitic rocks have been significantly observed from the crystalline remnants of Archean crust, and also other rocks like monzonites (An igneous intrusive rock with equal amount of plagioclase and alkali feldspar and less than 5% quartz) anorthosites (*mostly plagioclase feldspar (90–100%*)) and, too.
