4. Age dating and crustal evolution

Um Ba'anib gneissose granites in the core of Meatiq dome dated 626 [78] and 631 Ma [10]. Ali et al. [15] obtained a zircon age of 631 6 Ma for El-Shalul granitic gneiss. Kröner et al. [36] reported single zircon evaporation ages of 677 9 and 700 12 Ma for granitoid gneisses from the Hafafit gneiss complex and 704 + 8 Ma for migmatitic granitic gneiss from Wadi Bitan. Magmatic emplacement ages for samples from Wadi Beitan yielded 719 10, 725 9 and 744 10 Ma, indicating that the gneiss protoliths are Neoproterozoic [2].

The ophiolitic rocks of the ED have isotopic ages range from 890 to 690 Ma, documenting a 200 Ma year period of oceanic magmatism [79]. The Gerf ophiolites seem to be formed at 741 21 [80], 750 [41], and 730–750 [79]. The ages of the well-preserved ophiolitic rocks in Wadi Ghadir (746 19 Ma, [80]) and in Fawakhir (736.5 1.2 Ma [10]) in the CED are compatible with the 750 Ma crust forming event proposed by [49].

Stern and Hedge [6] date ED island-arc volcanics to 720–770 Ma. The mafic and felsic lavas of Shadli island arc metavolcanics yield Rb-Sr isochron age of 712 Ma

that probably represents the time of volcanic eruption [28]. Ali et al. [49] reported a protolith age of c. 750 Ma for the volcanic and volcanosedimentary rocks, and they considered that both the ophiolitic and island arc assemblages in the CED constitute an artifact of one (750 Ma) crust-forming event.

Gebel Umm Tawat, North Eastern Desert indicates its depositional age as

Geochemistry and Tectonic Setting of Neoproterozoic Rocks from the Arabian-Nubian Shield…

Gondwana to form the East African orogen [3]; and (3) post-collision stage

The tectonic evolution of the ANS is commonly divided into three major stages, namely: (1) subduction stage (\_870–635 Ma) during which oceanic crust, island arc volcano-sedimentary sequences, and plutonic rocks formed; (2) continental collision (640–650 Ma) resulting from continuing convergence between East and West

(580–540 Ma), evidenced by stabilization of ANS crust accompanied by the cutting of a vast peneplain [86]. Development of sedimentary basins and emplacement of increasingly alkaline igneous rocks took place during the last two stages (e.g., [87]). Finally, the ANS stabilized as continental crust by Early Cambrian time (525 Ma) [1]. The tectonic scenario for ED of Egypt can be summarized as follows [88]:

1. intra-oceanic island arcs were formed in the Mozambique Ocean (Figure 8A)

2. suture possibly by a continental block in the western Egypt (Figure 8B)

which, in turn, led to extension (Figure 8D).

dating of the different rock assemblages.

5. Concluding remarks

15

3. arc accretion led to substantial lithospheric thickening (Figure 8C). At this stage, conductive heating of the lithospheric root decreased the strength of the crust. The thickened crust became gravitationally unstable and collapsed,

4.Crustal thinning, through large low-angle normal shear zones, allowed the intrusion of A-type granites. The isostatic rebound and the intrusion of these granites contributed to the doming of the lower crust and the development of

metamorphic core complexes such as the Meatiq domes (Figure 8E). Sedimentary basins, bordered by normal faults, were formed at the upper crustal levels as a response to the extension and allowed the deposition of

5. post-orogenic molasse sequences as the Hammamat molasses group.

1. The Precambrian rocks of Egypt represent the northwestern part of the Arabian-Nubian Shield, which was formed during the Pan-African orogenic cycle (950–450 Ma) [89]. Island arc volcanic rocks and ophiolitic sequences formed between 700 and 800 Ma [6], and then, they were obducted in the earlier stage of the Pan-African orogeny. The Pan-African orogenic event in Egypt ended at about 615 Ma, and subsequent crustal uplifting and extensional collapse occurred within the 610–550 Ma time span [89]. This post-collision stage was characterized by the emplacement of large masses of Dokan volcanics (610–560 Ma) and shallow-level A-type granites (610–550 Ma) [6]. The most common rock units of the ED of Egypt are grouped into an ophiolitic suite/island arc assemblage and post-orogenic intrusions (Figure 9) [54]. Table 1 summarizes the geochemical characteristics, tectonic setting, and age

2. Collectively, the different types of the ED ophiolites fall geochemically and tectonically into two separate groups: MORB-like ophiolites formed in a backarc tectonic setting and SSZ ophiolites of fore-arc tectonic setting. The tectonic setting of the ophiolites changed from MORB to SSZ with time. Formation of an intra-oceanic island arcs and related volcaniclastic sediments is followed by

585 13 Ma [85].

DOI: http://dx.doi.org/10.5772/intechopen.82519

The emplacement of the Egyptian late- to post-tectonic younger granites covers a time span between 600 and 550 Ma, [6] and 600 and 475 Ma [15]. The underformed Um Had granite has a U-Pb zircon age of 590 3.1 Ma [10]. Some alkaline A-type granites in the NED of Egypt (Al-Missikat, Abu Harba, and Gattar) dated ca. 600 Ma [81]. Available isochron Rb▬Sr ages of alkaline granites are from Sinai fall in the range 550–600 Ma [82].

The transition in the tectonic style from compressional to strong crustal extension is at approximately 600 Ma [66]. Breitkreuz et al. [83] reported age range between 592 and 630 Ma (early Ediacaran) for acidic Dokan volcanics indicating that Dokan volcanism occurred over a 40 Ma time span.

Most of the Eastern Desert molasse basins were evolved between 650 and 580 Ma in individual basins with different individual tectonic settings (e.g., [84]). Rb-Sr whole-rock analyses give an age of 585 15 Ma that approximates the time of sedimentation [75]. U▬Pb dating of clastic zircons from the Hammamat group at

Geochemistry and Tectonic Setting of Neoproterozoic Rocks from the Arabian-Nubian Shield… DOI: http://dx.doi.org/10.5772/intechopen.82519

Gebel Umm Tawat, North Eastern Desert indicates its depositional age as 585 13 Ma [85].

The tectonic evolution of the ANS is commonly divided into three major stages, namely: (1) subduction stage (\_870–635 Ma) during which oceanic crust, island arc volcano-sedimentary sequences, and plutonic rocks formed; (2) continental collision (640–650 Ma) resulting from continuing convergence between East and West Gondwana to form the East African orogen [3]; and (3) post-collision stage (580–540 Ma), evidenced by stabilization of ANS crust accompanied by the cutting of a vast peneplain [86]. Development of sedimentary basins and emplacement of increasingly alkaline igneous rocks took place during the last two stages (e.g., [87]). Finally, the ANS stabilized as continental crust by Early Cambrian time (525 Ma) [1]. The tectonic scenario for ED of Egypt can be summarized as follows [88]:

