5. Concluding remarks

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

Applied Geochemistry with Case Studies on Geological Formations, Exploration Techniques…

The emplacement of the Egyptian late- to post-tectonic younger granites covers

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

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

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

A cartoon displaying the different stages of the evolution of the Arabian-Nubian Shield after [88].

a time span between 600 and 550 Ma, [6] and 600 and 475 Ma [15]. The

an artifact of one (750 Ma) crust-forming event.

from Sinai fall in the range 550–600 Ma [82].

Figure 8.

14

that Dokan volcanism occurred over a 40 Ma time span.


the incorporations of ophiolite fragments into the volcaniclastic matrix to form "ophiolitic mélange" through tectonic and/or concurrent sedimentary and tectonic processes to be formed in an interarc or back-arc basin [19]. Subsequent to this stage, the volcanic eruptions of bimodal-evolved island arcs are contemporaneously or shortly followed by deposition of volcaniclastic sediments in an arc-rift basin known as "intra-arc basin" [26].

Rock units Geochemical characters and tectonic

volcanic SSZ ophiolites

Granitoid rocks Old tonalite-granodiorite, calcalkaline, I-

setting

mafic rocks

Arc metavolcanics with slightly fractionated REE patterns of island arc affinity; bimodal metavolcanics with felsic lavas are slightly LREE-depleted, whereas basalt is slightly LREEenriched; arc-related volcaniclastic sediments (interarc to intra-arc basin)

type, volcanic arc granites, syn- to lateorogenic granitoids; most of younger granitoids are mainly alkaline, A-type granites, and of within-WPG tectonic

Generally, have steep LREE and nearly flat HREE; medium-K to high-K calcalkaline affinity and continental arc to within plate tectonic setting (transitional setting)

Greywackes with relatively high Zr, Nb, Y, and TH and relatively low Cr, Ni and V, and Sc; have enriched LREE pattern; of active continental margin or continental island arcs tectonic setting and appear to be formed in pull-apart intermontane basins; their sources are felsic source of evolved magmatic island arcs and active continental margin together with minor inputs from calcalkaline island arcs or ophiolitic

Summary of geochemical characteristics, tectonic settings, and age dating of the different Neoproterozoic rock

Granite gneiss core varies from enriched REE, to alkaline granite REE pattern; from enriched in HFSE, Rb, Ga, and total REE showing A-type characters and within plate tectonic setting, to granitic gneisses of calcalkaline and of I-type granites and of volcanic arc tectonic

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

MORB-like ophiolites formed in a backarc tectonic setting and SSZ ophiolites of fore-arc tectonic setting; flat REE MORB volcanic ophiolites, and depleted to slightly enriched LREE pattern for

Age dating

631 Ma for Meatiq granite gneisses [10]; 700 Ma for Hafafit granite gneisses [38]; Wadi Beitan migmatitic granitic gneisses yielded 719 10, and 744 10 Ma [2]

Wadi Gerf 741 21 Ma [80]; 730– 750 Ma [79]; Wadi Ghadir 746 19 Ma, [80]; Fawakhir 736.5 1.2 Ma [10]

Island-arc volcanics to 720–770 Ma [6]; Shadli island arc metavolcanics 712 Ma [28]; 750 Ma for the volcanic and volcano-sedimentary rocks [49]

monumental granite intruded at 606 Ma, respectively [38]; younger granites covers a time span between 600 and 550 Ma, [6]; A-type granites dated ca. 600 Ma [81]; 590 3.1 Ma [10] for Um

The Aswan Tonalite and the

592 and 630 Ma for acidic Dokan

Depositional age 585 13 Ma [75, 85]

Had younger granite

volcanics [83]

settings

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

setting

Granite gneisses, amphibolites and migmatites

Ophiolite assemblage

Island arc assemblage

Dokhan volcanics

Hammamat molasse sediments

Table 1.

17

assemblages of the ED of Egypt.



#### Figure 9.

Lithostratigraphy, major tectonic events, and ages of the basement complex in the Eastern Desert of Egypt from [54].


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

#### Table 1.

the incorporations of ophiolite fragments into the volcaniclastic matrix to form "ophiolitic mélange" through tectonic and/or concurrent sedimentary and tectonic processes to be formed in an interarc or back-arc basin [19].

Applied Geochemistry with Case Studies on Geological Formations, Exploration Techniques…

Subsequent to this stage, the volcanic eruptions of bimodal-evolved island arcs are contemporaneously or shortly followed by deposition of volcaniclastic

3. The "gneissic domes" are metamorphic core complexes that were formerly interpreted to have been formed either in a compressional setting or in an extensional regime. However, the obtained age data indicated that the ED

4.Granitoid rocks in the ED include (1) old calcalkaline, I-type, syn- to lateorogenic granitoid assemblages (880–610 Ma) and (2) younger commonly alkaline, post-orogenic to anorogenic granitoid assemblages emplaced between 600 and 475 Ma. Most of the older granitoids and phase I younger granitoids are of I-type character, displaying metaluminous, calcalkaline geochemical characteristic plot in the area of volcanic arc granites (VAG), whereas younger phase granitoids are mainly alkaline, of A-type granites, and of within-plate tectonic setting (WPG). Generally, I-type granitoids were interpreted to result from melting of an amphibolitic crust and dated at approximately 760–650 Ma. The origin of A-type granites is consistent with the melting of a juvenile Neoproterozoic mantle source that assimilated some older crustal materials or

5. Dokan volcanics, sedimentary basins, and post-orogenic A-type granites were interpreted to have been formed in an extensional or rifting regime. This rifting event may have created accommodation space for the Hammamat molasse sediments that accumulated in a structurally controlled intermontane basin.

6.More geochemical and age dating studies are required to characterize the different rock units and to determine their ages, compositional variations, and consequently, to construct the tectonic evolution of the Neoproterozoic crust

Lithostratigraphy, major tectonic events, and ages of the basement complex in the Eastern Desert of Egypt

sediments in an arc-rift basin known as "intra-arc basin" [26].

granite gneissic and migmatitic rocks are juvenile in origin and

as anatectic melts of various crustal sources.

Neoproterozoic.

through time.

Figure 9.

from [54].

16

Summary of geochemical characteristics, tectonic settings, and age dating of the different Neoproterozoic rock assemblages of the ED of Egypt.

Applied Geochemistry with Case Studies on Geological Formations, Exploration Techniques…

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