2.2 Ophiolite-island arc assemblages

The CED and SED of Egypt are characterized by the widespread distribution of Neoproterozoic ophiolite, ophiolitic mélanges, and intra-oceanic island arc metavolcanic assemblages, along with volcaniclastic metasediments and banded iron formations (e.g., [17–20]). Locally, nearly complete ophiolitic sequences can be observed including serpentinized peridotites, gabbros, sheeted dykes, pillow lavas, and deep-sea sedimentary rocks such as in Ghadir, Muweilih, Esel, El Sid areas [21]. Sheeted dikes are only locally preserved in some localities, whereas pillowed metabasalts are widespread. The ophiolitic peridotites are almost completely serpentinized and are typically altered to talc-carbonate and quartz-carbonates (listwanite) bodies along shear zones. El Bahariya [20] classified the Neoproterozoic ophiolites of the Central Eastern Desert of Egypt based on field geology and mode of occurrences, together with compiled geochemical data into three types: (i) intact MORB ophiolites, (ii) dismembered ophiolites (dismembered blocks and fragments within the mélanges and ophiolites along structural contacts), and (iii) arc-associated ophiolites. The best preserved and nearly intact MORB ophiolites are represented by Wadi Ghadir and Muweilih ophiolites. The arc-associated ophiolite sequences are exposed in Abu Dahr ophiolite, Esel, and El Sid occurrences. Dismembered ophiolites occur either as individual blocks and sheets tectonically emplaced along tectonic contacts or as blocks and fragments within a sheared matrix of volcaniclastic metasediments or metapyroclastics forming "ophiolitic mélange" [19].

The island arc assemblages are concentrated mainly in the CED and SED. They include:


The metamorphosed volcanic island arc assemblages are widespread in the CED and SED [22–24]. They are composed of metavolcanics and related volcaniclastic metasediments. The metavolcanics include metabasalts, metandesites, metadacites, metarhyodacites, and metarhyolites, together with their metapyroclastic counterparts. The volcaniclastic metasediments comprise meta-mudstones, metasiltstones, metagreywackes, metaconglomerates, and schists. The volcaniclastic metasediments together with the metapyroclastics constitute the matrix of the "ophiolitic mélange" [19]. The exotic fragments within melanges are mainly ophiolites of variable sizes and shapes, which include serpentinite and metamorphosed ultramafic rocks, metagabbros, pillowed and massive metabasalts, and minor sheeted dykes and pelagic sedimentary rocks. El Bahariya [19] documents different occurrences of Neoproterozoic ophiolitic melanges in the CED of Egypt and classified the ophiolitic mélanges into:

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

(i) tectonic mélange, (ii) olistostrome, and (iii) olistostromal mélange. Ophiolitic melanges are also recorded in different occurrences in the SED such as Atshan Ophiolite, Gerf, and Abu Dahr [25].

The bimodal metamorphosed island arc assemblage comprises mafic and felsic volcanic intercalations and arc-related volcanoclastics sediments. They are regionally metamorphosed up to the greenschist facies, locally transformed into schists and amphibolites and commonly associated with banded iron formations and massive sulfides [13, 26, 27]. The metavolcanics together with intra-arc volcaniclastic metasediments occur in different localities in the CED and northern part of SD such as Um Khariga and metapyroclastics, Sodamine, Um Samuky, and El Shadly metavolcanics. The Shadli metavolcanics host some polymetallic massive sulfide mineralizations, e.g., Um Samiuki and Abu Gurdi [28].

## 2.3 Granitoid rocks

Beitan domes [11–15]. They are generally surrounded by low-grade supracrustal assemblages, and the contact between superstructure and infrastructure is sometimes an intrusive contact and sometimes a high-strain mylonitic zone [16]. The Meatiq Dome consists of Um Baanib deformed granite (cataclastic gneissose granite) forming the core of the dome, followed outward by schists with variable degrees of intercalated amphibolites, together with local mylonites along thrust faults [13]. The Neoproterozoic migmatitic rock association at Wadi Abu Higlig in the Hafafit region is composed of diatexites and schlieric granites (foliated or gneissic granite) in the core of a domal structure flanked by metatexites and pre-

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

The CED and SED of Egypt are characterized by the widespread distribution of

metavolcanic assemblages, along with volcaniclastic metasediments and banded iron formations (e.g., [17–20]). Locally, nearly complete ophiolitic sequences can be observed including serpentinized peridotites, gabbros, sheeted dykes, pillow lavas, and deep-sea sedimentary rocks such as in Ghadir, Muweilih, Esel, El Sid areas [21].

Neoproterozoic ophiolite, ophiolitic mélanges, and intra-oceanic island arc

Sheeted dikes are only locally preserved in some localities, whereas pillowed metabasalts are widespread. The ophiolitic peridotites are almost completely serpentinized and are typically altered to talc-carbonate and quartz-carbonates (listwanite) bodies along shear zones. El Bahariya [20] classified the Neoproterozoic ophiolites of the Central Eastern Desert of Egypt based on field geology and mode of occurrences, together with compiled geochemical data into three types: (i) intact MORB ophiolites, (ii) dismembered ophiolites (dismembered blocks and fragments within the mélanges and ophiolites along structural contacts), and (iii) arc-associated ophiolites. The best preserved and nearly intact MORB ophiolites are represented by Wadi Ghadir and Muweilih ophiolites. The arc-associated ophiolite sequences are exposed in Abu Dahr ophiolite, Esel, and El Sid occurrences. Dismembered ophiolites occur either as individual blocks and sheets tectonically emplaced along tectonic contacts or as blocks and fragments within a sheared matrix of volcaniclastic metasediments or metapyroclastics forming "ophiolitic mélange" [19].

The island arc assemblages are concentrated mainly in the CED and SED.

The metamorphosed volcanic island arc assemblages are widespread in the

metandesites, metadacites, metarhyodacites, and metarhyolites, together with their metapyroclastic counterparts. The volcaniclastic metasediments comprise meta-mudstones, metasiltstones, metagreywackes, metaconglomerates, and schists. The volcaniclastic metasediments together with the metapyroclastics constitute the matrix of the "ophiolitic mélange" [19]. The exotic fragments within melanges are mainly ophiolites of variable sizes and shapes, which include serpentinite and metamorphosed ultramafic rocks, metagabbros, pillowed and massive metabasalts, and minor sheeted dykes and pelagic sedimentary rocks. El Bahariya [19] documents different occurrences of Neoproterozoic ophiolitic melanges in the CED of Egypt and classified the ophiolitic mélanges into:

i. metamorphosed volcanic island-arc assemblage and

ii. metamorphosed bimodal volcanic island arc assemblage.

CED and SED [22–24]. They are composed of metavolcanics and related volcaniclastic metasediments. The metavolcanics include metabasalts,

served amphibolites and metagabbros [14].

2.2 Ophiolite-island arc assemblages

They include:

4

The granitoid rocks constitute about 50% of the basement complex of Egypt. They can, in general, be classified into older and younger granitoids based on their composition, color, and relative age [29]. The older granitoids (850–635 Ma) comprise trondhjemites, tonalites, granodiorites, and rarely granites, whereas the younger granitoids (630–540 Ma) are predominated by granites and alkali feldspar granites [6]. The younger granites are further classified according to their geological setting and petrography [30] into: (i) phase I granodiorites with minor monzogranites, (ii) phase II (monzogranites and syenogranites), and phase III (alkali feldspar granites). Recently, part of the Younger granites (phase III) are classified as A-type granites [31].

## 2.4 Nonmetamorphosed rocks

### 2.4.1 The Dokan volcanic rocks

The later stage of the crustal evolution of the NED and CED is characterized by the eruption of the Dokhan volcanics, which typically include basaltic andesite, andesite, dacite, and rhyolite, together with tuffs, ignimbrite, and agglomerates [32].

#### 2.4.2 Hammamat sediments

The best exposures of the Hammamat molasses sediments found in Wadi Hammamat area of the CED of Egypt [29], where the sedimentary rocks unconformably overlie other old rock units and consist of unmetamorphosed thick sequences of unsorted conglomerates, sandstones, and siltstones. Most of the Hammamat fragments were derived from the Dokhan volcanics and their thickness varies between 4000 m in Wadi Hammamat and 7500 m thick in the Kareim basin. Locally, the Hammamat sediments are sheared and metamorphosed [33].

## 3. Geochemistry

The compiled available chemical data from of the ED of Egypt are used for the purpose of understanding the geochemistry of Neoproterozoic rocks, and to clarify their geochemical characteristics and tectonic settings. The overall geochemical characteristics of the different rock units are presented as follows.
