2. Geologic overview of the Eastern Desert of Egypt

The Precambrian basement rocks of Egypt constitute the northern part of the socalled the Arabian Nubian Shield (ANS), which are exposed mainly in the Eastern Desert (ED) and the Sinai Peninsula (Figure 1). The general geological settings of the rock assemblages of the CED were grouped into two major lithotectonic units [8]. The structurally lower one, the "infrastructure," is composed of gneisses and migmatites that crop out in dome structures, such as the Meatiq, Sibai, and Hafafit domes. The overlying unit, the "suprastructure," includes the Neoproterozoic ophiolite complexes and island arc-related metavolcanic and metasedimentary rocks. The suprastructure is also known as the Pan-African Nappe Complex [9]. The juvenile crust in the ED of Egypt is characterized by four main rock units: (i) a gneiss assemblage that comprises the core complexes, (ii) an ophiolite-island arc assemblage, (iii) granitoid intrusions, and (iv) nonmetamorphosed to weakly metamorphosed Dokhan volcanics and Hammamat molasses sediments that unconformably overlie the suprastructure in places [10]. Most of the rock sequences are generally deformed and metamorphosed due to the Neoproterozoic East African orogeny. Below, the most important geological aspects of the main rock units are briefly summarized:

#### 2.1 Granite gneisses and migmatites

A number of medium- to high-grade core complexes or "gneissic domes" have been described in the ED. These infrastructures consist of upper amphibolite facies gneisses, amphibolites, migmatites as well as granitic gneisses. They exposed in several places in the ED, including the Meatiq, El Shalul, the Migif-Hafafit, and

Neoproterozoic crust, now widely exposed in parts of Egypt, Saudi Arabia, Sudan,

Neoproterozoic between 900 and 550 Ma through the accretion of intra-oceanic arcs during the closure of the Mozambique Ocean and the amalgamation of Gondwana [2]. These accretion processes led to the formation of well-defined arc-arc and continent-arc suture zones [3, 4]. The ANS was essentially stable continental crust by Early Cambrian time at 530 Ma [5]. The ANS and its surroundings has been the object of geologic investigations for a wide range of geological economic and scien-

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

(a) Inset geological sketch map of NE Africa showing the Arabian-Nubian Shield, the Saharan Metacraton, and Archaean and Palaeoproterozoic crust that was remobilized during the Neoproterozoic and (b) geological

map of the Eastern Desert of Egypt showing study areas [15].

Eritrea, Ethiopia, Yemen, and Somalia. The ANS was formed during the

tific reasons.

Figure 1.

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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 preserved amphibolites and metagabbros [14].

(i) tectonic mélange, (ii) olistostrome, and (iii) olistostromal mélange. Ophiolitic melanges are also recorded in different occurrences in the SED such as Atshan

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

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

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

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].

The best exposures of the Hammamat molasses sediments found in Wadi Hammamat area of the CED of Egypt [29], where the sedimentary rocks

Locally, the Hammamat sediments are sheared and metamorphosed [33].

characteristics of the different rock units are presented as follows.

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.

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

Ophiolite, Gerf, and Abu Dahr [25].

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

2.3 Granitoid rocks

classified as A-type granites [31].

2.4 Nonmetamorphosed rocks

2.4.1 The Dokan volcanic rocks

2.4.2 Hammamat sediments

3. Geochemistry

5

mineralizations, e.g., Um Samiuki and Abu Gurdi [28].
