3.3 Geochemistry of granitoid rocks

The geochemistry of both older and younger granites is briefly presented. The REE patterns of the older granodiorites (Figure 5a) show enrichment in the LREE relative to HREE, Lan/Ybn values vary from 7.08 to 35.21 (mostly between 7.08 and 19.37) and with Eu anomalies ranging from (Eu/Eu\* = 0.701.13) [56]. The slightly concave HREE pattern of some biotite suggests hornblende fractionation. The younger alkali feldspar granites are characterized by LREE-enrichment (Lan/Ybn = 5.28 13.46), moderately fractionated LREE, flat heavy REE patterns (Figure 5a and b), and moderately to strongly negative Eu anomalies (Eu/Eu\* = 0.140.63).

Most of the younger granites are LILE-enriched calcalkaline to mildly alkaline rocks commonly of A-type signatures. The younger granites, except phase I exhibit within plate tectonic setting [61], due to their high contents of HFS elements (Figure 5c). The phase-III younger granitoids (A-type) are characterized by higher SiO2, Rb, Y, and Nb and lower MgO, Sr, and Ba contents than other phases of younger granites [38]. The A-type granites are enriched in SiO2, Na2O<sup>+</sup> K2O, FeO\*/ MgO, Ga/Al, Zr, Nb, Ga, Y, Ce, Rb, and REE and low in CaO, MgO, Ba, and Sr. They are classified as alkaline, and peralkaline to mildly peraluminous A-type granites (e.g., [37]). They are generally enriched in Rb (104–198 ppm), Nb

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

(27–53 ppm), Y (35–79 ppm), Zr (348–750 ppm), and Ga (21–29 ppm), compared to average continental crustal rocks. The overall geochemical characteristics of the A-type granitic rocks of the ED and Sinai are consistent with a within-plate tectonic settings (Figure 5c). The A-type granites are eligible for A1-A2 discrimination diagrams after [62] and classified mainly as A2 types (Figure 5d), implying that the A-type granites formed mainly in a post-collisional setting. The alkaline A-type granites are generally regarded as the product of either extensive fractional crystallization of mantle-derived mafic magmas (e.g., [63]) or partial melting of various

The Dokan volcanic rocks display well-defined major and trace element trends and a continuum in composition with wide ranges in SiO2 (54–76%), CaO (8.19– 0.14%), MgO (6.96–0.04%), Sr. (983–7 ppm), Zr (328–95 ppm), Cr (297–1 ppm), and Ni (72–1 ppm). The rocks are enriched in LILEs (Rb, Ba, K, Th, Ce) relative to HFSE (Nb, Zr, P, Ti) and have high total REEs with LREE enriched and display variable degrees of enrichment according to rock type (Figure 6a) [64]. The intermediate volcanics are characterized by moderate total REE and moderately fractionated patterns with slightly negative Eu-anomalies. Similarly, the REE pattern for the rhyolites is almost identical but with relatively lower content of REE. Generally, the Dokhan volcanics have steep LREE and nearly flat HREE and the large negative Eu anomalies in the rhyolite rocks than those of other varieties indicating formation under condition of relatively low temperature and pressure and/or low

The geochemistry of the Dokhan volcanic rocks indicates medium-K to high-K calcalkaline affinity, and their tectonic setting is suggested to be: (i) subduction related [65], (ii) extensional setting/rift system (e.g., [66]), and (iii) transitional stage between subduction and extension (e.g., [67, 68]). However, the Dokan lavas mostly plot in an overlap zone between the volcanic arc and within-plate settings on the binary SiO2▬Nb diagram of Pearce and Gale [69] (Figure 6b), suggesting a

It is of great importance to assess the composition and nature of the source rocks of the Hammamat molasses sediments geochemically, and to determine their tectonic settings. The HFSE are incompatible during most igneous processes; therefore, they tend to be enriched in felsic relative to mafic rocks. Also, they are generally resistant to changes during weathering and alteration processes [70]. The greywackes of the Hammamat molasses sediments have relatively high Zr, Nb, Y, and TH and relatively low Cr, Ni and V, and Sc. Figure 7a shows that Um Hassa

crustal sources (e.g., [37, 49]).

water content in the melt.

transitional tectonic setting.

11

3.4.2 Geochemistry of Hammamat molasse sediments

3.4 Geochemistry of nonmetamorphosed rocks

3.4.1 Geochemistry of Dokan volcanic rocks

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

Geochemistry of older granites reveals that they are metaluminous to slightly peraluminous and have calcalkaline affinity. The older granite can be classified as I-type granites and of volcanic-arc-granite tectonic setting (Figure 5c) (e.g., [57]). In the ANS, the I-type granitoids were generally interpreted to result from melting of an amphibolitic crust (e.g., [58]). Moreover, older I-type granites can form through fractionation from mantle-derived, LILE-enriched basaltic melts in subduction settings (e.g., [59]), or from remelting of mafic to intermediate igneous lower crust [60].

#### Figure 5.

(a) REE of older granites and (b) REE of younger and A-type granites (from [56]); (c) tectonic setting of granitoid rocks using diagram of Pearce et al. [61], data of older and younger granitoids from [38]; (d) division of A-type granites [62], field of Egyptian A-type granites from combined data from [37, 60] and references therein.

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

Most of the younger granites are LILE-enriched calcalkaline to mildly alkaline rocks commonly of A-type signatures. The younger granites, except phase I exhibit within plate tectonic setting [61], due to their high contents of HFS elements (Figure 5c). The phase-III younger granitoids (A-type) are characterized by higher SiO2, Rb, Y, and Nb and lower MgO, Sr, and Ba contents than other phases of younger granites [38]. The A-type granites are enriched in SiO2, Na2O<sup>+</sup> K2O, FeO\*/ MgO, Ga/Al, Zr, Nb, Ga, Y, Ce, Rb, and REE and low in CaO, MgO, Ba, and Sr. They are classified as alkaline, and peralkaline to mildly peraluminous A-type granites (e.g., [37]). They are generally enriched in Rb (104–198 ppm), Nb (27–53 ppm), Y (35–79 ppm), Zr (348–750 ppm), and Ga (21–29 ppm), compared to average continental crustal rocks. The overall geochemical characteristics of the A-type granitic rocks of the ED and Sinai are consistent with a within-plate tectonic settings (Figure 5c). The A-type granites are eligible for A1-A2 discrimination diagrams after [62] and classified mainly as A2 types (Figure 5d), implying that the A-type granites formed mainly in a post-collisional setting. The alkaline A-type granites are generally regarded as the product of either extensive fractional crystallization of mantle-derived mafic magmas (e.g., [63]) or partial melting of various crustal sources (e.g., [37, 49]).

#### 3.4 Geochemistry of nonmetamorphosed rocks

#### 3.4.1 Geochemistry of Dokan volcanic rocks

The Dokan volcanic rocks display well-defined major and trace element trends and a continuum in composition with wide ranges in SiO2 (54–76%), CaO (8.19– 0.14%), MgO (6.96–0.04%), Sr. (983–7 ppm), Zr (328–95 ppm), Cr (297–1 ppm), and Ni (72–1 ppm). The rocks are enriched in LILEs (Rb, Ba, K, Th, Ce) relative to HFSE (Nb, Zr, P, Ti) and have high total REEs with LREE enriched and display variable degrees of enrichment according to rock type (Figure 6a) [64]. The intermediate volcanics are characterized by moderate total REE and moderately fractionated patterns with slightly negative Eu-anomalies. Similarly, the REE pattern for the rhyolites is almost identical but with relatively lower content of REE. Generally, the Dokhan volcanics have steep LREE and nearly flat HREE and the large negative Eu anomalies in the rhyolite rocks than those of other varieties indicating formation under condition of relatively low temperature and pressure and/or low water content in the melt.

The geochemistry of the Dokhan volcanic rocks indicates medium-K to high-K calcalkaline affinity, and their tectonic setting is suggested to be: (i) subduction related [65], (ii) extensional setting/rift system (e.g., [66]), and (iii) transitional stage between subduction and extension (e.g., [67, 68]). However, the Dokan lavas mostly plot in an overlap zone between the volcanic arc and within-plate settings on the binary SiO2▬Nb diagram of Pearce and Gale [69] (Figure 6b), suggesting a transitional tectonic setting.

#### 3.4.2 Geochemistry of Hammamat molasse sediments

It is of great importance to assess the composition and nature of the source rocks of the Hammamat molasses sediments geochemically, and to determine their tectonic settings. The HFSE are incompatible during most igneous processes; therefore, they tend to be enriched in felsic relative to mafic rocks. Also, they are generally resistant to changes during weathering and alteration processes [70]. The greywackes of the Hammamat molasses sediments have relatively high Zr, Nb, Y, and TH and relatively low Cr, Ni and V, and Sc. Figure 7a shows that Um Hassa

3.3 Geochemistry of granitoid rocks

(Eu/Eu\* = 0.140.63).

lower crust [60].

Figure 5.

therein.

10

The geochemistry of both older and younger granites is briefly presented. The REE patterns of the older granodiorites (Figure 5a) show enrichment in the LREE relative to HREE, Lan/Ybn values vary from 7.08 to 35.21 (mostly between 7.08 and 19.37) and with Eu anomalies ranging from (Eu/Eu\* = 0.701.13) [56]. The slightly concave HREE pattern of some biotite suggests hornblende fractionation. The younger alkali feldspar granites are characterized by LREE-enrichment (Lan/Ybn = 5.28 13.46), moderately fractionated LREE, flat heavy REE patterns (Figure 5a and b), and moderately to strongly negative Eu anomalies

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

Geochemistry of older granites reveals that they are metaluminous to slightly peraluminous and have calcalkaline affinity. The older granite can be classified as I-type granites and of volcanic-arc-granite tectonic setting (Figure 5c) (e.g., [57]). In the ANS, the I-type granitoids were generally interpreted to result from melting of an amphibolitic crust (e.g., [58]). Moreover, older I-type granites can form through fractionation from mantle-derived, LILE-enriched basaltic melts in

subduction settings (e.g., [59]), or from remelting of mafic to intermediate igneous

(a) REE of older granites and (b) REE of younger and A-type granites (from [56]); (c) tectonic setting of granitoid rocks using diagram of Pearce et al. [61], data of older and younger granitoids from [38]; (d) division of A-type granites [62], field of Egyptian A-type granites from combined data from [37, 60] and references

#### Figure 6.

(a) Chondrite-normalized REE patterns for the Wadi Fatira Dokan volcanics from [64]; (b) SiO2 vs. Nb diagram after [69] for Wadi Um Sidra and Um Asmer Dokan volcanics from [68].

However, there are minor inputs from island arcs and mafic rocks or ophiolites as reworked clasts from the oldest rocks or from the mélange. The Hammamat molasse area appears to have been deposited in a retroarc foreland basin [71] or appear to be

(a) Chondrite-normalized REE patterns for Um Hassa greywackes from [71]; (b) REE patterns of the Hammamat lithologies from [72]; (c) K2O/Na2O vs. SiO2 after [75] and (d) TiO2 vs. Fe2O3 + MgO after [73] (field of Um Esh-Um Seleimat Hammamat sediments based on data from [33], plots of Um Hassa

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

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

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

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

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

accumulated in intermountain basins or foreland molasse basins [76, 77].

4. Age dating and crustal evolution

Figure 7.

greywackes from [71].

that the gneiss protoliths are Neoproterozoic [2].

forming event proposed by [49].

13

greywackes of Hammamat molasses sediments have LREE-enriched chondritenormalized patterns similar to post-Archean Australian shale (PAAS) and UCC patterns [71]. Upper continental crust-normalized patterns for the Um Hassa greywackes reveal significant enrichment of Cr (234–434 ppm) and Ni (49– 72 ppm) but depletions in Nb (4.1–7.7 ppm), Rb (33–63 ppm), and Th (3.64– 8.92 ppm) relative to UCC values (35, 20, 25, 112, and 10.7 ppm, respectively).

The shale is enriched in REE relative to the coarser sediments (Figure 7b), but has a markedly greater Eu anomaly. Chondrite-normalized Ce/Yb ratios are very similar for the shale, the siltstone, and the sandstone ((Ce/Yb)n = 9.811.0) [72]. The relatively high K2O (3.0%), Rb (79 ppm), Ba (1014 ppm), and LREE-enriched pattern ((Ce/Yb)n = 10.3) indicate that the rocks were derived from an LIL and LREE-enriched source. Plausible candidates for this enriched source include the Dokhan volcanics and the Pink younger granite, both of which occur as clasts in the conglomerates and breccias.

There is a close relationship between the tectonic setting of depositional basins and the geochemical characteristics of their sandstones [73–75]. The greywackes from Hammamat molasses sediments plot within the field of active continental margin or continental island arcs (Figure 7c and d) and appear to be formed in pullapart intermontane basins of continental margin [33, 71]. The source rocks of the Hammamat molasses sediments are represented mainly by calcalkaline to alkaline felsic source of evolved magmatic island arcs and active continental margin together with minor inputs from calcalkaline island arcs or mafic rocks [33].

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

Figure 7.

greywackes of Hammamat molasses sediments have LREE-enriched chondritenormalized patterns similar to post-Archean Australian shale (PAAS) and UCC patterns [71]. Upper continental crust-normalized patterns for the Um Hassa greywackes reveal significant enrichment of Cr (234–434 ppm) and Ni (49– 72 ppm) but depletions in Nb (4.1–7.7 ppm), Rb (33–63 ppm), and Th (3.64– 8.92 ppm) relative to UCC values (35, 20, 25, 112, and 10.7 ppm, respectively). The shale is enriched in REE relative to the coarser sediments (Figure 7b), but has a markedly greater Eu anomaly. Chondrite-normalized Ce/Yb ratios are very similar for the shale, the siltstone, and the sandstone ((Ce/Yb)n = 9.811.0) [72]. The relatively high K2O (3.0%), Rb (79 ppm), Ba (1014 ppm), and LREE-enriched pattern ((Ce/Yb)n = 10.3) indicate that the rocks were derived from an LIL and LREE-enriched source. Plausible candidates for this enriched source include the Dokhan volcanics and the Pink younger granite, both of which occur as clasts in the

diagram after [69] for Wadi Um Sidra and Um Asmer Dokan volcanics from [68].

(a) Chondrite-normalized REE patterns for the Wadi Fatira Dokan volcanics from [64]; (b) SiO2 vs. Nb

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

There is a close relationship between the tectonic setting of depositional basins and the geochemical characteristics of their sandstones [73–75]. The greywackes from Hammamat molasses sediments plot within the field of active continental margin or continental island arcs (Figure 7c and d) and appear to be formed in pullapart intermontane basins of continental margin [33, 71]. The source rocks of the Hammamat molasses sediments are represented mainly by calcalkaline to alkaline felsic source of evolved magmatic island arcs and active continental margin together with minor inputs from calcalkaline island arcs or mafic rocks [33].

conglomerates and breccias.

Figure 6.

12

(a) Chondrite-normalized REE patterns for Um Hassa greywackes from [71]; (b) REE patterns of the Hammamat lithologies from [72]; (c) K2O/Na2O vs. SiO2 after [75] and (d) TiO2 vs. Fe2O3 + MgO after [73] (field of Um Esh-Um Seleimat Hammamat sediments based on data from [33], plots of Um Hassa greywackes from [71].

However, there are minor inputs from island arcs and mafic rocks or ophiolites as reworked clasts from the oldest rocks or from the mélange. The Hammamat molasse area appears to have been deposited in a retroarc foreland basin [71] or appear to be accumulated in intermountain basins or foreland molasse basins [76, 77].
