An Overview on the Classification and Tectonic Setting of Neoproterozoic Granites of the Nubian Shield, Eastern Desert, Egypt

2.1 General outlines

Granites are of wide distributions between the different rock units of the Egyptian Neoproterozoic rocks, constituting approximately 60% of its plutonic assemblage [35]. The main exposures of granitic masses are concentrated in the ED of Egypt, where a huge masses of granite plutons intruded into the pre-existing country rocks (Figure 3; based on the Geological map of Egypt 1981; [36]). Granitoid rocks of the ED are classified into older (750–610 Ma)and younger (620–540 Ma)granites based on their composition, color, and relative age [37]. They further classified as: (1) Subduction-related older granites; (2) suture-related or Post-orogenic younger granites and (3) intraplate anorogenic younger granites [35].The Older granites (OG)comprise mainly tonalites and granodiorites, and minor trondhjemite and quartz diorites. The Younger granites(YG) classified according to their geological setting and petrography [38] 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 (commonly phase III) is classified as A-type granites [39]. Stern and Hedge [31] proposed a major tectonic transition from a compressive to an extensional regime at 600 Ma. They concluded that the Egyptian granites are belonging to two main phases of the Pan-African Orogeny: (1) The older group (715–610 Ma) comprises syn- to late-tectonic granites forming batholithic masses that exhibit wide compositional variations (trondhjemites to granodiorites with minor granites), and (2) The younger group (600–540 Ma) comprises post-tectonic pluton to stock-sized granitic bodies, generally rich in K-feldspars and sometimes associated with rare metal mineralization. Bentor [32] classified the granites of the Arabian Nubian Shield into two groups: an older Syn- to late-orogenic granites (880–610 Ma), and younger post-orogenic to anorogenic granite (600–475 Ma. Loizenbauer et al. [40] identified three magmatic pulses in the Central Eastern Desert, dated as: 680 Ma; (2) 620 Ma; and (3) 585 Ma.

2.2 Geological setting

The older granitesof the ED constitute about 27% it’s basement outcrops [33]. They occur as low relief igneous mountains (Figure 4a). They intrude the oldest rock types such as metavolcanics, and metasediments and have commonly have foliated margins concordant to wall rock structure. The granites include rounded to subrounded microgranular mafic enclaves of variable sizes (few cm up to meter (Figure 4b). The rocks are of gray to whitish gray colors, medium to coarse grained and composed mainly of tonalites and granodiorites. Examples of such granites are: Abu Ziran granites in the Central Eastern Desert and the Shaitian granite in the southern Eastern Desert.

The Younger granitesare of wide distribution across the ED, where they form high relief bold mountains (Figure 4c). They intrude the earlier exposed rocks with sharp contacts and they commonly possess steep walls and oval or elongated outlines. They enclose mafic xenoliths, enclaves and roof pendants of country rocks with sharp contacts with the enclosing granitic rocks (Figure 4d). The rocks are of pink and red colors, medium to coarse grained and comprise monzogranites, syenogranite and alkali feldspar granites, even granodiorites are seldom reported. They are classified according to their geological setting and petrography into: (i) phase I granodiorites with minor monzogranites, (ii) phase II (monzogranites and syenogranites), and phase III (alkali feldspar granites) [38]. Examples of younger granites are El Sibai granite in the Central ED and Gattar granite in the northern ED.

2.3 Petrography

The Older granites are composed mainly of tonalite and granodiorites. They are consisting of quartz, plagioclase and K-feldspar as essential minerals as well as variable proportions of biotite and hornblende. Quartz occur as anhedral crystals, whereas plagioclase forms tabular, lamellar oligoclase and is occasionally zoned (Figure 5a,b). K-feldspar occurs as tabular microcline and/or Carlsbad orthoclase. Hornblende forms long prismatic crystals of green or yellowish green colors. Biotite occur as platy crystals of yellow or yellowish-brown colors.

The younger granites comprise monzogranite, syenogranite and alkali feldspar granite. They are composed of quartz, k-feldspar and plagioclase as essential minerals, together with subordinate biotite, muscovite, hornblende, riebeckite and arfvedsonite (Figure 5c,d). Quartz occurs as anhedral large crystals interstitial to other mineral constituents. Potash-feldspars include tabular orthoclase and microcline perthite crystals. Primary K-feldspar minerals are usually altered to sericite and clay minerals and corroded by quartz and plagioclase. Plagioclase is represented by subhedral tabular crystals with distinct albite-lamellar twining of albite to oligoclase composition and occasionally intergrown with the K-feldspar forming perthitic texture (Figure 5c). In alkali feldspar granite, quartz is actively intergrowth by adjacent feldspar leaving blebs of quartz inside the replacing alkali feldspar forming micrographic and myrmekitic textures (Figure 5c,d). Biotite is subordinate and occurs as subhedral flaky or platy crystals with inclusions of zircon, apatite and sphene.

2.4 Geochemical characteristics

Compiled data of whole-rock major, trace and REE of representative granitic samples from different occurrences are presented [24, 41, 42, 43, 44]. The OG are metaluminous to slightly peraluminous, and have calc-alkaline affinity, whereas most of YG have a peraluminous character and slightly metaluminous and peralkaline [24, 45]. According to Frost et al. [15], the majority of OG analyses fall within the magnesian field, while analyses of YG plot in ferroan field with few exceptions (Figure 6). The younger granites are either of calc-alkaline character, LILE-enriched, highly fractionated I-type granites, or alkaline rocks of A-type character. Commonly, the trace element characteristics for YG are marked by enriched contents of K, Rb, Ta, Th, Nb and Zr and depletion in Sr., Ba, P and Ti. They are enriched in the HFSE, Nb, Ta, Zr, Hf, Y, U, Th and total REEs and relatively depleted in Ba, Sr. with LREE-enriched to almost flat and prominent negative Eu anomaly. Noweir et al. [46] classified younger granites into four groups: (1) Group I of calc-alkaline to weakly alkaline I-type granites; (2) Group II alkaline A-type monzogranite to syenogranite; (3) Group III strongly alkaline alkali feldspar A-type granites; and (4) Group IV apogranites, enriched in Na₂O. The Phase I younger granites of Samadi and Um Rus are calc alkaline, I-type and formed in a compressional regime or continental arc setting [47]. The phase-III younger granitoids (A-type) are characterized by higher SiO₂, Rb, Y, Nb and REE and lower CaO, MgO, Sr., and Ba contents than other phases of younger granites [45]. They are classified as alkaline, and peralkaline to mildly peraluminous A-type granites [48]. Commonly, the calc-alkaline rocks of YG are enriched in Sr. and Ba, but relatively depleted in Zr, Nb, Y, Zn and K in comparison with the alkaline suite granitoids (A-type granites). They are considered as fractionated I-type to A-type granites, magnesian and peraluminous to metaluminous, whereas the alkaline suite ones (A-type) are ferroan and commonly peralkaline.

The REE patterns of the granodiorites of the OG (Figure 7a) show enrichment in the LREE relative to HREE with small negative Eu anomaly (average Eu/Eu* = 0.674) [24]. The rocks display enrichment of LILE) HFSE, K, Sr., Rb, Ba, Th and Ta are, which are compatible with calc-alkaline trends found in island/continental arc settings. The alkali feldspar granites of the YG are characterized by LREE-enrichment or moderately fractionated LREE, flat heavy REE patterns (Figure 7b), and moderately to strongly negative Eu anomalies (Eu/Eu* = 0.14–0.63) [49]. REE patterns for YG show high contents of total REE and are enriched in LREEs, and depleted in HREEs, with negative Eu/Eu* [44]. Collision-related granites exhibited moderately negative Eu anomaly (Eu/Eu* = 0.093–0.436) whereas pattern of within plate granites showed moderately to strong negative Eu/Eu* values of approximately 0.026–0.211 for A1-type and 0.004–0.382 for A2-type (Figure 6c,d). Also, the calc-alkaline granites of the YG are characterized by higher Eu/Eu* values (0.5–1), giving rise to shallow negative anomalies. The alkaline granites appear to be differentiated from the calc-alkaline granitoids by higher RREE and much lower Eu/Eu* values [50].

2.5 Tectonic setting

Generally, the Egyptian granitoids can be classified into: (1) synorogenic calc-alkaline granitoids, (2) late- to post-orogenic calc-alkaline granitoids, and (3) post-orogenic alkaline granites [51], or moreover, granitic rocks subdivided into (1) subduction-related granites, (2) collision-related granites, (3) A2-type intraplate granites and (4) A1-type intraplate granites. Petro et al., [52] constructed the AFM triangular diagram to discriminate between granites formed in compressional (e.g., subduction related) and extensional regimes (e.g., intraplate environment). The data of the OG are plotted inclined to the AF side of the AFM diagram (Figure 8a) suggesting a compressional tectonic setting, while the data from the YG are plotted parallel to the AF line implying their extensional trend and that they formed in within plate environment. Pearce and Gale, [53] suggested Nb (ppm) versus SiO₂ (wt. %) diagram (Figure 8b) to discriminate tectonic environment of granites, in which the data of OG fall in the field of volcanic arc granites, whereas those of the YG plot in the field of the within–plate environment. The tectonic settings of the Egyptian granites can be deduced by using the discrimination diagrams of Pearce et al. [8]. In the Rb versus Y + Nb discrimination diagram (Figure 9a), the data of OG (tonalities, granodiorites) and phase I of younger granites plot within the volcanic-arc granite, while most obtained data of phase II and phase III of YG plot in the within-plate granite field. So, the YG include highly fractionated calc-alkaline I-type volcanic arc granites (postorogenic or collision-related) as well as A-type, intraplate granites. In other words, the younger granites, except phase I exhibit within plate tectonic setting. Most of the geochemical data of the A-type granites of the ED are generally consistent with a within-plate tectonic settings (Figure 9a). However, the majority of A-type granites such as El Atawi, Homrit are classified as A2 types with crustal sources, whose ratio Y/Nb is above 1.2 (Figure 9b) and appear to be formed mainly in a post-collisional setting., while A1-type granites such as Gattar, Abu Harba and Um Ara granites show Y/Nb ratio less than 1.2 and is characterized by continental intraplate environment [44]. Alkaline granitesform mostly in intraoceanic system ocean islands or intracontinental rifts near the divergent boundaries of lithospheric plates in post-tectonic stage around 540 Ma [54]. The chemical characteristics of the A2-type intraplate granites indicate that they were derived from transitional stage between orogenic and anorogenic regimes, i.e., post-collision calc-alkaline granites. The change from compressional volcanic arc settings to extensional intraplate setting is likely to have occurred around 650–630 [55].

2.6 Origin and source of magma

Previous studies on granitoid rocks of Egypt suggested several models and scenarios for the sources of magmas in different tectonic sites. Some workers favor fractional crystallization of mantle-derived mafic magma [56]. Others suggested instead partial melting of various crustal sources [57] or combine mantle and crustal source components [58, 59]. Moreover, Lundmark et al. [54] also suggested three magmatic pulses with refined age brackets at 705–680, 660 and 635–630 Ma.

The older granitesare classified as I-type granites and of volcanic-arc-granite tectonic setting [60]. Two contrasting petrogenetic models have been proposed for the origin of the OG: (1) magmas generation by partial melting of mantle sources previously metasomatized by slab-derived fluids through fractionation from mantle-derived, LILE-enriched basaltic melts in subduction settings with some crustal contamination [61, 62, 63, 64]; (2) Fractional crystallization of mafic crustal melts generated by partial melting of amphibolite and mafic to intermediate igneous of lower crust [57]. However, the parental magmas may instead reflect mixing of mantle and crustal melts. The intermediate magma derived, by partial melting of the lower crust may promoted by heat from mantle-derived mafic melts producing a hybridized intermediate magma, that form tonalite and granodiorite by fractional crystallization during its ascent and cooling [65]. This metaluminous magnesian-rich magma eventually produce subduction-related granites (diorites, tonalities, granodiorites and minor monzogranites). Hassan and Hashad [34] proposed that emplacement of subduction-related granites was linked to three magmatic pulses at 850–800 Ma, 760–710 Ma and 630 Ma.

The magma of the younger granitesappears to be derived by high degree of partial melting of crustal materials including: mafic lower crustal rocks [48], middle crust granodiorites-tonalites [66], metasedimentary protolith [67]. Alternatively, others-maintained mixing/mingling; of juvenile mantle-derived magma with felsic crustal melts [58]. Post-collisional granitesof alkaline affinity are dominated at the final stages of the Pan-African orogeny [59]. The hot asthenosphere and crustal uplift causing extensive decompression melting and basaltic underplating and the heat promote partial melting of the lower crustal rocks to form the post-collisional calc-alkaline magma [68].

The alkaline A-type granites are considered as the product of either extensive fractional crystallization of mantle-derived mafic magmas [69] or partial melting crustal sources [70]. The A2-type intraplate granites appear to be derived from transitional stage between orogenic and anorogenic regimes. The A1-types are considered to be derived from differentiation of melts similar in composition to oceanic island basalts. Most of the A2 granites present in anorogenic environments and plot in the field of crust-derived A-type granites, which originate from a wide range of sources. However, the A2-type granites can also form at the sites of convergent (collision) plate boundaries [71]. Formation of A1-type intraplate granites is related to faulting [72] or magma is suggested to be produced by a melting process of lower crustal rocks in extension setting [73].

2.7 Age dating of granitoid rocks

Ages obtained for Older Granites from the Eastern Desert are younger than 750 Ma [74]. Stern and Hedge [31] gave Rb/Sr. whole-rock age of 674 ± 13 Ma for the time of intrusion of Wadi El-Miyah gray granites; and U/Pb zircon age 614 ± 8 Ma for the age of emplacement of Abu Ziran tonalite and granodiorite. Crystallization age for granodioritic batholiths from Humr Akarim and Humrat Mukbid, Eastern Desert of Egypt, is 630–620 Ma [75]. A comparable crystallization age (643 ± 9 Ma) of the Um Rus tonalite-granodiorite was determined by Zoheir et al. [76] although it is classified as phase I younger granite [38, 47].

Younger Granites (YG) include evolved island arc postorogenic and within plate anorogenic tectonic setting. The emplacement of the Egyptian late- to post-tectonic younger granites covers a time span between 600 and 550 Ma, [31] or 600 and 475 Ma [75]. The 635–580 Ma or 610 and 590 Ma period are characteristic for the postcollisional younger granite emplacement [77, 78]. They have been emplaced as two separates, but partially overlapping calc-alkaline and alkaline suites at 635–590 Ma and 608–580 Ma, respectively [79]. The Um Had granite has a U–Pb zircon age of 590–3.1 Ma [74]. Alkaline A-type granites from the ED (such as Al-Missikat, Abu Harba, and Gattar) dated ca. ~600 Ma [58].