Geochemistry of the Um Had Plutonites, Central Eastern Desert, Egypt: Implications for Magma Evolution, and Tectonic Setting

Geochemistry of gabbroid and granitoid plutonites from the Um Had area indicates island arc subalkaline basic magma with tholeiitic affinity and calc-alkaline, metaluminous and slightly peraluminous magma, respectively. Although different in age both plutonite types were emplaced under compressional regime, where subduction-related environment was dominant. They were formed under relatively low to moderate water-vapour pressure (1–5 k-bars) at moderate depths (20–30 km). Biotite granites were formed at a relatively high temperature range (800–840 C), while biotite-muscovite granites were formed under relatively moderate temperature conditions (760–800 C). These two units may represent evolution from island arc to active continental margin. It is suggested that island arc gabbros might have sourced the late subduction-related calc-alkaline granitoids during the waning stages of the pan-African orogeny. The I-type nature of the investigated plutonites in the study area and elsewhere suggests the juvenile character of the basement complex of the Eastern Desert of Egypt.

In this study, the geochemical characteristics of the Um Had-Um Shegila plutonites are addressed to clarify the tectonic environment that the Central Eastern Desert represents.

The Um Had Granitoids
The Um Had area contains a large elliptical structure trending northwest outlining a core of medium-to high-grade partly gneissic rocks enclosed by a domed thick mylonitic shear zone (e.g., Fowler, 2001; Figure 1).The core is up-heaved by the Um Had granitoid intrusion.The latter and its equivalents scattered throughout the Egyptian Eastern Desert was emplaced during Pan-African orogeny.These granitoids are referred to as late-to post-orogenic calc-alkaline to transitional A-type granitoids, interpreted by Black and Liegeois (1993) as being the result of delamination of the continental lithospheric mantle following the tectonic collisions of East and West Gondwana.These granitoids are characterised by unfoliated, small, nearly circular to elongate epizonal plutons with sharp intrusive contacts.
The southern, SW and SE contacts of this intrusion dip generally at moderate angles outwards, whereas, the western and eastern contacts are steep and associated with syn-intrusion faulting.On the other hand, the northern edge is defined by the northernmost significant dykes cropping out along Wadi Shegila.

Methods
Thirty seven samples (23 from the granitoid rocks and 14 from the gabbroic rocks) were carefully selected for the chemical analyses to avoid the effects of secondary alteration.The samples were then submitted to ACME Analytical Labs ltd.(Canada).Major element data were acquired using X-ray fluorescence spectrometry (XRF) on fused beads and pressed pellets.Trace element data, including rare earth-elements (REE) were obtained using Inductively Coupled Plasma Mass Spectrometry (ICP-MS).Based on replicate analyses and international standards, the precision is better than ± 1% for major elements, and ± 5% for most trace elements.Data of chemical analyses are illustrated in (Tables 1 and 2).

Geochemistry of the Granitoids
The Um Had granites reveal general decrease of major oxides (TiO 2 , Al 2 O 3 , MgO, Fe 2 O 3 and CaO) in relation with SiO 2 , which can be attributed to fractional crystallisation processes.These granitoid rocks can be classified by using normative An-Ab-Or ternary diagram of Barker (1979) and Q-ANOR of Streckeisen and Matiere, (1979), where all samples fall within the monzogranite field (Figures 2a and 2b).On the Zr versus 10 4 Ga/Al binary diagram of Whalen et al. (1987), the studied samples fall within I & S Type granites field (Figure 2c).Chappell and White (1974) used the Mol.Al 2 O 3 / (CaO + Na 2 O + K 2 O) ratio against Rb/Sr ratio to differentiate between I-type and S-type granites.On this binary diagram (Figure 2d) all the studied granitoid samples fall within the I-type field, except one biotite granite sample lies at the boundary separating between the two fields of I-type and S-type granites.Moreover, White and Chappell (1983) as well as Liew et al. (1989) used the Na 2 O-K 2 O binary diagram (Figure 3a) to discriminate between I-type, S-type and T-type granites.Plots of the all granitoid samples lie in the I-type granite field indicating igneous source materials for the studied granites, except two biotite granite samples fall within the transition (T-type) field.
All plots of the investigated granitoids fall within the calc-alkaline field (Figure 3b) on the AFM ternary diagram of Irvine and Baragar (1971).Moreover, Petro et al. (1979) used the AFM ternary diagram to discriminate between compressional and extensional suites.Plots of the studied granitoids mostly have the trend of compressional suite.On the SiO 2 vs. K 2 O diagram of Peccerillo and Taylor (1976), the studied granitoid samples fall within the field of high-K calc-alkaline series (Figure 3c).Maniar and Piccoli (1989), all plots of these younger granites are of peraluminous to metaluminous nature (Figure 3d).Plots of studied granitoids on the Y vs. Nb and Y+Nb vs. Rb binary diagrams (Figures 4a and 2b) of Pearce et al. (1984) reveal that these granites were formed within volcanic arc tectonic environment.
On the normative An-Ab-Or ternary diagram of Barker (1979) all plots of biotite granite and the two mica granite samples fall in a region of low to moderate water-vapour pressure ranging from 1 to 5 k-bars.Meanwhile, the biotite granites were emplaced under high temperature conditions, ranging from about 800 to 840 C, whereas the two mica granites were emplaced under moderate temperature conditions ranging from about 760 to 800 C (Figure 4c).The Rb-Sr binary diagram (after Condie, 1973) shows that the Um Had granites have emplaced at relatively shallow to moderate depths between 20 and 30 km (Figure 4d).

Geochemistry of the Gabbros
The total alkalis vs. silica classification diagrams (after Le Bas et al., 1986;Cox et al., 1979) were used to classify the studied gabbros.All plots of gabbros fall within the basalt (gabbro) field (Figures 5a & b).Plotting of the studied gabbros on Na 2 O+K 2 O vs. SiO 2 and AFM diagrams of Irvine and Baragar (1971) favors that these gabbros were derived from subalkaline magma (Figure 5c), with tholeiitic nature (Figure 5d).Also, on the SiO 2 vs. FeOt/MgO diagram of Miyashiro (1974) all the plotted samples of the different gabbroic members lie in the tholeiite field (Figure 5e).
There are two models that have been proposed for the geotectonic environment and magma sources of the younger gabbros.These are the tensional environment or rift-related (El Gaby et al., 1988;El-Mettwaly, 1992) and the subduction-related or island-arc environment (Mohamed & Hassanen, 1996).On the AFM diagram, Petro et al. (1979) defined two tectonic trends discriminating between the compressional and extensional suites.
The studied gabbros are concordant with the compressional trend indicating their origin in compressional environment (subduction-related or island-arc environment).Plots of the studied gabbros on the Y vs. Cr discrimination diagram (after Pearce, 1982) favor the arc-related tectonic setting for these gabbros, where all plots of gabbros fall within the field of volcanic-arc basalts (Figure 5f).

Summery and Conclusions
The late to post-tectonic granites of Um Had area are typically monzogranites (biotite granites and biotite-muscovite granites).They have calc-alkaline, metaluminous to slightly peraluminous nature and potassic affinity.Both biotite-granites and biotite-muscovite granites have emplaced during late-orogenic regime in compressional suite.They were formed under relatively low to moderate water-vapour pressure (1-5 k-bars) at moderate depths (20-30 km).Biotite granites were formed under relatively high temperature range (800-840 C), while biotite-muscovite granites were formed under relatively moderate temperature conditions (760-800 C).
The gabbros have been derived from subalkaline basic magma exhibiting tholeiitic nature.They were emplaced in compressional subduction-related environment having characters of the island-arc and active continental margin.Although caution is required in using the tectonic setting discrimination diagrams (Verma, 2010), results and graphical plots of the investigated rocks seem reasonable and convincing.It is therefore, concluded that island arc gabbros might have sourced the late subduction-related calc-alkaline granitoids are the convergent plate setting was long prevailed during the waning stages of the pan-African orogeny.

Figure 1 .
Figure 1.Geological map of the Urn Had granitoid intrusion, Central Eastern Desert of Egypt

Table 1b .
Major oxides and trace elements of granitoid rocks, Um Had area (Biotite granite)

Table 2 .
Major oxides and trace elements of gabbroic rocks, Um Had area