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This work aims to utilize oxygen and hydrogen stable isotopes as tracers for determining the groundwater sources of recharge in the central Nile delta. The analyzed water samples were collected from canals and drains as surface water, soil water and shallow, intermediate and deep zones of the groundwater. The isotopic data indicated that surface water samples have higher isotopic content than recent Nile river water (&#948 18O = 2.39‰ and &#9482 H = 22‰) which reflect the influence of an evaporation process that takes place in surface water bodies during its flow. Soil water samples were affected by evaporation process during water infiltration via soil. It was estimated that soil water samples have higher enrichment isotopic contents than groundwater and less than surface and recent Nile water. Studied groundwater is composed of mixed source from groundwater recharged before and that recharged after High Dam construction. Few groundwater samples have depleted isotopic content which suggested a mixing source between the rainwater and the Nile River water before High Dam construction. The evaporation is indicated in the shallow groundwater by the deviation away from the global meteoric line. In the deep groundwater, more deviation from global meteoric line was noticed which reflects the effect of mixing between freshwater and seawater. The intermediate groundwater samples average line showed an intermediate deviation indicating the effect of both evaporation and seawater intrusion.

Wadi Ghadir ophiolite in the Egyptian Eastern Desert, which forms the northern part of the Nubian Shield, is considered as one of the best-preserved segments of the Neoproterozoic oceanic lithosphere on Earth. Primary melt calculations and thermodynamic modeling of new geochemical data for the Wadi Ghadir pillow lavas and dike complexes, integrated with comprehensive petrographic investigations, are here employed to unravel the geodynamic evolution of this ophiolitic section. Whole-rock geochemical and clinopyroxene data indicate that the pillow lavas and sheeted dike complexes preserve geochemical signatures of N-MORB, E-MORB and OIB-like basalts. Less abundant, discrete dikes have a distinctly LILE-enriched island arc tholeiite (IAT) composition. The pillow lavas and sheeted dike complexes replicate mixed liquid lines of descent, whereas the discrete dikes exhibit a calc-alkaline differentiation trend. The pillow lavas were produced by limited (5–7%) mantle partial melting as a result of isothermal decompression over a wide pressure range (5 – 19 kbar) and restricted mantle potential temperatures (Tp ≈ 1260–1300 °C), suggesting ponding of rising melts from depths of ∼60 to 15 km in dry mantle. Primary melts of the sheeted dike complexes were produced by 9–11% partial melting at P ≈ 9.7 ± 2.5 kbar, Tp ≈ 1290 °C, and P ≈ 14.7 ± 1.4 kbar, Tp ≈ 1325 °C, suggesting a role of varying hydrostatic pressure. The magma source for the discrete dikes occurred at 10.7 ± 0.6 kbar and Tp of 1230–1300 °C by 11–13% partial melting of a metasomatized mantle region. The calculated Tp ranges for the different melts are consistently lower than temperatures of sub-ridge ambient mantle. Together with the variable concentrations of K2O and other incompatible elements, the estimated low temperatures of melt generation emphasize mixed sources and wet peridotite melting. The modeled primary melts designate crustal growth during back‐arc basin opening and closure. During the basin opening, decompression melting of mildly enriched mantle produced high-Ti tholeiitic (pillow lavas) and transitional (sheeted dike complexes) melts, whereas subduction initiation during basin closure promoted melt-peridotite interaction. Lowering of the solidus by subduction-related components triggered melting of a spinel lherzolitic mantle and produced low-Ti calc-alkaline melts that sourced the discrete dikes. The mixed MORB and SSZ geochemical characteristics of the studied volcanic rocks, coupled with the various modeled melts and lack of significant thermal anomalies in the mantle conditions, are here interpreted as manifestations of mantle source heterogeneity in a marginal oceanic basin during the accretionary stages of the Arabian-Nubian Shield.

The East-West Gondwana collision (0.75–0.5 Ga), by oblique convergence, was likely accompanied by high hydrothermal fluid flux and dispersed orogenic gold in the Arabian-Nubian Shield. Gold-bearing quartz veins along steep-dipping shear zones bordering or cutting through the ophiolitic and island arc rocks in the central Allaqi-Heiani belt in Wadi Defeit area deserved the interest of ancient miners. Field and remote sensing data revealed the structural and lithological controls of gold-bearing quartz veins, providing means of exploration for new targets in the larger Wadi Allaqi environ. The auriferous shear zones are attributed to D3 (a non-coaxial continuation of early NE-SW compression regime), and D4 (slip reactivation by E-W compression and transcurrent deformation). Microscopic investigations of quartz veins reveal the association of gold blebs and specks with fissure-filling galena-spahlerite-tetrahedrite assemblage and deformed Fe-As-sulfides (pyrite and arsenopyrite). As a gold-only province with abundant, variably deformed and carbonated metabasic rocks, metamorphic devolatilization is suggested to explain the discrete gold occurrences by fluid focusing in complex shear intersections. The available fluid inclusion data affirm the low salinity aqueous-carbonic composition of the ore fluids and mesothermal conditions of ore deposition, consistent with metamorphic devolatilization. Structures related to late ductile deformation in the post-accretionary stages of the evolution of the shield are, therefore, considered high priority zones for future exploration programs.

The assembly and disruption of Gondwana supercontinent involve several complex processes that induce major changes in chemical structures of the Arabian-Nubian Shield mantle reservoir. These processes include melting, depletion and metasomatic refertilization of the upper mantle peridotites. The geodynamic evolution of these processes is addressed based on petrological, mineral chemistry and geochemical data from a selected set of ophiolitic serpentinized peridotites from the Eastern Desert of Egypt (G. El-Maiyit, Um El-Saneyat and W. Atalla). The examined rocks represent mantle residual harzburgites after different degrees of partial melting. The most intact Cr-spinel cores and whole-rock trace and rare earth elements data show that mantle protoliths of the studied peridotites underwent different partial melting degrees of ∼23% to 35% (G. El-Maiyit rocks), ∼15% to 20% (Um El-Saneyat rocks) and ∼18% to 25% (W. Atalla rocks). The variations of partial melting degrees between different areas suggested mantle heterogeneity beneath the Arabian-Nubian Shield. The melt extraction took place under oxidizing conditions. All the studied samples are enriched in light rare earth elements (LREEs) relative to the middle (MREEs) and the heavy (HREEs) which is attributed most probably to induction of serpentinization-related hydrothermal fluids. The data conclude that the studied peridotites have different tectonic settings, including the forearc for G. El-Maiyit, mid-ocean ridge for Um El-Saneyat and mid-ocean ridge-arc transition setting for W. Atalla. The coexistence of the mid-ocean ridge peridotites with those of the supra-subduction zone reflects the transitional evolutionary history of the Neoproterozoic Mozambican oceanic lithosphere beneath the Eastern Desert of Egypt. A similar geodynamic evolution is recognized for the Mesozoic ophiolitic mantle peridotites which concludes that the construction of the Neoproterozoic mantle in the Eastern Desert of Egypt is might be similar to those of the younger age.

Late Precambrian intrusive rocks in the Arabian–Nubian Shield emplaced within and around the Najd Fault System of Saudi Arabia feature a great compositional diversity and a variety of degrees of deformation (i.e. pre-shearing deformed, sheared mylonitized, and post-shearing undeformed) that allows placing them into a relative time order. It is shown here that the degree of deformation is related to compositional variations where early, usually pre-shearing deformed rocks are of dioritic, tonalitic to granodioritic, and later, mainly post-shearing undeformed rocks are mostly of granitic composition. Correlation of the geochemical signature and time of emplacement is interpreted in terms of changes in the source region of the produced melts due to the change of the stress regime during the tectonic evolution of the Arabian–Nubian Shield. The magma of the pre-shearing rocks has tholeiitic and calc-alkaline affinity indicating island arc or continental arc affinity. In contrast, the syn- and post-shearing rocks are mainly potassium rich peraluminous granites which are typically associated with post-orogenic uplift and collapse. This variation in geochemical signature is interpreted to reflect the change of the tectonic regime from a compressional volcanic arc nature to extensional within-plate setting of the Arabian–Nubian Shield. Within the context of published geochronological data, this change is likely to have occurred around 605–580 Ma.

The southeastern sector of the Hafafit Metamorphic Complex, southern Eastern Desert of Egypt comprises infrastructural orthogneisses of tonalite and syenogranite parentage, amphibolites, and a volcano-sedimentary association. These are overthrust by an obducted suprastructural ophiolite nappes via the Nugrus thrust. The protolith of the biotite–hornblende-gneisses was formed during island-arc accretion, while that of the garnet–biotite gneisses were formed in a within-plate regime, consistent with a transition to a post-collisional setting. The volcano-sedimentary association comprises interbedded and intercalated highly foliated metapelitic schists, metabasites, and leucocratic gneisses, deposited in a back-arc basin. The metapelites and the leucocratic gneisses originated from immature Fe-shales and arkoses derived from intermediate-mafic and acidic igneous rocks, respectively, via weak chemical weathering in a tectonically active island arc terrane. The intercalated amphibolites were derived from tholeiitic basalts generated in a back-arc setting. The volcano-sedimentary association was metamorphosed under upper-amphibolite facies conditions with pressures of 9–13 kbar and temperatures of 570–675 °C, as derived from conventional geothermobarometry and pseudosection calculation. A steep, tight clockwise P–T path is constrained and a geothermal gradient around 20 °C/km is estimated for the peak metamorphism. We assume that deformation and metamorphism are due to crustal thickening during the collision of East and West Gondwana, where peak metamorphism took place in the middle to lower crust at 33 km average crustal depth. This was followed by a subsequent quasi-isothermal decompression due to rapid exhumation during wrench tectonics. Sinistral transcurrent shearing with extensional denudation resulted in vertical ductile thinning that was accompanied by heat input from magmatism, as indicated by a higher geothermal gradient during retrograde metamorphism and exhumation of the complex. U–Pb data from magmatic zircons yields protolith ages of 731 ± 3 Ma for the biotite–hornblende gneisses and 646 ± 12 Ma for the garnet–biotite gneisses. Conforming to field evidence, our geochronology data point to a depositional age of the volcano-sedimentary cover at around 650 Ma. The age of metamorphism is constrained by a low Th/U ratio of a zircon grain crystallized at an age of 597 ± 6 Ma.

In active tectonic regions, shear zones play an important role in re-configuring the structure of the lithosphere. One of the largest shear zones on Earth is the Najd Fault System of the Arabian–Nubian Shield. The main active phase of this shear zone was during the last stages of the Pan-African Orogeny (ca. 630–540 Ma). Six samples of intrusive rocks that were emplaced into the shear zone at different stages during its active phase are used to illustrate the progressive evolution of the Ajjaj shear zone. A sample of coarse-grained diorite, with an intercept U–Pb zircon age of 696 ± 6 Ma, shows very weak deformation. Two samples from deformed granodiorite–tonalite intrusions at the border of the Ajjaj shear zone show conspicuous degrees of deformation, and define two U–Pb clusters of concordia ages at 747 ± 12 Ma–668 ± 8 Ma and 742 ± 5 Ma–702 ± 12 Ma. Two samples of granites show mylonitic foliation with flattened quartz and biotite parallel to the trend of the shear zone. These samples yield U–Pb ages of 601 ± 3 Ma–584 ± 3 Ma. Another granite sample is undeformed and shows cross-cutting relations with the shear foliation of the Ajjaj shear zone. It yields a concordia age of 581 ± 4 Ma. The metamorphic rocks of the Hamadat complex host the Ajjaj shear zone, and have been useful in determining the metamorphic P-T conditions attending the activity of the shear zone. The peak metamorphism of the Hamadat Complex is 505–700 °C at two ranges of pressure 8–11 and 14.5 ± 2 kbar. New data confine the activation of the Ajjaj shear zone in a limited period of time between 604 Ma and 581 Ma and the operation at different crustal levels with a maximum depth of 58 km.

Accepted manuscript version, licensed CC BY-NC-ND 4.0. The bedrock of Mühlig-Hofmannfjella, central Dronning Maud Land in eastern Antarctica, is part of the high-grade Maud Belt and comprises a deep-seated metamorphic-plutonic complex. The P-T-t evolution of anatectic supracrustal gneisses has been recovered through a study of mineral assemblages, textural relationships and U-Pb ID TIMS geochronology on zircon and monazite followed by pseudosection modelling. Peak conditions reached granulite facies conditions (T ≥ 810–820 °C) at moderate crustal depths (P = ca. 8 kbar) and resulted in partial melting. Peak-pressure conditions were followed by isothermal decompression at elevated temperatures. After exhumation to crustal levels of about 4–5 kbar, the area underwent a final near-isobaric cooling, which is documented by a secondary growth of garnet. Zircons indicate a period of growth at 570–566 Ma, whereas monazite ages range from 610 to 525 Ma. A likely heat source for the granulite facies metamorphism is decay of radioactive heat-producing elements in the core of the orogen. The combined geochronology and metamorphic data indicate a prolonged, clockwise P-T path, which reflects collision and formation of a long-lived orogenic plateau.

The Arabian–Nubian Shield is the largest tract of juvenile continental crust of the Neoproterozoic. This juvenile crust is composed of intra-oceanic island arc/back arc basin complexes and micro-continents welded together along sutures as the Mozambique Ocean was closed. Some of these sutures are marked by ophiolite decorated linear belts. The Sol Hamed ophiolite (808 ± 14 Ma) in southeastern Egypt at the Allaqi-Heiani-Onib-Sol Hamed-Yanbu arc–arc suture represents an uncommon example of rocks that might be less deformed than other ophiolites in the Arabian–Nubian Shield. In order to understand fluid–rock interactions before and during arc–arc collision, petrological, mineral chemistry, whole-rock chemistry and thermodynamic studies were applied to the Sol Hamed serpentinized ophiolitic mantle fragment. These studies reveal that the protolith had a harzburgite composition that probably originated as forearc mantle in the subducted oceanic slab. We propose that these rocks interacted with Ti-rich melts (boninite) in suprasubduction zone, which latter formed the Sol Hamed cumulates. Spinel’s Cr# associated with the whole rock V–MgO composition suggest that the harzburgites are highly refractory residues after partial melting up to 29%. The melt extraction mostly occurred under reducing conditions, similar to peridotites recovered from the subducted lithosphere. Protolith alteration resulted from two stages of fluid–rock interaction. The first stage occurred as a result of infiltration of concentrated CO 2 -rich fluid released from carbonate-bearing sediments and altered basalt at the subduction zone. The alteration occurred during isobaric cooling at a pressure of 1 kbar. The fluid composition during the isobaric cooling was buffered by the metamorphic reactions. The second stage of fluid–rock interactions took place through prograde metamorphism. The increase in pressure during this stage occurred as a result of thrusting within the oceanic crust. In this process the forearc crust was loaded by roughly 20–30 km of overthrust rocks.

Recently published age data indicate that the Sa’al–Zaghra metamorphic complex in Sinai, Egypt contains the oldest rocks found in the northernmost Arabian-Nubian Shield, preserving evidence for a 1110–1030 Ma rift-related volcanic system formed during Rodinia break-up ( Be’eri-Shlevin et al., 2012 ). As such, its metamorphic evolution provides evidence for an important part of the geological history of the shield. Here we use petrographic, mineral chemistry and thermodynamic modeling, in combination with structural data from the field, to derive a P–T–D–t path for the complex. It is shown that the metamorphic rock of the complex equilibrated during an early deformation event that involves a flat lying fabric and is interpreted as an extensional event. P–T conditions attained during this event are between 370–420 °C and around 3 kbar. These conditions correspond to a geothermal gradient of 38–41 °C/km which is much higher than that documented elsewhere in the metamorphic complexes of Sinai (i.e. 25–27 °C/km). We suggest that this is because metamorphism in the Sa’al–Zaghra complex records an earlier stage of metamorphism and deformation during breakup of Rodinia, whereas the lower gradients documented elsewhere is related to the Gondwana collision. During the subsequent East-West-Gondwana collision, the Sa’al–Zaghra complex remained at shallow crustal levels (

Calc-silicates have proved to be important rock types to place constraints on the fluid behaviour in high grade metamorphic rocks. Here we describe amphibolite facies calc-silicate rocks of Wadi Solaf, Egypt which is one of the highest grade basement complexes of Egypt and was exhumed in close connection with the Najd fault system – one of the largest pre-Mesozoic fault systems on the Earth. Calc-silicates formed around 7–8 kbar in temperature range of 600–720 °C and can be classified into three groups (CS 1 , CS 2 and CS 3 ). CS 1 and CS 2 are characterized by the presence of garnet porphyroblasts which contain concentrically arranged wollastonite inclusions. CS 3 has a similar paragenesis but is characterized by the presence of clinozoisite/epidote and the absence of wollastonite as well as a pervasive late overgrowth of prehnite. Garnet in CS 1 lies along a grossular–andradite solid solution, while the garnet composition of CS 2 and CS 3 is hydrogrossular and hydroandradite. There is a positive correlation between Al 2 O 3 and Na 2 O as well as between Al 2 O 3 and Fe 2 O 3 from CS 2 through CS 1 to CS 3 possibly indicating a successive increase in fluid flow between the different calc-silicate types. During the peak metamorphism, the X CO 2 is 0.02–0.08 and 0.2–0.44 for the CS 1 and CS 3 , respectively. The mineral assemblage of CS 1 and CS 2 buffered the composition of the fluids along the reaction: q + cc = wo + CO 2 . During post-peak metamorphism the rocks were intruded by syn-tectonic granites. CS 2 records the contact metamorphic conditions. The rocks reached a peak contact metamorphism conditions at temperature 790–828 °C and X CO 2 = 0.22–0.41. Finally, the rocks reached a temperature range 296–311 °C during the cooling path.