Abstract Ophiolitic peridotites exposed in the Eastern Desert (ED) of Egypt record multiple stages of evolution, including different degrees of partial melting and melt extraction, serpentinization, carbonatization and metamorphism. The present study deals with metaperidotites at two selected localities in the central and southern ED, namely Wadi El‐Nabá and Wadi Ghadir, respectively. They represent residual mantle sections of a Neoproterozoic dismembered ophiolite that tectonically emplaced over a volcano‐sedimentary succession that represents island–arc assemblages. The studied metaperidotites are serpentinized, with the development of talc‐carbonate and quartz‐carbonate rocks, especially along shear and fault planes. Fresh relics of primary minerals (olivine, orthopyroxene and Cr‐spinel) are preserved in a few samples of partially‐serpentinized peridotite. Most of the Cr‐spinel crystals have fresh cores followed by outer zones of ferritchromite and Cr‐magnetite, which indicates that melt extraction from the mantle protolith took place under oxidizing conditions. The protoliths of the studied metaperidotites were dominated by harzburgites, which is supported by the abundance of mesh and bastite textures in addition to some evidence from mineral and whole‐rock chemical compositions. The high Cr # (0.62–0.69; Av. 0.66) and low TiO 2 (<0.3 wt%) contents of the fresh Cr‐spinels, the higher Fo (89–92; Av. 91) and NiO (0.24–0.54 wt%, Av. 0.40) contents of the primary olivine relics, together with the high Mg # (0.91–0.93; Av. 91) and low CaO, Al 2 O 3 and TiO 2 of the orthopyroxene relics, are all comparable with depleted to highly depleted forearc harzburgite from a suprasubduction zone setting. The investigated peridotites have suffered subsequent phases of metasomatism, from ocean‐floor hydrothermal alteration (serpentinization) to magmatic hydrothermal alteration. The enrichment of the studied samples in light rare earth elements (LREEs) relative to the heavy ones (HREEs) is attributed to most probably be due to the contamination of their mantle source with granitic source hydrothermal fluids after the obduction of the ophiolite assemblage onto the continental crust. The examined rocks represent mantle residue that experienced different degrees of partial melting (∼10% to 25% for W. El‐Nabá rocks and ∼5% to 23% for W. Ghadir rocks). Variable degrees of partial melting among the two investigated areas suggest mantle heterogeneity beneath the Arabian‐Nubian Shield (ANS).

Protolith

Peridotite

Metasomatism

Ultramafic rock

Amphibole

10.1111/1755-6724.14688

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Citations (17)

An example of post-collisional appinitic magmatism with an arc-like signature: the Wadi Nasb mafic intrusion, north Arabian–Nubian Shield, south Sinai, Egypt

Figshare (2017)

Hisham A. Gahlan Mohamed A. Obeid Mokhles K. Azer Paul D. Asimow

We present new data for the Neoproterozoic mafic intrusion exposed in Wadi Nasb, south Sinai, Egypt (northernmost Arabian–Nubian Shield; ANS). The Nasb mafic intrusion (NMI) intrudes metasediments, Rutig volcanics, and diorite/granodiorite, and is intruded in turn by younger monzogranite and quartz-monzonite. Available geochronological data for the country rocks of the NMI provide a tight constraint on its age, between 619 and 610 Ma, during the hiatus between the lower and upper Rutig volcanics. The NMI is neither deformed nor metamorphosed, indicating post-collisional emplacement, and uralitization by late-magmatic and sub-solidus alteration is restricted to the margins of the intrusion. A quantitative fractionation model indicates a fractionating assemblage of 61% primary amphibole, 10% clinopyroxene, 28% plagioclase, 1% biotite, 0.4% apatite, and 0.15% Fe-Ti oxide. Contrary to the recent studies, we find that the nearby diorite of Gebel Sheikh El-Arab is not co-genetic with the appinitic gabbro of the NMI. Although there are volcanic xenoliths in the NMI, we find no chemical evidence requiring contamination by continental crust. A subduction-related signature in a post-orogenic intrusion requires the inheritance of geochemical tendencies from a previous subduction phase. Given that the fine-grained gabbro of the NMI is consistent with a near-primary mantle melt, we attribute this inheritance to persistence and later melting of the slab-modified mantle domains, as opposed to partial melting and assimilation of the juvenile continental crust. The fine-grained gabbro composition indicates derivation at temperature and pressure conditions similar to the sources of mid-ocean ridge basalts: mantle potential temperature near 1350°C and extent of melting about 7%. Such temperatures, neither so high as to require a plume nor so low as to be consistent with small degrees of melting of a volatile-rich source, are most consistent with a lithospheric delamination scenario, allowing the upwelling of fertile, subduction-modified asthenosphere to depths ≤50 km.

Wadi

10.6084/m9.figshare.5307103

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Provenance of conglomerate clasts from the volcano-sedimentary sequence at Wadi Rutig in southern Sinai, Egypt as revealed by SIMS U–Pb dating of zircon

Gondwana Research (2010)

M.D. Samuel Yaron Be’eri-Shlevin Mokhles K. Azer Martin J. Whitehouse Hilmy E. Moussa

Conglomerate

Pebble

10.1016/j.gr.2010.11.021

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Citations (58)

Supplementary Table 1k: Micas. The Atud Gabbro–Diorite Complex: Glimpse of the Cryogenian Mixing, Assimilation, Storage, and Homogenization Zone beneath the Eastern Desert of Egypt

Figshare (2020)

Robert J. Stern Kamal A. Ali Paul D. Asimow Mokhles K. Azer Matthew I. Leybourne

Supplementary Table 1k: Micas.

Diorite

Assimilation (phonology)

Homogenization

10.6084/m9.figshare.12220277

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Mineralogy and Geochemistry of Jasperoid Veins in Neoproterozoic Metavolcanics: Evidence of Silicification, Pyritization and Hematization

Minerals (2024)

Mohamed Zaki Khedr Mahmoud A. Sayed Shehata Ali Mokhles K. Azer Yuji Ichiyama

The Wadi Ranga sulfidic jasperoids in the Southern Eastern Desert (SED) of Egypt are hosted within the Neoproterozoic Shadli metavolcanics as an important juvenile crustal section of the Arabian Nubian Shield (ANS). This study deals with remote sensing and geochemical data to understand the mechanism and source of pyritization, silicification, and hematization in the host metavolcanics and to shed light on the genesis of their jasperoids. The host rocks are mainly dacitic to rhyolitic metatuffs, which are proximal to volcanic vents. They show peraluminous calc-alkaline affinity. These felsic metatuffs also exhibit a nearly flat REE pattern with slight LREE enrichment (La/Yb = 1.19–1.25) that has a nearly negative Eu anomaly (Eu/Eu* = 0.708–0.776), while their spider patterns display enrichment in Ba, K, and Pb and depletion in Nb, Ta, P, and Ti, reflecting the role of slab-derived fluid metasomatism during their formation in the island arc setting. The ratios of La/Yb (1.19–1.25) and La/Gd (1.0–1.17) of the studied felsic metatuffs are similar to those of the primitive mantle, suggesting their generation from fractionated melts that were derived from a depleted mantle source. Their Nb and Ti negative anomalies, along with the positive anomalies at Pb, K, Rb, and Ba, are attributed to the influence of fluids/melt derived from the subducted slab. The Wadi Ranga jasperoids are mainly composed of SiO2 (89.73–90.35 wt.%) and show wide ranges of Fe2O3t (2.73–6.63 wt.%) attributed to the significant amount of pyrite (up to 10 vol.%), hematite, goethite, and magnetite. They are also rich in some base metals (Cu + Pb + Zn = 58.32–240.68 ppm), leading to sulfidic jasperoids. Pyrite crystals with a minor concentration of Ag (up to 0.32 wt.%) are partially to completely converted to secondary hematite and goethite, giving the red ochre and forming hematization. Euhedral cubic pyrite is of magmatic origin and was formed in the early stages and accumulated in jasperoid by epigenetic Si-rich magmatic-derived hydrothermal fluids; pyritization is considered a magmatic–hydrothermal stage, followed by silicification and then hematization as post-magmatic stages. The euhedral apatite crystals in jasperoid are used to estimate the saturation temperature of their crystallization from the melt at about 850 °C. The chondrite (C1)-normalized REE pattern of the jasperoids shows slightly U–shaped patterns with a slightly negative Eu anomaly (Eu/Eu* = 0.43–0.98) due to slab-derived fluid metasomatism during their origin; these jasperoids are also rich in LILEs (e.g., K, Pb, and Sr) and depleted in HFSEs (e.g., Nb and Ta), reflecting their hydrothermal origin in the island arc tectonic setting. The source of silica in the studied jasperoids is likely derived from the felsic dyke and a nearby volcanic vent, where the resultant Si-rich fluids may circulate along the NW–SE, NE–SW, and E–W major faults and shear zones in the surrounding metavolcanics to leach Fe, S, and Si to form hydrothermal jasperoid lenses and veins.

10.3390/min14070647

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Genesis and geodynamic evolution of serpentinized ultramafics and associated magnesite deposits in the Al-Wask ophiolite, Arabian Shield, Saudi Arabia

American Journal of Science (2020)

Hisham A. Gahlan Mokhles K. Azer Paul D. Asimow Khaled Al‐Kahtany

Situated along the Yanbu Suture Zone, the Al-Wask ophiolite is one of the largest and best-preserved ophiolite sequences in the Proterozoic Arabian shield. A mantle section of serpentinized ultramafics is structurally overlain by a crustal section of gabbros and pillow lavas. The whole ophiolite sequence is capped by pelagic sedimentary cover, and tectonically emplaced over a metamorphosed island-arc volcano-sedimentary succession. The Al-Wask ultramafic rocks are strongly deformed, metamorphosed, and altered by carbonatization and silicification. Samples dominated by antigorite indicate upper greenschist to lower amphibolite facies peak metamorphic grade, whereas samples dominated by lizardite and magnesite preserve lower grade conditions that we interpret as a cooling path buffered to low CO₂ activity by the increasing stability of magnesite with decreasing temperature. Nearly all the primary silicate minerals have been replaced by serpentine minerals, leaving only relics of primary olivine and chromian spinel. Petrographic observation of relict olivine and spinel and of mesh and bastite textures in the serpentines suggest that the peridotite protoliths were mainly harzburgite with minor dunite. Whole-rock compositions of serpentinites show low CaO (<0.1 wt.%), Al₂O₃ (<1.5 wt.%), and Y (<0.4 ppm) combined with high Mg# (0.90–0.92), Ni, Co, and Cr contents; all these indicate a highly refractory mantle protolith. The mineral chemistry of relict primary spinel and olivine provides additional petrogenetic and geodynamic indicators. The high Cr# (> 60) and low TiO₂ (≤0.2 wt. %) of spinel and high forsterite contents (90–92) of associated olivine indicate residual mantle that underwent extensive partial melt extraction. The whole-rock and mineral chemistry of the serpentinized ultramafic rocks are both consistent with extracted melt fractions from ∼32 to 38 percent. This extent of melting is typical of fore-arc supra-subduction zone settings, which is the most likely tectonic environment for formation and preservation of the Al-Wask ophiolite. Two types of magnesite deposits can be distinguished in the Al-Wask mantle section: an early generation of massive magnesite and a later generation of magnesite veins. Hence the Al-Wask ophiolite underwent multiple stages of carbonatization, likely involving different sources of CO₂-bearing fluids. The massive magnesite likely formed at relatively high temperature during cooling from peak metamorphic condition from CO₂-bearing fluid probably derived from decomposition of subducted carbonates. Using thermodynamic calculations in the simple MgO-SiO₂-H₂O-CO₂ system, we constrain the path of the reaction boundary where lizardite and magnesite can coexist at equilibrium. On the other hand, the cryptocrystalline magnesite veins fill tectonic fractures and likely formed at low temperature and shallow levels, after serpentinization and ophiolite obduction.

Peridotite

Protolith

Ultramafic rock

Magnesite

Forearc

Greenschist

10.2475/03.2020.02

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Neoproterozoic concentric intrusive complex of gabbro-diorite-tonalite-granodiorite association, Rahaba area, southern Eastern Desert of Egypt: Implications for magma mixing of arc intrusive rocks

Lithos (2021)

Abdel‐Aal M. Abdel‐Karim Mokhles K. Azer Moustafa M. Mogahed