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Abstract: Submarine tholeiitic and calc-alkaline basalt and andesite with subordinate dacite, metamorphosed to greenschist facies, are exposed at Wadi Kareim and Wadi El Dabbah, Central Eastern Desert of Egypt. These metavolcanic sequences are representative of early stages of Neoproterozoic crust formation in the northern part of the Arabian-Nubian Shield. The magmas were primitive to moderately fractionated, as indicated by average Mg# (54), Cr (191ppm) and Ni (69ppm). Their REE patterns are slightly fractionated [(La/Yb)N =1.3–3.9], and multi-element diagrams show Ba, Sr, K and Rb enrichments and Nb and Ta depletions, typical of modern convergent margin igneous rocks. Tectonic discrimination diagrams also show affinities with modern convergent margin magmas, suggesting that Wadi El Dabbah lavas formed in a volcanic arc that experienced cryptic crustal contamination, whereas Wadi Kareim lavas formed in a back-arc basin. Positive initial ɛNd (+5.1 to +8.9) and Nd model ages (0.64–0.79Ga) indicate that the crust is juvenile and was extracted from a depleted mantle source. Wadis Kareim and El Dabbah metavolcanic rocks are dated at ∼750Ma (U–Pb SHRIMP) but contain abundant pre-Neoproterozoic zircons which might be derived from underlying sediments or perhaps older continental crust. These results indicate that a major ∼750Ma crust-forming event involving ophiolite generation and eruption of juvenile melts was important for the formation of the crust. [Copyright &y& Elsevier]

Most Neoproterozoic ophiolites of the Arabian-Nubian Shield show compositions consistent with formation in a suprasubduction zone environment, but it has not been clear whether this was in a forearc or back-arc setting. Ophiolitic serpentinites are common in the Eastern Desert of Egypt, but their composition and significance are not well understood. Here we report new petrographic, mineral, chemical, and whole-rock compositional data for serpentinites from Wadi Semna, the northernmost ophiolitic serpentinites in the Eastern Desert, and use these to provide insights into the significance of other Eastern Desert serpentinite locales. The Wadi Semna serpentinites are composed essentially of antigorite, chrysotile, and lizardite, with minor carbonate, chromite, magnetite, magnesite, and chlorite, and they were tectonically emplaced. The alteration of chrome spinel to ferritchromite was accompanied by the formation of chloritic aureoles due to the release of Al from spinel. Major-element compositions indicate that, except for the addition of water, the serpentinites have not experienced extensive element mobility; these were originally CaO- and A12O3-depleted harzburgites similar to peridotites from modern oceanic forearcs. High Cr# (Cr/(Cr+Al)) in the relict spinels (average = 0.69) indicates that these are residual after extensive partial melting, similar to spinels in modern forearc peridotites. These characteristics of Wadi Semna serpentinites also typify 22 other Eastern Desert serpentinite localities. We infer that Eastern Desert ophiolitic serpentinites, except perhaps Gebel Gerf, originated by forearc seafloor spreading during subduction initiation associated with the closing of the Neoproterozoic Mozambique Ocean. [ABSTRACT FROM AUTHOR]

We use the name Nubian Swell to refer to a complex, east–west trending structural high in southern Egypt and northern Sudan. This 500km wide zone of uplifted Neoproterozoic crystalline basement and Paleozoic sediments and parallel troughs extend westward for more than 800km from the flanks of the Red Sea Hills. The Nile in this region is called the Cataract Nile and is in a youthful stage, particularly in northern Sudan where it is incised in the Neoproterozoic crystalline basement. The northern Cataract Nile flows through the rapids of the Batn el Hajar or ‘Belly of Stones’ region, characterized by structurally controlled 90° turns, frequent bifurcation and disruption by several cataracts, and near-absence of floodplains. Orbital imaging radar has advanced our understanding of the Nubian Swell, through the discovery and mapping of paleochannels and faults that indicate tectonic uplift during Cenozoic time. Earthquakes in southern Egypt during the early 1980s provide evidence that portions of the Nubian Swell are still tectonically active, with recent seismic activity concentrated where E–W trending structures intersect N–S trending structures of the Aswan corridor. We conclude that the Nubian Swell is an important tectonic feature of North Africa, with episodic but continuing uplift. [Copyright &y& Elsevier]

Ultramafic portions of ophiolitic fragments in the Arabian–Nubian Shield (ANS) show pervasive carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Notwithstanding the extent of the alteration, little is known about the processes that caused it, the source of the CO2or the conditions of alteration. This study investigates the mineralogy, stable (O, C) and radiogenic (Sr) isotope composition, and geochemistry of suites of variably carbonate altered ultramafics from the Meatiq area of the Central Eastern Desert (CED) of Egypt. The samples investigated include least-altered lizardite (Lz) serpentinites, antigorite (Atg) serpentinites and listvenitic rocks with associated carbonate and quartz veins. The C, O and Sr isotopes of the vein samples cluster between −8.1‰ and −6.8‰ for δ13C, +6.4‰ and +10.5‰ for δ18O, and87Sr/86Sr of 0.7028–0.70344, and plot within the depleted mantle compositional field. The serpentinites isotopic compositions plot on a mixing trend between the depleted-mantle and sedimentary carbonate fields. The carbonate veins contain abundant carbonic (CO2±CH4±N2) and aqueous-carbonic (H2O-NaCl-CO2±CH4±N2) low salinity fluid, with trapping conditions of 270–300°C and 0.7–1.1 kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile elements relative to primitive and depleted mantle. The extensively carbonated Atg-serpentinites contain significantly lower concentrations of these elements than the Lz-serpentinites suggesting that they were depleted during carbonate alteration. Fluid inclusion and stable isotope compositions of Au deposits in the CED are similar to those from the carbonate veins investigated in the study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived CO2-bearing fluids caused break down of Au-bearing minerals such as pentlandite, releasing Au and S to the hydrothermal fluids that later formed the Au-deposits. This is the first time that gold has been observed to be remobilized from rocks during the lizardite–antigorite transition. [ABSTRACT FROM PUBLISHER]

The Korab Kansi and Abu Ghalaga Neoproterozoic mafic-ultramafic intrusions in the South Eastern Desert (SED) of Egypt host economic Fe–Ti–V oxide deposits, ∼41 million tons for Abu Ghalaga ores. The Korab Kansi deposits are composed of titanomagnetite with subordinate ilmenite layers in dunites, gabbros and troctolites. The Abu Ghalaga ores consist of hemo-ilmenite and ilmenite with subordinate magnetite and titanomagnetite lenses in norites, gabbros and anorthosites. The difference in calculated oxygen fugacity (f O2 : ∼ ΔFMQ–1.24 to −3.28 for Korab Kansi ores and +0.21 to −0.3 for Abu Ghalaga ores) during subsolidus re-equilibration is consistent with different dominant iron and titanium oxides, magnetite and hematite, respectively. Ilmenite in both deposits is enriched in Nb, Ta, Zr, Hf and V, but is poor in Cr and Ni relative to coexisting magnetite, which belongs to magmatic Fe–Ti–V deposits. The Korab Kansi and Abu Ghalaga ore deposits are rich in Ti, Fe, V, Nb, Ta and Hf with subordinate S, Cu, Ga and Zn at high f O2. The Abu Ghalaga gabbroic intrusion crystallized from ferrobasaltic magmas of tholeiitic affinity at lower temperature (∼1082 °C) and pressure (5.1 kbar) than the Korab Kansi intrusion (∼1180 °C, 8.3 kbar). The Korab Kansi ore deposits mainly formed by fractional crystallization of olivine followed by in situ crystallization of Fe–Ti oxides from ferropicritic/ferrobasaltic parent melts at the floor of the magma chamber. The Abu Ghalaga ores formed as a result of more advanced fractional crystallization of ferrobasaltic parent melts. The Abu Ghalaga Fe–Ti oxides in massive ores at the base of the intrusion grew in situ, whereas most ore lenses in the middle to the top of the intrusion precipitated from immiscible oxide melts that separated from the parental magma in association with floatation of plagioclase and sulfide crystals at a late magmatic stage. Both Fe–Ti ore deposits formed from similar mantle-derived magmas but Korab Kansi ores formed as early fractionates, whereas Abu Ghalaga ores formed late. The Korab Kansi ilmenite is rich in vanadium (up to 3.8 wt% V 2 O 5) relative to that of other magmatic deposits. Its parental melt may have formed from locally Fe–Ti–V enriched part of the Neoproterozoic mantle due to interaction with upwelling asthenosphere or a mantle plume. Controlling factors for formation of economic SED Fe–Ti–V deposits were the V-rich ferropicritic/ferrobasaltic compositions of the parental magmas, and addition of H 2 O to cause high magma ƒ O2. [Display omitted] • Microtexture, mineralogy and chemistry of Fe–Ti–V ore deposits in mafic-ultramafic intrusions. • Evidence for fractional crystallization of V-rich melts and formation of Fe–Ti oxide deposits. • Role of in situ crystallization and immiscible separation for genesis of Fe–Ti–V ore deposits. • Controlling factors for economic Fe–Ti–V deposits in Egypt, and explaining high V in deposits. [ABSTRACT FROM AUTHOR]

[Display omitted] • Two ophiolitic peridotites in the SE Desert of Egypt provide a window into the Neoproterzoic mantle. • Geochemistry of peridotite and its mineral chemistry suggest an origin in the arc-related setting. • SED peridotites are refractory mantle residues (25–40% melting), like modern fore-arc peridotites. • Geochemical aspects of peridotite indicate changing from extension during subduction initiation. Korab Kansi and Abu Dahr Neoproterozoic ophiolitic peridotites in the Southern Eastern Desert (SED) of Egypt, are parts of the largest ophiolitic nappes in the Arabian-Nubian Shield. They mainly comprise refractory harzburgites with minor dunites that are commonly altered to serpentinites and talc carbonates. Olivine relics in serpentinized peridotites of the two complexes have olivines with high forsterite (Fo 90.6-92.2) and NiO (0.39 wt%) and low MnO (0. 13 wt%) contents, similar to those of mantle olivines. High Mg# (0.91–0.95) of orthopyroxene is consistent with that of depleted harzburgites. Primary spinel Cr# (0.55–0.75) and TiO 2 contents (<0.04 wt%) of the two peridotite complexes are similar to forearc peridotite spinels. The estimated degree of partial melting of Korab Kansi peridotites (25–40% melting) is slightly lower than that of Abu Dahr (30–40% melting), consistent with modern forearc peridotites. Korab Kansi peridotites exhibit lower oxygen fugacity (Δlog ƒO 2 , FMQ + 0.7 on average) and equilibrium temperature (610–710 °C) than those of Abu Dahr (T , 750–900 °C at Δlog ƒO 2 , FMQ + 2.1), both suggesting subduction-modified peridotites. The low concentrations of incompatible elements, U-shaped REE patterns and high spinel Cr# of both peridotites indicate highly depleted mantle protoliths, reflecting multiple metasomatic and melting episodes in the mantle wedge. Enrichment of fluid mobile elements (e.g. B, Li, Th, U, Pb) relative to high-field strength elements (e.g. Nb, Ta, Ti, Zr) may reflect interaction between slab-derived fluids and peridotites. Calculated parental melts in equilibrium with Korab Kansi and Abu Dahr peridotite spinels have tholeiitic (MORB-like) and boninitic affinities, which were generated during proto-forearc spreading to a mature arc stage. Variations in melt compositions suggest a transitional setting from early proto-forearc spreading of mantle beneath Korab Kansi to the mature arc stage of Abu Dahr peridotites. Subduction initiation possibly started in the W and/or SW with proto-forearc spreading, and progressed to the E and/or NE where a mature arc stage with boninitic melts above an E- and/or SE-dipping subduction zone. [ABSTRACT FROM AUTHOR]

Abstract: The Kid Group is one of the few exposures of Neoproterozoic metavolcano-sedimentary rocks in the basement of southern Sinai in the northernmost Arabian–Nubian Shield. It is divided into the mostly metamorphosed volcaniclastic Melhaq and siliciclastic Um Zariq formations in the north and the mostly volcanic Heib and Tarr formations in the south. The Heib, Tarr, and Melhaq formations reflect an intense episode of igneous activity and immature clastic deposition associated with core-complex formation during Ediacaran time, but Um Zariq metasediments are relicts of an older (Cryogenian) sedimentary sequence. The latter yielded detrital zircons with concordant ages as young as 647±12Ma, which may indicate that the protolith of Um Zariq schist was deposited after ~647Ma but 19 concordant zircons gave a 206Pb/238U weighted mean age of 813±6Ma, which may represent the maximum depositional age of this unit. In contrast, a cluster of 11 concordant detrital zircons from the Melhaq Formation yield a weighted mean 206Pb/238U age of 615±6Ma. Zircons from Heib Formation rhyolite clast define a 206Pb/238U weighted mean age of 609±5Ma, which is taken to approximate the age of Heib and Tarr formation volcanism. Intrusive syenogranite sample from Wadi Kid yields a 206Pb/238U weighted mean age of 604±5Ma. These constraints indicate that shallow-dipping mylonites formed between 615±6Ma and 604±5Ma. Geochemical data for volcanic samples from the Melhaq and Heib formations and the granites show continuous major and trace element variations corresponding to those expected from fractional crystallization. The rocks are enriched in large ion lithophile and light rare earth elements, with negative Nb anomalies. These reflect magmas generated by melting of subduction-modified lithospheric mantle, an inference that is further supported by εNd(t)=+2.1 to +5.5. This mantle source obtained its trace element characteristics by interaction with fluids and melts from subducting oceanic crust during the Late Cryogenian time, prior to terminal collision between fragments of East and West Gondwana at ~630Ma. Positive εNd(t) values and the absence of pre-Ediacaran zircons in all but Um Zariq metasediments indicate minor interaction with Cryogenian and older crust. A model of extensional collapse following continental collision, controlled mainly by lithospheric delamination and slab break-off is suggested for the origin of the post-collision volcanics and granites at Wadi Kid. No evidence of pre-Neoproterozoic sources was found. Kid Group Ediacaran volcanic rocks are compositionally and chronologically similar to the Dokhan Volcanics of NE Egypt, which may be stratigraphic equivalents. [Copyright &y& Elsevier]

The Humr Akarim and Humrat Mukbid plutons, in the central Eastern Desert of Egypt, are late Neoproterozoic post-collisional alkaline A-type granites. Humr Akarim and Humrat Mukbid plutonic rocks consist of subsolvus alkali granites and a subordinate roof facies of albite granite, which hosts greisen and Sn-Mo-mineralized quartz veins; textural and field evidence strongly suggest the presence of late magmatic F-rich fluids. The granites are Si-alkali rich, Mg-Ca-Ti poor with high Rb/Sr (20-123), and low K/Rb (27-65). They are enriched in high field strength elements (e.g., Nb, Ta, Zr, Y, U, Th) and heavy rare earth elements (La/Yb = 0.27-0.95) and exhibit significant tetrad effects in REE patterns. These geochemical attributes indicate that granite trace element distribution was controlled by crystal fractionation as well as interaction with fluorine-rich magmatic fluids. U-Pb SHRIMP zircon dating indicates an age of ~630-620 Ma but with abundant evidence that zircons were affected by late corrosive fluids (e.g., discordance, high common Pb). εNd at 620 Ma ranges from +3.4 to +6.8 (mean = +5.0) for Humr Akarim granitic rocks and from +4.8 to +7.5 (mean = +5.8) for Humrat Mukbid granitic rocks. Some slightly older zircons (~740 Ma, 703 Ma) may have been inherited from older granites in the region. Our U-Pb zircon data and Nd isotope results indicate a juvenile magma source of Neoproterozoic age like that responsible for forming most other ANS crust and refute previous conclusions that pre-Neoproterozoic continental crust was involved in the generation of the studied granites. [ABSTRACT FROM AUTHOR]

The Neoproterozic Allaqi–Heiani suture in southeastern Egypt is the western extension of the Allaqi–Heiani–Onib–Sol Hamed–Yanbu suture that represents one of arc–arc sutures in the Arabian–Nubian Shield. It extends for more than 250 km from the N-trending Hamisana Shear Zone in the east to Lake Nasser in the west. It separates the 750-Ma-old Southeastern Desert terrane in the north from the 830–720-Ma-old Gabgaba terrane in the south. Structural studies supported by remote sensing investigations including Landsat Thematic Mapper (TM) images show that the western part of Allaqi–Heiani suture zone constitutes three S- to SW-verging nappes in the north overriding an autochthonous block to the southwest. SW-verging, low-angle thrust sheets and folds, forming a 10-km wide imbrication fan, dominate the northern upper nappe (northern allochthon). These folds and thrusts deform shelf metasedimentary rocks including psammitic metasediments, marble and subordinate conglomerate. Volcanic rocks including rhyolites and felsic tuffs dominate the upper part of the northern allochthon. The contacts between the metasedimentary rocks on the one hand and the rhyolites and felsic tuffs on the other hand are extrusive. This allochthon overrides an internally deformed nappe (central allochthon) dominated by arc and ophiolitic assemblages now preserved as felsic and mafic schist, talc schist, serpentinites, and metagabbros. This allochthon is characterized by NW-trending, upright folds, which deform the earlier sub-horizontal structures. The structurally lower nappe (southern allochthon) is dominated by NNE-trending folds which deform amphibolite facies schistose metavolcanic and metavolcanoclastic rocks. The NNE-trending folds deform earlier NW-trending folds to produce crescentic dome interference pattern with well-developed NE-trending axial planar cleavage, consistent with ESE–WNW bulk shortening. The southernmost structural unit is an autochthonous block dominated by arc-related volcanic and volcanoclastic rocks. This block has suffered only minor deformation compared to the nappes to its north. The consistent SW-vergence of the structures indicates tectonic transport from northeast to southwest, followed by ESE–WNW shortening similar to that found in the Hamisana Shear Zone, further east. Collision between the Gabgaba–Gebeit terrane and the Southeastern Desert terrane along the Allaqi–Heiani suture, after the consumption of a marginal basin probably over an N-dipping subduction zone, led to the formation of EW- to NW-trending folds and thrusts. This was followed by ESE–WNW tectonic shortening to form NNE-trending folds, which are found to be overprinting the earlier structures. This latest shortening might be due to collision between the Arabian–Nubian Shield and the Saharan Metacraton along an N-trending arc–continent suture represented farther south by the Keraf suture. [Copyright &y& Elsevier]