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Serpentinites are metamorphic rocks that are widely applied as aggregates in the production of radiation-shielding concrete. Different varieties of massive serpentinite mountains located in Egypt exist without real investment. Hence, this study aims to evaluate the radiation shielding efficacy of three varieties of serpentinite rocks from different geological perspectives: mineralogical, geochemical, and morphological characteristics. X-ray diffraction, transmitted-light microscopy, and thermal analysis were required to characterize their mineralogical composition, while X-ray fluorescence was necessary to investigate their geochemical features. Moreover, scanning electron microscopy was used to detect their morphological characteristics. On the other hand, the PuBe source and stilbene detector were employed for the experimental determination of fast neutrons and γ-ray attenuations, which were conducted at energy ranges of 0.8–11 and 0.4–8.3 MeV, respectively. Based on the mineralogical, geochemical, and morphological characteristics of these rocks, the radiation attenuation capacity of lizardite > antigorite > chrysotile. However, these serpentinites can be applied as a natural alternative to some radiation-shielding concrete in radiotherapy centers and other counterpart facilities.

In the Wadi Um Had area, Central Eastern Desert, Egypt, NE-trending metapelitic and molasse-type successions are exposed. The metasediments bear the geochemical signature of a first depositional cycle in two distinct continental island arc settings that derived from incipiently-to moderately-weathered intermediate to felsic sources under generally warm and humid conditions. The metapelitic succession records three distinct episodes of metamorphism, M1–M3, whereas the molasse-type succession records only the last metamorphic episode, M3. M1/D1 records an amphibolite facies tectono-metamorphic event that has been dated at 625 ± 5 Ma, whereas M2/D2 records a greenschist facies subduction-related event. Collision of the two domains during a NE–SW shortening D3, culminated in formation of the macroscopic NW–SE-trending folds. D2 and D3 correlate with the gneiss-forming event, which is constrained at

Neoproterozoic ophiolites in the Eastern Desert (ED) of Egypt are pervasively carbonated and listvenitized. Two types of carbonation are recognized: 1) intergrown magnesite (and to lesser extent dolomite) with serpentine and talc that in cases form pure carbonate veins, and 2) cryptocrystalline magnesite veins filling the fractures crosscutting other ophiolitic host rocks. Few studies address the conditions of carbonate alteration of ultramafic rocks, especially the temperature of altering fluids. We employ clumped isotope thermometry on natural dolomite and magnesite from 17 variably carbonated ophiolitic rocks and veins in the ED. Five samples of antigorite-bearing serpentinite, talc-carbonate, and associated carbonate veins yield wide range temperatures of magnesite and dolomite between 213 to 426°C (285±73°C). These temperatures are comparable with previous fluid inclusion thermometry carried out on some of the vein samples (homogenization temperature between 225 to 383°C; Boskabadi et al. 2017). Ten samples of fully quartz-carbonate altered peridotites (i.e. listvenites) record even a wider range of clumped isotope carbonation temperatures between 90 and 452°C (227±112°C). In contrast, two samples of late-stage veins of cryptocrystalline magnesite record lower temperatures of 19 and 28°C. While the constraints on the pressure of carbonation are lacking, the wide range of temperatures for the carbonates in antigorite-bearing serpentinite, talc-carbonate, and listvenite lithologies suggest that carbonation probably occurred at variable depths, whereas the low temperature of cryptocrystalline magnesite veins points to conditions nearer the surface most likely associated with post-obduction processes. Therefore, different sources of carbon and CO2-bearing fluids should have been responsible for the formation of high- and low-temperature carbonates in the region. Boskabadi et al. 2017. International Geology Review 59, 391–419.

The igneous rocks of the Wadi Hamad area are exposed in the northernmost segment of the Arabian–Nubian Shield (ANS). These rocks represent part of crustal section of Neoproterozoic continental island arc which is intruded by late to post-collisional alkali feldspar granites. The subduction-related intrusives comprise earlier gabbro–diorites and later granodiorites–granites. Subduction setting of these intrusives is indicated by medium- to high-K calc-alkaline affinity, Ta–Nb troughs on the spider diagrams and pyroxene and biotite compositions similar to those crystallized from arc magmas. The collisional alkali feldspar granites have high-K highly fractionated calc-alkaline nature and their spider diagrams almost devoid of Ta–Nb troughs. The earlier subduction gabbro–diorites have lower alkalis, LREE, Nb, Zr and Hf values compared with the later subduction granodiorites–granites, which display more LILE-enriched spider diagrams with shallower Ta–Nb troughs, reflecting variation of magma composition with arc evolution. The later subduction granitoids were generated by lower degree of partial melting of mantle wedge and contain higher arc crustal component compared with the earlier subduction gabbro–diorites. The highly silicic alkali feldspar granites represent extensively evolved melts derived from partial melting of intermediate arc crustal sources during the collisional stage. Re-melting of arc crustal sources during the collisional stage results in geochemical differentiation of the continental arc crust and the silicic collisional plutonism drives the composition of its upper part towards that of mature continental crust.

A suite of Neoproterozoic island arc volcanic and plutonic rocks are exposed in the Hamash area of the south Eastern Desert of Egypt. The volcanic assemblage includes metabasalt, a group of andesites with subordinate basalts (the Hamash Volcanics), and felsic volcanic porphyry. The plutonic rocks include a diorite-tonalite series, fine-grained granite, and granite porphyry. The volcanic rocks are generally sub-alkaline and feature a strong depletion in Nb and Ta. The metabasalt belongs to the low-K tholeiitic series and has nearly flat rare-earth element patterns. Its compositional characteristics indicate generation by partial melting of a depleted mantle source in an immature oceanic island arc setting. On the other hand, the basalt, andesite and felsic volcanic porphyry have low- to medium-K calc-alkaline affinity and are enriched in light rare-earth elements and large ion lithophile elements. Their parental magmas likely formed by fluid-assisted partial melting of a less depleted mantle source with major subduction input, in a mature island arc setting. The andesite cannot be derived from the basalt by fractional crystallization; their parental magmas likely represent different degrees of melting underneath crust of different thickness. Estimation of the arc crustal thickness during formation of these rocks yields ~ 5 km for the metabasalt, ~20 km for the Hamash basalt, and ~ 30 km for the Hamash andesite, consistent with a progression in maturity of the arc over time. The geochemical characteristics of the diorite-tonalite series suggest that it represents an intrusive equivalent of the medium-K calc-alkaline Hamash andesite, and the fine-grained granite represents residual liquid after continued fractionation of the parental magma of the andesite and diorite-tonalite. We present thermodynamic fractionation models that constrain the conditions of fractionation for the andesite, diorite-tonalite, and fine-grained granite suite. The compositional similarity of the granite and rhyolite porphyries, especially their SiO2 and Al2O3 contents and Y/Nb ratios, indicate generation by partial melting of similar crustal sources, although greater assimilation of upper crust is evident in elevated Rb/Sr ratios and K2O and light rare-earth element concentrations in the granite porphyry. We conclude that the magmatic rocks of the Hamash area represent an arc crustal section that records the evolution of a Neoproterozoic oceanic island arc. The arc was later caught up in the assembly of the northern Arabian-Nubian Shield.

To estimate the oxygen fugacity (fO2) of the Neoproterozoic and Cretaceous suprasubduction zone mantle, and to evaluate the possible secular changes in the upper mantle oxidation state, the compositions of spinel, olivine and orthopyroxene of Neoproterozoic (Egypt and Saudi Arabia) and late Cretaceous (Iran) mantle rocks were determined. For accurate estimation of fO2, spinel ferric iron was calculated after correcting the electron microprobe data using a set of spinel standards for which the ferric iron content was measured by Mӧssbauer spectroscopy. The Neoproterozoic samples record strongly heterogenous fO2 values ranging from moderately oxidized (FMQ +0.54) to ultra-reduced (FMQ -4.73) for harzburgites, from highly oxidized (FMQ +1.49) to moderately reduced (FMQ -0.60) for dunites as well as one highly reduced (FMQ -1.61) value for chromitite. Such heterogeneity is not apparent in the late Cretaceous harzburgites that record fO2 values ranging from slightly oxidized (FMQ +0.45) to moderately reduced (FMQ -0.85). The fO2 of the Neoproterozoic forearc mantle is most easily explained by melt-mantle interaction and deep-mantle recycling, while that of the late Cretaceous forearc mantle can be attributed to variable degrees of melt-mantle interaction. The estimated fO2 values of Neoproterozoic/Cretaceous mantle unaffected by melt-rock interaction and deep-mantle recycling, and published values of Precambrian and Modern mantle suggest a consistent upper mantle oxidation state from Proterozoic to present day.

Large outcrops of ultramafic rocks are common in the Central Eastern Desert (CED) of Egypt and represent residual mantle sections and cumulates of dismembered Neoproterozoic oceanic lithosphere. This study integrates field observations, petrography, geochemistry and mineral chemistry to investigate ultramafic rocks of the dismembered ophiolitic rocks at four areas distributed from east to west across the CED, with the aim of gaining a better understanding the tectonic settings and mantle conditions under which the ultramafic rocks formed. The ultramafic masses studied are composed of serpentinized peridotites, serpentinites and pyroxenites, and occasionally host podiform chromitites. Preserved textures, relict primary minerals and whole-rock compositions indicate that the protoliths of these ultramafic rocks were dominated by harzburgites. The Al2O3- and CaO-depleted nature of residual harzburgites, the high Fo and NiO contents of primary olivines and the high Cr# and low TiO2 contents of fresh Cr-spinels suggest that these ultramafic rocks are remnants of depleted to highly depleted forearc mantle of suprasubduction zone (SSZ) system. Temperatures from olivine-spinel geothermometry (668–778 °C) and Al-in-orthopyroxene geothermometry (984 °C) estimated for residual harzburgites are comparable to those reported for modern forearc mantle peridotites. The range of oxygen fugacity [ΔlogƒO2 (FMQ)] values (−3.01 to +0.32) calculated for the residual harzburgites reveals heterogeneity in the Neoproterozoic mantle oxidation state and suggests that SSZ mantle was not consistently more oxidized than MOR mantle. The podiform chromitites of different ultramafic masses were formed following the interaction of residual mantle peridotites with forearc basaltic and boninitic melts, implying the evolution of Neoproterozoic SSZ system from subduction initiation to more mature island arc stage. Integration of the analyses carried out in this study shows a Neoproterozoic SSZ system where slab-derived melts evolve in composition and oxidation state from early reduced forearc basalt in the east to more oxidised boninitic melts in the west.

The Neoproterozoic intrusive rocks of the Wadi Ranga area, Nubian Shield of Egypt, comprise gabbros and granitoids emplaced during oceanic island arc and post-collision stages. The plutonic rocks of the island arc stage include hornblende gabbros (Dabbah pluton), trondhjemite (Abu Ghalaga pluton) and tonalites with subordinate quartz gabbro and quartz diorite (Reidi and Abu Ghusun plutons), whereas the post-collision intrusives include granodiorite and monzogranite (Helifi-Hamata pluton). The gabbros and granitoids of the island arc stage are largely calcic, low-K rocks which have either tholeiitic (gabbro and trondhjemite) or transitional tholeiitic to calcalkaline nature (tonalites). On the other hand, the granitoids of the post-collision stage are medium to high-K calcalkaline rocks. All the investigated granitoids are metaluminous. The spider diagrams, with enrichment in LILE and strong Nb depletion, and the almost flat to slightly LREE-depleted REE patterns of the gabbro and trondhjemite are similar to those of the Wadi Ranga low-K tholeiitic basalts and silicic volcanics, respectively, suggesting that the gabbro and trondhjemite are the plutonic equivalents of the Wadi Ranga immature island arc extrusives, and they were derived from mantle source at the early immature island arc stage. Similar to the trondhjemite, the tonalites show LILE enrichment and strong Nb depletions on the MORB-normalized spider diagrams. However, the tonalites have REE patterns which are enriched in LREE (La/Yb = 1.71–5.54). The derivation of the tonalites through fractionation of the same magma produced the trondhjemite seems unlikely. Therefore, high degree partial melting of juvenile basaltic arc crust is favoured for the origin of tonalites during a late immature island arc stage. The post-collision granitoids show considerable enrichment in LILE and to a lesser extent in HFSE, slight negative Nb anomaly and strong negative P and Ti anomalies relative to N-MORB. Their REE patterns are LREE-enriched (La/Yb = 5–19), with negative Eu anomaly. These characteristics are consistent with origin through lower degrees of partial melting of old basaltic arc crust and subsequent fractional crystallization. The geochemical characteristics of the trondhjemite and tonalites, and the granodiorite–monzogranite classify them as M-type and I-type granitoids, respectively. The partly tholeiitic intrusives of the Wadi Ranga area (South Eastern Desert) have lower K2O, Rb and LREE compared to the M-type calcalkaline intrusives of the North Eastern Desert, implying northwardly dipping subduction zone. The geochemical similarities between the intrusives of Neoproterozoic and Phanerozoic oceanic island arcs imply that they share similar style of subduction, which differs from that of the Archaean. The generation of high SiO2 (up to 74.5 wt%), low K2O (0.56–1.78 wt%) and slightly LREE-depleted trondhjemite in early immature oceanic island arc setting supports the arc origin of the primitive continental crust. Silicic magma production through partial melting of the early arc volcanic rocks during the evolution of the arc and the post-collision stage, drives the middle and upper oceanic arc crust towards a composition closer to that of the continental crust. The present study indicates that the intra-oceanic island arcs continued to play a role in the generation of the continental crust after the Archaean.

The ophiolitic metabasalts (pillowed and sheeted dikes) of Wadi Ghadir area, Eastern Desert, Egypt, were analyzed for their major, trace and rare earth elements, Nd and Sr isotopes and the chemistry of their plagioclase, amphibole and chlorite was also reported. Geochemically these rocks range from tholeiitic basalt to basaltic andesite. The generally low MgO, Cr and Ni and high Zr contents are consistent with derivation of these rocks from an evolved magma. The high TiO2 contents (mostly between 1.76% and 2.23%) classify Wadi Ghadir ophiolitic metabasalts as MORB ophiolite. The chondrite-normalized REE patterns of most samples display small LREE-enrichment with (La/Yb)n ranging from 1.44 to 2.56. The MORB-normalized spider diagram shows variable LILE abundances, which are either similar to or enriched relative to MORB, and most samples display small Nb depletion. The abundances of some LILE (Ba, Rb and K) as well as Na and Si were modified by post-magmatic seafloor hydrothermal alteration. Enrichment of the least mobile LILE (Th & U) indicates that Wadi Ghadir ophiolitic metabasalts are akin to C (contaminated)-MORB. These geochemical characteristics are similar to BABB modified by contamination. Wadi Ghadir metabasalts have low initial Sr ratios (0.7010–0.7034) which are similar to those of MORB, while their eNd(t) values (+7.7–+4.5) are either more or less positive than the value of depleted mantle (DM). The more positive eNd(t) values indicate DM source for these basalts, while the less positive eNd(t) values reflect the involvement of slightly older component in Wadi Ghadir ophiolite. We suggest that the parent magma of metabasalts was contaminated by slightly older material, most probably oceanic-arc crustal rocks, which caused enrichment in LREE, and by analogy LILE, but did not significantly affect Nd isotopic systematics or modify Sr isotopes. Such contaminated MORB character also revealed by other ophiolitic metavolcanics in the Central Eastern Desert, contrasting the N-MORB character of the Gerf ophiolite in the South Eastern Desert. Moreover, the present work suggests the increase of the degree of contamination of the ophiolitic metabasalts from south to north in the Central Eastern Desert.