The Evolution of Neoproterozoic Mantle Peridotites Beneath the Arabian–Nubian Shield: Evidence from Wadi Sodmein Serpentinites, Central Eastern Desert, Egypt

Abstract

Serpentinites make up one of the most significant rock units associated with primary suture zones throughout the ophiolite sequence of the Arabian–Nubian Shield. Wadi Sodmein serpentinites (WSSs) represent dismembered parts of the oceanic supra-subduction system in the central Eastern Desert of Egypt. In this context, we present whole-rock major, trace, and rare earth elements (REE) analyses, as well as mineral chemical data, to constrain the petrogenesis and geotectonic setting of WSS. Antigorite represents the main serpentine mineral with minor amounts of chrysotile. The predominance of antigorite implies the formation of WSS under prograde metamorphism, similar to typical metamorphic peridotites of harzburgitic protolith compositions. The chemistry of serpentinites points to their refractory composition with notably low Al₂O₃, CaO contents, and high Mg# (90–92), indicating their origin from depleted supra-subduction zone harzburgites that likely formed in a forearc mantle wedge setting due to high degrees of hydrous partial melting and emplaced owing to the collision of the intra–oceanic arc with Meatiq Gneisses. Spinels of WSS generally exhibit pristine compositions that resemble those of residual mantle peridotites and their Cr# (0.625–0.71) and TiO₂ contents (<0.05 wt%) similar to forearc peridotite spinels. Moreover, WSS demonstrates a significant excess of fluid mobile elements (e.g., Th, U, Pb), compared to high-field strength elements (e.g., Ti, Zr, Nb, Ta), implying an interaction between mantle peridotites and fluids derived from the oceanic subducted-slab. The distinct U-shaped REE patterns coupled with high Cr# of spinel from WSS reflect their evolution from mantle wedge harzburgite protolith that underwent extensive melt extraction and re-fertilized locally.

1. Introduction

Arabian–Nubian Shield (ANS) is characterized by abundant Neoproterozoic ophiolites that are aligned along numerous suture zones (Figure 1) and exhibit different ages varying from 690 to 890 Ma [1,2,3]. Geochemical investigation of the ANS oceanic crust (e.g., ophiolite sequences) provides valuable evidence regarding melt generation events during the formation of oceanic crust. Additionally, these studies aid in interpreting the tectonic processes that occurred during the accretion stage of arc-basin systems within the ANS [4].

Serpentinites are widely distributed in ANS and are accepted to have been derived essentially from peridotites [5,6]. Serpentinite masses of the Eastern Desert of Egypt occur as an integral part of the ophiolite sequence and/or randomly distributed fragments of varying size in a mélange with sedimentary matrix [7]. Isolated masses of serpentinite are mainly composed of serpentine group minerals and talc-carbonate rocks together with minor chromitites, chloritites, and rare relics of peridotite and dunites parentage [8].

Figure 1. (a) Distribution of ophiolites in the Eastern Desert of Egypt [9]. (b) Geological map of the Wadi Sodmein area showing the distribution of serpentinites (Modified after [10]).

Figure 1. (a) Distribution of ophiolites in the Eastern Desert of Egypt [9]. (b) Geological map of the Wadi Sodmein area showing the distribution of serpentinites (Modified after [10]).

Ophiolites and serpentinites of the ANS were divided into subduction-related and subduction-unrelated types according to the classification of [11]. The first type is classified as a volcanic arc and supra-subduction zone (SSZ) ophiolites that are associated with the closing of ocean basins. On the other hand, subduction-unrelated ophiolites exhibit mid-ocean ridge basalts (MORBs) chemical characteristics and were interpreted to have been emplaced during sea-floor spreading [12,13,14]. Although the SSZ ophiolites are believed to occur in both forearc and back-arc basin settings (e.g., [15,16]), the former is characterized by severe deformation of mantle rocks and intensive melt/fluid-rock interaction (e.g., [17,18,19,20,21,22,23]). Fluid/melt-mantle interaction is regarded as the prominent factor that modifies the chemistry of the sub-arc mantle. The arc magma generated due to the extensive partial melting of the mantle wedge commonly preserves its geochemical imprints of subduction inputs into the mantle, mantle wedge metasomatism, and serpentinization [21]. Accordingly, SSZ serpentinites are valuable for detecting mantle processes and the degree to which fluids and melts interacted with the mantle wedge during the initiation of subduction and fore-arc spreading [16,22].

The ANS was interpreted to have formed during the late Neoproterozoic period through the closure of the Mozambique Ocean and the collision between East and West Gondwana, where the Pan-African Mountain building event took place (e.g., [1,23]). The Mozambican lithospheric mantle underwent partial melting through the process of decompression and the mantle upwelling in regions experiencing lithospheric extension and/or melting through hydration of the mantle wedge above subducting slabs (e.g., [24,25]). Following the collision, many suture zones were formed and continents were thrusted by parts of the oceanic lithosphere exposing numerous ophiolitic complexes along the major faults (Figure 1a).

The tectonic settings of the Egyptian ophiolites are still debated, various tectonic settings were recognized for the ophiolitic peridotites from ANS (e.g., mid-oceanic ridge tectonic setting, remnants of back-arc basins, or fore-arc setting due to seafloor spreading during initiation of subduction process) (e.g., [26,27,28,29,30,31,32]). However, these controversial models are attributed to vulnerable chemical changes in primary minerals and modification of whole-rock chemical compositions of peridotites due to extensive serpentinization processes. These ophiolites are suggested to be emplaced in a supra-subduction zone environment and a forearc setting (e.g., [28,33] and many others). Several authors, (e.g., [13,15,34]) agreed that serpentinites resulted from systematic changes and modification of the geochemical characteristics of their parent peridotite; therefore, they could be used as an important clue for understanding the melting and metasomatic process within the mantle. The geochemical behavior of incompatible trace elements in serpentinites could be used for interpreting the geochemical modifications that resulted during fluid/melt–mantle rock interactions [35]. Moreover, it is believed that the distribution of rare earth elements (REE) and ratios between heavy-(HREE), middle-(MREE), and light-(LREE) could be used to attest to the depletion and melting in the melt of the mantle peridotites (e.g., [36]).

Although serpentinites are essential components of the Pan-African ophiolites in the Eastern Desert of Egypt that formed during the Neoproterozoic, their magmatic processes and relations in accordance with the geodynamic evolution of mantle are still ambiguous. This paper aims to provide new insights clarifying these issues by examining the serpentinites found in the Eastern Desert and their implications regarding the magmatic and tectonic history of the underlying mantle during Precambrian time. Consequently, we introduce new geochemical and mineral chemical data on Wadi Sodmein serpentinites (WSS) which are regarded as one of the best representatives for the dismembered ophiolitic sequence within the central Eastern Desert (CED) of Egypt. These data provide significant scientific evidence into the origin and geotectonic setting of WSS, shedding light on the processes of mantle activity, partial melting, and interaction between melts and rocks that occurred during the evolution of the WSS. Furthermore, these findings contribute to a better understanding of the development of the Neoproterozoic mantle beneath the ANS. It is important to highlight that the WSS has received limited attention in the geological literature, with most publications focusing on the petrogenesis and geochemistry of metasediments, metavolcanic rocks, and their associated fluorite deposits, as well as the formation of felsite and A-type granites in the area [37,38].

2. Geologic Setting

The WSS lies in the CED and constitutes nearly the whole succession of the Egyptian Pan-African Neoproterozoic basement complex. The Wadi Sodmein area lies to the northwest of Quseir City on the Red Sea coast between latitudes 26°08′13″ and 26°11′50″ N and longitudes 33°50′00″ and 33°54′57″ E. The Precambrian basement rocks exposed in the Wadi Sodmein area mainly consist of gneisses, ophiolitic rocks, metavolcanic-metasedimentary rocks of island-arc assemblages, syn- to late- and post-orogenic granitoids, and Dokhan volcanics (Figure 1b). The Meatiq gneisses are primarily composed of gneisses, amphibolite, and gneissose granite, which exhibit varying degrees of deformation and cataclasis [39,40]. These gneisses form an elongated belt oriented in the NW–SE direction and have been thrust over by ophiolitic rocks originating from the east along a major NW–SE thrust fault (Figure 1b and Figure 2a,b). On the other hand, they have been intruded by Baanib granites from the west. Within this geological setting, elongated masses of ophiolitic rocks can be observed, extending parallel to the NW-SE direction (Figure 1b). The ophiolitic rocks consist of serpentinites, some of which have undergone local transformation into talc-carbonate rocks (Figure 2c), as well as metagabbros and metavolcanics. These rocks have been amalgamated within a highly sheared matrix of metasediments and metapyroclastics. Along both sides of Wadi Sodmein, numerous isolated serpentinite lenses are frequently oriented in the NW-SE and N-S directions, conforming to the regional structures. Some of these lenses are integral parts of the ophiolitic rocks (Figure 2c). The serpentinites exist as isolated masses and tectonic slices within the folded metavolcanic and metasediments, as well as along thrust faults (Figure 2d–f). Locally, the serpentinite lenses have undergone partial replacement by talc-carbonate rocks, which are more prevalent along the dissected shear zones (Figure 2e).

The serpentinites are composed of varying proportions of antigorite serpentinites, antigorite schist, and talc-carbonate rocks, where relics of ultramafic protoliths are not encountered. In the intensely sheared and foliated serpentinites, the schistosity is frequently run NW and conformable with the schistosity of the enclosed metavolcanics. Although these serpentinites are structurally controlled [27], they are not related to mélanges as they generally occur conformable with the inherent weakness planes of their surrounding rocks and along thrust faults.

Figure 2. Representative field photographs exhibiting (a,b) gneisses thrusted by ophiolitic rocks (serpentinites) along an NW–SE major thrust fault; (c) talc-carbonate rocks representing the alteration product of serpentinite; (d–f) sharp contacts between serpentinites and their alteration products with (d,e) metasediments and (f) metavolcanics.

Figure 2. Representative field photographs exhibiting (a,b) gneisses thrusted by ophiolitic rocks (serpentinites) along an NW–SE major thrust fault; (c) talc-carbonate rocks representing the alteration product of serpentinite; (d–f) sharp contacts between serpentinites and their alteration products with (d,e) metasediments and (f) metavolcanics.

3. Petrography

Serpentinites of WSS are medium to fine-grained, greenish grey to brownish grey, and essentially constituted by serpentine minerals with sporadic occurrences of chromite (Figure 3). Minor amounts of secondary chlorite and carbonates are also present. Serpentine minerals are mainly represented by antigorite with minor chrysotile (Figure 3a). Antigorite generally exhibits massive aggregates (Figure 3a) but occasionally and sometimes occurs as subhedral small flakes (Figure 3b) that approximately intersect at right angles, reflecting the original cleavage planes of pyroxenes (Figure 3b).

Figure 3. Petrographical features of serpentinite rocks showing (a) chrysotile veinlets within antigorite; (b) pseudomorphic interpenetrating texture in serpentinites consisting of optically distinguishable blades of antigorite and chlorites, where antigorite laths are disposed nearly at right angles along the inherited pyroxene cleavage plains; (c) serpentinites composed mainly of antigorite exhibiting with magnesite replacing chrysotile veins; (d) backscattered electron (BSE) image of serpentinite exhibiting mesh texture and anhedral spinel; (e) serpentinite exhibiting well-developed interlocking textures with fine patches of magnesites (f) subhedral spinel marginally altered to magnetite of black color and dissected by fractures that filled with antigorite and magnesite inherited from the original olivine; (g) BSE image of spinel with unaltered core (dark gray) and altered ferritchromite rim (white) and exhibiting well developed fractures that are filled with antigorite and magnesite inherited from the original olivine; (h) serpentinites composed of antigorite and chrysotile with subhedral grains of magnetite and patches of magnesite. Abbreviations: (Atg) antigorite, (Ctl) chrysotile, (Chl) chlorite, (Spl) spinel, (Mgs) magnesite, (Fe-Ch) ferritchromite, (Mag) magnetite.

Figure 3. Petrographical features of serpentinite rocks showing (a) chrysotile veinlets within antigorite; (b) pseudomorphic interpenetrating texture in serpentinites consisting of optically distinguishable blades of antigorite and chlorites, where antigorite laths are disposed nearly at right angles along the inherited pyroxene cleavage plains; (c) serpentinites composed mainly of antigorite exhibiting with magnesite replacing chrysotile veins; (d) backscattered electron (BSE) image of serpentinite exhibiting mesh texture and anhedral spinel; (e) serpentinite exhibiting well-developed interlocking textures with fine patches of magnesites (f) subhedral spinel marginally altered to magnetite of black color and dissected by fractures that filled with antigorite and magnesite inherited from the original olivine; (g) BSE image of spinel with unaltered core (dark gray) and altered ferritchromite rim (white) and exhibiting well developed fractures that are filled with antigorite and magnesite inherited from the original olivine; (h) serpentinites composed of antigorite and chrysotile with subhedral grains of magnetite and patches of magnesite. Abbreviations: (Atg) antigorite, (Ctl) chrysotile, (Chl) chlorite, (Spl) spinel, (Mgs) magnesite, (Fe-Ch) ferritchromite, (Mag) magnetite.

Chrysotile is found in elongated fibrous veinlets that cut across the serpentine massive matrix (Figure 3a), indicating a prolonged process of serpentinization and subsequent crystallization of chrysotile under static conditions (e.g., [41]). The presence of well-developed magnesite (Figure 3c) and pseudomorphs after primary minerals such as olivines and pyroxenes in the serpentinites (Figure 3b) provide petrographic evidence for their origin. Furthermore, the WSS exhibits mesh-like (Figure 3d) and interlocking textures (Figure 3e). The mesh texture (Figure 3d) is indicative of the formation of serpentine as an alteration product after olivine, whereas bastite is indicative of serpentine after pyroxene ([42]; Figure 3b). Small aggregates of chlorite are enveloped by an antigorite matrix (Figure 3b). Spinels showing blood-red subhedral and irregularly brecciated fractured grains with few grains transformed into ferritchromite along peripheries, which are highly porous and dissected by microfractures filled with antigorite and magnesites (Figure 3f,g). Carbonate minerals present as veins (Figure 3c), sparse crystals (Figure 3h), disseminated patches, and fine aggregates of subhedral magnesite enclosing antigorite laths (Figure 3c). Magnetite forms subhedral granules and their streaks are disposed and conformable with the regional schistosity (Figure 3h).

4. Materials and Methods

Whole-rock major oxides (wt%) were analyzed on fused glass beads (0.4 g sample powder mixed with 4 g of lithium tetraborate) by an X-Ray Fluorescence spectrometer (Rigaku ZSX-100e, Rigaku Holdings Corporation, Tokyo, Japan) at the Instrumental Analytical Center of Kagoshima University, Kagoshima, Japan. The 14 standard reference materials of the GSJ [43] were used for analytical calibrations and matrix corrections. Detection limits of the XRF during the analyses were 0.01% for major elements. LOI was determined on powdered samples at 650 °C, heated for 6 h, and the heated sample powders were used for XRF analysis.

Concentrations of trace and rare earth elements (REE) were determined via ICP-mass (Agilent7700ICP-MS) spectrometry (Agilent Technologies, Santa Clara, CA, USA) at the Primorsky Center of Local Elemental and Isotopic Analysis of Far East Geological Institute, Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia. About 50 mg power of each sample was dissolved in acid-washed Teflon containers by refluxing in hot (250 °C) 3:1 nitric and hydrofluoric acid for at least 8 hrs. The external precision and accuracy of the analyses were assessed by measuring unknown four rock standards: BEN and BIR basalts, JP-1 peridotite, and UBN serpentinite (Supplementary Table S1). Our results show good agreement between measured values and expected values for the international standards, and external reproducibility is within 0%–5% for all the analyzed elements.

Mineral compositions were determined using a JEOL JXA-8200 electron probe microanalyzer (JEOL Ltd., Akishima, Japan) at the Institute of Geosciences, University of Potsdam, Potsdam, Germany. Operating conditions involved a beam current of 10 nA, a spot diameter of 2 μm, and an accelerating voltage of 15 kV. Elemental peaks were counted for 20 s with a background subtraction for 10 s. For every 100 readings, calibration was performed again using a set of synthetic oxides and natural silicates from the Smithsonian Institute.

5. Results

5.1. Mineral Chemistry

The analyzed minerals include serpentines, Cr-spinel, chlorites, and magnesite where their chemical composition is presented in Supplementary Tables S1–S4.

5.1.1. Serpentine Minerals

Chemical analyses of serpentine minerals from the studied samples are presented in Supplementary Table S1. Generally, serpentine minerals are distinctly variable in their chemical compositions owing to the different degrees of serpentinization [44]. The analyzed serpentine minerals possess 42.45–43.95 wt% SiO₂, 37.49–41.97 wt% MgO, 0.13–0.92 wt% Al₂O₃, 1.01–3.16 wt% FeO, 0.05–0.11 wt% Cr₂O₃, and 0.15–0.36 wt% NiO. Based on their chemical compositions, the studied serpentine minerals are distinguished as antigorite (Figure 4a) and their silica contents are consistent with antigorite of Pan-African ophiolitic serpentinites (SiO₂ < 45 wt%) [45,46]. The obvious SiO₂ enrichment with Al₂O₃ depletions is remarkable for chrysotile/lizardite to antigorite transition (Figure 4a; e.g., [47,48]). Most of the analyzed serpentine minerals are characterized by low Cr₂O₃ and Al₂O₃ contents (Supplementary Table S1) that are typical for the composition of mesh serpentines, whereas few samples correspond to the composition of orthopyroxene-bastite (Figure 4b), indicating their derivation from a harzburgitic protolith. Moreover, their NiO and Cr₂O₃ contents resemble the mesh serpentine minerals of fore-arc serpentinites (Figure 4c; [49]), implying their formation from olivine and orthopyroxene, which additionally emphasizes their harzburgitic protolith origin. In addition, the remarked excess of SiO₂ and MgO contents of the analyzed serpentine minerals are consistent with those pseudomorphic serpentines that consist of antigorite with a interpenetrating texture (Figure 4d,e; [47]).

5.1.2. Chromian-Spinels

Chromite is regarded as the only magmatic mineral that retains its original chemistry during serpentinization [50]. Accordingly, it can be used as a petrogenetic and geotectonic indicator (e.g., [51,52]). The analyzed chromite crystals display uniform chemical compositions (Supplementary Table S2), exhibited by high Cr# (Cr/Cr + Al) and Mg# (Mg/Mg + Fe²⁺), which are consistent with those of mantle-derived peridotites [53].

Based on their chemical constituents, the analyzed Cr-spinels are plotted in the mantle chromite field that is related to ophiolitic spinels of forearc peridotites (Figure 5a). Moreover, they are classified into chromite and Magnesiochromite (Figure 5b), whereas their Cr# and Mg# are related to those commonly derived from mantle peridotite (Figure 5c). The very low TiO₂ concentrations (0.01–0.04 wt%, Supplementary Table S2) suggest that their parent rocks possibly originated through a depleted mantle (Figure 5d; e.g., [54]). Furthermore, the analyzed spinels are homogenous and reddish brown in color (Figure 3f), similar to those of residual mantle peridotites [55,56]. In addition, the analyzed spinels plot in the residual peridotite field (Figure 5e) is consistent with their low TiO₂ contents. They have low MnO (0.22–0.46 wt%), TiO₂ (<0.05 wt%), and YFe³⁺ (<0.05) but high Fe²⁺/Fe³⁺ (>5.44) (Supplementary Table S2; Figure 5f), which are indicative for least alteration and consistent with mantle spinel compositions [57]. The analyzed spinels from the WSS plot in the space of primary forearc peridotite spinel and far away from spinel fields of greenschist facies and low-amphibolite facies (Figure 5g). They neither plot in the secondary spinel field (Figure 5f) nor along with alteration trend (Figure 5g); thus, we suggest that examined spinels have pristine compositions and resemble those of primary spinels in depleted peridotites (Figure 5c,f). Consequently, they plot in the mantle array field, suggesting their residual origin is similar to those of Pan-African ophiolitic podiform chromitites, particularly those associated with harzburgites (Figure 5h).

Figure 4. Microprobe analyses of serpentine phases in the studied samples are plotted in (a) Al₂O₃ vs. SiO₂ (wt%). Fields of different serpentine minerals after [58]. (b) Cr₂O₃ vs. Al₂O₃ after [47]. (c) NiO vs. Cr₂O₃ (wt%). Fields for olivine and orthopyroxene after [49]. (d) MgO vs. SiO₂ after [59]. (e) MgO vs. SiO₂ after [60].

Figure 4. Microprobe analyses of serpentine phases in the studied samples are plotted in (a) Al₂O₃ vs. SiO₂ (wt%). Fields of different serpentine minerals after [58]. (b) Cr₂O₃ vs. Al₂O₃ after [47]. (c) NiO vs. Cr₂O₃ (wt%). Fields for olivine and orthopyroxene after [49]. (d) MgO vs. SiO₂ after [59]. (e) MgO vs. SiO₂ after [60].

5.1.3. Chlorite

Chlorites from the investigated WSS samples are relatively Mg-rich and classified as pycnochlorite and diabantite (Figure 6a) and have higher Mg numbers (0.72–0.77; Supplementary Table S3). Their Si concentrations range between 6.07 and 6.55 apfu (based on 28 oxygens; Supplementary Table S3), whereas Mg from 6.41 to 7.12, Al^iv from 2.35 to 2.70, and Fe from 2.09 to 2.61, which most probably reflect their formation during retrogressive metamorphism [61].

5.1.4. Magnesite

Magnesite represents the alteration product from magnesium-rich igneous and metamorphic rocks [62]. The chemical analyses of magnesite from WSS exhibit a general enrichment of MgO with notable depletion of FeO (Supplementary Table S4). The presence of magnesite in most of the studied samples reflects the significant occurrence of carbonates resulting from the percolation of CO₂-rich fluids from the mantle [63].

Figure 5. Microprobe analyses of spinel and chlorite in the studied samples plotted in the (a) Cr-Al-Fe³⁺ ternary diagram after [64]; spinel fields of abyssal peridotites [55,56] and forearc peridotites [21,65]; (b) Cr# vs. Mg# of spinel after [66]; (c) Mg# versus Cr# variation diagram for the studied spinel. Field (I) represents Cr-spinels of mantle peridotite, field (II) represents magnetite from metamorphic rocks, and field (III) is magnetite from unmetamorphosed igneous rocks (fields after [53,67]); (d) Cr# versus TiO₂ diagram for the chromian spinels. Fields of boninites, forearc peridotites, and highly depleted to depleted peridotites are after [51,55,68]; (e) Cr-Al-Fe³⁺ ternary diagram for the analyzed spinels. The superimposed fields are taken from [68]; (f) MnO versus Cr# for chromian spinel to distinguish between primary and secondary types. Fields of primary and secondary chromian spinel are after [69]. (g) Cr–Al–Fe³⁺ ternary diagram Cr-spinel metamorphic facies after [64] are shown for comparison; (h) Al₂O₃ vs. Cr₂O₃ diagram after [70]. Data sources of Cr-spinel of Pan-African harzburgites after [29,71].

Figure 5. Microprobe analyses of spinel and chlorite in the studied samples plotted in the (a) Cr-Al-Fe³⁺ ternary diagram after [64]; spinel fields of abyssal peridotites [55,56] and forearc peridotites [21,65]; (b) Cr# vs. Mg# of spinel after [66]; (c) Mg# versus Cr# variation diagram for the studied spinel. Field (I) represents Cr-spinels of mantle peridotite, field (II) represents magnetite from metamorphic rocks, and field (III) is magnetite from unmetamorphosed igneous rocks (fields after [53,67]); (d) Cr# versus TiO₂ diagram for the chromian spinels. Fields of boninites, forearc peridotites, and highly depleted to depleted peridotites are after [51,55,68]; (e) Cr-Al-Fe³⁺ ternary diagram for the analyzed spinels. The superimposed fields are taken from [68]; (f) MnO versus Cr# for chromian spinel to distinguish between primary and secondary types. Fields of primary and secondary chromian spinel are after [69]. (g) Cr–Al–Fe³⁺ ternary diagram Cr-spinel metamorphic facies after [64] are shown for comparison; (h) Al₂O₃ vs. Cr₂O₃ diagram after [70]. Data sources of Cr-spinel of Pan-African harzburgites after [29,71].

5.2. Whole-Rock Geochemistry

Whole-rock (major, trace, and REE) compositions with some calculated normative components of serpentinites are listed in Supplementary Table S5. The analyzed samples have high loss on ignition (LOI) values that vary from 12.06 to 14.96 wt%, reflecting an extensive degree of hydration and serpentinization, which is consistent with the petrographic investigation, where the olivine and pyroxene are absent. In addition, the high LOI values (Supplementary Table S5) are consistent with those of subduction zone serpentinites [25]. Major element oxides were calculated on an anhydrous basis in order to compensate for the effect of element modification resulting from the serpentinization process (Supplementary Table S5). Due to the effects of serpentinization on the studied rocks, the classification scheme of Streckeisen [72], which relies on the modal proportions of Ol, Cpx, and Opx, is deemed inadequate. Consequently, the normative whole-rock chemistry indicates a harzburgite protolith (Figure 6b). Moreover, the serpentinite samples plot on the harzburgite field (Figure 6c,d) are akin to the Alpine type field (Dunite-Peridotite; Figure 6e). The average contents of major oxides in serpentinites show lower contents of TiO₂, Al₂O₃, MnO, Na₂O, K₂O, and P₂O₅ (generally <than 1 wt%). In addition, they have high Mg# (90.14–92.91) that are similar to forearc peridotites and Pan-African serpentinites [9,73,74]. Bonatti and Michael [74] argued that Al₂O₃ contents seem to be unaffected during serpentinization; thus, the whole-rock Al content can reflect their original primary concentrations. The Al₂O₃ contents of the studied serpentinites (0.45–1.05 wt%) are conformable with those found in fore-arc and Pan-African serpentinites (e.g., [46,75,76,77]). The relatively depleted Al₂O₃ and CaO contents in the studied serpentinites can be linked with metamorphic peridotite of the ophiolitic complex and resemble harzburgites from Izu-Bonin-Mariana forearc (Figure 6f). Furthermore, they possess a low CaO/Al₂O₃ ratio (<0.75), which reflects the relatively depleted-mantle source [78].

The studied serpentinites have relatively higher concentrations of Ni (2131–2591 ppm) and Cr (1765–3234 ppm) compared with the primitive mantle (Ni > 400 ppm and Cr > 800 ppm). Moreover, the studied samples have relatively low concentrations of TiO₂ and V resembling harzburgites from the Izu-Bonin-Mariana forearc (Supplementary Table S5).

Chondrite-normalized REE patterns of the analyzed WSS samples (Figure 6g) exhibit nearly U-shaped REE patterns with small negative Eu anomalies. Moreover, the normalized REE patterns (Figure 6g) are characterized by strong LREE/MREE fractionations with (La/Sm)_N = 2.12–4.29 and (Gd/Yb)_N = 0.34–2.38 indicating MREE depletion with relative enrichment of LREE-HREE, which is analogous with that of serpentinized peridotites from South Sandwich forearc (Figure 6g). Primitive mantle-normalized multi-element abundances of the studied samples show negative anomalies of Ti, Nb, and Zr, where U, Th, and Pb exhibit positive anomalies, with slight enrichment in La and Sr (Figure 6h). The observed depletion of High Field Strength Elements (HFSEs; e.g., Nb, Zr, Ti, and Y) coupled with enrichments of Large Ion Lithophile Elements (LILEs; e.g., Cs, Rb, Th, and U) is conformable with subduction zone serpentinites [25].

Figure 6. (a) Classification diagrams for the analyzed chlorite after [79]; (b) Ol–Cpx–Opx normative compositions diagrams after [72]; (c) SFM ternary diagram of [80], where S = SiO₂ + Al₂O₃ + Na₂O, F = FeO + CaO + Al₂O₃ + 2 Na₂O and M = MgO + CaO + Al₂O₃ + 2 Na₂O; (d) variation diagrams of Ni versus MgO in the serpentinite rocks [81]; (e) Ni/Co versus Ni diagrams in comparison with ratios of alpine-type and Bushveld-layered ultramafic [82]; (f) Al₂O₃-CaO-MgO plots for mantle peridotites from the study area. Field of mantle peridotite is after [83]. Data from harzburgite from Izu-Bonin-Mariana forearc after [21] have also been plotted in Figure (e); (g) chondrite-normalized REE patterns; (h) primitive mantle-normalized trace element patterns, normalization values are from McDonough and Sun [84]. Forearc peridotites are from Pearce et al. [85] and mantle-wedge peridotites are from Deschamps et al. [25] and Khedr et al. [35] were used for comparison.

Figure 6. (a) Classification diagrams for the analyzed chlorite after [79]; (b) Ol–Cpx–Opx normative compositions diagrams after [72]; (c) SFM ternary diagram of [80], where S = SiO₂ + Al₂O₃ + Na₂O, F = FeO + CaO + Al₂O₃ + 2 Na₂O and M = MgO + CaO + Al₂O₃ + 2 Na₂O; (d) variation diagrams of Ni versus MgO in the serpentinite rocks [81]; (e) Ni/Co versus Ni diagrams in comparison with ratios of alpine-type and Bushveld-layered ultramafic [82]; (f) Al₂O₃-CaO-MgO plots for mantle peridotites from the study area. Field of mantle peridotite is after [83]. Data from harzburgite from Izu-Bonin-Mariana forearc after [21] have also been plotted in Figure (e); (g) chondrite-normalized REE patterns; (h) primitive mantle-normalized trace element patterns, normalization values are from McDonough and Sun [84]. Forearc peridotites are from Pearce et al. [85] and mantle-wedge peridotites are from Deschamps et al. [25] and Khedr et al. [35] were used for comparison.

6. Discussion

6.1. Effects of Serpentinization

The ultramafic rocks related to the Egyptian ophiolitic section in the Eastern Desert are generally metamorphosed and transformed to serpentinized peridotites and/or serpentinites that consist mainly of serpentine, talc, chlorite, carbonates, and magnetite (e.g., [9,30,46,86]). Although serpentinization in mantle peridotites neglects modification on their major elements, except for Ca [87], it results in significant enrichments of H₂O, as well as increasing fluid-mobile elements (FMEs; e.g., Pb, U, Sr and Ba; [88,89]). The concentrations of FEMs incorporated within the serpentinites depend mainly on physicochemical conditions, fluid compositions, and reaction ratios between fluid and rocks [25,90].

The high LOI values (12.06–16.23; Supplementary Table S5) in the studied serpentinites point to different degrees of post-magmatic alteration. Although the serpentinization process on peridotites led to mineralogical transformations, bulk-rock geochemical studies on subduction zone serpentinites, indicate, in general, negligible modifications of major elements (excluding Ca [25]). The degree of serpentinization could be assured based on the LOI contents of serpentinized samples that lack other hydrous minerals (e.g., amphibole, chlorite, talc, brucite, or clay minerals; [25]). Consequently, the LOI contents of the WSS samples were plotted against some major and trace elements contents to trace the effects of serpentinization through the analyzed samples (Figure 7). The studied samples show no correlation of most of the major elements with LOI (Figure 7) except Si and Mg, which exhibit a weakly positive correlation with LOI, indicating that their contents might have been slightly modified due to serpentinization. The relatively lower CaO contents (0.03–0.50 wt%) in the studied serpentinites are comparable with those of highly depleted mantle contents [91] and refute the modification of Ca through serpentinization [63]. There is no correlation between LOI and CaO (Figure 7), which argues against the possibility of Ca-metasomatism. A CaO depletion trend seems to appear through the compilation of subduction-related serpentinites. Nevertheless, it is difficult to assess if this trend is representative of serpentinization-related depletion or more related to the depletion in CaO of protoliths [25]. Furthermore, the relatively low contents of CaO within WSS < 1% led us to exclude the possible removal of CaO during serpentinization as serpentinized lherzolite has high CaO (>1.5 wt%; [25]). Moreover, Al₂O₃ contents reflect their original primary concentration as they exhibit no correlation with LOI, indicating that Al₂O₃ contents are not affected by serpentinization or the metamorphic process [74]. On the other hand, all the trace elements (including REE) exhibit irrelevant correlation with LOI (Figure 7), refuting the possibility of elemental modification due to serpentinization and metamorphism (e.g., [46,92]). The LREE are not correlated with LOI (Figure 7), indicating they were not affected by the serpentinization process due to their high incompatibility [93]. The studied WSSs exhibit very high Mg# values (88.01 < Mg# < 92.68; Supplementary Table S5), which are higher than the primitive mantle value of 0.89 [94]. They possess very low concentrations of Al₂O₃, CaO, Na₂O, and TiO₂, resembling those of supra-subduction zone (SSZ) peridotites [18]. Although CaO is highly mobile during serpentinization, Al₂O₃ is very immobile [34]; the Al₂O₃ concentration was used as the denominator of a depletion index [95]. The studied WSS exhibit good correlation trends of Al₂O₃ with some major and trace elements (Figure 8), which can be attributed to the melt/fluid–rock interaction rather than serpentinization (e.g., [93,96]) and a high degree of partial melting and depletion [95]. Accordingly, we infer that the bulk chemical compositions of the studied serpentinites were not affected by serpentinization or metamorphic process, but the modification might have taken place due to other processes such as mantle metasomatism.

Figure 7. Variation diagrams of LOI (wt%) vs. selected major, trace and rare earth elements of bulk composition of the WSS.

Figure 7. Variation diagrams of LOI (wt%) vs. selected major, trace and rare earth elements of bulk composition of the WSS.

Figure 8. Variation diagrams of Al₂O₃ (wt%) vs. selected major, trace, and rare earth elements of the bulk composition of the WSSs. Composition of primitive mantle (PM) after [84]; and depleted MOR mantle (DMM) after [97]. Depletion trend after [95].

Figure 8. Variation diagrams of Al₂O₃ (wt%) vs. selected major, trace, and rare earth elements of the bulk composition of the WSSs. Composition of primitive mantle (PM) after [84]; and depleted MOR mantle (DMM) after [97]. Depletion trend after [95].

6.2. Mantle Metasomatism (Melt/Rock Interaction)

Mantle metasomatism (e.g., melt/rock interaction) is a significant phenomenon in mantle peridotites of supra-subduction zone environments (e.g., [96,98]).

The studied serpentinites are characterized by concave chondrite-normalized REE patterns, indicating that their protolith was affected by mantle metasomatism (Figure 6g,h; e.g., [25,99,100]). Moreover, the relative enrichments of LREE and HREE relative to MREE, resulting in the U-shaped REE patterns in the studied serpentinites (Figure 6g), are similar to subduction zone ophiolitic serpentinites and highly depleted peridotites. These geochemical features are assumed to have resulted from extensive and/or repetitive hydrous partial melting, accompanied by metasomatic or refertilizated processes involving subduction-related LREE-rich melts (e.g., [20,95,99,100,101]). Furthermore, the LREE enrichments coupled with slight depletion of MREE (Figure 9a) in depleted mantle peridotites are generally attributed to reaction between melts and rock or refertilization processes prior to serpentinization (e.g., [19,102,103]). Generally, HFSE is considered relatively immobile during serpentinization and/or hydrothermal processes; accordingly, they could be considered a proxy to estimate whether the LREE-MREE modification is of magmatic or hydrothermal origin [34,104]. The significant fractionation observed between LREE and HFSE points toward a hydrothermal (i.e., aqueous) signature, while the absence of fractionation between HFSE and LREE suggests the influence of magmatic processes (Figure 9b; e.g., [34,104,105]). Trace element concentrations, normalized to the primitive mantle, of the studied WSS samples (Figure 6h) exhibit moderate fractionation between LREE and HFSE, where La/Nb and Nd/Zr ratios generally exceed primitive mantle values by approximately 0.39–11.4 and 4.06–11.95 times, respectively (Figure 6h). These features reflect the magmatic nature of HFSE enrichment of the studied serpentinites’ protolith (Figure 9b).

The pronounced positive anomaly of Pb (Figure 6h) is conformable with serpentinites attributed to SSZ mantle peridotites of forearc tectonic setting rather than abyssal and mantle wedge serpentinites [25]. The studied serpentinites are characterized by high peaks of Pb and low Ce/Pb ratios (0.05–0.60; Supplementary Table S5), referring to fluid/rock interaction prior to serpentinization [25,106,107]. The studied serpentinites exhibit strong U anomalies (Figure 6h), without a significant correlation between LOI and U (Figure 7), which refutes the increment of U during serpentinization. Moreover, the studied samples share characteristics with refertilized mantle wedge serpentinites as they plot away from the terrestrial array (Figure 9c), implying that the melt/tock interaction was the main cause of the prominent U enrichment in the studied samples [89]. Consequently, the serpentinization process has neglectable effects on the bulk-chemical composition of the studied samples and their effects were restricted to the increasing LOI contents resulting from adding water, whereas major and trace elements almost remained steady. Accordingly, the bulk-rock data can be used to constrain the petrogenesis of the studied serpentinites.

6.3. Metamorphism and Incipient Carbonation of Serpentinite

The studied serpentinites are formed of imbricated lensoidal masses and thick sheets of variable sizes thrust westward over the calc-alkaline metavolcanics (Figure 1b). The absence of a thermal impact from WSSs on the surrounding country rocks, along with their marked association with thrust faults, points to their tectonic emplacement. The studied serpentinites consist mainly of antigorite, along with varying amounts of chrysotile, magnetite, chromite, magnesite, talc, and chlorite. The dominance of antigorite compared to other serpentine minerals indicates progressive metamorphism or that their ultramafic protoliths were serpentinized under higher pressures. This process most likely occurred during their tectonic emplacement onto the continental margin [62,108,109]. The analyzed serpentine minerals exhibit substantial variability in the concentrations of SiO₂, MgO, Al₂O₃, FeO, and Cr₂O₃, which probably can be attributed to multiple generations of serpentine. The analyzed serpentines exhibit low silica contents (SiO₂ < 45 wt%) comparable to those of Pan-African ophiolitic serpentinites, which underwent regional metamorphism [45]. In addition, the remarked enrichment of MgO and SiO₂ contents indicates that the studied serpentinites are akin to pseudomorphic serpentines (Figure 3a and Figure 4b; [47]).

Coleman [83] stated that pseudomorphic serpentines generally occur due to retrograde replacement of olivine, orthopyroxene, and clinopyroxene by serpentine. On the Al₂O₃ versus SiO₂ diagram (Figure 4a), the analyzed serpentine minerals are mainly antigorite, indicating that its parent minerals were first retrogressed to form lizardite and chrysotile (pseudomorphic serpentines); then, they were subjected to progressive metamorphism that resulted in the formation of antigorite (Figure 4d). Moreover, the prevailing antigorites and their contribution within interpenetrating texture (Figure 4e) further confirm that WSSs were mainly formed during prograde metamorphism [60,62].

Schwartz et al. [58] demonstrated that below 300 °C, lizardite and locally chrysotile are the dominant serpentine species commonly forming a mesh texture and that between 320 °C and 390 °C, lizardite is progressively replaced by antigorite, which is the sole stable serpentine mineral at temperatures > 390 °C. The extent of spinel alteration suggests similar alteration conditions, with thin outer ferritchromite rims in sharp contact with the Cr-spinel core (Figure 3g) suggesting lower-greenschist facies condition [10]. This alteration zoning of Cr-spinel together with a predominance of higher temperature serpentine phase (i.e., antigorite) indicates lower-greenschist facies conditions and temperatures between 390–500 °C [10,50].

Carbonation occurs when silicate rocks (e.g., ultramafic) interact with CO₂-bearing fluids, promoting alteration reactions that precipitate carbonates and other minerals (i.e., magnesite). Textural observations show that magnesite in WSS preferentially occurs replacing serpentine group minerals (Figure 3c,e). These ubiquitous textural relations in WSS indicate that magnesite was not formed by direct precipitation from a Mg- and CO₂-rich fluid phase, but by fluid-mediate coupled dissolution precipitation reactions that dissolved serpentine minerals similar to antigorite serpentinite of Advocate ophiolite complex [110]. A key question concerning the formation of such magnesite is the potential sources of fluids for the carbonation of serpentinite, which are seawater, primary magmatic fluids, meteoric water, and diagenetic or metamorphic fluids derived from the devolatilization of carbonate or organic carbon-rich rocks, or a combination of these sources. The source(s) of fluids for carbonation of WSS need to be assisted using stable carbon isotopes, which are not available during the preparation of the current paper. Therefore, we recommend the suggested scenario of Boskabadi et al. [111] for the altered ultramafics from the Meatiq area of the Central Eastern Desert (CED) of Egypt, which represents the southwest extension of our studied area. They concluded that stable (C, O) and radiogenic (Sr) isotope compositions suggest that the carbonating fluid was a mantle-derived CO₂-rich fluid mixed with surficial fluid. The extent of carbonate alteration suggests that the fluxing of mantle CO₂ was significant [111].

6.4. Geochemical Characterization of Serpentinites

Geochemical compositions of serpentinites are controlled by several factors such as temperature, nature of hydrating fluids, and tectonic setting. Within the subduction zone, three distinct groups of serpentinites can be identified: abyssal, mantle wedge, and subducted serpentinites [25]. Abyssal serpentinites are formed through the process of hydration of oceanic peridotites by seawater alteration and seafloor hydrothermal activity. On the other hand, mantle wedge serpentinites represent hydrated mantle peridotites by fluid released from the subducted slabs. Subducted serpentinites can be formed through the hydration of abyssal peridotites at ridges, trenches, or within the subduction channel or from the oceanic–continent transition zone [25]. The Cr-spinel of the studied serpentinites possess boninitic affinity (Figure 9d). The boninitic geochemical affinities for the serpentinites are used to attest their linkage with the initiation of oceanic plate subduction at convergent margin settings and SSZ affinity [112,113,114]. Accordingly, WSS samples are inferred to be associated with a sub-arc mantle wedge in an intra-oceanic subduction zone resembling most peridotites and their derivatives of the Egyptian South Eastern Desert (SED) (Figure 9d; [115]), serpentites and serpentinite; therefore, they could be attested as mantle wedge serpentinites. Moreover, the relatively high Cr# and lower TiO₂ contents of the spinel from the WSS (Figure 9d), resemble spinels found in the Izu-Bonin-Marian fore-arc peridotites, which are believed to have originated from primitive island-arc depleted melts with boninitic affinity in an SSZ environment (e.g., [116]).

Figure 9. (a) Geochemical records of interactions between melt/fluid and residues of the WSS through Gd/Lu vs. Th diagram after [25]; (b) Primitive mantle (PM) normalized Nb vs. La contents of the WSS to discriminate between hydrothermal and magmatic origin serpentinites. Primitive mantle values are from [84]; (c) MgO/SiO₂ vs. Al₂O₃/SiO₂ diagram. Primitive and depleted mantle values are from [84]. The “terrestrial array” represents the bulk silicate Earth’s evolution [117,118]. Fields of abyssal and fore-arc peridotite are after [21,34,85]; ANS ophiolitic peridotite field is after [29,92]; (d) Mg# vs. Cr# of spinel from WSS. Fields of chromian spinel in forearc peridotites [65], boninites [64], abyssal peridotites [55], Mariana forearc peridotites [119], SED peridotites [115]; (e) plots of the analyzed spinels on TiO₂ vs. Cr# variation diagram after [85].

Figure 9. (a) Geochemical records of interactions between melt/fluid and residues of the WSS through Gd/Lu vs. Th diagram after [25]; (b) Primitive mantle (PM) normalized Nb vs. La contents of the WSS to discriminate between hydrothermal and magmatic origin serpentinites. Primitive mantle values are from [84]; (c) MgO/SiO₂ vs. Al₂O₃/SiO₂ diagram. Primitive and depleted mantle values are from [84]. The “terrestrial array” represents the bulk silicate Earth’s evolution [117,118]. Fields of abyssal and fore-arc peridotite are after [21,34,85]; ANS ophiolitic peridotite field is after [29,92]; (d) Mg# vs. Cr# of spinel from WSS. Fields of chromian spinel in forearc peridotites [65], boninites [64], abyssal peridotites [55], Mariana forearc peridotites [119], SED peridotites [115]; (e) plots of the analyzed spinels on TiO₂ vs. Cr# variation diagram after [85].

The comparatively higher Mg# (90.29–92.92) in WSS samples is typical of mantle samples [74] and their high Mg, Ni, Cr, and Co contents and depletion of incompatible trace elements (Supplementary Table S5) are consistent with highly depleted and refractory nature of their mantle protolith [21,73,92,120,121]. Moreover, their low values of whole rock MgO/SiO₂ and Al₂O₃/SiO₂ (Figure 9c), as well as high Cr# and low TiO₂ contents in their Cr-spinels (Supplementary Table S2), are consistent with residues after a high degree of partial melt extraction. In addition, the concave REE patterns of the studied serpentinites, exhibiting slight LREE and HREE enrichments relative to MREE [(La/Sm)_N = (2.12–4.29) and (Gd/Lu)_N = (0.25–0.41)], coupled with slightly negative Eu anomalies (Figure 6g) resemble those of serpentinized mantle wedge harzburgites (e.g., [25,104]). The LREE enrichments of the studied serpentinites are attributed to the effects of extensive partial melting and subsequent percolation of LREE-rich fluids or melts through the mantle wedge [95,122,123]. In addition, the relative enrichment of HREE within the studied serpentinites is identical to those commonly found in ultramafic rocks that experienced a high degree of melt extraction [102]. Consequently, the distinct U-shaped REE patterns for the studied samples (Figure 6g) reflect the evolution of the studied serpentinites from mantle wedge harzburgite protolith through extensive melt extraction, rendering a depleted mantle residue, followed by fertilization of the refractory mantle residue by the infiltration of melts released from hydrated subducted slab [124].

6.5. The Nature of Mantle and Melting Conditions

The nature of the mantle and its melting conditions can be estimated from the chemistry of Cr-spinel and several trace elements in the whole rock (e.g., Yb, Ti, V, and Cr, Figure 10). The Cr# values of spinels represent a proxy for the degree of melt extraction of mantle peridotites (e.g., [125,126]). Hellebrand et al. [127] suggested a logarithmic equation [F = 10Ln (Cr#) + 24] of spinel for estimating the degree of fractional melting (F in percent). By applying the equation of [127] on the studied serpentinites, their estimated partial melting degree is generally >21 (Supplementary Table S2), which is consistent with the range of melt extraction (15%–40%) of SSZ peridotites [85,128]. Moreover, due to the immobility of HREE in the whole rock, they can be used as a proxy to quantify extensive partial melting and melt extraction in mantle peridotite [99,129]. Accordingly, the HREE contents of the studied serpentinites exhibit a concentrated range relative to partial melting degrees > 20% (Figure 10a), which is compatible with the obtained results from the Cr-spinel composition (Supplementary Table S2). In addition, the variation in Tb/Yb ratios with respect to Al₂O₃ could be used to assign the mantle domain where the partial melting occurred [130]. The studied samples point out that spinel, as a stable aluminous phase in the mantle residue (Figure 10b), indicates partial melting at a shallow level in the compositional domain of spinel peridotite.

The Yb_N variations compared to (Sm/Yb)_N of bulk residue from anhydrous and hydrous fractional melting within the spinel peridotite field were utilized (Figure 10c) to detect the extent of melting and the relationship between melting and the effects of fluids during melt extraction in the analyzed serpentinites. The calculated (Sm/Yb)_N variations for WSS samples (Figure 10c) indicate a general trend of increased fluid addition with a higher degree of melting for the analyzed serpentinites (~20–25 wt%; Figure 10c) following the initial spinel peridotite melting process, which contrasts with typical abyssal peridotites. The studied serpentinites are characterized by U-shaped REE patterns (Figure 6g), recognized as a hallmark of partial melting in conjunction with interactions between the melt and the host rock. This type of phenomenon is observed in various ophiolitic mantle peridotites worldwide (e.g., [99,101,131,132,133]).

Moreover, the Ti–Yb plot for the studied samples further confirms their origin from a highly depleted mantle residue following a high degree of partial melting (22%–26% melting; Figure 10d). These results are comparable to the spinel peridotite melting curve of the Izu-Bonin fore-arc. The V–Yb plot (Figure 10e) and WSS samples plot on the QFM + 1 melting curve reflect evolution through an oxidizing regime, which indicates that the studied serpentinites originated through an interaction between arc lithosphere and arc melts resembling the Izu-Bonin fore-arc basin system [85]. In addition, the studied serpentinites exhibit a high concentration of LREE with (La/Sm)_N > 2, and they straddle along the interaction curve on the (La/Sm)_N vs. (1/Sm)_N diagram (Figure 10f). These results suggest a significant influence of melt percolation and enrichment of refractory mantle residue through fluid-mantle, melt-mantle, and crust-mantle interaction in the SSZ setting that is observed in ophiolites of the Eastern Desert (Figure 10f). Moreover, significant negative anomalies of Nb and Zr of WSS (Figure 6h) refute the possibility of crustal contamination (e.g., [134,135]). Moreover, the deviation of the MgO/SiO₂ ratio from the terrestrial array (Figure 9c) reveals that their magmatic process was controlled by major element chemistry rather than alteration effects, therefore preserving the mantle signatures without any loss of Mg.

Figure 10. Partial melting modelling using (a) Chondrite-normalized REE patterns of WSS. The shaded area is shown for comparison and the range of model residual mantle compositions is based on Aldanmaz and Koprubasi [136]; (b) Tb/Yb vs. Al₂O₃ plot for mantle peridotite from south Andaman ophiolite suite. The trends for spinel and garnet melting are from [130]; (c) Chondrite-normalized Yb vs. Sm/Yb of the studied serpentinites. Normalizing values from [94]. Anhydrous and hydrous melting curves are modeled after [137,138]; (d) Ti-Yb and (e) V-Yb co-variations in the studied serpentinites. Fore-arc peridotite fields from the South Sandwich and Izu-Bonin-Mariana are shown for comparison. Fields and fractional melting trends for different oxygen fugacities annotated from [21,85]. FMM points to fertile MORB mantle; (f) Chondrite-normalized (La/Sm)_N vs. (1/Sm)_N diagram after [139] for the studied serpentinites. MORB: Mid-ocean ridge basalts; UM: Upper mantle composition; CC: Continental crust composition: UDM: Ultra-depleted melt composition; HZ1, HZ2, and HZ3: Model harzburgite compositions. Fields of ophiolites from the Egyptian eastern Desert is shown for comparison [8,92].

Figure 10. Partial melting modelling using (a) Chondrite-normalized REE patterns of WSS. The shaded area is shown for comparison and the range of model residual mantle compositions is based on Aldanmaz and Koprubasi [136]; (b) Tb/Yb vs. Al₂O₃ plot for mantle peridotite from south Andaman ophiolite suite. The trends for spinel and garnet melting are from [130]; (c) Chondrite-normalized Yb vs. Sm/Yb of the studied serpentinites. Normalizing values from [94]. Anhydrous and hydrous melting curves are modeled after [137,138]; (d) Ti-Yb and (e) V-Yb co-variations in the studied serpentinites. Fore-arc peridotite fields from the South Sandwich and Izu-Bonin-Mariana are shown for comparison. Fields and fractional melting trends for different oxygen fugacities annotated from [21,85]. FMM points to fertile MORB mantle; (f) Chondrite-normalized (La/Sm)_N vs. (1/Sm)_N diagram after [139] for the studied serpentinites. MORB: Mid-ocean ridge basalts; UM: Upper mantle composition; CC: Continental crust composition: UDM: Ultra-depleted melt composition; HZ1, HZ2, and HZ3: Model harzburgite compositions. Fields of ophiolites from the Egyptian eastern Desert is shown for comparison [8,92].

6.6. Tectonic Implications

Generally, ophiolites and related rocks could be classified as subduction-related and subduction-unrelated types [11]. The first type is divided into volcanic arc and SSZ ophiolites that are associated with the closure of ocean basins and exhibit subduction characteristics. On the other hand, subduction-unrelated ophiolites exhibit MORB-like chemical characteristics and are generally emplaced during sea-floor spreading [12,13,14]. Tectonic settings of the Egyptian ophiolites are still debated. Some of these ophiolites are thought to have formed in a MORB-like setting (e.g., [26,140]) while others exhibit back- or fore-arc tectonic settings and exhibit transitional geochemical features between MORB and island arcs (e.g., [2,27,29,30,92,141,142,143,144,145]).

Accordingly, the studied WSS samples are attributed to supra-subduction-related peridotites (Figure 11a) of a forearc tectonic setting similar to the Neyriz ophiolite of Iran [22]. Furthermore, chondrite-normalized REE patterns (Figure 6g) characterized by U-shaped patterns and LREE-HREE enrichment is accompanied by MREE depletion, consistent with peridotites from South Sandwich forearc (Figure 11b). The Mg/Si vs. Al/Si variations (Figure 11c) emphasize their residency of forearc tectonic settings. Generally, the Al₂O₃ contents remain steady during serpentinization and metamorphism; therefore, it has the potential to serve as a tool for distinguishing between various tectonic environments [74]. The studied WSS are characterized by relatively low contents of Al₂O₃ and CaO (0.45–1.05 wt% and 0.03–0.61 wt%, respectively) with low SiO₂/MgO (0.96–1.06; Supplementary Table S5), which are comparable to the composition of intraoceanic forearc peridotites (Figure 10d; [65,74]) and are related to ophiolitic peridotites as same as the other Eastern Desert ophiolitic ultramafics (Figure 10e; e.g., [28,46,75,77,92,101,146,147]) and worldwide (e.g., [19,25,100,102]).

Accordingly, WSSs possess geochemical characteristics similar to those originated from highly depleted SSZ harzburgites generated by high degrees of hydrous partial melting in a forearc mantle wedge environment and emplaced as a consequence of the intra-oceanic arc colliding with Meatiq gneisses (Figure 1). Hence, the investigated serpentinites exhibit resemblances to the presumed tectonic environments of the majority of Egyptian ophiolites. These environments entail the emplacement of oceanic lithosphere fragments above a subduction zone within a forearc setting (Figure 10d; e.g., [28,46,73,101,145,148]).

Consequently, the tectono-magmatic evolution of the Neoproterozoic serpentinites in the Arabian–Nubian Shield, such as WSSs (Figure 12), could be summarized as (i) during the opening of the Mozambican Ocean between East and West Gondwana, the Neoproterozoic mantle reservoir underwent anhydrous melting at the mid-ocean ridge (Figure 12a); (ii) the initiation of intraoceanic subduction by the sinking of old oceanic crust into the mantle that likely produced rapid extinction within the lithospheric upper plate, which in turn induced the formation of a proto-forearc spreading center (Figure 12b) during the subduction initiation stage [33,101], (iii) the sequential upwelling of depleted asthenospheric mantle beneath the spreading forearc zone adjacent to the nascent intraoceanic subduction system induced attenuation of overlying lithosphere. Anhydrous decompression melting of fertile asthenospheric mantle wedge material under these conditions generated tholeiitic forearc basalts through peridotite partial melting, leaving a refractory harzburgite compositional residue (Figure 12c), and (iv) the progressive initiation of subduction melts, along with the infiltration of fluids induce flux melting of the forearc mantle, were generated at the water-saturated solidus. Extensive interactions between those melts and the surrounding rocks within the mantle resulted in widespread refertilization. The extraction of these melts, which contributed to the formation of the upper oceanic crust, left behind a highly depleted harzburgitic residue. This residue exhibited low concentrations of both LILE and LREE (Figure 12d). Following the extraction of the upper crustal melts, the refractory mantle section was abducted onto the continental margin in the form of an ophiolite complex. The proceeding of hydrous melting of refractory harzburgitic restite at a shallow forearc mantle wedge was enhanced by the action of subducted-slab released fluids [78,149,150].

Figure 11. (a) Cr-TiO₂ plot to distinguish between SSZ and MOR peridotites, based on Tethyan Ophiolites [151]; (b) Chondrite-normalized REE patterns for studied serpentinites. Compositional field for the south Sandwich forearc peridotites is from [85] and boninite field is adopted from [152]; (c) Plot of weight ratios of Mg/Si vs. Al/Si for mantle peridotites, in comparison with fields of peridotites of different origins. The diagram was based on the compositional variation in mantle peridotites compiled by [153] and abyssal peridotites by [34]. The expected compositional change in residual mantle peridotites during partial melting is shown with a thick gray arrow originating from the primitive mantle values [84]. Mariana forearc peridotites are from [65]. (d) Al₂O₃ contents of the whole-rock of the studied serpentinites compared with those from other tectonic settings [9,25,31,41,73,142,154]; (e) SiO₂/MgO ratios versus the Al₂O₃ diagram. Fields of ophiolitic peridotites, as well as MORB are from [102]. Data from the Eastern Desert [8,28,41,86,92,101,148] and Zimmer et al. [26] are shown for comparison.

Figure 11. (a) Cr-TiO₂ plot to distinguish between SSZ and MOR peridotites, based on Tethyan Ophiolites [151]; (b) Chondrite-normalized REE patterns for studied serpentinites. Compositional field for the south Sandwich forearc peridotites is from [85] and boninite field is adopted from [152]; (c) Plot of weight ratios of Mg/Si vs. Al/Si for mantle peridotites, in comparison with fields of peridotites of different origins. The diagram was based on the compositional variation in mantle peridotites compiled by [153] and abyssal peridotites by [34]. The expected compositional change in residual mantle peridotites during partial melting is shown with a thick gray arrow originating from the primitive mantle values [84]. Mariana forearc peridotites are from [65]. (d) Al₂O₃ contents of the whole-rock of the studied serpentinites compared with those from other tectonic settings [9,25,31,41,73,142,154]; (e) SiO₂/MgO ratios versus the Al₂O₃ diagram. Fields of ophiolitic peridotites, as well as MORB are from [102]. Data from the Eastern Desert [8,28,41,86,92,101,148] and Zimmer et al. [26] are shown for comparison.

Figure 12. Proposed tectonic model for the evolution of the studied serpentinites and Arabian–Nubian Shield. (a) Opening of the Mozambican Mozambique ocean between East and West Gondwana (b) subduction initiation and associated stage, in which partial melting occurred in the mantle wedge during subduction of the Mozambique oceanic crust beneath the fore-arc. Arrows show the approximate direction of flow and released fluid from the subducting slab; (c) development of mature subduction or a mature-arc stage, which is associated with tremendous slab-derived fluids and a high degree of partial melting forming refractory peridotites. (d) The depleted mantle wedge peridotite is refertilized and serpentinized by the subduction-derived fluids. This carton model was modified from after [9,154].

Figure 12. Proposed tectonic model for the evolution of the studied serpentinites and Arabian–Nubian Shield. (a) Opening of the Mozambican Mozambique ocean between East and West Gondwana (b) subduction initiation and associated stage, in which partial melting occurred in the mantle wedge during subduction of the Mozambique oceanic crust beneath the fore-arc. Arrows show the approximate direction of flow and released fluid from the subducting slab; (c) development of mature subduction or a mature-arc stage, which is associated with tremendous slab-derived fluids and a high degree of partial melting forming refractory peridotites. (d) The depleted mantle wedge peridotite is refertilized and serpentinized by the subduction-derived fluids. This carton model was modified from after [9,154].

7. Conclusions

Wadi Sodmein serpentinites (WSSs) represent dismembered fragments of ophiolitic rocks in the central Eastern Desert of Egypt. Petrographic and chemical compositions of major mineral phases (serpentine, spinel, and chlorite) revealed that antigorite was the dominant serpentine mineral with lesser occurrences of chrysotile and lizardite. This mineral assemblage indicates that WSSs formed under prograde metamorphism, akin to a typical metamorphic peridotite of the harzburgitic protolith. Bulk chemical compositions of WSS samples were not affected by serpentinization or metamorphic process, but the modification might have taken place due to other processes such as mantle metasomatism. The harzburgitic protolith corresponds to mantle restite that was subjected to extensive melt extraction. The WSS samples, characterized by significant LILE-LREE enrichment, HFSE depletion, and U-shaped chondrite-normalized REE patterns, reflect the influence of various petrogenetic processes including refertilization of a depleted refractory mantle wedge by subduction-derived fluids and melts operative in an intraoceanic fore-arc environment. The petrogenetic evolution of WSSs involves hydration, metasomatism, and serpentinization of the depleted sub-arc wedge mantle residue. This evolution occurs as a result of the hydrous melting of refractory harzburgitic restite at a shallow forearc mantle wedge, which is further enhanced by the influence of fluids released from the hydrated subducted slab.