Introduction

The Arabian-Nubian Shield (ANS) ophiolites are considered to be thrust sheets of oceanic lithosphere that were tectonically obducted onto the continental margin during closure of the Mozambique Ocean (Stern et al. 2004; El Bahariya 2018; Abuamarah 2020; Abdel-Karim et al. 2021; Moussa et al. 2022; Abuamarah et al. 2023; Ali et al. 2023). The ophiolites are widely distributed along major shear zones in both the central and southern sectors of the Eastern Desert of Egypt (Fig. 1). The complete ophiolitic sequence of the Egyptian ophiolites consists of a mantle unit of serpentinized peridotite accompanied by pyroxenite, overlain by a Moho transition zone and a crustal section composed of layered and isotropic gabbro, sheeted dykes, and pillowed metabasalt, with local pelagic sediments (e.g., El Sharkawy and El Bayoumi 1979; Abd El-Rahman et al. 2009a, b; Basta et al. 2011). Most of the Egyptian ophiolites are dismembered and lack one or more of these distinctive lithologies due to their obduction and subsequent deformation (Azer and Stern 2007; Akaad et al. 1997; El Bahariya 2006, 2018; Azer et al. 2013; Gahlan et al. 2021; Moussa et al. 2021). The ophiolitic mélanges in the Eastern Desert have been classified, based on their mode of occurrence, into tectonic mélange, olistostrome and olistostromal mélange (El Bahariya 2008; 2012).

Fig. 1
figure 1

Map of the Eastern Desert of Egypt [modified after Shackleton (1994)], showing distribution of ophiolitic rocks and location of Fig. 2; the insert map shows the location of Fig. 1 within the Arabian/Nubian Shield

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Previous geological and petrological studies on the ophiolitic rocks at Al-Barramiya were focused mainly on the serpentinized peridotite and its alteration products (e.g., Zohier and Lehmann 2011; Ali-Bik et al. 2012; Salem et al. 2012; Khedr and Arai 2017; Ali et al. 2020), whereas less attention has been given to the metapyroxenite and metagabbro. Therefore, the spatial and petrogenetic relationships between them and other ophiolitic components are the focus of the present research. Fieldwork and petrographical studies, together with mineral and whole-rock chemical data, are herein presented for the metapyroxenite and metagabbro to constrain their tectonic environment with respect to whether or not that formed in a subduction setting.

Geological setting

Ophiolites and ophiolitic mélanges are important rock units in the Al-Barramiya district and are a characteristic component of the Neoproterozoic rocks in the ANS (Zohier and Lehmann 2011; Ali et al. 2020). The area under investigation, Wadi Al-Barramiya, lies between latitudes 25° 04′ 15″ and 25° 06′ 15″ N and longitudes 33° 45′ and 33° 49 E and is located on the northern side of the Idfu-Mersa Alam Road, midway between Idfu and Mersa Alam (Figs. 1, 2): Idfu is located outside the mapped area to the west on the River Nile. Al-Barramiya forms the northeast part of the east-northeast trending Barramiya-Um Salatit ophiolitic belt and is characterized by a moderate relief with surface outcrops of Neoproterozoic rock units consisting of ophiolites, island-arc assemblages, metagabbro-diorite complexes and late intrusive granitic rocks, as well as Nubian sandstone of Cretaceous age (Fig. 2). Ophiolitic rocks in the area comprise extensively serpentinized peridotite, metagabbro, lenses of coarse-grained metapyroxenite, talc carbonate, magnesite and listvenite. Exotic ophiolitic blocks and fragments of serpentinized peridotite, metapyroxenite and metagabbro are dispersed in a sheared matrix of metasedimentary rocks forming ophiolitic mélange with an ENE-trending foliation.

Fig. 2
figure 2

Detailed geological map of the study area—Barramiya et al. [modified after Said (2006)]

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Serpentinite is the most abundant rock type in the ophiolite and all serpentinite masses have tectonic contacts and have undergone strong brecciation and shearing. They occur as large thrust sheets forming topographic ridges which are elongated in a general ENE–WSW direction, concordant with the foliation in the surrounding matrix. They are commonly massive, but along fault planes and shear zones they are strongly carbonated and altered to talc carbonate, magnesite and listvenite. The contacts between talc carbonates and the parent serpentinites can be gradational, but are commonly sharp, with lenses of serpentinite set in talc carbonate (Fig. 3a). Some lenses of coarse-grained metapyroxenite are also present within serpentinite (Fig. 3b), especially at Sin El-Salatit (Fig. 2).

Fig. 3
figure 3

Field photographs of a serpentinite body within talc carbonates; b lens of coarse-grained pyroxenite (sample no. Px14); c veinlets of magnesite in talc-rich rocks and d sheared metagabbro (sample no. Bz51)

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The talc carbonate rocks range in color from creamy white to light grey to yellowish-white or buff, and are characterized by cavernous weathering. Veinlets, nodules and irregular lenses of magnesite are observed in the sheared serpentinites and talc-rich rocks (Fig. 3c). The veinlets are generally orthogonal to the foliation in the talc-rich bodies.

Listvenite is infrequent in the study area, occurring as reddish-brown ridges, and as irregular lenses and masses along shear zones and fault planes. It forms positive relief features because of its resistance to weathering. Some listvenite outcrops have a porous texture as a result of supergene oxidation (Gahlan et al. 2018). Locally, listvenite is brecciated and fractured, with fractures filled with carbonate veinlets and fine quartz ribbons.

Ophiolitic metagabbros occur as scattered masses distributed throughout the ophiolitic mélange or as large masses and small hillocks that locally overlie serpentinite. The metagabbro is mostly grayish-green to black in color. A few pegmatitic gabbro (appinite) masses are present within more isotropic metagabbro. Some metagabbros are sheared (Fig. 3d) and overthrust by serpentinites. Abundant veins of carbonate, quartz and epidote cut the metagabbro. In the northern part of the study area (Fig. 2), the metagabbros are intruded by syn- to late-orogenic granodiorite.

The island-arc assemblages are widely distributed in the region and have thrust contacts with the ophiolitic rocks, especially with mélange and serpentinite. These rocks include metavolcanics, volcaniclastics and metasediments. All of the ophiolitic units and the island-arc assemblages are intruded by syn- to late-orogenic granodiorite. The Cretaceous Nubian sandstone unconformably overlies the Precambrian rocks in the western part of the mapped area (Fig. 2). It represents the base of the Phanerozoic sedimentary succession that extends westward towards the Nile Valley. It occurs either as continuous or separate small outcrops and is characterized by conspicuous, nearly horizontal beds with some cross-bedding.

Petrography

Serpentinite

Serpentinites are fine-grained and massive, but are locally sheared. They consist mainly of antigorite and lizardite, together with carbonate, talc, chlorite, opaque minerals and subordinate relicts of olivine and Cr spinel. Using Raman spectroscopy, antigorite was identified as the main serpentine mineral, with minor chrysotile occurring as fibrous lamellae or filling veinlets. The presence of bastite replacing orthopyroxene, as well as a more uniform mesh texture (Fig. 4a) indicates harzburgite and dunite protoliths, respectively. Fresh relicts of olivine also occur dissected by serpentine veinlets (Fig. 4b) and locally contain inclusions of Cr-spinel. Kink banding is observed in some olivine crystals. Magnesite is the main carbonate, with minor calcite, and occurs as rare crystals, patches or veinlets cutting through the serpentine matrix. Cr-spinel and magnetite are the main opaque minerals, together with minor sulphides. Cr-spinel is subhedral to anhedral, and has distinct zoning in some larger crystals. Magnetite forms disseminated crystals and as rims around olivine and Cr-spinel grains. Talc occurs in a few samples as anhedral patches and aggregates, and is colorless to very light brown. Chlorite forms aggregates of fine anhedral flakes or as a crust developed around Cr-spinel. Sulfides are present as minute crystals of pyrite and chalcopyrite.

Fig. 4
figure 4

Photomicrographs of metapyroxenite and metagabbro from the Al-Barramiya ophiolite (all photomicrographs are cross-polarized transmitted light, except (e), which is in plane polarized light). a Serpentine mesh texture after olivine in serpentinite; b fresh relict of olivine in highly serpentinized peridotite; c clinopyroxene altered to amphibole and other secondary minerals, but with some fresh relicts; d relicts of olivine in highly-serpentinized metapyroxenite (sample no. Px17); e cracked, anhedral crystal of Cr-spinel altered along the margins to ferritchromite and Cr-magnetite (sample no. Px10); f plagioclase showing intense alteration to secondary minerals in the core (sample no. Bz8b); g bundles of fibrous secondary amphibole, with hornblende relicts (sample no. Bz43); and h fresh clinopyroxene within amphibole in metagabbro (sample no. Bz15)

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Metapyroxenite

Metapyroxenite forms lenses within serpentinite and has undergone variable degrees of serpentinization. It consists mainly of clinopyroxene (> 90%) with minor plagioclase, together with trace amounts of olivine, magnetite and Cr-spinel. The clinopyroxene forms anhedral tabular crystals that are strongly altered along their rims to serpentine, secondary amphibole, chlorite, carbonate or talc, with a few relicts of primary clinopyroxene (Fig. 4c). The outlines of completely altered pyroxene crystals can be identified from the alignment of iron oxides along the cleavage planes. Some fresh relicts of clinopyroxene have exsolution lamellae and contain anhedral grains of serpentine after olivine. Olivine grains are mainly interstitial to large clinopyroxene crystals, but also occur as inclusions in clinopyroxene, or within serpentinized domains (Fig. 4d). Secondary amphibole forms bundles of tremolite-actinolite that replaces clinopyroxene along the margins, cleavage or cracks. Serpentine is mostly found as thin crusts around clinopyroxene or replacing it along cracks. Carbonates occur as patches and veinlets of calcite cutting through the rock. Opaque minerals are magnetite and Cr-spinel with the former commonly oriented parallel to the cleavage planes of clinopyroxene. Cr-spinel occurs as anhedral crystals that are altered along their margins to ferritchromite and Cr-magnetite (Fig. 4e).

Metagabbro

Metagabbro is essentially composed of plagioclase, amphibole and pyroxene relicts. Apatite, zircon, titanite and opaques are accessory minerals, whereas chlorite, epidote, sericite, calcite and kaolinite are secondary minerals. An ophitic to sub-ophitic texture is preserved in a few samples. Plagioclase is the most common mineral and occurs as large subhedral tabular crystals that show intense alteration to epidote and sericite, especially in the cores (Fig. 4f). The amphibole comprises hornblende and tremolite-actinolite, with the latter representing pesudomorphs after pyroxene or alteration of hornblende. Hornblende forms long prismatic crystals and aggregates with a pale to darker green pleochroism. Tremolite-actinolite occurs as fibers with ragged edges or as fine aggregates that form bundles (Fig. 4g).

Clinopyroxene is extensively altered to green hornblende, tremolite-actinolite or chlorite. A few remnants of fresh clinopyroxene occur surrounded by amphibole (Fig. 4h). Chlorite forms irregular flakes due to alteration of pyroxene and amphibole.

Secondary epidote, sericite, kaolinite and calcite occur as anhedral granular aggregates replacing plagioclase. Some carbonate forms patches and veinlets. The opaque mineral content is variable and ranges between 3 and 6 vol%. It mainly consists of Fe-Ti oxides with minor sulfides. The former occur as separate crystals or as fine aggregates associated with the altered mafic minerals, and mainly consist of ilmenite, with rare magnetite. Sulphides include both pyrite and chalcopyrite. Some magnetite is altered to goethite, whereas chalcopyrite is altered to covellite. Apatite occurs as small prismatic crystals in amphibole.

Analytical techniques

Mineral chemical analyses were obtained from carbon-coated polished thin-sections using a JEOL JXA-8530F field-emission electron microprobe (EMPA) housed at the Department of Earth Sciences, University of Western Ontario, Canada. Operating conditions were 20 kV probe current; 40–60 nA accelerating voltage; 1–2 μm diameter beam; and counting time of 5–10 s. Natural and synthetic mineral standards were used for calibration, and a ZAF matrix correction routine program was applied. The standards were orthoclase for K, albite for Na and Al, anorthite for Ca, rutile for Ti, zircon for Si, forsterite for Mg and fayalite for Fe. Calculation of cations in the structure of the analyzed minerals was performed using Excel sheets and Minpet software. The mineral formulae were normalized to a specific number of oxygens, depending on each mineral as atoms per formula unit (a.p.f.u.).

Based on the petrographic studies, 19 samples were selected out of a total of sixty for chemical analysis at Northwest University, Xi'an, China. A portion of the powdered samples was heated at 1000 °C in an oven for 50 min to determine the Loss on Ignition (LOI). Another portion of the powdered sample was analyzed by X-ray fluorescence (XRF, Rigaku RIX2100) by mixing 0.7 g of sample, 0.3 g LiF, 0.4 g NH4NO3, 3.6 g Li2B4O7, and 2–3 drops of 1.5% (w/w) LiBr solution. This mixture was melted in a non-wetting precious metal crucible and poured into a glass disk. In addition, 50 mg of sample powder was mixed with HClO4, HNO3 and HF and digested at 190 °C for 48 h in steel bombs with polytetrafluoroethylene sleeves. After drying, the residues were taken up in 80 mL of 2% HNO3 with 10 µg/g of Rh as an internal standard. These solutions were analyzed by ICP-MS (Agilent 7500a) for trace and rare earth elements (REEs). The analytical protocol was monitored using international rock standards (BCR-2, BHVO-1 and AGV-1). The analytical precision, based on sample replicates, was better than 1% for major oxides and 5–10% for REE and trace elements.

Mineral chemistry

Metapyroxenite

Four different mineral phases (olivine, clinopyroxene, Cr-spinel and serpentine) were analysed from two metapyroxenite samples (BZ9 and UM13) by electron microscopy (EM). Fresh relicts of olivine were analyzed and their chemical compositions and calculated structural formulae are listed in supplementary table ST1. All the analyzed olivine crystals are unzoned forsterite and show little variability in their composition, with SiO2 ranging from 40.29 to 41.21 wt%, MgO from 49.39 to 51.23 wt%, and NiO between 0.41 and 0.48 wt%, with negligible amounts of TiO2 (< 0.02 wt%), Al2O3 (< 0.04 wt%), Cr2O3 (< 0.07 wt%), and CaO (< 0.05 wt%). Olivine has a high forsterite content of Fo = 0.91–0.93 and NiO content of 0.41–0.48 wt%, similar to mantle olivine (Takahashi et al. 1987). On the olivine mantle array diagram, the olivine mostly plots in the mantle olivine array, and all plot in the field of the ANS ophiolites (Fig. 5a).

Fig. 5
figure 5

a NiO vs. Fo showing where fresh olivine relicts in metapyroxenite plot compared with the olivine mantle array (Takahashi et al. 1987), ophiolites of the ANS (Khalil et al. 2014; Gahlan et al. 2015, 2018; Obeid et al. 2016; Azer et al. 2019), and layered intrusions of the ANS (Helmy and Mahallawi 2003; Abd El-Rahman et al. 2012; Abdel-Halim et al. 2016; Azer et al. 2017; Gahlan et al. 2023); b En-Wo-Fs triangular diagram (after Morimoto et al. (1988), with isotherms after Lindsley (1983). showing where metapyroxenite and metagabbro from Al-Barramiya plot; c SiO2 vs. Al2O3 diagram of Le Bas (1962) for clinopyroxene; d Cr–Al–Fe3+ plot of fresh and altered Cr-spinel. e Al2O3 vs. Cr2O3 diagram (after Franz and Wirth 2000), showing where fresh relicts of Cr-spinel plot; f MgO vs. SiO2 classification diagram for serpentine polymorphs (Bahrambeygi et al. 2019); g Or–Ab–An ternary diagram (Deer et al. 1992) with analyzed feldspars plotted; and h classification diagram for chlorite-group minerals (Hey 1954)

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The chemical composition of fresh pyroxene relicts (Supplementary table ST2) are homogeneous and also show limited compositional variation. According to the classification of Morimoto et al. (1988), the pyroxenes in the metapyroxenite plot mainly in the diopside field (Fig. 5b). They have 0.37–1.12 wt% Cr2O3 and Al2O3 contents of 1.02–2.68 wt%, confirming their primary origin, because metamorphic clinopyroxenes have lower Cr2O3 (< 0.4 wt%) and Al2O3 (< 0.5 wt%) contents (Nozaka 2010). The CaO content is in the range of 21.82–24.21 wt% and TiO2 contents are lower than 1 wt%, which is characteristic to non-alkaline rocks (Le Bas 1962). The Mg# is high (0.93–0.95 wt%) and similar to that of ophiolitic ultramafic rocks in the ANS (Obeid et al. 2016; Azer et al. 2019; Gahlan et al. 2020), as well as worldwide (Parlak et al. 2000; 2013; Allahyari et al. 2014). On the Al2O3 vs. SiO2 discrimination diagram of Franz and Wirth (2000), the metapyroxenite clinopyroxenes plot as subalkaline (Fig. 5c).

The chemical composition and structural formulae of Cr-spinel and its alteration products are given in supplementary table ST3. It shows compositional zoning, with fresh Cr-spinel cores and an outer zone of ferritchromite that is commonly altered to an outer rim of Cr-magnetite. Going from core to rim, FeO(t) content increases, while Al2O3, Cr2O3 and MgO decrease. On the Cr–Al–Fe3+ diagram (Fig. 5d) the fresh cores plot along or near the Al-Cr line whereas the altered rims plot along the Cr-Fe3+ line, indicating the loss of both Cr2O3 and Al2O3 and a consequent increase in the Fe2O3 content as a result of metamorphism under hydrothermal conditions (Barnes 2000; Proenza et al. 2004). The fresh Cr-spinel cores have Cr# (molar Cr/(Cr + Al)) ranging between 0.63 and 0.75, with an average of 0.67, and Mg# (molar Mg/(Mg + Fe2+)) ranging from 0.41 to 0.64, with an average of 0.56. Relative to fresh spinel cores, the altered rims have higher Cr# (0.90–0.96; av.0.93) and lower Mg# (0.11–0.52; av. 0.32). Generally, the fresh Cr-spinel cores have Al2O3 and Cr2O3 contents that are similar to those of Cr-spinel of mantle peridotite and most plot in or near the mantle array, but extending into the arc cumulate field (Fig. 5e).

The chemical composition and structural formulae of the serpentine minerals in the metapyroxenite are given in Supplementary table ST4. On the SiO2 versus MgO diagram of Bahrambegi et al. (2019), they range from antigorite to chrysotile in composition (Fig. 5f).

Metagabbro

The metagabbro minerals include plagioclase, pyroxene, amphibole, chlorite and ilmenite. The chemical composition and structural formula of plagioclase are listed in Supplementary table ST5. They range in composition from labradorite to albite (Fig. 5g), with An contents varying from 57.20 to 4.07. The low anorthite content of some plagioclase is attributed to its albitization during metamorphism.

Fresh pyroxene relicts are rare, and their chemical composition and calculated structural formula are listed in Supplementary table ST6. They are calcic in composition and according to the classification of Morimoto et al. (1988), they mainly straddle the boundary between the diopside and augite fields and extend to hedenbergite in composition (Fig. 5b). On the SiO2 vs. Al2O3 diagram of Le Bas (1962) for clinopyroxene, they plot in the subalkaline field (Fig. 5c), similar to the metapyroxenite crystals.

Amphiboles in the metagabbro include primary magmatic minerals (hornblende) and subsequent alteration products (tremolite-actinolite). Their chemical analyses and structural formulae are presented in Supplementary table ST7. According to the amphibole classification scheme of Hawthorne et al. (2012), they are calcic amphiboles; namely magnesio- to ferri-hornblende and actinolite.

Chlorite forms due to alteration of pyroxene and amphibole. Representative chemical analyses and the structural formula are given in Supplementary table ST8. They show limited variation in composition and, according to the scheme of Hey (1954), they are classified as brunsvigite and ripidolite (Fig. 5h).

Ilmenite is the main Fe-Ti oxide mineral in the metagabbro, and its representative chemical composition and structural formula are given in Supplementary table ST9. Chemically, it is homogenous and has low MgO contents indicating formation by low-grade metamorphism, after most MgO had been sequestered in other phases.

Whole-rock geochemistry

Nineteen representative samples of metagabbro (10 samples) and metapyroxenite (9 samples) from the ophiolitic rocks cropping out in the Al-Barramiya area were analyzed for their major oxides, trace and rare earth elements. The whole-rock chemical analyses are presented in supplementary table ST10. Some of the trace elements are below the detection limit of the analytical protocol used. Overall, the metapyroxenite samples have high LOI (4.21–5.24 wt%) compared with the metagabbro samples (2.37–4.01 wt%).

Metapyroxenite

The metapyroxenite samples have 47.72–51.30 wt% SiO2, 0.08–0.16 TiO2 wt%, 15.45–18.72 MgO wt%, 1.34–2.87 Al2O3 wt% and 15.83–19.88 CaO wt%. They have high concentrations of compatible elements (Supplementary table ST10), such as Cr (1120–2120 ppm), Ni (401–711 ppm), Co (67–91 ppm) and Sc (161–276 ppm). Due to the serpentinized nature of the metapyroxenite, the classification scheme based on modal percentages of olivine, clinopyroxene and orthopyroxene is unsuitable. Therefore, using the normative whole-rock composition (Supplementary table ST10), the metapyroxenite can be classified as websterite and olivine websterite, but very close to the clinopyroxenite field (Fig. 6a). On the SiO2/MgO versus Al2O3 diagram (Bodinier and Godard 2003), the metapyroxenite samples plot as low-Al pyroxenite (Fig. 6d). On the primitive mantle (PM)-normalized trace element diagram, the metapyroxenite samples show positive peaks for Cs, Ba, U, Pb and Eu (Fig. 7a), with marked depletion in Rb, Nb, and Hf. In the chondrite-normalized REE diagram (Fig. 7b) the metapyroxenites show depletion in LREEs (3.52–5.69 ppm) and flat MREE to HREE patterns. They have (La/Sm)n = 0.35–0.39 and (La/Yb)n = 0.28–0.37. The depletion in LREEs is comparable with other ophiolitic clinopyroxenites in the ANS (Gahlan et al. 2012).

Fig. 6
figure 6

a Metapyroxenites plotted in the Ol-Opx-Cpx normative composition diagram (after Coleman 1977); b Metagabbros plotted in the TAS diagram of Cox et al. (1979), adapted by Wilson (1989) for plutonic rocks; c Metagabbros plotted in the AFM diagram of Irvine and Baragar (1971); d metapyroxenites and metagabbros plotted in the SiO2/MgO versus Al2O3 diagram (Bodinier and Godard 2003); e metapyroxenites and metagabbros plotted in the Y vs. Zr diagram after Barrett and McLean (1994); and f Metagabbros plotted in the Th–Zr–Nb ternary diagram, modified after Wood (1980)

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Fig. 7
figure 7

a Primitive mantle-normalized trace element patterns for metapyroxenite and metagabbro from Al-Barramiya, and b chondrite-normalized rare earth element patterns for metapyroxenite and metagabbro; normalization values for a and b from McDonough and Sun (1995)

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Metagabbro

The metagabbro samples have slightly lower SiO2 (46.50–50.87 wt%) and much lower MgO (7.34–8.73 wt%) contents than the metapyroxenite, but have higher Al2O3 (16.23–17.74 wt.%) and TiO2 (0.98–1.62 wt.%) contents. The whole-rock Mg# [molar Mg/(Mg + Fe2+)] ranges from 0.59 to 0. 76, lower than that in the metapyroxenite samples. They also have lower compatible elements (Supplementary table ST10), with (Cr = 111–182 ppm, Ni = 40–128 ppm, Co = 44–58 ppm, and Sc = 36–48 ppm). On the TAS discrimination diagram of Cox et al. (1979) (Fig. 6b), the samples plot in the field of gabbro with a slight spread into the sub-alkalic field. On the AFM diagram (Irvine and Baragar 1971), they straddle the boundary between the tholeiitic and calc-alkaline fields (Fig. 6c). On the SiO2/MgO versus Al2O3 diagram (Bodinier and Godard 2003), the metagabbro samples plot within the ophiolitic gabbro field (Fig. 6d). On the Zr versus Y diagram, they plot in the tholeiitic field but extend down into the transitional field (Fig. 6e), whereas in the Th–Zr–Nb ternary diagram (Fig. 6f), they plot in the field of island-arc tholeiite.

The primitive mantle (PM)-normalized trace element patterns for the metagabbro are essentially similar to those of the metapyroxenite, but with higher total contents and stronger Pb enrichment (Fig. 7a). They show similar depletion in Rb and Nb, but contrast with the metapyroxenite in showing enrichment in Hf and Ti, but depletion in Eu. The positive Ti anomalies can be attributed to higher amounts of Fe-Ti oxide minerals in these samples. Both metapyroxenite and metagabbro samples have negative Nb anomalies similar to those of other ophiolitic metagabbros in the Eastern Desert of Egypt (Gahlan et al. 2015; Obeid et al. 2016). In the chondrite-normalized REE plot (Fig. 7b), they show slightly enriched LREEs, but with flat MREE to HREE patterns (Gd/Lu = 0.98–1.18) and (La/Lu)n = 1.50–1.88. They have weakly negative to slightly positive Eu anomalies [Eu/Eu* = 0.84–1.07], indicating a general lack of plagioclase fractionation. Overall, the REE patterns of the Al-Barramiya metagabbro are similar to ophiolitic metagabbros elsewhere in the ANS (Gahlan et al. 2015; Obeid et al. 2016).

Discussion

Alteration and metamorphism

Most ophiolites in the Eastern Desert of Egypt are metamorphosed to greenschist or lower amphibolite facies (e.g., Khalil and Azer 2007; El Bahariya 2006, 2008, 2018; Farahat 2008; Boskabadi et al. 2017; Abuamarah et al. 2020, 2023). However, it remains controversial when metamorphism of the Egyptian ophiolites took place, i.e. whether it was during or after their emplacement in the continental crust. For the ophiolitic rocks in the Al-Barramiya area, it has been suggested that they underwent various degrees of post-magmatic alteration either by seafloor hydrothermal activity or during the obduction processes (Ali et al. 2020). Although some of the original igneous textures of the ophiolitic metagabbro are still preserved, the mineralogy has been largely altered, with the primary minerals (plagioclase, amphibole and pyroxene) altered to secondary/metamorphic ones (albite, epidote, tremolite/actinolite, chlorite and titanite). Data presented here indicates that the ophiolitic metagabbro at Al-Barramiya is rich in LILEs and Th, most likely as a result of element mobility during slab dehydration (Pearce and Peate 1995).

Mineral assemblages in most samples of serpentinite, metapyroxenite and metagabbro suggest that they underwent regional metamorphism under greenschist to amphibolite facies conditions (e.g., Suita and Streider 1996; Mellini et al. 2005; El Bahariya 2006; El Bahariya and Arai 2003; Azer et al. 2019; Whattam et al. 2022). Ferritchromite in the metapyroxenite is enriched in FeO(T) (49.43–75.26 wt%) and depleted in Al2O3 (0.42–2.43 wt%) and MgO (1.82–8.04 wt%), reflecting the increase in Fe2O3 and loss of Al2O3 and Cr2O3 due to metamorphism (Azer and Stern 2007). The alteration of Cr-spinel to ferritchromite possibly took place during the late magmatic stage, and continued later due to serpentinization (Gahlan et al. 2018). The presence of ferritchromite rims around fresh Cr-spinel cores with a sharp compositional change likely indicates upper greenschist to lower amphibolite facies metamorphism (Evans and Frost 1975; Suita and Streider 1996; Barnes and Roeder 2001; Mellini et al. 2005; Arai et al. 2006). This is supported by the chemical data of Cr-spinel in the metapyroxenite on the Cr-Fe3+-Al triangular diagram (Fig. 8a), where fresh Cr-spinel relicts and ferritchromite rims plot mostly in the lower amphibolite facies field. Also, serpentinite is dominated by antigorite, with subordinate chrysotile, indicating serpentinization during heating and burial (Moody 1976; Deer et al. 1992), or retrograde formation of chrysotile followed by a second cycle of prograde recrystallization to antigorite. The dominance of antigorite over chrysotile in the serpentinite suggests formation at 400 − 600 °C during an early stage of serpentinization at depth (Evans 2010).

Fig. 8
figure 8

a Cr–Fe3+–Al3+ diagram for fresh and altered Cr-spinel in metapyroxenite; the fields of the different metamorphic facies from Purvis et al. (1972), Evans and Frost (1975) and Suita and Streider (1996); b Hf/3-Th-Nb/16 discrimination diagram after Wood (1980) for the Al-Barramiya metagabbro; A = N-MORB, B = enriched MORB and within-plate tholeiites, C = within-plate basalts and D = destructive plate basalts; c Th/Yb vs. Nb/Yb diagram after Pearce (2008) for the Al-Barramiya metagabbros and metapyroxenites; and d Dy/Yb vs. La/Yb diagram after Xu et al. (2001) for the metagabbros and metapyroxenites of the Al-Barramiya ophiolite

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Petrogenesis and tectonic setting

Dilek and Furnes (2014) classified ophiolites simply into subduction-related and subduction-unrelated types. The former include fore-arc and back-arc supra-subduction zone and volcanic arc types, whereas the latter comprise continental margin, mid-ocean ridge and plume-types. Several studies have been dedicated to determining the origin and tectonic setting of the Egyptian ophiolites because they represent key components in constructing the geodynamic model for the Pan-African belt of the ANS. Unfortunately, the metamorphosed nature of the Egyptian ophiolites makes it more difficult to infer their tectonic setting. Indeed, a wide range of tectonic settings has been proposed for these rocks, ranging from mid-ocean ridge (e.g., Shackleton et al. 1980; Zimmer et al. 1995), to supra-subduction zone (fore-arc or back-arc setting) (e.g., Ahmed et al. 2001, 2012; Azer and Stern 2007; Ahmed 2013; Abdel-Karim et al. 2018). The Neoproterozoic ophiolites of Egypt have also been classified into high Ti-MORB and low Ti-SSZ ophiolites (e.g. El Bahariya 2012, 2018). However, most recent studies are in favor of a supra-subduction zone setting, specifically in the fore-arc (Ali et al. 2020; Abdel-Karim et al. 2021; Moussa et al. 2021).

Because of metamorphism, the tectonic setting of igneous rock protoliths has to be inferred using elements immobile during metamorphism, including Ti, Zr, Y and Nb (e.g. Pearce and Cann 1973; Meschede 1986; Pearce 2008, 2014; Baziotis et al. 2014). In the Hf–Th–Nb diagram, the metagabbro samples plot in the destructive plate margin field (Fig. 8b). On the Th/Yb vs. Nb/Yb diagram (Pearce 2014), both the ophiolitic metapyroxenite and metagabbro plot above the MORB-OIB array toward the volcanic-arc array (Fig. 8c). Moreover, the metagabbro samples are relatively enriched in LREEs and LILEs, similar to magmatic rocks formed at convergent plate margins (e.g., Saunders et al. 1991; Hawkesworth et al. 1993). According to Geng et al. (2011), the metagabbro and metapyroxenite of the Al-Barramiya ophiolite have low Zr (35.88–42.60 ppm in metagabbro and 4.2–7.03 ppm in metapyroxenite), coupled with low Nb (1.96–2.75 ppm in the metagabbro and 0.04–0.08 ppm in the metapyroxenite), indicating a depleted mantle source: our new data support this assertion. Furthermore, the Dy/Yb ratios (1.74–1.92 in metapyroxenite and 1.43–1.62 in metagabbro) and La/Yb ratios (0.41–0.55 in metapyroxenite and 2.36–2.70 in metagabbro) indicate up to 30% partial melting of a spinel lherzolite source (Fig. 8d).

The serpentinized clinopyroxenite has fresh relicts of primary silicates (e.g., olivine and pyroxene) and Cr-spinel, which are important petrogenetic indicators of the protolith (Barnes and Roeder 2001; Arif and Jan 2006). The very low TiO2 content in the Cr-spinel from the metapyroxenite samples confirm their highly depleted character and indicate their association with SSZ tectonic settings. In residual peridotites, the Cr# of spinel reflects the degree of partial melting (Arai 1994). The high Cr# of spinel from the metapyroxenite samples therefore indicates a high degree of melting, as would be expected in the SSZ (Parkinson and Pearce 1998). Hence, on the Cr# versus TiO2 plot the fresh Cr-spinel plots in the depleted field (Fig. 9a). The chemical composition of fresh Cr-spinel can be successfully used to distinguish between a back-arc and a fore-arc setting (Barnes and Roeder 2001; Ohara et al. 2002; Stern et al. 2004; Azer and Stern 2007 and many others), with the Cr# versus Mg# plot of fresh Cr-spinel in the serpentinized metapyroxenite indicating that most of the analysed Cr-spinels are similar to those of modern forearc peridotites (Fig. 9b).

Fig. 9
figure 9

Tectonic discrimination diagrams for metagabbro and metapyroxenite based on mineral chemistry. a Cr# vs. TiO2 diagram for fresh Cr-spinels in metapyroxenite. The fields are according to Dick and Bullen (1984), Jan and Windley (1990) and Arai (1992); b Cr# vs. Mg# diagram for fresh Cr-spinels (after Stern et al. 2004); the field boundaries are from Dick and Bullen (1984), Bloomer et al. (1995) and Ohara et al. (2002). The melting trend of experimental equilibrium (melting %) is from Hirose and Kawamoto (1995); c TiO2–Na2O–SiO2/100 diagram for clinopyroxene in the metapyroxenite and metagabbro (after Beccaluva et al. 1989). WOPB within-ocean plate basalts, MORB mid-ocean ridge basalts, IAT island arc tholeiites, BON + BA–A boninites + basaltic andesites and andesites from intraoceanic fore-arcs; d Cr2O3 vs. Al2O3 diagram for clinopyroxene from the metapyroxenites; e Al2O3 vs. Mg# plot showing clinopyroxene from the metapyroxenite; and f Cr# vs. Fo for spinel coexisting with olivine in the metapyroxenites (olivine-spinel mantle array; OSMA); fields of fore-arc and mid-ocean ridge peridotites, and partial melting trend are from Arai (1994). Compositional fields for pyroxene in b and c are adopted from Pagé et al. (2008) and Xiong et al. (2018)

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In addition, clinopyroxene can provide further insight into the petrogenesis of the metapyroxenite from Al-Barramiya. Here, clinopyroxenes in the metapyroxenite and metagabbro plot in the fields of boninite and island-arc tholeiite (Fig. 9c), confirming the fore-arc setting, as boninite is endemic to fore-arcs (e.g., Bedard 1999; Beccaluva et al. 2004). Likewise, the clinopyroxene in the metapyroxenite is characterized by low Al2O3 contents, plotting in the field of fore-arc peridotites on the Cr2O3 versus Al2O3 and Mg# versus Al2O3 diagrams (Fig. 9d, e). In addition, clinopyroxene in the metapyroxenite has a Mg# (0.93–0.95) similar to those reported for ophiolitic pyroxenite [Mg# = 0.90–0.91 (Parlak et al. 1996); 0.86–0.91 (Whattam et al. 2011); 0.89–0.91 (Singh et al. 2017)] Furthermore, the negative correlation between Al2O3 and Mg# in the clinopyroxene (Supplementary table ST2) is similar to that reported for the ultramafic cumulates from the Troodos and Mersin ophiolites (Hébert and Laurent 1990; Parlak et al. 1996).

Fresh relicts of magmatic olivine are Fo-rich (0.91–0.93), indicating they have a mantle source and were residual after extensive melting, similar to forearc peridotites (e.g. Stern et al. 2004; Coish and Gardner 2004). The high Mg content of olivine (Fo) is similar to forearc peridotites investigated elsewhere in the ANS (e.g. Stern et al. 2004; Khalil et al. 2014; Obeid et al. 2016; Abuamarah 2020 and others), where the relationship between the Fo content and the Cr# of coexisting Cr-spinel displays a partial melting trend (Fig. 9f), supporting a residual origin (Arai 1994) that is a similar to forearc peridotites elsewhere in the world (e.g. Pearce et al. 2000).

Mantle pyroxenites have been interpreted as: (1) the product of melt and/or fluid interaction with peridotite (Garrido and Bodinier 1999; Bodinier et al. 2008; Marchesi et al. 2013; Laukert et al. 2014; Rogkala et al. 2017; Le Roux and Liang 2019); (2) cumulates of more primitive melts (Wilshire and Shervais 1975; Batanova et al. 2011); (3) metamorphosed products of recycled oceanic crust (Pearson et al. 1993; Barth et al. 2001; Morishita et al. 2003, 2004) or metamorphic segregations of peridotite (Dick and Sinton 1979); and (4) assimilation and fractional crystallization where olivine was assimilated by percolating melt, followed by in situ fractional crystallization of pyroxenes (Laukert et al. 2014). Cumulate and metamorphic origins for the Al-Barramiya pyroxenites are not in general agreement with field and petrographic evidence or with the mineral and whole-rock geochemistry. Also, the available data do not support assimilation and fractional crystallization for their formation. Recently, the melt/rock reaction model has become the most adopted model for pyroxenite formation (Akizawa et al. 2016; Saka et al. 2019; Xiong et al. 2020). According to this model, olivine in harzburgite wall-rock dissolves through interaction with a Si-enriched melt, causing pyroxene to precipitate (Garrido and Bodinier 1999; Bodinier et al. 2008; Marchesi et al. 2013; Laukert et al. 2014; Rogkala et al. 2017; Le Roux and Liang 2019). We consider that this model is most likely applicable to the ophiolitic rocks in the Al-Barramiya area, and illustrate this in Fig. 10. Here, we show the sequence of events for formation of the Al-Barramiya ophiolitic section due to fore-arc spreading and obduction of the ophiolite sequence.

Fig. 10
figure 10

Sequence of events for the formation of the Al-Barramiya ophiolite: a fore-arc spreading and formation of ophiolitic mantle section, b close-up of emplacement of pyroxenite and gabbro in the fore-arc, and c obduction of the ophiolite sequence resulting in brittle shear deformation

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Conclusions

  • The dismembered ophiolites in the Wadi Al-Barramiya area comprise serpentinized peridotite, metapyroxenite and metagabbro. The serpentinized peridotites are strongly altered to talc carbonate, magnesite and listvenite, but fresh relicts of primary minerals, including olivine and Cr-spinel, remain. The ophiolitic assemblage was metamorphosed under greenschist to lower amphibolite facies conditions.

  • Ophiolitic metapyroxenite occurs as irregular coarse-grained lenses within serpentinite, whereas the metagabbros occur as scattered masses distributed throughout the ophiolitic mélange. Metapyroxenite is classified as websterite and olivine websterite and is mainly composed of altered clinopyroxene and serpentine, with fresh relicts of olivine, clinopyroxene and Cr-spinel. Metagabbro is comprised mainly of altered plagioclase, pyroxene, and amphibole.

  • Metapyroxenite samples have low total REEs (3.52–5.69 ppm) and display LREE-depleted patterns [(La/Sm)n = 0.35–0.39; (La/Yb)n = 0.28–0.37) accompanied by weakly fractionated HREE patterns (Gd/Lu = 1.21–1.30).

  • The fresh Cr-spinel relicts in the metapyroxenite have high Cr# (0.63–0.75), which together with their low TiO2 contents (0.04–0.24 wt%) are similar to highly refractory peridotites that evolve in fore-arcs. This is supported by the high forsterite content (Fo = 0.91–0.93) of fresh olivine and high Mg# (0.93–0.95) of fresh clinopyroxene.

  • The metapyroxenite was formed as a magmatic rock derived from ultramafic magma at the base of the Neoproterozoic crust of the ANS due to melt-rock interaction, where olivine in harzburgite wall-rock dissolved through interaction with a Si-enriched melt, causing pyroxene to precipitate.

  • Metagabbro samples are characterized by enrichment in LILE over HFSE and they are tholeiitic with a calc-alkaline affinity. They have flat REE patterns, with weak enrichment in LREE relative to HREE, and are similar to metagabbros associated with fore-arc ophiolites elsewhere in the ANS and around the world.