Petrogenesis and Tectonic Evolution of Kab Amiri Ophiolites and Island-Arc Assemblages, Central Eastern Desert, Egypt: Petrological and Geochemical Constraints

: The Kab Amiri area in the Central Eastern Desert (CED) of Egypt comprises ophiolitic rocks, island arc metasediments, and granitic rocks. This study deals with the petrography and geochemistry of the ophiolitic and island arc rocks to understand their petrogenesis and geodynamic evolution of the CED ophiolitic belt. The Kab Amiri ophiolite is dismembered, comprising serpentinites and metabasalt (spilite). Serpentinites have low contents of Al 2 O 3 (1.14 wt%) and CaO (0.65 wt%), suggesting they are depleted peridotite protoliths (e.g., depleted harzburgites to dunites). The high L.O.I. value (13.7 wt%) of serpentinite rocks indicates intense hydration and serpentinization during alteration processes. Petrographic and geochemical studies suggest that serpentinites were likely formed after depleted peridotites in a supra-subduction zone (SSZ) setting (e.g., a fore-arc setting). Spilitic basalt shows a tholeiitic afﬁnity of the depleted mantle source. The arc-related metasediments are represented mainly by schists and slate. Many samples of metasediments are relatively low in alumina (Al 2 O 3 < 15%), suggesting a low clay content and formation in an island arc setting. In contrast, protoliths of island-arc metabasalts and meta-andesites crystallized from calc-alkaline magmas in the immature oceanic arcs.


Introduction
The Eastern Desert of Egypt is divided into the northern (NED), central (CED), and southern (SED) sectors by two arbitrary lines that were drawn from Qena to Safaga and from Idfu to Marsa Alam (Figure 1a).The CED supracrustal sequences are an oceanic assemblage consisting of various Neoproterozoic ophiolitic rocks, primarily with greenschist facies, arc volcanic, as well as volcaniclastic rocks, banded iron formations (BIF), and diamictite.Ophiolitic ultramafic and island arc of the Neoproterozoic age are widespread in the CED and SED sectors of the Eastern Desert of Egypt [1][2][3].Throughout the CED, there are many Neoproterozoic ophiolites.Figure 1a shows the distribution of various lithologies [3][4][5].Ophiolite rocks of the Neoproterozoic age 800 and 730 Ma [6,7] are considered the most significant and widespread units with granitoid rocks in the Egyptian basement rocks of the Eastern Desert (ED) of Egypt.They are vestiges of oceanic lithosphere with intraoceanic island arcs that are connected to the opening and closing of the Neoproterozoic Mozambique oceanic basin, and they constitute the northern extent of the East African Orogen 550-850 Ma [8,9].Neoproterozoic ophiolites of the CED of Egypt are divided into The exposed serpentinites shape elongated bodies generally extending in the E-W direction southwest and southeast of Gabal Kab Amiri granites.Figure 2b shows a structural contact between serpentinites and metasediments.They are embedded in the metasediments mainly along axial planes of large anticlinal folds.The talc-carbonate rocks are found along faults and shear zones within the mountain range.Asbestos and calcite occur as veins about 5 cm thick along the fractures and shear zones, suggesting mixed H2O-CO2 introduction during deformation.

Geologic Setting
Kab Amiri area is situated in the Central Eastern Desert of Egypt, south of the Qena-Safaga Road and mainly comprises Precambrian exposures.The main wadies draining in the Kab Amiri area include Wadis Kab Amiri, Abu Grahishi, El Bohlog, and Kab Um El Abas (Figure 1b).It is encircled by Latitudes 26 • 15 and 26 • 25 N and Longitudes 33 • 30 and 33 • 40 E. The lowest relief is south of Gabal Kab Amiri, while the highest relief (911 m above sea level) is at Gabal Kab Um El Abas, east of Gabal Kab Amiri.The volcano-sedimentary associations have been deformed and metamorphosed under greenschist to lower amphibolite facies conditions [23].The ophiolitic rocks in the study area are represented by serpentinites and metabasalts (spilite).While island arc assemblages are comprised of metasediments and metavolcanic and followed by granitoid rocks and different post-orogenic dykes (pegmatite, basaltic) (Figure 2b).Several significant faults that cut across the Kab Amiri stretch mostly in the NW direction (Figure 1b) consist of the Najid fault trend, with a smaller number of faults trending in the NE and E-W directions [17,23].The Kab Amiri ophiolitic metabasalts are related to the older metavolcanic, which occupy substantial exposures in the northwestern sector of the study area, as depicted in Figure 1b.These metabasalts are elliptical (elongated) and irregularly curved pillow lobes, Dismembered ophiolitic rocks are concentrated in the northern and central parts of the study area.It is noticeable in this region that the serpentinites turned into talc carbonate rocks (Figure 2a).Serpentinites are grey to greyish on the weathered surface, while the fresh surfaces have dark green.They have moderate to high relief.They vary in size and shape and always have tectonic contact with country rocks without thermal effects.The exposed serpentinites shape elongated bodies generally extending in the E-W direction southwest and southeast of Gabal Kab Amiri granites.Figure 2b shows a structural contact between serpentinites and metasediments.They are embedded in the metasediments mainly along axial planes of large anticlinal folds.The talc-carbonate rocks are found along faults and shear zones within the mountain range.Asbestos and calcite occur as veins about 5 cm thick along the fractures and shear zones, suggesting mixed H 2 O-CO 2 introduction during deformation.
The Kab Amiri ophiolitic metabasalts are related to the older metavolcanic, which occupy substantial exposures in the northwestern sector of the study area, as depicted in Figure 1b.These metabasalts are elliptical (elongated) and irregularly curved pillow lobes, ranging in size from 40 to 70 cm in length, with a chilled margin visible along the peripheral boundary, as shown in (Figure 2c,d).The metabasalts are surrounded by a rim rich in epidote, chlorite, and other minerals, indicative of alteration processes on the seafloor and through subsequent hydrothermal activity, as depicted in Figure 2c.The metabasalts exhibit moderate topographic relief and are characterized by a dark grey to greenish-grey colouration, as depicted in (Figure 2d).
The metasediments and metavolcanic comprising the island arc assemblage constitute the majority of the rocks in the study area.The metasediments are primarily located in the southern portion of the mapped site, forming a curvilinear belt that encircles the Kab Amiri granite from the south, as depicted in Figure 1b.These rocks are characterized by medium-grained immature sediments, sometimes containing banded iron formations, and are primarily foliated, folded, and exhibit a greyish-green colouration with moderate to high relief.The lowest relief is observed in the southern region of Gabal Kab Amiri, while the highest relief, reaching 911 m above sea level, is present at Gabal Kab Um El Abas, located to the east of Gabal Kab Amiri.The metasediments are related to the Abu Fannani schist around the Meatiq dome [24].The contacts between the metasediments and metavolcanic are tectonic in nature, particularly along Wadi El Saqia.The interaction between the dismembering ophiolite and Abu Fannani schist is tectonic, with the ophiolite rocks thrust over the Abu Fannani schist, which has been described as a tectonic mélange by [25].These rocks exhibit an advanced schistosity and are bisected by quartz veins and veinlets, as depicted in Figure 2e.
The metavolcanic rocks are represented by substantial exposures located in the southern and northeastern parts of the mapped area and are characterized by tectonic contact with the metasediments.These rocks are related to the young metavolcanic (YMV), visible along Wadi El Saqia and extending east, as depicted in Figure 1b.These rocks form moderate to high relief ridges with dark grey to greenish-grey and pink colour.They are highly fractured, altered, sheared, and traversed by some mafic and felsic dykes (Figure 2f).The metavolcanic and metasediments are structural, while their contacts with Kab Amiri granites are intrusive.

Analytical Methods
In the study area, 12 samples were procured from ophiolitic rocks, 4 samples of serpentinites, 4 samples of talc-carbonates and 4 samples of metabasalts, and 11 samples from the island arc assemblage, including 7 samples of metasediments and 4 samples of metavolcanic.These samples were subjected to microscopic analysis using 15 thin sections prepared at the Laboratories of the Nuclear Materials Authority (NMA).A Nikon polarizing microscope (Nikon, Tokyo, Japan) equipped with an automatic photomicrographic and mechanical stage attachment was used to identify the minerals and textures.
Serpentinites are composed primarily of serpentine minerals (over 90% by volume), with minor amounts of other minerals such as olivine, chromite spinel, carbonates, and opaque minerals.The serpentine minerals antigorite, chrysotile, and lizardite are characterized by colourless, weakly birefringent, and parallel extinct crystals.Antigorite often occurs as elongated blade-like crystals, sometimes arranged in a parallel fashion, creating a pseudomorph after orthopyroxene minerals, known as the bastite texture.When the geometric configuration of the original mafic minerals is preserved, a pseudomorphic texture is created.In contrast, lizardite is a fine-grained matrix filling the pseudomorphs of olivine and is characterized by a mesh texture after olivine.Olivine is partially serpentinized and eventually altered to talc and carbonates.Chrysotile is a serpentine mineral present as randomly oriented fibrous crystals and associated with antigorite.Chromite is the primary opaque Mineral, forming subhedral fractured crystals filled with carbonates (Figure 3c).
Talc-carbonate rocks are massive and schistose a fine-grained texture.Their hues range from pale green to greenish-grey-coloured rocks and are often spotted with a brownish colour.These rocks comprise talc and carbonate with small quantities of opaque.Talc presents as fine-grained fibrous aggregates of parallel arrangement characterized by asbestos structure replacing the tremolite and actinolite (Figure 3d).It is colourless and displays strong birefringence with low relief, parallel extinction, and high interference colours.Talc also presents as a few crystals associated with the serpentine, while antigorite occasionally presents as blade-like crystals in the talc rocks (Figure 3e).The opaque are present as fine anhedral crystals of iron oxides disseminated throughout the rock (Figure 3f).
The ophiolitic metabasalts (spilite) are dark green consisting of plagioclase, chlorite, carbonate, and epidote.They are cryptocrystalline with an amygdaloidal texture that is oriented parallel to the schistosity planes.Plagioclase microphenocrysts are predominant in the rocks and occupy about 50%-70% of the studied rock volume.The crystals are intensively altered and albuminized, where sodium ions replace the calcium ions to form albite and liberate Ca form epidote and carbonate as secondary minerals (Figure 3g).Carbonates are common in this rock and constitute about 20 modal vol.% of the rock; they occur in two forms: as an aggregate of calcite associating the main constituents resulting from the process of albitization of the plagioclase (Figure 3h) and as amygdale fillings during seawater infiltration usually associated with the secondary quartz (Figure 3i).The rock primarily comprises chlorite, which makes up about 10% of the rock and is associated with plagioclase and mafic minerals.
Minerals 2023, 13, x 8 of 30 5e).Biotite and muscovite are found as minute flakes in the groundmass and altered to chlorite (Figure 5f), while epidote accounts for about 10% of the rock.The plagioclase crystals are present as porphyroblastic clasts and fine lathes in the groundmass, making up 55% of the rock and are characterized by zoning and lamellar twinning with partial saussuritization.The metasediments of Kab Amiri can be categorized according to the grain size into two types, schist and slate.These types are characterized by schistose texture, where the constituting minerals are elongated and foliated by stress.
Quartz-actinolite-chlorite schists are composed of amphibole, biotite, plagioclase, and quartz.Amphibole minerals are represented by hornblende, tremolite and actinolite (Figure 4a).Hornblende is found as hypidioblastic prisms commonly foliated and dark brown intensively altered to the tremolite-actinolite association and finally to chlorite.Actinolite is characterized by a brownish-green colour with pleochroism from pale green to greenish black.It is mostly associated with tremolite and is considered an alteration product of amphibole minerals.Tremolite, the most common amphibole, exists as fibrous crystals characterized by asbestos structure and exhibits third-order interference colours (Figure 4b).Biotite is less common and occurs as hypidioblastic pale brown flakes up to 0.4 mm long (Figure 4c).Sometimes, biotite occurs as porphyritic flakes surrounded by finer crystals of quartz, epidote, and carbonate (Figure 4c).Chlorite is more dominant and associated with mica minerals.It occurs as greenish folia with polysynthetic structure (Figure 4d) and as pseudomorphs after the earlier mafic minerals.Plagioclase is moderately saussuritized and altered to epidote and saussurite minerals, where its longer dimension is parallel to the schistosity (Figure 4c).Epidote is the most common secondary Mineral; it is found as oval crystals associating zoisite and chlorite as alteration products (Figure 4c).Quartz is found as elongated crystals with rounded to sub-rounded shapes.It also occurs as porphyritic elongate crystals with chlorite following the schistose texture (Figure 4d).The rock encloses a few crystals of primary calcite that are characterized by twinkling.
Slate is a fine-grained metamorphic rock produced by low-grade regional metamorphism for shale or mudstone, varying in colour from buff to brown and black.The main foliation is defined by aligned mafic minerals, such as chlorite showing well-developed schistosity.The slate is composed mainly of quartz, sericite, and chlorite.It appears banded due to the elongation and orientation of its constituents (Figure 4e).Quartz is the main constituent, comprising about 60 vol% of rocks and occurs as rounded to sub-rounded porphyroblastic crystals with diameters of about 0.6 mm (Figure 4e) or as minute crystals prevailing the groundmass.Chlorite exists as xenomorphic secondary flakes, associated with quartz, epidote and carbonates showing schistosity (Figure 4e).Chloritization is dominant, so the rock's texture yields bands of green colour.Carbonates occur as cross-cutting micro veinlets of calcite (Figure 4f).Opaque minerals are represented by fine deep black grains disseminated through the rock.Sericite occurs as minute shreds and acicular crystals scattered throughout the groundmass (Figure 4e).
Metabasalts are fine-grained rocks with dark grey colour characterized by porphyritic texture.They are composed mainly of plagioclase and mafic minerals.Plagioclase occurs as hypidioblastic to xenoblastic crystals up to 0.6 mm in length and 0.3 mm in width (Figure 5a); it is partially altered to saussurite (Figure 5a) and epidote.Plagioclase also occurs as porphyritic crystals embedded in a fine-grained groundmass composed of plagioclase microlites and chlorite, exhibiting phenocryst texture.Mafic minerals are represented by actinolite, chlorite, and tremolite.The alteration minerals are also present as kinked xenoblastic crystals (up to 0.4 mm long) of chlorite tremolite (Figure 5b) and (Figure 5c) associated with epidote, iron oxides and relict biotite and hornblende.The mafic minerals are foliated and associated with carbonates as alteration products that are partially dissolved, replaced by secondary polycrystalline quartz (Figure 5a), and associated with epidote (Figure 5c).
The metapyroclastic rock is an andesitic and composed primarily of porphyritic clasts of plagioclase, quartz, carbonate, and perthite.The clasts are embedded in a fine-grained groundmass of chlorite and epidote.Plagioclase is the dominant Mineral, making up about 55% of the rock's volume, and is present as porphyroblastic clasts and fine lathes in the groundmass.The plagioclase crystals have zoning and lamellar twinning and have been partially saussuritized (Figure 5d).The potash feldspar is present in the metapyroclastic rock as porphyritic clastic perthite with a string-like shape and measures 1.3 mm in length and 1.1 mm in width (Figure 5d), constituting about 20% of the rock.Few crystals of quartz mantled by plagioclase are recorded as porphyritic (Figure 5e).Quartz is also present as rounded to subrounded porphyroblastic crystals and fine crystals in the groundmass, accounting for about 15% modal vol.%.The porphyroblastic crystals are characterized by boundaries assimilated by the thermal effect of the groundmass (Figure 5e).Biotite and muscovite are found as minute flakes in the groundmass and altered to chlorite (Figure 5f), while epidote accounts for about 10% of the rock.The plagioclase crystals are present as porphyroblastic clasts and fine lathes in the groundmass, making up 55% of the rock and are characterized by zoning and lamellar twinning with partial saussuritization.26,27].Furthermore, the majority of MnO, Na 2 O, K 2 O, TiO 2 and P 2 O 5 contents in the serpentinites are lower than 1 wt% and show the depletion degree of the mantle of investigated samples [28].Volatile components are reported as a loss on ignition (L.O.I.) and can be used to measure the degree of serpentinization.The average L.O.I. content varies from 10.43 to 20.76 wt%, with an average of 13.7 wt%.The high value of L.O.I. in serpentinite rocks reflects intense hydration and serpentinization during alteration processes and abundance in talc and carbonates.According to [29], L.O.I. values of more than six wt% are regarded as changed because either (1) these samples are partially dehydrated during their subduction and prograde metamorphism or (2) a higher amount of antigorite.
Trace element analyses show that the samples tend to be richer in Cr (1989 ppm), Ni (2130 ppm) and Co (33.7-101.4ppm) than the other analyzed trace elements.According to [30], serpentinites have a higher concentration of lithium than saltwater, which is consistent with this property [31].Bulk serpentinites have lithium contents ranging from 1.3 to 18.9 ppm.According to [32,33], the processes that took place before serpentinization may have contributed to the Li enrichment, and serpentinization itself ought to lower the bulk sample's Li concentration.The high mobile arsenic, antimony, and lead in fluids made these elements represent potential tracers of the nature of fluids during serpentinization.The serpentinites show (0.4 < As < 10.2 ppm; 0.08 < Sb < 1 ppm)that they are moderate to highly enriched in these elements compared to the primitive and depleted mantle (0.1 < As < 10 ppm; 0.001 < Sb < 1 ppm) [34,35].They showed that they are transferred by aqueous fluids from the slab to the mantle wedge and are incorporated into serpentinites under oxidized conditions at shallow depths (~25 km) [36].The serpentinites show Pb concentrations from 0.49 to 10.89 ppm.Nb/La and Nb/Ce value ranges of the serpentines are (0.23-0.80) (0.13-0.32), with averages (of 0.44, 0.21), respectively.These values are lower than that of the primitive mantle (PM 1.02 and 0.40, respectively [33,37], 1.04 and 0.40, respectively, [38] and the average bulk crust (0.69 and 0.33, respectively), which could be evidence of possible crustal contamination.Mantle-derived magmas are characterized by high Ce/Pb (25 ± 5) and Nb/U (47 ± 10), low Lu/Yb (0.14-0.15), and relatively lower La/Sm (<4.5), [39].Continental crust has relatively low Ce/Pb (<15) and Nb/U (~9.7), along with higher Lu/Yb (0.16-0.18) and La/Sm (>4.5).Thus, these ratios can be used to reveal crustal contamination.All samples of serpentinites samples have a La/Sm ratio of <4.5, indicating a mantle derived source [39].In addition, the Nb/U ratio is lesser than 9.7, in addition, all serpentinite samples have Ce/Pb ratio of <15, which also indicates some crustal contamination.According to the Ol-Opx-Cpx ternary diagram [40], the serpentinites and talc-carbonates are mainly formed after harzburgites, with minor dunites (Figure 6a) because of the serpentinites alteration and lack of primary mineral phases, low content of Al 2 O 3 and CaO and higher MgO.The (AFM) (Na 2 O + K 2 O)-FeO t -MgO diagram [41] can discriminate tholeiitic and calc-alkaline affinity.Plotting the investigated serpentinites and talc-carbonates on this diagram revealed that the samples plot within the calc-alkaline field (Figure 6b).The most significant variations among the major oxides are usually in the abundances of K 2 O and SiO 2 .According to the binary diagram of SiO 2 and K 2 O of [42], the serpentinites and talc-carbonates plot in the low-K tholeiitic field (Figure 6c).

Tectonic Implication of Serpentinites and Talc-Carbonates
The studied serpentinites and talc-carbonates are low in Ti and high in Cr# [19] (Figure 7a,b).The Cr-spinel in serpentinites lies in fore-arc peridotites [2,27], but Cr-spinel in dunites and some highly depleted harzburgites plots in the boninite field.In the Cr# [19] and Mg# diagram (Figure 7c), the serpentinites and talc-carbonates lie in fore-arc peridotites, showing a good negative Cr# − Mg# trend reflecting the partial melting trend from harzburgites to dunites.Based on the variation diagram of [43] of SiO2/MgO versus Al2O3, the serpentinites plot in the peridotite field (Figure 7d).
In the MgO/SiO2 and Al2O3/SiO2 diagrams (Figure 8a), each sample examined plotted the "terrestrial mantle array" trend and showed a trend of magmatic depletion or enrichment from a primitive mantle to a highly depleted harzburgitic composition [44][45][46][47].This shift could be attributed to Mg loss due to seafloor weathering during serpentin-

Tectonic Implication of Serpentinites and Talc-Carbonates
The studied serpentinites and talc-carbonates are low in Ti and high in Cr# [19] (Figure 7a,b).The Cr-spinel in serpentinites lies in fore-arc peridotites [2,27], but Crspinel in dunites and some highly depleted harzburgites plots in the boninite field.In the Cr# [19] and Mg# diagram (Figure 7c), the serpentinites and talc-carbonates lie in fore-arc peridotites, showing a good negative Cr# − Mg# trend reflecting the partial melting trend from harzburgites to dunites.Based on the variation diagram of [43] of SiO 2 /MgO versus Al 2 O 3 , the serpentinites plot in the peridotite field (Figure 7d).
In the MgO/SiO 2 and Al 2 O 3 /SiO 2 diagrams (Figure 8a), each sample examined plotted below the "terrestrial mantle array" trend and showed a trend of magmatic depletion or enrichment from a primitive mantle to a highly depleted harzburgitic composition [44][45][46][47].This shift could be attributed to Mg loss due to seafloor weathering during serpentinization [45,48].The Al 2 O 3 /SiO 2 and MgO/SiO 2 ratios are comparable to those of Arabian shield and fore-arc peridotites (Figure 8a; [22,27,49]).When plotting compiled serpentinites and talc-carbonates in an Al 2 O 3 versus CaO (wt%) diagram (Figure 8b) of [27,49] all samples plotted in the fore-arc peridotite field.The serpentinites have low Al 2 O 3 and CaO contents, like depleted fore-arc peridotite [2,27].On the SiO 2 /MgO versus Al 2 O 3 diagram (Figure 8c), they are affiliated with ophiolitic peridotites, as are the other Eastern Desert ophiolitic ultramafics [43].The investigated serpentinites differ significantly from those found elsewhere in Egypt, as seen in Figure 8c.The Al 2 O 3 content seems to be relatively uninfluenced by serpentinization, demonstrating that the bulk-rock Al content typically reflects its original primary concentration [28,45].Low Al 2 O 3 abundances (average 1.14 wt%) define the investigated serpentinites, like peridotites from the fore-arc setting and Neoproterozoic serpentinized peridotites from the Eastern Desert, Egypt (Figure 8d).The chemistry of serpentinites (low in Al 2 O 3 , CaO and TiO 2 , but high in MgO, Ni, Cr and Co) suggests that they were formed after depleting harzburgite protoliths in fore-arc settings.
Minerals 2023, 13, x 15 of 30 shield and fore-arc peridotites (Figure 8a; [23,27,49]).When plotting compiled serpentinites and talc-carbonates in an Al2O3 versus CaO (wt.%) diagram (Figure 8b) of [27,49] all samples plotted in the fore-arc peridotite field.The serpentinites have low Al2O3 and CaO contents, like depleted fore-arc peridotite [2,27].On the SiO2/MgO versus Al2O3 diagram (Figure 8c), they are affiliated with ophiolitic peridotites, as are the other Eastern Desert ophiolitic ultramafics [43].The investigated serpentinites differ significantly from those found elsewhere in Egypt, as seen in Figure 8c.The Al2O3 content seems to be relatively uninfluenced by serpentinization, demonstrating that the bulk-rock Al content typically reflects its original primary concentration [28,45].Low Al2O3 abundances (average 1.14 wt.%) define the investigated serpentinites, like peridotites from the fore-arc setting and Neoproterozoic serpentinized peridotites from the Eastern Desert, Egypt (Figure 8d).The chemistry of serpentinites (low in Al2O3, CaO and TiO2, but high in MgO, Ni, Cr and Co) suggests that they were formed after depleting harzburgite protoliths in fore-arc settings.The contents of the bulk-rock Al2O3 (wt.%) of Kab Amiri serpentinites were compared with those from different tectonic settings and the Pan-African serpentinites [58].Symbols as in Figure 6.

Classifications and Magma Type of Basalts
The present samples show sub-alkali basalt affinity on the Zr/TiO2 × 0.0001 vs. Nb/Y diagram [59] (Figure 9a).While on the variation diagram of (Na2O + K2O) vs. SiO2 [60], all samples plot in the subalkaline field (Figure 9b).The Ti/Y versus Nb/Y ratio diagram discriminates between the tholeiitic and alkaline magmas [61].The metabasalts samples lie in the tholeiitic field (Figure 9c).These features indicate that the parental magma of basalt was tholeiitic in composition.

Tectonic Significance and Petrogenesis of Basalts
The geochemical characteristics of the ophiolite suite of rocks, in general, show variations in their major and trace elemental concentration, which could be due to mantle heterogeneity without any influence of crustal contamination [45,62].In the Th/Yb vs. Nb/Yb discrimination diagrams of [63], all the metabasalts rocks fall above the N-MORB array suggesting arc-related magmatism (Figure 9d). Figure 10a shows that there is considerable overlapping between N-MORBs and E-MORBs, and between E-MORBs, P-MORBs, and ABs.Th and Nb behave similarly during both partial melting and fractional crystallization processes (see trends in Figure 10b).In the Th-Nb diagram, basalts generated in oceanic subduction-unrelated settings, rifted margins, and OCTZ can be distin-  [43,54,55], respectively.The black line is the terrestrial array after [46].The blue area highlights a global compositional range of abyssal peridotites [43].(b) Al 2 O 3 versus CaO binary diagram of [49].(c) SiO 2 /MgO ratios versus Al 2 O 3 diagram.Fields of ophiolitic gabbros and peridotites as well as MORB are from [43].Data from the Eastern Desert are shown for comparison [56,57].(d) The contents of the bulk-rock Al 2 O 3 (wt%) of Kab Amiri serpentinites were compared with those from different tectonic settings and the Pan-African serpentinites [58].Symbols as in Figure 6.

Classifications and Magma Type of Basalts
The present samples show sub-alkali basalt affinity on the Zr/TiO 2 × 0.0001 vs. Nb/Y diagram [59] (Figure 9a).While on the variation diagram of (Na 2 O + K 2 O) vs. SiO 2 [60], all samples plot in the subalkaline field (Figure 9b).The Ti/Y versus Nb/Y ratio diagram discriminates between the tholeiitic and alkaline magmas [61].The metabasalts samples lie in the tholeiitic field (Figure 9c).These features indicate that the parental magma of basalt was tholeiitic in composition.
ing PM define an array of mantle depletion without contribution from subduction-derived components Figure 10a,b.

Tectonic Significance and Petrogenesis of Basalts
The geochemical characteristics of the ophiolite suite of rocks, in general, show variations in their major and trace elemental concentration, which could be due to mantle heterogeneity without any influence of crustal contamination [45,62].In the Th/Yb vs. Nb/Yb discrimination diagrams of [63], all the metabasalts rocks fall above the N-MORB array suggesting arc-related magmatism (Figure 9d). Figure 10a shows that there is considerable overlapping between N-MORBs and E-MORBs, and between E-MORBs, P-MORBs, and ABs.Th and Nb behave similarly during both partial melting and fractional crystallization processes (see trends in Figure 10b).In the Th-Nb diagram, basalts generated in oceanic subduction-unrelated settings, rifted margins, and OCTZ can be distinguished from subduction-related basalts with a misclassification rate of <1% (Figure 10b).Three different types of convergent plate settings can be discriminated on the Th N vs.   Chondrite-normalized rare Earth elemental (REE) patterns of the metabasalts are shown in (Figure 11).The studied rocks have rare earth element (REE) contents (ΣREE = 19.16-45.2ppm).These metabasalts show moderate LREE/HREE fractionation (La/Yb) N = 0.5 to 1.5 and (Gd/Yb) N = 0.8 to 1.4 with Eu/Eu* = 0.9-1.2 and display flat pattern without an Eu anomaly, similar to typical N-MORB.The chemical analyses of seven representative samples of the i ments and four representative samples of the island arc metavolcan area are listed in Table 2.The metasediments are characterized by high 49.20 to 64.52 wt.% and A12O3 12.57 to 16.36%, which reflects the dom clay minerals.The percentages of MgO, CaO, and Na2O in metasedim to 7.30, 1.32 to 9.6, and 1.93 to 6.38, respectively.The Ni, Cr, Sc, and V as compatible ferromagnesian trace elements, exhibit quite variable metasediments.Ni is in the range from 18 to 94 ppm, while Cr conte 145 ppm.Alternatively, Sc and V abundance ranges are from 19.8 to 342 ppm, respectively.The Large ion lithophile elements (LILEs) Cs, R centrations are variable.Cs have lower content and vary from 0.1 to from 3.6 to 30.8 ppm, and Ba from 3.98 to 2.83 ppm.Sr is depleted an 258 ppm.Depletion of Sr is connected to the low abundance of CaO, metasediments' source material is plagioclase poor.Th, U, Zr and Y s ranging from 0.2 to 0.6 ppm, 0.1 to 0.4 ppm, 15.2 to 100.3 ppm, and spectively.In addition, normal to low concentration of Zr, which is co and increases with maturity, infers immaturity to semi-maturity for t (Table 2).The related pairs Nb and Ta show exceptional coherence an samples, and they range from 0.67 to 7.0 ppm, and 0.1 to 3 ppm, metasediments have ΣREE contents varying from 15.5 to 30.1 ppm,  [64] for the studied metabasalts.

Elemental Distribution in Schists
The chemical analyses of seven representative samples of the island arc metasediments and four representative samples of the island arc metavolcanic of the Kab Amiri area are listed in Table 2.The metasediments are characterized by high silica contents from 49.20 to 64.52 wt% and A1 2 O 3 12.57 to 16.36%, which reflects the dominance of aluminous clay minerals.The percentages of MgO, CaO, and Na 2 O in metasediments range from 1.03 to 7.30, 1.32 to 9.6, and 1.93 to 6.38, respectively.The Ni, Cr, Sc, and V elements, regarded as compatible ferromagnesian trace elements, exhibit quite variable abundances in the metasediments.Ni is in the range from 18 to 94 ppm, while Cr content varies from 17 to 145 ppm.Alternatively, Sc and V abundance ranges are from 19.8 to 30.2 ppm and 112 to 342 ppm, respectively.The Large ion lithophile elements (LILEs) Cs, Rb, Ba, K and Sr concentrations are variable.Cs have lower content and vary from 0.1 to 0.9 ppm, Rb ranges from 3.6 to 30.8 ppm, and Ba from 3.98 to 2.83 ppm.Sr is depleted and ranges from 87 to 258 ppm.Depletion of Sr is connected to the low abundance of CaO, implying that these metasediments' source material is plagioclase poor.Th, U, Zr and Y show concentrations ranging from 0.2 to 0.6 ppm, 0.1 to 0.4 ppm, 15.2 to 100.3 ppm, and 6.2 to 21.8 ppm, respectively.In addition, normal to low concentration of Zr, which is concentrated in zircon and increases with maturity, infers immaturity to semi-maturity for these metasediments (Table 2).The related pairs Nb and Ta show exceptional coherence and are depleted in all samples, and they range from 0.67 to 7.0 ppm, and 0.1 to 3 ppm, respectively.These metasediments have ΣREE contents varying from 15.5 to 30.1 ppm, LREE contents from 11 to 17.9 ppm, HREE from 4.4 to 11.8 ppm, and slightly negative to positive Eu/Eu* anomalies ranging from 0.3 to 1.3, suggesting vigorous changes in the physic-chemical conditions.[64] was used to determine the origin of metasedimentary samples.Plotting of the investigated metasediments on this diagram revealed that they fall in the greywacke field, which suggests a sedimentary origin (Figure 12a).The FeO t -MgO-Al 2 O 3 ternary diagram of [65] supports the metamorphic origin of these rocks (Figure 12b).According to the (SiO 2 /Al 2 O 3 )-(K 2 O/Na 2 O) binary diagram of [66], the metasediments plot in the pelitic greywackes field (Figure 12c).Based on the log (FeO/K 2 O) versus log (SiO 2 /Al 2 O 3 ) of [67], the samples fall within the Fe-shale field, indicating high content of iron oxides (Figure 12d).K 2 O versus Na 2 O of [68] divided the greywacke into quartz rich, quartz intermediate, and poor quartz varieties; the studied samples are fitted with quartz intermediate and poor fields.(Figure 12e).Chondrite normalized rare Earth elemental (REE) patterns of the metasediments are shown in Figure 13.The metasedimentary samples are characterized by a moderate degree of REE fractionation, as revealed by their contents (La/Yb) N ratios (1.2 to 8.3 ppm).The degree of LREE fractionation is quite low, and (La/Sm) N content ranges from 1.0 to 3.6 ppm, while the heavy REEs are weakly fractionated and (Gd/Yb) N content range from 0.9 to 5.3 ppm.

Classifications and Magma Type of Island-Arc Metavolcanic
According to Middlemost's Na 2 O + K 2 O vs. SiO 2 diagram [69], metavolcanic samples fall in the basaltic-andesite field (Figure 14a).On the variation diagram of Na 2 O + K 2 O vs. SiO 2 [60], all samples plot in the subalkaline magma field (Figure 14b).The (AFM) Alkalis-FeO t -MgO diagram proposed by [60] can discriminate tholeiitic and calc-alkaline affinity.In this diagram, the samples plot within the calc-alkaline field (Figure 14c).On the SiO 2 versus FeO t /MgO variation diagram of [70], the metavolcanic samples plot in the calc-alkaline field (Figure 14d).The investigated metavolcanic in chemistry resembles Shadli island-arc metavolcanic [71].5.6.Tectonic Setting and Petrogenesis of the Island-Arc Metavolcanic Rocks According to Nb versus SiO 2 variation diagram of [72] to discriminate between the active continental margin, orogenic volcanic terrains and immature island arcs.All metavolcanic samples plot in the immature island arcs (Figure 14e).Based on the Th/Yb versus Nb/Yb variation diagram of [73], all samples plot in the oceanic arc field (Figure 14f), suggesting an arc-related (island-arc) setting [72].

Tectonic Model and Emplacement Mechanism
Egypt's Central Eastern Desert (CED) is divided into two ophiolite belts with controversial genetic systems.The first belt is called Ghadir-Mubarak-Barramiya and is located in the south [26], while the second belt is the Wadi Semna-Fawakhir-Um Gheig belt in the north of CED [21].The study area is located in the northern belt of CED (Figures 15 and 16), which is affected by NW obduction-related thrust faults resulting from the structure evolution of the Najd fault system (NFS).
The two Cryogenian ophiolitic belts in the CED and the accretionary history of the Eastern Desert of Egypt were overprinted by the Najd fault system (NFS) with fore-arc and back-arc geochemical characteristics (Figure 15a); Abd El-Rahman et al. propose that an intra-oceanic island connected to a NE-dipping subduction zone formed during the tectonic history of the CED [4].A fore-arc ophiolite belt, however, stretches from El Barramiya [26] in the southern portion of the CED (SCED) to Fawakhir in the northern section of the CED (NCED).In addition, a back-arc basin parallel to the fore-arc belt formed to the east (in modern coordinates), and the back-arc basin was closed by a WSW-dipping subduction zone [21] (Figure 15b).Egypt's Central Eastern Desert (CED) is divided into two ophiolite belts with controversial genetic systems.The first belt is called Ghadir-Mubarak-Barramiya and is located in the south [26], while the second belt is the Wadi Semna-Fawakhir-Um Gheig belt in the north of CED [21].The study area is located in the northern belt of CED (Figures 15 and  16), which is affected by NW obduction-related thrust faults resulting from the structure evolution of the Najd fault system (NFS).
The two Cryogenian ophiolitic belts in the CED and the accretionary history of the Eastern Desert of Egypt were overprinted by the Najd fault system (NFS) with fore-arc and back-arc geochemical characteristics (Figure 15a); Abd El-Rahman et al. propose that an intra-oceanic island connected to a NE-dipping subduction zone formed during the tectonic history of the CED [4].A fore-arc ophiolite belt, however, stretches from El Barramiya [26] in the southern portion of the CED (SCED) to Fawakhir in the northern section of the CED (NCED).In addition, a back-arc basin parallel to the fore-arc belt formed to the east (in modern coordinates), and the back-arc basin was closed by a WSW-dipping subduction zone [21] (Figure 15b).(a) Model I after [74]; (b) Model II after [4] showing the distribution of serpentinite in the Eastern Desert of Egypt.Zones with blue colour in both models are the fore-arc association, while the zones with red colour are the back-arc association.
Alternatively, some studies argue that the Eastern belt (toward the Southern Eastern Desert) is a fore-arc, and the Western belt (toward the Northern Eastern Desert) is a backarc, both of which have opposing directions.This theory is more consistent with the overall structural patterns (Figure 16a).The metasomatized serpentinites of Kab Amiri match the majority of Egyptian ophiolites' proposed tectonic settings, which featured oceanic lithosphere fragments deposited over a subduction zone in a fore-arc setting, according to Alternatively, some studies argue that the Eastern belt (toward the Southern Eastern Desert) is a fore-arc, and the Western belt (toward the Northern Eastern Desert) is a back-arc, both of which have opposing directions.This theory is more consistent with the overall structural patterns (Figure 16a).The metasomatized serpentinites of Kab Amiri match the majority of Egyptian ophiolites' proposed tectonic settings, which featured oceanic lithosphere fragments deposited over a subduction zone in a fore-arc setting, according to the aforementioned data (Figure 16b) [75][76][77].
In the suggested models for the CED's development, the presence of the paired belts is indicated by ophiolite belts.Having geochemical affinities in the fore-arc and back-arc [4].The first piece of evidence for this concept is the age of the fore-arc belt, which should be older than the back-arc belt.However, the beginning of a back-arc basin happens later to the spreading at the front of the subduction zone at the fore-arc setting [21].

2.
Dismembered ophiolitic rocks are distributed in the northern and eastern parts of the study area and comprise serpentinites, talc carbonates and metabasalt rocks.Serpentinites in the present area are the oldest rock unit.Petrographically, the serpentinites are composed of serpentine minerals (90%), and olivine with minor amounts of carbonates and opaques.Geochemically, the serpentinites resemble those of depleted mantle peridotites (harzburgite-dunite) and tend to have abyssal geochemical characteristics.

3.
Kab Amiri metabasalt is related to the older metavolcanic (OMV), characterized by large exposure of pillow structures.They consist of plagioclase, chlorite, carbonate and epidote; they are also characterized by cryptocrystalline grain size and amygdaloidal texture.The ophiolitic metabasalt are sub-alkali basalt affinities, tholeiitic and formed in the fore-arc basin.The chondrite normalized rare Earth elemental (REEs) patterns of the metabasalts show low fractionated patterns.4.
Island arc assemblages are represented by metasediment and metavolcanic rocks.Metasediments rocks comprise quartz-actinolite-chlorite schist and slate.They are mostly foliated and highly folded.Microscopically, the schists are essentially composed of amphibole and mica minerals, plagioclase, and quartz.In contrast, slate samples are composed mainly of quartz, sericite, and chlorite and are related to oceanic island arc tectonic setting.

5.
Large exposures of andesitic basalt in the southern and northeast parts of the mapped area represent metavolcanic rocks.Microscopically, the island arc metavolcanic is classified as metabasalt (plagioclase and mafic minerals) and metapyroclastic (porphyroblasts of clastic plagioclase, quartz, carbonate and perthite embedded in a fine-grained groundmass of chlorite and epidote).Geochemically, the metavolcanic are basaltic-andesite andesite in composition, and calc-alkaline affinity is related to the island arc tectonic setting.Chondrite-normalized rare earth element (REE) patterns show slightly fractionated patterns.6.
The large serpentinite bodies are concentrated on the CED's western side, extending in an NNW-SSE direction.To the west of the arc-back-arc assemblages, arc-forearc assemblages are still visible, proving that the intra-oceanic island arc system was formed over an E-dipping subduction zone (present coordinates).Both assemblages define a suture zone in the CED by extending as NW-SE belts.7.
The geochemical signature of subduction increases in the direction of the south in arc-back-arc assemblages.Bimodal volcanism's prevalence in the south suggests that a back-arc basin has been gradually expanding from the north to the south, but the rift island arc assemblages farther south have been preserved.
Nb N diagram in Figure 10b.Island arcs with complex polygenetic crustal nature are primarily characterized by the occurrence of CABs, which are displaced to the highest Th-Nb values.Intra-oceanic arcs display a large variability in Th-Nb contents, which can be used for recognizing two sub-types of intra-oceanic arc basalts.The Th/Nb enrichment indicates subduction-mantle source interaction, whereas decreasing Th-Nb compositions concerning PM define an array of mantle depletion without contribution from subduction-derived components Figure 10a,b.

Figure 10 .
Figure 10.(a) Summary of the compositional variations of different post-Archean ophiolitic basaltic rock types on the ThN vs. NbN diagram.Vectors indicate the trends of compositional variations due to the main petrogenetic processes.Abbreviations: SSZ-E: supra-subduction zone enrichment; AFC: assimilation-fractional crystallization; OIB-CE: ocean island-type (plume-type) component enrichment; FC: fractional crystallization.Crosses indicate the composition of typical N-MORB, E-MORB and OIB [38].(b) Tectonic interpretation of ophiolitic basaltic types based on ThN-NbN systematics.Backarc A indicates backarc basin basalts (BABB) characterized by the input of subduction or crustal components (e.g., immature intra-oceanic or ensialic backarcs).In contrast, Backarc B indicates BABBs showing no input of subduction or crustal components (e.g., mature intra-oceanic backarcs).OCTZ: ocean-continent transition zone.In both panels, Nb and Th are normalized to the N-MORB composition [38].

Figure 10 .
Figure 10.(a) Summary of the compositional variations of different post-Archean ophiolitic basaltic rock types on the Th N vs. Nb N diagram.Vectors indicate the trends of compositional variations due to the main petrogenetic processes.Abbreviations: SSZ-E: supra-subduction zone enrichment; AFC: assimilation-fractional crystallization; OIB-CE: ocean island-type (plume-type) component enrichment; FC: fractional crystallization.Crosses indicate the composition of typical N-MORB, E-MORB and OIB [38].(b) Tectonic interpretation of ophiolitic basaltic types based on Th N -Nb N systematics.Backarc A indicates backarc basin basalts (BABB) characterized by the input of subduction or crustal components (e.g., immature intra-oceanic or ensialic backarcs).In contrast, Backarc B indicates BABBs showing no input of subduction or crustal components (e.g., mature intra-oceanic backarcs).OCTZ: ocean-continent transition zone.In both panels, Nb and Th are normalized to the N-MORB composition [38].

Figure 15 .
Figure 15.The controversial models for the evolution of the CED ophiolite belts according to [21];

Figure 15 .
Figure15.The controversial models for the evolution of the CED ophiolite belts according to[21]; (a) Model I after[74]; (b) Model II after[4] showing the distribution of serpentinite in the Eastern Desert of Egypt.Zones with blue colour in both models are the fore-arc association, while the zones with red colour are the back-arc association.

Minerals 2023, 13 , x 25 of 30 Figure 16 .Figure 16 .
Figure 16.(a) The proposed model of Neoproterozoic fore-arc ophiolite belts in the northern Arabian-Nubian Shield at Central Eastern Desert (CED), Egypt [21,74].(b) Tectonic model for the evolution of the studied ultramafic rocks and the Arabian-Nubian [76-78].6.Conclusions 1.Most previous studies mainly concern granitic rocks' geology, petrography, and chemistry.We carried out detailed field, petrographical and geochemical studies on the serpentinites, spilite, metasediments and metavolcanic (metabasalt) at Wadi Kab Amiri.2. Dismembered ophiolitic rocks are distributed in the northern and eastern parts of the study area and comprise serpentinites, talc carbonates and metabasalt rocks.Serpen-Figure 16.(a) The proposed model of Neoproterozoic fore-arc ophiolite belts in the northern Arabian-Nubian Shield at Central Eastern Desert (CED), Egypt [21,74].(b) Tectonic model for the evolution of the studied ultramafic rocks and the Arabian-Nubian [76-78].

Table 2 .
Whole-rock major oxides, trace and REEs element compositions of metasediments and metavolcanic rocks from Kab Amiri.