Petrogenesis of Neoproterozoic Ultramaﬁc Rocks, Wadi Ibib–Wadi Shani, South Eastern Desert, Egypt: Constraints from Whole Rock and Mineral Chemistry

: This contribution deals with new geology, petrography, and bulk-rock/mineral chemistry of the poorly studied ultramaﬁcs of Wadi Ibib–Wadi Shani (WI–WS) district, South Eastern Desert, Egypt. These ultramaﬁcs are dismembered ophiolitic rocks that can be subdivided into serpentinites and serpentinized peridotites. Primary minerals such as olivine and pyroxene are absent in serpentinites, but relics of them occur in serpentinized peridotites. Pseudomorph after olivine is indicated by common hourglass textures with less mesh, whilst schistose bastites reﬂect a pyroxene pseudomorph. Chromite can be subdivided into Cr-spinel and Al-spinel. Cr-spinel ranges from chromite to magnesochromite in composition, whereas Al-spinel belongs to the spinel ﬁeld. Cr-spinel includes YCr (Cr/(Cr+Al+Fe +3 ), YAl (Al/(Al+Cr+Fe +3 ), and YFe +3 (Fe +3 / (Fe +3 +Al+Cr), similar to forearc peridotite, whilst Al-spinel is more similar to abyssal peridotite, and may be formed during inanition of subduction processes in proto forearc environments. The main secondary minerals are tremolite, talc, and chlorite—which is subdivided into pycnochlorite and diabantite—and their temperature ranges from 174 to 224 ◦ C. The examined rocks had undergone high partial melting degrees (>25%), as indicated by the Cr# of their unaltered cores (Cr-spinel, >0.6), whole rocks (Al 2 O 3, SiO 2 , CaO, and MgO), trace and REEs, depleted Na 2 O, Al 2 O 3 , and Cr 2 O 3 of clinopyroxene, and high forsterite content ((Fo = 100 Mg/Mg + Fe), av. 95.23 mol%), consistent with forearc settings.


Introduction
Neoproterozoic ultramafic rocks are commonly distributed in central and southern sectors of the Egyptian Eastern Desert (ED) in the northern Nubian Shield. The Nubian Shield is considered the western side of the Arabian Shield, both of which are juvenile crusts forming the Arabian-Nubian Shield (ANS) of the northern East African Orogeny (EAO) [1]. These rocks constitute 5.3% of Egypt's crystalline outcrops, mostly as allochthonous nappes [2][3][4][5][6]. The field and geochemical signatures of the ultramafic magma provide clues as to the different tectonic settings. They are either older, dismembered ophiolitic fragments-which are partially or completely metamorphosed to serpentinite, mostly consisting of chromitites and magnesites of mid-ocean ridges or supra-subduction settings-or younger, unmetamorphosed ones [7,8]. The latter, with an age > 640 Ma [9], encompass fresh, concentrically arc-zoned dunites and clinopyroxenites of Alaskan type, mostly consisting of PGEs, Ni-Cu sulfide deposits, and fresh layered intrusions hosting Fe-Ti oxides [10,11].
Ophiolites are slices of the oceanic lithosphere thrust on the continental plates, which help in recognizing the tectonic processes in the mantle [12]. Serpentinites, carbonatized Ophiolites are slices of the oceanic lithosphere thrust on the continental plates, which help in recognizing the tectonic processes in the mantle [12]. Serpentinites, carbonatized serpentinites, talc-carbonates, and listwaenite series (carbonate-rich, silica-carbonate, and birbirites) rocks are the main alteration products of the ophiolitic harzburgite/dunite fragments, due to interaction with CO2-and SiO2-rich fluids [13]. These processes are widely observed along faults and shear zones. Ultramafic rocks are enriched with magnesium and iron silicate minerals; therefore, carbonation processes take place via the hydrolysis of these minerals, through eviction of Si and combination of these cations with carbonates [3,14]. The tectonic setting of Egyptian ophiolitic mantel rocks (ultramafics) is still a subject of debate; some authors suggest mid-ocean ridge (MOR), while others suggest suprasubduction zone (SSZ) [4,15].
MORs form abyssal mantle rocks of anhydrous melting at low degrees (5-15%), whilst more than 20% represents a high degree of melting, forming refractory or depleted rocks in SSZs [16]. The melting degree of the first type depends on the interaction between mantle rocks and melts, as well as the temperature and pressure at melting depth. Conversely, the melting degree of the latter represents a complex process, and depends on the interaction between melts and fluids generated from the subducted slab [17]. In this paper, new geological, mineralogical, and geochemical data are discussed, with a view to detecting the hydrothermal effects, origins and tectonic setting of the studied ultramafic rocks.
The examined ophiolitic ultramafic rocks are represented by the dominant serpentinite and less serpentinized peridotites, which are usually metamorphosed to greenschist facies. Field observations reveal that the examined ultramafic rocks cover ~100 km 2 , forming an extended N-S ridge with a high relief (950-1100 m above sea level) at the northsouth side of G. Abu Hadeida. They are exposed as mountainous blocks, which are mainly massive and grey-black in color. Furthermore, they exhibit a closely spaced system of joints trending NE-SW and NW-SE along the fault plane. They are thrust over the metasediments (Figure 2a), as indicated by dip directions, with some shearing and tightness of foliation close to the contact. These metasediments are fine-grained, highly foliated, laminated, and exhibit alternating mafic-rich and -poor layers of schists (Figure 2b). Ultramafics are partially to extensively sheared and highly altered to foliated (asbestiform), buff colored, talc-carbonate, carbonate-rich (listwaenite), and magnesite rocksespecially along fault planes and shear zones. These alteration products (listwaenite) occur either as patches or as dike-like with high relief relative to the host (serpentinite)especially along fault planes.  [15]. In addition, location of the examined area, and (c) detailed geological map of Wadi Ibib (WI)-Wadi Suwawrib (WS) [19].
The examined ophiolitic ultramafic rocks are represented by the dominant serpentinite and less serpentinized peridotites, which are usually metamorphosed to greenschist facies. Field observations reveal that the examined ultramafic rocks cover~100 km 2 , forming an extended N-S ridge with a high relief (950-1100 m above sea level) at the north-south side of G. Abu Hadeida. They are exposed as mountainous blocks, which are mainly massive and grey-black in color. Furthermore, they exhibit a closely spaced system of joints trending NE-SW and NW-SE along the fault plane. They are thrust over the metasediments (Figure 2a), as indicated by dip directions, with some shearing and tightness of foliation close to the contact. These metasediments are fine-grained, highly foliated, laminated, and exhibit alternating mafic-rich and -poor layers of schists ( Figure 2b). Ultramafics are partially to extensively sheared and highly altered to foliated (asbestiform), buff colored, talc-carbonate, carbonate-rich (listwaenite), and magnesite rocks-especially along fault planes and shear zones. These alteration products (listwaenite) occur either as patches or as dike-like with high relief relative to the host (serpentinite)-especially along fault planes.

Petrography
Generally, the examined mantle units can be divided into serpentinites and serpentinized peridotites, based on their mineralogical composition. Petrographic examination of 31 samples was carried out at NMA using a polarizing microscope (Olympus bx53). Serpentinites are commonly sheared (carbonatized), and exhibit pseudomorphic textures after olivine and pyroxene minerals. Pseudomorph after olivine is indicated by common hourglass textures with less mesh, whilst bastite reflects pyroxene pseudomorph. Primary minerals such as olivine and pyroxene are completely replaced by antigorite, magnetite, and carbonate minerals. Antigorite occurs as dense sherds of fibro-lamellar, irregular aggregates, ranging from colorless to pale green, fractured, and occupied by iron oxides and carbonates. Abundance of bastites suggest pyroxene pseudomorph, providing evidence for harzburgite protolith (Figure 3a). Scattered patches of carbonate are the main component in the examined serpentinites. Opaque minerals are essentially represented by magnetite ( Figure 3b). Magnetite is disseminated as fine, black, anhedral crystals.

Materials and Methods
Representative samples were crushed and pulverized using a vibration mill. Loss of ignition (LOI) was measured by the difference in weight after ignition. Whole-rock analysis (major, trace elements, and rare earth elements) of 25 samples was carried out via inductively coupled plasma emission spectrometry (ICP-ES) at Acme Lab, Vancouver, Canada. Detection limits for trace elements and major oxides were 0.01-0.5 ppm and 0.001-0.04 wt.%, respectively. The analytical precision, as calculated from replicate analyses, was 0.5% for major oxides, and varied from 2% to 20% for trace elements. Some samples were selected and analyzed for major elements (as a test) using fused pellets prepared as described in [20], using lithium tetraborate as a flux at the laboratories of the Nuclear Materials Authority (NMA), Cairo, Egypt. The mineral chemistry was carried out at the Microscopy and Microanalyses Facility, University of New Brunswick (UNB), Fredericton, New Brunswick, Canada. Samples were analyzed with a JEOL JXA-733 Electron Microprobe, equipped with ds spec and dQant32 automation (Geller Micro Analytical Labs, Canada). An accelerating voltage of 15 kV and a probe current of 30 nA were used. Peak counting times were 30 s (Al and Si), 90 s (Ti, V, Cr, and Cs), and 120 s (Na, Mg, K, Ca, Mn, Fe, and Ni). High and low backgrounds were counted for one-half of the peak counting times.

Petrography
Generally, the examined mantle units can be divided into serpentinites and serpentinized peridotites, based on their mineralogical composition. Petrographic examination of 31 samples was carried out at NMA using a polarizing microscope (Olympus bx53). Serpentinites are commonly sheared (carbonatized), and exhibit pseudomorphic textures after olivine and pyroxene minerals. Pseudomorph after olivine is indicated by common hourglass textures with less mesh, whilst bastite reflects pyroxene pseudomorph. Primary minerals such as olivine and pyroxene are completely replaced by antigorite, magnetite, and carbonate minerals. Antigorite occurs as dense sherds of fibro-lamellar, irregular aggregates, ranging from colorless to pale green, fractured, and occupied by iron oxides and carbonates. Abundance of bastites suggest pyroxene pseudomorph, providing evidence for harzburgite protolith (Figure 3a). Scattered patches of carbonate are the main component in the examined serpentinites. Opaque minerals are essentially represented by magnetite (Figure 3b). Magnetite is disseminated as fine, black, anhedral crystals.
Spinel is predominantly composed of opaque minerals, and reveals heterogenetic composition; it can be distinguished into Cr-spinel (Supplementary Materials, Table S3) and Al-spinel (Supplementary Materials, Table S4). Al-spinel is found in the picotitic field, whilst Cr-spinel straddles the chromite field, and both are related to ophiolitic spinels [29,30] (Figure 5a). Furthermore, Cr-spinel can be classified into chromite and magnesochromite, whereas Al-spinel is classified as spinel according to the Cr# versus Mg# binary discrimination diagram [31]  , compared to those of Cr-spinel. In addition, they are found in the mantle array field, suggesting their magmatic nature ( Figure 5c). The Cr# contents of Cr-spinel are similar to those of the Egyptian Eastern Desert [32]. Cr-spinels contain YCr (Cr/(Cr+Al+Fe +3 ), YAl (Al/(Al+Cr+Fe +3 ) and YFe +3 (Fe +3 /(Fe +3 +Al+Cr), similar to forearc peridotites [32], whilst the contents of Al-spinel are similar to those of abyssal peridotites, which may be formed during inanition of subduction processes in proto-forearc environments (Figure 5d). Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 22 The chrysotile phase has a chemical affinity similar to that of olivine, whilst antigorite comes from orthopyroxene and olivine, reflecting a dunite protolith for the former, and both harzburgite and dunite for the latter phase ( Figure 6e). The abundance of antigorite rather than chrysotile in the examined ultramafic rocks reflects prograde metamorphism [33,34].
Chlorite represents one of the main alteration products of the primary minerals (pyroxenes) in serpentinized peridotites. EPMA results show that the examined chlorites contain SiO2 ranges from 33 Table S6). On the Si (apfu) vs. Fe +2 +Fe +3 (apfu) binary diagram [35], the examined chlorite is classified as pycnochlorite and diabantite (Figure 7a). Al iv can be used to determine the chlorite temperature [36]. Using the equation in [37] (T = 106.2*Al iv + 17.5), the temperature of chlorite ranges from 174 to 224 °C.  (Figure 6b-d). The chrysotile phase has a chemical affinity similar to that of olivine, whilst antigorite comes from orthopyroxene and olivine, reflecting a dunite protolith for the former, and both harzburgite and dunite for the latter phase ( Figure 6e). The abundance of antigorite rather than chrysotile in the examined ultramafic rocks reflects prograde metamorphism [33,34].
Chlorite represents one of the main alteration products of the primary minerals (pyroxenes) in serpentinized peridotites. EPMA results show that the examined chlorites contain SiO 2 ranges from 33 Table S6). On the Si (apfu) vs. Fe +2 +Fe +3 (apfu) binary diagram [35], the examined chlorite is classified as pycnochlorite and diabantite (Figure 7a). Al iv can be used to determine the chlorite temperature [36].
Using the equation in [37] (T = 106.2*Al iv + 17.5), the temperature of chlorite ranges from 174 to 224 • C.  Table S7). They are classified as tremolite according to [40] (Figure 7b). They have Ti contents ranging from 0.003 to 0.014 apfu, with a mean value of 0.0036 apfu, reflecting their metamorphic nature. Furthermore, greenschist facies of the examined rocks are indicated by low Na and K contents of tremolite (av. 0.003 apfu) [41].
The investigated magnetites (serpentinites and serpentinized peridotites) were composed mainly of high FeO* contents (av. 93. 76 Table S8). They possess ulvospinel of less than 0.5 mol%, based on [42]. They are classified mainly as magnetites by using the FeO-TiO2-Fe2O3 ternary diagram (Figure 7c). In addition, the abundance of magnetites-either as patches, or filling fractures as veinlets-reflects the high degree of alteration of primary minerals, such as olivine [43].  Table S7). They are classified as tremolite according to [40] (Figure 7b). They have Ti contents ranging from 0.003 to 0.014 apfu, with a mean value of 0.0036 apfu, reflecting their metamorphic nature. Furthermore, greenschist facies of the examined rocks are indicated by low Na and K contents of tremolite (av. 0.003 apfu) [41].
The investigated magnetites (serpentinites and serpentinized peridotites) were composed mainly of high FeO* contents (av. 93. 76 Table S8). They possess ulvospinel of less than 0.5 mol%, based on [42]. They are classified mainly as magnetites by using the FeO-TiO 2 -Fe 2 O 3 ternary diagram (Figure 7c). In addition, the abundance of magnetites-either as patches, or filling fractures as veinlets-reflects the high degree of alteration of primary minerals, such as olivine [43].  Table S9). It is widely known that the talc forms as an alteration product after primary minerals (olivine and serpentine), suggesting high-temperature metamorphism.

Whole-Rock Geochemistry
The whole-rock (major, trace, and REEs) compositions of serpentinites and serpentinized peridotites are given in Table 2. Loss on ignition (LOI) varies from 5.27 to 11.5 wt.% for serpentinites, and from 4.16 to 10.7 wt.% for serpentinized peridotites, indicating a various and extensive degree of hydration and serpentinization. Therefore, bulk major oxides were recalculated to 100 wt.% based on an anhydrous to compensate for the variable serpentinization effect [18,44]. The average contents of major oxides in serpentinites and serpentinized peridotites reveal that the TiO2, Al2O3, MnO, Na2O, K2O, and P2O5 contents are less than 1 wt.%. In addition, the low CaO (av. 1.48 and 1.09 wt.%, respectively) and Al2O3 (av. 0.76 and 0.59 wt.%, respectively) contents of the examined serpentinites and serpentinized peridotites are similar to forearc peridotites and Pan-African serpentinites [18,45]-especially those in the Egyptian Eastern Desert [8,19,22]-suggesting their refractory origin [19] (Figure 8a,b).  Table S9). It is widely known that the talc forms as an alteration product after primary minerals (olivine and serpentine), suggesting high-temperature metamorphism.

Whole-Rock Geochemistry
The whole-rock (major, trace, and REEs) compositions of serpentinites and serpentinized peridotites are given in Table 2. Loss on ignition (LOI) varies from 5.27 to 11.5 wt.% for serpentinites, and from 4.16 to 10.7 wt.% for serpentinized peridotites, indicating a various and extensive degree of hydration and serpentinization. Therefore, bulk major oxides were recalculated to 100 wt.% based on an anhydrous to compensate for the variable serpentinization effect [18,44]. The average contents of major oxides in serpentinites and serpentinized peridotites reveal that the TiO 2 , Al 2 O 3 , MnO, Na 2 O, K 2 O, and P 2 O 5 contents are less than 1 wt.%. In addition, the low CaO (av. 1.48 and 1.09 wt.%, respectively) and Al 2 O 3 (av. 0.76 and 0.59 wt.%, respectively) contents of the examined serpentinites and serpentinized peridotites are similar to forearc peridotites and Pan-African serpentinites [18,45]-especially those in the Egyptian Eastern Desert [8,19,22]-suggesting their refractory origin [19] (Figure 8a,b). Table 2. Abundances of major (wt.%), trace, and rare earth elements (ppm) in ultramafic rocks. IA1  IA2  IA3  IA4  IA5  IA6  IA7  IA8  IA9  IA10  IA11   The SiO 2 /MgO ratio of the studied rocks ranges from 0.87 to 1.22, similar to the ophiolitic peridotites in [45] (Figure 8c). In terms of the CaO-Al 2 O 3 -MgO ternary diagram, the studied rocks are located in the dunite field, with a few samples in harzburgite (Figure 8d). Lack of alteration of clinopyroxene is associated with dissolution of Ca [27] in the examined rocks; therefore, they have very low CaO contents, below the values of the depleted (DM, 3.5 wt.%) [46] and primitive mantle (PM, 3.7 wt.%) [47]. Furthermore, the Al 2 O 3 contents of the studied rocks are less than that of the PM (Al 2 O 3 ca. 4.5 wt.%) [47]. Likewise, the average MgO content is high relative to PM composition, suggesting that the mantle melting is more than 20%. Conversely, SiO 2 and Fe 2 O 3 concentrations are lower than the values of the PM. 8d). Lack of alteration of clinopyroxene is associated with dissolution of Ca [27] in the examined rocks; therefore, they have very low CaO contents, below the values of the depleted (DM, 3.5 wt.%) [46] and primitive mantle (PM, 3.7 wt.%) [47]. Furthermore, the Al2O3 contents of the studied rocks are less than that of the PM (Al2O3 ca. 4.5 wt.%) [47]. Likewise, the average MgO content is high relative to PM composition, suggesting that the mantle melting is more than 20%. Conversely, SiO2 and Fe2O3 concentrations are lower than the values of the PM.   Table 2). Multiple elements of the primitive mantle (PM)-normalized diagram [51] (Figure 9a) show a strong negative Ta anomaly and slight enrichment of LFSEs relative to HFSEs, which are consistent with SSZ geochemical affinity [52]. Furthermore, they reveal a slightly positive Pb anomaly, similar to forearc peridotites [44]. This may be related to the effects of percolation of fluid during serpentinization processes [18]. They also possess high concentrations of transition elements, such as Ni and Co. Ni (1265-2510, av. 1982 ppm) and Co (92-158, av. 128 ppm) contents are higher in serpentinites than in serpentinized peridotites (Ni, av. 1825; av. Co, 114 ppm). Wide variation of Ni may be related to the dissemination of Ni in ferromagnesian minerals by serpentinization processes [53]. In addition, Ni 2+ substitutes Mg 2+ in olivine, and holds with Mg 2+ even during and after serpentinization processes.

Serpentinites
anomaly, similar to forearc peridotites [44]. This may be related to the effects of percolation of fluid during serpentinization processes [18]. They also possess high concentrations of transition elements, such as Ni and Co. Ni (1265-2510, av. 1982 ppm) and Co (92-158, av. 128 ppm) contents are higher in serpentinites than in serpentinized peridotites (Ni, av. 1825; av. Co, 114 ppm). Wide variation of Ni may be related to the dissemination of Ni in ferromagnesian minerals by serpentinization processes [53]. In addition, Ni 2+ substitutes Mg 2+ in olivine, and holds with Mg 2+ even during and after serpentinization processes. Furthermore, we noticed a narrow range of Co contents, which may be related to the substitution by Fe +2 -due to the similarity of ionic radii-and Mg +2 [53]. REEs are variable even in the same rock units, and reveal zigzag patterns (Figure 9b). The examined serpentinites are depleted in REEs (av. ∑REEs = 6.6 ppm), since their light rare earth element (LREE) contents range from 0.2 to 2.2, and their heavy rare earth element (HREE) contents range from 0.5 to 8.3 [51]. Likewise, the examined serpentinized peridotites contain ∑REEs ranging from 3.11 to 9.97 ppm, and slightly less fractionated LREEs ((La/Sm)N = 0.2-1.9) relative to HREEs ((Gd/Yb)N = 0.2-7.3), which is consistent with forearc serpentinites [18].

Protolith Changes and Element Mobility
Slight changes in the bulk major oxides are estimated from the (TiO2 + MgO + Al2O3 + Fe2O3 + CaO + MnO + K2O + Na2O + P2O5)/SiO2 ratios [18]. The calculated sum oxides/SiO2 ratio (1.08-1.46) is similar to those of [54]. The investigated rocks represent some of the metamorphosed Egyptian ultramafic rocks with variable amounts of serpentine, tremolite, and talc, and fewer chlorite minerals. The presence of antigorite and talc suggest high temperature [55]. REEs and HFS elements are considered immobile, especially during alteration, in comparison with LFSEs-such as Cs, Rb, K, Sr, and U-even during low alteration [18]. Serpentinites represent as an alteration products of low temperature of ultramafic rocks, making some changes of the protolith chemical composition by water addition [3,18,56]. In addition, some major oxides-such as CaO and SiO2 [49]-can be remobilized during serpentinization processes. According to [49], the completely serpentinite rocks are more depleted in CaO relative to partially serpentinized rocks. Some physical properties of the protolith-such as density and magnetism-are also changed by Furthermore, we noticed a narrow range of Co contents, which may be related to the substitution by Fe +2 -due to the similarity of ionic radii-and Mg +2 [53]. REEs are variable even in the same rock units, and reveal zigzag patterns (Figure 9b). The examined serpentinites are depleted in REEs (av. ∑REEs = 6.6 ppm), since their light rare earth element (LREE) contents range from 0.2 to 2.2, and their heavy rare earth element (HREE) contents range from 0.5 to 8.3 [51]. Likewise, the examined serpentinized peridotites contain ∑REEs ranging from 3.11 to 9.97 ppm, and slightly less fractionated LREEs ((La/Sm) N = 0.2-1.9) relative to HREEs ((Gd/Yb) N = 0.2-7.3), which is consistent with forearc serpentinites [18].

Protolith Changes and Element Mobility
Slight changes in the bulk major oxides are estimated from the (TiO 2 + MgO + Al 2 O 3 + Fe 2 O 3 + CaO + MnO + K 2 O + Na 2 O + P 2 O 5 )/SiO 2 ratios [18]. The calculated sum oxides/SiO 2 ratio (1.08-1.46) is similar to those of [54]. The investigated rocks represent some of the metamorphosed Egyptian ultramafic rocks with variable amounts of serpentine, tremolite, and talc, and fewer chlorite minerals. The presence of antigorite and talc suggest high temperature [55]. REEs and HFS elements are considered immobile, especially during alteration, in comparison with LFSEs-such as Cs, Rb, K, Sr, and U-even during low alteration [18]. Serpentinites represent as an alteration products of low temperature of ultramafic rocks, making some changes of the protolith chemical composition by water addition [3,18,56]. In addition, some major oxides-such as CaO and SiO 2 [49]-can be remobilized during serpentinization processes. According to [49], the completely serpentinite rocks are more depleted in CaO relative to partially serpentinized rocks. Some physical properties of the protolith-such as density and magnetism-are also changed by serpentinization processes. Protoliths have higher density and lower magnetism than serpentinites [56], due to magnetite formation by serpentinization, as represented by the following equation: There are two types of serpentinites based on the major serpentine minerals: (1) chrysotile is formed by retrograde metamorphism of primary minerals (olivine, orthopyroxene, and clinopyroxene) at low temperatures (<300 • C), and (2) antigorite serpentinites are formed by prograde metamorphism of lizardite and chrysotile at temperature ranges from 320 • C and 390 • C, and become stable above 390 • C [56]. Lizardite represents the most common serpentine mineral, whereas chrysotile is the least common [34]. They crystallize at identical temperature and pressure, but in different modes, where chrysotile precipitates a void space from a liquid state, whilst lizardite precipitates from a solid state on the surface of olivine [56]. Transition of lizardite or chrysotile to antigorite within water-saturated open systems takes place according to the following equation: Lizardite (Chrysotile) + SiO 2 (aq) → antigorite + H 2 O Major oxides (Al 2 O 3 ) and some trace elements-especially HFSEs (e.g., Zr, Th, Nb, Hf)-of serpentinites and serpentinized peridotites exhibit little change, tending to be immobile during the different metamorphic conditions, whilst LFSEs are expected to be mobile [13,57]. Fluid mobile elements (FMEs), encompasses LFSEs (Rb, Sr, Ba, U & Cs) and semi-volatile elements (e.g., Pb) are enriched in serpentinites relative to other of the same compatibility either serpentinites developed at supra-subduction or MOR [29]-due to high concentrations of these elements as a result of fluid-rock interaction leverage [17].
Serpentinization of mantle rocks is achieved via the formation of serpentine minerals and the addition of H 2 O at theoretical contents of up to 12.38wt.% ± 2.99 wt.% [18]. For this reason, the degree of serpentinization can be assessed by LOI. According to [57], samples with LOI of more than 6 wt.% are considered to be altered. LOI reached up to 11.5 wt.% in the studied serpentinites, and 10.7 wt.% in serpentinized peridotites, supporting the role of hydrothermal alteration. There was a varying degree of serpentinization processes in all samples, where LOI varied from 4.16 to 11.5 wt.%, reflecting moderately to completely serpentinized samples.

Partial Melting Processes
Whole-rock geochemistry and mineral chemistry of primary mantle minerals can be used to manifest different processes affecting the protolith, including partial melting and metasomatism [44,58]. It is noticeable that the examined rocks contain low concentrations of TiO 2 , ranging from 0.01 to 0.03 wt.%, suggesting a high degree of partial melting [59].
On the other hand, systematic changes in primary minerals (olivine, pyroxene, and spinel) can be used as a key to deduce historical processes such as metasomatism and melting degree [60]. It is noteworthy that Cr-spinels are more resistant primary minerals and can be used to infer the degree of partial melting [60]. The examined Cr-spinels of serpentinized peridotites have high Cr# (more than 6; av. 0.754), TiO 2 < 0.3, and Mg# (av. 0.42), reflecting a high degree of partial melting (>35%) (Figure 10a) that developed in a forearc setting [22,38,61,62]. In terms of Cr# vs. TiO 2 in Cr-spinel, they exhibit depleted mantle, and the examined sample was located in the fresh Cr-spinel of the Egyptian Eastern Desert and forearc fields (Figure 10b).
The Cr# content of both spinel types (especially Cr-rich) can be used to detect the degree of partial melting in the host rocks [62]. The degree of partial melting in spinel mantle can be calculated using the equation described in [60] (F = 10 × nCr# + 24), where F is the melting degree (wt.%). Based on this empirical equation, the examined rocks had experienced partial melting degrees ranging from 13.32 to 14.63% for Al-spinel, and from 17.34 to 24% for Cr-spinel, which is consistent with forearc affinity [61]. Moreover, the analyzed Al-spinels contained Al 2 O 3 ranges from 33.55 to 38.55 wt.%, and Cr# from 0.34 to 0.39, which may be related to decompression melting in the asthenosphere (Al content decreases with increases in the degree of partial melting) according to [17,38].
In  Figure 4a) content of the analyzed olivine relics, reflect depleted mantle that had been subjected to high partial melting in a forearc setting [38].

Protolith and Tectonic Setting
The presence of primary orthopyroxene, olivine, and spinel relics in the examined rocks, as well as enrichment of transition elements such as Ni (1265-2632 ppm) and Co (75-170 ppm), reflect their mantle origin. Bulk major contents of SiO 2 and Al 2 O 3 are immobile during alteration processes relative to CaO and LFSEs; therefore, they can be used as indices of mantle depletion [44,61]. The tectonic setting of the ultramafic rocks is still a subject of debate. Some of the earliest authors suggested a mid-ocean ridge origin for the Egyptian ophiolites, based on the geochemistry of their basaltic units [63,64]. Geochemical studies of the Egyptian ophiolites recognize a supra-subduction zone, which have a geochemical nature of spreading centers in back-arc basin [65] or fore-arc basin [8,22]. Al 2 O 3 concentration in the bulk-rock chemistry and mineral chemistry of spinel, olivine, and pyroxene can be used to determine typical tectonic settings via various discrimination diagrams [66].

Geothermometry
Different thermometer methods can be used to deduce the temperature of equilibrium of the examined serpentinized ultramafics (Supplementary Materials, Table S10). These methods include olivine-spinel thermometry using the calibrations described in [76,77], as well as Al in orthopyroxene-olivine-spinel thermometry [75]. The obtained average temperature of the studied serpentinized peridotites using the olivine-spinel thermometry [77] method was 781 °C, which is lower than the average temperature (852 °C) calculated using the calibration described in [76]. On the other hand, the average temperatures of the studied serpentinized peridotites calculated using the calibrations described in [75] (Al content in the orthopyroxene) was 859 °C, similar to the estimated average in [76]. In comparison with Pan-African forearc peridotites using the calibration described in [77], the estimated average temperature of the studied serpentinized peridotites (781 °C) is very similar to those found in Um Khariga (778 °C) [78]. In addition, the average temperature of the examined rocks obtained using Al in orthopyroxene thermometry (859 °C) is close to the average of Um Khariga peridotites (984 °C) using the same calibration.  [44,46,67], respectively. Mariana forearc and forearc are from [44,61], respectively. The thick yellow line represents a terrestrial array [71]. (b) Zr vs. Nb diagram from [68]; (c) Mg# versus Cr# binary diagram from [1]; partial melting degree is from [72]. (d) 100Cr# vs. TiO 2 of the examined spinels (Cr-and Al-spinel). Boninites, MORB, forearc peridotites, and depleted and highly depleted peridotites are obtained from [28,[73][74][75], respectively. The pink field represents fresh Cr-spinels of the Egyptian Eastern Desert [22].

Geothermometry
Different thermometer methods can be used to deduce the temperature of equilibrium of the examined serpentinized ultramafics (Supplementary Materials, Table S10). These methods include olivine-spinel thermometry using the calibrations described in [76,77], as well as Al in orthopyroxene-olivine-spinel thermometry [75]. The obtained average temperature of the studied serpentinized peridotites using the olivine-spinel thermometry [77] method was 781 • C, which is lower than the average temperature (852 • C) calculated using the calibration described in [76]. On the other hand, the average temperatures of the studied serpentinized peridotites calculated using the calibrations described in [75] (Al content in the orthopyroxene) was 859 • C, similar to the estimated average in [76]. In comparison with Pan-African forearc peridotites using the calibration described in [77], the estimated average temperature of the studied serpentinized peridotites (781 • C) is very similar to those found in Um Khariga (778 • C) [78]. In addition, the average temperature of the examined rocks obtained using Al in orthopyroxene thermometry (859 • C) is close to the average of Um Khariga peridotites (984 • C) using the same calibration.

Conclusions
Wadi Ibib-Wadi Shani (WI-WS) ultramafics represent relics of dismembered ophiolitic rocks in the southwestern extension of the largest Gerf ophiolitic nappe, South Eastern Desert, Egypt. Based on their petrographic description, the ultramafic rocks can be divided into serpentinites and serpentinized peridotites; the former consist mainly of serpentine minerals, whereas the latter comprise serpentine minerals with relics of primary minerals such as pyroxene, Cr-spinel, and olivine. These primary minerals are completely replaced by serpentine, magnetite, and carbonate minerals in serpentinites. The abundance of mesh and bastite textures reflects dunite and harzburgite protoliths. The composition of primary mantle minerals such as orthopyroxene (Mg# (Mg/Mg+Fe 2+ ), 89.65 to 93.18), clinopyroxene (depletion of Al2O3, Na2O, and Cr2O3), high Fo content of olivine, and high Cr# of Cr-spinel reflect a forearc setting. The calculated temperatures of the WI-WS serpentinized peridotites using olivine-spinel (av. 781 °C) and Al in orthopyroxene (av. 859 °C) thermometer calibrations are consistent with those of Egyptian forearc mantle rocks.

Supplementary Materials:
The following are available online at www.mdpi.com/xxx/s1: Table S1: Representative microprobe analysis of orthopyroxene in serpentinized peridotites; Table S2: Representative microprobe analysis of clinopyroxene in serpentinized peridotites; Table S3: Representative microprobe analysis of Cr-spinel in serpentinized peridotites; Table S4: Representative microprobe analysis of Al-spinel in serpentinized peridotites; Table S5: Representative microprobe analysis of serpentine; Table S6: Representative microprobe analysis of chlorite in serpentinized peridotites; Table S7: Representative microprobe analysis of tremolite in serpentinized peridotites; Table  S8: Representative microprobe analysis of magnetite; Table S9: Representative microprobe analysis of talc in serpentinized peridotites; Table S10: Average temperatures of serpentinized peridotites calculated using different calibrations.

Conclusions
Wadi Ibib-Wadi Shani (WI-WS) ultramafics represent relics of dismembered ophiolitic rocks in the southwestern extension of the largest Gerf ophiolitic nappe, South Eastern Desert, Egypt. Based on their petrographic description, the ultramafic rocks can be divided into serpentinites and serpentinized peridotites; the former consist mainly of serpentine minerals, whereas the latter comprise serpentine minerals with relics of primary minerals such as pyroxene, Cr-spinel, and olivine. These primary minerals are completely replaced by serpentine, magnetite, and carbonate minerals in serpentinites. The abundance of mesh and bastite textures reflects dunite and harzburgite protoliths. The composition of primary mantle minerals such as orthopyroxene (Mg# (Mg/Mg+Fe 2+ ), 89.65 to 93.18), clinopyroxene (depletion of Al 2 O 3 , Na 2 O, and Cr 2 O 3 ), high Fo content of olivine, and high Cr# of Cr-spinel reflect a forearc setting. The calculated temperatures of the WI-WS serpentinized peridotites using olivine-spinel (av. 781 • C) and Al in orthopyroxene (av. 859 • C) thermometer calibrations are consistent with those of Egyptian forearc mantle rocks.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.