## 6 Discussion
### 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 SiO2 and Al2O3 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]. Harzburgite-dunite complex protolithts are indicated by bastite and mesh textures, in addition to high contents of compatible elements such as Ni and Co. Ultramafics of supra-subduction zone affinity (forearc) are characterized by low TiO\({}_{2}\) contents (<0.05 wt.%), MgO/SiO\({}_{2}\) ratios (<1.4), Al\({}_{2}\)O\({}_{3}\) contents (<0.78 wt.%), and Al\({}_{2}\)O\({}_{3}\)/SiO\({}_{2}\) ratios (<0.02), according to [17; 49]. The Al\({}_{2}\)O\({}_{3}\)/SiO\({}_{2}\) ratios of the examined rocks show very narrow variance, ranging from 0.004 to 0.04, with an average of 0.014--lower than that of PM composition (~0.1) [67], but close to that of depleted mantle harzburgite (~0.02) [63]--suggesting that they underwent partial melting before serpentination processes [17]. Conversely, the average of MgO/SiO\({}_{2}\) ratio of these rocks ranged from 0.81-1.14, with an average of 0.99--higher than that of PM (~0.85). Al\({}_{2}\)O\({}_{3}\)/SiO\({}_{2}\) and MgO/SiO\({}_{2}\) ratios were not affected during hydration processes or peridotite metamorphism [46]. The studied samples lie above the trend of mantle terrestrial arrays and abyssal peridotites straddling the forearc field (Figure 10a). In addition, they have Al\({}_{2}\)O\({}_{3}\)/SiO\({}_{2}\) (0.004-0.04) and MgO/SiO\({}_{2}\) (av. 0.99) ratios similar to those of forearc peridotite [61]. Among the HFSEs, the studied peridotite are variably depleted in Nb, consistent with an SSZ geochemical signature [52]. According to the Zr vs. Nb binary diagram in [68], the examined rocks plot among depleted mantle sources (Figure 10b). On the other hand, the mineral chemistry of Cr-spinel can be suggestive of a forearc setting. The ternary Cr-Al-Fe diagram [18] (Figure 5d), Mg# vs. Cr# [1] (Figure 10c), and Cr# vs. TiO\({}_{2}\)[69] (Figure 10d) reveal a forearc signature. In addition, there is a positive relationship between the Cr# content of spinels and the partial degree of melting or melt/rock reaction, where those with Cr# > 6 are formed in supra-subduction (mostly forearc) environments with a high degree of partial melting, whilst those with Cr# < 6, with tholeiititic magma, are formed in abyssal zones such as mid-ocean ridges (MORs) [66; 70]. The average Cr# of the examined Cr-spinels was more than 0.6 (av. 0.689), which is consistent with those of modern forearc serpentinites [22]. Furthermore, high Fo content (av. 95.23 mol%) also reflects their forearc origins [22] (Figure 4a). The clinopyroxene compositions of the examined ultramafic rocks plot in the field characteristic of forearc settings, due to depletion of Al\({}_{2}\)O\({}_{3}\), Na\({}_{2}\)O, and Cr\({}_{2}\)O\({}_{3}\)[70] (Figure 4c,d). Likewise, orthopyroxenes are moderate in Al\({}_{2}\)O\({}_{3}\) (<3.23 wt.%) and low in Na\({}_{2}\)O (<0.04 wt.%), TiO\({}_{2}\) (<0.04 wt.%), and Cr\({}_{2}\)O\({}_{3}\) (<0.92 wt.%), consistent with forearc peridotite orthopyroxenes [24; 28] (Figure 4e).