Mineralogy and geochemistry of ophiolitic mafic-ultramafic rocks in the Southern Eastern Desert of Egypt: Constraints on low-T metasomatism by CO₂-rich fluids

The Neoproterozoic mafic–ultramafic rocks of G. Muqsim, G. Balamhindit, and G. Abu Dahr constitute well-preserved fragments of dismembered ophiolitic sequences in the Southern Eastern Desert (SED) of Egypt as part of the Arabian–Nubian Shield (ANS). This study integrates new mineralogical, whole-rock, and in-situ geochemical data to constrain low- T CO₂-rich metasomatism and its imprints during serpentinization and carbonation. The ultramafic assemblage comprises serpentinized and metasomatized peridotites, serpentinites hosting chromitite lenses, and ophiocarbonates after harzburgite protoliths, whereas the mafic assemblage is represented mainly by ophiolite metagabbros. Despite pervasive metasomatic overprinting, the immobile elements of ultramafic rocks, integrated with compositions of primary olivine (Fo_91–93), orthopyroxene (Opx Mg#: 91–94), clinopyroxene (Cpx), and Cr-spinel (Cr#: 0.6–0.8), suggest generation of their peridotite protoliths in a forearc setting. For both chromitites and their host peridotites, equilibrium melts calculated from primary Cr-spinel compositions display MORB-like affinities for G. Abu Dahr and boninitic affinities for G. Balamhindit and G. Muqsim ultramafic rocks, suggesting their formation at different stages during forearc evolution. Low- T CO₂-rich metasomatism is evidenced by hydrous inclusions in Cr-spinel and the occurrence of thin vein networks of carbonate, talc‑carbonate matrix, amphibole, and malachite, together with extensive serpentinization and carbonation of the ultramafic rocks. In contrast, clinopyroxenite veinlets within the G. Abu Dahr chromitites record localized melt-related metasomatism associated with melt impregnation. The metasomatic overprint is most pronounced in the G. Muqsim harzburgites, which are characterized by elevated U/Th ratios (0.43–0.6), enrichment in LREE and fluid-mobile elements (FME: Li, Rb, Pb, Sr, U, and Th). This is also evidenced by the high occurrence of carbonate matrix and low values of olivine Fo (Fo_86–89), Ni and Mn, and low Opx Mg# (0.87–0.89), Ca and Cr, but high spinel Ti (TiO₂: 1.11 wt%). In-situ analyses of G. Muqsim metasomatized olivine and Opx reveal enrichment in LREE and several FME relative to their primary counterparts. The investigated ultramafic rocks display U-shaped REE patterns, high Ba/La ratios (up to 104), and low Th/Nd ratios (0.3–1.2), whereas the associated metagabbros and their constituent amphibole and plagioclase are enriched in LILE (e.g., Li, Cs, Rb, Ba, Pb, and Sr) and LREE relative to HFSE (e.g., Nb, Ta, Zr, and Hf) and HREE, respectively. Collectively, the investigated ophiolitic rocks and the associated chromitites record low- T CO₂-bearing slab-derived fluid metasomatism during forearc evolution, with localized melt-related metasomatism.