Petrogenesis of Gerf Neoproterozoic carbonatized peridotites (Egypt): Evidence of convergent margin metasomatism of depleted sub-arc mantle

Gerf Neoproterozoic ophiolitic rocks in the Southern Eastern Desert of Egypt represent the largest ophiolite nappe in the Arabian-Nubian Shield and have been preserved as part of the N-S striking Allaqi-Heiani suture zone. Landsat-8 OLI/TIRS, ASTER and Sentinel-1B data successfully discriminate the Gerf ophiolitic section and the structural framework of the study area. This study has applied spectral transform approaches, consisting of principal component analysis (PCA), band ratio (BR) and minimum noise fraction (MNF) for lithological and structural mapping. NW-SE and N-S structural trends are dominant and control the distribution of talc‑carbonates and ophicarbonates. The mantle section comprises dominantly harzburgites with subordinate dunites and is rarely cut by dike-like bodies of clinopyroxenites; these peridotites have been partially to completely converted to serpentinites and related rocks. The primary Gerf peridotites are low in TiO2 (0.01 wt%), Al2O3 (0.50 wt%), and CaO (0.44 wt%) content on average, but are rich in Ni (up to 2758 ppm) and Cr (2906 ppm) relative to primitive mantle, suggesting their highly refractory residual nature after high degrees of partial melting, similar to forearc peridotites. This is confirmed by chemistry of their relic primary minerals, namely olivine (Fo: 91–93.7; NiO: 0.28–0.43 wt%), chromian spinel (Cr# 0.75 on average), orthopyroxene (Mg#: 0.92–0.93) and clinopyroxene (Mg#: 0.89–0.90). Gerf serpentinized peridotites also show ranges of oxygen fugacity (Δlog ƒO2, FMQ + 0.2 – FMQ + 1.4) and equilibrium temperature (770–900 °C), consistent with those of forearc peridotites. Bulk-rock analyses of ultramafic rocks and in-situ analyses of their pyroxenes reveal enrichment of fluid mobile elements (FME: e.g., B, Cs, Pb, Sr) relative to high-field strength elements (e.g., Nb, Zr, Ti, Ta), indicating intense metasomatism of Gerf peridotites by slab-derived fluids. The Gerf peridotites have been subjected to intense CO2 input from the subducted slab to form carbonate-rich rocks beneath the arc-forearc region. Carbonates (mainly magnesite) replace serpentine minerals and their formation synchronizes with the transition from lizardite to antigorite at temperatures, ~250° to ~350 °C. The CO2-rich fluids increase the Au content because of alteration and break down of Au-bearing sulphides, and this process formed Au mineralization in carbonate-rich rocks. The maximum amount of CO2 expulsion from the subducted slab increases with increasing mantle depth, and structural trends as ophicarbonates are abundant in thick parts of the ophiolite sequence in the Gerf area that was highly dissected by NW-SE, N-S and E-W striking faults. This confirms that structures control on distribution of the ophiocarbonate rocks in Gerf ophiolite. Calculated parental melts in equilibrium with Gerf peridotite spinels have boninitic affinities, suggesting their generation during the forearc stage, but parental melts of Gerf clinopyroxenite veins resemble N-MORB-like melts, indicating melt metasomatism of sub-arc mantle by impregnated mafic melts during early subduction initiation. Clinopyroxenite veins crystallized from MOR-like melts as a result of interaction between these melts and depleted peridotites in the sub-arc mantle.