A fore-arc setting of the Gerf ophiolite, Eastern Desert, Egypt: Evidence from mineral chemistry and geochemistry of ultramafites

• The Gerf ophiolite is the largest mantle-derived complex in the Arabian-Nubian Shield (ANS).
• The Gerf ophiolitic ultramafics comprise serpentinites, serpentinized peridotites and minor pyroxenites.
• They represent highly-depleted mantle residues after high degrees of melt extraction [high Cr# (> 0.84)].
• Gerf ophiolite suite developed in fore-arc setting in a supra-subduction zone (SSZ) environment.

The Gerf ophiolite is the largest mantle-derived complex in the Arabian–Nubian Shield (ANS). This ophiolitic complex extends for tens of kilometers in the south Eastern Desert (SED) of Egypt as part of the Allaqi–Heiani and Oneib-Sol Hamed suture zones. The ultramafic section of the Gerf ophiolite comprises serpentinites, serpentinized peridotites and minor pyroxenites. All rocks contain relics of original magmatic phases. The elevated Cr# (> 0.84) of Cr-spinels indicates that these rocks represent highly-depleted mantle residues after high degrees of melt extraction.Mineral and bulk-rock chemistry show that the Gerf ophiolite suite represents fragments of oceanic lithosphere that developed in fore-arc setting in a supra-subduction zone (SSZ) environment. The pyroxenites have a LREE-enriched pattern relative to the serpentinites while the serpentinized peridotites display depleted patterns [average (La/Yb)n = 0.56)]. Modeling of LREE suggests that the LREE-enriched pyroxenites and serpentinites could have been produced via contamination of their mantle source by crustal material and/or subduction-related slab fluids during the mantle evolution in a SSZ setting or soon after ophiolite assemblage obduction onto the continental crust. In contrast, the LREE-depleted serpentinized peridotites could have been generated through MORB melt/mantle rock reaction.