Fragments of a Neoproterozoic ophiolite sequence in the North Eastern Desert of Egypt, Arabian-Nubian Shield

The Abu Samuki ultramafic rocks are the first unambiguous example of a fragmented Neoproterozoic ophiolite in the North Eastern Desert of Egypt. They consist of variably serpentinized peridotite, serpentinite, and pyroxenite. The geochemical and textural characteristics of the Abu Samuki mantle peridotite samples require both melting and refertilization processes to explain their origin. Their residual character after extraction of >20% melt is indicated by the mineral chemistry of olivine (Fo ~ 91; NiO up to 0.60 wt%), orthopyroxene, and Cr-spinel (Cr# up to 81, low TiO₂) and by numerous aspects of the whole-rock composition such as high Mg# (average 91), low normative clinopyroxene abundance (≤3%), and depleted incompatible trace element contents. Such strong depletion is characteristic of peridotites generated in the fore-arc region of a supra-subduction zone environment. Evidence of melt-rock interaction in the residual peridotite include growth of orthopyroxene at the expense of olivine, the coexistence of Al-rich and primary Cr-spinel, and the presence of phlogopite. Estimation of melt compositions coexisting with the fresh and metasomatized Cr-spinel, respectively, indicate that the primary partial melt was boninitic whereas the refertilizing melt was MORB-like. The section also includes mantle pyroxenite dikes and cumulate ultramafic rocks, both pyroxenite and a cumulate harzburgite group that differs from the Abu Samuki residual harzburgite in having lower whole-rock Mg# (~90), less refractory olivine (Fo ~ 86; average NiO 0.26 wt%), and high TiO₂ in Cr-spinel. Mineralogy, texture, and mineral chemistry suggest that the parental magma of the cumulate harzburgite was a hydrous boninite, plausibly the same as the average partial melt extracted during depletion of the residual peridotite samples. The variable degrees of partial melting and melt-rock interaction recorded by the mantle peridotite and distinctive cumulate ultramafic rocks at Abu Samuki provide a window to understand the heterogeneity in the Neoproterozoic oceanic lithosphere.