Origin and evolution of Neoproterozoic metaophiolitic mantle rocks from the eastern Desert of Egypt: Implications for tectonic and metamorphic events in the Arabian-Nubian Shield

Abstract

The mantle rocks from Kadaboura and Madara areas represent sections of dismembered ophiolitic complexes developed during the Neoproterozoic in the Eastern Desert of Egypt, which is located in the northwestern corner of the Arabian–Nubian Shield. The Kadaboura mantle rocks comprise serpentinites and serpentinized dunites, whereas those of the Madara consist of serpentinites and serpentinized pyroxenites.
Despite the serpentinization of the studied mantle rocks, few relicts of primary chromite, olivine and pyroxene are preserved. Chromite is partly altered having unaltered Al-rich chromite cores surrounded by Fe-rich chromite and Cr-rich magnetite rims. The unaltered Al-rich chromite cores show compositions equilibrated at temperatures mostly below ~500-600°C, which is a temperature comparable to that estimated for primary chromite in greenschist up to lower amphibolite facies rocks. The high Cr# [100×Cr/(Cr+Al)= 47-76] of the unaltered chromite cores and the Mg-rich nature of the olivine relicts (Fo91–94) indicate that the studied mantle rocks were produced from a highly depleted mantle that experienced high degrees of melt extraction (mostly ~30-40%). This range of melt extraction resembles that estimated for supra-subduction zone peridotites, but higher than that in abyssal and passive margin peridotites. Furthermore, the clinopyroxene relicts show compositions comparable to those from the Mariana forearc peridotites. Bulk-rock geochemistry also reflects derivation from an extremely depleted and a highly refractory mantle source. Modelling of rare-earth elements suggests that the studied mantle rocks were possibly formed by the interaction of their highly depleted harzburgitic mantle precursors with subduction-related melts/fluids during their evolution in a fore-arc basin of the supra-subduction zone.
The proposed geodynamic model suggests that the oceanic lithosphere generated during the seafloor spreading of the Mozambique Ocean was emplaced in the upper plate of the intra-oceanic subduction zone, in which the formely depleted Neoproterozoic mantle of the Arabian-Nubian Shield experienced mature phases of hydrous melting, extreme depletion and enrichment.