Neoproterozoic serpentinites from the Eastern Desert of Egypt: Insights into Neoproterozoic mantle geodynamics and processes beneath the Arabian-Nubian Shield

Abstract

The assembly and disruption of Gondwana supercontinent involve several complex processes that induce major changes in chemical structures of the Arabian-Nubian Shield mantle reservoir. These processes include melting, depletion and metasomatic refertilization of the upper mantle peridotites. The geodynamic evolution of these processes is addressed based on petrological, mineral chemistry and geochemical data from a selected set of ophiolitic serpentinized peridotites from the Eastern Desert of Egypt (G. El-Maiyit, Um El-Saneyat and W. Atalla). The examined rocks represent mantle residual harzburgites after different degrees of partial melting. The most intact Cr-spinel cores and whole-rock trace and rare earth elements data show that mantle protoliths of the studied peridotites underwent different partial melting degrees of ∼23% to 35% (G. El-Maiyit rocks), ∼15% to 20% (Um El-Saneyat rocks) and ∼18% to 25% (W. Atalla rocks). The variations of partial melting degrees between different areas suggested mantle heterogeneity beneath the Arabian-Nubian Shield. The melt extraction took place under oxidizing conditions. All the studied samples are enriched in light rare earth elements (LREEs) relative to the middle (MREEs) and the heavy (HREEs) which is attributed most probably to induction of serpentinization-related hydrothermal fluids. The data conclude that the studied peridotites have different tectonic settings, including the forearc for G. El-Maiyit, mid-ocean ridge for Um El-Saneyat and mid-ocean ridge-arc transition setting for W. Atalla. The coexistence of the mid-ocean ridge peridotites with those of the supra-subduction zone reflects the transitional evolutionary history of the Neoproterozoic Mozambican oceanic lithosphere beneath the Eastern Desert of Egypt. A similar geodynamic evolution is recognized for the Mesozoic ophiolitic mantle peridotites which concludes that the construction of the Neoproterozoic mantle in the Eastern Desert of Egypt is might be similar to those of the younger age.