Petrology and geochemistry of ophiolitic ultramafic rocks and chromitites across the Eastern Desert of Egypt: Insights into the composition and nature of a Neoproterozoic mantle and implication for the evolution of SSZ system

摘要

Large outcrops of ultramafic rocks are common in the Central Eastern Desert (CED) of Egypt and represent residual mantle sections and cumulates of dismembered Neoproterozoic oceanic lithosphere. This study integrates field observations, petrography, geochemistry and mineral chemistry to investigate ultramafic rocks of the dismembered ophiolitic rocks at four areas distributed from east to west across the CED, with the aim of gaining a better understanding the tectonic settings and mantle conditions under which the ultramafic rocks formed. The ultramafic masses studied are composed of serpentinized peridotites, serpentinites and pyroxenites, and occasionally host podiform chromitites. Preserved textures, relict primary minerals and whole-rock compositions indicate that the protoliths of these ultramafic rocks were dominated by harzburgites. The Al2O3- and CaO-depleted nature of residual harzburgites, the high Fo and NiO contents of primary olivines and the high Cr# and low TiO2 contents of fresh Cr-spinels suggest that these ultramafic rocks are remnants of depleted to highly depleted forearc mantle of suprasubduction zone (SSZ) system. Temperatures from olivine-spinet geothermometry (668-778 degrees C) and Al-in-orthopyroxene geothermometry (984 degrees C) estimated for residual harzburgites are comparable to those reported for modern forearc mantle peridotites. The range of oxygen fugacity [Delta logfO(2) (FMQ)] values (-3.01 to +0.32) calculated for the residual harzburgites reveals heterogeneity in the Neoproterozoic mantle oxidation state and suggests that SSZ mantle was not consistently more oxidized than MOR mantle. The podiform chromitites of different ultramafic masses were formed following the interaction of residual mantle peridotites with forearc basaltic and boninitic melts, implying the evolution of Neoproterozoic SSZ system from subduction initiation to more mature island arc stage. Integration of the analyses carried out in this study shows a Neoproterozoic SSZ system where slab-derived melts evolve in composition and oxidation state from early reduced forearc basalt in the east to more oxidised boninitic melts in the west.