Petrology and Geochemistry of Ophiolitic Pyroxenite in the Eastern Desert of Egypt: Genesis of Ultramafic Cumulates and Implications for Neoproterozoic Supra-Subduction Seafloor Metamorphism

We describe and compare two outcrops of pyroxenite associated with Neoproterozoic ophiolite sequences in the Eastern Desert of Egypt: small masses in the Abu Samuki area of the North Eastern Desert and large belts in the Wadi El-Mireiwa area of the South Eastern Desert. This study presents field observations, petrographic descriptions, and data on mineral and whole-rock compositions as a basis for investigating the tectonic setting, nature, origin, and alteration history of this pyroxenite. Both pyroxenite bodies represent cumulates from the crustal sections of fragmented ophiolites, emplaced by thrusting above metasedimentary rocks and island arc assemblages, and later intruded by granite. Samples from both localities are mainly olivine clinopyroxenite. The compositions of olivine (Fo ∼ 79.7 and 0.13–0.49 wt% NiO), clinopyroxene, and fresh Cr-spinel cores, as well as the low whole rock Mg# (average 87.6 at Abu Samuki and 85.7 at Wadi El-Mireiwa) are all consistent with a cumulate origin for both pyroxenite suites. Moreover, the geochemical data all indicate that both pyroxenite suites are derived from fragments of oceanic lithosphere that developed in a fore-arc supra-subduction zone environment. The differences in mineral chemistry between the two locations suggest that the parental magma at Abu Samuki was low-Ti fore-arc basalt, while that at Wadi El-Mireiwa was a high-Ca boninite. The existence of green spinel, zoisite, Al-amphibole, pumpellyite and Mg-chlorite as secondary minerals in pyroxenite at Abu Samuki can be attributed to alteration and metasomatism by Al-Mg-Ca-bearing hydrothermal fluids. The preservation of deformation features in clinopyroxene and the absence of K-bearing secondary minerals suggest that this alteration was associated with circulation of seawater in fractured oceanic crust during the pre-obduction stage of ophiolite emplacement and not with the post-obduction granitoid intrusion.