Mineralogy, geochemistry, and geotectonic significance of the Neoproterozoic ophiolite of Wadi Arais area, south Eastern Desert, Egypt

摘要

The dismembered ophiolites in Wadi Arais area of the south Eastern Desert of Egypt are one of a series of Neoproterozoic ophiolites found within the Arabian-Nubian Shield (ANS). We present new major, trace, and rare earth element analyses and mineral composition data from samples of the Wadi Arais ophiolitic rocks with the goal of constraining their geotectonic setting. The suite includes serpentinized ultramafics (mantle section) and greenschist facies metagabbros (crustal section). The major and trace element characteristics of the metagabbro unit show a tholeiitic to calc-alkaline affinity. The serpentinized ultramafics display a bastite, or less commonly mesh, texture of serpentine minerals reflecting harzburgite and dunite protoliths, and unaltered relics of olivine, orthopyroxene, clinopyroxene, and chrome spinel can be found. Bulk-rock chemistry confirms harzburgite as the main protolith. The high Mg# (91.93-93.15) and low Al2O3/SiO2 ratios (0.01-0.02) of the serpentinized peridotite, together with the high Cr# (>0.6) of their Cr-spinels and the high NiO contents (0.39-0.49wt.%) of their olivines, are consistent with residual mantle rocks that experienced high degrees of partial melt extraction. The high Cr# and low TiO2 contents (0.02-0.34wt.%) of the Cr-spinels are most consistent with modern highly refractory fore-arc peridtotites and suggest that these rocks probably developed in a supra-subduction zone environment.