Genesis and Tectonic Implications of the Kabr El-Bonaya Ultramafic Rocks, Sinai Peninsula, Egypt: Constraints from Mineralogical and Geochemical Characteristics

The Kabr El-Bonaya mafic-ultramafic intrusion is exposed along the southeastern border of the Sinai Peninsula and the northernmost segment of the Arabian-Nubian Shield (ANS). It occurs as an elliptical intrusive body that is located along the major NE-SW trending fracture zones that prevail in the Kid metamorphic complex. The ultramafic rocks in the complex comprise ultramafic cumulates of peridotites (dunite, harzburgite and wehrlite) and pyroxenite. These rocks are generally unmetamorphosed and have intrusive contacts with the country rock. Mineral chemistry and whole-rock chemical compositions of these ultramafic rocks are mostly consistent with those of residual mantle peridotites from refractory supra-subduction tectonic settings. Based on the variations of the major elements, the studied ultramafic rocks are consistent with those of a supra-subduction zone mantle, as it seems to have melted at 1-2 GPa and 1300-1450 degrees C. Linear variations of Al2O3, CaO, V and Ni with MgO, coupled with incompatible and rare-earth-element depletion and mineral compositions, suggest prior events of partial melting in both wehrlites and harzburgites. The LREE enrichment in the harzburgite, as well as the development of Cr-rich spinel, is consistent with a history of melt-peridotite interaction. The calculated (Sm/Yb)(N) variations for the studied peridotites indicate a general increase in the addition of fluids with an increasing degree of melting from the wehrlite (similar to 13-15 wt% of fluid) in the source, after initial spinel peridotite melting to the harzburgite (similar to 20-25 wt% of fluid) in the same source, which is contrary to normal abyssal peridotites. The estimated equilibration temperature ranges from 1214 to 1321 degrees C for the studied wehrlites and from 1297 to 1374 degrees C for harzburgites. The Mg-rich nature of the analysed olivines from the studied ultramafic rocks (Fo = 81.41 to 91.77) reflect their primary composition and are similar to olivines in Alaskan-type ultramafic rocks. The Fo content of the analyzed olivines decrease slightly from the dunite to the harzburgite to the wehrlite and to pyroxenite, reflecting a fractional crystallization trend. The high Cr-# and very low TiO2 contents (0.03-0.12 wt%) of the Cr-spinels from the studied peridotites are mostly consistent with modern highly refractory fore-arc peridotites, indicating that these peridotites developed in a supra-subduction zone environment.