Ultramafic rocks in Gabal El-Rubshi, Central Eastern Desert, Egypt: petrography, mineral chemistry, and geochemistry constraints

Abstract

Petrographic, mineral chemistry, and geochemical studies of the ultramafic rocks from the El-Rubshi range, the main ophiolitic outcrop of the Neoproterozoic (Cryogenian) Complex in Central Eastern Desert of Egypt, provide new petrogenetic data evidence on the nature of the fluids that interacted with an original mantle peridotite. Certain primary features and chemical parameters are maintained even after their multistage histories of alteration, deformation, strain, and metamorphism. Chemo-metamorphic conditions and textures of the studied serpentinites and amphibolites show the principal events of recrystallization and metasomatism, but the diagnostic phases associated with the high pressure events related to subduction were erased. The occurrences of chrome spinels are relatively abundant in the oceanic domain ophiolites. The ophiolitic (serpentinite and amphibolite) rocks show extensive alteration, including hydration, carbonization, and serpentinization of the ultramafic rocks and amphibolization and chloritization of the basic rocks. Like arc-related rocks, the Ti contents of oceanic ophiolites are low in most samples but slightly higher in a few. Mantle heterogeneities may have caused the components with diverse geochemical signatures that are indicative of tholeiitic-komatiitic rocks, mid-ocean ridge basalts, and the enriched rocks of the oceanic islands or seamounts. Normalized trace element patterns of the studied serpentinite and amphibolite samples are characteristic of peridotites from both supra-subduction (SSZ) and mid-ocean ridge (MOR) zones. The pyroxenites are magmatic intruded within the El-Rubshi serpentinites and amphibolites, tholeiitic magma, and within-plate tholeiite environment.