The Neoproterozoic ophiolitic ultramafic rocks in Eastern Desert of Egypt: implications for petrogenesis and metasomatic processes

Abstract

ABSTRACT This paper deals with the mineral chemistry and geochemistry of ophiolitic peridotites from three localities in the Eastern Desert of Egypt, namely; Wadi Muweilha, Wadi Arayis and Gabal Kurbiyay. The present peridotites show variable degrees of alteration, including; serpentinized peridotites, serpentinites and talc-carbonate rocks with small-scale outcrops of chromitites, serpentinized pyroxenites and chloritites. Cr-spinel together with olivine and pyroxene relics represent the most common primary phases in the studied rock types. The investigated rocks represent mantle residues that experienced high degrees of partial melting. The Cr numbers of Cr-spinels reveal that serpentinized pyroxenites exhibit lower degrees of partial melting (~35%) than serpentinites and chromitites (> 35%). The whole-rock trace and rare earth elements (REE) data together with the elevated Cr# of Cr-spinels (>0.6), high Mg# of olivines (Fo = 90.82–92.61) and depleted composition of clinopyroxene are consistent with a fore-arc supra-subduction zone environment. The enrichment of light rare earth elements (LREEs) relative to heavy ones (HREE) in samples of W. Muweilha and W. Arayis could be attributed to the induction of serpentinization-related hydrothermal fluids. In contrast, the studied samples of G. Kurbiyay are depleted in LREE compared with HREE in common with a previously formed abyssal peridotite that later modified in a fore-arc region. The serpentinized pyroxenites and chloritites are speculated a genetic relationship with the rest of the mantle suite. The former is produced by the assimilation of subducted sediments in arc-related magmas. The latter is considered to be products of retrograde metamorphism simultaneously with the obduction and thrusting of the ophiolitic ultramafics. The investigated Cr-rich chromitites were formed from boninitic melts resulting from the high-degree melting of the sub-arc depleted mantle in the presence of slab-derived fluids at a mature-arc stage.