Geochemistry and tectonic evolution of the Neoproterozoic incipient arc–forearc crust in the Fawakhir area, Central Eastern Desert of Egypt

The geodynamic origin of the Neoproterozoic ophiolites of the Arabian-Nubian Shield exposed in the Eastern Desert of Egypt remains controversial. In this study, we present new geochemical and field data from the Fawakhir ophiolite and from some mélange blocks along the Qift-Qusier Road in order to constraint the tectonic evolution of this part of the Central Eastern Desert. The Fawakhir ophiolite contains most lithological units of a Penrose-type ophiolite sequence, and includes ultramafic rocks in the west overlain by isotropic gabbro, sheeted dikes and pillow basalt in the east. These ophiolitic units are enriched in LREE (light rare earth elements) and LILE (large ion lithophile elements) but depleted in high field strength elements (La/Smcn = 0.40–1.22, Th/Nbpm = 1.7–10.9, La/Nbpm = 1.4–6.6). Their magmas appear to have been derived from a depleted (N-MORB-like) mantle source, and their geochemical characteristics are comparable to those of the Izu-Bonin-Mariana forearc oceanic crust formed during the initiation of an intra-oceanic subduction zone. Pillow lava blocks in the eastern mélange have geochemical signatures similar to those of oceanic crust generated in back-arc basins. The decrease in the magnitude of mantle depletion and the change of the geochemical signature along the Qift-Qusier Road from a forearc in the west to a back-arc in the east suggest the formation of the Fawakhir intra-oceanic arc system over an east-dipping subduction zone. With continued subduction and arc migration, this intra-oceanic arc system finally collided with the passive margin of the West Gondwana (the Saharan craton), resulting in the accretion of the Fawakhir arc–forearc units. Following its tectonic accretion onto the West Gondwana continental margin, the Fawakhir ophiolite was intruded by calc-alkaline dikes, whose magmas were derived partly from partial melting of the sub-continental lithospheric mantle. These dikes have geochemical characteristics similar to those of modern active continental margin (Andean-type) rocks (La/Smcn = 2.13–2.48, Gd/Ybcn = 2.04–4.25, Th/Nbpm = 3.2–5.8, La/Nbpm = 2.5–4.9), suggesting that the West Gondwana passive margin (Atlantic-type) was converted to an Andean-type margin subsequent to the arc-continent collision. The inferred conversion of the Atlantic-type margin to an Andean-type margin resulted from the collision-induced reversal of the subduction direction.