New SIMS zircon U-Pb ages and oxygen isotope data for ophiolite nappes in the Eastern Desert of Egypt: Implications for Gondwana assembly

Ophiolite nappes, encompassing dismembered oceanic lithosphere sections, in the Eastern Desert of Egypt are mainly confined to Tonian-Cryogenian inter-terrane sutures. A cohesive geodynamic model of these ophiolites remains elusive in light of the variably obliterated field relationships and lack of ample geochronological information. Available geochemical data of the well-studied ophiolites lead to different interpretations of mid-ocean ridge, back-arc and/or fore-arc affinities. Relevant geochronological data are likewise heterogeneous in terms of methods and the dated lithologic units. Here, we attempt to constrain the timing and genesis of some Eastern Desert ophiolites by using new SIMS zircon U-Pb and oxygen isotope data of gabbros from four different ophiolites. The new zircon U-Pb ages and oxygen isotope values integrated with available geochemical data constrain ages of 737 ± 6 Ma and 720 ± 6 Ma and fore-arc origin of the Wadi El-Sid and Gabal Abu Dahr-Abu Siayil ophiolites, respectively. The δ18O(Zrn) values of these fore-arc ophiolites (+2.5 to +8.5‰) deviate significantly from primitive mantle values, suggestive of substantial degrees of subduction-induced metasomatism. The Gabal El-Rubshi and Wadi Ghadir back-arc ophiolites returned younger and mutually similar ages (702 ± 5 Ma and 698 ± 4 Ma, respectively) and less heterogeneous δ18O(Zrn) values (+3.1 to +6.1‰). Excluding outliers, most measured δ18O(Zrn) values are indistinguishable from the unaltered MORB values and the restricted mantle-like δ18O range. Results of the present study suggest multiple arc-fore-arc accretion events and an extensive late Tonian back-arc basin opening event in the evolution of the Eastern Desert accretionary belt. Subduction tectonics continued to stabilize the immature arc crust ∼750–720 Ma and a tapered back-arc spreading ridge engendered at ∼700 Ma. The final terrane accretion and suturing could have been associated with subduction-related melts that circulated and locally metasomatized back-arc ophiolites. This model is in broad agreement with the recently advocated ∼720 Ma plate reorganization in the closure of the Mozambique Ocean during the course of Gondwana assembly.