A Pan African origin and uplift for the gneisses and peridotites of Zabargad Island, Red Sea: a Nd, Sr, Pb, and Os isotope study

A Sr, Nd, Pb, and Os isotopic study of peridotites andgranulite-facies gneisses from Zabargad Island in the Red Sea suggests that thetectonothermal, petrogenetic, and geochemical evolution of these rocks occurredlargely during the Pan African Orogeny rather than the recent opening of theRed Sea. Sm-Nd model ages and whole rock errorchrons indicate that spinelIherzolites and gneisses differentiated from a common depleted mantle sourceabout 700 Ma. The Iherzolites were mylonitized, metasomatized, andamphibolitized during a structural event that juxtaposed the peridotites withthe gneiss complex and uplifted the gneiss/peridotite complex to relativelyshallow crustal levels. Most radiometric dating schemes suggest a Pan Africanage for this event. The gneisses generally have lower ¹⁴³Nd/¹⁴⁴Nd, ⁸⁷Sr/⁸⁶Sr, ²⁰⁸Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb,and ²⁰⁶Pb/²⁰⁴Pb ratios than the peridotites. Theyextend linear trends defined by the spinel and amphibole peridotites on Sr-Nd,Sm-Nd, and Pb-Pb diagrams, suggesting the gneisses were either the source orbuffering medium for the Pan African metasomatism. Only one post-Pan Africanevent had a significant effect on the geochemistry of the gneiss/peridotitecomplex: shallow level metasomatism by ultrahot (750–900°C) hypersalinesolutions with high ⁸⁷Sr/⁸⁶Sr (≈0.710) ratio led tothe development of gem-quality olivine crystals as well as low-pressure mineralassemblages in the peridotites, gneisses and younger rocks. Plagioclase-richassemblages with apparent igneous textures (“troctolites”) that are most commonin the southern peridotite body may have formed by interaction of these fluidswith peridotite (i.e., are “pseudo-troctolites”). Metasomatism changedthe ⁸⁷Sr/⁸⁶Sr, Sm/Nd, and Re/Os ratios of theplagioclase peridotites making them unsuitable representatives of the PanAfrican mantle.