Geotectonic significance of Neoproterozoic amphibolites from the Central Eastern Desert of Egypt: A possible dismembered sub-ophiolitic metamorphic sole

Abstract

Supra-subduction zone ophiolites in the Egyptian Central Eastern Desert (CED) occur as clusters in its northern (NCEDO) and southern (SCEDO) parts, displaying abundant island arc–boninitic and MORB/island-arc geochemical affinities, respectively. An amphibolite belt, including the investigated massive to slightly foliated Wadi Um Gheig (WUG) amphibolites, is exposed in the southeast most of the NCEDO thrusting over the El Sibai gneissic association and intruded by late- to post-orogenic granitoids and gabbros. The WUG rocks are metamorphosed under epidote amphibolite to common amphibolite facies. The amphiboles are calcic and represented by actinolitic hornblende to magnesio-hornblende in the epidote amphibolites and magnesio- to ferro-hornblende in the amphibolites. Plagioclase composition varies from pure albite (An3–8) in the epidote amphibolites to andesine and labradorite (An36–65) in the amphibolites. The estimated P–T conditions are in favor of their metamorphism under epidote amphibolite (c. 550–600°C and 2–3±1.5kbar) and amphibolite (c. 618–720°C and 3–6±1.5kbar) facies. The peak metamorphic conditions point to a burial depth of c.15–20km.Geochemically, the WUG amphibolites show basaltic to andesitic compositions of tholeiitic affinity. They display LILE-enriched MORB-normalized patterns with negative Nb anomalies characteristic of the subduction-related rocks. However, their chondrite-normalized rare-earth element (REE) patterns vary from LREE-depleted (LaN/YbN=0.29 to 0.49) to LREE-enriched (LaN/YbN=2.97 to 3.74). Few samples show major and trace element contents typical of boninitic rocks, including U-shaped REE pattern. On the standard tectonic discrimination diagrams the WUG amphibolites plot mostly in the island-arc fields with some samples of MORB and boninitic affinities. Greenschist facies metamorphosed NCEDO obviously share these geochemical characteristics, implying formation in the same tectonic environment, i.e. forearc basin. This argues that the WUG amphibolites likely represent remnants of a dismembered metamorphic sole beneath the NCEDO. Their formation possibly involves intra-forearc basin thrusting followed by emplacement of ophiolite as imbricated stack of dismembered thrust slices in an accretionary wedge setting. This revives interests in geotectonic model in which the CED represents a forearc–arc–back-arc system above a southeast-dipping subduction zone.