U-Pb zircon geochronology and Hf-Nd isotopic systematics of Wadi Beitan granitoid gneisses, South Eastern Desert, Egypt

Migmatitic granitoid gneisses are widespread in the southern Eastern Desert of Egypt, but their formation ages are poorly understood. They consist of granitoid gneiss ranging in composition from tonalite to granodiorite, with a distinct calc-alkaline chemical character. Zircons from three migmatitic gneiss samples from Wadi Beitan were dated on SHRIMP II and yielded magmatic emplacement ages of 719. ±. 10, 725. ±. 9 and 744. ±. 10. Ma, respectively, indicating that the gneiss protoliths are Neoproterozoic. The zircons yielded εHf(t) values of -. 4.8 to +. 12.5 and corresponding Hf crustal model ages ranging from 824 to 1753. Ma. These data indicate the involvement of both juvenile and older continental crust in protolith formation. Positive whole-rock εNd(t) values (+. 5.1 to +. 6.6) and corresponding Nd model ages of 690 to 830. Ma suggest a relatively young, juvenile Neoproterozoic crustal source for the Wadi Beitan granitic gneisses. However, a εNd(t) value of +. 5.1 (sample WB-23) is less than predicted for a depleted mantle source at ~. 700. Ma (εNd of about +. 6.5), perhaps indicating that there was minor contribution from old (pre-Neoproterozoic) crust. The chemical data and significant variations in both isotopic systems argue for source heterogeneity and may suggest that the Wadi Beitan granitoids formed along an active continental margin.