Sr–Nd isotopes and geochemistry of granite-gneiss complexes from the Meatiq and Hafafit domes, Eastern Desert, Egypt: No evidence for pre-Neoproterozoic crust

Neoproterozoic gneisses at Meatiq and Hafafit in the Eastern Desert of Egypt give Rb–Sr and U–Pb zircon ages of 600–750 Ma. These gneisses are interpreted by different workers to represent deeper levels of juvenile Neoproterozoic crust or Archaean/Palaeoproterozoic crust that was remobilized during Neoproterozoic time. Geochemical and Sr–Nd isotope compositions for these gneisses reported here are remarkably homogeneous: Initial 87Sr/86Sr (0.70252 ± 0.00056) and εNd (+6.4 ± 1.0). These values are best explained as reflecting derivation from depleted asthenospheric mantle sources during Neoproterozoic time, consistent with mean Nd model ages of 0.70 ± 0.06 Ga. The increasing recognition of old, xenocrystic zircons in juvenile ANS igneous rocks can be explained in several different ways. The participation of ancient crust is allowed as one of the explanations, but it is the isotopic composition of radiogenic elements such as Sr and Nd for whole-rock specimens that are the most reliable indicators of whether or not a given crustal tract is juvenile or reworked older crust. These isotopic data indicate that the protolith for the Meatiq and Hafafit gneisses were juvenile Neoproterozoic igneous rocks and sediments derived from them. There is no support in the isotopic data for any significant contribution of pre-Neoproterozoic crust in these two sections of Eastern Desert crustal infrastructure.