Common Pb systematics of Precambrian granitic rocks of the Nubian Shield (Egypt) and tectonic implications

Abstract

We report the Pb isotopic compositions for alkali feldspars separated from 34 granitic rocks from the late Proterozoic assemblages of the Eastern Desert and the Sinai Peninsula (Egypt) in the Nubian segment of the Arabian-Nubian Shield. The Eastern Desert shows a range of initial Pb isotopic compositions ( 206 Pb/ 204 Pb = 17.375-19.176; 207 Pb/ 204 Pb = 15.462-15.629; 208 Pb/ 204 Pb = 37.023-38.349) that extend from model mantle toward upper-crustal values. Initial Pb isotopic compositions do not define geographic provinces, except at Aswan and surrounding areas, where the most radiogenic values were obtained (Aswan, 207 Pb/ 204 Pb = 15.611; Gebel El Hudi, 207 Pb/ 204 Pb = 15.629; and Wadi Mariya, 207 Pb/ 204 Pb = 15.615). Samples from these areas are separated from the less radiogenic samples to the east by trails of serpentinites that we interpret from Landsat thematic mapper images as the extension of the Allaqi-Heiani ophiolitic belt. Our data support previous interpretations of the belt as a suture location and suggest that its western margin might coincide with the boundary between the Nubian Shield and the old African continent. We interpret the Pb isotopic compositions from the Eastern Desert and the previously identified Group I and II Pb (Stacey and others, 1980) from the Arabian-Nubian Shield to indicate various degrees of mixing between mantle-derived, juvenile oceanic component(s) and pre-Pan-African crustal material. On 207 Pb/ 204 Pb- 208 Pb/ 204 Pb plots, the feldspar and galena data display variations characteristic of modern island-arc magmas, consistent with models that relate Shield assembly to accretion of island arcs. The crustal Pb isotopic signature of sample from Aswan and surrounding areas is best explained by interaction of arc magmas with an overriding crustal plate, whereas the less radiogenic nature of the Eastern Desert Pb9s is probably related to subduction of sediments derived from adjacent continent or deposition of these sediments in near-arc basins. Accounting for recycled pre-Pan-African crustal material in the Shield results in normal growth rates (0.44 km 3 /yr), using reasonable assumptions for initial Pb isotopic compositions and concentrations.