COMMON PB SYSTEMATICS OF PRECAMBRIAN GRANITIC-ROCKS OF THE NUBIAN SHIELD (EGYPT) AND TECTONIC IMPLICATIONS

We report the Pb isotopic compositions for alkali feldspars separated from 34 granitic rocks from the late Proterozoic assemblages of the Ea stern 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 (Pb-206/Pb-204 = 17.375-19.176; Pb-207/Pb-20 4 = 15.462-15.629; Pb-208/Pb-204 = 37.023-38.349) that extend from mod el mantle toward upper-crustal values. Initial Pb isotopic composition s do not define geographic provinces, except at Aswan and surrounding areas, where the most radiogenic values were obtained (Aswan, Pb-207/P b-204 = 15.611; Gebel EI Hudi, Pb-207/Pb-204 = 15.629; and Wadi Mariya , Pb-207/Pb-204 = 15.615). Samples from these areas are separated from the less radiogenic samples to the east by trails of serpentinites th at we interpret from Landsat thematic mapper images as the extension o f the Allaqi-Heiani ophiolitic belt. Our data support previous interpr etations 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 P b (Stacey and others, 1980) from the Arabian-Nubian Shield to indicate various degrees of mixing between mantle-derived, juvenile oceanic co mponent(s) and pre-Pan-African crustal material. On Pb-207/Pb-204-Pb-2 08/Pb-204 plots, the feldspar and galena data display variations chara cteristic of modern island-arc magmas, consistent with models that rel ate Shield assembly to accretion of island arcs. The crustal Pb isotop ic signature of samples from Aswan and surrounding areas is best expla ined by interaction of arc magmas with an overriding crustal plate, wh ereas the less radiogenic nature of the Eastern Desert Pb's is probabl y related to subduction of sediments derived from adjacent continent o r deposition of these sediments in near-arc basins. Accounting for rec ycled pre-Pan-African crustal material in the Shield results in normal growth rates (0.44 km3/yr), using reasonable assumptions for initial Pb isotopic compositions and concentrations.