Abstract

Two Late Proterozoic granitic bodies from the Eastern Desert of Egypt, the ca. 578 Ma Nakhil and the ca. 595 Ma Aswan granites, provide insights into processes of crust formation in the Arabian-Nubian shield. Evidence for involvement of an older crustal component in the formation of the Nakhil granite includes (1) U/Pb zircon data that establish a crystallization age of 578 ±15 Ma and indicate the presence of inherited zircons possibly as old as 1.6 Ga; (2) an elevated model initial 87Sr/86Sr (0.7136); and (3) an elevated initial 207Pb/204Pb (15.561) relative to model mantle compositions at 578 Ma. Evidence for involvement of an older crustal component in the Aswan granite comes from the elevated initial 207Pb/204Pb (15.611). In contrast, extensive crustal contamination is not reflected in the high initial ϵNd(+5.7) for the Nakhil and the low initial 87Sr/86Sr (0.7029) for the Aswan granite. The contrasting inferences from the different isotopic systems can be explained by the high whole-rock Nd and Sr concentration for the Nakhil (87 ppm Nd) and the Aswan (173 ppm Sr) granites, respectively, that suggest that the Nd and Sr isotopic composition of the older component has been overshadowed by the more primitive material. Similar contrasts in Ph, Sr, and Nd isotopic data from the eastern and western shield margins can be interpreted in the same manner and might suggest widespread involvement of older crustal components in the formation of the Late Proterozoic Arabian-Nubian shield.

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