U-PB (ZIRCON) AGES FOR THE GNEISSIC TERRANE WEST OF THE NILE, SOUTHERN EGYPT

The first U - Pb zircon ages are reported for the gneissic bedrock inl iers previously interpreted as part of the Nile Craton. The inliers cr op out in the Egyptian Western Desert, east of the Uweinat area and we st of the Eastern Desert. Multi- and single-grain zircon analyses of g ranitoid gneiss and migmatite from Gebel Um Shagir, Aswan, and another locality approximately 160 km south-west of Aswan, yield simple disco rdia with near modern day Pb loss trajectories, and the following Neop roterozoic crystallization ages: 626 + 4/-3, 634 +/- 4 and 741 +/- 3 M a. In contrast, multi- and single-grain U - Pb analyses (zircon and sp hene) from an anorthositic gabbro at Gebel Kamil (22-degrees 46'N 26-d egrees 21'E) and an anorthosite at Gebel El Asr (22-degrees 46'N 31-de grees 10'E) yield Archean and Paleoproterozoic emplacement ages. The f ormer yield a crystallization age of > 2.67 Ga and a metamorphic age o f almost-equal-to 2.0 Ga; the latter a metamorphic age of 0.69 Ga and an inheritance age of 1.9 - 2.1 Ga. Because high grade gneiss and migm atite of Neoproterozoic, Paleoproterozoic and Archean age crop out wes t of the Nile, pre-Neoproterozoic crust should no longer be identified by its metamorphic grade. By contrast, mapping the anorthosite and re lated rocks might provide first-order estimates for the extension of p re-Neoproterozoic crust in north-east Africa. It is suggested that Arc hean and Paleoproterozoic crust of the Uweinat and Congo Craton are co ntiguous because these U - Pb (zircon) data show no evidence for a Neo proterozoic thermal overprint in the Gebel Kamil area and there is no pronounced Neoproterozoic magmatic activity south of the Uweinat inlie r and north of the Congo Craton.