干旱区生态环境知识资源中心

A sample from near the base of the sedimentary Hammamat Group at Gebel Umm Tawat, North Eastern Desert, Egypt, contains detrital zircons with sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages as young as 585 +/- 13 Ma. This age is within error of recent SHRIMP dates from the adjacent Dokhan Volcanic Series and establishes that they contributed material to the sediments. It is also c. 45 Ma younger than any zircon analysed from the upper part of the sedimentary sequence, suggesting that the volcanic source was either totally eroded or lay buried beneath the evolving Hammamat Group. Zircon populations in both samples show age peaks at 640 and 680 Ma; the former not previously identified as the time of major igneous activity in the North Eastern Desert. Both samples also contain zircons with ages ranging between c. 750 and c. 2630 Ma, indicating contributions from Proterozoic and Archaean sources unknown in the North Eastern Desert. The source of these zircons may lie to the SE in the Central and South Eastern Desert, to the SW in the Nile Craton, where sparse zircons of this age have previously been recorded, or they may be derived from further afield in the Arabian-Nubian Shield, from earlier continental nuclei incorporated during crustal accretion. Alternatively, as sedimentary clasts in the Hammamat samples indicate erosion of earlier sedimentary rocks, also unknown in the area, these may have been the direct source of the older zircons. However, whichever interpretation is correct, it appears likely that the Hammamat Group was deposited in a major fluvial system of continental proportions and not in isolated basins as previously believed.