Age and geochemistry of late precambrian sediments of the hammamat series from the Northeastern desert of Egypt

Abstract

Late Precambrian crustal evolution in the Northeastern Desert of Egypt culminated in a major episode of strong extension, accompanied by bimodal igneous activity and deposition of clastic sediments in restricted terrigenous basins. These sediments, known as the Hammamat Series, record the uncovering of the evolving crust of the region. We have undertaken petrographic, geochemical, and Rb-Sr and K-Ar geochronologic studies of Hammamat samples collected along a single traverse. The lower half of a 400 m thick section consists of coarse detritus shed from nearby ensimatic terranes, while the upper half was derived from rapid reworking of bimodal igneous rocks similar to those of the Dokhan Volcanics and Pink Granites, units that are preserved within the region. Rb-Sr whole-rock analyses give an 8-point isochron age of 585 ± 15 Ma with an initial 87Sr/ 86Sr of 0.70323 ± 0.00013, interpreted to approximate closely the time of sedimentation. This is very similar to the K-Ar ages of the coarser clay fractions (0.5–2 μm) (567–588 Ma). This result is consistent with stratigraphic considerations and other geochronological studies indicating that Hammamat deposition occurred at ∼ 590 Ma. Rb-Sr analyses of leached and unleached size fractions indicate thermal resetting at 524 ± 17 Ma accompanied or followed by fluxing of a radiogenic Sr-rich pore fluid ( 87Sr/ 86Sr=0.7124). This is further indicated by K-Ar ages of the 0.2–0.5 μm size fraction of 532–542 Ma.