A late Precambrian 710 Ma) high volcanicity rift in the southern Eastern Desert of Egypt

The late Precambrian Shadli Metavolcanics of SE Egypt constitute a slightly metamorphosed bimodal sequence that has been previously interpreted as manifesting volcanic activity at an island arc. We report the first Rb-Sr geochronologic, trace element (including REE), and Nd isotopic data for these rocks. Two types of basalt are recognized, the stratigraphically lower suite having compositions like N-MORB ferrobasalt while the overlying basalt is similar to slightly fractionated E-MORB. The two basalt types were derived from melting of a strongly depleted source, most likely within the upper 60-75 km of the upper mantle. The origin of the felsic melts is problematic, and these could either have fractionated from a mafic melt or resulted from melting of juvenile crust. The mafic and felsic lavas yield a Rb-Sr isochron age of 712 ___ 24 Ma that probably represents the time of volcanic eruption. The trace element characteristics of both mafic and felsic members of the Shadli Metavolcanics show few of the hallmarks of subduction-related melts, and we reject the hypothesis that these formed at an island arc. Instead, the field and geochemical data are most consistent with the hypothesis that these rocks originated in a magmatic rift, where the eruption of large volumes of lava accompanied large-scale lithos*) Authors' addresses: R. J. STERN, The University of Texas at Dallas, Geosciences, E O. Box 830 688, Richardson, Texas 75083-0688, U.S.A, A. KRONER, Institut fiir Geowissenschafien, Universitfit Mainz, Postfach 3980, D-6500 Mainz, A. A. RASHWAN, Egyptian Geological Survey and Mining Authority, 3, Salah Salem St., Abbasiya, Cairo, Egypt. Manuscript received: 02. 10. 89; accepted: 08. 10. 90 156 R.J. STERN, A. KRONER & A. A. RASHWAN pheric extension. This inference suggests that the tectonic setting of the important 700-715 Ma crust-forming event in NE Africa and Arabia needs to be critically reexamined.