Growth of the Egyptian Crust in the Northern East African Orogen: a Review of Existing Models and Proposed Modifi Cations
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Growth of the Egyptian crust in the northern East African Orogen 317 N. Jb. Miner. Abh. 2007, Vol. 183/3, p. 317–341, Stuttgart, April 2007, published online 2007 © by E. Schweizerbart’sche Verlagsbuchhandlung 2007 Growth of the Egyptian crust in the northern East African Orogen: A review of existing models and proposed modifi cations Mohamed M. El-Sayed, Alexandria, Harald Furnes, Bergen, and Abou Shagar, S., Alexandria With 10 fi gures Abstract: The crystalline basement complex of Egypt in NE Africa is exposed extensively in the Eastern Desert and Sinai as well as in smaller, isolated exposures in the southern part of the Western Desert extending west of the River Nile to the Uweinat area. The Egyptian crust can be divided into the Eastern Desert terrain which is a part of the Nubian Shield and the Western Desert terrain which is a part of the Saharan metacraton. The two terrains are lithologically, structurally and geochronologically different. The rock assemblages of the Western Desert terrain were produced predominantly by remelting of the older, pre-Pan-African crust, while those of the Eastern Desert terrain exhibit juvenile character. However, the basement complex in the southern Western Desert was strongly reworked by Neoproterozoic thermal and deformational events. The evolution of the Egyptian crust within the eastern Desert can be classifi ed into two main regimes, i.e. an older (780– 620 Ma) compressional regime, and a younger (620–450 Ma) compressional/transpressional to tensional regime. During the compressional regime, initial island-arc build-up (780–730 Ma) and arc-splitting and back-arc spreading (730–620 Ma) stages were represented. The fi rst stage is characterised by generation of the intrusive metagabbro-diorite complex, intrud- ing continental shelf facies metasediments, and the early Pan-African Shadli metavolcanics. This stage is followed by arc- splitting and back-arc spreading producing the ophiolite sequences and older granitoids. The fi nal compressional/transpres- sional to tensional regime (620–450 Ma) was characterised by the evolution of late Neoproterozoic post-orogenic A-type younger granites, the Dokhan volcanics, the molasse-type sediments and exhumation of the metamorphic core complex- es. Emplacement of the ring complexes in Egypt is mainly controlled by faults and shear zones and took place during an ex- tended period of time between closing of island arcs at the end of the Pan-African orogeny and opening of the Red Sea Rift. Key words: Egypt, Eastern Desert terrain, Western Desert terrain, geotectonic model. Introduction proterozoic juvenile crust of the Arabian-Nubian Shield (ANS) to the east and the Archean continental crust of the Much of the African crust is made up of Archean Congo craton in the south. The region of the Saharan met- cratons, such as the Kalahari craton, the Congo craton, acraton constitutes pre-Neoproterozoic continental crust and the West African craton (Fig. 1, inset) surrounded overprinted by Neoproterozoic tectonic events as well by younger mobile belts. These blocks were unaffected as containing Neoproterozoic juvenile material (Abdel- or only little affected by the Neoproterozoic Pan-African salam et al. 2002). orogenic cycle (900–450 Ma; Kröner 1984). On the oth- Recent geochronologic data indicate the presence er hand, the Saharan metacraton is geologically complex of two major Pan-African tectonic events in East Africa and was severely affected by the Pan-African orogenic namely the East African Orogeny (800–650 Ma; Stern cycle (Abdelsalam et al. 2002). Geochronological and 1994) and Kuunga Orogeny (550 Ma; Meert & van der isotopic data (Stern 1994, Liégeois et al. 1994, Küster Voo 1996) which represents the fi nal suturing of the Aus- & Liégeois 2001) indicate that the crust of the Saharan tralian and Antarctic segments of the Gondwana continent. metacraton is heterogeneous and contrasts with the Neo- The Arabian-Nubian Shield forms the suture between East DOI: 10.1127/0077-7757/2007/0078 0077-7757/07/0078 $ 6.25 © 2007 E. Schweizerbartsche Verlagsbuchhandlung, D-70176 Stuttgart.