干旱区生态环境知识资源中心

Strongly deformed and locally migmatized gneisses occur at several places in the southern Eastern Desert of Egypt and in Sinai and have variously been interpreted as a basement to Pan-african (almost-equal-to 900 to 600 Ma) supracrustal and intrusive assemblages. A suite of grabbroic to granitic gneisses was investigated in the Hafafit area, which constitutes an I-type calc-alkaline intrusive assemblage whose chemistry suggests emplacement along an active continental margin and whose granitoid members can be correlated with the so-called ’Older Granites’ of Egypt. Pb-207/Pb-206 single zircon evaporation from three samples of the Hafafit gneisses yielded protolith emplacement ages between 677 +/- 9 and 700 +/- 12 Ma and document granitoid activity over a period of about 23 Ma. A migmatitic granitic gneiss from Wadi Bitan, south-west of Ras Banas, has a zircon age of 704 +/- 8 Ma, and its protolith was apparently generated during the same intrusive event as the granitoids at Hafafit. Single zircons from a dioritic gneiss from Wadi Feiran in south-west Sinai suggest emplacement of the protolith at 796 +/- 6 Ma and this is comparable with ages for granitoids in north-east Sinai and southern Israel. None of the above gneisses is derived from remelting of older continental crust, but they are interpreted as reflecting subduction-related calc-alkaline magmatism during early Pan-african magmatic arc formation.