AGE AND TECTONIC SETTING OF GRANITOID GNEISSES IN THE EASTERN DESERT OF EGYPT AND SOUTH-WEST SINAI

Strongly deformed and locally migmatized gneisses occur at several pla ces in the southern Eastern Desert of Egypt and in Sinai and have vari ously been interpreted as a basement to Pan-african (almost-equal-to 9 00 to 600 Ma) supracrustal and intrusive assemblages. A suite of grabb roic to granitic gneisses was investigated in the Hafafit area, which constitutes an I-type calc-alkaline intrusive assemblage whose chemist ry suggests emplacement along an active continental margin and whose g ranitoid 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-w est of Ras Banas, has a zircon age of 704 +/- 8 Ma, and its protolith was apparently generated during the same intrusive event as the granit oids at Hafafit. Single zircons from a dioritic gneiss from Wadi Feira n in south-west Sinai suggest emplacement of the protolith at 796 +/- 6 Ma and this is comparable with ages for granitoids in north-east Sin ai and southern Israel. None of the above gneisses is derived from rem elting of older continental crust, but they are interpreted as reflect ing subduction-related calc-alkaline magmatism during early Pan-africa n magmatic arc formation.