Geochemical constraints on the tectonomagmatic evolution of the late Precambrian Fawakhir ophiolite, Central Eastern Desert, Egypt

-The Fawakhir ophiolitic sequence is composed mainly of serpentinites, metagabbros, boninitic metagabbros and metavolcanics. The serpentinites have geochemical features similar to those of depleted-mantle peridotites. Major and trace element characteristics show that the ophiolitic metagabbro and metavolcanic rocks are of tholeiitic to calc-alkaline affinity. REE abundances in the metagabbros and metavolcanics are characterised by flat to fractionated patterns. The Fawakhir ophiolitic sequence shows a spectrum of compositions ranging from arc-type lavas, P-type MORB and boninitic rocks, suggesting that these rock units developed in a back-arc environment. The evolution of the Fawakhir ophiolite sequence from early tholeiites to later boninitictype magmas may suggests that these rocks could be generated by fractional fusion and melt extraction during ascent of the magma source within a mantle melting column. The boninites were generated from a more refractory mantle source relative to the other rocks. © 1999 Elsevier Science Limited. All rights reserved. RI~SUMI ~ La s6quence ophiolitique de Fawakhir se compose surtout de serpentinites, m~tagabbros, m~tagabbros boninitiques et m6tavolcanites. Les serpentinites ont des caract~,res chimiques semblables aux p6ridotites de manteau appauvri. Les caract~ristiques des dl6ments majeurs et en traces montrent que le m~tagabbro ophiolitique et les roches m~tavolcaniques ont des affinit6s thol6iitiques & calco-alcalines. Les teneurs en Terres Rares des m6tagabbros et des m~tavolcanites se caract~risent par des spectres plats ~ fractionn6s. La sdquence ophiolitique de Fawakhir montre un spectre de compositions avec des laves de type arc, des MORB de type P et des boninites, suggdrant que ces roches correspondent ~ un environnement d'arri~re-arc. L'dvolution depuis des thol~iites pr6coces ~ des magmas tardifs de type boninitique peut sugg6rer que ces roches proviennent de la fusion fractionn~e et de I'extraction des liquides durant I'ascension de la source magmatique ~ I'intdrieur d'une colonne de manteau en d~but de fusion. Les boninites sont issues d'une source mant61ique plus r~fractaire que les autres roches. © 1999 Elsevier Science Limited. All rights reserved. (Received 3/11/97: revised version received 14/5/98: accepted 2/6/98) INTRODUCTION The Arabian-Nubian Shield (ANS) is a part of the Afro-Arabian crust which was strongly affected by tectonogenetic processes during the Pan-African episode (950-450 Ma). Kr6ner et al. (1987) classified the basement rocks of the ANS into four principal rock associations: (1) older shelf sequences, (2) arc assemblages, (3) ophiolit ic suites, and (4) granitoid intrusives. The ophiol i te sequences of the AfroArabian Shield occur as al lochthonous and commonly dismembered ultrabasic and basic bodies, and include ophiol i t ic melanges, serpent in i te thrust melanges and fault-bounded inliers. The basement * Corresponding author Journal of African Earth Sciences 515