Résumé
The Maghrebian, Betics and Apenninic chains outcrop around the mediterranean basin and constitute dismembered fragments of the Alpine orogen. Conflicting geodynamic models have been proposed in order to explain the different paleogeographic settings from which these fragments derived. Crust-mantle interactions following subduction of Jurassic oceanic crust and collision-related tectonic events of Eocene age in relation to the northward motion of Africa have been demonstrated by numerous works. The incorporation of mafic/ultramafic rocks into the basement is evidenced in various peri-mediterranean areas, in particular at c. 22 Ma. This work is focused on the basic and ultrabasic rocks from the easternmost internal part of the Maghrebides. An extensive petrological and geochemical study has been performed on three distinct outcrops, i.e. Bou Maiza gabbros, amphibolites from La voile Noire and Sidi Mohamed peridotites. Peridotites display a primitive character (Mg number >85), but slightly enriched trace elements patterns (1 to 10 times CHUR) characterized by negative Nb anomalies and flat to slightly LREE-depleted patterns. Associated isotopic constraints suggest a possible continental contamination of the peridotites by the surrounding gneisses. These ultrabasic rocks are interpreted as parts of the lithospheric mantle incorporated into the continental crust during a late Burdigalian extensional event that opened the Algerian basin. The Bou Maiza gabbros and La Voile Noire amphibolites show complementary trace elements spectra suggesting derivation from a common MORB source reservoir, but without filiation with the Sidi Mohamed peridotites. Such affinities suggest they represent a fragment of the Neotethys lithosphere obducted onto the North African margin during Miocene times.