Nouvelles donnees petrologiques sur les massifs ultramafiques-mafiques de la region de Laouni (Hoggar central, Algerie)

The western Laouni ultramafic-mafic intrusions are composed of two massifs, both displaying well-preserved concentric structures with an outermost uralitized gabbro ring and an north-south elongation parallel to the late-Pan-African transcurrent faults. The massifs cut a series of Pan African syntectonic migmatitic granites and allochthonous metamorphic series reworked during the Pan-African orogeny. Moreover, they are free of any metamorphic re-crystallization. On the other hand, the massifs are crosscut by dykes of post-tectonic granites. All these features suggest the late- to post-tectonic emplacement of the basic massifs with respect to the paroxysm of Pan-African orogeny estimated at about 600 Ma. Consequently, a relationship between the emplacement of the massifs and the opening of the pre-Pan-African ocean at 800-700 Ma is definitively ruled out. The general lithology and petrography of the cumulate pile share many similarities with the late- to post-tectonic layered intrusions in spite of considerable differences in size. The existence of a basic magmatism post-dating the peak of the Pan-African orogeny, in the central Hoggar, is proposed here for the first time in the central Hoggar. One possible way for this magma to intrude a thickened continental crust is local zones of extension occurring during the relaxation of lithospheric tectonic stress. -from English summary