A 17 Ma onset for the post-collisional K-rich calc-alkaline magmatism in the Maghrebides: Evidence from Bougaroun (northeastern Algeria) and geodynamic implications
Résumé
Bougaroun is the largest pluton (~ 200 km2) in the 1200 km-long Neogene magmatic belt located along the Mediterranean coast of Maghreb. New U–Pb dating on zircons and K–Ar ages on whole rocks and separated minerals document its emplacement at 17 Ma within the Lesser Kabylian basement, a continental block that collided with the African margin during the Neogene. This Upper Burdigalian intrusion is therefore the oldest presently identified K-rich calc-alkaline massif in the whole Maghrebides magmatic lineament and marks the onset of its activity. The Bougaroun peraluminous felsic rocks display a very strong crustal imprint. Associated mafic rocks (LREE-enriched gabbros) have preserved the “orogenic” (subduction-related) geochemical signature of their mantle source. Older depleted gabbros cropping out at Cap Bougaroun are devoid of clear subduction-related imprint and yielded Ar–Ar hornblende ages of 27.0 ± 3.0 Ma and 23.3 ± 3.2 Ma. We suggest that they are related to the Upper Oligocene back-arc rifted margin and Early Miocene oceanic crust formation of the nearby Jijel basin, an extension of the Algerian basin developed during the African (Tethyan) slab rollback. The fact that the Bougaroun pluton intrudes exhumed Kabylian lower crustal units, mantle slices and flysch nappes indicates that the Kabylian margin was already stretched and in a post-collisional setting at 17 Ma. We propose a tectono-magmatic model involving an Early Miocene Tethyan slab breakoff combined with delamination of the edges of the African and Kabylian continental lithospheres. At 17 Ma, the asthenospheric thermal flux upwelling through the slab tear induced the thermal erosion of the Kabylian lithospheric mantle metasomatized during the previous subduction event and triggered its partial melting. We attribute the strong trace element and isotopic crustal signature of Bougaroun felsic rocks to extensive interactions between ascending mafic melts and the African crust underthrust beneath the Kabylie de Collo basement.