Miocene incorporation of peridotites into the lower crust during opening of the Algerian basin: insight from U-Pb LA-ICP-MS analyses

Sizeable pieces of deep mantle material croup-out worldwide within orogenic belts involving major continental collisions or within regions with rift-thinned continental margins. From their original mantle position these peridotites have been transferred into the crust by tectonic processes involving subduction, collision or extension and, although minor components of most metamorphic belts, the understanding of how and when they were emplaced within the continental crust and their subsequent exhumation is paramount. A U-Pb age of 17.84±0.12 Ma (2σ) was obtained from monazites separated from a diatexite migmatite collected in close proximity to a small peridotite massif incorporated into the lower crustal sequence of the Edough massif (eastern Algeria), a southern segment of the peri-Mediterranean Alpine Belt. Monazites, extracted from a neighbouring deformed leucogranite, yield a similar age of 17.4±1.3 Ma (2σ) whereas coexisting zircon with magmatic characteristics yield an age of 308±7 Ma (2σ) interpreted as dating the magmaticcrystallisation of the leucogranite during the hercynian orogeny. These results emphasize the polycyclic evolution of basement rocks preserved in the crystalline units of the Western Mediterranean and indicate that part of their metamorphic features was inherited from older events. Taken together with published Ar-Ar datings, the late Burdigalian age of monazites, indicates a fast cooling rate of c. 300°C/Ma and is regarded as closely approximating the emplacement of the peridotites into the hercynian basement. This age issignificantly younger than those recorded for orogenic peridotites from the Betic-Rif orocline and for lithospheric extension forming the Alboran sea. It is also younger than rifting and back-arc extension opening the Liguro-Provençal basin. The late Burdigalian age is interpreted as dating the incipient rifting event that opened the Algerian basin. At the scale of the Western Mediterranean, opening of the Algerian basin provides a link between the late Oligocene-early Miocene extension in the western part (Alboran basin, Valencia trough and Liguro-Provençal basin) and the upper Miocene extension in the eastern part (Thyrrenian basin) of the western Mediterranean