Characterisation of the pre-Mesozoic basement within the Tell Orogeny (Northwestern Algeria ): Implications for the Tethys realm

The Alpine Tell and Rif orogenic belts of northern Algeria and Morocco formed in response to the southward closure of the Tethys Ocean from the Late Cretaceous onward. During the Cenozoic, it was associated with the coeval opening of the western Mediterranean basin and the collision between the AlKaPeCa blocks and the North African rifted margin.

While the pre-Mesozoic basement is accessible south of the Tell-Rif front, this basement is poorly exposed within the Tell-Rif orogenic belt where it remains largely unknown. Yet, the Tell-Rif basement bears key informations : (1) on the late Variscan collision between European blocks and Gondwana mainland with the potential existence of a Paleotethyan domain, and (2) on the Maghrebian Tethys evolution from Triassic rifting to Cenozoic closure.

In the Western Tell, especially in the Oran region, remnants of the North African margin basement occur in two types of outcrops: (1) a variety of xenoliths from the basement, including metamorphic and mafic rocks, that can be found within the Triassic salt-related structures and brought to the surface by the salt and (2) the “external metamorphic massifs”, affected by a subduction-related metamorphism of maximum Oligocene age, often associated with ultramafic rocks. Although these two complementary features offer a rare opportunity to sample the North African margin basement, it has almost never been studied.

In this work, we focused first on the xenoliths. An extensive sampling has been done within a dozen of Triassic salt related structures in the Oran region. These rocks are ranging from magmatic to high-temperature metamorphic rocks and can be either mafic or felsic. This vast diversity allowed us to do petrological studies as well as geochronological work (U-Pb on zircons) to characterise the basement, with a particular interest to the highest grade metamorphic rocks such as sillimanite-rich micaschists,  kinzigites and mafic granulites.

The results provide a unique opportunity to better understand the North African margin basement composition and its Palaeozoic to Cenozoic geodynamic evolution.