An exotic terrane of Laurussian affinity in the Mauritanides and Souttoufides (Moroccan Sahara)
Résumé
The Moroccan Sahara includes the Dhlou and Adrar Souttouf Massifs, both of which belong to the Souttoufide belt and are located on the western margin of the Archean Reguibat Shield. The Adrar Souttouf Massif has previously been assumed to be part of the Variscan Mauritanian-Appalachian system. New zircon ages from the two units of the Adrar Souttouf Massif have nevertheless allowed us to hypothesise a complex polyphased history. The Massif comprises four NNE-SSW trending units (listed here from east to west). The Sebkha Matallah unit represents the eastern margin of the Adrar Souttouf Massif and is thrust over the Ordovician to Devonian sedimentary Dhloat Ensour Group to the east. A central (Dayet Lawda) unit consisting of mafic and ultramafic rocks is interpreted as a possible remnant of Neoproterozoic oceanic crust or mafic terranes. The western Sebkha Gezmayet and Oued Togba units are mainly composed of granitoids and orthogneisses. Tonian-Stenian (1400-1000 Ma) zircon ages recorded in the Oued Togba and Sebkha Gezmayet units suggest an Avalonian-Meguman-like relationship. The other three age groups obtained in these two units are 610 to 570 Ma (Pan-African), 530 to 490 Ma (Cambrian) and 440 to 270 Ma. The latter population cannot result from Variscan orogeny alone, and is possibly linked to the Salinic and Acadian orogenies of Laurentia. Zircon ages of ~ 3000 Ma and ~ 2650 Ma determined for two granite samples from the Archean foreland corroborate data already published. Lower intercept ages of between 226 Ma (Upper Triassic) and 158 Ma (Upper Jurassic) identified for discordia lines are interpreted as a Mesozoic thermal overprint of the area attributed to the break-up of Pangaea and the initial rifting of the Atlantic Ocean. The ages obtained support a new tectonic model describing the formation of the units of the Adrar Souttouf Massif.