The Archean to Late-Paleozoic architecture of the Oulad Dlim Massif, the main Gondwanan indenter during the collision with Laurentia

Abstract The Oulad Dlim Massif, located west of the Archean part of the Reguibat Rise of the West African Craton, has an areal extent close to 36,000 km2. Despite its tectonic importance, as it may have been the main indenter during the late Paleozoic Laurentia-Gondwana collision that formed supercontinent Pangea, this region has remained unstudied for a long time. Based on previous research done by Spanish geologists before 1965, the pioneering work done by Rjimati and Zemmouri (2002 and personal communication) between 2000 and 2010, and new fieldwork conducted after 2010 in collaboration with Moroccan exploration geologists, we carried out a massif-wide study that includes sensitive high-resolution ion mass spectrometry (SHRIMP) zircon U Pb geochronology, modern petrography, geochemistry, and isotope geology. Here we present these results and discuss their interpretation in the context of other circum-Atlantic orogens and terranes. The Oulad Dlim Massifs is composed of an Archean terrain divided by an NNE-SSW trending Ediacaran (Pan-African) intracontinental rift marked by a voluminous bimodal mafic-felsic magmatism, and a Silurian-Devonian belt in the west. The massif is affected by an intense shear deformation that likely marks its main assembly. Deformed and undeformed Silurian-Devonian granites indicate that such deformation was active at ≈420 Ma, declined at ≈410 Ma, and ended at ≈404 Ma. The whole massif is metamorphosed. The metamorphism is especially intense in the Ediacaran Adrar Suttuf Metamafic Complex, i.e., the rift-related mafic magmatic rocks, which have a crystallization age of 603 ± 1 Ma and probable metamorphic age of 570 Ma to 550 Ma. There is also evidence of a later thermal event, likely related to the formation of the Silurian-Devonian belt, that reset the Rb Sr system of the Ediacaran leucogranites to ≈ 420 Ma and metamorphosed them in the greenschist facies. We have also found an enigmatic strip of Late Carboniferous rocks in the Silurian-Devonian belt, and evidence indicating that the latest structures and low-grade metamorphism affecting the Silurian-Devonian granites likely were of this age. Therefore, we suggest that off-shore and west of the Silurian-Devonian belt, underneath the Cenozoic cover and on the continental shelf there may be more evidence for Late Carboniferous deformation equivalent to the eastern North America Alleghanian Orogen, which, like this, would have resulted from the Laurentia-Gondwana collision during the Late Carboniferous.