Initial Pangean rifting north of the West African Craton: Insights from late Permian U-Pb and 40Ar/39Ar dating of alkaline magmatism from the Eastern Anti-Atlas (Morocco)

Abstract

Numerous mafic dykes, sills and laccoliths crop out in the southern part of the Tafilalt basin (Eastern Anti-Atlas, Morocco). These rocks intrude the mildly folded Ordovician to Early Carboniferous formations, consisting mainly of lamprophyric dolerites and camptonites with minor gabbros and syenodiorites. Previous geochemical studies have shown that the Tafilalt magmatism of sodic-alkaline affinity has been produced by low degrees of partial melting from an enriched deep mantle source within the garnet stability field. However, the age and the geodynamic context of these rocks were presently unknown since no isotopic dating had so far been made of the Tafilalt dolerites. To resolve this issue, we present here the first 40Ar/39Ar biotite and U-Pb zircon dating from the Tafilalt alkaline magmatism. Three samples (biotite separates) yielded well-defined 40Ar/39Ar plateau ages of 264.2 ± 2.7 Ma, 259.0 ± 6.3 Ma and 262.6 ± 4.5 Ma whereas 206Pb/238U dating of zircon from one of these samples yielded an age of 255 ± 3 Ma. These ages coincide within the dating error, and indicate that this magmatism occurred in the late Permian. Considering geochronological and geochemical data, we propose that the Tafilalt magmatism reflects an early-rift magmatic activity that preceded the Triassic rifting heralded by the Central Atlantic Magmatic Province. This magmatic activity is recorded in both sides of the future Atlantic Ocean by small-volume alkaline magmatism that started in the late Permian and extends into the Triassic. The alkaline magmas are probably generated in response to an increase in the mantle potential temperature underneath the Pangea supercontinent.