A precise U–Pb zircon age for the Bleïda granodiorite, Anti-Atlas, Morocco: implications for the timing of deformation and terrane assembly in the eastern Anti-Atlas

Abstract

Precambrian rocks within the Bou Azzer inlier, southern Morocco expose the boundary between the West African Craton (WAC) and dismembered parts of the Anti-Atlas Pan-African orogenic belt. A pervasive greenschist fabric, is variably developed within Pan-African units, generally orientated 120/60° NNE. This fabric has been interpreted as recording thrusting of the Pan-African arc-complex onto the West African Craton. In the SE of the inlier an undeformed intrusion, the Bleïda granodiorite, is emplaced within the allocthonous units of the arc-complex. The Bleïda granodiorite crosscuts the predominant fabric elements in the host rocks and does not itself contain any pervasive fabric development. Because the host-rock fabric has regional tectonic significance, emplacement of the undeformed Bleïda granodiorite necessarily occurred after cessation of thrusting and assembly of the tectonic blocks in the region. It hence constrains the cessation of the main period of Pan-African deformation in the eastern anti-Atlas. Two well constrained ages of 579.4 ± 1.2 Ma and 578.5 ± 1.2 Ma, based on U–Pb analyses of groups of ⩽5 zircons, have been determined for two samples of the Bleïda granodiorite. Penetrative regional deformation associated with collision of the arc-complex with the WAC was thus completed before ca. 580 Ma. These new ages are closely similar to the ages of post-tectonic granites in the neighboring Sirwa inlier and therefore suggest that penetrative deformation in the eastern Anti-Atlas as a whole was near completion by ca.580 Ma.