Quartzite crests in Paleoproterozoic granites (Anti-Atlas, Morocco); a hint to Pan-African deformation of the West African Craton margin

摘要

South of the Pan-African suture zone of the Anti-Atlas, the Agadir Melloul inlier exhibits several crests of nearly vertical conglomeratic quartzite beds inserted in Paleoproterozoic granites. These enigmatic rocky walls have been considered as Early Neoproterozoic platform sediments pinched within the 2.03 Ga-old granites during brittle faulting events at ca. 570 Ma. The data presented here contradict this interpretation. The quartzite crests correspond to parts of tight synclines pinched along brittle-ductile, strike-slip shear zones involving the granitic basement itself. This deformation occurred in low-grade greenschist facies conditions at T = 260-280 degrees C. The detrital zircon grains from the vertical quartzite beds are mostly grouped around 2 Ga, with the youngest grains at 1.8 Ga. Thus, the studied quartzites exhibit the same detrital zircon barcode as the other siliciclastic platform formations of the southwestern Anti-Atlas (Taghdout Group) whose age has been recently constrained between 1.8 and 1.750 Ga. Ar-40-Ar-39 age measurements were unsuccessful to determine the age of crystallization of small syn-tectonic white mica grains within the quartzite samples, but the large white mica grains of granite samples outside the quartzite-bearing shear zones yielded ages of similar to 1.777 and 1.830 Ga. We interpret the enigmatic quartzite structures as the result of two major events that the Paleoproterozoic foreland of the Pan-African mobile belt likely suffered, i.e., an extensional event around 800-750 Ma, coeval with the opening of the PanAfrican Ocean, and a compressional event during its final closure at about 650-600 Ma. We propose that the quartzite formations detached then from the granitic basement of the Pan-African foreland in the frame of a complex, thick-and thin-skinned tectonics.