Zircon U–Pb geochronology and Sm–Nd and Rb–Sr isotope systematics of Neoproterozoic granitoïds from Bou Azzer (Anti-Atlas - Morocco): The obduction trigger of the central Anti-Atlas terrane

Abstract

This study presents new U–Pb zircon ages for the Neoproterozoic granitoids of the Bou Azzer inlier at the central part of the Anti-Atlas chain (Morocco), in the northwestern edge of the West African Craton (WAC). Analyzes of zircon grains yield the following ages: 658 ± 3 Ma for Ait Ahmane pluton, 647 ± 5 Ma for Bou-Izbane intrusion, 656 ± 3 Ma for Bou Azzer quartz-diorites, 583 ± 12 Ma in the Oustrate intrusion and 644 ± 4 Ma for the Ait Ahmane leucogranite. These ages confirm a major magmatic event between 640 and 660 Ma contemporaneous with the obduction of oceanic crust marking the formation of the ophiolites of Bou Azzer and Sirwa. The emplacement of granitoids would have led to a decrease in the density of the oceanic crust and triggered the process of obduction. The granitoids display the geochemical features of volcanic arc magmas and have positive initial ϵNd (+4.67 to +6.57) and low initial 87Sr/86Sr (0.7002–0.7055). The quartz-diorite group displays young Depleted Mantle Model (TDM) ages that range from 904 to 1065 Ma. The leucogranites have younger TDM of 869–884 Ma. These new data attest to their juvenile nature and the absence of any crustal contribution. The NNW-SSE extended inlier band of Bou Azzer - Sirwa constitutes an exotic terrane of Neoproterozoic age and of a juvenile nature wedged between the SW and NE domains, respectively, formed by or surmounting a Meso-Paleoproterozoic or even Archean basement. The Bleida granodiorite, dated at 559 ± 4, displays older U–Pb ages from inherited zircon cores at 1234, 1748 and 2085 Ma. The Bleida granodiorite was emplaced during a later magmatic episode that belongs to the Ouarzazate Group, generated during the post-collisional metacratonic evolution of the northern edge of the West African craton.