Petrology of the early Neoproterozoic Bou Azzer–El Graara Tachdamt–Bleida igneous formations (Central Anti-Atlas, Morocco): geodynamic evidence for NW Gondwana

Abstract

The Tachdamt and Bleida formations from the Bou Azzer–El Graara inlier (Central Anti-Atlas, Morocco) record two distinct Neoproterozoic magmatic episodes that were previously grouped as a single unit. In this study, we present new whole rock major and trace elements geochemistry alongside samarium-neodymium (Sm-Nd) isotopic data for representative volcanic and volcano-sedimentary rocks from both formations. Uranium-lead (U-Pb) zircon dating from previous studies confirms emplacement ages of ~883 Ma for the Tachdamt Formation and ~700 Ma for the Bleida Formation. The temporal separation and geochemical distinctions between these formations demonstrate two independent magmatic events. The ~883 Ma Tachdamt Formation coincides in age with the Munali magmatism (~880–860 Ma) in the Greater Congo Craton; although a direct geodynamic link remains uncertain, this age similarity suggests both events formed during a broader Neoproterozoic extensional phase. Geochemical signatures further differentiate the two pulses. The Tachdamt basalts exhibit tholeiitic affinities, high iron-titanium (Fe-Ti) contents, enrichment in LILE, and positive neodymium isotopic values [εNd(t) = +3.2 to +4.5], consistent with plume-related magmatism in a Tonian large igneous province (LIP). In contrast, the Bleida basalts display negative niobium (Nb) anomalies, greater geochemical variability, and older depleted mantle model ages (TDM = 1.1–1.3 Ga), indicating an arc-influenced lithospheric mantle source in a foreland basin setting. These contrasting signatures record a geodynamic transition from extensional, plume-driven rifting (Tachdamt) to subduction modified magmatism (Bleida) along the northwestern margin of the West African Craton (WAC). Additional dolerite dykes crosscut both formations, marking a younger magmatic phase and exhibiting orientations similar to the ~860 Ma Manso dyke swarm; further U–Pb dating is required to clarify their provenance. Overall, our new geochemical and isotopic dataset refines the Neoproterozoic tectonic evolution of the Anti-Atlas and provides critical constraints on the broader magmatic and rheological history of Rodinia’s break-up.