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In the southern Moroccan Atlas, abundant volcanic and sedimentary formations, dated from the Ediacaran to Cambrian time, were set at the northwestern Paleo-Gondwanan margin, after the main Pan-African orogenic event. The Precambrian-Cambrian geodynamic transition is characterized by an Early Cambrian marine transgression. We examine the tectonic conditions of this transgression and the magmatic signatures of the volcanic rocks that were produced just before and around the Precambrian-Cambrian boundary. Significant angular unconformities are evidenced, between the Late Neoproterozoic formations and the Cambrian deposits, in the central and eastern Anti-Atlas, which are due to a late Ediacaran NNE-SSW compressional event. The Late Neoproterozoic formations are related to an intracontinental volcanic chain of andesitic to rhyolitic lavas dated to the Ediacaran period. These calc-alkaline rocks were generated by melting of the mantle, previously metasomatized during the Pan-African orogenic stage, and of continental crust. The Late Ediacaran to Early Cambrian formations are analyzed in the Agoundis-Ounein and Toubkal areas, southwest of the old block of High-Atlas. An important basaltic pile unconformably overlies the Ediacaran rhyolitic formation and is overlain by Tommotian sediments. These basalts are continental tholeiites generated by melting of a normal subcontinental mantle. They outpoured from an important N 30°-trending fissural system over a basin floor. Some lherzolite fragments have been sliced along southwest-northeast faults, in the Lower Cambrian sediments. They originated from a transitional mantle between continental and oceanic domains. Farther east of the central Anti-Atlas, the Tommotian Djbel Boho volcano exhibits olivine basalts having an intraplate enriched asthenospheric signature type of ocean island basalt. The magmatic characteristics of the Late Ediacaran to Early Cambrian volcanic rocks, the structural features, and the presence of lherzolite fragments are consistent with a volcanic passive margin rift setting in a WNW-ESE extension regime. The meaning of this extensional event is discussed in relationships to the opening of a Cambrian basin and the drifting of the Avalonian terranes.

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