In the western High Atlas, major volcanic activity supplied andesitic and dacitic lava and pyroclastic flows interbedded with pelitic sandstones and limestones. All these formations were isoclinally folded and metamorphosed into the greenschist facies, very likely during the Pan-African orogeny. The lavas include: 1) a few back-arc basin basalts depleted in the most incompatible elements, 2) andesites weakly to moderately enriched in incompatible elements, and 3) dacites. This magmatism is assigned to a volcanic arc context, which we consider as being earlier than the continental intra-plate extensive context of the Cambrian magmatism.Dans le Haut-Atlas occidental, un important volcanisme a donne des coulees de laves et de pyroclastites andesitiques et dacitiques, intercalees dans des formations gresopelitiques et carbonatees. L'ensemble est deforme en plis isoclinaux synschisteux et epimetamorphises, vraisemblablement au Panafricain. Les laves comprennent: 1) quelques basaltes de bassin d'arriere-arc appauvris en elements les plus incompatibles; 2) des andesites a enrichissement faible a modere en elements incompatibles et 3) des dacites. Ce magmatisme est attribue a un contexte d'arc volcanique, que l'on situe anterieurement au contexte extensif intra-continental du magmatisme cambrien.