Le volcanisme permien et mésozoïque inférieur du bassin d'Argana (Haut-Atlas occidental, Maroc): un magmatisme intraplaque associé à l'ouverture de l'Atlantique central

Abstract

From petrographical and geochemical studies of volcanism in the Permian-Mesozoic Argana Basin, three eruptive episodes have been distinguished in relation to intracontinental rifting preceeding the opening of the Central Atlantic Basin. Necks and dykes located south of Argana are probably Permian or Lower-Middle Triassic in age. They constitute a first event yielding an anorogenic character and alkaline to transitional affinities, and are clearly different to the orogenic calc-alkaline Lower Permian volcanism known in the Late Hercynian basins of central Morocco. These magmatic rocks mark an Upper Permian pre-rifting stage at the beginning the Triassic extension. The flows attributed to the Upper Trias-Lower Lias constitute a second event. Eruptions occurred in aerial to sub-aerial contexts during two major periods. Their chemical characteristics are similar to continental flood basalts and comparable to the Triassic-Liassic tholeiites of the rest of Morocco. This effusive volcanic activity marks the Triassic-Liassic rifting stage. The magmatism of Middle Jurassic age forms the third event. It is represented by the sill of Amlal, the neck of Imi n'Tiguirt and the flows of th Bigoudine area. Their chemical characteristics are very similar to the first event. These alkaline to transitional basalts indicate a low extensional regime in relation to the abortion of the Atlas rifting, due to the change of tectonic regime, which became compressive.