LOWER MESOZOIC AND PERMIAN VOLCANISM OF THE ARGANA BASIN (WESTERN HIGH ATLAS, MOROCCO) - INTRAPLATE MAGMATISM ASSOCIATED WITH THE OPENING OF THE CENTRAL ATLANTIC BASIN

From petrographical and geochemical studies of volcanism in the Permia n-Mesozoic Argana Basin, three eruptive episodes have been distinguish ed in relation to intracontinental rifting preceeding the opening of t he Central Atlantic Basin. Necks and dykes located south of Argana are probably Permian or Lower-Middle Triassic in age. They constitute a f irst event yielding an anorogenic character and alkaline to transition al 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 i n aerial to sub-aerial contexts during two major periods. Their chemic al characteristics are similar to continental flood basalts and compar able to the Triassic-Liassic tholeiites of the rest of Morocco. This e ffusive volcanic activity marks the Triassic-Liassic rifting stage. Th e magmatism of Middle Jurassic age forms the third event. It is repres ented 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 lo w extensional regime in relation to the abortion of the Atlas rifting, due to the change of tectonic regime, which became compressive. (C) 1 998 Elsevier Science Limited.