In the Anti-Atlas (Morocco), the Jurassic Great Dyke of Foum Zguid is NE-SW trending over 200 km and has an average thickness of one hundred meters. It consists, from rims to core, of basic, intermediate and acidic rocks. The contacts between the different facies are gradational. Mineralogical and geochemical studies have shown that these different rocks are generated by transverse magmatic differentiation. This evolution corresponds to fractional crystallization from olivine-dolerite at the rims to granophyres at the core.