Alkaline magmatism subsequent to collision in the Pan-African belt of the Adrar des Iforas (Mali)

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Abstract
Summary: The Pan-African Trans-Saharan belt in the Iforas displays a rapid switch from subduction and collision-related calc-alkaline to typical A-type magmatism, which is accompanied by transcurrent movements along major shear zones inducing weak distension. Detailed Rb-Sr geochronology and geochemical data point to different mantle sources for orogenic (lithospheric depleted mantle + oceanic crust) and within-plate magmatism (more primitive asthenospheric mantle). Both groups suffer lower-crustal contamination. A model is proposed whereby asthenospheric mantle originally underlying the subducted plate has risen to shallow depth beneath the continental lithosphere after the rupture of the cold plunging plate. This source, which is often proposed for alkaline rocks, explains the great similarity of oversaturated alkaline ring-complexes whatever their environment. The peculiarities of the alkaline province, for example the lack of Sn mineralization when compared with the Niger-Nigerian province, may be related to the nature and composition of the basement.