Several late-collision and intraplate features are not entirely integrated in the classical plate tectonic model. The Pan-African orogeny (730–550 Ma) in Saharan Africa provides some insight into the contrasting behaviour of cratons and mobile belts. Simple geophysical considerations and geological observations indicate that rigidity and persistence of cratons are linked to the presence of a thick mechanical boundary layer, the upper brittle part of the continental lithospheric mantle, well attached to an ancient weakly radioactive crust. The surrounding Pan-African mobile belts, characterized by a much thinner mechanical boundary layer and more radioactive crust, were the locus of A-type granitoids, volcanism, tectonic reactivation and basin development during the Phanerozoic. During oceanic closures leading to the assembly of Gondwana, lithosphere behaviour was controlled by its mechanical boundary layer, the crust being much less rigid. We suggest that the 5000 km wide Pan-African domain of Saharan Africa, a collage of juvenile and old reactivated basement terranes, has suffered regional continental lithospheric mantle delamination during the early stages of this orogeny, as has been postulated for the more recent Himalayan orogeny in Tibet. Delamination of the continental lithospheric mantle and juxtaposition of crust against hot asthenosphere can explain many features of the late Pan-African (around 600 Ma): reactivation of old terrains, abundant late-tectonic high-K calc-alkaline granitoids, high temperature-low pressure metamorphism, important displacements along mega-shear zones and mantle-derived post-tectonic granitoids linked to a rapid change in mantle source. Recycling into the asthenosphere of large amounts of continental lithospheric mantle delaminated during the Pan-African, can provide one of the reservoirs needed to explain the isotopic compositions of ocean island basalts. Lastly, the lithospheric control over the location of the alkaline rocks enjoins us to consider the thermal boundary layer (the lower ductile part of the continental lithospheric mantle) as a major mixing source zone for these rocks.
Concepts developed in the recently published model of the Air region (eastern Tuareg shield; Niger, Africa) as a college of three displaced terranes integrated in a single geodynamic model lead us to propose a terrane map of the Tuareg shield (500000 km 2). The 23 terranes recognized have their own lithological, metamorphic, magmatic, and tectonic characteristics and are separated by subvertical strike-slip megashear zones that can be traced for hundreds of kilometres, or by m^jor thrust fronts. Some of these boundaries have ophiolitic assemblages or molassic deposits. The Tuareg shield was shaped and partly accreted during the Pan-African orogeny (750-550 Ma), but not as a homogeneous body.
SUM MARY: 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.
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