Article révisé par les pairs
Résumé : The Iforas alkaline province is composed of spectacular N-S-trending acid dyke swarms, plateaux of ignimbritic and rhyolitic lavas, and silica-oversaturated ring-complexes. This province was emplaced just after the Pan-African collision which occurred between the Tuareg shield and the West-African craton 600 Ma ago. It is devoid of Sn mineralization in comparison with similar Paleozoic-Mesozoic complexes in Niger and Nigeria. The Pb/1bSr/1bO data on the Timedjelalen ring-complex and on contemporaneous N-S dykes demonstrate the cogenetic mantle source for their parental magma, with a contaimination by the lower continental crust from 10% to 50%, on the basis of Pb isotopes. There is no evidence for incorporation of old upper continental crust. These alkaline rocks show little alteration except for hydrothermal activity at ring- or dyke-country-rock contacts, as shown by lower δ18O-values. The hydrothermal water involved appears isotopically heavy (in the range 0 to -3‰) and may have originated in a residual sea or lake. Lack of mineralization can be explained by the fact that the primary source is economically barren (mantle and lower crust) and that the interaction with country-rocks during the intrusion process was weak and mainly with barren mantle-derived Pan-African rocks. © 1986.