La province des complexes annulaires alcalins sursaturés de l'adrar des Iforas, Mali

Abstract

The Cambrian silica-saturated alkaline province of the Iforas is composed of about 15 massifs including typical large size ring complexes (20–30 km in diameter). Its originality is that it follows closely a regime of subduction blocked by the Pan-African collision (600 Ma) and that each stage of the transition from calcalkaline to alkaline can be related to structural elements in a geodynamic setting. Several complexes are described in some detail. The Kidal-Tibeljeljeline complex starts with the intrusion of syenite porphyries forming external ring dykes, often polygonal in outline, which are truncated by a central massif 30 km in diameter which is composed of a dozen major phases. It complexity is due to the individualization of several intrusive centres and to the fact that the erosional level coincides with that of the flat roof structures. Djounhane, Takellout and Timedjelalen (20–30 km in diameter) are simpler and composed of concentric intrusions youing inwards. Other occurrences are described briefly. The emplacement of all these intrusions at shallow depth in a rigid environment under a thick rhyolitic cover of fissural origin is contemporary to movement along a tear fault system with intermittent periods of distension as shown by spectacular associated dyke swarms. The study shows the presence of both peralkaline and aluminous trends, an evolution of the minerals (pyroxenes, amphiboles and micas), and geochemical characteristics that are quite comparable to those of anorogenic provinces such as Niger and Nigeria. It is important to note, however, the absence of cassiterite and the predominance of subsovus granites in the aluminous trend of the Iforas. The striking similarity between the provinces leads the authors to envisage a common mantle origin, the difference being due rather to the influence of the country rock.