The subduction- and collision-related Pan-African composite batholith of the Adrar des Iforas (Mali): A review

Abstract

A large composite calc-alkaline batholith, in the Iforas region, Mali, occurs close to the Pan-African suture between the 2000 Ma old West African craton and the Trans-Saharan mobile belt. Its location in an embayment of the West African craton is probably responsible for the important production of magma. The Iforas batholith intrudes the western border of an old continental segment affected by early nappe tectonics (D1 event) and is flanked to the west by the Tilemsi palaeo–island arc. The batholith comprises several successive stages. The cordillera (>620 Ma), probably post-dating the D1 event, is essentially composed of volcanosedimentary sequences. The collision (620–580 Ma) is marked by the production of abundant granitoids mostly emplaced by the end of the D2 EW compressional event. The post-collision tectonic stages (D3 and D4; 580–540 Ma) are characterized by strike-slip movements, reversals in the stress field, and a rapid switch from calc-alkaline to alkaline magmatism. Magmas corresponding to each step show distinctive geochemical trends but all share low 87Sr/86Sr initial ratios (0·7035–0·7061). The possible successive sources have been evaluated from different entities in the Inforas region: Eburnean granulites for lower crust, Tilemsi palaeo–island arc for depleted subduction source and the Tadhak undersaturated province for asthenospheric more primitive mantle. A geodynamic model is proposed where all the calc-alkaline groups originated from a classical subduction source (depleted upper mantle modified by hydrous fluids from the subducted oceanic plate) which, some fifty million years after the beginning of the collision, was taken over by an asthenospheric source producing the alkaline province.