Geochemistry of the Neoproterozoic Tilemsi belt of Iforas (Mali, Sahara): a crustal section of an oceanic island arc

Abstract

Abstract The pre-Pan-African Neoproterozoic Tilemsi belt of the western Iforas (Mali, Sahara) comprises a predominantly bimodal mafic and felsic volcanic assemblage associated with numerous intrusions mainly of mafic composition. The volcanics include island arc tholeiites and the calc-alkali rocks (rhyolites, dacites and andesites). The volcanic domain shows similarities to recent oceanic island arc systems as well as to Archaean greenstone belts. The Tilemsi magmatic sequence represents an example of a section through the middle and upper levels of a Neoproterozoic intra-oceanic island arc crust that is composed nearly exclusively of igneous material. The middle crustal section is mainly of mafic composition while the upper crustal level contains more silica-rich plutonic, volcanic and volcaniclastic assemblages. The mafic intrusions include a large proportion of cumulates indicating that fractional crystallization played an important role during the evolution of the oceanic arc suites. Compositionally, the arc changed through time from an immature to a relatively developed system. The rocks became more silica- and incompatible-element-rich with higher Sr isotopic ratios. This trend reflects the increasing growth and maturity of the arc.