A Paleoproterozoic ultramafic-mafic assemblage and associated volcanic rocks of the Boromo greenstone belt: fractionates originating from island-arc volcanic activity in the West African craton

Abstract

The Loraboué Birimian ultramafic-mafic assemblage, located in the Boromo greenstone belt (Burkina Faso), is interpreted as the remains of a magma chamber that crystallized at the base of an island arc. The ultramafic rocks exhibit an heteradcumulate texture, being generally made up of wehrlites and more rarely dunites. The crystallization sequence inferred from the cumulates is olivine+chromite followed by clinopyroxene+amphibole±orthopyroxene±biotite. The gabbroic rocks are mainly layered and grade into more differentiated facies with sub-pegmatitic texture, containing up to 70% modal plagioclase including zircon and, more commonly, apatite crystals. Textural relationships and mineral phase chemistry are indicative of crystallization at elevated pressures (>8 kbar), the parental magma being generated by a moderate to high degree of partial melting of a mantle source affected by previous metasomatic events above a subducted oceanic slab. The Loraboué volcanic formations exhibit a range of geochemical features. They consist dominantly of calc-alkaline basalts, pyroclastites and rhyolite and, more rarely, of basalt, dolerite and gabbro of tholeiitic affinity. These different types of basalt, as well as the dolerite and the isolated massive gabbro, show the classic features of arc magmatic suites, namely LILE and Pb enrichment, depleted HFSE patterns and high Ce/Nb and Th/Nb ratios. Thus, the calc-alkaline plutonic and volcanic assemblages of Loraboué could represent the roots of an island arc and the associated coeval volcanic rocks. The Paleoproterozoic crust of the West African craton was heterogeneous and was not the consequence of a single process of genesis. As some modern igneous province, the Birimian crust was generated by both volcanic arc accretion and oceanic plateau accretion.