Juvenile Paleoproterozoic crust evolution during the Eburnean orogeny (∼2.2–2.0 Ga), western Burkina Faso

Abstract

We have investigated three greenstone belts (Boromo, Houndé, and Banfora) and associated granitoid terrains, which form part of the Eburnean orogen, situated in western Burkina Faso. These belts expose tholeiitic basalts (basal unit) followed by calc-alkaline intermediate predominantly effusive volcanic and sedimentary suites geochemically reminiscent of present-day volcanic island arc environments. The basal mafic unit probably corresponds to a juvenile arc crust or oceanic plateau. It contains unusual megacrystic tholeiitic basalts, allowing us to correlate the western margin of the Boromo belt with the eastern margin of the Houndé belt. These two N–S trending belt-parallel tholeiitic units are interpreted as limbs of a crustal scale anticline, intruded and partially obliterated by tonalite–trondhjemite–granodiorite (TTG) and granite intrusions. Three deformation events (D1–D3) can be distinguished in western Burkina Faso. The first deformation phase (D1) operated under an E–W to WNW-oriented compression. Regional greenschist to lower amphibolite facies metamorphism and intense folding characterize early-Eburnean deformation phases, during which time the crust was thickened by lateral shortening of volcanic island arcs and concomitant magma input. The crustal-scale antiform between the Boromo and Houndé belts is attributed to the D1 event. Shallow water detrital Tarkwaian-type sediments were deposited during the late D1 event within the Houndé belt, in a belt-parallel basin extending for 400 km. The subsequent D2 phase overprints the structural grain of the study area, and is best visible in airborne magnetic data. It is characterized by N to NE-trending transcurrent shear zones, which are considered preferred host structures for gold mineralization. We suggest that the newly formed and thickened crust reached the maximum thickness supportable by a weak and hot mantle during the D1 phase, and the pure shear dominated compressional regime switched to simple shear dominated transpression during the subsequent D2 phase. Granitoid diapirism played an important role at all stages of the Eburnean crustal growth processes in particular through early volume addition to the newly formed orogen and through later accommodating part of the lateral shortening. Pluton emplacement contributed to the greenstone belt structuration at local scales; however, the regional scale system geometry was controlled by coaxial shortening of the viscous volcanic units (basalts, gabbros, and andesites) of the greenstone belts, supported by coeval magma input. The last D3 deformation, which is either late-Eburnean or perhaps even Pan-African in age, is characterized by shallow N or S dipping minor thrust faults or an E-W trending steeply dipping spaced crenulation cleavage and kink folds, occurring mainly in highly anisotropic lithologies across the study region.