Timing and geodynamical setting of Paleoproterozoic events in the Birrimian of the West African Craton: Insights from the Hounde Greenstone Belt (SW Burkina Faso)

This study presents geochemical (major and trace elements, Pb-Sr-Nd isotopes on whole rocks) and geochronological (U-Pb on zircon) analyses on granitoids and volcanics from the Hounde Greenstone Belt (HGB). The HGB is one of the three Greenstone Belts outcropping in the southwestern part of Burkina Faso with, from east to west, the Boromo, Houndé and Banfora Greenstone belts. All three greenstone belts are separated by granitic domains and are mined for gold. In the HGB, the preserved sequence is constituted by a 6 km thick basaltic and gabbroic pile followed upward by intermediate (andesite) to felsic (rhyolite) volcanics with intercalated volcaniclastic and sedimentary material. Fine-grained sediments are more and more abundant upward. The HGB is limited on the East by the N-S Boni shear zone, which controlled deposition of “Tarkwaian-type” detrital sediments (conglomerate, sandstones and phyllites).

Geochemical analyses indicate that the oldest dated plutonic samples (2195 ± 6 and 2183 ± 7 Ma) have a trondhjemitic affinity and metaluminous or peraluminous character. Similar ages at 2172 ± 5 and 2191 ± 5 Ma were also obtained on volcanics (andesite and rhyolite) from within the HGB indicating coeval development of GB and basement rocks. The trondhjemite have geochemical characteristics consistent with magma production in a convergent plate setting and the 2.20 – 2.17 Ga period is interpreted as corresponding to arc build-up and thickening of the arc crust. Cross-cutting leucocratic veins in the trondhjemite indicates partial melting of the arc crust at 2144 ± 6 Ma, which may be related either to a regional metamorphic event or triggered by the gradual thickening of the crustal section. Granites and granodiorites (and their volcanic equivalents) yield younger ages in the range 2132 ± 3 Ma to 2095 ± 9 Ma, and geochemical features typical of volcanic arc granites although the youngest granitoids, in the 2120 – 2100 Ma range, indicate a syn-collisional setting. Nd isotope data show large variations (eNd ranging from +0.7 to +14.5) and a broad general trend of increasing eNd values with decreasing ages. This trend may reflect increasing proportions of juvenile mantle-derived material to the source regions of the younger granitoids. However some samples have significantly older T_DM model ages ranging from 2310 to 2430 Ma suggesting the involvement of early Birrimian components that could have been added to the source regions, either by assimilation at lower crustal levels or by sediment subduction.