Hayman, Patrick1, Asmussen, Pascal1, Senyah, Gloria1, Tegan, Eudes2, Coulibaly, Inza3, Denyszyn, Steven4, Jessell, Mark4
1Queensland University of Technology, Brisbane, Australia, 2 Institut National Polytechnique Felix Houphouet Boigny, Yamoussoukro, Côte d’Ivoire, 3Université Nangui Abrogoua, Abidjan, Côte d’Ivoire, 4University of Western Australia, Perth, Australia
Despite many similarities between late Archean and Paleoproterozoic greenstone belts, volcanological, compositional, temporal and geometric variations reflect differences in lithospheric strength and geodynamics. We present new field, geochemical and geochronological (U-Pb LA-ICPMS and ID-TIMS) data of the Toumodi Greenstone Belt (Ivory Coast, West Africa) to reconstruct the stratigraphic, paleo-depositional environment and structural history of a well-preserved greenstone belt that represents one of the first post-Archean supracrustal sequences. We then compare and contrast results with a typical 2.7 Ga greenstone belt (Agnew, Western Australia) and highlight some important differences across the A-P boundary. The Toumodi Belt is 130 km long and >5 km wide and preserves a sequence 5-10 km thick, for which there is no evidence of pre-existing continental basement. The stratigraphy consists of three main stages: I) an initial (ca. 2.35-2.30 Ga) tholeiitic lava flow and sill succession intercalated with minor cherts and black mudstones, representing a mafic event in an open, anoxic and deep-water setting (mid-oceanic ridge basalt or oceanic island?); II) a diverse volcano-sedimentary package (ca. 2.30-2.15 Ga) of basalt/andesite lavas and turbidites that formed in deep water, as well as pyroclastic (scoria/tuff cones) and epiclastic deposits, including debris avalanche flow deposits, that formed in a subaerial setting; and III) an uppermost (ca. 2.15-2.05 Ga) sequence of felsic pyroclastic and epiclastics rocks that formed in a subaerial setting, as well as coeval and distal turbidites. Stages II and III form a synclinal sequence, while stage I forms only on the western edge of the belt. Similarities with 2.7 Ga greenstones include transitions from mafic to felsic volcanism and subaqueous to subaerial environments. Important differences include the protracted history (>250 vs 70 Myrs), belt asymmetry, and abundance of emergent stratigraphy (especially during stage II) that reflect multiple thermal events, accretionary tectonics, and stiffer continents, respectively.
Dr Hayman’s principal research focuses on field data and geochemical techniques to resolve volcanic, terrane and ore forming processes of the Earth at a range of scale, from outcrops to terranes.