First 40Ar/39Ar geochronology of lateritic manganiferous pisolites: Implications for the Palaeogene history of a West African landscape

A pisolitic superficial formation deriving from the geochemical degradation of a previous massive manganiferous crust has been investigated in West Africa using 40Ar/39Ar laser probe analysis on different generations of cryptomelane from Fe-Mn pisolites and their embedding lateritic matrices. Prior to the 40Ar-39Ar geochronological analysis, a detailed petrographical study has allowed the identification of successive assemblages from Mn-pisolites to (Fe,Mn)-pisolites embedded in Mn-rich and Fe-rich matrices, respectively. The age spectra obtained for pisolites, including cores, cortices and matrices enables the definition of three age clusters around 56-59, 44-47, and 24-27 Ma. The first two age clusters with the petrographical and geochemical results obtained on the different assemblages indicate that the development of the pisolitic crust first induced Mn-leaching and Fe-Al accumulations due to warmer and wetter climatic conditions in the age interval 56-47 Ma that characterizes the Tertiary greenhouse effect period propitious to bauxite formation in West Africa. The absence of 40Ar/39Ar ages between 44 and 27 Ma only means that geochemical conditions for cryptomelane crystallisation were not fulfilled, but could still be favourable to Fe- and/or Al-oxy-hydroxides formation, before drier climatic conditions became rather effective at the Oligocene period to sustain mechanical erosion rather than chemical weathering processes. The age cluster 24-27 Ma indicates however a reactivation of the manganiferous lateritic weathering late Oligocene. The 40Ar/39Ar dating results are discussed in terms of climatic condition changes during the Palaeogene that were favourable to the genesis of either Al- and Fe- or Mn-oxy-hydroxides in the course of development and evolution of the pisolitic formation. The 40Ar/39Ar dating also provides a new highlight to large-scale geomorphological patterns of West Africa.