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

• Published in: Earth and planetary science letters Vol. 238; no. 1; pp. 172 - 188
• Main Authors: Colin, F., Beauvais, A., Ruffet, G., Hénocque, O.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 30.09.2005
• Subjects: 40Ar/ 39Ar-geochronology; Burkina Faso; Cryptomelane; Lateritic pisolites; Paleoclimate; Planation landsurface; West Africa; Burkina Faso; West Africa; Paleoclimate; 40Ar/ 39Ar-geochronology; Planation landsurface; Cryptomelane; Lateritic pisolites
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2005.06.052

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.