Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Gbambie Isaac Bertrand Mbowou, Claudial Lagmet, Sébastien Nomade, Ismaïla Ngounouno, Bernard Deruelle, Daniel Ohnenstetter

Petrology of the Late Cretaceous peralkaline rhyolites (pantellerite and comendite) from Lake Chad, Central Africa

Journal of Geosciences, volume 57 (2012), issue 2, 127 - 141

DOI: http://doi.org/10.3190/jgeosci.118

Late Cretaceous peralkaline rhyolites (69.4 ± 0.4 Ma (2σ analytical, 1.9 Ma full external uncertainty, ⁴⁰Ar/³⁹Ar sanidine single crystal laser dating) from Lake Chad are the oldest lavas of the “Cameroon Hot Line”. These lavas (pantellerite and comendite), consisting of quartz ± alkali feldspar ± fluoro-arfvedsonite ± augite-hedenbergite ± aegirine ± fayalite (Fa_97-99) and ilmenite, are characterized by a progressive increase in the total REE contents and the magnitude of the negative Eu anomaly owing to the alkali feldspar-dominated fractionation. Two groups of peralkaline rhyolites have been distinguished according to mineralogical and geochemical data. Both, group-1 (La_N/Yb_N: 8.4-9.0; Zr/Nb: ~ 8) and group-2 (La_N/Yb_N: 10.7-12.6; Zr/Nb: ~ 6) are genetically related. The peralkaline rhyolites from Lake Chad could not be solely the product of fractional crystallization from a basaltic parental magma. The influx of F-rich fluids could have greatly modified the composition of rhyolitic magmas as attested by the presence of fluoro-arfvedsonite.