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

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 (Fa97-99) 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 (LaN/YbN: 8.4-9.0; Zr/Nb: 8) and group-2 (LaN/YbN: 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.