Petrology and geochemistry of ignimbrites and associated enclaves from Mount Bambouto, West-Cameroon, Cameroon Volcanic Line

Abstract

Geochemical and mineralogical analyses data are presented for ignimbritic deposits from the Bambouto volcano. These deposits are characterized by their welded and non-welded massive lapilli tuff and massive lithic breccias facies. The ignimbrites generally display vitroclastic and/or eutaxitic texture characterized by whole or broken minerals, subangular fragments of trachytes, rhyolites, ignimbrites, as well as fiammes, and fragments of glass. The geochemistry of the ignimbrites of Mount Bambouto indicates that they are alkaline and peralkaline with rhyolitic and trachytic composition. The behaviour of the major and trace elements suggest that ignimbrites, as well as felsic and mafic lavas of the Bambouto volcano have an evolution dominated by fractional crystallization (FC). The La/Yb, Ce/Zr and Th/Nb ratios for Mt Bambouto ignimbrites (19.92–52.66; 0–0.48; 0.10–0.12, respectively) and the whole Western Cameroon Highlands (WCH) and Mt Manengouba lavas (15.58–34.29; 0–0.61; 0–0.27, respectively) also reflect the dominant role of FC. This suggests a relatively constant degree of partial melting during the petrogenesis of the lavas and deposits. The high Rb/Sr (0.52–28.98) and the low ratios of LILE/HFSE (Rb/Nb: 0.08–0.95; Rb/Zr: 0.01–0.18; Th/Ta: 1.38–2.1; Ba/Nb: 0.06–8.27) in Mt. Bambouto ignimbrites, compared with the very high value of these ratios in the granitic basement rocks of the area, preclude the petrogenesis of the ignimbrites from the melting of the Pan-African basement. The weak values of Th/La and La/Nb in the studied ignimbrites and other WCH lavas suggest that crustal contamination was insignificant during ascent and storage of the magma in shallow magma chambers prior to their release to the surface. The similarities in Nb/Ta, Zr/Hf and Nb/U ratios in the Mt Bambouto ignimbrites (Nb/Ta: 13.54–20.93; Zr/Hf: 37.39–49.95; Nb/U: 26.04–55.70) and the lavas of the WCH (Nb/Ta: 11.3–19.57; Zr/Hf: 33.16–55.01; Nb/U: 19.83–75.35) corroborated with their similar source origin. The enrichment of light REE relative to heavy REE or their Dy/Yb > 2 (2.03–2.97), (La/Yb)N > 5 (14.29–46.69) and (Tb/Yb)N > 1.9 (1.91–2.9) suggest melting in a garnet-bearing mantle.