Geochemical features and petrology of ignimbrite deposits from Bamenda volcano, Western Highlands of the Cameroon Volcanic Line

This study presents mineralogical, major, and trace element data for ten samples of ignimbrite matrix, one of fiamme and fourteen samples of rock enclave from Bamenda volcano ignimbritic formations situated in the Western Cameroon Highland (central part of Cameroon Volcanic Line). The mineralogy of the deposits comprises alkali feldspars, plagioclase, quartz, clinopyroxene, amphibole, biotite, and oxides (titanohematite, titanite, goethite, pseudorutile, ilmenorutile, and ilmenite). Subangular fragments of trachytes, rhyolites, ignimbrites, and fiammes and debris of glass are found in these deposits. The representative matrix samples and enclaves are rhyolitic in composition and both peralkaline (PI > 1) and alkaline (PI ≤ 1). Like other felsic and mafic lavas of the Bamenda volcano, the studied ignimbrites and their enclaves show an evolution dominated by fractional crystallization (FC). The later petrogenetic process is highlighted by the minor variation in Ce/Zr, Th/Nb, and La/Yb ratios for the Bamenda volcano ignimbrites (0.12–0.81, 0.08–0.33, 12.60–45.40, respectively) and the Western Cameroon Highlands (WCH) lavas (0.05–1.11, 0.04–0.2, 16.1–37.91, respectively). The low La/Nb (0.5–3.45) and Th/La (0.04–0.25) values in the ignimbrites, positive to negative Zr–Hf (0.25–1.73) and Nb–Ta (0.3–1.14) anomalies, negative Eu spikes, and low Th/Ce ratios (0.02–0.12) underline an intervention of crustal contamination in the petrogenetic processes, though their high Rb/Sr ratios (0.30–182.88) and weak ratios of LILE/HFSE compared to the very high value of the same ratios in the Pan-African country rocks preclude their melting from this Pan-African basement.