Petrogenesis of a silicic magma system: Geochemical evidence from Bamenda Mountains, NW Cameroon, Cameroon Volcanic Line

Abstract

The concentrations of major and trace elements and the isotopic compositions were measured in felsic lavas of the Bamenda Mountains, an extinct volcanic center of the West Cameroon Highlands (WCH), in order to assess the relative roles of mantle versus crustal magma sources in the evolution of silicic magmatic systems. These silicic volcanics are composed of benmoreites, trachytes and rhyolites associated with alkaline basaltic lavas and have characteristics of A-type magmas. Chemical diagrams show two magmatic lineages based on the ratios of HFSE/LILE (e.g. Hf/Rb: 0.19–0.34 and 0.11–0.23), the degree of fractionation (La/Sm: 5.2–6.4 and 6.5–7.5) and the contents of REE; they are respectively referred to as less-differentiated felsic lavas (LDFL) and high-differentiated felsic lavas (HDFL). Their evolution is mainly governed by alkali feldspar-dominated fractional crystallization. The K/Ar ages on trachytes range from 27.40 ± 0.50 to 12.74 ± 0.25 Ma with the HDFL being mostly younger. Their ε Nd range mostly between −0.88 and +2.43 with one value at −3.43 precluding their derivation from melting of a continental crust. Besides the most negative value (−3, 43) obtained in trachyte BA63 remains high compared to −20.28 calculated for a Bamemda Pan-African granitoid suggesting very low crustal contamination. However, the fact that this lower ε Nd in BA63 is not followed by the corresponding low Ce/Pb and high La/Nb rather indicates a selective role of crustal contamination. This feature seems characteristic of the felsic lavas of the whole Cameroon Volcanic Line. The isotopic and trace element variations observed for the Bamenda felsic lavas are most consistent with a source in the continental lithospheric mantle.