Elemental and Sr–Nd–Pb isotopic compositions, and K–Ar ages of transitional and alkaline plateau basalts from the eastern edge of the West Cameroon Highlands (Cameroon Volcanic Line)

Abstract

This study presents the first geochronological and geochemical data from volcanic rocks on the eastern edge of the West Cameroon Highlands, which is the southern continental sector of the Cameroon Volcanic Line. Whole-rock petrography and geochemistry indicate that the rocks are transitional and alkaline basalts. KAr ages show that the transitional lavas are older (36.67 ± 1.88 Ma) than the alkaline lavas (14.86 ± 1.1 to 8.89 ± 0.7 Ma). The Sr–Nd–Pb isotopic ratios of the transitional lavas (87Sr/86Sri = 0.70380–0.70384, 143Nd/144Ndi = 0.51268–0.51277, 206Pb/204Pb = 18.06–18.65, 207Pb/204Pb = 15.52–15.62, and 208Pb/204Pb = 38.15–39.07) are different to those of the alkaline lavas (87Sr/86Sri = 0.70300–0.70330, 143Nd/144Ndi = 0.51287–0.51298, 206Pb/204Pb = 19.67–19.89, 207Pb/204Pb = 15.63–15.66, and 208Pb/204Pb = 39.35–39.59). Chondrite-normalized rare earth element (REE) and primitive-mantle-normalized trace element patterns of the lavas from the study area are similar to those of oceanic islands basalts (OIB). The transitional lavas have Nb/Y ratios of 0.67–1.18, (La/Yb)N ratios of 4.96–5.84, and Mg# = 29–39, and the alkaline lavas have Nb/Y ratios of 1.65–3.55, (La/Yb)N ratios of 7.76–20.13, and Mg# = 35–58. Both the transitional and alkaline lavas are enriched in Nb, Ta, Ti, and Ba, and depleted in K and Pb on normalized incompatible element diagrams. The geochemical differences may be due to distinct source compositions. The trace elements and REE patterns suggest that garnet and amphibole played a significant role in the generation of the lavas. Batch partial melting modelling suggests that the transitional lavas were formed by higher degrees of partial melting (3%–5%) than the alkaline lavas (0.6%–2.0%). The geochemical and geochronological data imply that the transitional and alkaline volcanism was related to exchange between the upwelling asthenosphere and the lithospheric mantle which might have triggered lithospheric extension during the Cenozoic.