Ar-Ar Dating of the Nephelinite-to-Basanite Transition at Etinde and Mount Cameroon (Cameroon Volcanic Line, West Africa) Provides Insights Into the Origin of Intraplate Magmatism

Abstract

Intraplate magmatism is widespread in continental and oceanic domains. Some occurrences, such as Hawai'i, fit the predictions of the mantle-plume hypothesis well. However, many occurrences require alternative explanations. The Cameroon volcanic line (CVL) in West Africa is one of the most voluminous and long-lived non-plume intraplate magmatic provinces, providing an ideal location for testing alternative intraplate magma-generation hypotheses. Two CVL volcanoes, Etinde and Mount Cameroon, located on the African continental margin are composed mostly of nephelinite and basanite, respectively. We present 12 new Ar-Ar dates and show that the Etinde mafic nephelinites (0.572 ± 0.032 and 0.5152 ± 0.0073 Ma) predate the Mount Cameroon basanites (0.442 ± 0.014 Ma to present). Basanite samples from Etinde had much younger ages (0.113 ± 0.019 and 0.073 ± 0.011 Ma) and likely originated in the Mount Cameroon magmatic system. Indistinguishable radiogenic isotope ratios and similar primitive-mantle-normalized incompatible-element patterns indicate that the magmas feeding the two volcanoes share the same mantle source. The temporal progression from nephelinite (Etinde) to basanite (Mount Cameroon and minor, late eruptions on Etinde) is marked by a reduction in La/Yb and an increase in SiO₂ content in the most mafic magmas. These features are consistent with the progressive melting of a common carbonate-enriched mantle source in which the proportion of carbonate in the melt declined with increasing melt fraction. We propose that the carbonate-enriched mantle flowed outwards from beneath Africa and decompressed as it encountered a thinner lithosphere at the continent-ocean boundary, leading to magmatism at Etinde and Mount Cameroon.