Mantle Dynamics: A Nd, Sr, Pb and O Isotopic Study of the Cameroon Line Volcanic Chain

The Cameroon line comprises a 1600-km long Y-shaped chain of < 30 m.y. old volcanoes and <70 m.y. old plutons extending into mainland Africa from the Atlantic island of Pagalu. The distribution of basaltic volcanic centres is ideal for comparing sub-continental and sub-oceanic sources for basalts and for studying the influence of the lithosphere on magma generation. We report Nd, Sr, Pb and O isotopic data for more than thirty (principally basaltic) samples from all the main volcanic centres together with data for two granulite facies xenoliths. Those basalts which display no obvious evidence of crustal contamination yield initial 87Sr/86Sr ratios ranging from 0.7029 to 0.7035, εNd between +2 and +7 and 206Pb/204Pb between 19·0 and 20·6. The Nd and Sr isotopic compositions define a field on the left hand side of the ‘mantle array’ (that is with relatively unradiogenic Sr) and include some data which show overlap with compositions observed for St. Helena. 208Pb/204Pb ratios extend to 40·4—amongst the more radiogenic observed for alkali basalts. The Nd and Sr isotopic data are similar in oceanic and continental sectors indicating that the magmas are derived from generally similar mantle sources. Despite this overall simple picture, the source of the Cameroon line volcanics has in fact been variable in both time and space. Pb is less radiogenic and Sr is more radiogenic in transitional to hypersthene-normative compositions. There is a progression to more radiogenic lead isotopic compositions with time for the Cameroon line as a whole that is most strikingly displayed in the 30 m.y. eruptive history of Principe. These space-time data are difficult to reconcile with conventional plume models or with some dispersed ‘plum pudding’ models. The heterogeneities require isolation times considerably longer than the age of the south Atlantic sea floor (120 Ma). The eruptive lavas with the most radiogenic Pb observed (accompanied by unradiogenic Nd) precisely straddle the continental edge (i.e. occur in both oceanic and continental sectors) with no dependency on Nd and Pb concentrations. A model is proposed which links these observations with the destruction of lithosphere, and the impregnation of the uppermost mantle by the St. Helena hot spot during the formation of the South Atlantic ocean. This mantle was subsequently melted to form the Cameroon line which appears to be derived from a rising hot zone initiated by the early plume activity. The magmatic products reflect the mantle mixing that took place during continental breakup, the consequent cooling and thickening of the lithosphere and the continued interaction between rising plume components and this lithosphere. The depth from which magmas are currently being tapped at the continent/ocean boundary is estimated at less that 150 km.