Abstract

Sr-Nd-Pb isotopic studies have been carried out on the lavas of Mt. Oku, the central part of the continental sector of the Cameroon volcanic line (CVL). The lavas define a considerable range of Sr-Nd-Pb isotopic compositions characterized by ¹⁴³Nd/¹⁴⁴Nd = 0.512626 – 0.512937,⁸⁷Sr/⁸⁶Sr = 0.699411 - 0.704279 and more radiogenic isotopic ratios ²⁰⁶Pb/²⁰⁴Pb = 18.3800 – 19.4480. ²⁰⁷Pb/²⁰⁴Pb = 15.4216 – 15.6448, ²⁰⁸Pb/²⁰⁴Pb = 37.1114 – 39.7507, with radiogenic Pb similar to the HIMU of the continent ocean boundary of the Cameroon volcanic line. These isotopic data display a linear array on the ¹⁴³Nd/¹⁴⁴Nd vs ⁸⁷Sr/⁸⁶Sr diagrams trending from MORB towards bulk earth. ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr vs. ²⁰⁶Pb/²⁰⁴Pb show a continuum of compositions that suggest differing contributions to the parental magmas from three end-members. One end-member corresponds to a mixture of high uranium high lead (HIMU) and depleted mantle (DM) source components while the other end-member corresponds to an enriched mantle source thought to be the MORB source or sub-lithospheric source. The enriched geochemical signatures (EM) of the Oku magmas are unlikely to be the result of crustal assimilation during magma ascent. Rather, they are thought to be derived from a sub-continental lithospheric mantle enriched in incompatible trace elements by ancient metasomatic processes. The HIMU component is thought to have been inherited from the fossil plume that underlay the Equatorial Atlantic during the period 130 to 100 Ma before migrating to its present site. The radiogenic initial ²⁰⁶Pb/²⁰⁴Pb ratios of the samples (~18.3800 – 19.44804) furthermore require the involvement of an ancient HIMU mantle plume in the magmatism and this may have been formed by metasomatism of fluids derived from St. Helena hot plume ca 120 Ma.

Key words:  Oku, isotopes, metasomatism, HIMU, enriched geochemical signatures (EM), mantle, enrichment.