Temporal variation and carbonatite contamination in primitive ocean island volcanics from São Vicente, Cape Verde Islands

Abstract

Primitive samples (MgO>5%) that are representative of the volcanic stratigraphy of São Vicente were analysed for major and trace element compositions and Nd, Sr and Pb isotopes. Mantle-normalized trace element patterns of the samples have negative anomalies of large ion lithophile elements such as Rb and K and positive anomalies for Nb, Ti and Zr, as is typical for some ocean island basalts (OIB). The oldest volcanics (OV) and the youngest volcanics (RV) are picrobasalts and basanites, whereas the intermediate age volcanics (IV) are nephelinites. Carbonatite dykes intrude the OV but are not observed in IV, indicating intrusion in the late part of the OV phase. OV have the most radiogenic Sr, least radiogenic Nd, and highly radiogenic Pb. Thus, OV comprise the most direct representation of a plume-derived HIMU component. The radiogenic isotopes of IV vary between depleted mantle (DM) and HIMU components. The isotopic compositions of RV indicate a predominance of DM. Isotopic differences between carbonatites and basaltic magmas are evidence of separate HIMU sources for these two magma types. Large negative Δ8/4 ( 208Pb/ 204Pb deviation from the Northern Hemisphere Reference Line: NHRL) is a feature of the carbonatites and basaltic rocks younger than OV. The IV also show an overall enrichment in incompatible elements, and have anomalously high CaO, P 2O 5 and Sr, and high Sr/Nd, P/Nd and P/Zr, without concomitant enrichments in Ti and Zr. The negative Δ8/4 and element enrichments of basaltic rocks younger than OV indicate assimilation of carbonatite. The assimilation of carbonatite is thought to have occurred while the basaltic magmas passed through lithosphere that was previously traversed by carbonatite.