Role of the sub-continental lithosphere in magma genesis at active continental margins

Volcanic are basalts can usually be distinguished from basalts erupted in other settings by their selective enrichments in LIL elements and also, in some cases, in light rare-earth elements and P. Ibis enrichment component is independent of the nature of the overlying mantle wedge and is assumed to be subduction-derived. However, basalts erupted at active continental margins carry an additional geochemical component, which is not observed in basalts front oceanic island arcs. This component, which contains Nb, Ta, Zr and Hf in addition to the elements mentioned above, cannot realistically have a subduction or crustal origin and is thus assumed to be derived from trace element enriched metasomatized sub-continental lithosphere. The relative contributions of the subduction and lithosphere components to the composition of active continental margin basalts can be calculated from their MORB-normalized geochemical patterns. In a typical basalt, from Central Chile, the subduction zone can be shown to make the dominant contribution to its content of Rb, K, Ba, 71 and Sr, whereas the lithosphere contributes the greater proportion of its content of Ce, Sm and P and all its content of Ta, Nb, Zr, Hf, Ti, Y and Yb.