Basalts within mafic complexes in southern Chile, interpreted as remnants of the mafic portion of the floor of an Early Cretaceous extensional back-arc basin, exhibit a tholeiitic differentiation trend. The least differentiated basalts are olivine normative. Both Chilean basalts and the dominant basalt type sampled from present-day back-arc basins resemble, with respect to FeO*/MgO, Ni, and Cr, ocean ridge rather than island arc tholeiites. A wide range of K/Rb, Rb/Sr, and (Ce/Yb)N is exhibited by Chilean basalts as well as basalts from present-day back-arc basins. The observed variations in K/Rb, Rb/Sr, and (Ce/Yb)N in Chilean basalts are interpreted as secular compositional changes. Relatively low K/Rb?250, high Rb/Sr?0.075, and high (Ce/Yb)N?1.7 basalts were emplaced during the early stages of development, and relatively high K/Rb?850, low Rb/Sr?0.020, and low (Ce/Yb)N?0.6 basalts, compositionally indistinguishable from 'normal' ridge segment oceanic tholeites, were emplaced furing later stages of development of the back-arc basin. The observed differences in K/Rb, Rb/Sr and (Ce/Yb)N for basalts of similar FeO*/MgO could not have resulted from different depths, degrees or single-stage processes of melting of a chemically homogeneous mantle. Instead, these differences must reflect compositional differences in the mantle source region, as has also been inferred for 'nornal' and 'plume' segment ocean ridge tholeiites. Secular variations in th composition of the mantle source may result from sequential tapping of different depths of a chemically layered mantle but may also result from progressive depletion in large-ion lithophile elements of an initially homogeneous mantle due to removal of successive partial melt fractions. The data suggest that a limited volume of mantle source material which evolves compositionally by progressive partial melting and magma extraction is an appropriate model for the source regions of back-arc basin basalts.