Mafic Mg-olivine (Fo88)+hornblende lamprophyre dikes, with Ni similar to 190 ppm and Cr similar to 390 ppm, cut late Miocene lavas in the Quebrada los Sapos a few kilometers west of the El Teniente Cu-Mo mine. These dikes have petrochemical affinities with other less primitive, Pliocene (2.9-3.9 Ma), olivine-free lamprophyres previously described from both within and in the vicinity of El Teniente. The mafic mantle-derived lamprophyre dikes from Quebrada los Sapos have La/Yb ratios of 10-13, higher than the ratios of 4-9 for older Late Miocene El Teniente Mafic Complex olivine basalts, suggesting a temporal decrease in the percent of partial mantle melting, consistent with the observed decrease in the volume of igneous rocks through time at this latitude, as well as the ultimate cessation of magmatism and >40 km eastward arc migration by the Late Pliocene. Less primitive olivine-free lamprophyres have higher La and lower Yb, resulting in higher La/Yb ratios of 15-44, due to crystal-liquid fractionation involving hornblende, but not plagioclase, the crystallization of which is suppressed by the high H2O contents of the lamprophyres. The lamprophyre dikes, as well as younger (1.8-2.3 Ma) olivine-bearing basaltic-andesite lava flows in the valley of the Cachapoal river, have Sr-87/Sr-86=0.7041 to 0.7049, epsilon(Nd)=+1.2 to -1.1 and Pb-206/Pb-204=18.60 to 18.68, while Middle to Late Miocene (6.5-13.9 Ma) El Teniente Volcanic and Plutonic Complex igneous rocks have lower Sr-87/Sr-86=0.7039 to 0.7041 and Pb-206/Pb-204=18.56 to 18.59, and higher epsilon(Nd)=+1.9 to +3.8, and older Oligocene to Early Miocene (>15 Ma) Abanico or Coya-Machali Formation volcanic and plutonic rocks in the region have even lower Sr-87/Sr-86=0.7033 to 0.7039 and Pb-206/Pb-204=18.45 to 18.57, and higher epsilon(Nd)=+3.8 to +6.2. The data indicate a significant progressive temporal evolution, between the Oligocene and the Pliocene, to higher Sr-87/Sr-86 and Pb-206/Pb-204, and lower epsilon(Nd) for mantle-derived mafic magmas, and by implication their mantle source region. Significantly Sr, Nd and Pb isotopic compositions are independent of SiO2 content for rocks in each age group, which precludes contamination by isotopically heterogeneous Paleozoic and Mesozoic continental crust during evolution of intermediate and silicic rocks from mantle-derived mafic magmas. The Oligocene to Pliocene isotopic evolution of the mantle source of the mafic magmas may be explained by an increase from 1% to 6% in the extent of mantle source region contamination by subducted components, including continental crust tectonically eroded off the continental margin. We attribute this to both decreasing subduction angle and increasing rates of subduction erosion associated with the southward migration of the locus of subduction of the Juan Fernandez Ridge. The lamprophyres also imply increased hydration of the mantle below this portion of the arc by the Pliocene, which may have played an important role in producing oxidized volatile-rich magmas and mineralization at El Teniente.