Isotopic data for Quaternary volcanic centers from lat 17.5° to 22°S in northern Chile provide evidence for crustal contributions to arc magma genesis in the central Andes and delineate basement domains. Crustal thickness, distance from trench, height of volcanic edifices above the seismically active subduction zone, and sediment supply to the trench are all constant along this segment of the arc. The only significant variable is crustal age (Paleozoic in the south, Proterozoic in the north). Pb isotopic compositions of Quaternary lavas in the northern sector are lower (206Pb/204Pb = 17.89-18.28) and Sr and O isotope ratios are less variable (87Sr/86Sr = 0.7058-0.7077, δ18O = +6.5‰-+8.4‰) than in the southern sector (206Pb/204Pb = 18.59-18.79, 87Sr/86Sr = 0.7053-0.7083, δ18O = +6.4‰-+9.3‰) for rocks similar in bulk composition, from 53 to 74 wt% SiO2. Together, these features are interpreted to reflect interaction of magmas with crustal contaminants of different isotopic character, rather than changes in the mantle source of central Andean are magmas resulting from either subduction erosion or involvement of old enriched lithosphere.