Geochemical evolution of Andean arc magmatism over 215 Myr: Influence of South Atlantic opening and variations in subduction-eroded material

The Andean orogen represents a classic example of long-lived subduction-related magmatism accompanied by major geochemical and geodynamic changes. To trace the evolution of the full Andean subduction cycle, we present major and trace elements and Sr-Nd-Pb-Hf isotope data for 99 lavas, dikes and plutonic rocks from the Southern Central Volcanic Zone between 26 and 29°S. The majority of samples exhibit low Nb/(Th, La) N ratios and high fluid-mobile to immobile trace element ratios, indicating a persistent subduction-zone signature throughout the past ∼215 Ma. Radiogenic isotope data define coherent mixing trends that converge toward a common enriched MORB-like component, interpreted as a long-lived subduction-modified mantle source. Isotopic compositions evolved from enriched continental-like signatures in the Triassic (radiogenic 87 Sr/ 86 Sr, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb at a given 206 Pb/ 204 Pb, and unradiogenic 143 Nd/ 144 Nd and 176 Hf/ 177 Hf), toward increasingly depleted, enriched MORB-like compositions during the Jurassic-Cretaceous, followed by a progressive return to more continental-like signatures during the Cenozoic. Correlations between SiO 2 and radiogenic isotope ratios indicate that crustal assimilation was an important control on this isotopic evolution, whereas isotopically enriched primitive samples additionally require recycling of continental material into the mantle source through continental margin sediment subduction and forearc subduction erosion. The geodynamic and geochemical evolution reflects successive stages of plateau accretion, slab break-off, subduction re-initiation, and transitions from compressional to extensional phases. Compression resulting from plateau accretion promoted continental crust-like isotopic signatures, whereas extension resulting from slab break-off increased asthenospheric input and generated increasingly depleted Jurassic-Cretaceous magmatism. A shift to compression during the Cretaceous, potentially linked to opening of the South Atlantic, enhanced progressive lithospheric thickening, crustal assimilation, and subduction erosion, resulting in more enriched continental crust-like compositions. Temporal variations in the composition of subduction-eroded material and its recycling into the melting source, from dominantly oceanic to continental material, provide an additional mechanism to explain the pronounced contrast between Jurassic-Cretaceous depleted arc magmatism and Cenozoic continental crust-like signatures.