Arc volcanoes with back-arc signatures: Geochemistry of five Quaternary composite volcanoes at the southern end of the Central Volcanic Zone of the Andes (26.5–27.1°S)

Volcanism along continental subduction zones commonly consists of a main arc front dominated by composite volcanoes and a rear- or back-arc region characterized by more dispersed, mafic, and geochemically distinct volcanism. At ∼27°S in the southern Central Volcanic Zone (CVZ) of the Andes, however, composite volcanoes located along an inflection of the main arc towards the back-arc, spatially associated with the projection of the subducting Copiapó Ridge, display geochemical features transitional between arc and back-arc magmatism. We present new whole-rock major and trace element data, together with Sr-Nd-Pb isotope compositions, of lavas from five Quaternary composite volcanoes that define this anomalous arc segment: Peinado, El Cóndor, Falso Azufre, San Francisco and Incahuasi. Major and trace element geochemistry indicates magmatic evolution within the upper crust dominated by fractional crystallization of plagioclase and amphibole, assimilation of crustal material, and mafic recharge, similar to other main arc composite volcanoes along the southern CVZ. However, isotopic compositions, in particular relatively high 206Pb/204Pb and 143Nd/144Nd, and relatively low 87Sr/86Sr, point to the involvement of a radiogenic Pb isotopes mantle source, more typical of back-arc mafic volcanism. This back-arc affinity is further supported by elevated K2O contents and trace element ratios such as low Ba/La, indicative of lower slab-fluid input. Temporal constraints suggest that the contribution of this radiogenic Pb mantle component has increased with time since ∼1 Ma. Older lavas from Incahuasi record a more typical arc-like signature, consistent with mixing between depleted mantle and crustal components, whereas younger lavas, particularly from Peinado and El Cóndor, require an additional radiogenic Pb component, similar to the source inferred for mafic monogenetic centers in the back-arc Puna region. We propose that the source of this radiogenic Pb component may be a subcontinental lithospheric mantle reservoir akin to that sampled in the Cretaceous rift system of NW Argentina. Its involvement since ∼1 Ma in the study region may be related to extensional tectonism, which in turn could be associated to lithospheric delamination.