Oceanic ridge subduction is known to generate anomalous magmatism before and during the opening of the slab window. High Sr/Y intermediate magmas and slightly alkaline basalts with transitional arc-to-intraplate (OIB-like) signatures have been reported worldwide in such tectonic settings. Southernmost South America has been affected by the subduction of several spreading ridges from the late Oligocene to the present. Here we present new geochronological, petrographic, geochemical, and isotopic data to evaluate the petrogenesis of two Lower Miocene volcanic units from the Fuegian archipelago at ~56° S, formed during subduction of the Nazca-Phoenix spreading ridge. Comparison with igneous suites related to subduction of the Chile Ridge (currently being subducted beneath central Patagonia at ~46.5° S) reveals notable similarities in timing relative to slab-window formation and in chemical signatures. We interpret the high Sr/Y chemistry as resulting from amphibole fractionation and delayed plagioclase crystallization at depth from calc-alkaline melts, with input of terrigenous material via mantle contamination or lower crust assimilation. The slightly alkaline magmas reflect lower degrees of partial melting due to a different thermal structure of the slab-mantle wedge.