Sampling of the Pacific-Antarctic Rise close to the Foundation hotspot in the South Pacific reveals that silicic lavas erupt together with basalts on a 130-km-long part of this mid-oceanic ridge, representing the most extensive occurrence of andesites on the submarine spreading system. Most of these tholeiitic andesites and dacites have negative Nb and Ta anomalies, a signature that has so far been attributed only to subduction-related magmas and the continental crust. The silicic lavas formed by fractional crystallization from basalts and assimilation of melts from hydrothermally altered amphibolite in the oceanic crust. The presence of andesites with negative Nb and Ta anomalies on the Pacific-Antarctic Rise implies that such magmas are not restricted to subduction zones but can form at a plume-influenced mid-oceanic ridge. The andesites and dacites from the Pacific-Antarctic Rise may represent analogs to some Precambrian volcanic rocks and early continental crust.