A comparative study of two exposed sections of lower crust in a magmatic arc setting has shown an impressive similarity between the primitive magmas that feed continental and oceanic arcs, and has highlighted the critical role that intracrustal evolution plays in producing some of the characteristic chemical signatures of arc magmas erupted at the surface.  This study has focused on the Tonsina-Nelchina assemblage, a cross-sectional exposure from upper mantle-lower crust to volcanic carapace of the Jurassic Talkeetna island arc, located in south-central Alaska, and the Fiambala gabbro, a pluton that was intruded into the lower crust of the South American active continental margin in the Ordovician.