The late Miocene Cordillera Blanca Batholith lies directly over thick (50 km) crust, inboard of the older Cretaceous Coastal Batholith. Its peraluminous ‘S’ type mineralogy and its position suggest recycling of continental crust, which is commonly thought to be an increasingly important component in magmas inboard of continental margins. However, the peraluminous, apparent ‘S’ type character of the batholith is an artefact of deformation and uplift along a major crustal lineament. The batholith is a metaluminous ‘I’ type and the dominant high-silica rocks (>70%) are Na rich with many of the characteristics of subducted oceanic slab melts. However, the position of the batholith and age of the oceanic crust at the trench during the Miocene preclude slab melting. Instead, partial melting of newly underplated Miocene crust is proposed. In this dynamic model newly underplated basaltic material is melted to produce high-Na, low HREE, high-Al ‘trondhjemitic’ type melts with residues of garnet, clinopyroxene and amphibole. Such Na-rich magmas are characteristic of thick Andean crust; they are significantly different from typical cole-alkaline, tonalite-grano-diorite magmas, and their presence along the spine of the Andes provokes questions about models of trondhjemite genesis by melting of subducted oceanic crust, as well as any generalized, circum-Pacific model involving consistent isotopic or chemical changes inboard from the trench.