Generation of sodium-rich magmas from newly underplated basaltic crust

Plutonic and volcanic rocks from the Cordillera Blanca complex in Peru are described which have characteristics of the high-Al trondhjemite - tonalite - dacite (TTD) suite but which were produced above a subduction zone containing a 60-Myr-old slab. Evidence is presented that the complex formed by partial melting of newly underplated basaltic crust. It is argued that this mechanism should be considered more generally as an additional way of generating sodium-rich arc magmas.