Generation of sodium-rich magmas from newly underplated basaltic crust

SODIUM-RICH rocks of trondhjemite-tonalite-dacite (TTD) or -granodiorite (TTG) suites form much of Precambrian continental crust¹. They are thought to have formed by partial melting of subducted oceanic crust ^2,3 - a process that would have been much more widespread early in Earth history than at present, owing to the higher thermal gradients prevailing at that time⁴. Phanerozoic TTD suites do exist, however, and seem also to relate to subduction zones⁵. Defant and Drummond⁶ proposed that these suites form where young (<25 Myr), hot oceanic lithosphere is subducted and melts, thus locally simulating the conditions that led to widespread crustal growth in the Archaean. Here we describe plutonic and volcanic rocks from the Cordillera Blanca complex in Peru, which have characteristics of the high-Al TTD suite but which were produced above a subduction zone containing a 60-Myr-old slab. We present evidence that the complex formed by partial melting of newly underplated basaltic crust, and argue that this mechanism should be considered more generally as an additional way of generating sodium-rich arc magmas.