A 60-Myr record of continental back-arc differentiation through cyclic melting

Continental crust forms and evolves above subduction zones as a result of heat and mass transfer from the mantle below. The nature and extent of this transfer remain debated. Although it has been recognized that arc magmatism at active continental margins can be cyclical at 50- to 1-Myr timescales, such cyclicity has not been recognized in the back-arc. Here we investigate the melting of sedimentary rocks in the continental back-arc of the western Gondwana margin during the Cambrian–Ordovician Famatinian orogeny of Northwest Argentina. We determine the U-Pb ages of zircons that formed during crustal melting and find that they range from 505 to 440 million years ago, concentrated into age groups spaced by 10–15 Myr. This suggests multiple and cyclical melting events in the continental back-arc, which matches the more than 60 Myr duration of the magmatic activity in the arc and demonstrates that thermal and magmatic cyclicity also extends to the continental hinterland. We conclude that back-arc melting reflects a long-lasting, pulsating, cyclical heat transfer from mantle to crust that leads to thorough crustal reworking, transforming sedimentary rocks into crystalline continental crust. Thus, while magma transfer from the mantle promotes crustal growth in convergent margins, cyclical melting promotes crustal maturation of the back-arc.