MELTARC · Modelling and geochemical implications of intra-arc melting: consequences for the composition of the continental crust

Arcs are expected to be the fundamental brick of the Continental Crust (CC). The main problem with this hypothesis is that the bulk CC has an andesitic composition while most arcs are formed by successive inputs of basaltic magmas.If island arcs accretion is indeed the main mechanism of crustal growth, one must explain how arcs can reach an intermediate composition and if these processes are effective in the nature.Intra-arc differentiation takes place in their lower crustal section. Removal of magmatic cumulates is not a sufficient process to drive the bulk composition of island arcs towards intermediate ones. Because melting and dehydration lead to the production of dense iron-rich melting and/or dehydration residues, it could causes delamination of mafic-ultramafic rocks into the mantle due to gravitational instability in the lowermost arc crust. Melting at the basis of arcs is most probably a necessary conditions to evolve towards andesitic bulk composition.The mechanisms of arc differentiation and their consequences are still poorly understood