Growth of early continental crust controlled by melting of amphibolite in subduction zones.

It is generally accepted that the first continental crust formed by melting of either eclogite or amphibolite, either at subduction zones or on the underside of thick oceanic crust. The compositions of early crustal gneisses and experimental studies support these interpretations, but our understanding of trace element partitioning could not distinguish between them up to now. Here, we show a clear contrast in trace element ratios of melts derived from amphibolites and those from eclogites. Partial melting of low-Mg# (<70) amphibolite can explain low Nb/Ta and high Zr/Sm in melts, as required for the early continental crust, whereas melting of eclogite cannot. This indicates formation of the earliest continental crust by melting of amphibolites in subduction zones and not by melting of eclogite nor Mg-rich amphibolites (Mg#>80) in the lower part of thick ocean crust. The low Nb/Ta seen in subduction zone igneous rocks of all ages is compelling geochemical evidence that melting of rutile-eclogite has never been a volumetrically important process.