Growth of early continental crust by partial melting of eclogite

• Published in: Nature Vol. 425; no. 6958; pp. 605 - 609
• Main Authors: Rapp, Robert P, Shimizu, Nobumichi, Norman, Marc D
• Format: Journal Article
• Language: English
• Published: London Nature Publishing 09.10.2003; Nature Publishing Group
• Subjects: Accretion; Basalt; Continental crust; Crystalline rocks; Dehydration; Earth sciences; Earth, ocean, space; Ecology; Exact sciences and technology; Experimental petrology; Geochemistry; Granite; Island arcs; Melting; Plate tectonics; Rocks; Soil and rock geochemistry; Trace elements; experimental studies; eclogite; gneisses; schists; tonalite; continental accretion; basaltic composition; Precambrian; imbrication; granites; Terranes; dehydration; thrust sheets; trondhjemite; granodiorites; granitoids; igneous rocks; metamorphic rocks; partial melting; greenstone; crustal growth; subduction; island arcs; plutonic rocks; tectonic setting; Archean; plate convergence
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature02031

The tectonic setting in which the first continental crust formed, and the extent to which modern processes of arc magmatism at convergent plate margins were operative on the early Earth, are matters of debate. Geochemical studies have shown that felsic rocks in both Archaean high-grade metamorphic ('grey gneiss') and low-grade granite-greenstone terranes are comprised dominantly of sodium-rich granitoids of the tonalite-trondhjemite-granodiorite (TTG) suite of rocks. Here we present direct experimental evidence showing that partial melting of hydrous basalt in the eclogite facies produces granitoid liquids with major- and trace-element compositions equivalent to Archaean TTG, including the low Nb/Ta and high Zr/Sm ratios of 'average' Archaean TTG, but from a source with initially subchondritic Nb/Ta. In modern environments, basalts with low Nb/Ta form by partial melting of subduction-modified depleted mantle, notably in intraoceanic arc settings in the forearc and back-arc regimes. These observations suggest that TTG magmatism may have taken place beneath granite-greenstone complexes developing along Archaean intraoceanic island arcs by imbricate thrust-stacking and tectonic accretion of a diversity of subduction-related terranes. Partial melting accompanying dehydration of these generally basaltic source materials at the base of thickened, 'arc-like' crust would produce compositionally appropriate TTG granitoids in equilibrium with eclogite residues.