Archaean Cratons Record a Duality of Melting Regimes During Early Continental Growth

Abstract

The tectonic setting and pressure-temperature conditions responsible for the formation of felsic crust on the early Earth remain debated. Rare earth elements (REE) have been extensively used to study the formation of tonalite-trondhjemites-granodiorites (TTGs)- the building blocks of the early felsic crust, but conclusive interpretations based on the chondrite-normalized REE patterns have not materialised because of the inability to distinguish subtle differences. Here we apply a polynomial approach that quantifies the REE patterns by describing their slope and curvature using shaping coefficients to the TTG compositions from five different Archaean cratons. In combination with partial melting modelling, this enables an assessment of the effects of variations in pressure-temperature, degree of melting and residual mineral assemblages on the formation of TTGs. The REE composition of the Archaean TTGs display two distinct trends: (1) a horizontal-trend suggesting their formation in the presence of garnet-poor amphibolitic residues, possibly formed at the base of a thickened crust; and, (2) an inclined-trend consistent with their formation in equilibration with amphibole-poor, but garnet-rich residues at convergent settings (but not necessarily related to plate tectonics). These different melting regimes coexisted during the Paleoarchaean to Neoarchaean and provide direct evidence for a duality of petro-tectonic regimes of felsic crust formation on the early Earth.