Abstract

Archean tonalite-trondhjemite-granodiorite (TTG) magmatic suites represent the oldest coherent pieces of felsic continental crust. We present results of long-duration dehydration-melting experiments on amphibolites to show that melting depths of >48 km are required to have sufficient garnet in the residuum for generating the degree of heavy rare earth element (REE) depletion documented in Early Archean TTG. This depth constraint is inconsistent with early crust evolution models that posit melting at the base of oceanic plateaus or oceanic crust to explain the origin of Early Archean continental crust. Alternative explanations for this early crust, e.g., subduction models, are problematic in that they do not readily provide a mechanism for initiation of subduction in a hotter Archean Earth. We hypothesize that intraoceanic subduction systems in the Archean originated due to gravitational instabilities produced by compositional and density contrasts between converging oceanic plateau and normal oceanic lithosphere. Our model differs from earlier models of subduction initiation in that subduction of oceanic lithosphere occurs through the ‘hot’ mantle residuum (protomantle lithosphere) at the base of newly formed oceanic plateau crust. Under a high Archean geothermal gradient, subducted oceanic crust would melt to produce TTG. This model explains the origin of subduction systems, TTG, TTG–mafic and/or ultramafic magma association, stabilization of continental crust, and the broadly coeval formation of cratons and their lithospheric roots.

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