Petrogenetic processes at the tipping point of plate tectonics : Hf-O isotope ternary modelling of Earth’s last TTG to sanukitoid transition.

Abstract

Modern style plate tectonics is characterized by one-sided subduction and continental margin basaltandesite-dacite-rhyolite (BADR) magmatism, whereas continental magmatic rocks in the Archaean record had tonalite-trondjhemite-granodiorite (TTG) composition. Their main difference is the absence (in the modern style) and the presence (in the ancient style) of abundant juvenile, basalt-derived felsic magmatism. The diversity of modern continental magmas depends partly on metasomatic processes in the mantle wedge providing sediment input (melt/fluid). Contrasting scenarios in the rock record therefore are the presence or absence of (1) basalt-derived melt (TTG) and (2) sedimentary input to the magmas. A late, “Archaean-style” tectonic regime is recorded in the Palaeoproterozoic Mineiro Belt (Brazil) using whole-rock geochemistry of its plutons coupled to zircon Hf and O isotopes from these rocks and from detrital grains from the local (meta)sediments. Increasing δ18O with decreasing εHf(t) values in zircon indicate oceanic crust recycling and input of sediments to the mantle wedge during the Palaeoproterozoic. Since some form of early subduction including the formation of a mantle wedge has occurred since the Mesoarchaean, it seems that the onset of subduction and the final establishment of modern plate tectonics at the global scale are temporally distinct. The latter did not occur until the Palaeoproterozoic. Prior to this time (“ante-plate tectonics”), punctuated subcretion/subduction marks a gestational stage that took c. 800 Myr to transition diachronously to a truly global mechanism. The TTGsanukitoid transition, which signals the opening of a mantle wedge, occurred in the Mineiro Belt during the early Proterozoic “magmatic lull” and thus represents the last gasp of the ante-plate tectonics regime.