Crustal Convection Turned Out a Superior Craton—and Zimbabwe, Pilbara and Ukraine Archean Cratons Too

The size and spacing of TTG batholiths is remarkably uniform within and amongst Archean granitoid-greenstone terrains. On craton-spanning maps, the average distance between centres of batholiths is, on average, twice the present-day depth to the Moho in the Zimbabwe, Ukraine, Superior and eastern Pilbara cratons, wherein the present erosional surface that reveals the granitoid-greenstone cellular pattern well corresponds to paleo-depths near the brittle-ductile rheologic transition (~520C). It is a fundamental principle of fluid dynamics that an array of convection cells in a horizontal layer convecting by Rayleigh-Bénard (temperature-induced) or Rayleigh-Taylor (composition-induced) density instabilities has convection cell radii (half the distance between centres of cells) that scale near 1:1 with the thickness of the convecting layer, as is the case in all these granite-greenstone terrains. Heavy oxygen isotopic compositions of mafic granulites (intercalated with paragneisses) in the Archean lowermost crust exposed in the Kapuskasing Zone (south-central Superior), the Vredefort structure (Kaapvaal craton), and as Archean xenoliths in Tertiary diatremes in western Wyoming craton show them to be seafloor-altered metavolcanics that migrated to the base of the crust during the Archean. Metamorphic peak P-T measurements in supracrustal rocks spanning the prehnite-pumpellyite to garnet granulite facies in the Wawa-Kapuskasing crustal section preserve a precisely resolved, steep, conductive geotherm of ~40°C/km to ~12 km paleodepth and a much flatter gradient (~11°C/km) deflected toward an adiabatic gradient in convecting ductile rocks at ~12-40 km paleodepth. Metamorphic fluids released from supracrustal rocks that migrated to the base of the crust and underplated hotter overlying rocks lowered the solidus temperature of the fluid-metasomatised overlying rocks and induced production of silicic partial melts (TTGs) having spidergram spikes in relative abundance of water-soluble elements like spidergrams of Phanerozoic arc magma. The arc-like spidergram patterns are absent in most coeval greenstones, which have spidergram patterns resembling MORB and OIB melts of asthenospheric mantle. Arc-like spidergram patterns in Archean TTGs can be well explained without plate tectonics. Craton-scale cellular arrays of greenstone belts and TTG batholiths are inconsistent with plate tectonics.