Contrasting sources for upper and lower continental crust: The greenstone connection

Greenstones may be useful in tracking the role of oceanic plateaus in the growth of continents. Lithologic proportions, Th/Ta ratios, and Ni concentrations in greenstone basalts of all ages show that arc-related greenstones greatly exceed oceanic plateau and MORB-related greenstones in abundance. This distribution may be accounted for by the preferential obduction of arcs formed on top of oceanic plateaus during collision of plateaus with continents. Because thick oceanic plateaus resist subduction, a significant volume may be accreted to continental margins during collisions; and over time, these plateaus may evolve into lower continental crust. This idea has important implications for continental development in that the lower continental crust may come chiefly from accreted oceanic plateaus, while upper continental crust forms by subduction-related processes. Consistent with this model are high seismic wave velocity layers in the lower crust of Proterozoic cratons similar to high-velocity layers in the lower crust of oceanic plateaus. Mafic xenoliths from the lower continental crust also have high measured seismic wave velocities. Relatively low Th/Ta ratios and high Ni contents of many lower crustal mafic xenoliths are also consistent with the model. The model can account for the common lack of a positive Eu anomaly in lower crustal rocks. Wrangellia, an oceanic plateau accreted to the American Cordillera in the Cretaceous, may provide us with a young and still-evolving example of continental crust forming from two sources: the lower crust from an accreted oceanic plateau and the upper crust from subduction-related processes. Growth of the lower continental crust during the Archean may have occurred rapidly as buoyant oceanic crust and oceanic plateaus were accreted to existing continents, especially during the Late Archean when a supercontinent may have formed. The first continents in the Archean may have formed by collision of ocean ridge/plateau crustal blocks with each other followed by subduction zones developing around their margins leading to the production of felsic upper crust.