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 are-relate d greenstones greatly exceed oceanic plateau and MORE-related greensto nes in abundance. This distribution may be accounted for by the prefer ential obduction of arcs formed on top of oceanic plateaus during coll ision of plateaus with continents. Because thick oceanic plateaus resi st subduction, a significant volume may be accreted to continental mar gins during collisions; and over time, these plateaus may evolve into lower continental crust. This idea has important implications for cont inental development in that the lower continental crust may come chief ly 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 plateau s. Mafic xenoliths from the lower continental crust also have high mea sured seismic wave velocities. Relatively low Th/Ta ratios and high Ni contents of many lower crustal mafic xenoliths are also consistent wi th the model. The model can account for the common lack of a positive Eu anomaly in lower crustal rocks. Wrangellia, an oceanic plateau accr eted 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 con tinental crust during the Archean may have occurred rapidly as buoyant oceanic crust and oceanic plateaus were accreted to existing continen ts, especially during the Late Archean when a supercontinent may have formed. The first continents in the Archean may have formed by collisi on of ocean ridge/plateau crustal blocks with each other followed by s ubduction zones developing around their margins leading to the product ion of felsic upper crust.