CONTINENTS AS LITHOLOGICAL ICEBERGS - THE IMPORTANCE OF BUOYANT LITHOSPHERIC ROOTS

An understanding of the formation of new continental crust provides an important guide to locating the oldest terrestrial rocks and minerals . We evaluated the crustal thicknesses of the thinnest stable continen tal crust and of an unsubductable oceanic plateau and used the resulti ng data to estimate the amount of mantle melting which produces perman ent continental crust. The lithospheric mantle is sufficiently deplete d to produce permanent buoyancy (i.e., the crust is unsubductable) at crustal thicknesses greater than 25-27 km. These unsubductable oceanic plateaus and hotspot island chains are important sources of new conti nental crust. The newest continental crust (e.g., the Ontong Java plat eau) has a basaltic composition, not a granitic one. The observed stru cture and geochemistry of continents are the result of convergent marg in magmatism and metamorphism which modify the nascent basaltic crust into a lowermost basaltic layer overlain by a more silicic upper crust . The definition of a continent should imply only that the lithosphere is unsubductable over greater than or equal to 0.25 Ga time periods. Therefore, the search for the oldest crustal rocks should include rock s from lower to mid-crustal levels.