GEOCHEMICAL PROBING OF CONTINENTAL DYNAMICS

The Earth’s continents appear to be unique in our solar system. These regions of thick and buoyant evolved rock ride high relative to their neighboring thinner and denser basaltic counterparts beneath the ocean basins. Whereas a half a century of oceanographic study has largely revealed the processes by which oceanic lithosphere is formed and recycled, the processes that create and modify continental lithosphere remain enigmatic, despite greater accessibility and an even longer period of scientific interrogation. As the EarthScope initiative to probe the structure, composition and evolution of North America gets underway, I review here the insights into continental evolution afforded by complementary geochemical investigations. The evolved bulk composition of the continental crust (andesite, on average) requires the recycling of mafic to ultramafic residues of crust formation into the deep Earth. This recycling can be accomplished in different ways: 1) through Wilson-cycle recycling of oceanic lithosphere (if evolved melts of oceanic crust create continental crust), 2) through the geochemical transformation of mafic rocks during subareal weathering and transport of soluble cations to the oceans, where some may be fixed in altered oceanic crust and thus preferentially recycled during subduction, 3) through deep lithosphere recycling during collisional orogeny. All three processes may have helped shaped the continents as we see them today and all three may be evaluated through geochemical studies. The third process, in particular, can be investigated through studies of deep-seated xenoliths and evolving magmatic products associated with orogeny, as illustrated by a case study from the eastern block of the North China craton. Here, thick, buoyant lithosphere typical of Archean cratons persisted through the Paleozoic, but was destroyed during Mesozoic tectonism, as witnessed by the presently high heat flow and seismic activity, a profound change in the composition of mantle xenoliths from the Paleozoic through the Cenozoic and eruption of unique magmatic products during the Mesozoic that appear to derive from foundered lower crust.