ROOT FOUNDERING IN CORDILLERAN ARCS - AN UPDATED REVIEW OF OBSERVATIONAL EVIDENCE, EXPERIMENTAL BACKGROUND AND THEORETICAL MODELING

Numerous lines of evidence point to an interesting mechanism of mass transfer from the Earth's crust to the mantle in magmatic arcs - it has been called delamination, convective removal, or foundering. The process, first described by Kay and Kay (1991) is distinct from thermally driven convective instabilities that have been long postulated for mantle materials in collisional orogens, although the same names are being used for both processes. Large composite batholiths, such as the ones formed along coastal North and South America generate roots that are, under some circumstances denser than the underlying mantle. Deep, dense roots are primarily eclogitic and comprise garnet and two-pyroxenes, whereas shallower, buoyant roots are granulitic and comprise plagioclase and pyroxenes. An added complexity to the composition of arc roots is the presence of amphibole in the residue of some arcs. Deep, feldspar-free roots are unstable and prone to foundering in the mantle. In this presentation, I will briefly review key observations and recent progress towards understanding arc root removal, outline several unresolved issues and directions for future research. Worldwide examples from arcs with dense roots and without dense roots will be given. A special emphasis will be put on reviewing recent efforts to predict the composition of arc roots using petrological models, and understand the fate of these residues, using geodynamic modeling constraints.