BATHOLITHS: AN INTEGRATED GEOLOGICAL, GEOCHEMICAL AND GEOPHYSICAL STUDY OF THE FORMATION OF CONTINENTS

The Batholiths Project includes 15 investigators (US and Canada) focused on understanding the evolution of Cordilleran magmatic arcs. We are focusing on Cordilleran arcs because they provide the key to understanding the distillation of continental crust from the mantle. Recent work on the Coast plutonic complex in the Canadian Cordillera has shown that the thickness of the felsic arc is as much as 30 km. Studies of the metamorphic carapace indicate crustal thicknesses in excess of 50 km during batholith formation. Experimental studies and geochemical modeling of arc magmas imply that such thick arcs should be underlain by ultramafic garnet pyroxenite residues produced by extraction of the felsic batholiths. The integrated composition of the batholith and its residues results in a crustal column with a basaltic bulk composition. The fate of the residues is critical to understanding the evolution of continents. These residues have seismic properties that are indistinguishable from typical mantle lithologies. However, their density is higher than typical mantle rocks. This leaves two possible end member fates for these residues: 1) The residues reside hidden below the seismic Moho, and 2) The residues founder into the mantle (delaminate) due to their high density. If the first process is dominant, then our understanding of the composition of continental lithosphere needs to be reevaluated. If the second process occurs, then the formation of continents results from a complex two-stage recycling process.