Combined field, seismic, xenolith, and heat flow data from the Four Corners region of the Colorado Plateau permit construction of a four-layer crustal model in which a thin sediment layer is underlain sequentially by Early Proterozoic supracrustal rocks and granitoids, intermediate plutonic rocks, and finally by mafic granulites and amphibolites. Both upper and lower crust show Nb-Ta depletion, Pb enrichment, and negative Eu anomalies, suggesting an arc origin in which plagioclase was an important fractionating phase throughout the crust. The relative abundance of felsic rocks in the section suggests that the arc formed on continental lithosphere. The bulk crust has approximately andesitic composition, and mass balance calculations indicate that significant volumes of mafic cumulate and/or restite must have been recycled into the mantle during crustal formation in the Proterozoic. Neither the xenolith nor the seismic data support mafic underplating of the Colorado Plateau crust since Proterozoic crustal assembly.