The object of this study is to construct a lithologic and geochemical model of the crust in the central Colorado Plateau, based on the combined input of field, xenolith, seismic, and heat flow data. The Four Corners area straddles a proposed Proterozoic crustal boundary (fig. 1; Silver 1968; Bennett and DePaolo 1987; Condie 1992; Karlstrom and Bowring 1993), and we make use of differing xenolith populations on opposite sides of the boundary (Selverstone et al. 1999) to test the sensitivity of our model to varying contributions of metasedimentary and igneous components in the crust.