The crust of the Colorado Plateau: New views of an old arc

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 Pr oterozoic supracrustal rocks and granitoids, intermediate plutonic rocks, a nd finally by mafic granulites and amphibolites. Both upper and lower crust show Nb-Ta depletion, Pb enrichment, and negative Eu anomalies, suggesting an are origin in which plagioclase was an important fractionating phase th roughout the crust. The relative abundance of felsic rocks in the section s uggests that the are 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 r ecycled into the mantle during crustal formation in the Proterozoic. Neithe r the xenolith nor the seismic data support mafic underplating of the Color ado Plateau crust since I)Proterozoic crustal assembly.