Lahar deposits within the Late Cretaceous McDermott Formation (Maastrichtian) contain abundant trachyandesite volcanic clasts with a narrow range of whole rock compositions. The outcrop pattern and thickness variations of the McDermott Formation suggest a source located in the general area of the present-day La Plata Mountains. Major and trace element trends indicate a possible petrogenetic relationship between the McDermott trachyandesite and La Plata Mountains intrusive rocks. Incompatible trace element compositions show subduction signatures in the trachyandesite and La Plata intrusive rocks, as well as in lower crustal xenoliths from the nearby Navajo volcanic field. Trace element patterns of Proterozoic units from the adjacent San Juan uplift rule out involvement of these upper crustal units in McDermott petrogenesis. Near zero ∊Ndt (−1.94 and 0.47) values rule out a Precambrian crustal source for these magmas as crustal values typically range from −6 to −18. Ba/La vs. Nb/La plots and ∊Ndt data suggest a subduction-modified continental lithospheric mantle source for these rocks. This lithospheric mantle may have acquired its orogenic trace-element signature during formation of the Proterozoic Yavapai terrane, which underlies the region.