The eastern and central Beartooth Mountains of Montana and Wyoming are composed of 2,800-m.y.-old granitoids that intruded older rocks of variable age and composition. The most abundant rock type among these older units is a 3,000-m.y.-old amphibolite with a major-element composition similar to modern andesites. The concentrations of the incompatible trace elements, particularly the light rare earth elements, however, are much higher than those of most modern or ancient andesites. In addition, the range in concentration of many major and trace elements is large, and strong correlations for both mobile and immobile elements are evident. This coherence suggests that the observed element-abundance patterns in these rocks are not the result of metamorphic or other postcrystallization, nonisochemical processes. The compositional features of these andesitic rocks seem to be explained most easily as the result of variable degrees of partial melting of a source enriched in incompatible elements, followed by variable extents of crystallization of the resulting magmas. The mafic compositions of these magmas and limited Sm-Nd isotopic data suggest that the magmas were of mantle origin and that they were generated with the aid of incompatible-element–rich or metasomatic fluids.