The two-mica, sillimanite-bearing Silver Plume and St. Vrain batholiths of Colorado are representative of a distinctly peraluminous section of the 1.4 to 1.5 Ga anorogenic province of North America. Indicative of fundamental variations of lower-crust composition in this region of the continent, these granitic magmas were generated from an oxidized peraluminous quartzofeldspathic source at P ⩾ 10 kbar (⩾36 km). Despite mineralogic similarities, the rocks remain distinct from S-type orogenic granitoids in a high abundance of K, Ba, light rare earth elements, and other incompatible elements. This enrichment, although less than in some coeval metaluminous granites, results from limited partial melting under vapor-undersaturated conditions. Emplacement occurred at a depth possibly as shallow as 8 to 9 km (2.3 kbar) at a PH2O of 487 to 560 bar and temperature of 740 to 760 °C. These temperatures and pressures fall well beyond the conventional stability field of muscovite-melt equilibria and point to an enhanced stability of muscovite owing to its ferric and titaniferous nature in plutonic occurrences.