Felsic plutonic rocks in the northwestern portion of the Ruby geanticline of north-central Alaska occur as two distinct rock suites: (1) silica-saturated monzonite and syenite and (2) silica-oversaturated granite. K–Ar mineral ages for each suite average 107 ± 6 Ma, and both suites plot along a single Rb–Sr whole-rock isochron that yields an age of 112 ± 4 Ma and an initial 87Sr/86Sr ratio of 0.70778 ± 0.00006. Whole-rock δ18O values range from + 6.75 to + 7.30‰. The two suites cannot be related by simple fractionation, yet they appear to have a chemically and isotopically similar parental magma. Based on chemical mixing calculations, observations of magma – wall-rock interaction and xenolith abundances, we suggest a model whereby a small amount of assimilation of siliceous wall rock by a portion of an originally silica-saturated magma caused an increase in silica saturation, which led to two distinct fractionation suites. The almost exactly silica-saturated composition and extremely high Sr concentrations indicate that the silica-saturated magma formed at high pressure. This evidence, combined with the initial 87Sr/86Sr ratio and δ18O values, leads us to suggest a lower crust or enriched mantle source for the magma. The silica-saturated suite may be representative of a parental magma that contributed to the melting of upper crustal siliceous wall rock throughout the Ruby geanticline. Whereas a small amount of crustal contamination led to the silica-oversaturated rocks that occur in the northwestern part of the Ruby geanticline, larger amounts of assimilation may have formed the largely anatectic granites of the same age that are found in the interior of the Ruby geanticline.