Generic provenance data from the Montgomery Creek Formation, northern California, even in light of paleocurrent data and regional stratigraphic patterns, do not uniquely uniquely establish its sediment source. Dating individual detrital mica grains with the 40Ar/39Ar laser probe shows that the source area for the upper Montgomery Creek included the Idaho batholith. Unaltered muscovite grains yield data that provide limits on timing and rate of uplift of the batholith. Combined with paleocurrent data, the inferred provenance constrains the timing of tectonic rotation in the southeastern Klamath orogen. Dating variably altered biotite grains elucidates the age and environment of alteration and establishes a distinct source for some of the lower Montgomery Creek.