K–Ar mineral ages from a post-tectonic pluton give a minimum age of 1690 m.y. for deformation and metamorphism of the Precambrian Yavapai Series near Jerome, Arizona. Rb–Sr measurements of whole-rock samples of silicic metavolcanic rocks indicate an age of 1610 ± 85 m.y. for one locality of the Deception Rhyolite and an 'age' of 1640 ± 38 m.y. for a suite of samples from the Ash Creek Group. Samples of the Alder Group give ages of less than 1600 m.y., but since the data and field relations suggest that these samples have lost radiogenic strontium, this age is probably a minimum.