Potassium-argon age determinations in the Tucson Mountains, Pima County, Arizona, indicate two main periods of volcanic activity: the Laramide (57 m.y.–75 m.y. ago); and mid-Tertiary (20 m.y.–30 m.y. ago). The Laramide interval also produced some large plutons. Igneous activity was sparse or absent at other times.

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