⁴⁰Ar/³⁹Ar age determinations for the Carrizo Mountains laccolith, Navajo Nation, Arizona
— Steven C. Semken and Wiliam C. McIntosh

We have obtained Late Cretaceous (73.8-70.65 Ma) ages for hornblende separates from three specimens of diorite porphyry from the Carrizo Mountains laccolith, located on the Navajo Nation in northeastern Arizona, using the ⁴⁰Ar/³⁹Ar step-heating method. The age spectra show no evidence of excess ⁴⁰Ar or contamination by older xenocrysts, both of which have called some published K-Ar ages from the Four Corners area into question. The Carrizo Mountains intrusive center is thus confirmed as an extension of the Laramide-age Colorado mineral belt into Arizona, first suggested by Armstrong (1969).