A suite of igneous rocks from the Sawatch Range, Colorado, known to have a Rb-Sr whole-rock age of 1650 ± 35 m. y., has been dated by the nuclear fission-track method. Analysis yielded an age of about 50 ± 12 m. y. for some samples of apatite; other apatite samples yielded ages of 183 ± 44 m. y. to 248 ± 68 m. y. Ages of 45 to 50 m. y. agree closely with the age of Laramide igneous events in the Rocky Mountains and suggest that partial to complete annealing of spontaneous fission tracks in the apatites occurred about 50 m. y. ago. Spontaneous fission-track densities in most other minerals are too low to be useful for dating; zircons are strongly metamict (Church, 1967).
For assumed durations of the Laramide annealing event, an extrapolated annealing temperature-time curve for apatite provides minimum temperatures. If, for example, the Laramide event lasted 10 m. y., the apatite samples must have been heated to at least 150°C. Rb-Sr ages of minerals from these rocks were not measurably affected by Laramide events; thus, subject to assumptions regarding the duration of the heating, a minimum temperature for Rb-Sr redistribution may be determined.