The McKinley pluton is one of the northern-most plutons of the Alaska-Aleutian Range batholith. This biotite granite intrusion makes-up most of the 6,194 meter high Mt. McKinley massif. Previous K-Ar and apatite fission-track studies suggest that the pluton has had a protracted cooling/uplift history since its emplacement at ~56 Ma. The great local relief available for sampling makes the McKinley pluton an ideal intrusion for a thermochronometric study through 40Ar/39Ar dating of K-feldspars and biotites. The samples analyzed were collected from a range in elevation of 2,100 to 5,990 meters. The K-feldspars can be modeled as having multiple domains, thus, cooling histories spanning ~35 Ma can be estimated for each elevation.
Six biotites and one primary white mica have relatively flat age spectra with plateau ages between 55.1 and 55.8 Ma. They show a very slight age/elevation dependence with the lower biotites being younger. These consistent mica ages most likely reflect uniform cooling of the pluton at this time.
The K-feldspars exhibit saddle-shaped age spectra that vary significantly from each other. Cycled heating experiments carried out on the K-feldspars resulted in arrhenius plots that show the samples may contain 3-4 separate diffusional domains. The arrhenius plots and age spectra were fitted using the programs of Lovera (1992). The results show that the pluton cooled very slowly with rates of 10deg. to 4deg. C/Ma, with the lower samples cooling at the slower rates. The pluton came into equilibrium with the geothermal gradient at ~30 Ma and at a temperature of ~ 250deg. C. The K-feldspars record uplift rates of 0.13 mm/yr from 43 Ma to 25 Ma. Even the small domains of the K-feldspars do not record the rapid uplift of Mt. McKinley that began at 5 Ma.
Internal funds. Scientific personnel: Paul Layer and Andrew West