40Ar/39Ar ages determined for 24 mineral separates from the Pleasant Lake, Hunt Ridge, and Cochrane Lake plutons bear on the timing of the main phase of the Acadian orogeny in northern Maine. Six hornblende samples and one clinopyroxene sample have only minor disturbances in their release spectra. The increments with concordant ages form overlapping plateaus at 403 ± 4 Ma. Many of the biotite samples have discordant release spectra. The average of their total gas ages is 400 ± 6 Ma. Calculated closure temperatures for the hornblendes and biotites are 550 °C and approximately 350 °C, respectively. The difference between closure temperatures and concordancy in ages demonstrates that the plutons cooled very rapidly upon emplacement. Therefore, the 40Ar/39Ar ages date both the time of argon retention in the minerals and the time of emplacement of the plutons. The best estimate for their time of emplacement is the average of the combined ages of the micas, hornblendes, and clinopyroxene, 402 ± 6 Ma.
The plutons are undeformed and crosscut upright, open, Acadian folds in the Carys Mills, Smyrna Mills, and Madrid(?) Formations. The age of the Pleasant Lake, Hunt Ridge, and Cochrane Lake plutons therefore places a lower limit of 402 ± 6 Ma on the time of Acadian folding in northern Maine.