The high-level McGerrigle Mountains plutonic complex consists of a compositionally diverse (gabbro to granite) hybrid suite at its northern end and a relatively homogeneous granite suite in its southern half. Field and geochemical evidence support formation by hybridization between at least three end-member magmas. New U–Pb zircon and titanite ages from these end members indicate that intrusion of all components took place at 391.3 ± 3.4 Ma. Eighteen new K–Ar mineral ages, which cover the range 370–390 Ma, suggest that the granite suite cooled quickly to < 500 °C in less than 2 Ma, whereas temperatures between 600 and 500 °C may have been maintained over an extended period (5–15 Ma) within the hybrid suite. The long-lived thermal anomaly associated with the hybrid suite may explain the large epimetamorphic (< 300 °C) halo surrounding this suite and the existence of hydrothermal mineralization (Madelaine (Cu) and Candego (Pb–Zn–Ag) deposits) adjacent to it.