The formation of metamorphic core complexes may be triggered by plutonic activity during episodes of continental extension. Pulses of ductile deformation have taken place during short-lived thermal events initiated by the heat input from intruded plutons, sills, or dikes. Such intrusions may be the underlying cause for differential uplift of the footwall during tectonic denudation of metamorphic core complexes. Fast cooling inferred from 40Ar/39Ar apparent ages may have taken place after periods of magma arrival, and need not be the result of rapid erosional or tectonic denudation. Heterogeneity of 40Ar/39Ar apparent ages can be explained by rapid cooling of deforming mylonites formed at shallow crustal levels in the thermal aureoles of intruded sills (or sill swarms). Under such circumstances, apparently isothermal decompression paths may actually link pressure-temperature points set during periods of transient mineral growth during and after periods of igneous activity.