The origins of metamorphism (∼600 °C and 3–4 kbar) in the western Mount Isa inlier, Australia, remain controversial for a number of reasons. (1) No synmetamorphic intrusive bodies can be recognized; (2) high temperatures appear to be sustained for periods >100 m.y.; and (3) metamorphism follows an extended phase of thermal subsidence. We show that the burial of granite batholiths enriched in radiogenic elements beneath the thick insulating sedimentary succession of the Mount Isa Group (deposited in response to repeated rift-sag cycles) was capable of generating steep upper crustal thermal gradients immediately prior to the Isan orogeny. These gradients are appropriate to peak metamorphic conditions, such that the ensuing Isan orogeny required no significant additional heat input. This result is significant in that it may provide a mechanism for understanding the origins of high-temperature metamorphism in other terranes where the involvement of transient heating is not obvious.