Partially molten diapirs and domes in Earth's continental crust can be an effective means of transporting heat from lower to higher levels, often producing pronounced prograde metamorphic aureoles. Our numerical thermomechanical models show that this classical thermal scenario is violated by the diapirs of partially molten metasedimentary rocks to 8 km in diameter that penetrate the Bushveld Complex, the world's largest layered intrusion. Here, diapirism was triggered by the emplacement of an 8-km-thick, hot, and dense mafic magma over a cold and less dense sedimentary succession. These diapirs promoted cooling of the giant magma chamber, bringing cooler material into higher crustal levels. Comparison between numerical results and geological observations indicates that diapir nucleation is crucially dependent on the presence of initial topographic disturbances between 800 and 1000 m in height in the floor of the magma chamber, and also allows critical parameters of initial geometry and temperature distribution of the Bushveld event to be outlined.