High radiogenic heat-producing granites and metamorphism - An example fromthe western Mount Isa inlier, Australia

The origins of metamorphism (similar to 600 degrees C and 3-4 kbar) in the western Mount Isa inlier, Australia, remain controversial for a number of r easons. (1) No synmetamorphic intrusive bodies can be recognized: (2) high temperatures appear to he sustained for periods >100 m.y.; and (3) metamorp hism follows an extended phase of thermal subsidence. We show that the buri al of granite batholiths enriched in radiogenic elements beneath the thick insulating sedimentary succession of the Mount Isa Group (deposited in resp onse to repeated rift-sag cycles) was capable of generating steep upper cru stal thermal gradients immediately prior to the Isan orogeny. These gradien ts are appropriate to peak metamorphic conditions, such that the ensuing Is an orogeny required no significant additional heat input. This result is si gnificant 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.