High-Mg, low-Ti diorites in the Cambrian–Ordovician Glenelg River Complex (southeastern Australia) have compositional affinities with the boninitic lavas erupted in intraoceanic arcs, but occur in an anatectic environment mingled with crustal melts. Boninitic rocks have not previously been recorded from such a setting. Field and petrologic evidence suggests that the parental boninitic liquids are approximated by hornblende diorites, crystallization from which yielded complementary cumulates and evolved tonalitic derivatives. The presence of these rocks, and associated arc-like intrusions, compromises collisional tectonic models for the Glenelg River Complex and instead implies early development of the terrane above a subduction zone. Boninitic magmas were subsequently trapped at deep crustal levels during convergent orogenesis.