Sr and Nd isotopic analyses of granulite-facies lower-crustal xenoliths from northern Queensland, Australia, fall on a mixing array on a vs. diagram at 300 Ma. This was a time of intense orogeny in northern Queensland, attended by granitic magmatism in the upper crust and granulite-facies metamorphism in the lower crust. Ion microprobe zircon dating indicates that most of the xenoliths formed at this time. The Sr and Nd isotopic compositions of these rocks are therefore interpreted to have resulted from large-scale mixing in the lower crust between mantle-derived basaltic melts and isotopically evolved, early Proterozoic continental crust. Both end-members are represented in the xenolith suite. Depleted-mantle Nd model age (tDM ages) for meta-igneous granulite xenoliths having appropriate (i.e. between 0.12 and 0.15) range from 1.9 to 2.2 Ga for mafic melt-like xenoliths and 1.2 to 1.4 Ga for felsic melt-like xenoliths. However, zircon ages on these same samples show that their protoliths crystallized during the Permo-Triassic orogeny. Thus the old tDM ages for the mafic xenoliths do not accurately reflect their crustal residence ages, possibly due to crustal assimilation or their derivation from an enriched mantle source. This is a common feature of mafic granulite xenoliths the world over and highlights the difficulty in determining the age of mafic underplates, hence the timing of crustal growth, from tDM ages. Because the felsic melt-like xenoliths lie on the mixing array, their mid-Proteozoic tDM ages are likely to have resulted from mixing and therefore represent an average crustal residence age. The combined data suggest that significant crustal growth and reworking occurs in the lower crust during Phanerozoic crustal orogenies.