Growth and recycling of early Archaean continental crust: Geochemical evidence from the Coonterunah and Warrawoona Groups, Pilbara Craton, Australia

In the Pilgangoora Belt of the Pilbara Craton, Australia, the Coonterunah Group and Carlindi granodiorite underlie the Warrawoona Group beneath an erosional unconformity, providing evidence for emergent continental crust at ~3.47 Ga. The basalts either side of the unconformity are remarkably similar, with N-MORB-normalised enrichment factors for LILE, Th, U and LREE greater than those for Ta, Nb, P, Zr, Y and M-HREE. Geological and geochemical evidence shows that both the Coonterunah and Warrawoona Groups were erupted onto continental basement, and that the basalts assimilated up to 25% crustal material. Geochemical modelling suggests that the mantle source of the basalts was as depleted in incompatible elements as the present-day mantle. Therefore, the early development of the Pilbara Craton records the interaction of depleted mantle with pre-3.5 Ga continental basement. Since only very small relics of pre-3.52 Ga rocks have been discovered in the Pilbara, it is likely that the pre-3.5 Ga continental basement has been substantially recycled. This suggests that the recycling of continental material was an important process in the Archaean, and consequently that many estimates of continental growth rates in the early Archaean are too low. The evidence is most consistent with steady-state crustal evolution models in which large volumes of continental crust were extracted from the mantle early in Earth's history, and then efficiently recycled. (C) 2000 Elsevier Science B.V. All rights reserved.