Kemp, Dr Tony1
1University Of Western Australia, , Australia
Long-lived radiogenic isotope systems such as 147Sm-143Nd and 176Lu-176Hf suggest that large volumes of the Earth’s continental crust formed in the Archean Eon (> 2.5 Ga). The onset of substantial continent stabilization in the geological record is marked by the distinctive ‘granite-greenstone’ terranes that are the hallmarks of Archean crustal blocks. Yet, to what extent generation of the buoyant, silica-rich (i.e. continental) components in these terranes involved the re-melting of pre-existing, primordial crust as opposed to rapid differentiation of new mantle additions, remains uncertain. Establishing the composition of the mantle source from which early crust was extracted, and comparing this with the compositions of felsic crust, is key to this question. The geochemical signatures of ancient, unambiguously mantle-derived rocks are, however, susceptible to modification by later metamorphism. Here, hafnium and neodymium isotope data are reported for well preserved mafic-ultramafic and felsic igneous rocks of the Pilbara and Yilgarn Cratons, Western Australia. Comparing the mantle and crustal records of Archean continent formation in these cratons reveals a striking isotopic link that endured over 500 million years. In the Pilbara Craton, this linkage is interpreted to reflect the efficient transformation of new mafic inputs from the mantle into felsic continental crust throughout the history of the craton. In contrast, broadly coeval rocks in the Yilgarn Craton formed by remelting older rocks, although the crustal evolutionary records of both cratons converge in the Neoarchean. The possible reasons for the cratonic contrasts are considered.
Currently in the School of Earth Sciences at the University of Western Australia