EVOLUTION OF THE KAAPVAAL-CRATON AS VIEWED FROM GEOCHEMICAL AND SM-ND ISOTOPIC ANALYSES OF INTRACRATONIC PELITES

Precambrian cratonic pelites from the Kaapvaal Craton in southern Africa have similar REE patterns with relative LREE enrichment and absence of significant depletion in HREEs. They have a narrow range of Sm-147/Nd-144 ratios with a mean value of 0.118, which is identical to the mean value of approximate to 450 worldwide fine-grained samples of all ages obtained by isotopic dilution analyses. This value is probably the best estimate for the upper continental crust. The Kaapvaal pelites also have distinct Cr/Th ratios, but overlap in Eu/Eu* ratios, suggesting that variable provenance and sedimentary recycling were important both during and after the Archean. Because the light REE budget is controlled chiefly by granitoids, which mask contributions of mafic-ultramafic components, the relatively uniform Sm/Nd ratios in sediments do not indicate a near-constant composition for the upper continental crust. Most Kaapvaal pelites have negative epsilon Nd(T) values, indicating important contributions of older crustal sources. Overall, there is a slight decrease of epsilon Nd(T) values with decreasing age, but no clear distinction is apparent at the A/P boundary at 2.5 Ga. Almost all of Kaapvaal pelites have T-DM ages greater than their depositional ages but younger than 3.6 Ga, suggesting an absence of rocks older than 3.6 Ga in the Kaapvaal Craton. The debate on growth or no-growth of continents depends much on the choice of parameters in model calculations. The crucial parameters include sediment flux in subduction zones and delaminated lower crust, and the Sm/Nd ratio of continental crust. Unfortunately, the available data are ambiguous in modelling studies. Neodymium isotopic data and Sm/Nd ratios cannot be taken as a robust argument against the no-continental-growth model advocated by R. L. Armstrong (1991).