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

Precambrian cratonic pelites from the Kaapvaal Craton in southern Afri ca 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 m ean value of approximate to 450 worldwide fine-grained samples of all ages obtained by isotopic dilution analyses. This value is probably th e best estimate for the upper continental crust. The Kaapvaal pelites also have distinct Cr/Th ratios, but overlap in Eu/Eu ratios, suggest ing that variable provenance and sedimentary recycling were important both during and after the Archean. Because the light REE budget is con trolled chiefly by granitoids, which mask contributions of mafic-ultra mafic components, the relatively uniform Sm/Nd ratios in sediments do not indicate a near-constant composition for the upper continental cru st. Most Kaapvaal pelites have negative epsilon Nd(T) values, indicati ng 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 a ll of Kaapvaal pelites have T-DM ages greater than their depositional ages but younger than 3.6 Ga, suggesting an absence of rocks older tha n 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 calculati ons. 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. Ar mstrong (1991).