AN INTERPRETATION OF KAROO PICRITE BASALTS IN TERMS OF INTERACTION BETWEEN ASTHENOSPHERIC MAGMAS AND THE MANTLE LITHOSPHERE

The previously reported "mantle isochron" [20] from the picritic basalts of the Nuanetsi-N Lebombo area is interpreted as a mixing line between asthenospheric and mantle lithospheric components. The former, despite its probable derivation from a plume, is identified as a depleted, MORB-like, composition. On the basis of trace element characteristics the latter is demonstrated to be similar to lamproitic magmas, which appear to have acted as contaminants to the asthenospheric melts. Correlation between major elements and incompatible trace elements and isotopes, found in data sets of restricted MgO content, appear to result from differing crystal fractionation histories depending on the K content (i.e., amount of lithospheric contaminant) of different magma batches. Karoo geochemical provinciality may be partly a consequence of geographical position relative to the plume axis.