Contrasted trace-element variations in two Karroo Cumulus complexes

Abstract

The plotting of Ti, P, Sr, Y and Zr against MgO yields linear relationships in the case of the layered Elephant's Head intrusion, and the calculated regression lines converge on the MgO values characteristic of the cumulus olivine. Levels of incompatible elements therefore lie along mixing lines linking pure olivine with the parental magma. The distribution of Ni and Co shows similar features, although the high distribution coefficient for Ni causes the later cumulates in the exposed section to leave the mixing line. Both major- and trace-element data for the large Insizwa layered complex indicate retention of 20% parental Karroo magma as an intercumulus liquid within cumulus picrites, but attention is focussed on a further class of cumulates which are strongly enriched in incompatible elements and depleted in Sr, but “normal” with respect to major elements, Co, Cr, Ni and Zn. It is probable that displacement of interstitial liquid by heteradcumulus growth of plagioclase in adjacent layers is responsible for this anomalous geochemistry.