Kaapvaal craton mantle composition

Figure 18 Normative olivine and opx (garnet Uierzo-Ute norm) versus rock Mg showing low-T cratonic mantle relative to batch melt extraction trends from 3 GPa to 7 GPa (see Figure 6). Open circles are compositions from the Kaapvaal, Siberian, and Slave cratons, and the filled circles are compositions from the Tanzanian, Canadian, and Greenland cratons.

Here we explore the idea that the discrepancy between the average composition of the continental crust, which is andesitic, and the modern flux from the mantle to the continental crust, which is basaltic, can be explained in terms of a change in the composition of the crust-mantle flux over time. The hypothesis adopted here is that Archaean crust had a TI G composition, formed from a TTG melt, and was not fractionated into lower basaltic and upper felsic components. Modern crust on the other hand has a basaltic bulk composition but has been modified to andesitic through the fractionation and the removal of a mafic lower crustal component (Rudnick Taylor, 1987). Evidence for the absence of a mafic lower crust in the Archaean comes from Archaean lower crust preserved as granulite terrains, such as the Lewisian (Rollinson Tarney, 2005), the Limpopo Belt (Berger Rollinson, 1997), and the lower crust of the Kaapvaal Craton... 

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