The growth of the Zimbabwe craton during the Neoarchaean

New geochemical data demonstrate the similarity between Neoarchaean TTGs found in the middle crust of the Zimbabwe Craton and the lower crust of the Northern Marginal Zone of the Limpopo Belt. Hence the Neoarchaean segment of the Zimbabwe Craton provides a complete crustal section—from lower crustal granulites to the supracrustal rocks of the greenstone belts. Mantle activity recorded in the basalts of the greenstone belts is interpreted as plume volcanism, setting a context for the process of crustal generation. However, published isotopic data and new trace element data reported here show that magma genesis in Zimbabwe Craton in the Neoarchaean incorporated up to 20% of older crust, indicating that not all the granitoids are juvenile. This paper seeks to identify the trace element composition of the parental melts to the TTGs of the Zimbabwe Craton, define the petrological processes by which they were generated and set limits on possible petrological models for the process of crust generation. The least contaminated TTGs have low SiO2 and moderately fractionated trace element patterns. Calculations indicate that they were derived by the partial melting of a garnet amphibolite source at ca 10 kb depth. Rare intrusive diorites represent a low fraction melt of an enriched mantle source derived from refertilised depleted mantle. These observations lead to a model in which the source material of the TTGs was basaltic and the thermal energy to melt the basalts came from continued basaltic and komatiitic magmatism. Partial melts of old felsic crust were incorporated into the TTG magmas. The resulting ‘enriched’ TTGs provided a fertile lower crust from which an extensive suite of late tectonic granites could be generated. These data do not support a modern-style plate tectonic model for the growth of the Zimbabwe Craton in the Neoarchaean.