Abstract

The results of P-wave velocity and petrographic studies of lithospheric nodules from the Venetia kimberlite pipes indicate that at this locality, supracrustal lithologies related to the Central Zone of the Limpopo Belt, southern Africa, extend to seismic, gravity, and geoelectrical discontinuities at approximately 10 km. Below these discontinuities to approximately 80 km, the lithosphere is mafic to ultramafic. Textures indicate that amphibolite extending from approximately 10 km to approximately 32 km is restitic, resulting from partial melting of primary amphibolite to create large volumes of tonalitic-trondhjemitic-granodioritic magmas. Heat to produce this partial melting may have been introduced into the lithosphere by intrusion of mafic to ultramafic magma, now represented by the zone of cumulate rocks at between approximately 42 and approximately 57 km depth. No age measurements have been made on the restitic amphibolite. However, comparison of the emplacement ages of tonalitic-trondhjemitic-granodioritic rocks in the Limpopo Belt and adjacent Zimbabwe and Kaapvaal Cratons, which on the basis of their REE compositions could be the products of such melting, suggests that partial melting occurred during the Archaean (> approximately 2.6 Ga ago). By implication, the present lithospheric structure dates from that time. However, in marked contrast to the marginal zones of the Limpopo Belt and the adjacent cratons, tonalitic and trondhjemitic rocks are volumetrically minor in the Central Zone and partial melting of amphibolite could not have produced the widespread approximately 2.57 Ga granitic rocks of the Central Zone. Seismic and gravity data suggest that rocks of the Zimbabwe Craton extend beneath the Central Zone, and hence the products of partial melting of amphibolite beneath the Venetia kimberlite pipes, should be found among those rocks. REE compositions of the voluminous approximately 2.7 to approximately 2.6 Ga igneous enderbite and charnoenderbite of the Northern Marginal Zone and the Razi granite are consistent with being that product. If true, major high-grade metamorphism and tectonism in the Central Zone of the Limpopo Belt between approximately 2.03 and 1.9 Ga are not reflected in rocks below approximately 10 km, indicating that the rocks of the Central Zone were tectonically emplaced into their present position after this high-grade metamorphism. Thus, the discontinuities at approximately 10 km mark the base of Central Zone lithologies and reflect an important decollement.

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