Geochemistry of amphibolites and quartzofeldspathic gneisses in the Pan-African Zambezi belt, northwest Zimbabwe: Evidence for bimodal magmatism in a continental rift setting

The Zambezi belt separates the Congo and Kalahari cratons in southern Africa and is a key part of the regional Pan-African orogenic framework related to amalgamation of Gondwana in the Neoproterozoic-early Palaeozoic. Several thick, probably correlative, supracrustal sequences are preserved in the belt in Zimbabwe and Zambia. The Makuti Group, a major assemblage of supracrustal rocks within the belt in northwestern Zimbabwe, consists dominantly of amphibolite-facies quartzofeldspathic gneisses of supracrustal origin interlayered with horizons of marble, calc-silicate rock, quartzite, and pelitic schist. Numerous thick, concordant amphibolites derived from mafic sills and/or lava flows are intercalated within the supracrustal sequence. Major- and immobile trace-element geochemistry indicates dominantly tholeiitic affinities for the amphibolites, with some samples showing transitional to alkaline affinities. High-field-strength trace-element contents and LREE-enriched patterns are consistent with a within-plate setting for the mafic rocks. Major- and trace-element data show the quartzofeldspathic gneisses to be dominantly of igneous origin. Their protoliths are inferred to be mainly peralkaline rhyolites and trachytes. High Zr contents (up to 1500 ppm) are a diagnostic signature for these rocks. The bimodal nature of the magmatism and the abundance of peralkaline felsic rocks point to a continental rift zone as the setting for the Makuti Group. Other examples of pre-orogenic, mafic or bimodal magmatic rocks are found in the Zambezi belt elsewhere along strike in Zambia and Zimbabwe. All these rocks are inferred to represent widespread, rift-related magmatism associated with initiation of the depositional basin within which the Neoproterozoic sequences of the Zambezi belt accumulated