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

Tectonic evolution of the Zambezi orogenic belt: geochronological, structural, and petrological constraints from northern Zimbabwe

In southern Africa, the Zambezi belt forms the eastern part of a transcontinental orogenic system that connects with the East African orogen and records interactions between the Congo and Kalahari cratons during collisional assembly of the Gondwana supercontinent at the end of the Neoproterozoic. We report the results of reconnaissance studies in the eastern part of the Zambezi belt in northern Zimbabwe, where thick-skinned thrusting has inverted a crustal column comprising a Neoproterozoic supracrustal sequence tectonically overlain by rocks exhumed from the lower crust. An extensive felsic gneiss with A-type geochemical signatures that is inferred to represent a metarhyolitic unit within the supracrustal sequence has yielded a U–Pb zircon crystallization age of ca. 795 Ma, which helps to constrain the timing of supracrustal deposition in this part of the Zambezi belt. The dominant ductile structures in the area record south-vergent thrusting, during which the supracrustal rocks underwent prograde, amphibolite-facies metamorphism as they were overridden by a crystalline thrust stack partly preserving high-pressure granulite-facies assemblages. Complex U–Pb zircon geochronological results for a layered, metagabbroic to meta-anorthositic intrusive complex in the upper part of the thrust stack are interpreted to indicate a minimum igneous crystallization age of ca. 1830 Ma. Polydeformed orthogneisses in the lower part of the thrust stack were derived from granitoid protoliths with U–Pb zircon crystallization ages of ca. 1050 and 870 Ma. The younger granites are inferred to be parts of an A-type magmatic province that can be recognized throughout the Zambezi belt. U–Pb zircon and titanite geochronological results from the layered complex and the underlying orthogneisses are interpreted to record metamorphism associated with thrust emplacement at ca. 550–530 Ma. This is consistent with isotopic age data from adjacent areas, indicating that a major orogenic event affected the Zambezi belt in this time frame, during assembly of central Gondwana