Petrology, geochemistry, and zircon U-Pb geochronology of the Zambezi Belt in Zimbabwe: Implications for terrane assembly in southern Africa

Abstract

The Zambezi Belt in southern Africa has been regarded as a part of the 570–530 Ma Kuunga Orogen formed by a series of collision of Archean cratons and Proterozoic orogenic belts. Here, we report new petrological, geochemical, and zircon U-Pb geochronological data of various metamorphic rocks (felsic to mafic orthogneiss, pelitic schist, and felsic paragneiss) from the Zambezi Belt in northeastern Zimbabwe, and evaluate the timing and P-T conditions of the collisional event as well as protolith formation. Geochemical data of felsic orthogneiss indicate within-plate granite signature, whereas those of mafic orthogneiss suggest MORB, ocean-island, or within-plate affinities. Metamorphic P-T estimates for orthogneisses indicate significant P-T variation within the study area (700–780 °C/6.7–7.2 kbar to 800–875 °C/10–11 kbar) suggesting that the Zambezi Belt might correspond to a suture zone with several discrete crustal blocks. Zircon cores from felsic orthogneisses yielded two magmatic ages: 2655 ± 21 Ma and 813 ± 5 Ma, which suggests Neoarchean and Early Neoproterozoic crustal growth related to within-plate magmatism. Detrital zircons from metasediments display various ages from Neoarchean to Neoproterozoic (ca. 2700–750 Ma). The Neoarchean (ca. 2700–2630 Ma) and Paleoproterozoic (ca. 2200–1700 Ma) zircons could have been derived from the adjacent Kalahari Craton and the Magondi Belt in Zimbabwe, respectively. The Choma-Kalomo Block and the Lufilian Belt in Zambia might be proximal sources of the Meso- to Neoproterozoic (ca. 1500–950 Ma) and early Neoproterozoic (ca. 900–750 Ma) detrital zircons, respectively. Such detrital zircons from adjacent terranes possibly deposited during late Neoproterozoic (744–670 Ma), and subsequently underwent high-grade metamorphism at 557–555 Ma possibly related to the collision of the Congo and Kalahari Cratons during the latest Neoproterozoic to Cambrian. In contrast, 670–627 Ma metamorphic ages obtained from metasediments are slightly older than previous reports, but consistent with ∼680–650 Ma metamorphic ages reported from different parts of the Kuunga Orogen, suggesting Cryogenian thermal events before the final collision.