A connection between the Neoproterozoic Dom Feliciano (Brazil/Uruguay) and Gariep (Namibia/South Africa) orogenic belts - evidence from a reconnaissance provenance study
A provenance study of Neoproterozoic siliciclastic successions in the stratigraphically and tectonically lowermost and uppermost parts of the Pan-African Gariep Belt (Stinkfontein Subgroup and Oranjemund Group, respectively) in southwestern Africa, as well as in the Rocha Group of the Punta del Este Terrane (DomFeliciano Belt) in Uruguay, revealed that the Oranjemund and Rocha Groups can be correlated and most likely formed in the same basin. Thus, the Rocha Group is considered to represent the fill of the westernmost part of a re-activated Ediacaran Gariep Basin. The lower parts of the Oranjemund and Rocha Groups reflect erosion of mafic rocks, whereas the upper parts are derived from a predominantly felsic source area. First major and trace element geochemical data are consistent with oceanic islands of within-plate geochemistry in the immediate vicinity as the most likely source of the mafic input into the lower part of the Oranjemund Group, whereas a combination of continental margin and island arc is indicated as source for the bulk of the younger sediments in the upper Oranjemund Group. Age spectra obtained by SHRIMP U–Pb analyses of detrital zircon grains from the Stinkfontein Subgroup (Port Nolloth Group), the Oranjemund Group, and the Rocha Group are very similar, except for a lack of the youngest age group around 600 Ma in the Stinkfontein Subgroup. In all three units, zircon grains of 1000–1200 Ma dominate, with a further peak in the age distribution between 1700 and 2000 Ma. These ages compare well with the pre-Gariep basement geology in southwestern Africa, where the former age range corresponds to magmatic and high-grade metamorphic activity in the Mesoproterozoic Namaqua-Natal Belt and the latter to an extensive Palaeoproterozoic Andean-type volcanic arc (Richtersveld Terrane). Comparable ages are conspicuously absent in the basement of the Rio de la Plata Craton in South America. Derivation of the Rocha Group sediments from a similar source as the contemporaneous Oranjemund Group sediments is therefore suggested. The most likely source of the youngest detrital zircon grains in these two stratigraphic units is the 640–590 Ma magmatic arc of the DomFeliciano Belt. This interpretation explains the continental island arc component in the sediment provenance and takes into account the absence of comparable magmatism in the Gariep Belt. Two stages of basin evolution are distinguished for the Gariep Basin, an early failed continental rift that subsequently became re-activated as back-arc basin related to the Cryogenian DomFeliciano arc. Consequently, the Gariep Basin does not represent the main suture between the Rio de la Plata and the Kalahari Cratons. This suture is now suspected to be west of the DomFeliciano volcanic arc, and might be connected with the Sierra Ballena – Major Gercino Lineament.
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