Provenance of the Arroyo del Soldado Group (Ediacaran to Cambrian, Uruguay): Implications for the paleogeographic evolution of southwestern Gondwana

The Ediacaran to Lower Cambrian Arroyo del Soldado Group represents a thick marine sequence cropping out in the eastern border of the Rio de la Plata Craton in central Uruguay. This work addresses the provenance of different siliciclastic units in the Arroyo del Soldado Group in terms of petrography, geochemistry, Nd isotope and detrital zircon geochronologic data. The lower part of the Arroyo del Soldado Group, the Yerbal Formation and the overlying carbonates of the Polanco Formation, are related to an open shelf which formed after rifting along eastern border of the Rio de la Plata Craton. U-Pb zircon dating of reworked detrital material of the Yerbal Formation shows a provenance with mainly Palaeoproterozoic ages (2.0-2.2 Ga). The Yerbal Formation shows T-DM model ages of 2.1 Ga and epsilon(Nd)(t) of -15. The middle part of the Arroyo del Soldado Group is characterized by deposition of coarse sediments (Barriga Negra Formation), that could be related to a sea level fall during non-global glaciation. The zircon ages reflect a major contribution from the Palaeoproterozoic and secondly from Neoproterozoic basement rocks, and constrain the maximum age of deposition to 566 Ma. T-DM model ages reflect Archaean (2.7 Ga) and Palaeo-Mesoproterozoic (1.6-1.9 Ga) mantle extraction ages for the sources. The Archaean component shows geochemical similarities with a mafic source characterized by high concentration of Cr (350 ppm) and low Th/Sc (0.7). Up section, during a sea level rise the deposition of BIF, shale and chert of the Cerro Espuelitas Formation occurred. Tom ages of 2.1 Ga suggest a provenance mainly derived from the Rio de la Plata Craton. U-Pb zircon dating of the Cerros San Francisco Formation, representing the upper part of the Arroyo del Soldado Group, shows a provenance mainly from Palaeoproterozoic (2.0-2.1 Ga) and Archaean (2.5-2.9-3.5 Ga) rocks. The scarcity of Neoproterozoic zircons along with T-DM ages of 2.2-2.9 Ga and the extreme recycling that affected the Cerros San Francisco Formation as indicated by petrography and geochemistry do not fit with the hypothesis that the Arroyo del Soldado Group was deposited close to a syntectonic magmatic arc. A tectonic scenario in which the Arroyo del Soldado Group was deposited on a passive margin before strike-slip collision with the Cuchilla Dionisio Pelotas Terrane at ca. 530 Ma is supported. (c) 2009 Elsevier B.V. All rights reserved.