Abstract

The Paraguay Belt in central South America developed in response to the collision of the Amazonian craton, the São Francisco craton, and the Paranapanema block. The alleged “Brasiliano” age (ca. 620 Ma) of orogenesis has recently been questioned by paleomagnetic and radioisotopic ages that indicate the closing stages of orogenesis occurred well into the Cambrian. We investigated the timing of deposition and source areas for these sedimentary rocks overlying the Amazonian craton using integrated U-Pb and Hf isotope data of detrital zircons from within this sequence. In total, 742 detrital zircon U-Pb ages were analyzed from samples taken from the base to the top of this sedimentary succession. Maximum depositional ages from the uppermost part of this sequence of rocks, the Diamantino Formation, indicate that final deposition began no earlier than 560 ± 13 Ma and possibly as young as the Cambrian. Given that zircon inheritance in these rocks continues up until this age and that known Amazonian craton ages are older than ca. 950 Ma, we considered other potential sources for these sediments. This was achieved by integrating the U-Pb detrital zircon data with Hf isotopic data from these zircons that have εHf values ranging from –18 to 12. The εHf signature is consistent, with a predominantly Amazonian source until the early Neoproterozoic, at which point the signal becomes significantly more evolved. These data, when combined with other evidence discussed here, are consistent with an ocean to the east of the present-day Amazonian craton that did not close until the latest Ediacaran–Cambrian.

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