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The Neoproterozoic Paraguay Belt in mid-eastern Mato Grosso, western Brazil, records the geological evolution of the margins of western Gondwana. The lack of absolute geochronology for the Paraguay Belt has hampered both regional and global stratigraphic correlations. Here we determine the depositional age for the Peresopolis black slate that belongs to the base of the Cuiabá Group in the Paraguay Belt using Re-Os geochronometry. We obtain a narrow Re-Os age interval of 784 ± 8 Ma and a juvenile initial 187 Os/ 188 Os composition (Osi) of 0.15 ± 0.03. These data indicate that the Peresopolis black slate is Tonian in age and was deposited in a basin strongly influenced by the weathering of juvenile material, and potentially linked to the early rifting of Rodinia on western Gondwana. These slates record a time interval that is poorly represented in Brazil and with few counterparts in other Neoproterozoic basins globally. As such, our work is a foundation for future geochemical correlations and paleoenvironmental reconstructions of this critical transition of Earth history in Brazil and elsewhere.

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