The Neoproterozoic southern passive margin of the Sao Francisco craton: Insights on the pre-amalgamation of West Gondwana from U-Pb and Hf-Nd isotopes
Résumé
Sedimentary basins are, in many cases, the only preserved record of the geological processes active through geological time. These deposits preserve age, isotopic and geochemical signatures of the source rocks, providing insights into tectonic setting. In this context, we investigate the lithostratigraphy, provenance and tectonic environment of the Carrancas Group, a meta-sedimentary sequence characterised by a basal psammitic unit (Lower Unit) passing up into interbedded psammo-pelites (Intermediate Unit), which are overlain by a further psammitic unit (Upper Unit). This succession, along with the Canastra, Paranoá and Vazante groups (Brasília orogen), comprises a mature passive margin developed at the western-southern margin of the São Francisco craton during the Neoproterozoic. Detrital zircon U-Pb ages and Nd-Hf isotope data enable revised stratigraphic correlations across the basin, highlighting variations in the source of detritus. Deposition began after 0.92 Ga, with the sedimentation of the Lower Unit, with input of juvenile ∼2.1 Ga detritus (εNdt = −0.1 to +3.2) derived from the São Francisco craton. A marine transgression characterises the transition to the Intermediate Unit, which gradually covered over juvenile ∼2.1 Ga terrains and rendered them unavailable as possible source rocks for detrital zircons. Evolved Archaean-Palaeoproterozoic basement rocks, the Espinhaço Supergroup and evolved Neoproterozoic granitoids, all belonging to the São Francisco craton, acted as sources for the Intermediate and Upper units (U-Pb = 2.7 Ga to 0.9 Ga; εNdt = −10.4 to +7.1). The Goiás massif might also have acted as a source, providing juvenile Mesoproterozoic detritus. The sediments of the Upper Unit register a significant shift in source, with incoming of juvenile Mesoproterozoic and crustal 1.9 – 1.7 Ga detritus. This change in the ages of the detrital material may be related to exhumation of the Espinhaço Supergroup during the evolution of the passive continental margin basin. The whole-rock Nd TDM model ages become progressively younger from the base to the top of the sequence (Nd TDM = 2.6–1.8 Ga). The collisional events between Amazonian and São Francisco cratons and the Paranapanema block set the minimum age of deposition along this passive continental margin at around 0.67 Ga.