Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

Detrital zircons from quartzites of the São Roque Domain were subject of studies of crystal growth and stratigraphy, using in situ LA-ICPMS trace element geochemistry and U–Pb dating to deduce their provenance and contribute to understand the tectonic evolution of the Proterozoic metasedimentary successions of southeast Brazil. Trace element chemistry of most detrital cores indicates derivation from plagioclase-rich felsic rocks (e.g., tonalites to granodiorites); however, a small proportion may derive from gabbroic sources, as indicated by a combination of features (strong positive Ce anomaly, high LuN/SmN, low U/Yb; concave-down shape of the intermediate REE patterns). Thin metamorphic overgrowths are chemically distinct (e.g., enriched in several trace elements, especially the LREE), and were dated at 584 ± 47 Ma, reflecting the Ediacaran metamorphism and granite magmatism that affected the São Roque Domain. The detrital zircon U–Pb ages for five quartzite samples from different stratigraphic positions within the São Roque Domain are restricted to the Paleoproterozoic and Archean (∼1730–3440 Ma). A striking feature of the dataset is a main peak at ∼2.2 Ga which is characteristic of all samples; the age and abundance of Archean peaks allow distinguish two groups of samples: Jaraguá and Japi, with 41–43% Archean zircons and a peak at 2.6–2.7 Ga; and Voturuna, Pirucaia and Serra da Viúva, with 16–21% Archean zircons and a peak close to the Archean-Paleoproterozoic boundary (2.4–2.5 Ga). A limited contribution of late Paleoproterozoic (Statherian) sources places an upper limit for the deposition of the sedimentary successions that is consistent with the estimated age of the metaconglomerate-metarkose rift sequence of the São Roque Group (Boturuna Formation), based on U–Pb dating of interlayered meta-trachydacites (1.75 Ga). Our data demonstrate that the different sedimentary successions of the São Roque Domain have an essentially continental (cratonic) provenance which is very similar to those of other sequences in the Ribeira Belt (Açungui and possibly also Embu Domain). The ∼2.2 Ga age of the volumetrically most important sources, also indicated by the age of granite pebbles in metaconglomerates, is a diagnostic signature, consistent with the age of regional basement (e.g., orthogneisses from few basement nuclei in the Açungui Domain). Similarities in provenance and tectonic evolution of the metasedimentary sequences are consistent with the interpretation that the Ribeira Belt is part of the reworked border of the São Francisco Paleoplate, displaced southwestward as a result of significant dextral orogen-parallel tectonics during the Neoproterozoic collision, and may imply that a suture zone between this paleoplate and another stable continental area to the SW lies below the Phanerozoic sediments of the Paraná Basin.