Using U-Pb and Hf data to unravel the provenance and high-grade metamorphic overprint of the Neoproterozoic distal passive margin of the São Francisco paleocontinent, Ribeira Orogen, SE Brazil

The Raposos Group, the distal unit of the Andrelândia basin, crops out at the uppermost thrust system (Juiz de For a thrust system Occidental terrane, Central Ribeira Belt (CRO).  The distal sector of the passive margin comprises two major lithostratigraphic units: a basal meta-psammitic it with banded paragneisses with rutile-tourmaline-feldspar quartzite and garnet gneisses/schists lenes; and a top unit made up by pelitic sillimanite- garnet- biotite gneisses with quartzite, graphite and Mn-rich rocks, and calcsilicate rocks lenses. Metabasic rocks transformed in amphibolites and mafic granulites occur as lenses within the two units.  Intense deformation and high metamorphic overprint made this unit as a proxy for validate U-Pb and Hf provenance studies.

Detailed textural observations, cathodoluminescence and backscattered images, coupled with preliminary zircon chemistry studies suggest high intensity of recrystallization and/or growth by interstitial magmatic fluids and liquids during granulite facies metamorphism. Obtained results testify the importance of this kind of investigation to interpret U-Pb and Hf data, to unravel not only provenance as well maximum ages of sedimentation and tectonic settings.

Filtered LA_ICPMS U-Pb undisturbed data indicate robust contribution of Paleoproterozoic sources with main peak at ca. 2.2 Ga and 2.1 Ga, with but significant contribution from Archean sources of ca. 2.8 Ga and 2.6 Ga. The Lu-Hf data applied to Paleoproterozoic and Archean populations point to positive ԑHf(t) values (+0.6 to +10), suggesting juvenile contribution and, with minor proportions, a group that presented negative ԑHf(t) values (-19 to - 0.5) indicating crustal contribution. Results data suggest that the rocks of the Mantiqueira and Juiz de Fora complexes must have been the detrital sources. Neoproterozoic zircon grains, with clearly preserved primary textures, yielded   ages between 0.96 Ga and 0.69 Ga, suggestive for syn-depositional magmatic contributions, such as data reported from mafic dykes between 1.0 and 0.9 Ga, as well as metabasite ages of ca. 766 Ma (Heilbron et al., 2019) and 760 Ma to 653 Ma (Nunes et al., 2022) discovered for the distal segment of the basin.

Finally, new data reinforces the importance of understanding high-temperature metamorphic processes (above 700 ºC) in the re-balancing of old zircon crystals, as well as in the possibility of growth of new grain related with discrete melt films as well leucosomes within the studied rocks. Disturbance of the U-Pb system is related to metamorphic overprint related to an Ediacaran collisional between the reworked São Francisco passive margin and colliding arc-related terranes.