Evolução tectônica Toniana da margem sudoeste do Paleocontinente São Francisco: geoquímica e geocronologia U-Pb de rochas metassedimentares da nappe de passos e magmatismo félsico contemporâneo, Orógeno Brasília meridional

Abstract

Sedimentary deposits from passive margins can provide information about magmatic events and paleoenvironmental conditions during the tectonic evolution of ancient continental blocks. At the end of the orogenic cycle, however, the sedimentary rocks that compose the passive margins may be subducted and their primary characteristics and stratigraphic relationships lost due to metamorphism and deformation. This is the case of the Southern Brasília orogen, which currently exposes rocks that composed the SW passive margin of the São Francisco paleocontinent. The Passos Nappe is an allochthonous structure in the Southern Brasília orogen formed by metasedimentary rocks and metaigneous rocks metamorphosed in greenschist to granulite facies conditions. This study aims to investigate part of the evolution of the passive margin of the São Francisco paleocontinent. Sedimentary provenance analysis was performed based on elemental geochemistry and Nd isotopes in whole rock, and U-Pb in detrital zircon of metasedimentary rocks representative of the Passos Nappe units, in addition to investigations in coeval meta-igneous rocks. The Passos Nappe can be divided into three main sedimentary successions (S1 to S3) based on abrupt differences in sedimentary provenance. S1 comprises quartzite and muscovite schists with high chemical alteration index (CIA > 60%), predominantly felsic source rocks, TDM model ages of Nd isotopes between 2.0 and 2.4 Ga and detrital zircon populations with ages predominantly ca. 2.1 Ga, and subordinately ca. 1.7 Ga, ca. 2.4 Ga and ca 3.0 Ga. S2 comprises intercalations of biotite gneisses and biotite feldspathic schists with CIA between 48 and 62%, geochemical characteristics similar to rocks of TTG associations of Paleoproterozoic magmatic arcs, TDM between 2.0 and 2.4 Ga and unique detrital zircon age modes of ca. 2.05 Ga and ca. 2.10 Ga. S3 comprises intercalations of (kyanite)-garnet-muscovite schists, biotite feldspathic schists and biotite gneisses. These biotite schists have intermediate to felsic source rocks and detrital zircon populations with ages of ca. 0.82 to 0.72 Ga, ca. 1.53 to 0.93 Ga and ca. 2.29 to 1.64 Ga. The ages 792 ± 9 Ma and 727 ± 10 Ma were calculated as maximum depositional ages for this succession. Samples with detrital zircon populations from ca. 790 to 730 Ma are characterized by TDM between 1.2 and 1.8 Ga, indicating syn-depositional magmatism with a relatively juvenile character. This study indicates that felsic magmatism is associated with the reactivation of extensional processes during the passive margin evolution. Part of the metasedimentary rocks deposited in a distal shelf environment are also related to the continental rifting, including a new chapter in the evolution of the southwestern passive margin of the São Francisco during the second half of the Tonian period.