Tectono-stratigraphic evolution of a subducted Tonian continental margin in the western São Francisco paleocontinent (Passos Nappe, SE Brazil): insights from lithogeochemistry, Sr-Nd isotopes, and detrital zircon U-Pb ages

Sedimentary provenance studies reveal stratigraphic relationships obscured by orogeny that can be critical for testing tectonic models and for unravelling the geodynamic evolution of ancient mountain belts. Multiproxy analyses are applied in this study for reconstructing paleo-tectonic events in the central portion of the Neoproterozoic Brasilia orogen in Brazil. The Passos Nappe incorporates metasedimentary rocks and minor metabasic rocks that represent a subducted portion of a Tonian passive margin. Nine lithological units (A to I) comprising metapsammites and metapelites show a common deformational history and inverted metamorphic gradient from greenschist to granulite facies. The paleoenvironments and deposition period of these metasedimentary rocks are unknown, even though they are key to understanding the evolution of the São Francisco continental margin. This study presents new data including U-Pb ages of detrital zircon, whole-rock major and trace element and Sm-Nd-Sr isotope compositions for the Passos Nappe units. The results indicate that the Passos Nappe may be divided into three tectonic-stratigraphic levels (TS1 to TS3) characterised by abrupt shifts in the sedimentary provenance pattern. In TS1, the main zircon population yields ages between c. 2.15 and 2.05 Ga with a minor c. 2.6 Ga component, while T_DM varies between 2.69 and 2.14 Ga. Biotite-rich rocks in TS1 yield a low chemical index of alteration (49.4 - 61.9 mol%), high La_n/Yb_n ratios (21.2 - 35.1), and Sr/Y ratios (27.2 - 85.6), indicating derivation from Paleoproterozoic basement (magmatic arc) sources. TS2 yields detrital zircon with ages varying from c. 0.82 to 0.72 Ga, c. 1.53 to 0.93 Ga, and c. 2.29 to 1.64 Ga with a maximum depositional age of c. 0.79 Ga. The T_DM varies between 1.21 and 2.44 Ga, and samples with a conspicuous mid-Tonian detrital zircon contribution also yield εNd_{790 Ma} between -7.0 and +0.8. The comparatively juvenile character of the magmatism is supported by an orthogneiss sample with a crystallisation age of 795 ± 8 Ma and εNd_{790 Ma} of +2.1. TS3 is characterised by detrital zircon between c. 1.06 to 0.65 Ga, c. 2.45 to 1.73 Ga, and c. 2.91 to 2.53 Ga, with a maximum depositional age of 654 ± 6 Ma. The εNd_650Ma ranges from -7.5 to +0.5 and low Th/Sc ratio (0.33 to 0.66) indicate the input of a relatively juvenile Neoproterozoic source. These new results indicate that mantle-derived active magmatism and a shift in the provenance of source rocks accompanied the deposition of TS2 during the mid-Tonian continental rifting of the precursor block of the São Francisco paleocontinent. Moreover, TS3 is now recognized as a syn-collisional metasedimentary succession that is structurally in the basal portion of the Passos Nappe.