Provenance shift from a continental margin to a syn-orogenic basin in the Neoproterozoic Araxá nappe system, southern Brasília belt, Brazil

Abstract

In the southern Brasília belt of central Brazil, prominent east-verging systems of stacked nappes caused the thrusting and tectonic interleaving of metasedimentary rocks of different provenance, age and metamorphism. Detrital zircon U-Pb analysis of metasedimentary rocks of the Araxá Group show very distinct age spectra and provenance patterns for samples collected in different sites of the Brasília belt. While samples collected in the external zone of the belt yielded zircon age peaks mainly from 790 Ma to 2.5 Ga, samples of the internal zone of the belt, close to the Goiás Magmatic Arc, yielded important age peaks around 650 Ma. This suggests an important provenance shift recorded by the metasedimentary rocks of the Araxá nappe system. The lower and easternmost rocks might represent the western passive margin of the São Francisco paleocontinent, and thus are correlated to the Canastra Group, while the upper and westernmost rocks show a striking influence of the late Cryogenian (ca. 650 Ma) continental magmatic arc of the Brasília belt, and thus were probably deposited in a syn-orogenic basin. The variability of source areas is also recorded by distinct Sm-Nd TDM ages, between 1.4 Ga and 2.0 Ga, and 87Sr/86Sr ratios from 0.719 to 0.891. Low-grade metasedimentary rocks of the Ibiá Group in the Araxá nappe system show even younger detrital zircons, around 580 Ma, suggesting they were deposited after the metamorphic peak of the Brasília belt and represent the erosion of the thrust fronts during late-stage nappe extrusion and stacking. Therefore, the Araxá nappe system is actually composed of a tectonically interleaved package of metasedimentary rocks deposited in distinct basins during distinct evolutionary stages and, finally, assembled and metamorphosed during collisional thrusting. Recognizing such metasedimentary rocks that are petrographically similar but show distinct deposition timing, sediment source and tectonic setting is an important challenge to understand the evolution of the Brasília belt in the scenario of West Gondwana amalgamation.