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

摘要

In the southern Brasilia 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 Araxa Group show very distinct age spectra and provenance patterns for samples collected in different sites of the Brasilia 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 Goias Magmatic Arc, yielded important age peaks around 650 Ma. This suggests an important provenance shift recorded by the metasedimentary rocks of the Araxa nappe system. The lower and easternmost rocks might represent the western passive margin of the Sao 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 Brasilia belt, and thus were probably deposited in a syn-orogenic basin. The variability of source areas is also recorded by distinct Sm-Nd T-DM ages, between 1.4 Ga and 2.0 Ga, and Sr-87/Sr-86 ratios from 0.719 to 0.891. Low-grade metasedimentary rocks of the Ibia Group in the Araxa nappe system show even younger detrital zircons, around 580 Ma, suggesting they were deposited after the metamorphic peak of the Brasilia belt and represent the erosion of the thrust fronts during late-stage nappe extrusion and stacking. Therefore, the Araxa 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 underst