Provenance of passive-margin and syn-collisional units: Implications for the geodynamic evolution of the Southern Brasília Orogen, West Gondwana

Abstract

Collisional orogens are one of the most conspicuous features of plate tectonics on Earth. They record the dynamics of continent-continent amalgamation, often confined to linear deformational belts. The Southern Brasília Orogen is one of the most well-studied and complete orogens of West Gondwana. Newly obtained isotopic data, coupled with previously published analyses, expand the occurrence of metamorphosed passive margin sequences (maximum depositional age of ~900 Ma), with a main source in the São Francisco paleocontinent, to large sections of the orogen previously interpreted, by some investigators, as related to younger active margin deposits. This has implications for the location of the suture zone: as most of the upper and lower nappes are dominated by metamorphosed passive margin deposits, the suture is probably located at the base of the uppermost nappe, related to the active margin (Paranapanema paleocontinent). Zircon (U-Pb and Lu-Hf) and whole-rock (Sm-Nd and geochemistry) data obtained for syn-collisional meta-graywackes (~650–630 Ma), that uncomformably overlie passive-margin-related successions, point to a bimodal provenance from both the passive (São Francisco paleocontinent) and active (Paranapanema paleocontinent) margins, depending on the structural level from which the samples were collected. Despite the conspicuous zircon age differences, the maximum depositional age is the same for samples from the upper and the lower nappes. Furthermore, the meta-graywackes from the lower and upper nappes are petrographically and geochemically similar. This suggests concomitant deposition in a common collisional setting. Two metamorphic ages were obtained by U-Pb monazite dating at ~633 and 606 Ma, indicating that protracted medium-grade metamorphic conditions affected the Southern Brasília Orogen.