Tectonic significance of the late-Ediacaran syn-orogenic basin in the easternmost portion of the Paraguay Belt, Tocantins Province, central Brazil

Abstract

Sedimentary provenance studies using detrital zircon U–Pb ages represent an important tool to investigate the evolution of orogenic basins and to suggest tectonic settings and paleogeography reconstructions when it comes to supercontinent modeling. Syn-orogenic basins worldwide are characterized by a large proportion of zircons with ages close to the maximum depositional period, reflecting the proximity of recently formed magmatic rocks. In this work, we combine field observations, detrital zircon U–Pb ages, whole-rock Sm–Nd and mica Ar–Ar isotopic analyses to constrain the final tectonic evolution of a restricted basin located in a poorly studied region at the easternmost limit of the Paraguay Belt in the Tocantins Province, central Brazil. The investigated area corresponds to the boundary sector of the Paraguay Belt and the Brasília Belt and is also transected by the strike-slip fault system associated with the Transbrasiliano Lineament. This sector was previously mapped as part of the Cuiaba Group, internal zone of the Paraguay Belt. We provide 465 new detrital zircon U–Pb data with ages distributed from the early Cambrian to the Archean, with approximately 60% of the analyzed grains derived from Ediacaran-Cryogenian sources. The maximum depositional age of the basin is defined at ca. 590 Ma, constrained by the youngest age peak and an evident provenance shift in detrital input in the Paraguay Belt was demonstrated. The syn-orogenic character of the basin is inferred based on the main peak of the detrital zircon population age distribution around 600 Ma, which is very close to the maximum depositional period and points to a short time between erosion, deposition, and burial processes. The muscovite 40Ar/39Ar age of ca. 536–546 Ma obtained for muscovite schist metamorphosed under greenschist facies conditions indicates that the regional thermal regime was maintained up to the early-Cambrian in the area. The minimum fast cooling rate of 25 °C/Myr defined in the investigated area is constrained by the 40Ar/39Ar analysis of biotite (549.16 ± 1.30 Ma) from a syn-to late-kinematic granodiorite intrusion. The very rapid magma emplacement occurred into relatively shallow crustal levels through the associated strike-slip faults. The Transbrasiliano Lineament would have facilitated the rapid unroofing of the study area and therefore the syn-orogenic deposition in a foreland domain at the final phase of West Gondwana amalgamation. The studied metasedimentary rocks represent a transition phase from the passive margin sedimentation (Cuiabá Group) to an orogenic phase, constraining an inversion event in the West Gondwana around ca. 590 Ma. The data provide evidence of a late orogenic basin that was formed contemporaneously with the evolution of an active margin between the Amazonian Craton and the eastern blocks/cratons (São Francisco-Congo and Rio de la Plata cratons and Paranapanema Block) close to Cambrian times. Our results support the existence of the younger Clymene Ocean and the subsequent final assembly of West Gondwana in the Cambrian.