U–Pb geochronology of the southern Brası́lia belt (SE-Brazil): sedimentary provenance, Neoproterozoic orogeny and assembly of West Gondwana

Abstract

Abstract The Brasilia belt borders the western margin of the Sao Francisco Craton and records the history of ocean opening and closing related to the formation of West Gondwana. This study reports new U–Pb data from the southern sector of the belt in order to provide temporal limits for the deposition and ages of provenance of sediments accumulated in passive margin successions around the south and southwestern margins of the Sao Francisco Craton, and date the orogenic events leading to the amalgamation of West Gondwana. Ages of detrital zircons (by ID–TIMS and LA-MC-ICPMS) were obtained from metasedimentary units of the passive margin of the Sao Francisco Craton from the main tectonic domains of the belt: the internal allochthons (Araxa Group in the Araxa and Passos Nappes), the external allochthons (Canastra Group, Serra da Boa Esperanca Metasedimentary Sequence and Andrelândia Group) and the autochthonous or Cratonic Domain (Andrelândia Group). The patterns of provenance ages for these units are uniform and are characterised as follows: Archean–Paleoproterozoic ages (3.4–3.3, 3.1–2.7, and 2.5–2.4 Ga); Paleoproterozoic ages attributed to the Transamazonian event (2.3–1.9 Ga, with a peak at ca. 2.15 Ga) and to the ca. 1.75 Ga Espinhaco rifting of the Sao Francisco Craton; ages between 1.6 and 1.2 Ga, with a peak at 1.3 Ga, revealing an unexpected variety of Mesoproterozoic sources, still undetected in the Sao Francisco Craton; and ages between 0.9 and 1.0 Ga related to the rifting event that led to the individualisation of the Sao Francisco paleo-continent and formation of its passive margins. An amphibolite intercalation in the Araxa Group yields a rutile age of ca. 0.9 Ga and documents the occurrence of mafic magmatism coeval with sedimentation in the marginal basin. Detrital zircons from the autochthonous and parautochthonous Andrelândia Group, deposited on the southern margin of the Sao Francisco Craton, yielded a provenance pattern similar to that of the allochthonous units. This result implies that 1.6–1.2 Ga source rocks must be present in the Sao Francisco Craton. They could be located either in the cratonic area, which is mostly covered by the Neoproterozoic epicontinental deposits of the Bambui Group, or in the outer paleo-continental margin, buried under the allochthonous units of the Brasilia belt. Crustal melting and generation of syntectonic crustal granites and migmatisation at ca. 630 Ma mark the orogenic event that started with westward subduction of the Sao Francisco plate and ended with continental collision against the Parana block (and Goias terrane). Continuing collision led to the exhumation and cooling of the Araxa and Passos metamorphic nappes, as indicated by monazite ages of ca. 605 Ma and mark the final stages of tectonometamorphic activity in the southern Brasilia belt. Whilst continent–continent collision was proceeding on the western margin of the Sao Francisco Craton along the southern Brasilia belt, eastward subduction in the East was generating the 634–599 Ma Rio Negro magmatic arc which collided with the eastern Sao Francisco margin at 595–560 Ma, much later than in the Brasilia belt. Thus, the tectonic effects of the Ribeira belt reached the southernmost sector of the Brasilia belt creating a zone of superposition. The thermal front of this event affected the proximal Andrelândia Group at ca. 588 Ma, as indicated by monazite age. The participation of the Amazonian craton in the assembly of western Gondwana occurred at 545–500 Ma in the Paraguay belt and ca. 500 Ma in the Araguaia belt. This, together with the results presented in this work lead to the conclusion that the collision between the Parana block and Goias terrane with the Sao Francisco Craton along the Brasilia belt preceded the accretion of the Amazonian craton by 50–100 million years.