ARAGUAINHA IMPACT STRUCTURE: A STRATIGRAPHIC WINDOW TO THE BASEMENT TO THE PARANÁ BASIN

The 40-km-diameter Araguainha impact structure in central Brazil, is the largest impact structure of South America. It was formed in the northeasternmost part of the intracontinental Paraná Basin, at the Permian-Triassic boundary. The structure with an erosion of ~200 m, represents a stratigraphic window, that allow to see the basin sequences and, in the central uplift, into the uplifted basement. The Precambrian basement below the Paraná Basin in the northeast part is believed to be relate to the Tocantins Structural Province, comprising the Paraguay and Brasília and Araguaia mobile belts. The Paraguay Belt consists of Neoproterozoic and Cambrian metasedimentary/sedimentary strata deposited in a passive margin environment. It constitutes four major lithostratigraphic units: fine-grained metasedimentary rocks of the Cuiabá Group; glacial diamictite-turbidites from the Cuiabá Group and the Puga Formation; carbonates of the Araras Group; and epiclastic, unmetamorphosed sedimentary rocks of the Alto Paraguay Group. A borehole drilled to investigate the Paraná Basin 30 km WSW of the Araguainha impact structure reached a depth of 1.9 km and encountered metasedimentary rock associated with the Cuiabá Group. This basement is locally intruded by apophyses from the Cambrian Araguainha alkali granite and subrounded by the lower units of the Paraná Basin. In the Araguainha central uplift only metasandstone and phyllites have been related to the Cuiabá Group. Extensive new outcrops exposed along the MT-100 state road recently, led to the identification of additional lithotypes of the basement.  In order to better understand the stratigraphy bellow the Paraná Basin, U-Pb zircon provenance analyses were obtained into four (meta)sedimentary  samples: CA-22, a dark grey siltstone with quartz veins of ca. 35 cm width; CA-111, a dark brown slate with quartz veins up to 30 cm width and in  erosional unconformity with conglomerate from a lower unit of the Paraná Basin; GAR-15-0, a black phyllite of varied coherence - sampled between two apophyses of altered granite; GAR-15-G10; and a light-grey metasandstone, in contact with greywacke and altered impact melt rock after a granitic precursor. Zircon were dated by MC-LA-ICP-MS at the Geochronological Research Center (CPGEO) at São Paulo University (USP). Under BSE and CL imaging, zircons exhibited magmatic zonation, metamorphic overgrowths, and/or inherited metamorphic cores. Nearly 290 concordant U-Pb isotope data were obtained. The provenance analyses of samples show a nearly bimodal zircon distribution diagram with the main populations in the Cambrian-Neoproterozoic boundary – peaks at ~600 and 778 Ma - and a second group with ages spready in between ~1000 and ~2500 Ma, with a small peak at ~2007 Ma. The maximum depositional ages, taking into account the youngest statistically significant peak is 561 Ma. These results, together with published data from the Paraguay Belt and lower units of the Paraná Basin were analyzed by multidimensional scaling (MDS) and kernel density estimation (KDE) to evaluate the proximity between populations The samples show a similarity with the lower part of the Paraná Basin, the Rio Ivaí Group, Furnas and Ponta Grossa formations but not with the Alto Paraguay or Cuiabá groups. All samples were taken into the multilithological megablock zone of the collapsed central uplift and this geological context can complicate the interpretation of the real stratigraphy of the area. Nevertheless, samples CA-22 and CA-111 are clearly metasedimentary and deposited, probably at the time when Cuiabá Group was deposited, but with provenance from proximal sources, like the Goiás magmatic arc. Even so, these results will be compared with U-Pb in zircon provenance data of polymict impact breccias formed in the Araguainha impact event, with the aim to obtain information about the main protoliths for these impact breccias.