Fácies, petrografia, geocronologia Pb-Pb e geoquímica isotópica de Sr, C e O da formação Guia, Neoproterozóico da Faixa Paraguai Norte, região de Paranatinga-MT

Abstract

The occurrence of extensive carbonate platforms (cap carbonates) at the base of Ediacaran overlapping glacial diamictites that reached low latitudes at the end of the Cryogenian is explained by the Snowball and Slushball Earth hypotheses. These cap carbonates are found at the base of the Araras Group, Northern Paraguay Fold belt, southern border of the Amazonian Craton, represented by the Mirassol d'Oeste and Guia formations. The Guia Formation consists of fine and bituminous limestones, interspersed with black shales rich in organic matter and hydrocarbons. In the study area, Mato Grosso State, east portion of the fold belt, the best exposures of the Guia Formation ocurr in the Emal quarry and in dissected hills at the base of Serra Azul hills. The main lithotypes are represented by fine grayish limestones with preserved sedimentary structures intercalated by gray to black shales, with fine sandstone that occurs subordinately. Calcareous breccia are also described, with tabular and/or elongate clasts. Nine facies/sedimentary microfacies were grouped into three facies associations (FA). Association 1 (FA1)- tidal flat/intertidal: mudstone with planar lamination; mudstone with wavy lamination and massive mudstone. Association 2 (FA2)- shoreface/offshore transitional deposits: Hummocky cross-laminated sandstone; Shale with terrigenous lamination; mudstone/wackstone with terrigenous. Association 3 (FA3)- offshore deposits: Breccia with tabular clasts - Rudstone and Limestone breccia - Floatstone. The Pb-Pb geochronological study provided an age of 476 ± 93 Ma for the Guia Formation in the Paranatinga-MT region and 87Sr/86Sr isotopic ratios of 0.70934 and 0.707744 for this unit in region of Paranatinga- MT and in Bom Jardim-MT, respectively. These 87Sr/86Sr ratios indicate an Ediacaran to Cambrian age when correlated to the evolution curve of the oceanic Sr in the Neoproterozoic, which is corroborated by the Pb-Pb age.The evaluation of the C and O isotopic signal for the Guia Formation in the Bom Jardim Region revealed 13C values ranging from -0.27 to -1.33 ‰ and 18O between -7.21 to -0.92 ‰, for the Paranatinga region the 13C values range from -3.01 to -5.29 ‰ and 18O -5.13 to -9.58 ‰. The predominance of carbonaceous facies and absences of dolomitic, siliciclastic and evaporitic facies, supported by 13C values, suggests that the Paranatinga and Bom Jardim outcrops are, respectively, representative of the base and top sucessions of the Guia Formation. The 13C values are similar to those described for the Nobres and Serra do Quilombo formations. However, its diagenetic characteristics differ from the others, corroborates the characteristics of the Guia Formation. In addition, the 13C values obtained are related to the top portion of the Guia Formation than to the Serra do Quilombo Formation, which in turn presents positive 13C values, which are not identified for the Guia xi Formation in this or in previous works. The Guia Formation, in the Tangará da Serra-MT region (Amazonian Craton), was the target of Pb-Pb and isotopic geochronological study of Sr, with a 622 ± 33 Ma age, as well as 0.70709 to 0,70729 ratio of 87Sr/86Sr interpreted as primary isotopic signature of the carbonate deposition environment. However, the geochronological Pb-Pb and Sr isotopic data for the Guia Formation in the Paranatinga and Bom Jardim regions are different from the avaliable data for this formation in the context of the Amazon Craton. This suggests that the difference in age obtained represents a rejuvenated age, not reflecting the age of deposition of these rocks, but a reopening of the U-Pb isotope system related to the evolution of the Northern Paraguay River and installation of the Paleozoic basins of Paraná and Parecis.