Detrital zircon ages and geochronological constraints on the Neoproterozoic Puga diamictites and associated BIFs in the southern Paraguay Belt, Brazil

The Paraguay belt comprises a thick sedimentary succession deposited on the southwestern border of the Amazonian Craton and the Rio Apa Block. The base of the succession in the southern Paraguay belt is marked by a level of glacially derived deposits from the Puga Formation associated with banded iron formations, which has been assumed to be end-Cryogenian in age (635Ma) by previous authors is spite of the lack of geochronological data. Here we present the first U-Pb SHRIMP ages on detrital zircon grains separated from the matrix of six samples of these diamictites two different localities (Puga Hill and Bodoquena area). U-Pb ages determined from two samples (ca. 130 grains) of Puga Hill show a large variation between 970Ma and 2100Ma. Rocks with these ages can be found in the Amazonian Craton suggesting that it is the most probable source of the sediments. Detrital zircons (ca. 230 grains) from the Bodoquena area (about 200km south of Puga Hill) range from 706 to 1990Ma. The 1760Ma source is significantly more important in these samples, comprising more than 70% of analyzed grains, and indicates provenance from the adjacent Rio Apa Block. The youngest zircon was dated at 706±9Ma, thus constraining the maximum depositional age for the Puga Formation. Possible sources for this younger population could be either the juvenile Mara Rosa magmatic arc in the Brasilia belt, or the rocks from the Laurentian external fold belts located to the west of the sampled area in Neoproterozoic paleogeographic reconstructions. The maximum depositional age of the diamictites (and associated BIFs), together with cap carbonate carbon and strontium isotope data (δ13C=-5.0 and 87Sr/86Sr=0.7077) in Puga Hill, indicate that they were deposited after 700Ma, suggesting that they may represent the end-Cryogenian event

Redox variations and bioproductivity in the Ediacaran: Evidence from inorganic and organic geochemistry of the Corumbá Group, Brazil

Stable isotope ratios combined with elemental compositions and molecular biomass data provide a powerful tool in Neoproterozoic palaeoenvironmental interpretations. Here, we report the results of an extensive organic and inorganic geochemical study performed in the Ediacaran sedimentary succession of the Corumba Group (CG) from SW-Brazil, deposited in a shallow marine basin in southwestern Gondwana. This sedimentary succession and in particular the Tamengo Formation, a unit bearing metazoan fossils, has been investigated by means of stable isotopes from carbonates (delta C-13(car) and delta O-18) and associated organic matter (delta C-13(ker) and delta N-15(ker)) together with hydrocarbon distribution and concentrations of major, trace and rare earth elements (REE). A short post-glacial delta C-13(car) negative excursion, interpreted as a period of water mixing, is recorded in the cap carbonates overlying diamictites of the Puga Formation, related to Gaskiers or end-Cryogenian glaciation. The overlying Tamengo Formation by contrast, represents a return to redox-stratified conditions in the basin before the Precambrian-Cambrian boundary. Two distinct biogeochemical modes alternate during deposition of Tamengo sediments: 1) an eutrophic, redox-stratified basin well defined by carbonaceous marls from the middle part of the unit but also recorded upwards, at the transition between bioclastic limestones and calcisiltites. 2) An anoxic basin well characterized in the shallow facies, particularly by bioclastic limestones of the upper Tamengo Formation. A positive Delta C-13(car-ker) (Delta C-13(car-ker) = delta C-13(car) - delta C-13(ker)) excursion of similar to 5% in the carbonaceous marls is explained by enhanced primary productivity in surface waters probably related to an increase of pCO(2), nutrient supply and possibly also changes of the primary producer communities. Abundant pyrite, a biomarker distribution characterized by the occurrence of gammacerane and a low Pr/Ph ratio (similar to 0.7) are also remarkable signatures of these facies, most probably associated with a sulfate-reducing microbial consortium in an anoxic and sulfidic (euxinic) environment. However, low concentrations in redox-sensitive trace elements in these fades suggest a largely oxygenated water column, thus constraining the euxinic setting to the sediments and/or bottom waters. The shallow-water bioclastic limestones record higher concentrations of redox-sensitive elements and Sigma REE as well as a positive Ce anomaly supporting reducing conditions. Oxygenated conditions and dominance of eukaryotic algae characterize the overlying Guaicurus Formation. Principal component analysis (PCA) was used to assess the major geochemical associations. The most significant component combines parameters involved in primary production, such as P concentrations and delta C-13(ker) values. The bio-chemostratigraphic variations in this part of SW-Gondwana point to a stratified ocean with oxic surface waters, alternating periods of high and low bioproductivity and anaerobic conditions at the bottom waters, in the aftermath of younger Neoproterozoic glaciations and close to the Precambrian-Cambrian boundary. (C) 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved

Iron-rich formations at the Cerro Manom\uf3 region, Southeastern Bolivia: remnant of a BIF?

In Southeastern Bolivia, the field relations, geochemical and isotopic data collected in the Manom\uf3 area, along with the occurrence of apatite-rich lenses, carbonate blocks and the extensive enrichment in U, Th and REE probably point to a complex history: 1. An old BIF-like formation with E-W orientation, formed and metamorphosed during Mesoproterozoic times, cuts an older compressive structure at the border of the Amazonic craton; 2. Probable intrusion and fenitization by carbonatitic magma in (Early, Late?) Cretaceous times; 3. Lateritization processes and formation of a thick duricrust in Tertiary-Quaternary times. A more detailed field sampling, as well as additional systematic and extensive geological, mineralogical, petrographic and geochemical studies are required to obtain a more accurate regional picture