Sr isotope geochemistry and Pb-Pb geochronology of the Neoproterozoic cap carbonates,Tangara da Serra, Brazil

摘要

The Mirassol d'Oeste Formation (cap dolomites) and the base of the Guia Formation (cap limestones) represent the cap carbonates of the Neoproterozoic Araras Group, located along the southern border of Amazonian Craton, central Brazil. Petrographical and microfacies descriptions together with geochemical and mineralogical data of the top of the Mirassol d'Oeste and the base of the Guia formations in the Tangara da Serra area allowed us to identify the main diagenetic features (neomorphism, chemical compaction, and fractures) and to evaluate the terrigenous contribution. Dolomitization has been documented in a limestone horizon of the lower part of Guia Formation through petrography, X-ray diffractometry, and geochemistry. Despite diagenetic alteration, sedimentary structures of the Tangara da Serra cap carbonate are well preserved (laminar bedding and crystal fans). Pb-Pb dating of the Guia limestones yielded an age of 622 ± 33 million years. This early Ediacaran age is considered as the deposition age and constitutes further evidence that the cap carbonates of the Araras Group are related to the Marinoan glaciation, which took place at the end of the Cryogenian. The assessment of the primary (87)~Sr/(86)~Sr signature of the Guia limestones has been improved by sequential leaching of diluted acetic acid, which allowed the elimination of highly radiogenic Sr contributions due to terrigenous grams. The (87)~Sr/~(86)Sr ratios of 0.70709-0.70729 are regarded as the primary Sr isotopic signature when the carbonates precipitated. The terrigenous contribution can explain the discrepancy with the more radiogenic (87)~Sr/~(86)Sr ratios previously obtained on the cap carbonate. At a global scale, the Sr isotopic signature around 0.7071-0.7073 of the cap carbonates of the Araras Group is compatible with Sr marine evolution curves at the end of the Marinoan glaciation. However, such low Sr values suggest that the abrupt rise could have happened asynchronously and heterogeneously in the early Ediacaran oceans.