The Neoproterozoic cap carbonates have been the subject of countless paleoenvironmental and chemicalstratigraphy studies in almost all Cratons around the world. Those were deposited after global glacial events deposits, are composed of dolostones and limestones, overlying glacial diamictites with specific features as stromatolits, tubes like structures, megaripples and crystal fans of calcite. In Brazil this ocucurrence, principal objetive of this work is exposed in Mirassol d ́Oeste and Tangará da Serra towns at Southeast of Mato Grosso State, Southeastern of Amazon Craton, recovering glacial diamictites with age of ≈ 630 Ma, The cap carbonate is part of base of Araras Group compoused by dolostone of Mirassol d ́Oeste and limestones of base of the Guia Formation. The main goal of this work is consolidate an Early Ediacaran age for the cap carbonate of the Tangará da Serra area by Pb-Pb geochronology and bring new Sr isotopic data to understand the paleoemviromental conditions as well as the increasing Sr marine evolution curve of during Neoproterozoic. Were collected 36 samples from top of Mirassol d ́Oeste (13 samples) and base of Guia Formation (23 samples), for petrography, X-ray difractometry, X- fluorescence as well as Sr isotopic analyses and Pb-Pb dating, using mass spectrometer TIMS and ICP-MS. The petrography allow recognize diagenetic features as stylolits, dolomite grains recristalized,
spatic dolomite, iron oxides and autigenic quartz in dolostones of Mirassol d ́Oeste Formation. In Guia Formation, were observed partial substitution from crystal fans of calcite to spatic dolomite, stilolits as well as microfractures filled by calcite and secondary dolomite. The X-difractometry complemented the determination of mineralogical assemblage about carbonates, identificating the presence of secondary dolomite as well as quartz and feldspar in limestones of Guia Formation. The X-ray fluorescence allows determinate the teors of Fe, Mn, Ca and Sr, which were used to help the interpretation of Sr isotopic signatures. The Sr isotopic signature in 5 samples in dolostones of Mirassol d ́Oeste Formation and 6 limestones samples of base of Guia Formation, were determinated by sequential leaching with acetic
acid. For dolostones the 87 Sr/ 86 Sr ratios ranging 0.708 to 0.709. For limestones the 87 Sr/ 86 Sr ratios ranging 0.707093 to 0.707289, after eliminate the leaching by probably terrigenous contribution for 87 Sr/ 86 Sr ratios until 0.7114. The lowest 87 Sr/ 86 Sr ratios were found in samples with lowest Mn/Sr ratios (0.31-0.45) and Fe/Sr (6.03-9.8). The differences in analytical procedure used to obtain the 87 Sr/ 86 Sr ratios, allow to explain the differences a out the isotopic Sr signatures in limestones from Tangará da Serra área (sequential leaching) with which, more radiogenic from limestones from Mirassol d ́Oeste área (total dissolution), previously published. The 87 Sr/ 86 Sr ratios ranging 0,70709 – 0,70729 in limestones from Tangará da Serra area, are positioned above from Sr marine curve in late Neoproterozoic, in transition from Cryogenian to Ediacaran, before the abrupt rise in early Ediacaran. Six dolostones samples and 17 limestones samples were analyzed, for determination of isotopic Pb composition, the samples were leached with HCl. The isotopic Pb signatures are homogeneous in dolostones (19.05< 206 Pb/ 204 Pb < 19.50; 15.69 < 207 Pb/ 204 Pb < 16.89) and were not used for elaborate the Pb-Pb isochron. From 17 limestones samples just 15 provided and age of 622 ± 33 Ma (2σ), despite de limited isotopic variations of the isotopic composition (18.77 < 206 Pb/ 204 Pb < 31.18; 15.71 < 207 Pb/ 204 Pb < 16.46). This result reinforces the Ediacaran early age to cap carbonate of Araras Group and check their association with the events after the last Criogenian glaciation at South of Amazon Craton.
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