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首页> 外文期刊>Precambrian Research >Meso-Neoproterozoic isotope stratigraphy on carbonates platforms in the Brasilia Belt of Brazil
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Meso-Neoproterozoic isotope stratigraphy on carbonates platforms in the Brasilia Belt of Brazil

机译：巴西巴西利亚带碳酸盐岩台地中新元古代同位素地层

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Carbonate platforms were present worldwide during the Proterozoic Eon, and variations in their C and Sr isotope ratios are commonly used as a correlation tool particularly through the Neoproterozoic, when rapid secular change in marine C and Sr isotope values permit distinction between Cryogenian glacial events. In central Brazil, the late Mesoproterozoic and Neoproterozoic eras are represented by a thick succession of sedimentary rocks that were deposited, and later deformed, along the eastern margin of the S?o Francisco Craton. These strata are divided into the three major groups: (1) the Paranoá Group， which consists of a succession of sandstone, siltstone, rhythmite, and selected intervals of carbonate, (2) the Macaúbas Group, which consists of a glacial diamictite (Jequitaí Formation)， and (3) the Bambuí Group with an important carbonate-bearing succession that includes characteristic "cap carbonate" facies in its lower strata. Carbonate facies of Paranoá and Bambuí Groups typically occur in unconformable contact， and when the diamictite of Jequitaí Formation is absent, it can be difficult to determine the stratigraphic position of these lithological similar groups. Furthermore, uncertainties in the age of the Bambuí Group has lead to several distinct interpretations regarding the age of the Jequitaí glacial diamictites. We investigated the C, O, and Sr isotopes and chemical composition of carbonate rocks in five measured sections, including both pre- and post-glaciation carbonate successions. The δ~(13)C (‰pdb) values in the upper Paranoá Group occur in a narrow interval between +0.6 and +3.6, whereas the post-glacial Bambuí Group begins with substantially negative values (as low as -5.7‰) in cap dolomite facies and rises to values up to +11 permil in limestone of the upper Sete Lagoas Formation. Similarly, carbonate rocks of the Paranoá and Bambuí groups are distinct in terms of their ~(87)Sr/~(86)Sr ratio. Generally non-radiogenic ratios between 0.7056 and 0.7068 are recorded in the upper Paranoá Group, and ratios between 0.7074 and 0.7080, occurring within the Bambuí Group. The stratigraphic pattern of the C and Sr ratios indicates distinctive isotopic characteristics for these two carbonate successions. The isotopic data for the cabonates in the Paranoá Group are consistent with a sedimentation age in upper Mesoproterozoic or lowermost Neoproterozoic, preceding the first Cryogenian glaciation. Carbonate facies and the isotopic data for the lower Bambuí Group suggest a relationship with the second Cryogenian glaciation.
机译：碳酸盐岩平台在元古代的早期存在于世界各地，当海洋C和Sr同位素值的长期快速变化可以区分低温冰河事件时，它们的C和Sr同位素比的变化通常被用作相关工具，尤其是通过新元古代。在巴西中部，中元古代和新元古代时代以沉积岩的厚层序为代表，这些沉积岩沿圣弗朗西斯科·克拉顿的东缘沉积，后来变形。这些地层分为三个主要组：（1）帕拉诺亚组，它由一系列的砂岩，粉砂岩，脉动石和选定的碳酸盐岩层组成;（2）玛卡巴斯组，它由一个冰川铁矾土组成（Jequitaí地层），以及（3）班布伊组具有重要的含碳酸盐岩相演替，其下地层包括特征性的“盖层碳酸盐岩”相。 Paranoá和Bambuí组的碳酸盐相通常发生在不整合面的接触中，并且当Jequitaí组的铁矾土不存在时，可能很难确定这些岩性相似组的地层位置。此外，Bambuí集团年龄的不确定性导致了有关Jequitaí冰川铁矾土年龄的几种不同解释。我们在五个测量的剖面中研究了碳酸盐岩的C，O和Sr同位素以及化学成分，包括冰期前后的碳酸盐岩演替。上Paranoá组的δ〜（13）C（‰pdb）值出现在+0.6到+3.6的狭窄范围内，而冰川后的Bambuí组开始时基本为负值（低至-5.7‰）。覆盖塞特拉各斯组上部石灰岩中的白云岩相，并使其值上升至+11 permil。同样，Paranoá和Bambuí组的碳酸盐岩的〜（87）Sr /〜（86）Sr比也不同。通常在帕拉诺阿组中记录到的非放射源比率在0.7056和0.7068之间，而在班布依组中则记录在0.7074和0.7080之间。 C和Sr比的地层模式表明这两个碳酸盐岩演替的独特同位素特征。帕拉诺亚群中碳酸盐的同位素数据与首次冰山成冰作用之前中元古界或最低新元古代的沉积年龄相一致。较低的班布伊群的碳酸盐相和同位素数据表明与第二次低温成冰作用有关。

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