The Neoproterozoic carbonate rocks of the Araras Group (Amazon Craton) and the Sete-Lagoas and Salitre Formations (São Francisco Craton) share a statistically indistinguishable single-polarity (reversed) characteristic direction. This direction is associated with paleomagnetic poles that do not align with the expected directions for primary detrital remanence. We employ a combination of classical rock magnetic properties and micro imaging/chemical analysis (in thin sections) using synchrotron radiation to examine these remagnetized carbonate rocks. Magnetic data indicate that most samples lack the anomalous hysteresis properties typically associated with carbonate remagnetization (except for distorted loops). Through a combination of Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS), X-ray Fluorescence (XRF), and X-ray Absorption Spectroscopy (XAS), we identified subhedral/anhedral magnetite, or spherical grains with a core-shell structure of magnetite surrounded by maghemite. These grains are within the pseudo-single domain size range, as do most of the iron sulfides, and are spatially associated with potassium-bearing aluminosilicates. While fluid percolation and organic matter maturation play a role, smectite-illitization appears to be crucial for the growth of these phases. X-ray diffraction analysis, in addition, identifies these silicates as predominantly highly crystalline illite, suggesting exposure to epizone temperatures. These temperatures were likely reached during the final stages of the Gondwana assembly (Cambrian), but remanence was only locked in afterward, in successive cooling events during the Early Middle Ordovician. This is supported by the carbonates' paleomagnetic pole positions compared to Gondwana's apparent polar wander path, and the absence of reversals, contrasting with the high reversal frequency of the Late Ediacaran/Cambrian.

中文翻译：

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粘土矿物和大陆规模再磁化：南美新元古代碳酸盐岩案例研究


Araras 群（亚马逊克拉通）和 Sete-Lagoas 组和 Salitre 组（圣弗朗西斯科克拉通）的新元古代碳酸盐岩具有统计上无法区分的单极性（反向）特征方向。该方向与古地磁极相关，而古地磁极与初级碎屑剩磁的预期方向不一致。我们采用经典岩石磁性和显微成像/化学分析（在薄片中）的组合，使用同步加速器辐射来检查这些再磁化的碳酸盐岩石。磁性数据表明，大多数样品缺乏通常与碳酸盐再磁化相关的异常磁滞特性（扭曲的环除外）。通过扫描电子显微镜与能量色散 X 射线光谱 (SEM-EDS)、X 射线荧光 (XRF) 和 X 射线吸收光谱 (XAS) 的结合，我们鉴定了半面体/反面体磁铁矿，或具有磁赤铁矿包围的磁铁矿核壳结构。这些颗粒与大多数硫化铁一样，在伪单域尺寸范围内，并且在空间上与含钾铝硅酸盐相关。虽然流体渗滤和有机质成熟发挥了作用，但蒙脱石-伊利石化似乎对这些相的生长至关重要。此外，X 射线衍射分析表明这些硅酸盐主要是高度结晶的伊利石，表明暴露于外层温度。这些温度可能是在冈瓦纳大陆组装（寒武纪）的最后阶段达到的，但剩磁仅在中奥陶世早期的连续冷却事件中被锁定。 与冈瓦纳大陆明显的极地漂移路径相比，碳酸盐岩的古地磁极位置以及没有反转（与晚埃迪卡拉纪/寒武纪的高反转频率形成鲜明对比）都支持了这一点。