Abstract The biogenicity of proposed stromatolite structures from Eoarchean (ca. 3.71 Ga) rocks of the Isua Supracrustal Belt (ISB) in West Greenland is under debate. The structures in question are found within a suite of multiply deformed greenschist- to amphibolite-facies metamorphic rocks. To assess their premise as primary sedimentary features – as opposed to products of strain localization in layered, variably ductile rocks – we report new field mapping at the appropriate scale and resolution from the original discovery Sites reported by Nutman et al. (2016) . Our new map was used to guide micro- and macro-structural investigations and comprehensive geochemical sampling. Here, we report detailed field characterization and structural analysis to show that the structures are linear inverted ridges aligned with azimuths of local and regional fold axes and parallel to linear structures; they are not deformed conical stromatolites. Combined major element (e.g., Ca, Mg, Si) scanning μXRF maps, and electron backscatter diffraction (EBSD) patterns collected on fresh surfaces cut perpendicular and parallel to the ridges attest to the lack of any residual sedimentary laminae (e.g., compositional layering) within these structures' cores. Internal layering previously inferred for these features instead arises from variable weathering of outcrop surfaces that otherwise conceals granoblastic quartz ± dolomite cored boudins that sit between semi-continuous competent layers of enveloping quartzite in a calc-silicate schist. The morphology of boudins reflects viscosity contrasts of the different ductile layers during deformation. Therefore, these features are not of sedimentary origin. Furthermore, discontinuous field relationships and absence of primary sedimentary structures that could serve as way-up indicators preclude confident assignment of these outcrops as being structurally overturned, as originally argued. Collectively, our results show that the Isua structures are the expected result of a tectonic fabric that preserves no fine-scale primary sedimentary structures and were probably never stromatolites.

中文翻译：

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重新评估所谓的约。3.7 来自 Isua 上壳带（西格陵兰）的 Ga 叠层石来自详细的化学和结构分析

摘要 西格陵兰岛 Isua 上地壳带 (ISB) 的始太古代 (ca. 3.71 Ga) 岩石中提出的叠层石结构的生物成因正在争论中。所讨论的结构是在一组多次变形的绿片岩至角闪岩相变质岩中发现的。为了评估它们作为主要沉积特征的前提——而不是层状、可变延展性岩石中应变定位的产物——我们报告了来自 Nutman 等人报告的原始发现地点的适当比例和分辨率的新现场绘图。(2016)。我们的新地图用于指导微观和宏观结构调查以及综合地球化学采样。这里，我们报告了详细的场特征和结构分析，以表明这些结构是线性倒脊，与局部和区域折叠轴的方位角对齐并平行于线性结构；它们不是变形的锥形叠层石。组合主要元素（例如，Ca、Mg、Si）扫描 μXRF 图和在垂直和平行于山脊切割的新鲜表面上收集的电子背散射衍射 (EBSD) 图案证明没有任何残留的沉积层（例如，成分分层）在这些结构的核心内。先前为这些特征推断的内部分层相反是由露头表面的可变风化引起的，否则这些露头表面隐藏了花岗岩石英±白云岩芯的布丁，它们位于钙硅酸盐片岩中的半连续包裹石英岩层之间。Boudins 的形态反映了变形过程中不同韧性层的粘度对比。因此，这些特征不是沉积成因。此外，不连续的油田关系和可作为上升指标的主要沉积结构的缺乏排除了这些露头的可靠分配，正如最初所争论的那样。总的来说，我们的结果表明 Isua 结构是构造结构的预期结果，该构造结构没有保留精细的原始沉积结构，并且可能永远不是叠层石。不连续的油田关系和缺乏可以作为上升指标的主要沉积结构妨碍了将这些露头确定为结构翻转的可能性，正如最初所争论的那样。总的来说，我们的结果表明 Isua 结构是构造结构的预期结果，该构造结构没有保留精细的原始沉积结构，并且可能永远不是叠层石。不连续的油田关系和缺乏可以作为上升指标的主要沉积结构妨碍了将这些露头确定为结构翻转的可能性，正如最初所争论的那样。总的来说，我们的结果表明 Isua 结构是构造结构的预期结果，该构造结构没有保留精细的原始沉积结构，并且可能永远不是叠层石。