The Holy Cross Mountains (Poland) terranes palaeoposition and depositional environment in Silurian—new insights from rock magnetic studies
The Holy Cross Mountains (Poland) terranes palaeoposition and depositional environment in Silurian—new insights from rock magnetic studies
复制标题
圣十字山脉（波兰）志留纪地体古位置和沉积环境——岩石磁性研究的新见解
DOI：
10.1093/gji/ggad129
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发表时间：
2023
影响因子：
2.8
通讯作者：
Szaniawski, R
中科院分区：
文献类型：
--
作者：
Niezabitowska, D K;Szaniawski, R
位于波兰的圣十字山脉（HCM）是一个孤立的古生代岩石的自然露头，位于跨欧洲缝合带内，该缝合带是波罗的海托恩奎斯特边缘附近的大陆缝合带的构造拼贴。这种独特性使HCM成为古地理研究的目标。根据相的差异，HCM被划分为两个主要单元，南部（基尔切单元）和北方（Chuysogóry单元）部分（分别为SHCM和NHCM）。在志留纪时期，它们与波罗的海大陆的关系仍然是一个讨论的问题，无论是HCM的两个部分都是沿着波罗的海边缘分开的岛屿，还是它们在共同的古地理历史中共享。在这里，我们提出了综合岩石磁性测量的结果，作为一种工具来解释沉积和埋藏过程中的古环境条件，因此允许讨论的相对位置。为了识别志留系笔石质页岩的磁性矿物组成和结构，进行了几种岩石磁性测量，包括低温饱和等温剩磁，三分量磁化率和磁滞测量的热退磁，以及磁化率的各向异性（AMS）。岩石样品来自HCM的两个单元。在所有分析的样品中，我们发现单域（SD）化学计量磁铁矿的主要成岩（即沉积后）的起源和针铁矿可能导致风化。反过来，碎屑磁铁矿，即使观察到在以前调查的志留纪岩石从波罗的海边缘，在这项研究中没有确定，我们的属性在深水环境中的成岩作用期间的溶解。仅在NHCM中观察到SD赤铁矿和磁赤铁矿颗粒，我们将其解释为碎屑成因。由于这些颗粒在海洋沃茨中的溶解抗性，它们被保存在NHCM次盆地底部沃茨的亚氧环境中。考虑到两个HCM部分的沉积条件（近底区的氧化）相当相似，我们将风成赤铁矿颗粒的存在仅在NHCM岩石中与Llandovery（志留纪）晚期波罗的海大陆的NHCM比SHCM更近的位置相关联。这一结论与现有的一些古地理模式相一致。除了侧重于分析铁磁体的岩磁研究外，还进行了AMS测量。结果表明，磁化率主要受顺磁性矿物控制，主要是层状硅酸盐，铁磁性贡献较小。扁圆形AMS椭球体和明显的层理平行叶理表明主要的沉积-压实排列。观测到的构造成因的弱应变磁线理，可能与华力西期变形有关。
The Holy Cross Mountains (HCM) in Poland, is an isolated natural outcrop of Palaeozoic rocks located within the Trans-European Suture Zone, a tectonic collage of continental terranes adjacent to the Tornquist margin of the Baltica. This uniqueness made the HCM a target for palaeogeographic research. Based on the facies differences, the HCM had been divided into two major units, the southern (the Kielce Unit) and northern (the Łysogóry Unit) part (SHCM and NHCM, respectively). Their position in relation to each other and the Baltica continent during Silurian times is still a matter of discussion, whether both parts of the HCM were separated terranes located along the Baltica margin or they shared in common palaeogeographic history. Here, we present the results of comprehensive rock magnetic measurements applied as a tool to interpret palaeoenvironmental conditions during deposition and burial and therefore allow discussion about the terranes’ relative position. To recognize the magnetic mineral composition and texture of studied Silurian graptolitic shales several rock magnetic measurements were conducted including low-temperature Saturated Isothermal Remanent Magnetization, thermal demagnetization of three-component IRM and hysteresis measurements, as well as anisotropy of magnetic susceptibility (AMS). The sampled rocks come from both units of the HCM. In all analysed samples we found single domain (SD) stoichiometric magnetite of mostly diagenetic (i.e. post-depositional) origin and goethite resulting likely from weathering. In turn, detrital magnetite, even if observed in previously investigated Silurian rocks from the Baltica margin, was not identified in this study, what we attribute to dissolution during diagenesis in the deep-water environment. Solely in the NHCM, SD hematite and maghemite grains were observed, which we interpret as detrital in origin. These grains have been preserved in the suboxic environment of the NHCM sub-basin bottom waters due to their resistance to dissolution in marine waters. Considering the deposition conditions (oxygenation of the near-bottom zone) rather similar for both HCM parts, we associate the presence of aeolian hematite grains solely in the NHCM rocks with a more proximal position of the NHCM than the SHCM in relation to the Baltica continent during late Llandovery (Silurian). This conclusion agrees with some existing palaeogeographic models. In addition to petromagnetic studies focused on the analysis of ferromagnets, AMS measurements were also carried out. The results indicate that the magnetic susceptibility is mainly governed by paramagnetic minerals, mostly phyllosilicates with small ferromagnetic contributions. Oblate AMS ellipsoid and distinct bedding parallel foliation indicate prevailing sedimentary-compactional alignment. Observed magnetic lineation of tectonic origin resulting from weak strain is related presumably to Variscian deformations.