The Holy Cross Mountains (Poland) terranes palaeoposition and depositional environment in Silurian—new insights from rock magnetic studies

Abstract

SUMMARY 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.