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New palaeomagnetic data from the Palaeozoic carbonates of the Moravo-Silesian Zone (Czech Republic): evidence for a timing and origin of the late Variscan remagnetization

Grabowski J., Bábek O., Nawrocki J., Tomek Č.

Geological Quarterly

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2008

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Vol. 52, No 4

321-321

EN

Palaeomagnetic studies were carried out in the Devonian–Early Carboniferous carbonates of the Moravo-Silesian Zone — MSZ (Czech Republic) in order to evaluate the timing and origin of late Variscan magnetic over printing. Sampling localities were spread out along the strike of the MSZ from the SW to NE. Previously published thermal maturity data have demonstrated a significant gradient from SW (burial temperatures 150–200gradeC) to NE of the region (250–300gradeC). A late Variscan remagnetization direction (component A), carried by magnetite, was identified in 6 localities. Three phases of the remagnetization in the MSZ might be distinguished which might be assigned to Early to Late Carboniferous, Late Carboniferous and Early Permian. They are coeval with remagnetization events distinguished in Ardennes. A correlation exists between thermal indices and un blocking temperature spectra of component A. Thermal activation nomograms show that component A might be either a thermoviscous or thermochemical remanent magnetization acquired due to a thermal event (deep burial) of 1–10 My duration and stabilized during subsequent uplift. A more ancient component B, identified in the SW part, previously interpreted as primary, is shown to be a synfolding remagnetisation. Itindicates 70grade clockwise rotations before the acquis tion of the component A .

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A vestige of an Early Devonian active continental margin in the East Sudetes (SW Poland) - evidence from geochemistry of the Jegłowa Beds, Strzelin Massif

Szczepański J.

Geological Quarterly

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2007

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Vol. 51, No 3

271-284

EN

The Early Devonian metasandstones of the Jegłowa Beds (Strzelin Massif, NE Bohemian Massif) are low- to medium-grade metamorphosed siliciclastic deposits showing differences in modal composition, especially in the amount of micas and feld spars. Despite the similarity in relative concentrations of trace elements, three chemical groups can be distinguished among the metasandstones that differin the total amounts of major and trace ele ments. The negative Ta and Nb anomalies visible on a spider plot normalised to the average upper continental crust and spe cific ratios of e.g. Th, Zr, Hf, La/Th and Ti/Sc characterize the majority of the metasandstones, indicating the provenance of their sedimentary precursors from a subduction-related tectonic setting. However, few of the samples analysed show strongly differentiated geochemical characteristics with high Zr and Hf contents. This suggests input of relatively strongly reworked mate rial. Thus, it seems that a back-arc setting can account for the mixed nature of the inferred source ar eas with old basement and arcrelated detritus respectively as the end-members of the mix tures. The composition of the metasandstones indicates deposition of the siliciclastic material near an Early to Middle Devonian volcanic arc that was super imposed on rocks originally forming a part of the pre-De vo nian continental margin of the Brunovistulicum microplate. A plausible tectonic scenario in volves formation of the Devonian arc due to east-directed subduction of an oceanic domain between the Brunovistulian microcontinent in the east and the Central Sudetic terrain located fur ther west.

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Proweniencja materiału okruchowego piaskowców karbońskich z waryscyjskich basenów przedpola w południowo-zachodniej Polsce i na Morawach

Krzemiński L.

Biuletyn Państwowego Instytutu Geologicznego

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2005

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nr 417

27-108

PL

Opracowanie zawiera wyniki badań petrograficznych i geochemicznych około 300 próbek piaskowców wizenu górnego-namuru dolnego, pochodzących z 13 otworów wiertniczych położonych w Wielkopolsce, a ponadto z wychodni w strefie morawsko-śląskiej. Materiał okruchowy tych piaskowców, należących do sukcesji fliszowej wschodniej części waryscyjskiego pasma fałdowo-nasunięciowego, pochodził z waryscyjskiego orogenu kolizyjnego, tzn. bloku przedsudeckiego i Sudetów. Obszary źródłowe obejmowały głównie plutoniczne/gnejsowe korzenie łuków magmowych wraz z ich osłoną łupkową i pokrywami wulkanicznymi. Podrzędny był udział pasma fałdowo-nasunięciowego oraz kompleksów ofiolitowych, reprezentujących litosferę oceaniczną basenu marginalnego. Zapis osadowy odzwierciedla szybko postępującą ekshumację w obszarach źródłowych, polegającą na usuwaniu przez erozję pokryw wulkanicznych i odsłanianiu głębszych poziomów fundamentu. Związana z tym główna zmiana petrofacjalna w wielkopolskim basenie przedpola nastąpiła w czasie odpowiadającym poziomowi GoBeta. Dane analityczne dają podstawy do hipotetycznego przyjęcia istnienia kontynentalnych łuków magmowych w Sudetach i na bloku przedsudeckim. Nie jest jasne czy częściowo łukowe charakterystyki detrytusu są związane z wczesnokarbońską sytuacją geotektoniczną tej części orogenu, czy odzwierciedlają erozję starszych łuków magmowych.

EN

The study contains the results of petrographic and geochemical research of ca. three hundred samples of Upper Visean-Lower Namurian sandstones from thirteen boreholes situated in the Wielkopolska area and from outcrops in Moravo-Silesian Zone. These sandstones, belonging to the flysch succession in the eastern part of the Variscan fold-and-thrust belt, were sourced from the Variscan collisional orogen, i.e. from the Fore-Sudetic Block and the Sudetes. The source areas mainly comprised plutonic/gneissic roots of the magmatic ares together with their schistose envelopes and volcanic covers. Variscan fold-and-thrust belt and ophiolitic complexes, representing oceanic lithosphere of marginal basin, were of minor importance. The sedimentary record reflects rapid exhumation in the source area consisting in erosional removing of the volcanic covers and unroofmg of deep basement. The major change of detrital modes of sandstones in Wielkopolska foreland basin took place during GoBeta zone. The analytical data may indicate the presence of continental magmatic ares in the Sudetes and Fore-Sudetic Block. It is not clear whether, in part, arc characteristics of detritus are related to Early Carboniferous geotectonic setting in this part of the orogen, or whether they reflect an erosion of any older arcs.

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The Variscan overthrust of the Lower Palaeozoic gneiss unit on the Cadomian basement in the Strzelin and Lipowe Hills massifs, Fore-Sudetic Block, SW Poland: is this part of the East-West Sudetes boundary?

Oberc-Dziedzic T., Madej S.

Geologia Sudetica

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2002

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Vol. 34

39-58

EN

The problem of the position of the boundary between the geological structures of the West and the East Sudetes has been a topic of discussion since 1912, when F.E. Suess developed the concept of the Moldanubian overthrust as a boundary between the Moldanubian and Moravo-Silesian zones. The West Sudetes comprise gneisses of Cambro-Ordovician protolith age with inclusions of high pressure metamorphic rocks. The Cadomian basement, referred to as the Brunovistulian and overlain by Devonian rocks, is characteristic of the East Sudetes. The location of the East-West Sudetes boundary is well-defined in the mountainous part of the Sudetes but still a matter of debate in the Fore-Sudetic Block. This paper puts forward a new approach to this problem. The Proterozoic Strzelin gneiss with its Proterozoic (the older schist series) and Devonian envelope (the Jegłowa beds) are tectonically overlain by the Early Palaeozoic Gościęcice gneiss and the light Stachów gneiss with its envelope. The former occurs in the footwall and the latter in the hanging wall rocks of the Strzelin Thrust. This juxtaposition resembles the situation along the East-West Sudetes boundary separating two domains with contrasting protolith ages. Consequently, the Strzelin Thrust is considered part of the border zone between the East and West Sudetes, i.e. the northern continuation of the Ramzova/Nyznerov thrust to the Fore-Sudetic Block. At the present erosion level, the hanging wall rocks of the Strzelin Thrust are separated from their roots and form klippen. The minimum transport distance along the thrust is estimated at 10 km. The Strzelin Thrust forms a generally shallowly dipping domed surface. It becomes steeper east of the Strzelin massif, where it is hidden beneath Cenozoic sediments, and west of the Lipowe Hills, where it follows the eastern border of the Kamieniec Ząbkowicki Metamorphic Complex. The hanging wall is probably rooted in the strongly mylonitised mica schists exposed along the Mała Ślęza river. The thrust zone is a wide mylonitic belt carrying relatively HT and HP garnet-bearing amphibolites in the northern part of the Strzelin massif and the strongly mylonitised Henryków gneiss and quartzites in the southern part at the Lipowe Hills. This paper discusses the problem of the East-West Sudetes boundary, mainly in the framework of the Strzelin massif. The attitude of this boundary in other parts of the Fore-Sudetic Block is still unclear because of poor exposure and numerous faults of E-W and NW-SE orientation that displace it from its original position.