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Geochemistry and geochronology of the Jawornik granitoids, Orlica-Śnieżnik Dome, Sudetes, Poland

Białek Dawid

Geological Quarterly

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2020

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Vol. 64, No. 4

942--957

EN

The Jawornik granitoids intrude, in vein-like form, a sequence of a polymetamorphic metavolcanic and metapelitic rocks of the Orlica-Śnieżnik Dome, Sudetes, Poland. This paper provides whole-rock geochemical data, sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon geochronological data as well as 40Ar-39Ar age determinations to constrain the genetic and temporal relationships of the different rock types forming these veins. Based on macroscopically visible features of the granitoids and their relationship with tectonic structures visible in the country rocks, four varieties of the Jawornik granitoids have been distinguished: amphibole- and biotite-bearing granites (HBG), biotite-bearing granites (BG), biotite- and muscovite-bearing granites (BMG) and muscovite-bearing granites (MG). The Jawornik granitoids as a whole show a limited but significant variation in major element chemical composition, with SiO2 ranging from 65 to 76 wt.% (average 69.16 wt.%, n = 24). They are subalkalic, peraluminous and calc-alkaline [average A/CNK = 1.07, average (Na2O + K2O) = 7.75, average (Fe2O3t/(Fe2O3t + MgO) = 0.59]. Close inspection of their geochemical parameters showed that the samples investigated can be subdivided into two groups. The first group, the HBG, BG, and BMG varieties, comprising most of the granitoids in the Złoty Stok-Skrzynka Tectonic Zone, were formed by melting of greywackes or/and amphibolites. The MG, belonging to the second group, were formed by partial melting of a more felsic source. The HBG yielded a zircon U-Pb age of 351 ±1.3 Ma and well-defined 40 Ar-39Ar plateau ages for hornblende (351.1 ±3.9 Ma) and coexisting biotite (349.6 ±3.8 Ma), indicating probably the oldest magmatic event in this region. Zircons from the MG, the youngest rock variety on the basis of their relationship with the tectonic structures in the host rocks yielded a U-Pb age of 336.3 ±2.4 Ma, though based on three points only. The biotites and muscovites from the BMG have 40 Ar-39Ar plateau ages of 344.1 ±4.7 Ma and 344.6 ±3.8 Ma, respectively. These data, in combination with already published isotopic ages, suggest that the Jawornik granitoids intruded host rocks of the Orlica-Śnieżnik Dome in three stages, at ~350, ~344 and ~335 Ma.

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Palaeozoic orogeneses in the Sudetes: a geodynamic model

Cymerman Z.

Geological Quarterly

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2000

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Vol. 44, Nr 1

59-80

EN

The Palaeozoic geodynamic evolution of the Sudetes resulted from two successive orogenic events: (1) Ordovician-Silurian geotectonic processes (pre-Variscan stages), and (2) the Variscan orogeny. Early Palaeozoic rifting of Cadomian crustal segments and opening of the Ligerian (Galicia-Massif Central) and/or Saxothuringian Ocean occurred in Ordovician-Silurian times on the northern (peri-Gondwana) periphery of the Bohemian Massif. At the same time, the Góry Sowie terrane with a magmatic arc affinity quite probably developed on the SW margin of Baltica due to subduction of the Tornquist Ocean. Two major structural events characterised the Variscan evolution of the Sudetes: (1) regional-scale ductile thrusting of Late Devonian-Early Carboniferous age, and (2) Early Carboniferous-Early Permian regional extension. Ductile thrusting is characterised by: (I) a general NNE-directed, dextral transpressional stacking of ductile nappes due to oblique collision of the Moldanubian and Moravian microplates in the Eastern Sudetes, and (II) SW- to NW-directed, sinistral transpressional stacking of ductile nappes due to westward lateral extrusion of continental crust in the Central and Western Sudetes, itself a result of oblique indentation of the Central Sudetic oceanic lithosphere. The first Variscan deformation in the Sudetes might reflect a purely convergent setting that evolved into a transpressive setting during oblique convergence. Special attention is given to the geometry and kinematics of intraplate tectonic escape and a model of indentation processes in the Sudetes. The presented new geotectonic model for the Variscan evolution of the Sudetes is consistent with lateral escape of the Saxothuringian terrane as an important way of accommodating Variscan strain in the NE part of the Bohemian Massif. This model explains the lateral expulsion (escape) process as due to the indentation of the Central Sudetic terrane along with the Góry Sowie terrane and by the oblique subduction of the Ligerian/Saxothuringian Ocean(s) (now tectonically dismembered ophiolitic rocks of the Central Sudetic terrane).

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Genetic relationships between metabasalts and related gabbroic rocks: an example from the Fore-Sudetic Block, SW Poland

Dziedzic K., Dziedzic H.

Geologia Sudetica

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2000

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Vol. 33, nr 1

33-48

EN

The metabasalts occurring within the gneisses of the eastern part of the Góry Sowie gneiss block, have either enriched or depleted LREE patterns. However, these two types of metabasic rock are indistinguishable in terms of their major elements and many trace elements. Their intimate association indicates that their parent magmas erupted nearly contemporaneously. They originated through the decompressional, two-stage incremental melting of a mantle diapir source. The LRRE enriched variety of the basalts was formed from a spinel/garnet peridotite melt mixture which was followed by spinel peridotite melts. From the latter melts, cumulate gabbros crystallized and extracted portions of these melts provided the LRRE depleted variety of basalts. This strongly suggests that the metabasalts are compatible in age with the gabbros. Both of the metabasalts varieties developed mainly by AFC processes involving mantle source melts and lower continental crust components. From a comparison of these metabasalts with those in adjacent areas, it is possible to draw the conclusion that volcanic activity in the whole region had the same time-span. The metabasalts were metamorphosed to LP hornblende granulites, synchronously with the surrounding amphibolite facies gneisses of the margins of the Góry Sowie Block, which were metamorphosed to LP-HT cordierite gneisses. They originated due to the transition from high-grade amphibolite to granulite facies conditions associated with a near-isothermal decompression, during the time of the late Variscan (Carboniferous).

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The fabric of the Culm conglomerates in the eastern parts of the Nizký Jesenik and Drahany Uplands, eastern margin of the Bohemian Massif, the Czech Republic

Havir J.

Geologia Sudetica

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1999

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Vol. 32, nr 1

1-11

EN

The fabric of the Culm conglomerates was studied by geometrical strain analysis methods in the eastern parts of the Nízký Jeseník and the Drahany Uplands on the eastern margin of the Bohemian Massif. The shapes and orientations of pebbles were used as strain markers and the average final ellipsoids (Shimamoto & Ikeda, 1976) were calculated (final fabric ellipsoids representing both depositional and deformational influence on the sediments). The geometric strain analysis shows a predominantly depositional or only partially deformational fabric. Only at some sites in the eastern part of the Nízký Jeseník Uplands a deformational fabric of the conglomerates occurs. But there is evidence of a very weak effect of ductile shortening in the Drahany Upland probably connected with rotation of the more rigid pebbles in the viscous graywacke matrix of the conglomerates. The orientations of the long axes of the final fabric ellipsoids are more or less uniform. The long axes are predominantly orientated N-S to NE-SW in the eastern parts of the Nízký Jeseník and the Drahany Uplands, which means nearly parallel to the major structures of the Variscan orogen in this region.

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Fluvial sedimentology of the Biały Kamień Formation (Upper Carboniferous, Sudetes, Poland)

Kurowski L.

Geologia Sudetica

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1998

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Vol. 31, nr 1

69-77

EN

The Biały Kamień Formation (Namurian B-C) is part of a molasse sequence deposited in the post-Variscan Intra-Sudetic basin. The formation differs from the adjacent deposits by its coarse-grained nature and possession of fining-up cycles, consistent with deposition in a braided gravel-bed river (GII and GIII facies of Miall's 1978 classification). Fine-grained sediments and coal seams are present in the upper part of the formation. The cyclicity mostly seems to reflect autocyclic depositional mechanisms within the tectonically controlled Intra-Sudetic molasse basin.