Andrzej Żelaźniewicz

The eastern termination of the Variscides: terrane correlation and kinematic evolution

Abstract Analysis of tectonostratigraphic units in the West Sudetes reveals the same geological events as in the areas west of the Elbe Fault Zone: a late Proterozoic (Cadomian) orogenic event, Cambro-Ordovician to Devonian rift-drift, and late Devonian to early Carboniferous subduction-collision. There is no conclusive evidence of an Ordovician orogenic event. Tectonic units in the Sudetes are shown to be related to terranes defined in western parts of the Bohemian Massif. The Lausitz-Izera Block, the Orlica-Śnieżnik Unit and the Staré Město Belt represent easterly continuations of the Saxo-Thuringian Terrane. The Rudawy Janowickie Unit and the Sudetic Ophiolite contain fragments of the Saxo-Thuringian Ocean. The protoliths of the Görlitz-Kaczawa Unit, the South Karkonosze Unit, the Góry Sowie and the Kłodzko Units either belong to the Bohemian Terrane or else were welded onto it during mid-late Devonian metamorphism and deformation. Relicts of the Saxo-Thuringian Foreland Basin are marked by flysch with olistoliths in the Görlitz-Kaczawa Unit and in the Bardo Basin. The spatial array of terranes in and around the Bohemian Massif reveals a disrupted orocline, dissected by dextral transpression along the Moldanubian Thrust. This orocline was formed when central parts of the Variscan belt were accommodated in an embayment of the southern margin of the Old Red Continent.

Structure and evolution of the Bohemian Arc

Abstract Tectonic zones and palaeogeographic units (terranes) in the German segment of the Variscides correlate with equivalents in the Sudetes at the NE margin of the Bohemian Massif. This correlation defines an arcuate structure with an opening angle of about 90°. The structure is truncated to the SE by a crustal scale. NE-trending fault zone with dextral transpression, the Moldanubian Thrust (MT). The arc cannot have been formed by northeastward indentation of the Bohemian Massif, since there is no evidence of a fault zone on the NW flank of the notional indenter, and little evidence for northeastward tectonic transport. Kinematic and age constraints on the main fault zones instead suggest that the structural array was formed by a complex sequence of events. Northwestward displacement along the margin of the East European Platform (EEP) with clockwise rotation was followed by large southwestward movements along the Moldanubian Thrust, and renewed northwestward displacement along the SW margin of the East European Platform.

Rb–Sr and U–Pb geochronology of migmatitic gneisses from the Góry Sowie (West Sudetes, Poland): the importance of Mid–Late Devonian metamorphism

The Góry Sowie Block, West Sudetes, SW Poland consists of a gneiss–migmatite complex with minor amphibolites, calc-silicate rocks, ultrabasic rocks and granulites. Migmatitic gneisses underwent a complex polyphase history of deformation and metamorphism, each deformation phase (D1 to D5) being associated with synkinematic amphibolite-facies metamorphism (M1 to M5). For a better understanding of the P–T–t-deformation path three types of gneisses, which represent distinct stages of deformation and metamorphism (D2–M2, D3–M3 and D5–M5), have been studied by means of the Rb–Sr and U–Pb isotopic systems. U–Pb monazite and xenotime ages of c. 380 Ma for rocks migmatized and deformed during the D3–M3 and D5–M5 stages constrain the timing of the last metamorphic–migmatitic event in the Góry Sowie Block. Rb–Sr thin-slab and mica–whole-rock dating for samples with D2, D3 and D5 characteristics provide ages between 362 and 375 Ma which overlap within error (2ó). No correlation between deformational characteristics and apparent Rb–Sr ages was recognized. The Rb–Sr data are interpreted as time constraints for post-peak metamorphic cooling. The new geochronological data indicate the significance of Devonian high-temperature metamorphism in the Sudetes and support a similar interpretation of earlier studies. The isotopic results are also consistent with the interpretation, based on field, structural and petrological data, that at least four (D2–D5) structural episodes occurred in Devonian times within a very short time interval, the details of which cannot be further resolved with the methods used.

Protolith age and provenance of metasedimentary rocks in Variscan allochthon units: U–Pb SHRIMP zircon data from the Orlica–Śnieżnik Dome, West Sudetes

New U–Pb sensitive high-mass resolution ion microprobe (SHRIMP) data from detrital zircons within the Orlica–Śnieznik Dome provide new insights into the stratigraphic and palaeogeographic position of assumed relict Precambrian basement preserved in the Variscan collisional orogen of the West Sudetes, SW Poland. Hitherto, the Mlynowiec Formation and the Stronie Formation within the Orlica–Śnieznik Dome were assumed to represent two metavolcano-sedimentary successions of Proterozoic and early Palaeozoic age, respectively. However, when previous U–Pb data and mapping data from the Orlica–Śnieznik Dome are combined with the new detrital zircon isotopic ages both from paragneisses within the Mlynowiec Formation and from light-coloured quartzites and mica schists within the Stronie Formation, the result strongly suggests that the protoliths of these two formations actually form a continuous succession. This continuous succession is herein designated the Mlynowiec–Stronie Group. The rocks of this group were deposited during middle Cambrian–early Ordovician times ( c. 520–470 Ma), presumably at the northern edge of West Gondwana after the 10–20 Ma period of tectonic quiescence that followed the terminal stage of the Cadomian collisions. Monotonous Mlynowiec metagreywackes form the lower part of the succession, and the lithologically diverse schistose Stronie Formation forms its upper part. The change from greywacke (Mlynowiec) to volcano-sedimentary (Stronie) facies coincided with the onset of rather short-lived volcanic activity which climaxed around 505–495 Ma and which supplied the volcanogenic components to the Stronie Formation. No ‘Cadomian unconformity’ has been observed in the region. Xenocrystic zircons from the Mlynowiec–Stronie Group retain records of Archaean (3.0–2.3 Ga), Palaeoproterozoic (2.1–1.8 Ga) and Neoproterozoic to early Cambrian (660–530 Ma) zircon-forming events. These zircon ages, together with the lack of 1.7–1.2 Ga zircon ages, suggest that the source areas for the metasedimentary rocks may have been the West Africa craton, which therefore differs from the Amazonian affinity of the adjacent Brunovistulia Terrane. Nevertheless, two zircons, c. 1.0 and 1.1 Ga old, respectively, indicate that the Mlynowiec–Stronie Group sedimentary basin must have still been within the delivery reach of detritus ultimately derived from the Grenvillian-age belt(s). The detrital components of the supracrustal formations of the Orlica–Śnieznik Dome were mainly derived from Neoproterozoic zircon-bearing crystalline rocks that were accreted to, and included in, the Cadomian basement in several intrusive pulses that culminated at 660–640 Ma, 620 Ma and 570–530 Ma.

Pre-Variscan basement rocks in southern Poland: Where is the southeastern margin of eastern Avalonia (Cadomia)?

SummaryPre-Variscan basement in southern Poland is poorly exposed and thus known mostly from subsurface data. The availability of the latter is reviewed for terrains located between the Sudetes and the East European Platform. In these terrains the following relationships have been documented: Cadomian granitoids capped by Variscan flysch, Palaeozoic platform strata, Palaeozic folded and partly thermally altered successions, and low-grade metamorphic rocks overlain by Middle Cambrian strata. In view of their interrelationships the location of the Avalonia-Baltica suture in southeastern Poland is uncertain.