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Geologia Sudetica

Vol. 37 (2005) Abstracts

Geologia Sudetica, 37: 1-26.

The tectonometamorphic evolution of the marbles in the Lądek-Śnieżnik Metamorphic Unit, West Sudetes

Mirosław Jastrzębski

Institute of Geological Sciences of the Polish Academy of Sciences, Department of Geology of the Sudetes, Podwale 75, 50–449 Wrocław, Poland, e-mail: mjast@interia.pl

Key words:  marbles, tectonism, metamorphism, Variscan orogeny, Lądek-Śnieżnik Metamorphic Unit, Sudetes

Together with the adjacent rocks, the marbles of the Lądek-Śnieżnik Metamorphic Unit (LSMU), West Sudetes, SW Poland underwent a polyphase structural evolution that occurred in metamorphic conditions changing from medium-grade to low-grade and in deformation regimes changing from ductile to semi-brittle. The structural evolution of the marbles began with E-W subhorizontal shortening resulting in the tilting of the Cal-Dol layering (which had generally originated as a pre-tectonic and pre-metamorphic feature) and its transposition to a steeply dipping metamorphic S1 foliation. The subsequent vertical flattening occurred at the temperature peak of regional metamorphism and produced the N-S trending tight, recumbent F2 folds. This event is also documented by the subhorizontal S2 axial-plane carbonate grain shape fabric and the parallel alignment of Phl-Ms±Tr±Czo in the marbles, and the S2 axial-plane schistosity in the adjacent mica schists. The temperature increase was associated with the progressive mineral sequence Phl → Tr → Di in the dolomite-bearing marbles, which probably initially equilibrated at low to moderate X(CO2). Under peak temperatures, the observed arrangement of the metamorphic zonation of the Stronie formation developed, overprinting the folded planes. Subsequently, under retrogressive conditions, younger deformations were localised in the dynamically recrystallised shear zones that mostly reactivated the S2 planes and were associated with a late top-to-the-N (NE) directed tectonic transport. The D3 mylonitisation was associated with the elongation and size reduction of carbonate grains within the S-C' or S-C mylonites. It produced the S3 planes and the N-S trending L3 stretching lineation. Both groups of the tectonic structures and D2-established mineral isograds (Tr-in and Di-in) were together reoriented during the late compressional stages D4 and D5, related respectively to the SW-NE (WSW-ENE) and NW-SE (NNW-SSE) directed tectonic shortenings. This is visible in the large scale F4 folds, the diversity of the D2-and D3-related mineral assemblages, and the temperature estimations related to both tectonic stages, which indicate decreasing metamorphic conditions from ≥ 600°C in the SE to ≥ 490°C in the NE of the LSMU during D2, and from ca. 510°C to 430°C for the respective domains during D3. The incomplete pattern of the Di-in and Tr-in isograds, which still refers to the geometry of gneiss-schist boundary, confirms that the macrostructures of the LSMU mainly developed in tectonic event(s) following the temperature peak of metamorphism.

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Geologia Sudetica, 37: 27-33.

New paleontological data from the Turonian and Coniacian of the Opole Trough (SW Poland)

Robert Niedźwiedzki* & Agata Godlewska**

* Wrocław University, Institute of Geological Sciences, ul. Cybulskiego 30, 50-205 Wrocław, Poland; e-mail: rnied@ing.uni.wroc.pl
** Arcadis Ekokonrem Sp. z o.o., ul. Tarnogajska 18, 50-512 Wrocław, Poland; e-mail: a.godlewska@arcadis.pl

Key words:  solitary corals, nautilids, echinoids, Upper Cretaceous, Silesia

This report is on the first Micrabaciidae (Micrabacia sp.) solitary corals to have been found in the Upper Cretaceous deposits of the Opole Trough. We also present descriptions of the first nautilids (Cymatoceras sp.; smooth-shelled Nautilidae), regular echinoids (Gauthieria radiata (Sorignet, 1850) and solitary corals (Parasmilia sp.) to have been found in the Lower Coniacian of the Opole region.

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Geologia Sudetica, 37: 35-51.

Cadomian versus younger deformations in the basement of the Moravo-Silesian Variscides, East Sudetes, SW Poland: U-Pb SHRIMP and Rb-Sr age data

Andrzej Żelaźniewicz*, Izabella Nowak**, Robert Bachliński***, Alexander N. Larionov**** & Sergey A. Sergeev****

* Instytut Nauk Geologicznych PAN, Podwale 75, 50-449 Wrocław, Poland; pansudet@pwr.wroc.pl
  Instytut Geologii, Uniwersytet Adama Mickiewicza, ul. Maków Polnych 16, 61-616 Poznań

** Instytut Nauk Geologicznych PAN, Podwale 75, 50-449 Wrocław, Poland
*** Instytut Nauk Geologicznych PAN, Twarda 51/55, 00-818 Warszawa, Poland
**** Centre of Isotopic Research, All Russian Geological Research Institute, St. Petersburg, Russia

Key words:  Desná, Keprník, dome, zircon, tholeiite, pegmatite, muscovite, Vrbno Group, Rejvíz series, Bohemian Massif

U-Pb SHRIMP dating of zircons from a metapegmatite vein which cross-cuts amphibolite facies paragneisses confirms ~580 Ma magmatism in the basement of the northern part of the Moravo-Silesian Zone (Jeseníky Mts.). Structures older than the felsic vein set are interpreted as a record of the Cadomian orogeny. This has been represented by N-trending, W-vergent folds followed by a top-to-the east shearing that occurred at T = 600°C and P = 5 kbar in the Neoproterozoic. The subsequent tectonic overprint led to folding and shearing of the pegmatite, which took place at similar P-T conditions but was associated with top-to-the west kinematics and shortening at a high angle to the foliation. This event likely developed during early stages of Variscan convergence when the Moravo-Silesian crust (Brunovistulia) was subducted and forced down below the approaching upper plate composed of terranes of the Bohemian Massif. Alternatively, it may have occurred around 500 Ma, related to crustal extensional (break-up of Gondwana margins in Cambrian times). Although the first option is favoured, presumably the two may have actually happened. The last ductile deformation was a top-to-the-east younger shearing localized in zones of various widths, assigned to the Variscan collision and reverse movement of the basement rocks. The latter two events occurred at temperatures that allowed in the metapegmatite for the crystal plastic deformation of quartz grains from which the strain was removed by subsequent static recrystallization, and that were high enough to reset the Rb-Sr system in this rock. Consequently, the obtained Rb-Sr isochron age of 290 Ma is considered to reflect the time of uplift. Such late regional uplift is characteristic of the northern part of the Moravo-Silesian Zone, which is the footwall to the Moldanubian Thrust, which separates the Bohemian Massif terranes from the Brunovistulia terrane. It follows from this study that in the East Sudetes basement rocks, structures which are often classified as Variscan may in fact be Cadomian and that the Cadomian record in these rocks is richer than previously assumed.

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