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The acid metavolcanic rocks of the Orlica-Śnieżnik Dome (Sudetes): their origin and tectono-metamorphic evolution

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Murtezi M.

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tom Vol. 38

1-38

EN

The Orlica-Śnieżnik Dome (OSD) belongs to the mosaic of geological units forming the Sudetic part (NE margin of the Bohemian Massif) of the Variscan belt. Acid metavolcanic rocks referred to as leptites appear in the OSD as intercalations in the mica schists of the Stronie Formation - the supracrustal cover of the gneiss core of the unit. Two types of acid meta-volcanogenic rocks have been distinguished: massive leptites (ML) and biotite-bearing leptites (BBL); together they are referred the leptite series. The ML originate from acid volcanic rocks that could have contained only small amounts of sedimentary material. The BBL are metamorphosed tuffites with a variable share of sedimentary material. All the analysed leptites from the entire OSD area reveal similar geochemical characteristics. The high concentration of strongly incompatible elements such as REE, as well as the enrichment in large-ion-lithophile elements indicates the continental crust affinities of the leptites' protolith. On the other hand, ascertained the high Th/Ta ratios, negative Nb and Ta anomalies, and low Ti, Zr and Hf content can be interpreted as indicators of magmatism connected with a suprasubduction environment. Such ambiguous characteristics may indicate that an ensialic rift environment, considered as the place of origin for the magmatic rocks dated at c. 500 Ma and occurring in the OSD, developed due to extension behind a collisional zone of Lower Palaeozoic age. An extensional environment created during this event could be the location for the postulated early deformation and metamorphism of the Stronie Formation. The Śnieżnik metagranites would have been emplaced into such, already deformed, supracrustal cover. The signature of an active margin found in these continental crust rocks that were stretched and melted due to the increased heat transfer resulted from an earlier (Neoproterozoic?) tectono-thermal episode. The reconstructed path of the tectono-metamorphic evolution of the leptites entirely results from a Variscan collisional episode occurring in the borderland of the West and the East Sudetes. The sequence of the deformations established for the studied rocks is interpreted as an effect of the continuous convergence of crustal units in an E-W direction. Structural and phase-equilibria lines of evidence show that the tectono-metamorphic evolution of the Stronie Formation was that of early burial (D1) to a depth not exceeding upper amphibolite facies conditions, and subsequent uplift connected with subvertical shortening and flattening strain (D2). As a result of the continuous convergence, thrusting took place along the edges of the colliding units. The final adaptation of the colliding crustal slices led to zonal lateral oblique displacements (D3). This latter event is treated as the cause for disturbance of the isograd and isotherm pattern observed within the rocks of the Stronie Formation. It is also responsible for zonal shearing, generally taking place in a N-S direction. During the collision that occurred along the eastern margin of the OSD, the rocks of the Stronie Formation reached the highest P-T conditions of metamorphism at c. 9 kbar and 650°C. The presented data shows a considerable decrease in the peak-metamorphic conditions from the east to the west of the OSD.

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Cyrkon jako wskaźnik genezy łupków z Czarnowa we wschodniej okrywie granitu Karkonoszy

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Murtezi M. , Kryza R.

Przegląd Geologiczny

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2001

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tom Vol. 49 nr 9

815-819

PL

Analiza cech morfologicznych, analiza typologiczna metodą Pupina, oraz mikrosondowe badania chemiczne cyrkonów z łupków kwarcowo-skaleniowych oraz łupków łyszczykowych reprezentujących tzw. "formację łupków z Czarnowa" wskazują na urozmaicony charakter protolitów tych skał. Koncentraty cyrkonowe z pięciu różnych, wyselekcjonowanych próbek były poddane analizie typologicznej: trzy próbki z drobnoziarnistych łupków kwarcowo-skaleniowych z północnej części jednostki Czarnowa, dwie próbki z ciemniejszych i grubiej-ziarnistych łupków łyszczykowych z południowej części tej jednostki. Populacje cyrkonów ze wszystkich analizowanych próbek wykazują duże podobieństwo pod względem ogólnych cech morfologicznych. Znaczące różnice zostały jednak zaobserwowane przy analizie typologicznej: próbki łupków kwarcowo-skaleniowych zawierają cyrkony z dominacją słupa {100} i piramidy {101} (typy S24, S23, S19 oraz S18), podczas gdy w cyrkonach łupków łyszczykowych dominują formy {110} i {211}. Zmienność typologiczna odpowiada zmienności obserwowanej w chemicznej charakterystyce cyrkonów: badane ziarna z łupków łyszczykowych wykazują wyższą zawartość Hf, przeważnie z brzegami wzbogaconymi w Hf względem środka. Duża proporcja kryształów euhedralnych oraz generalny brak tzw. pozornego kąta wygaszania w cyrkonach z wszystkich próbek sugerują pierwotne magmowe źródło pochodzenia materiału tych skał. Lupki kwarcowo-skaleniowe z północy terenu prawdopodobnie reprezentują skały wulkanogeniczne, natomiast łupki łyszczykowe z części południowej zawierają cyrkony typowe dla granitoidów typu S lub dla materiału osadowego otrzymanego z tego typu magmowego protolitu.

EN

The quartzo=eldspathic rocks (so-called "leptynites ") and mica schists of the Czarnów unit in the eastern Karkonosze Complex (West Sudetes) exhibit problematic origin and age. They may represent felsic metavolcanics and metasediments of Neoproterozoic (?) age, and form apparently the country rocks of the Kowary orthogneiss that intruded at ca. 500 Ma ago. To highlight the problematic origin of the Czarnów schists, zircons from a set of representative samples of these rocks were studied. This study included description of morphological features, Pupin 's typological classification and electron-microprobe analyses. Zircon concentrates from 5 different rock samples were first studied using a typology method. Three samples come from fine-grained quartzo-feldspatic schists from the northern part of the Czarnów unit, whereas two samples from coarser-grained and darker mica schists in the southern part of the area. All the studied samples reveal zircon populations with many similarities in morphology. However, considerable differences are ascertained in typological analysis: samples from the quartzo-feldspatic schists have types with dominating {100} prism and {101} pyramide (types S24, S23, S19 and S18), whereas in those from the mica schists forms {110} and {211} prevail. This tvpological variation corresponds to differences observed in chemical characteristics of the zircons: the studied grains from the mica schists display higher Hfcontents, usually with rims richer in Hf The observed large proportion of idiomorphic crystals and a general lack of "apparent extinction angle " in all samples suggest their igneous origin. The quartzo-feldspathic schists of the northern area most probably represent acid volcanogenic rocks, whereas the mica schists in the southern part contain zircons ivpical of S-type granitoids or sedimentary material derived from such igneous protoliths.

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The 3rd meeting of the Central European Tectonic Group and the 10th meeting of the Czech Tectonic Studies Group, Felsotarkany, Hungary, April 14-17, 2005

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Jastrzebski M. , Murtezi M.

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tom 37