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Vestiges of Cambro-Ordovician continental accretion in the Carpathian-Balkan orogen: First evidence of the ‘Cenerian’ event in the central Serbo-Macedonian Unit

Spahić Darko, Bojić Zoran, Popović Danica, Gaudenyi Tivadar

Acta Geologica Polonica

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2021

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Vol. 71, no. 2

219--247

EN

In the Balkans, the Serbo-Macedonian Unit (SMU), Serbia, is thrust bounded by the composite Tethyan Vardar Zone and the Carpatho-Balkanides. The SMU actually emerges from beneath the Neoalpine Miocene-Pliocene deposits. Both provenance and geodynamic position of the SMU are poorly known and still debated. This paper reviews the data hitherto published and includes some new field data interpretations. The SMU is composed of a Neoproterozoic-Cambrian high-grade (para- and ortho-) gneiss with peraluminous magmatic arc components (560-470 Ma). The SMU is in the contact with Neoproterozoic upper Ordovician-Carboniferous low-grade metasedimentary succession of an accretionary wedge assembly represented by the Supragetic basement. The SMU basement became folded, sheared and metamorphosed around 490-450 Ma. Paleomagnetic data point to high southern latitudes and a peri-Gondwanan position of the SMU at that time, which concurs with glaciomarine evidence recorded from the upper Ordovician sediments at the base of an accretionary wedge succession. Based on the published data and field survey in the Stalać region, we correlate the SMU with the pre-Mesozoic gneiss terrane exposed in the Strona-Ceneri zone of the Alps. This terrane, identified as the Cenerian orogen of the Alaskan subduction type, developed at an active margin of Gondwana during middle Ordovician times. The SMU basement, with augen and migmatitic gneisses and arc-related peraluminous magmatic bodies, developed at this margin as part of the Cenerian belt or its equivalent. Such an orogenic edifice proved transient and in the earliest Silurian the SMU fragments drifted away being bound for Baltica (amalgamated Moesian microplate and Danubian terrane) to which they became accreted in the Carboniferous and included in the southern European branch of the Variscan orogen (Marginal Dacides/Carpatho-Balkanides). Despite consider-able Variscan and Alpine reworking, the pre-Variscan, Cenerian-type crustal assembly along with an inferred boundary between the magmatic arc and the accretionary wedge, accompanied by back-arc/forearc deposits, are still decipherable in the Western Balkan countries.

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An insight into a gneiss core of the Orlica–Śnieżnik Dome, NE Bohemian Massif : new structural and U-Pb zircon data

Redlińska-Marczyńska A., Żelaźniewicz A., Fanning C. M.

Geological Quarterly

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2016

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Vol. 60, No. 3

714--736

EN

The Orlica–Śnieżnik Dome in the Sudetes, the NE Bohemian Massif, embraces two formations of felsic gneisses of controversial origin and evolution. Our study shows that despite similar geochemical signatures, they carry systematic minor differences in mineral, isotope, zircon and geothermobarometric characteristics. Four variants of the Gierałtów gneisses include migmatites and have a longer structural history than the Śnieżnik augen orthogneisses. U-Pb SHRIMP analyses yielded U-Pb ages of ~500 Ma for cores and ~498 Ma for wide outer parts of zircon grains in the twice-folded Gierałtów gneisses, and an age of ~500 Ma for a discordant neosome vein. Neoproterozoic metasediments were among precursors of the lithologically diversified Gierałtów Gneiss Formation. First deformation, metamorphism, and migmatisation of these rocks occurred at 515–475 Ma and overlapped with the development and emplacement of a porphyritic S-type granite precursor to the Śnieżnik Gneiss Formation. The metagranite (= Śnieżnik augen orthogneiss) embraced migmatitic xenoliths. Zircon grains from such xenoliths revealed distorted and replaced cores which yielded U-Pb ages that dispersed around 507–487 Ma, whilst wide darker poorly zoned outgrowths yielded ages from ~343 Ma to ~325 Ma (mean ~340 Ma). These outgrowths were interpreted as a record of Carboniferous metamorphism assisted by rich Zr- and U-carrying fluids. The Variscan metamorphic overprint was heterogeneous, and variously affected rocks of the two gneiss formations.

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Charakterystyka morfologiczna i geochemiczna kryształów cyrkonu z migmatytu ze Smreczyńskiego Wierchu (Tatry Zachodnie)

Burda J.

Przegląd Geologiczny

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2005

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Vol. 53, nr 2

127--133

EN

In the Polish part of the metamorphic envelope of the granitoid Rohače pluton, migmatites are the predominant component. Migmatisation took place in the amphibolite facies conditions (T = 690–780C ; P =7.5–11 kbar). To observe the response of zircon crystals to the melt formation, zircon from the anatectic group of migmatites was investigated (both from leucosome and mesosome of stromatitic migmatite). Different generations of zircon present in leuco- and mesosome reflect different geological processes acting during the rock formation. Zircon from leucosome represent mainly the euhedral varieties, with two typological maxima: S1-2 (I generation) and S21-22 (II generation). Zircon from mesosome represents the anhedral varieties with one maximum at S1 (I gen-eration). In CL images the first generation (S1-2) comprises an inherited core surrounded by younger metamorphic and magmatic rims. The microchemical analyses showed that the inherited cores have higher Zr/Hf ratios than the overgrowing rims. The second genera-tion (S21-22) revealed only oscillatory zonation. The zones with strong luminescence are characterized by a decrease of Hf and U con-tent. Zircon showing polyphase internal structure might represent a component of mesosome which survived the dissolution in granitic melt, while zircon with oscillatory zonation might have crystallized from anatectic melt.