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The eastern part of the Saxothuringian Terrane characterized by zircon and monazite data from the Doboszowice Metamorphic Complex in the Sudetes (SW Poland)

Jastrzębski Mirosław, Żelaźniewicz Andrzej, Stawikowski Wojciech, Budzyń Bartosz, Krzemińska Ewa, Machowiak Katarzyna, Madej Stanisław, Białek Dawid, Sláma Jiří, Czupyt Zbigniew, Jaźwa Aleksandra

Annales Societatis Geologorum Poloniae

2023

Vol. 93, No. 3

229--249

In the Variscan Bohemian Massif, orthogneiss complexes of different sizes, derived from early Palaeozoic granitoids, are accompanied by metavolcano-sedimentary successions. They are common in the Sudetes, SW Poland, and belong mainly to the Saxothuringian Terrane. In order to better characterize the evolution of the latter, new U-Pb and O isotopic zircon analyses, combined with zircon typology and new U-Th-Pb isotopic monazite analyses, were performed. The present data show that the S-type granitic precursors of the Doboszowice orthogneisses (Fore-Sudetic Block) and the Śnieżnik gneisses (Sudetes) were formed at ca. 495 Ma from differently evolved magmas. Protoliths of the Doboszowice orthogneisses developed entirely in the crust, whereas the precursors to the Śnieżnik gneisses received some mantle input. Metasediments that accompany the Doboszowice orthogneisses reveal zircon spectra, which point to a late Cambrian-Early Ordovician sedimentary age. These spectra, in addition to predominant grains with Ediacaran ages, contain up to 10% of zircons dated at ca. 1.0 Ga. Therefore, it is suggested that the parent basin was supplied with detritus coming from areas, located in the Libyan-Nubian part of North Africa. Using also earlier published data, such a provenance is assigned to the units that now occur in the eastern part of the Fore-Sudetic Block. Late Cambrian-Early Ordovician granite intrusions and the concurrent accumulation of sediments originated in an extensional setting of the peri-Gondwana rifted continental margin or a back-arc setting. The Pb/U and Pb/Th monazite data constrain the ca. 346–341-Ma peak of the Variscan regional metamorphism in the eastern Saxothuringian Terrane and the prolonged juxtaposition of now adjacent tectonic units at least until 330 Ma.

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

Białek Dawid

Geological Quarterly

2020

Vol. 64, No. 4

942--957

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.