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Journal of the Geological Society

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The orthogneisses of the Orlica–nienik complex (West Sudetes, Poland): geochemical characteristics, the importance of pre-Variscan migmatization and constraints on the cooling history

Ulrich Lange¹, Michael Bröcker¹, Richard Armstrong², Andrzej elaniewicz³, Endres Trapp¹ and Klaus Mezger¹

^{1 1}Institut für Mineralogie, Zentrallaboratorium für Geochronologie, Universität Münster, Corrensstrasse 24, D-48149 Münster, Germany (e-mail: brocker@nwz.uni-muenster.de)
^{2 2}Research School of Earth Sciences, The Australian National University, Canberra A.C.T. 0200, Australia
^{3 3}Institute of Geological Sciences, Polish Academy of Sciences, ul. Podwale 75, PL 50-449, Wrocaw, Poland

Geochemical analyses in combination with Rb–Sr (whole-rock, phengite, biotite) and U–Pb zircon ages provide important constraints on the magmatic, metamorphic and structural evolution of the nienik and Gieratów gneisses from the Orlica–nienik complex (West Sudetes). These two gneisses have been considered to represent distinct petrogenetic units; however, their major and trace element compositions as well as their Sr–Nd isotope characteristics show no systematic differences that are indicative for different protoliths. This striking similarity leads to the conclusion that the petrographic variability is caused by modifications superimposed during deformation and migmatization. Most Nd₅₀₀ values are in the range between –3.3 and –5.7 and suggest derivation of the protoliths from pre-existing continental crust. Two-stage T_DM model ages mostly fall in the range between 1.4 and 1.6 Ga and closely correspond to other orthogneiss occurrences in the Bohemian Massif. The Rb–Sr whole-rock system is disturbed on a regional scale, to variable degrees, resulting in dates (c. 450 Ma and c. 395 Ma) considerably younger than the time of magmatic crystallization (c. 500 Ma). Secondary ionization mass spectrometry (SIMS) U–Pb analyses provide two groups of ²⁰⁶Pb/²³⁸U ages (364–341 Ma and 527–472 Ma), which largely correlate with previously established ages for protolith formation and Variscan high-temperature metamorphism. No geochronological evidence for pre-Variscan (‘Caledonian’) events was found. For phengite and biotite, the Rb–Sr system yields ages of c. 340–320 Ma, which provide further constraints on the regional cooling history.

KEYWORDS: Orlica–nienik complex, west Sudetes, orthogneisses, migmatization, absolute age

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