SHRIMP zircon geochronology and geochemistry of the Orlica-Snieznik gneisses (Variscan belt of Central Europe) and their tectonic implications

The Orlica-Snieznik dome comprises large orthogneiss bodies interbedded with amphiblite-grade metasediments and minor metavolcanics. New U-Pb and Pb-Pb SHRIMP zircon ages for two major gneiss units of the dome, the Snieznik and Gieraltow gneiss, yielded similar ages of ca. 500 Ma. This is interpreted to reflect the magmatic crystallization age from the same or similar igneous precursors, in agreement with the geochemical characteristics of these rocks. Some zircon cores in both gneisses, interpreted to be inherited xenocrysts, have ages of ca. 530-540 Ma, and, additionally, of ca. 565 Ma and 2,6 Ga in the Snieznik; gneiss. Igneous grains in both gneiss types have high-U rims, which are dark under cathodoluminescence. They are much better developed in the Gieraltow gneiss and they yield a well-defined weighted mean U-Pb age of 342 +/- 6 Ma. These high-U rims are interpreted to have grown close to the peak of HT metamorphism which is responsible for the migmatitic texture of the Gieraltow gneiss. The Visean HT-LP metamorphism in the Orlica-Snieznik dome is interpreted as a result of rapid uplift and decompression due to overthrusting; of high grade rocks over the Moravo-Silesian nappe pile. Our data support geodynamic models that ascribe a predominant influence in the tectonic evolution of the West Sudetes to the Variscan orogenic events. This is suggested by the inheritance of zircon xenoclysts from the Cadomian basement and by the Late Cambrian-Early Ordovician magmatic event, both typical of the Armorican terrane assemblage, as well as by the Early Carboniferous age of the metamorphism (C) 2000 Editions scientifiques et medicales Elsevier SAS.