Ordovician passive continental margin magmatism in the Central-European Variscides: U-Pb zircon data from the SE part of the Karkonosze-Izera Massif, Sudetes, SW Poland

Approximately 500-Ma-old orthogneisses are widespread in the eastern part of the Variscan belt and are commonly interpreted to have intruded mica-schist series of assumed Neoproterozoic age. New SHRIMP zircon ages of quartzofeldspathic metavolcanogenic rocks of the mica schist series in the eastern part of the Karkonosze-Izera Massif (SW Poland) indicate that they are late Cambrian/early Ordovician rather than Neoproterozoic in age, based on the zircon age spectra distributed mainly between ca. 500 and 660 Ma (with a few Proterozoic inherited minimum ages of ca. 970 and 1,825 Ma). Younger zircon dates, dispersed between ca. 412 and 464 Ma, are interpreted as a result of Pb-loss likely caused by subsequent metamorphism. Consequently, the felsic metavolcanogenic rocks appear to be roughly contemporaneous with the intrusion of ca. 500-Ma-old orthogneiss protoliths (with the pooled concordia age of 487 +/- A 8 Ma interpreted as the best approximation of the protolith intrusive age). Field relationships, petrological and geochemical features of the felsic and mafic rocks studied support a model in which the accompanying mica schist series are not the original country rocks to the ca. 500 Ma granite intrusions, and indicate that their recent close proximity is the result of tectonic juxtaposition. However, both the mica schists enclosing the bimodal metavolcanic rocks, and the orthogneisses, are interpreted to represent a Cambro-Ordovician passive continental margin sequence being part of the Saxothuringian domain. They are tectonically overlain to the east by HP/T metamorphic units, comprising MORB-type metaigneous rocks, and delineating a tectonic suture separating the Saxothuringian block in the west from an assumed continental block (Tepla-Barrandian) to the south-east.