Protolith age and provenance of metasedimentary rocks in Variscan allochthon units: U-Pb SHRIMP zircon data from the Orlica-Snieznik Dome, West Sudetes

摘要

New U-Pb sensitive high-mass resolution ion microprobe (SHRIMP) data from detrital zircons within the Orlica-Snieznik Dome provide new insights into the stratigraphic and palaeogeographic position of assumed relict Precambrian basement preserved in the Variscan collisional orogen of the West Sudetes, SW Poland. Hitherto, the Mlynowiec Formation and the Stronie Formation within the Orlica-Snieznik Dome were assumed to represent two metavolcano-sedimentary successions of Proterozoic and early Palaeozoic age, respectively. However, when previous U-Pb data and mapping data from the Orlica-Snieznik Dome are combined with the new detrital zircon isotopic ages both from paragneisses within the Mlynowiec Formation and from light-coloured quartzites and mica schists within the Stronie Formation, the result strongly suggests that the protoliths of these two formations actually form a continuous succession. This continuous succession is herein designated the Mlynowiec Stronie Group. The rocks of this group were deposited during middle Cambrian-early Ordovician times (c. 520-470 Ma), presumably at the northern edge of West Gondwana after the 10-20 Ma period of tectonic quiescence that followed the terminal stage of the Cadomian collisions. Monotonous Mlynowiec metagreywackes form the lower part of the succession, and the lithologically diverse schistose Stronie Formation forms its upper part. The change from greywacke (Mlynowiec) to volcano-sedimentary (Stronie) facies coincided with the onset of rather short-lived volcanic activity which climaxed around 505-495 Ma and which supplied the volcanogenic components to the Stronie Formation. No 'Cadomian unconformity' has been observed in the region. Xenocrystic zircons from the Mlynowiec Stronie Group retain records of Archaean (3.0-2.3 Ga), Palaeoproterozoic (2.1-1.8 Ga) and Neoproterozoic to early Cambrian (660-530 Ma) zircon-forming events. These zircon ages, together with the lack of 1.7-1.2 Ga zircon ages, suggest that the source areas for the metasedimentary rocks may have been the West Africa craton, which therefore differs from the Amazonian affinity of the adjacent Brunovistulia Terrane. Nevertheless, two zircons, c. 1.0 and 1.1 Ga old, respectively, indicate that the Mtynowiec-Stronie Group sedimentary basin must have still been within the delivery reach of detritus ultimately derived from the Grenvillian-age belt(s). The detrital components of the supracrustal formations of the Orlica-Snieznik Dome were mainly derived from Neoproterozoic zircon-bearing crystalline rocks that were accreted to, and included in, the Cadomian basement in several intrusive pulses that culminated at 660-640 Ma, 620 Ma and 570-530 Ma.