Cadomian versus younger deformations in the basement of the Moravo-Silesian Variscides, East Sudetes, SW Poland: U-Pb SHRIMP and Rb-Sr age data

Abstract

U-Pb SHRIMP dating of zircons from a metapegmatite vein which cross-cuts amphibolite facies paragneisses confirms ~580 Ma magmatism in the basement of the northern part of the Moravo-Silesian Zone (Jeseníky Mts.). Structures older than the felsic vein set are interpreted as a record of the Cadomian orogeny. This has been represented by N-trending, W-vergent folds followed by a top-to-the east shearing that occurred at T = 600°C and P = 5 kbar in the Neoproterozoic. The subsequent tectonic overprint led to folding and shearing of the pegmatite, which took place at similar P-T conditions but was associated with top-to-the west kinematics and shortening at a high angle to the foliation. This event likely developed during early stages of Variscan convergence when the Moravo-Silesian crust (Brunovistulia) was subducted and forced down below the approaching upper plate composed of terranes of the Bohemian Massif. Alternatively, it may have occurred around 500 Ma, related to crustal extensional (break-up of Gondwana margins in Cambrian times). Although the first option is favoured, presumably the two may have actually happened. The last ductile deformation was a top-to-the-east younger shearing localized in zones of various widths, assigned to the Variscan collision and reverse movement of the basement rocks. The latter two events occurred at temperatures that allowed in the metapegmatite for the crystal plastic deformation of quartz grains from which the strain was removed by subsequent static recrystallization, and that were high enough to reset the Rb-Sr system in this rock. Consequently, the obtained Rb-Sr isochron age of 290 Ma is considered to reflect the time of uplift. Such late regional uplift is characteristic of the northern part of the Moravo-Silesian Zone, which is the footwall to the Moldanubian Thrust, which separates the Bohemian Massif terranes from the Brunovistulia terrane. It follows from this study that in the East Sudetes basement rocks, structures which are often classified as Variscan may in fact be Cadomian and that the Cadomian record in these rocks is richer than previously assumed.