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Syntectonic Lower Ordovician migmatite and post-tectonic Upper Viséan syenite in the western limb of the Orlica-Śnieżnik Dome, West Sudetes: U-Pb SHRIMP data from zircons

Żelaźniewicz A., Nowak I., Larionov A.N., Presnyakov S.

Geologia Sudetica

2006

Vol. 38

63-80

In the Orlica-Śnieżnik Dome, the West Sudetes, metagranites of 515-480 Ma age occur as coarse-grained augen gneisses (Śnieżnik type) in the middle of the dome, whereas fine-grained, often migmatitic gneisses (Gierałtów type) are located more externally. Both the origin and genetic relationships of the gneisses have been disputed for many years. In a quarry near Zdobnice, in the western part of the dome, migmatitic gneisses and a post-tectonic dyke of unfoliated biotite-hornblende high-K syenite occur. The migmatititc gneiss has mesosome with relic minerals, notably Ca-Fe garnet and pseudomorphs after Al2SiO5 polymorph (?), indicative of an early granulitic metamorphism at considerably high pressure and temperature. Retrogression at still high temperature of ~720-750°C under the upper amphibolite facies conditions was accompanied by migmatization which among others produced cross-cutting neosome veins of graphic granite. Zircons from the melt derived neosome and from the syenite dyke were analysed with SHRIMP II. The former yielded a concordia age of 485š12 Ma which is taken to constrain the waning stage of the Late Cambrian-Early Ordovician migmatization. Migmatitic gneisses may have represented a metasedimentary-metaigneous Neoproterozoic crust that underwent multistage metamorphism, granulite facies inclusive, and then yielded to extensive partial melting between 515 Ma and 480 Ma. Our new data shows that the migmatization in the Orlica-Śnieżnik Dome was concurrent with the intrusion of a granitic precursor of the augen gneisses and does not support the views that the migmatitic gneisses can be a derivative of the ~500 Ma granite. In the Late Cambrian-Early Ordovician, the porphyritic granite intruded in migmatitic country rocks which mantled the granitic core. Both lithologies were later ductilely sheared and deformed under lower conditions of the amphibolite and greenschist facies during the Variscan orogeny. Four zircon grains from the post-tectonic syenite dyke yielded a concordia age of 326š3 Ma, which is interpreted as the time of its intrusion. This constrains the ductile Variscan events in the studied region.

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

Żelaźniewicz A., Nowak I., Bachliński R., Larionov A.N., Sergeev S.A.

Geologia Sudetica

2005

Vol. 37

35-51

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.