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Gierałtów versus Śnieżnik gneisses - what is the real difference?

Redlińska-Marczyńska Aleksandra

Geologos

2011

Vol. 17, No. 2

71--96

Structural and petrographic study applied to the gneisses from the eastern part of the Orlica-Śnieżnik Dome, indicate that two different types of gneiss are present. The Śnieżnik gneisses are porphyrithic granites, constricted and sheared into L-S tectonites, most commonly with augens; the Gierałtów gneisses are sheared migmatites, porphyroblastic gneisses and banded gneisses, with two sets of metamorphic foliation, intrafolial folds and lensoid leucosome aggregates or metamorphic porphyroblasts. Both lithologies were later zonally sheared and transformed into more or less deformationally advanced mylonites, difficult to be distinguished from one of the two types. Identification of the Śnieżnik and Gierałtów gneisses is possible only between zones of the late (Variscan) shearing, in which the original, pre-kinematic structures are preserved.

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.