Gierałtów versus Śnieżnik gneisses - what is the real difference?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.

The Orlica-OEnie¿nik Dome in the Sudetes, the NE Bo he mian Mas sif, em braces two for ma tions of fel sic gneiss es of con trover sial or i gin and evo lu tion. Our study shows that de spite sim i lar geo chem i cal sig na tures, they carry sys tem atic mi nor dif ferences in min eral, iso tope, zir con and geothermobarometric char ac ter is tics. Four vari ants of the Giera³tów gneiss es in clude migmatites and have a lon ger struc tural his tory than the OEnie¿nik augen orthogneisses. U-Pb SHRIMP anal y ses yielded U-Pb ages of ~500 Ma for cores and ~498 Ma for wide outer parts of zir con grains in the twice-folded Giera³tów gneiss es, and an age of ~500 Ma for a dis cor dant neosome vein. Neoproterozoic meta sedi ments were among pre cur sors of the lithologically di ver si fied Giera³tów Gneiss For ma tion. First de for ma tion, meta mor phism, and migmatisation of these rocks oc curred at 515-475 Ma and over lapped with the de vel op ment and em place ment of a por phy ritic S-type gran ite pre cur sor to the OEnie¿nik Gneiss For ma tion. The metagranite (= OEnie¿nik augen orthogneiss) em braced migmatitic xe no liths. Zir con grains from such xe no liths re vealed dis torted and re placed cores which yielded U-Pb ages that dis persed around 507-487 Ma, whilst wide darker poorly zoned out growths yielded ages from ~343 Ma to ~325 Ma (mean ~340 Ma). These out growths were in ter preted as a re cord of Car bon if er ous meta mor phism as sisted by rich Zr-and U-car ry ing flu ids. The Variscan meta mor phic over print was het er o ge neous, and var i ously af fected rocks of the two gneiss for ma tions.