Geologic constraints on isotopic
age data for some rocks in the West Sudetes
Geologiczne ograniczenia interpretacji
izotopowego wieku niektórych skał w Sudetach Zachodnich
Instytut Geologii, Universytet im. Adama Mickiewicza , Poznań
Instytut Nauk Geologicznych PAN, Wrocław
Abstract: Recent isotopic studies of several rocks in the Sudetes, undertaken to put numerical age constraints on earlier recognized tectonothermal events, eventually brought numbers which sometimes cannot be explained so conventionally as put in the proposed interpretations, because isotopic data must not contradict basic geological information. In the Orlica-Snieżnik dome (OSD), eclogites together with their country rocks appear as xenoliths in a porphyritic granite, thus they must be older than the latter. Therefore, a Sm-Nd WR isochrone age of c. 330-350 Ma for the OSD eclogites is in conflict with an U-Pb zircon age of c. 500 Ma for the granite. Gabbros of the massifs neighbouring Góry Sowie block shed detritus to the Upper Devonian conglomerates, therefore the Sudetic ophiolite in general cannot be c. 350 Ma old as proposed on the basis of the Sm-Nd isotopic numbers. Quartzofeldspathic granulites in the Góry Sowie block (GSB) have ductilely sheared retrogressed contacts to the surrounding gneisses, and were migmatized during the progressive upper amphibolite facies metamorphism of these gneisses. Therefore, they must have developed prior to the regional metamorphic peak, that is earlier in the deformational history of the GSB gneiss complex.
Key words: Sudetes Variscides, isotopic age, tectonothermal evolution, eclogite
