Cambro-Ordovician vs Devono-Carboniferous geodynamic evolution of the Bohemian Massif: evidence from P–T–t studies in the Orlica–Śnieżnik Dome, SW Poland

Abstract

AbstractThe pressure–temperature–deformation–time (P–T–d–t) record of metagranitic rocks and adjacent diverse rocks of the metavolcano-sedimentary group from the Orlica–Śnieżnik Dome (OSD) in SW Poland is examined. The study aims to better understand the course of the break-up of northern Gondwana and the overprinting Variscan tectonometamorphism in the NE Bohemian Massif. We test the existing hypotheses that explain the Cambro-Ordovician thermal event recorded in the meta-supracrustal group by (i) syn-deformational regional metamorphism or (ii) the contact metamorphism of the (meta)sedimentary rocks around the intruding ~490–500 Ma granitic magmas. In addition, we check the extent and timing of the Variscan prograde and retrograde medium-pressure metamorphism in the OSD. The results imply that Early Palaeozoic monazites, rarely preserved in both rock groups, document ~490–500 Ma volcanic and plutonic events related to the Gondwana's break-up and following disturbance of the Th–U–Pb system during younger, Variscan events. The monazite geochronology reveals no distinct Cambro-Ordovician thermal aureole around the post-granitic orthogneisses. However, no large-scale Variscan juxtaposition is evident between the two main OSD rock groups or within the meta-supracrustal rocks. Consistent P–T–d–t results for the meta-supracrustal rocks and the orthogneisses suggest that their precursors contacted before the Variscan tectonometamorphism. The directly contiguous ortho- and paragneisses together experienced tectonometamorphic processes at maximum depths that correspond to 7.5–8.0 kbar and maximum temperatures of ~600–620°C, as a result of the Variscan collision of Gondwana and Euramerica. The continental collision-related events intensified at ~360 Ma and ~330–340 Ma.