Migmatization and large-scale folding in the Orlica–Śnieżnik Dome, NE Bohemian Massif: Pressure–temperature–time–deformation constraints on Variscan terrane assembly

Abstract

New research indicates that there is stratigraphic and structural continuity between the Młynowiec and Stronie Formations of the Orlica–Śnieżnik Dome (OSD), Bohemian Massif, and reveals the primary importance of the collision of Armorican Terrane Assemblage and Brunovistulian terrane in Variscan development of the OSD. The Młynowiec Formation is a locally migmatized metasedimentary unit and understanding its tectonometamorphic history is crucial for making regional-scale Variscan reconstructions. Structural studies, pseudosection modelling in MnNCKFMASH system and microprobe monazite dating revealed that this Variscan collision led to crustal thickening of the OSD, tectonic burial of the Młynowiec Formation to depths corresponding to 10–11kbar and progression of regional metamorphism at 361±5Ma. Continuous underthrusting of the Brunovistulian terrane led to exhumation of the OSD metasediments and the formation of subhorizontal, N–S-trending tight folds and a peak-metamorphic axial planar foliation. This episode took place at 650°C and 6–7kbar in the non-migmatized rocks of structurally upper parts of the Młynowiec Formation, and at 700°C and 6–7kbar in its structurally lower parts, where metamorphism was accompanied by localised in-situ migmatisation. During final consolidation of a mosaic of terranes, top-to-the-NE tectonic movements reactivated former fabrics, produced further uplift and cooled the rocks to greenschist facies. This pervasive shearing could be also responsible for the metasomatic event at ~330Ma. The continuation of the NE-transpression of the OSD involved its interactions with adjacent crustal units in the east and in the north, which led to large-scale folding represented now by synforms and associated antiforms. The metamorphic structure of the rocks subjected to the Barrovian-type metamorphism was rebuilt so that the migmatitic rocks outcrop in the cores of large-scale antiforms, and the lithological contacts, penetrative foliation and metamorphic isograds dip locally at different angles (shallow, moderate to high) and at different azimuths (E-, NE-, N, NW and W).