A Variscan continental collision of the West Sudetes and the Brunovistulian terrane: a contribution from structural and metamorphic record of the Stronie Formation, the Orlica-Śnieżnik Dome, SW Poland

Abstract

A Variscan continental collision of the West Sudetes and the Brunovistulian terrane: a contribution from structural and metamorphic record of the Stronie Formation, the Orlica-Ś nie_ znik Dome, SW Poland Abstract Structural, petrological and geochronological data from marbles and mica schists combined with those from highly disputable gneisses and eclogites of the Orlica-Ś nie_ znik Dome (OSD), gave new insights into Variscan evolution of eastern borderland of the West Sudetes. It is shown that the Variscan tectonometamorphic evolution of the OSD began with E-W oriented subhorizontal shortening (D1 stage) related to the collision of the West Sudetes terranes and the Brunovistulian terrane. The shortening led to generally upright folding, which resulted in formation of the steep N-S trending metamorphic planar fabric S1, thickening of the lithosphere and burial of the Stronie Formation under greenschist facies conditions. As a consequence of subsequent, near-coaxial gravity-controlled vertical shortening (D2), the S1 foliation was deformed in tight recumbent folds F2. The flattening strain was associated with the progression to amphibolite-facies conditions (from ca. 510 upto ca. 620°C) and uplift from depths corresponding to 9-10 kbar to depths corresponding to 7-8 kbar. On microscale, the flattening strain is documented by rotation of the mineral fabric overgrown by syn-D2 prograde garnet porphyroblasts. The Sm-Nd Grt-WR isochron age, correlated with the D2 event yields 346.5 ± 4.4 Ma. Further deformation and progressive metamorphism led to development of the subhorizontally disposed S2 axial plane schistosity, which terminated at the metamorphic temperature peak. During retrogression but still under ductile conditions of deformation, the S2 planes were reactivated during successive top-to-the-N shear movement of the OSD (D3 stage). Due to the mutual interaction of the OSD with the adjacent terranes the shear deformations were localized within marginal parts of these units. Finally, as a result of the NE-SW and the NW-SE oriented shortenings (D4 stage and D5 stage, respectively), both the structural surfaces and metamorphic isograds were regionally folded with W(NW)-ward plunges. A synthesis of new and existing data shows an overall similarity in sequence of Variscan deformations (D1-D2-D3) in the West Sudetes borderland to that observed in the eastern flank of the Moldanubian domain.

The juxtaposition of eclogite and mid‐crustal rocks in the Orlica–Śnieżnik Dome, Bohemian Massif

“…, 2011b). The peak metamorphism is associated with a Sm–Nd garnet–whole‐rock age of 346.5 ± 4.4 Ma for a Stronie micaschist (Jastrzębski, 2009).…”

“…Previous metamorphic studies of metapelites in the OSD have attempted to decipher the relationships between the structures and the degree of metamorphism (Jastrzębski, 2005, 2009; Mazur et al. , 2005; Murtezi, 2006; Skrzypek et al.…”

“…Peak P – T conditions subsequently attained in the superimposed subvertical fabric show a continuous increase from the chlorite to the kyanite zone from the west to the east of the dome. They are estimated at 5–9 kbar and 460–650 °C (Murtezi, 2006; Jastrzębski, 2009; Skrzypek et al. , 2011a,b).…”

Protolith age and provenance of metasedimentary rocks in Variscan allochthon units: U–Pb SHRIMP zircon data from the Orlica–Śnieżnik Dome, West Sudetes

“…Despite the differences and uncertain ages, all the metasedimentary rocks in the Orlica-Śnieżnik Dome have often been treated as a single metasedimentary succession (e.g. Wojciechowska, 1975;Dumicz, 1979;Don et al 1990;Murtezi, 2006;Jastrzębski, 2009). No structural differences between the two formations are apparent, but these might have been obliterated by the later regional polyphase Variscan tectonometamorphic overprint (e.g.…”

“…Oberc, 1968;Ansilewski, 1966;Smulikowski, 1979). Pressure-temperature conditions established for various lithological units of the Stronie and Młynowiec formations show that they underwent metamorphism under mid-amphibolite facies conditions with maximum locally attained at 600-640 • C and 8-9 kbar (Murtezi, 2006;Jastrzębski, 2009). Wojciechowska, 1975;Dumicz, 1979;Don et al 1990;Murtezi, 2006;Jastrzębski, 2009).…”