Quantitative structural analyses and numerical modelling of ...

650 O. LEXA ET AL.followed by moderate crustal thickening and magmaunderplating during the Carboniferous (Variscan)orogeny (Sˇtı´pska´ et al., 2001). The results of a structural,petrological and geochronological study allow apreliminary distinction of two metamorphic events,which exhibit similar P–T conditions. The structuraland geochronological arguments presented here arenot sufficiently unambiguous to distinguish betweenmetamorphic fabrics developed during the Cambro-Ordovician thinning and the Carboniferous moderatethickening. Therefore, quantitative textural analysis ofpairs of rock samples with similar mineralogical compositionshas been used to show the influence of theCambro-Ordovician and Carboniferous events on finaltextures.GEOLOGICAL SETTINGThe Stare´ Meˇsto belt was a Cambro-Ordovician rift,separating high-grade gneisses of a thickened continentalcrust (Orlica-Snieznik dome) in the west from theNeo-Proterozoic continental margin (Silesian domain)in the east (Fig. 1), at the eastern margin of theBohemian Massif (Kro¨ ner et al., 2000a,b). The Stare´Meˇsto belt consists of a lower crustal complex whichwas later affected by the Variscan collisional tectonics(Kro¨ ner et al., 2000a,b; Sˇtípska´ et al., 2001). Theoverall structure of this unit is marked by NE–SWtrending lithologies generally dipping to the west(Figs 2 & 3). The top of the tectonic sequence and theboundary with the rocks of the Orlica-Snieznik dome isrepresented by a layer of strongly sheared metagabbro(504.9 ± 1.0 Ma; Kro¨ ner et al., 2000a,b). Thisboundary represents a ductile shear zone along which aVariscan granodiorite sill was emplaced (Parry et al.,1997). Zircon from the northern and southern parts ofthe granodiorite sill were dated at 339.4 ± 1.1 and344.5 ± 0.4 Ma, respectively, reflecting the time ofmagma crystallization. The banded amphibolite complexlocated structurally below comprises a layeredsequence of alternating banded amphibolites and finegrainedquartzofeldspathic rocks (503 ± 2 and502.1 ± 1.7 Ma; Kro¨ ner et al., 2000a,b), subordinatetonalitic gneisses (503.3 ± 0.8 and 501.9 ± 0.6 Ma;Kro¨ ner et al., 2000a,b) and high-grade metasedimentsshowing evidence of anatexis (age of metamorphism –c. 504 Ma; Kro¨ ner et al., 2000a,b). In the northernpart of the Stare´ Meˇsto belt the mylonitic metagabbrosare tectonically repeated underneath the bandedamphibolite complex (Fig. 1).(a)(b)Fig. 1. (a) The schematic outlines of the major units of the Bohemian Massif. The location of the studied area is indicated. Upper leftinset shows the position of the studied area in the frame of European Variscides. (b) Geological map of the Stare´ Město belt based ongeological maps 1:25 000 provided by courtesy of the Czech Geological Survey. Important thrust faults and normal faults areindicated. Frames mark location of maps shown in Fig. 2.Ó 2005 Blackwell Publishing Ltd232