Abstract
The Erzgebirge dome, exposing Proterozoic - Early Palaeozoic metagranitoids and metasedimentary rocks of the Saxothuringian domain in the Bohemian Massif, is formed by a tectonic stack of several UHP-HP and HT units exhumed during the Variscan continental subduction-collision process.
A systematic study of orthogneiss deformation microstructures carried out on a regional scale has been done to evaluate these units’ extent and relative structural position in the subduction complex. Five different microstructural types have been distinguished, revealing two overprinting sequences with 1) coarse-grained microstructures in the parautochthonous Kateřina-Reitzenhain Dome and 2) fine- grained microstructures in the allochthonous HP-HT Gneiss Eclogite Unit I. P-T conditions of the individual overprinting microstructures calculated by conventional thermobarometry suggest continuous deformation during exhumation along two contrasting P-T paths. The warmer one in the parautochthonous unit showed exhumation from ~13.5 kbar and ~820 ◦C to ~3 kbar and ~500 ◦C, while the colder one in the allochthonous unit showed exhumation from ~16 kbar and ~740 ◦C to ~3 kbar and ~400 ◦C. The overprinting microstructures are associated with vertical shortening interpreted as ductile thinning. This deformation started at depths of ~10 kbar and led to a temporal elevation of the crustal heat flow affecting the exhumation P-T paths of the structurally deeper units. We propose a model of continuous underplating of the former accretionary wedge by deeply subducted and exhumed continental material building up the orogenic wedge. Progressive heating of the orogenic wedge is explained by heat advection related to fast exhumation of the UHP/HP-HT units from the subduction channel followed by ductile thinning.
Another part of our research was focused on two individual quartzo-feldspathic rock bodies, Mnišská skála and Hohenstein, which are unique for their preserved mineral assemblages, which notably include garnets. These rock bodies are located in the allochthonous units, specifically in Gneiss Eclogite Unit II and Gneiss Eclogite Unit I.
The uniqueness of those localities is not only in the presence of garnet but also in the occurrence of different rock types including banded orthogneiss, mylonites and granofels with the clear transition of those rock types on a scale of less than 100 cm.
Pressure-temperature modeling, mineral chemistry of the studied samples and presence of coesite in the mylonite from one of the localities (Mnišská skála, Gneiss
Eclogite Unit I), suggesting UHP conditions. Our study thus highlights the importance of these two individual rock bodies in unravelling the complex evolution of the central part of the Erzgebirge Mountains and supporting our model of continuous
underplating of the former accretionary wedge by deeply subducted and exhumed continental material building up the orogenic wedge.
