Syn-deformational melt percolation through a high-pressure orthogneiss and the exhumation of a subducted continental wedge (Orlica-Śnieżnik Dome, NE Bohemian Massif)

• Published in: International journal of earth sciences : Geologische Rundschau 2020-06, Vol.109 (4), p.1213-1246
• Main Authors: Aguilar, Carmen, Štípská, Pavla, Chopin, Francis, Schulmann, Karel, Pitra, Pavel, Závada, Prokop, Hasalová, Pavlína, Martelat, Jean-Emmanuel
• Format: Article
• Language: English
• Subjects: Biotite; Decompression; Deformation; Domes; Earth and Environmental Science; Earth Sciences; Feldspars; Garnet; Geochemistry; Geology; Geophysics/Geodesy; Grain boundaries; High pressure; Massifs; Mica; Mineral Resources; Original Paper; Percolation; Plagioclase; Recrystallization; Rock; Rocks; Sciences of the Universe; Sedimentology; Structural Geology; Tectonics; Thermodynamic models; Vertical flow; Vertical mixing; Wedges
• ISSN: 1437-3254; 1437-3262
• DOI: 10.1007/s00531-020-01838-4

High-pressure granitic orthogneiss of the south-eastern Orlica–Śnieżnik Dome (NE Bohemian Massif) shows relics of a shallow-dipping foliation, reworked by upright folds and a mostly pervasive N–S trending subvertical axial planar foliation. Based on macroscopic observations, a gradual transition from banded to schlieren and nebulitic orthogneiss was distinguished. All rock types comprise plagioclase, K-feldspar, quartz, white mica, biotite and garnet. The transition is characterized by increasing presence of interstitial phases along like-like grain boundaries and by progressive replacement of recrystallized K-feldspar grains by fine-grained myrmekite. These textural changes are characteristic for syn-deformational grain-scale melt percolation, which is in line with the observed enrichment of the rocks in incompatible elements such as REEs, Ba, Sr and K suggesting open-system behaviour with melt passing through the rocks. The P – T path deduced from the thermodynamic modelling indicates decompression from ~ 15−16 kbar and ~ 650–740 ºC to ~ 6 kbar and ~ 640 ºC. Melt was already present at the P–T peak conditions as indicated by the albitic composition of plagioclase in films, interstitial grains and in myrmekite. The variably re-equilibrated garnet suggests that melt content may have varied along the decompression path, involving successively both melt gain and loss. The ~ 6–8 km wide zone of vertical foliation and migmatite textural gradients is interpreted as vertical crustal-scale channel where the grain-scale melt percolation was associated with horizontal shortening and vertical flow of partially molten crustal wedge en masse.