Abstract
High-pressure granitic orthogneiss of the south-eastern Orlica–Śnieznik 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.
Highlights
Recent petrological and microstructural studies show increased role of interplay between grain-scale melt transfer and deformation in various tectonic settings ranging from continental subduction (Závada et al 2018), extrusion of subducted crust in continental wedges (Štípská et al 2019) and in Cordilleran magmatic arc (Stuart et al 2018)
Závada et al (2018) suggested that prerequisite of pervasive porous flow of granitic melt is granular flow or cohesion-less grain boundary sliding of relictual parental grains enabling dynamic dilatancy of grain boundaries, typical for ultramylonitic cores of shear zones
Our goal is to examine a key outcrop section of the Králíky-Śnieżnik antiform where the continental subduction fabrics are almost completely transposed during simultaneous horizontal shortening and vertical melt transfer
Summary
Recent petrological and microstructural studies show increased role of interplay between grain-scale melt transfer and deformation in various tectonic settings ranging from continental subduction (Závada et al 2018), extrusion of subducted crust in continental wedges (Štípská et al 2019) and in Cordilleran magmatic arc (Stuart et al 2018). In all these settings melt passes through deforming crust exploiting grain boundaries mostly of felsic granitic protoliths without segregation into veins and dykes typical for metasedimentary migmatites (Collins and Sawyer, 1996; Brown and Solar, 1998; Weinberg, 1999; Vanderhaeghe, 2001). Quantitative petrological studies of simultaneously migmatized and deformed granitoids provide unique insight into mechanisms of exhumation of deeply buried crust as proposed already by pioneering works of Hollister (1993) and Brown and Solar (1998)
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