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

Abstract

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 2 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.

Detrital zircon analysis of metasedimentary rocks of the Staré Město Belt, Sudetes: implications for the provenance and evolution of the eastern margin of the Saxothuringian terrane, NE Bohemian Massif

“…In this area, a ma jor geo log i cal bound ary di vid ing two do mains with dis tinct geo log i cal char ac ter is tics was pos tu lated nearly 100 years ago (e.g., Cloos, 1922;Bederke, 1929), and its ex act po si tion and sig nif i cance have been dis cussed ex ten sively since that time (e.g., Oberc 1968;Cymerman, 1993;Schulmann and Gayer, 2000;Opletal and Pecina, 2004;Jastrzêbski et al, 2015). In the mod ern tec tonic sub di vi sion of the Bo hemian Mas sif, the Orlica-OEnie¿nik Dome is usu ally con sid ered to be part of the Saxothuringian terrane (e.g., Franke and ¯elaŸniewicz, 2000;Cho pin et al, 2012;Aguilar et al, 2020), although it is some times con sid ered to be part of the Moldanubian terrane (Matte et al, 1990;Cymerman et al, 1997). The Velké Vrbno Dome and Branná Belt be long to the Silesian do main, which is the north west ern part of the Brunovistulian (Brunia) microplate (e.g., Schulmann and Gayer, 2000;Štípská et al, 2006;Jastrzêbski et al, 2015;Oberc-Dziedzic et al, 2021;Collett et al, 2021;Fig.…”

The eastern part of the Saxothuringian Terrane characterized by zircon and monazite data from the Doboszowice Metamorphic Complex in the Sudetes (SW Poland)

“…1A). The OSD, intensely investigated for nearly 60 years (e.g., Don, 1964;Smulikowski, 1979;Żelaźniewicz, 1988;Don et al, 1990Don et al, , 2003Chopin et al, 2012), commonly has been linked with the Saxothuringian Terrane (e.g., Franke and Żelaźniewicz, 2000;Chopin et al, 2012;Aguilar et al, 2020) and also with the West African part of Gondwana (e.g., Jastrzębski et al, 2010;Mazur et al, 2012). Both the DMC and OSD contain orthogneisses, accompanied by amphibolite facies metasediments, yet their correlations still are not supported by detailed data.…”

Porous Melt Flow in Continental Crust—A Numerical Modeling Study

“…The overall amount of passing melt was large, but the melt fraction at any time was small, significantly lower than required for the breakdown of the solid crystal framework (∼25%-40% of melt, e.g., Rosenberg & Handy, 2005). Besides these mid-crustal migmatites, a similar process was identified several hundred kilometers away in exhumed felsic lower crust, which underwent subduction (Aguilar et al, 2020;Štípská et al, 2019;Závada et al, 2018Závada et al, , 2021. In addition, U-Pb monazite geochronology shows that the interaction between the melt and the solid rock was a long-lasting process that could have taken several millions of years (8-15 Myr;Závada et al, 2021).…”