Microstructural and metamorphic evolution of a high‐pressure granitic orthogneiss during continental subduction (Orlica–Śnieżnik dome, Bohemian Massif)

Abstract

A microstructural and metamorphic study of a naturally deformed medium-to high-pressure granitic orthogneiss (Orlica-S´nie_ znik dome, Bohemian Massif) provides evidence of behaviour of the felsic crust during progressive burial along a subduction-type apparent thermal gradient ($10°C km )1 ). The granitic orthogneisses develops three distinct microstructural types, as follows: type I -augen orthogneiss, type II -banded orthogneiss and type III -mylonitic orthogneiss, each representing an evolutionary stage of a progressively deformed granite. Type I orthogneiss is composed of partially recrystallized K-feldspar porphyroclasts surrounded by wide fronts of myrmekite, fully recrystallized quartz aggregates and interconnected monomineralic layers of recrystallized plagioclase. Compositional layering in the type II orthogneiss is defined by plagioclase-and K-feldspar-rich layers, both of which show an increasing proportion of interstitial minerals, as well as the deformation of recrystallized myrmekite fronts. Type III orthogneiss shows relicts of quartz and K-feldspar ribbons preserved in a fine-grained polymineralic matrix. All three types have the same assemblage (quartz + plagioclase + K-feldspar + muscovite + biotite + garnet + sphene ± ilmenite), but show systematic variations in the composition of muscovite and garnet from types I to III. This is consistent with the equilibration of the three types at different positions along a prograde P)T path ranging from <15 kbar and <700°C (type I orthogneiss) to 19-20 kbar and >700°C (types II and III orthogneisses). The deformation types thus do not represent evolutionary stages of a highly partitioned deformation at constant P)T conditions, but reflect progressive formation during the burial of the continental crust. The microstructures of the type I and type II orthogneisses result from the dislocation creep of quartz and K-feldspar whereas a grain boundary sliding-dominated diffusion creep regime is the characteristic of the type III orthogneiss. Strain weakening related to the transition from type I to type II microstructures was enhanced by the recrystallization of wide myrmekite fronts, and plagioclase and quartz, and further weakening and strain localization in type III orthogneiss occurred via grain boundary sliding-enhanced diffusion creep. The potential role of incipient melting in strain localization is discussed.

An insight into a gneiss core of the Orlica-Śnieżnik Dome, NE Bohemian Massif: new structural and U–Pb zircon data

“…The high est val ues were de ter mined for the Giera³tów type gneiss es which were in ter preted by these au thors as an ultramylonite de rived from the augen orthogneiss of OEnie¿nik type. Al though our ob ser va tions also re peat edly show that the OEnie¿nik metagranite (= augen gneiss) was meta mor phosed at lower tem per a ture and pres sure con di tions than the Giera³tów gneiss es and that rocks of the two for ma tions were zon ally mylonitised, we do not see ar gu ments which would sub stan ti ate the re la tion ship pro posed by Cho pin et al (2012a).…”

“…Higher val ues of <15kbar/<700°C to 19-20 kbar/>700°C were re cently ob tained from P-T pseudosections and compositional iso pleths for var i ous min erals by Cho pin et al (2012a) who, how ever, did not re fer to the clas si fi ca tion of gneiss es into the OEnie¿nik and Giera³tów types. The high est val ues were de ter mined for the Giera³tów type gneiss es which were in ter preted by these au thors as an ultramylonite de rived from the augen orthogneiss of OEnie¿nik type.…”

“…The old est fo li a tion that fol lowed bed ding planes in the metasedimentary man tle rocks was folded and trans posed to the subvertical ax ial pla nar fo li a tion due to an early E-W subhorizontal short en ing and then re folded dur ing subvertical short en ing. Such tec tonic tem plate is fa voured by most au thors al though iden ti fi ca tion and la bel ling of con sec u tive struc tures dif fer in de tails (Dumicz, 1979;Jastrzêbski, 2005Jastrzêbski, , 2009Murtezi, 2006;Skrzypek et al, 2011Skrzypek et al, , 2014Cho pin et al, 2012a, b). Sim i lar struc tural suc ces sion is of ten as sumed for the core gneiss es, which seems con tro versial and not nec es sar ily cor rect (¯elaŸniewicz at al., 2014a, b).…”

“…The lat ter can be in ter preted as in di ca tors of HP-HT ep i sode in ferred for the Giera³tów Gneiss For ma tion (Borkowska et al, 1990;Bröcker and Klemd, 1996;Grzeoekowiak, 2004;Stawikowski, 2006;Redliñska-Marczyñska and ¯elaŸniewicz, 2011;Cho pin et al, 2012a) and thus in migmatitic xe no liths en closed within the OEnie¿nik metagranite.…”

“…However, de tailed in te grated stud ies of the top ics un der point (4) are scarce. Very few pa pers pres ent struc tural char ac ter is tics of the gneiss es (Cymerman, 1997;Cho pin et al, 2012a;elaŸniewicz et al, 2013) and each deals with an other as pect of it. A more in te grated ap proach was at tempted by Redliñska-Marczyñska (2011) andRedliñska-Marczyñska andelaŸniewicz (2011), and needs to be con tin ued.…”