The juxtaposition of eclogite and mid‐crustal rocks in the Orlica–Śnieżnik Dome, Bohemian Massif

Abstract

Eclogite, felsic orthogneiss and garnet-staurolite metapelite occur in a 5 km long profile in the area of Międzygo´rze in the Orlica-S´nie_ znik dome (Bohemian Massif). Petrographic observations and mineral equilibria modelling, in the context of detailed structural work, are used to document the close juxtaposition of high-pressure and medium-pressure rocks. The structural succession in all lithologies shows an early shallow-dipping fabric, S1, that is folded by upright folds and overprinted by a heterogeneously developed subvertical foliation, S2. Late recumbent folds associated with a weak shallowdipping axial-plane cleavage, S3, occur locally. The S1 fabric in the eclogite is defined by alternation of garnet-rich (grs = 22-29 mol.%) and omphacite-rich (jd = 33-36 mol.%) layers with oriented muscovite (Si = 3.26-3.31 p.f.u.) and accessory kyanite, zoisite, rutile and quartz, indicating conditions of 19-22 kbar and 700-750°C. The assemblage in the retrograde S2 fabric is formed by amphibole, plagioclase, biotite and relict rutile surrounded by ilmenite and sphene that is compatible with decompression and cooling from 9 kbar and 730°C to 5-6 kbar and 600-650°C. The S3 fabric contains in addition domains with albite, chlorite, K-feldspar and magnetite indicating cooling to greenschist facies conditions. The metapelites are composed of garnet, staurolite, muscovite, biotite, quartz, ilmenite and chlorite. Chemical zoning of garnet cores that contain straight ilmenite and staurolite inclusion trails oriented perpendicular to the external S2 fabric indicates prograde growth, from 5 kbar and 520°C to 7 kbar and 610°C, during the formation of the S1 fabric. Inclusion trails parallel with the S2 fabric at garnet and staurolite rims are interpreted to be a continuation of the prograde path to 7.5 and 630°C in the S2 fabric. Matrix chlorite parallel to the S2 foliation indicates that the subvertical fabric was still active below 550°C. The axial planar S2 fabrics developed during upright folding are associated with retrogression of the eclogite under amphibolite facies conditions, and with prograde evolution in the metapelites, associated with their juxtaposition. The shared part of the eclogite and metapelite P-T paths during the development of the subvertical fabric reflects their exhumation together.

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

“…, 2012). When observed, the S2 fabric is defined by shear bands and axial‐planar cleavage of F2 folds intruded by granite, which is consistent with partial melting during the E–W lateral shortening (Štípská et al. , 2012; Chopin et al.…”

“…The foliation S1 and the N–S lineation L1 are defined by recrystallized augen and bands of quartz, plagioclase and K‐feldspar, and by the preferred orientation of mica. The top‐to‐the‐north sense of shear marked by K‐feldspar σ ‐clasts and sheathed folds (Štípská et al. , 2012) are attributed to the orogen parallel flow of the lower crust within the continental wedge (Pressler et al.…”

“…The S1 fabrics are affected by E–W‐directed D2 lateral shortening, which leads to the steepening of the N–S‐trending metamorphic fabric S1 and development of a new subvertical foliation S2 mainly in the vicinity of the eclogite (Figs 1c & 2; Chopin et al. , 2012; Štípská et al. , 2012).…”

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

“…1). Lithostratigraphic and tectonometamorphic char ac ter is tics of the OSD are used to show af fin ity al ter na tively with the Saxothuringian or Moldanubian ter ranes (zones) of the Eu ro pean Variscides (see Franke et al, 1993;Aleksandrowski and Mazur, 2002;elaŸniewicz et al, 2006;Jastrzêbski et al, 2010;Mazur et al, 2012Mazur et al, , 2013Štípská et al, 2012). In the Orlica-OEnie¿nik Dome, the gneissic core is man tled by a ~6000 m thick metasedimentary suc ces sion of Ediacaran-Early Pa leo zoic age, re ferred to as the M³ynowiec-Stronie Group (Gunia, 1974;Gunia and Wierzcho³owski, 1979;Jastrzêbski et al, 2010;Mazur et al, 2012;¯elaŸniewicz et al, 2014b).…”

“…Later geo chem i cal and geo chron ol ogi cal data fur nished the base for a hy poth e sis that all gneiss es in the dome came from a sin gle ~500 Ma gra nitic protolith. The orig inal gran ites were to be come sub stan tially trans formed and even tu ally di ver si fied dur ing Variscan de for ma tion and migmatisation be tween 360 Ma and 340 Ma, then cooled at 340-330 Ma (Maluski et al, 1995;Turniak et al, 2000;Lange et al, 2002Lange et al, , 2005Gordon et al, 2005;Bröcker et al, 2009Bröcker et al, , 2010Štípská et al, 2012) and zon ally sheared at ~321 Ma (Marheine et al, 2002).…”