Search

This comment addresses the incorrect treatment and presentation of data from laser ablation ICP-MS U–Pb age determinations of two samples of the Třebíč Pluton. This results in inaccurate ages and error assessment, invalidating the age interpretations of the authors. To corroborate our arguments, new high-precision chemical abrasion ID-TIMS data are presented that unequivocally define the emplacement age of the Třebíč pluton. [ABSTRACT FROM AUTHOR]

We combine structural observations, petrological data and 40Ar-39Ar ages for a stack of amphibolite facies metasedimentary units that rims high- P ( HP) granulite facies felsic bodies exposed in the southern Bohemian Massif. The partly migmatitic Varied and Monotonous units, and the underlying Kaplice unit, show a continuity of structures that are also observed in the adjacent Blanský les HP granulite body. They all exhibit an earlier NE− SW striking and steeply NW-dipping foliation (S3), which is transposed into a moderately NW-dipping foliation (S4). In both the Varied and Monotonous units, the S3 and S4 foliations are characterized by a Sil-Bt-Pl-Kfs-Qtz-Ilm±Grt assemblage, with occurrences of post-D4 andalusite, cordierite and muscovite. In the Monotonous unit, minute inclusions of garnet, kyanite, sillimanite and biotite are additionally found in plagioclase from a probable leucosome parallel to S3. The Kaplice unit shows rare staurolite and kyanite relicts, a Sil-Ms-Bt-Pl-Qtz±Grt assemblage associated with S3, retrogressed garnet−staurolite aggregates during the development of S4, and post-D4 andalusite, cordierite and secondary muscovite. Mineral equilibria modelling for representative samples indicates that the Varied unit records conditions higher than ~7 kbar at 725 °C during the transition from S3 to S4, followed by a P−T decrease from ~5.5 kbar/750 °C to ~4.5 kbar/700 °C. The Monotonous unit shows evidence of partial melting in the S3 fabric at P−T above ~8 kbar at 740-830 °C and a subsequent P−T decrease to 4.5-5 kbar/700 °C. The Kaplice unit preserves an initial medium- P prograde path associated with the development of S3 reaching peak P−T of ~6.5 kbar/640 °C. The subsequent retrograde path records 4.5 kbar/660 °C during the development of S4. 40Ar-39Ar geochronology shows that amphibole and biotite ages cluster at c. 340 Ma close to the HP granulite, whereas adjacent metasedimentary rocks preserve c. 340 Ma amphibole ages, but biotite and muscovite ages range between c. 318 and c. 300 Ma. The P− T conditions associated with S3 imply an overturned section of the orogenic middle crust. The shared structural evolution indicates that all mid-crustal units are involved in the large-scale folding cored by HP granulites. The retrograde P- T paths associated with S4 are interpreted as a result of a ductile thinning of the orogenic crust at a mid-crustal level. The 40Ar-39Ar ages overlap with U-Pb zircon ages in and around the HP granulite bodies, suggesting a short duration for the ductile thinning event. The post-ductile thinning late-orogenic emplacement of the South Bohemian plutonic complex is responsible for a re-heating of the stacked units, reopening of argon system in mica and a tilting of the S4 foliation to its present-day orientation. [ABSTRACT FROM AUTHOR]

This study answers the question of origin and evolution of a granulitic microstructure typically developed in felsic granulites of the European Variscan belt. It shows that the precursor of the Variscan felsic granulites was a high-pressure alkali feldspar-rich coarse-grained layered orthogneiss. Its S1 subhorizontal layering is defined by the alignment of alkali feldspar porphyroclasts alternating with monomineralic bands of quartz and bands rich in plagioclase and garnet. The alkali feldspar porphyroclasts contain inclusions of quartz, garnet, kyanite, biotite and rutile, reflecting peak P-T conditions of 1.6-1.8 GPa and 850 °C during S1 formation. Superimposed steep folds and steep cleavage, S2, are associated with recrystallization of alkali feldspar, plagioclase and quartz, and garnet chemistry modifications that correspond to 0.9-1.0 GPa and 800 °C. During exhumation, involving 0.8 GPa decompression and cooling, the probably perthitic alkali feldspar underwent an unusual process of heterogeneous decomposition along irregular reaction fronts forming a fine-grained matrix composed of plagioclase and K-feldspar grains. Regular grain distributions in the matrix, nucleation-dominated crystal size distribution and preservation of lattice orientation of the parental perthite crystals are all explained by a discontinuous precipitation process. This heterogeneous decomposition of alkali feldspar solid solution is controlled by chemically and strain induced grain-boundary migration. During exhumation and decompression, the fine-grained matrix underwent viscous deformation, forming the typical microstructure of the Variscan granulites. Random phase distributions, minor coarsening and feldspar textures are interpreted as a result of strain softening due to diffusion creep-accommodated grain-boundary sliding. Subordinate large quartz ribbons were rheologically stronger than the feldspar-dominated matrix due to the activity of different deformational mechanisms. Finally, in mid-crustal levels, the subvertical structure was overprinted by a perpendicular steep fabric associated with the growth of sillimanite, heterogeneous hydration and local partial melting, development of aggregate phase distributions and significant coarsening. This evolution is accompanied with the development of a strong lattice preferred orientation of quartz, K-feldspar and plagioclase, reflecting a switch to dislocation creep mechanism and a general hardening of the granulites under amphibolite facies conditions. [ABSTRACT FROM AUTHOR]

Reflection seismic section, field structural analysis and gravimetric modelling of orogenic lower crust in the core of a Carboniferous orogenic root reveal details of the polyphase process of exhumation. Subvertical amphibolite facies fabrics strike parallel to former plate margins that collided in the NW. The fabrics are developed in both mid-crustal and lower crustal high-pressure granulite units as a result of intensive NW-SE intraroot horizontal shortening driven probably by the west-directed collision. In granulites, the steep fabrics originated as a result of extrusion of orogenic lower crust in a ∼20 km wide vertical ascent channel from lower crustal depths at 350-340 Ma. The large granulite bodies preserve older granulite facies fabrics documenting a two-stage evolution during the exhumation process. Surface exposures of granulites coincide with the absence of subhorizontal seismic reflectors at depth, suggesting preservation of the ∼20 km wide subvertical tabular structure reaching Moho depths. Horizontal seismic reflectors surrounding the vertical channel structure corroborate a dominant flat migmatitic fabric developed in all tectonic units. This structural pattern is interpreted in terms of subhorizontal spreading of partially molten orogenic lower crust in mid-crustal levels (765 °C and 0.76 GPa) at 342-337 Ma. Large massifs of extruded and progressively dismembered felsic granulites disturbed mid-crustal fabrics in the surrounding horizontally flowing partially molten crust. The horizontal mid-crustal flow resulted in collapse of the supra-crustal Teplá-Barrandian Unit (interpreted as the orogenic lid) along a large-scale crustal detachment above the extruded lower crustal dome. The presence of felsic granulites at the bottom of the orogenic root is considered to be a key factor controlling the exhumation of orogenic lower crust in large hot orogens. [ABSTRACT FROM AUTHOR]