Thermal and mechanical behaviour of the orogenic middle crust during the syn- to late-orogenic evolution of the Variscan root zone, Bohemian Massif
Résumé
We combine structural observations, petrological data and 40Ar-39Ar ages for a stack of amphibolite-facies metasedimentary units that rims high-pressure (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-pressure 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 micas and a tilting of the S4 foliation to its present-day orientation.