Origin of felsic granulite microstructure by heterogeneous decomposition of alkali feldspar and extreme weakening of orogenic lower crust during the Variscan orogeny

Abstract

International audienceThis 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 degrees 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 degrees 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

Contrasting mafic to felsic HP-HT granulites of the Blanský les Massif (Moldanubian Zone of southern Bohemia): complexity of mineral assemblages and metamorphic reactions

“…5b-c, Tab. 3). The pressure estimate using Koziol and Newton (1988) barometry is 2.24 GPa (for 950 °C), a value notably higher than that obtained from this locality by the pseudosection method (1.6-1.8 GPa) (Franěk et al 2011b). Relict kyanite surrounded by plagioclase rim is illustrated in Fig.…”

“…% Grs), which is partly segregated into mm to 1 cm thick nearly monomineralic bands, and kyanite inclusions in large mesoperthite grains. A detailed compositional profile across such garnet was published by Franěk et al (2011b), showing a flat uniform pattern through most of the grain with a decrease in Ca and increase in Fe at the rims, possibly caused by diffusion during exhumation (Fig. 5a).…”

“…Even though the south Bohemian granulites have clearly recorded high-pressure conditions (at least 1.3-1.5 GPa: Pin and Vielzeuf 1988), the peak temperatures remain a matter of controversy, with estimates varying between c. 850 and 1 050 °C depending on the massif studied and the method used (Kröner et al 2000;Štípská and Powell 2005;O'Brien 2008;Kotková and Harley 2010;Franěk et al 2011b;Tajčmanová et al 2012).…”

“…5a). The main emphasis of previous studies from this locality was on evolution of the successive sets of deformation microstructures (Franěk et al 2011b) and origin of the internal structure of perthitic Kfs crystals (Tajčmanová et al 2012).…”

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

“…Crystal nucleation rate, growth rate and coarsening during metamorphism can be revealed using the technique of CSD developed by Randolph & Larson (1971). This technique has been used for metamorphic rocks, for example, by Cashman & Ferry (1988), Díaz Aspiroz & Fernández (2003) or, more recently, by Franĕk et al. (2011 and references therein).…”