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

Autor: Stanislav Ulrich, J. Franěk, Patricie Týcová, Karel Schulmann, Ondrej Lexa, J. Haloda, Pavla Štípská
Rok vydání: 2010
Předmět:
Zdroj: Journal of Metamorphic Geology. 29:103-130
ISSN: 0263-4929
DOI: 10.1111/j.1525-1314.2010.00911.x
Popis: 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.
Databáze: OpenAIRE
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