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Orthogneisses and metapelites from a polyphase tectonic zone - mesostructural versus microstructural evidence: an example from the Czerniawa Zdrój section (Izera-Karkonosze Block, West Sudetes)

100%

Czapliński W.

tom Vol. 31, nr 1

93-104

Within the gneisses of the northern part of the Izera-Karkonosze Block (West Sudetes), there are 4 belts of schistose rocks. The Stara Kamienica schist belt in the Czerniawa section consists of a sequence of orthogneisses deformed to various degree, occurring concordantly with metapelitic mica schists. Structural analysis of these rocks, including quartz c-axis analysis, allows four stages of deformation, common for both lithologies to be recognized. The quartz c-axis microfabrics are often incompatible with other elements of the structural record, which is interpreted as having resulted from multiple overprinting of older microfabrics by younger ones in a heterogeneous deformation regime. This heterogeneity concerns the geometry of the deformation as well as the mechanisms, which included subgrain-rotation recrystallization, grain-boundary migration, microfracturing and pressure-solution processes.

Quartz c-axis fabrics in constrictionally strained orthogneisses: implications for the evolution of the Orlica-Śnieżnik Dome, the Sudetes, Poland

51%

Żelaźniewicz A. , Kromuszczyńska O. , Biegała N.

Acta Geologica Polonica

2013

tom Vol. 63, no. 4

697--722

The Orlica-Śnieżnik Dome (OSD), NE Bohemian Massif, contains in its core several gneiss variants with protoliths dated at ~500 Ma. In the western limb of the OSD, rodding augen gneisses (Spalona gneiss unit) are mainly L>S tectonites with a prominent stretching lineation. The few quartz LPO studies have produced somewhat discrepant results. Reexamination of these rocks revealed that texture formation was a protracted, multistage process that involved strain partitioning with changing strain rate and kinematics in a general shear regime at temperatures of the amphibolite facies (450–600°C). Quartz c-axis microfabrics show complex yet reproducible patterns that developed under the joint control of strain geometry and temperature; thus the LPOs are mixed features represented by pseudogirdle patterns. Domainal differences in quartz microfabrics (ribbons, tails, quartzo-feldspathic aggregate) are common in the Spalona orthogneisses but uncommon in the sheared migmatitic gneisses. In the latter rocks, the constrictional strain was imposed on the originally planar fabric defined by high-temperature migmatitic layering. The constrictional fabric of the Spalona gneisses may have developed in the hinge zones of kilometer-scale folds, where the elongation occurred parallel to the fold axes. Other occurrences of rodding gneisses throughout the Orlica-Śnieżnik Dome are thought to occupy similar structural positions, which would point to the significance of large-scale folds in the tectonic structure of the dome.