Deformational history of metavolcanic rocks from the Kamieniec Ząbkowicki Metamorphic Belt (Fore- Sudetic Block, southwest Poland): a quartz [c]-axis lattice preferred orientation study

Abstract

The Kamieniec Ząbkowicki Metamorphic Belt (Fore-Sudetic Block, southwest Poland) consists mainly of mica schists. Based on mesostructural features, their deformational history has been interpreted in different ways and not quite consistently with the results of microstructural studies. Since the schist complex comprises acid metarhyolites – quartzo-feldspathic schists – which are well suited to microstructural analysis, we attempted to study quartz [c]-axis lattice preferred orientation (LPO) in these rocks in order to compare the results with those obtained by other authors and check against the proposed sequences of tectonic events in the region. The quartzo-feldspathic schists are fine- to medium-grained rocks with varying amounts of K-feldspar porphyroclasts. Most of these porphyroclasts are accompanied by symmetric or asymmetric pressure shadows. Their asymmetry uniformly indicates a top-to-the SSW shear sense along W-dipping foliation in a strike-slip regime. Quartz [c]-axis LPO indicates that the deformation took place mainly at plane strain with a non-coaxial component, with a top-to-the SSW or NNE sense of movement, and an apparent flattening field. The thermal conditions of deformation of the quartzo-feldspathic schists were within the limits of the amphibolite facies (550–600ºC). Integration of the micro- and mesofabric data indicated the superposition of two tectonothermal events: the main one with sinistral strike-slip, top-to-the SW kinematics consistent with the asymmetry of the porphyroclasts, and a subsequent overprint with dextral, transpressional, top-tothe NE kinematics. There is a visible dependence of the quartz LPO on the degree of mylonitization in the quartzofeldspathic schists. A westward increase in mylonitic deformation is observed in the study area.