Sillimanite deformation mechanisms within a Grt-Sil-Bt gneiss: effect of pre-deformation grain orientations and characteristics on mechanism, slip-system activation and rheology
2014 (English)In: Geological Society Special Publication, ISSN 0305-8719, E-ISSN 2041-4927, Vol. 394, 189-213 p.Article in journal (Refereed) Published
Sillimanite-bearing metapelites are common in the middle to lower crust and play an important role in the rheology of these crustal levels. We report deformation microstructures in a metasedimentary gneiss (Greenland) produced during simple shear at c. 700 degrees C and c. 4.5 kbar. Electron backscatter diffraction analysis shows activation of the (001) and (001) slip systems in large grains. Additionally, (100) and (010) are activated in needles with their c-axes parallel to lineation. Medium-grained sillimanite aggregates mixed with biotite show a weak crystallographic preferred orientation (CPO) suggesting activation of (001) and (001) together with grain boundary sliding (GBS). GBS is the dominant deformation mechanismin the outer parts of fine-grained clusters of sillimanite stemming from andalusite replacement. Associated features are phase mixing, clockwise dispersion of c-axis orientations and change of misorientation axes from original c-axis rotation to rotation close to the kinematic Y axis. The latter features which result in strain localization around these clusters allow their 'preservation'. Rheologically, the medium-grained aggregates together with zones around clusters take up most of the deformation. Our study shows that deformation mechanism and slip-system activation, and CPOs are strongly dependent on the orientation relationship between stress axes and pre-deformation grain orientations and characteristics.
Place, publisher, year, edition, pages
2014. Vol. 394, 189-213 p.
IdentifiersURN: urn:nbn:se:su:diva-109585DOI: 10.1144/SP394.10ISI: 000343702300012OAI: oai:DiVA.org:su-109585DiVA: diva2:765908
Conference on Deformation Mechanisms, Rheology and Tectonics (DRT), AUG 31-SEP 02, 2011, Oviedo, SPAIN