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Sillimanite deformation mechanisms within a Grt-Sil-Bt gneiss: effect of pre-deformation grain orientations and characteristics on mechanism, slip-system activation and rheology
Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Macquarie University, Australia.
Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
2014 (English)In: Geological Society Special Publication, ISSN 0305-8719, E-ISSN 2041-4927, Vol. 394, 189-213 p.Article in journal (Refereed) Published
Abstract [en]

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)[010] and (001)[100] slip systems in large grains. Additionally, (100)[001] and (010)[001] 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)[010] and (001)[100] 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.
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URN: urn:nbn:se:su:diva-109585DOI: 10.1144/SP394.10ISI: 000343702300012OAI: diva2:765908
Conference on Deformation Mechanisms, Rheology and Tectonics (DRT), AUG 31-SEP 02, 2011, Oviedo, SPAIN


Available from: 2014-11-25 Created: 2014-11-24 Last updated: 2014-11-26Bibliographically approved

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