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Deformation of textured hornblendite in the semi-brittle regime: an experimental investigation
Bhupesh Meher  1@  , Sarah Incel  2@  , Jörg Renner  3@  , Yuval Boneh  4@  
1 : Earth and Environmental Sciences, Ben-Gurion University of the Negev  (BGU)
2 : GFZ Helmholtz Centre for Geosciences
3 : Institute for Geology, Mineralogy and Geophysics, Ruhr-University Bochum  (RUB)
4 : Earth and Environmental Sciences, Ben-Gurion University of the Negev  (BGU)

Hornblende is a hydrous mineral of the amphibole group, often occurring in exhumed lower crust and associated with subduction zone-related metamorphism. Hornblende deforms by a variety of mechanisms in natural settings. However, it is experimentally challenging to induce hornblende plasticity due to its anisotropic nature and breakdown at relatively low temperatures (~ 850 °C). The lack of experimental analysis of hornblende plasticity hampers interpreting the deformation mechanisms of natural samples. We used samples of a strongly textured amphibolite from the Mamonia complex (Cyprus) cored at 0º, 45º, 60º, and 90º to the lineation. Deformation experiments were performed using a solid-medium Griggs rig at confining pressures of 0.5 to 2.0 GPa, temperatures of 400°C to 800°C, and strain rates of 10-5 to 10-4 1/s. Deformed samples exhibit marked rotation, accommodated by kink bands, gliding on foliation planes, and grain-scale fracturing: kink bands and gliding dominating for samples with intermediate angles to the foliation (45° and 60°), while localized shear and/or fractures accommodating deformation for those with 0° and 90°. Yet, we did not find simple relations between samples' initial orientation and their strength. We observe, however, a clear strength reduction with increasing temperature. The most detailed microstructural analysis was done on a set of samples inclined 60° to the foliation. With increasing temperature, a microstructural transition occurs which includes a decrease in fracture density, an increase in the intensity of intragrain misorientation, and a change in the dominant intragrain misorientation rotation axis from [001] to [010]. This transition, along with the temperature-dependent strength, suggests a shift from brittle to plastic deformation mechanisms. We discuss the geodynamic importance of the deformation structures of hornblende and the ability to use the observed transition in the dominant intragrain misorientation axis to interpret the transition from cataclastic to dislocation-mediated mechanisms in naturally deformed samples. 


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