Natural deformation processes of lower crustal prismatic sillimanite under high to ultra-high temperature conditions highlighted by EBSD investigation

Quartzo-feldspathic and restitic granulites from the Middle Atlas lower crust (Morocco) contain prismatic sillimanite crystals up to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:math> mm large, deformed under high temperature ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>T</mml:mi> <mml:mo>≥</mml:mo> <mml:mn>850</mml:mn> </mml:mrow> </mml:math> °C) and moderate pressure ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>P</mml:mi> <mml:mo>∼</mml:mo> <mml:mn>0</mml:mn> <mml:mo>.</mml:mo> <mml:mn>9</mml:mn> </mml:mrow> </mml:math> GPa) conditions. EBSD mapping reveals a heterogeneous deformation of sillimanite crystals in the quartzo-feldspathic samples depending on the minerals surrounding them. When they are included in sillimanite aggregates or constrained by large garnet grains, sillimanite deformation is strong and frequently reach dynamic recrystallization. Contrastingly, sillimanite crystals are significantly less deformed or even undeformed when isolated in the strongly deformed quartzo-feldspathic matrix. In the restitic samples almost all sillimanites display a homogeneous deformation but less frequent recrystallization. Deformed sillimanites display microstructures typical of dislocation-creep processes: misorientation, subgrain boundaries and subgrains, which point to recrystallization through subgrain rotation. Subgrain boundaries misorientation dominantly results from rotation around [100]. Crystallographic preferred orientation displays a concentration of [001] around the lineation. It is stronger in the restitic than in the quartzo-feldspathic samples, probably due to the heterogeneity of deformation in the latter. These data substantiate (010)[001] as the dominant dislocations slip system. This agrees with previous experimental deformation and transmission electron microscope observations. Our results differ from those obtained on fibrolitic sillimanite, which support (001)[010] and (001)[100] as dominant slip systems in large grains and a major effect of initial grain orientation relative to lineation. This difference may result from the contrasted <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>P</mml:mi> </mml:math> – <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>T</mml:mi> </mml:math> conditions of deformation: <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>≥</mml:mo> </mml:math> 850 °C and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>∼</mml:mo> </mml:math> 0.9 GPa for the Middle Atlas granulites, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>∼</mml:mo> </mml:math> 700 °C and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>∼</mml:mo> </mml:math> 0.45 GPa for the West Greenland ones.