Microstructures

First Previous Section 11 Next Last

Recrystallization of Amphibole

Click image to enlarge

37. Subgrains in amphibole - A rare example of preserved subgrains and recrystallized new grains of amphibole deformed at a high strain rate at middle amphibolite grade, in the core of the Vredefort Dome, near Potchefstroom, South Africa. Elongate subgrains within host amphibole crystals are not often preserved in naturally deformed rocks, as the elevated temperature required for plastic deformation of amphibole usually means that the microstructure rapidly anneals to more equant grain shapes, as in image #39, after the deviatoric stress is removed. The high strain rates known to have caused the deformation in Vredefort core rocks may have helped generate and preserve these amphibole microstructures.

FOV 1.5 mm, Nicols Crossed.

38. Rotation recrystallization of hornblende - Mylonitic amphibolite in shear zones at Castell Odair, northern Scotland, shows well-developed core-and-mantle texture around hornblende porphyroclasts. The original 'core' grain (mostly yellow) is on left of image. Equant recrystallized grains occur along the grain' s right-hand margin, and along a horizontal zone near it's upper edge. The small, new grains probably formed by rotation recrystallization, because they have an optical orientation that is very close to that of the core grain.

FOV 1.5 mm, Nicols Crossed.

39. Annealing recrystallization of hornblende - Another sample of mylonitic amphibolite from the same locality as image #38 shows complete recovery of all strain to a polygonal texture with straight grain boundaries.

FOV 1.5 mm, Nicols Crossed.

Recrystallization of Amphibole	Click image to enlarge
37. Subgrains in amphibole - A rare example of preserved subgrains and recrystallized new grains of amphibole deformed at a high strain rate at middle amphibolite grade, in the core of the Vredefort Dome, near Potchefstroom, South Africa. Elongate subgrains within host amphibole crystals are not often preserved in naturally deformed rocks, as the elevated temperature required for plastic deformation of amphibole usually means that the microstructure rapidly anneals to more equant grain shapes, as in image #39, after the deviatoric stress is removed. The high strain rates known to have caused the deformation in Vredefort core rocks may have helped generate and preserve these amphibole microstructures. FOV 1.5 mm, Nicols Crossed.
38. Rotation recrystallization of hornblende - Mylonitic amphibolite in shear zones at Castell Odair, northern Scotland, shows well-developed core-and-mantle texture around hornblende porphyroclasts. The original 'core' grain (mostly yellow) is on left of image. Equant recrystallized grains occur along the grain' s right-hand margin, and along a horizontal zone near it's upper edge. The small, new grains probably formed by rotation recrystallization, because they have an optical orientation that is very close to that of the core grain. FOV 1.5 mm, Nicols Crossed.
39. Annealing recrystallization of hornblende - Another sample of mylonitic amphibolite from the same locality as image #38 shows complete recovery of all strain to a polygonal texture with straight grain boundaries. FOV 1.5 mm, Nicols Crossed.