Microstructures

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Dynamic Recrystallization of Feldspar
Rotation recrystallization of Feldspar

Click image to enlarge

31. Recrystallized quartz and feldspar- Another view of the sample in images #28 and #50 illustrates the difference in recrystallized grain sizes of quartz and feldspar in the same rock deformed at middle greenschist facies. Foliation is horizontal, and the lower third of image shows equant, small grains of feldspar, quartz, epidote, biotite and white mica that forms much of the mylonite fabric. The large porphyroclast at left is a relict orthoclase grain, surrounded by a mantle of rotation-recrystallized new grains that have been swept from it into the matrix to form a fine-scale compositional banding. Quartz grains have recrystallized to equant, polygonal, new grains several times larger than recrystallized feldspar grains. Quartz is more easily deformed and undergoes recovery and grain growth more readily than feldspar under the same deformation conditions.

FOV 0.8 mm, Nicols Crossed.

32. Strain-induced removal of twin planes - Mylonitic granodiorite from the Borrego Springs mylonite zone, contains a plagioclase porphyroclast that has bent smoothly. Deformation occurred at upper greenschist facies. Original igneous twins have been locally removed by grain boundary migration and replaced by untwinned feldspar of the same composition. Some subgrain formation is visible in the untwinned, new feldspar at top of image. Smooth bending of feldspar grains is uncommon at temperatures below middle to upper amphibolite facies. Cleavage-plane slip (similar to bedding plane slip in flexural slip folds) may have facilitated bending.

FOV 0.8 mm, Nicols Crossed.

33.Lattice bending in microperthite - Microperthite in mylonitic, quartzo-feldspathic gneiss of Parry Sound, Ontario, Canada, shows undulatory extinction, formation of deformation bands, and subgrains, in addition to small (bright) exsolution blebs. Deformation was at uppermost amphibolite facies. Left and right edges of porphyroclast show rotation recrystallized new grains of the same size as sub-grains. Recrystallized feldspar grains in the matrix have a range of compositions and grain sizes, but all show good polygonal texture and 120° triple points, indicative of complete recovery. The microstructures preserved in this microperthite are similar to those in quartz deformed at middle greenschist facies (compare with images #20 and #21).

FOV 1.5 mm, Nicols Crossed.

34. Polygonal recrystallization of feldspar - Feldspar grains are recrystallized to equant polygons in this deformed adamellite from the alpine Maggia Nappe, Switzerland. Deformation was at epidote-amphibolite facies. Biotite and an elongate aggregate of recrystallized quartz with large grain size at top of image help define foliation, here horizontal. A few recrystallized feldspar grains contain twins, although many do not - this is typical of plagioclase deformed under these conditions.

FOV 0.8 mm, Nicols Crossed.

35. Rotation recrystallization of feldspar - Mylonitic, quartzo-feldspathic gneiss from Parry Sound, Ontario, Canada, contains many feldspar porphyroclasts like this one, which shows undulatory extinction, deformation bands, and subgrains. Deformation was at uppermost amphibolite facies. A ribbon of recrystallized quartz (dark grey, just above feldspar clast) helps define foliation, as does the elongate shape of the porphyroclast. This image is rather dark, like it's companion #36, because many grains in the rock have optic axes nearly parallel to the microscope barrel, indicating a very strong lattice preferred orientation. Smoothly curved boundaries of the porphyroclast, especially adjacent to the quartz ribbon at left, indicate that there has been phase boundary motion, with diffusional mass transfer of material along the feldspar-quartz interface.

FOV 3.2 mm, Nicols Crossed.

36. Rotation recrystallization of feldspar - Same image as #35 with gypsum plate inserted shows the very strong lattice preferred orientation in this sample.

FOV 3.2 mm, Nicols Crossed + Gypsum Plate.

Dynamic Recrystallization of Feldspar Rotation recrystallization of Feldspar	Click image to enlarge
31. Recrystallized quartz and feldspar- Another view of the sample in images #28 and #50 illustrates the difference in recrystallized grain sizes of quartz and feldspar in the same rock deformed at middle greenschist facies. Foliation is horizontal, and the lower third of image shows equant, small grains of feldspar, quartz, epidote, biotite and white mica that forms much of the mylonite fabric. The large porphyroclast at left is a relict orthoclase grain, surrounded by a mantle of rotation-recrystallized new grains that have been swept from it into the matrix to form a fine-scale compositional banding. Quartz grains have recrystallized to equant, polygonal, new grains several times larger than recrystallized feldspar grains. Quartz is more easily deformed and undergoes recovery and grain growth more readily than feldspar under the same deformation conditions. FOV 0.8 mm, Nicols Crossed.
32. Strain-induced removal of twin planes - Mylonitic granodiorite from the Borrego Springs mylonite zone, contains a plagioclase porphyroclast that has bent smoothly. Deformation occurred at upper greenschist facies. Original igneous twins have been locally removed by grain boundary migration and replaced by untwinned feldspar of the same composition. Some subgrain formation is visible in the untwinned, new feldspar at top of image. Smooth bending of feldspar grains is uncommon at temperatures below middle to upper amphibolite facies. Cleavage-plane slip (similar to bedding plane slip in flexural slip folds) may have facilitated bending. FOV 0.8 mm, Nicols Crossed.
33.Lattice bending in microperthite - Microperthite in mylonitic, quartzo-feldspathic gneiss of Parry Sound, Ontario, Canada, shows undulatory extinction, formation of deformation bands, and subgrains, in addition to small (bright) exsolution blebs. Deformation was at uppermost amphibolite facies. Left and right edges of porphyroclast show rotation recrystallized new grains of the same size as sub-grains. Recrystallized feldspar grains in the matrix have a range of compositions and grain sizes, but all show good polygonal texture and 120° triple points, indicative of complete recovery. The microstructures preserved in this microperthite are similar to those in quartz deformed at middle greenschist facies (compare with images #20 and #21). FOV 1.5 mm, Nicols Crossed.
34. Polygonal recrystallization of feldspar - Feldspar grains are recrystallized to equant polygons in this deformed adamellite from the alpine Maggia Nappe, Switzerland. Deformation was at epidote-amphibolite facies. Biotite and an elongate aggregate of recrystallized quartz with large grain size at top of image help define foliation, here horizontal. A few recrystallized feldspar grains contain twins, although many do not - this is typical of plagioclase deformed under these conditions. FOV 0.8 mm, Nicols Crossed.
35. Rotation recrystallization of feldspar - Mylonitic, quartzo-feldspathic gneiss from Parry Sound, Ontario, Canada, contains many feldspar porphyroclasts like this one, which shows undulatory extinction, deformation bands, and subgrains. Deformation was at uppermost amphibolite facies. A ribbon of recrystallized quartz (dark grey, just above feldspar clast) helps define foliation, as does the elongate shape of the porphyroclast. This image is rather dark, like it's companion #36, because many grains in the rock have optic axes nearly parallel to the microscope barrel, indicating a very strong lattice preferred orientation. Smoothly curved boundaries of the porphyroclast, especially adjacent to the quartz ribbon at left, indicate that there has been phase boundary motion, with diffusional mass transfer of material along the feldspar-quartz interface. FOV 3.2 mm, Nicols Crossed.
36. Rotation recrystallization of feldspar - Same image as #35 with gypsum plate inserted shows the very strong lattice preferred orientation in this sample. FOV 3.2 mm, Nicols Crossed + Gypsum Plate.