Microstructures

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Grain Shape Preferred Orientation (GSPO)

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49. GSPO in quartz - Quartz-rich zones in this example of mylonitic pegmatite from the Roses granodiorite on the southern edge of the Cap de Creus Peninsula in Spain, show a well-developed grain shape preferred orientation. Deformation was at lower greenschist facies and this sample was taken from one of many discrete left-lateral shear zones in the rock. The macroscopic foliation of the shear zone is aligned in the image parallel to horizontal. Quartz grains are elongate, with their long axes at an angle of about 30° to the foliation, and define a secondary foliation. Grain boundaries are irregular and many grains appear to be the result of coalescence of two or more adjacent grains of similar optical (and lattice) orientation. This type of fabric was called a Type II S-C mylonite by Lister and Snoke (1984).

At the same time as the recrystallized grains are deformed, rotation of the elongate grains toward the shear plane (main foliation) is constantly counterbalanced by the tendency of grain boundary migration to restore the grains to an equant shape. In addition, the incremental strain in the rock is acting to flatten all grains, especially the more equant ones, so that their long axes are parallel to the maximum incremental extension direction (for example in a simple shear zone this would be at 45° to the shear zone boundary). The result is that the elongate grains make an oblique angle to the foliation that tracks neither the finite strain nor the incremental strain axes, but something between the two. The obliquity angle is therefore a very powerful indicator of the sense of shear across the zone, but cannot be used to give quantitative strain estimates.

FOV 0.8mm, Nicols Crossed.

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50. GSPO in quartz and feldspar - Deformation of a pegmatite vein into mylonite in the Borrego Springs mylonite zone, southern California, has produced a well-developed grain shape preferred orientation in both the quartz and feldspar grains, but with different morphology in each case. Foliation in the mylonitic rock is oriented horizontally, sense of shear is left-lateral and deformation occurred under middle greenschist facies conditions. Quartz grains form most of the lower part of the image; they are polygonal and elongate at an oblique angle to foliation consistent with the left-lateral shear. Feldspar porphyroclasts (large, rounded grains in top half of image) have mantles of tiny new, rotation-recrystallized grains that have been swept away from the host grain and into the quartz-rich matrix. A narrow band of similarly formed recrystallized feldspar horizontally crosses the image about one-third way up. Individual grains in the feldspar-rich band are much smaller than matrix quartz grains on either side, and their long axes make a lower angle to foliation.

If the quartz and feldspar grains were passive markers in the rock, they would become more elongate with increasing shear strain and their long axes would be rotated so as to coincide with the finite maximum extension direction in the rock. In this case an interpretation could be that the feldspar grains had undergone more strain than the quartz grains. However, quartz is more easily deformed than feldspar under these deformation conditions, so we would expect the quartz to have undergone more cumulative strain. The resolution of this paradox is that the higher obliquity of quartz than feldspar grain long axes with respect to the foliation, and the more equant shape of the quartz grains, are a consequence of more active grain boundary migration in quartz than feldspar at middle greenschist facies conditions. As noted in the caption to image #49, the obliquity angle is a good indicator of the sense of shear across the zone, but cannot be used for strain estimates.

FOV 0.8 mm, Nicols Crossed.

Grain Shape Preferred Orientation (GSPO)	Click image to enlarge
49. GSPO in quartz - Quartz-rich zones in this example of mylonitic pegmatite from the Roses granodiorite on the southern edge of the Cap de Creus Peninsula in Spain, show a well-developed grain shape preferred orientation. Deformation was at lower greenschist facies and this sample was taken from one of many discrete left-lateral shear zones in the rock. The macroscopic foliation of the shear zone is aligned in the image parallel to horizontal. Quartz grains are elongate, with their long axes at an angle of about 30° to the foliation, and define a secondary foliation. Grain boundaries are irregular and many grains appear to be the result of coalescence of two or more adjacent grains of similar optical (and lattice) orientation. This type of fabric was called a Type II S-C mylonite by Lister and Snoke (1984). At the same time as the recrystallized grains are deformed, rotation of the elongate grains toward the shear plane (main foliation) is constantly counterbalanced by the tendency of grain boundary migration to restore the grains to an equant shape. In addition, the incremental strain in the rock is acting to flatten all grains, especially the more equant ones, so that their long axes are parallel to the maximum incremental extension direction (for example in a simple shear zone this would be at 45° to the shear zone boundary). The result is that the elongate grains make an oblique angle to the foliation that tracks neither the finite strain nor the incremental strain axes, but something between the two. The obliquity angle is therefore a very powerful indicator of the sense of shear across the zone, but cannot be used to give quantitative strain estimates. FOV 0.8mm, Nicols Crossed. Click here to view Flash animation in a new window.
50. GSPO in quartz and feldspar - Deformation of a pegmatite vein into mylonite in the Borrego Springs mylonite zone, southern California, has produced a well-developed grain shape preferred orientation in both the quartz and feldspar grains, but with different morphology in each case. Foliation in the mylonitic rock is oriented horizontally, sense of shear is left-lateral and deformation occurred under middle greenschist facies conditions. Quartz grains form most of the lower part of the image; they are polygonal and elongate at an oblique angle to foliation consistent with the left-lateral shear. Feldspar porphyroclasts (large, rounded grains in top half of image) have mantles of tiny new, rotation-recrystallized grains that have been swept away from the host grain and into the quartz-rich matrix. A narrow band of similarly formed recrystallized feldspar horizontally crosses the image about one-third way up. Individual grains in the feldspar-rich band are much smaller than matrix quartz grains on either side, and their long axes make a lower angle to foliation. If the quartz and feldspar grains were passive markers in the rock, they would become more elongate with increasing shear strain and their long axes would be rotated so as to coincide with the finite maximum extension direction in the rock. In this case an interpretation could be that the feldspar grains had undergone more strain than the quartz grains. However, quartz is more easily deformed than feldspar under these deformation conditions, so we would expect the quartz to have undergone more cumulative strain. The resolution of this paradox is that the higher obliquity of quartz than feldspar grain long axes with respect to the foliation, and the more equant shape of the quartz grains, are a consequence of more active grain boundary migration in quartz than feldspar at middle greenschist facies conditions. As noted in the caption to image #49, the obliquity angle is a good indicator of the sense of shear across the zone, but cannot be used for strain estimates. FOV 0.8 mm, Nicols Crossed.