Microstructures

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Shear Sense Indicators:
Sigma-type Porphyroclasts

Click image to enlarge

89. Sigma grains - Many of the sigma grains in this mylonitic granodiorite from the Santa Rosa Mylonite zone in southern California, have symmetrical, wedge-shaped tails that extend parallel to foliation. The tails do not wrap around the clast, as would delta-type tails. The large feldspar porphyroclast at center of image shows clear asymmetry of its tails, which step-up to the left, consistent with left lateral shear. Deformation was at upper greenschist grade.

FOV 6.4 mm, Plane Light.

90. Sigma grains - In this Borrego Springs mylonite sample the recrystallized tails on feldspar grains step-up to the left, consistent with left lateral shear. The wedge-shaped tails extend from the narrow end of the more elongate grains (at lower left and right center of image). Grain shape preferred orientation in recrystallized quartz also indicates left lateral shear sense. Deformation was at mid-upper greenschist grade.

FOV 1.5 mm, Nicols Crossed.

Click here to view Flash animation in a new window.

91. Mica Fish - Muscovite 'fish' from mylonitic granite of the Catalina metamorphic core complex, Arizona are oriented with their long dimensions oblique to the foliation (here very dark, oriented horizontal). As with sigma grains, the 'fish' step-up in the direction of shear; in this example shear sense is dextral. Note the absence of internal lattice deformation in the right-hand grain, despite the curvature of (001) planes. The left hand grain shows minor bending but in addition has a very planar and abrupt upper surface. These phenomena suggest that there was considerable diffusional mass transfer of material during deformation (see also image #72).

FOV 1.5 mm, Nicols Crossed. Section 20 µm thick.

92. Feldspar Fish - This is the same sample as image # 91. Small muscovite fish at top of image and a shear band from top left to middle right indicate right lateral shear.The plagioclase grain (dark blue at right) is relatively undeformed and has no recrystallized grains on its margin (the yellow and pink patches are included quartz grains). The K-feldspar grain at left (cyan colored) has developed myrmekitic exsolution textures along its margins. Like the mica fish in image #91, these feldspar fish step-up to the right and indicate a dextral shear.

FOV 3.2 mm, Nicols Crossed.

93. Biotite Fish - Biotite porphyroclasts often form 'fish'-like sigma grains, although the mechanisms involved in their formation may be different from those in muscovite fish. In mylonitic granodiorite from the Borrego Springs zone, biotite fish have tails of fine-grained, brittle biotite fragments. Three feldspar sigma grains are visible in the light-colored areas - two at upper right and one at lower left.

FOV 3.2 mm, Plane Light.

94. Back-rotated sigma grain - This back-rotated s-type feldspar grain is from the same sample as image # 93. The feldspar grain is elliptical in section and the wedge-shaped tails, here composed of white mica, have grown from the broad side of the grain, indicating that the grain has rotated clockwise even though the tails and quartz grain shape preferred orientation show that the overall sense of shear was left lateral (for explanation, see caption to image # 75).

FOV 3.2 mm, Nicols Crossed.

Click here to view Flash animation in a new window.

95. Back-rotated sigma grain - Two large feldspar porphyroclasts in ultramylonitic granodiorite from the Borrego Springs zone, deformed in a dextral shear zone. The wedge-shaped tails on both large grains have grown from the grains' broad sides, and even though the tails are still short, they clearly step-up to the right. Small feldspar grain in upper right of image is an incipient delta-type; its tails are too immature for the grain to be used as a shear sense indicator.

FOV 1.5 mm, Plane Light.

96. Sigma grains in ultramylonite - Feldspar porphyroclasts in this mid-greenschist facies, ultramylonitic quartz diorite from the Borrego Springs mylonite zone, have tails composed of fine-grained reaction products. The ultra-fine-grained matrix is dark and structureless. A band of fine-grained recrystallized feldspar and quartz across center of image shows a strong lattice preferred orientation. Most of the porphyroclasts are normal sigma-type, with tails that grow from the narrow ends of elongate grains. There is one back-rotated grain with tails growing from its broad sides (small blue grain, center of image).

FOV 6.4 mm, Nicols Crossed + Gypsum Plate.

Shear Sense Indicators: Sigma-type Porphyroclasts	Click image to enlarge
89. Sigma grains - Many of the sigma grains in this mylonitic granodiorite from the Santa Rosa Mylonite zone in southern California, have symmetrical, wedge-shaped tails that extend parallel to foliation. The tails do not wrap around the clast, as would delta-type tails. The large feldspar porphyroclast at center of image shows clear asymmetry of its tails, which step-up to the left, consistent with left lateral shear. Deformation was at upper greenschist grade. FOV 6.4 mm, Plane Light.
90. Sigma grains - In this Borrego Springs mylonite sample the recrystallized tails on feldspar grains step-up to the left, consistent with left lateral shear. The wedge-shaped tails extend from the narrow end of the more elongate grains (at lower left and right center of image). Grain shape preferred orientation in recrystallized quartz also indicates left lateral shear sense. Deformation was at mid-upper greenschist grade. FOV 1.5 mm, Nicols Crossed. Click here to view Flash animation in a new window.
91. Mica Fish - Muscovite 'fish' from mylonitic granite of the Catalina metamorphic core complex, Arizona are oriented with their long dimensions oblique to the foliation (here very dark, oriented horizontal). As with sigma grains, the 'fish' step-up in the direction of shear; in this example shear sense is dextral. Note the absence of internal lattice deformation in the right-hand grain, despite the curvature of (001) planes. The left hand grain shows minor bending but in addition has a very planar and abrupt upper surface. These phenomena suggest that there was considerable diffusional mass transfer of material during deformation (see also image #72). FOV 1.5 mm, Nicols Crossed. Section 20 µm thick.
92. Feldspar Fish - This is the same sample as image # 91. Small muscovite fish at top of image and a shear band from top left to middle right indicate right lateral shear.The plagioclase grain (dark blue at right) is relatively undeformed and has no recrystallized grains on its margin (the yellow and pink patches are included quartz grains). The K-feldspar grain at left (cyan colored) has developed myrmekitic exsolution textures along its margins. Like the mica fish in image #91, these feldspar fish step-up to the right and indicate a dextral shear. FOV 3.2 mm, Nicols Crossed.
93. Biotite Fish - Biotite porphyroclasts often form 'fish'-like sigma grains, although the mechanisms involved in their formation may be different from those in muscovite fish. In mylonitic granodiorite from the Borrego Springs zone, biotite fish have tails of fine-grained, brittle biotite fragments. Three feldspar sigma grains are visible in the light-colored areas - two at upper right and one at lower left. FOV 3.2 mm, Plane Light.
94. Back-rotated sigma grain - This back-rotated s-type feldspar grain is from the same sample as image # 93. The feldspar grain is elliptical in section and the wedge-shaped tails, here composed of white mica, have grown from the broad side of the grain, indicating that the grain has rotated clockwise even though the tails and quartz grain shape preferred orientation show that the overall sense of shear was left lateral (for explanation, see caption to image # 75). FOV 3.2 mm, Nicols Crossed. Click here to view Flash animation in a new window.
95. Back-rotated sigma grain - Two large feldspar porphyroclasts in ultramylonitic granodiorite from the Borrego Springs zone, deformed in a dextral shear zone. The wedge-shaped tails on both large grains have grown from the grains' broad sides, and even though the tails are still short, they clearly step-up to the right. Small feldspar grain in upper right of image is an incipient delta-type; its tails are too immature for the grain to be used as a shear sense indicator. FOV 1.5 mm, Plane Light.
96. Sigma grains in ultramylonite - Feldspar porphyroclasts in this mid-greenschist facies, ultramylonitic quartz diorite from the Borrego Springs mylonite zone, have tails composed of fine-grained reaction products. The ultra-fine-grained matrix is dark and structureless. A band of fine-grained recrystallized feldspar and quartz across center of image shows a strong lattice preferred orientation. Most of the porphyroclasts are normal sigma-type, with tails that grow from the narrow ends of elongate grains. There is one back-rotated grain with tails growing from its broad sides (small blue grain, center of image). FOV 6.4 mm, Nicols Crossed + Gypsum Plate.