Naturally deformed quartz-rich rocks


15 Quartz grain in micaceous quartzite. Quartz is dissolved on the top and bottom boundaries and new quartz-mica has precipitated in the pressure shadows on both sides of the quartz grain; the contact detrital grain-recrystallized aggregate is irregular and displays a concentration of subgrains showing a slight crystallographic misorientation relative to old grain; the jaggedness of the contact corresponds to the shape of new grains formed at the expense of the old grain, possibly through the process of progressive subgrain rotation, although the role of dissolution-reprecipitation is clearly important. (Sample WR 041, White Range, central Australia) (0.65 mm)
16 Detail of contact between original quartz grain and recrystallized quartz-mica fibers. The contact is sharp and appears as a 'reaction front' generated by the fluid phase that presumably reacted with the original quartz. The fibers consist of interlayered recrystallized quartz and mica flakes. The original quartz grain displays fractures subparallel to the quartz-mica fibers; some fractures appear stylolitic, and some are coated with fluid inclusions. The fractures are probably important in enhancing the recrystallization process. (Sample WR 041, White Range, central Australia) (0.65 mm)
17 Relatively undeformed Heavitree Quartzite of Sheet 1, Ruby Gap Duplex, central Australia. Slight foliation oriented NE-SW defined by fine micas grown in the pressure shadows of quartz grains. Some quartz grains consist clearly of the detrital grain over which a quartz cement has grown in the same crystallographic orientation. (Sample 59 A) (6.4 mm)
18 In mica-rich quartzite of sheet 2, Ruby Gap Duplex, central Australia, detrital quartz grains are flattened, stretched, and bent in a fine grained quartz-mica matrix. Some stretching of individual quartz grains may be apparent because dissolution at ~E-W oriented grain boundaries probably took place. NE-SW shear bands deform the dominant foliation and indicate top-to-the left (south) sense of shear. (Sample 704 A) (3.2 mm)
19 In mica-poor quartzite of sheet 2, Ruby Gap Duplex, central Australia, original detrital quartz grains are flattened and stretched and show severe undulose extinction due to the internal development of subgrains by lattice rotation. The large grains are surrounded by a mantle of fine recrystallized quartz grains ~50 microns in size; dislocation creep regime 2. (Sample 703 A) (3.2 mm)
20 Detail of 19. Original grains contain subgrains that grade into recrystallized grains toward their margins or in linear zones that may have been fractures or local shear zones. The similarity between low-angle subgrains and high-angle recrystallized grains in shape and size suggests that progressive rotation is the dominant process of dynamic recrystallization; dislocation creep regime 2. (Sample 703 A, Ruby Gap Duplex, central Australia) (1.3 mm)
21 Entirely recrystrallized quartzite in sheet 3, Ruby Gap Duplex, central Australia. Mica foliae, enhanced here with X-polars oriented diagonally relative to length of slide, define strong fabric and recrystallized quartz grains are elongate oblique to the mica fabric, in a manner consistent with a top-to-the-right (south) sense of shear; dislocation creep regime 3. (Sample SI-11 A) (3.2 mm)
22 Detail of 21. Quartz grains are elongated approximately 2:1, oblique to the mica foliation, consistent with dextral (top to the right) sense of shear. In mica-poor layers, quartz recrystallized grain size is larger: Recrystallized grain size is limited by mica grains pinning the grain boundaries; dislocation creep regime 3. (Sample SI-11 A, sheet 3, Ruby Gap Duplex, central Australia) (1.3 mm)
23 Quartz grains are elongated approximately 2:1, oblique to the mica foliation, consistent with top-to-the-right (south) sense of shear. In mica-poor layers, quartz recrystallized grain size is larger. Small and isolated white mica grains have pinned quartz grain boundaries, and grains boundaries appear to have moved in windows bewteen adjacent mica flakes; these microstructures suggest that recrystallization occurred at least partly by grain-boundary migration; dislocation creep regime 3. (Sample 157 A, sheet 3, Ruby Gap Duplex, central Australia) (0.65 mm)
24 Entirely recrystallized quartzite of sheet 5, Ruby Gap Duplex, central Australia. Mica foliae define strong macroscopic fabric (the foliation-lineation measured at the outcrop), and recrystallized quartz grains show little elongation or shape preferred orientation ; dislocation creep regime 3. (Sample 249 A) (3.2 mm)
25 Detail of 24. Recrystallized quartz grains show the typical microstructure of pinned boundaries. The recrystallized grain size is limited by the micas, although the smaller micas did not pin the boundaries which migrated past them; the distribution of small micas might indicate the characteristic recrystallized grain size at an earlier stage of the deformation-recrystallization process (possibly represented by photo 23); dislocation creep regime 3. (Sample 249 A, sheet 5, Ruby Gap Duplex, central Australia) (1.3 mm)
26 Undeformed chert at Marble Bar. Several generations of quartz veins crosscut the original chert sediment. The chert consists of patches of different quartz grain sizes. (Sample 76, chert, Pilbara, Western Australia) (3.2 mm)
27 The patchiness of quartz grain sizes persists through the early stages of deformation-recrystallization, and is enhanced by grain growth in the regions of more pure quartz. (Sample 110A, chert, Pilbara, Western Australia) (1.3 mm)
28 Deformed chert, Warrawoona Syncline. Dynamic recrystallization has resulted in coarsening of quartz grain size in chert. The foliation is defined by the alternation of recrystallized chert and transposed and recrystallized quartz veins showing the largest grain sizes. White mica pins the quartz boundaries, and in the layers of large quartz grain size, micas are contained within individual quartz grains. These microstructural relations suggest grain boundary migration. (Sample 606 A, chert, Pilbara, Western Australia) (3.2 mm)
29 Deformed chert, Warrawoona Syncline. Very strong macroscopic foliation and lineation. Grain size coarsening is a function of quartz purity, the quartz veins showing the largest grain size. Recrystallized quartz grains are elongate oblique to the main foliation, in a manner consistent with top-to-the-left (south) sense of shear . (Sample 119 A, chert, Pilbara, Western Australia) (3.2 mm)
30 Mica fish in quartzite mylonite. Gypsum plate inserted shows strong crystallographic preferred orientation as well as oblique foliation of elongate recrystallized quartz grains consistent with top-to-the-right (south) sense of shear from mica fish. The section is perpendicular to foliation and paralle to lineation; most c-axes lie in the foliation and are normal to lineation (red-purple grains) and a few define a part of a cross-girdle (blue and yellow grains) (Sample 428 A, Redbank shear zone, central Australia) (1.3 mm)