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Anorthosite metamorphosed in the granulite facies, Ceilidh Hill, central Australia. Associated gabbro contains
orthopyroxene and clinopyroxene. The texture is granoblastic. The bytownite plagioclase grains exhibit
polysynthetic twins that terminate within the grains defining them as secondary twins. Some grains show
significant lattice rotation across sharp deformation bands. (Sample CH 1) (3.2 mm)
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Shear zone affecting the meta-anorthosite shown in 1. Some large plagioclase porphyroclats are surrounded
by ~150 microns recrystallized grains that bear a slightly greater albitic composition. Old plagioclase grains
exhibit severe lattice distortion and probably subgrain development; the new grains have straight and polygonal
boundaries; equivalent to dislocation creep regime 2. (Sample CH 2, Ceilidh Hill, central Australia) (3.2 mm)
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Incipient recystallization in weakly deformed anorthosite. Core mantle-structures are common in the
plagioclase. Crystal plastic deformation of plagioclase is evident from bent twin lamellae and undulose extinction.
Other phases present: hornblende and garnet.
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Incipient recrystallization of a plagioclase porphyroclast. The large porphyroclast in the image is bent and
recrystallization takes place in zones of strain concentration (strongly bent zone to the left), and/or presumably
along fractures (to the right, parallel to the twin lamellae), and at grain boundaries. The right hand part, near the
strongly bent region, is deformed by twinning of the albite and pericline twin laws. 3.5 mm
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Example of largely recrystallized plagioclase in anorthosite. Only few porphyroclasts are in embedded in a
matrix of small new grains of plagioclase showing uniform grainsize. Some porphyroclasts are strongly elongated,
others have retained small aspect ratios.
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| 52 |
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Porphyroclast in the center of the image shows rectangular subgrains of a similar size as the recrystallized
matrix. The core-mantle structures and subgrain formation indicate that progressive subgrain rotation is the
dominant mechanism of dynamic recrystallization (regime 2). To the right of the image, this grain is segmented by
bands of recrystallized grains. 3.5 mm
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Segmentation of elongated porphyroclasts by recrystallization. Bands of recrystallized grains develop parallel
to the long axes of porphyroclasts, and may lead to their segmentation into narrow slivers. Thus, the aspect ratios
of such slivers cannot be used to infer the total elongation of porphyroclasts. 3.5 mm
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In mylonites, most of the plagioclase is dynamically recrystallized and only thin slivers are left of former
porphyroclasts. Relict porphyroclasts with small aspect ratios have a markedly different crystallographic
orientation compared to the thin slivers. 3.5 mm
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