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doi:10.2204/iodp.proc.304305.201.2009

Results

Microstructural characteristics

The deformed grains of all phases are elongate parallel to the foliation, although the amount of elongation varies between each phase in both samples (Fig. F3). Sample 304-U1309D-64R-2, 43 cm, represents the center of the shear zone, and the grains of all phases are deformed but exhibit very few subgrain boundaries. The grain sizes of the phases except diopside are larger in Sample 64R-2, 43 cm (center of shear zone), than in Sample 64R-2, 46 cm (edge of shear zone) (Table T1). Sample 64R-2, 46 cm, shows a slightly better development of subgrain boundaries inside the plagioclase and hornblende, but the sample does not show much internal substructure overall. Most of the high-angle grain boundaries exhibit ≥30° of misorientation across the boundary (black-colored boundaries).

Orientation data

Only orientation data for plagioclase is compared and presented in this section. Plagioclase was chosen for orientation analysis as it is the main mineral of the samples and is normally considered to be the weakest phase during deformation for a gabbro. Also, Sample 304-U1309D-64R-2, 43 cm, does not contain significant amounts of the other phases within the mapped area, so there is not enough data on the other phases to give a statistically viable crystallographic preferred orientation (CPO). The two samples show a similar CPO (Fig. F4). Sample 64R-2, 43 cm, from the center of the shear zone, exhibits a stronger texture than Sample 64R-2, 46 cm. In Sample 64R-2, 43 cm, a-axes form a distinct cluster 10°–20° from the y-direction, the b-axes are dispersed along a great circle on the lower hemisphere stereonet, and the c-axes are dispersed around the periphery of the upper hemisphere stereonet (Fig. F4A). In Sample 64R-2, 46 cm, a-axes form a distinct cluster 5°–15° from the y-direction, the b-axes are dispersed along a great circle on the lower hemisphere stereonet, and the c-axes are dispersed around the periphery of the stereonets (Fig. F4B).

Misorientation data

Statistically, the misorientation angle distribution (MAD) for plagioclase in both samples is dominated by a peak at 180° (Fig. F5A, F5B). An angle of 180° represents the rotation of the twin laws. A comparison between the neighbor-pair data and the theoretical random line shows that both plots also exhibit a small increase in the number of low-angle grain boundaries (<10°). Both MAD neighbor-pair distributions exhibit a lull of relative frequencies of boundaries with misorientation angles in the range of 10° to 30° for Sample 304-U1309D-64R-2, 43 cm, and in the range of 15° to 35° for Sample 64R-2, 46 cm. The relative frequencies then increase after the lull to the peak at 180° (bimodal distribution).

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