Abstract

Strain in a narrow mylonite zone has been estimated from deformed garnetiferous porphyroblastic aggregates and from preferentially oriented plagioclase porphyroclasts with high aspect ratios. In the undeformed metamafic dyke hosting the mylonite, the mineral aggregates have spheroidal to slightly oblate shapes, and plagioclase is nearly randomly oriented. In the mylonite, the mineral aggregates are prolate ellipsoids, and plagioclase in the aggregates and matrix is symmetrically oriented about the mylonitic planar fabric. Comparison with the radii of spheres of equal volume shows that the ellipsoidal mineral aggregates underwent triaxial strain, with maximum extension of 50–140% parallel to X and with shortening of up to −30 and −45% parallel to Y and Z, respectively. The maximum strain ratio varies between 1.9 and 4.2 (mean of 10 measurements = 3.1). The orientation and aspect ratios of elongate plagioclase grains measured in the XZ plane indicate an intermediate value (2.7) for the strain ratio. Plagioclase deformation was apparently accommodated by dislocation glide on (010), recovery processes (subgrain rotation), and microcracking. The effects of mechanical anisotropy in plagioclase, however, were subordinate to the strain regime, strain ratio, and initial aspect ratio of grains in determining the final aspect ratio and rest position of these porphyroclasts.Both the deformed garnetiferous aggregates and the plagioclase porphyroclasts record state of strain in the mylonite. This suggests that the preferred orientation of densely packed feldspars of high aspect ratio potentially may be used to estimate strain in tectonites.

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