Abstract

Smoothly curving, spiral-shaped inclusion trails may form in porphyroblasts during the overprinting of near-orthogonal foliations in schists that are relatively quartz rich and, in general, graphite poor. The smooth curvature results from the combined effects of (1) the protection from progressive shearing provided by projecting crystal faces and (2) greater competency and generally wider zones of progressive shearing in these rock types than in ones that are less quartz rich and more graphtic. Therefore, lack of truncations or abrupt changes in compositional zoning profiles and the presence of smoothly curving, spiral-shaped inclusion trails do not necessarily signify that the porphyroblast containing them rotated as it grew. This has considerable significance for shear-sense determinations in deformed and metamorphosed rocks.

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