Abstract

The classical macrotextural subdivision of quartzo-feldspathic mylonitic rocks yields only three rock types: protomylonite, mylonite, and ultramylonite. This restriction impedes detailed mapping of the internal textural transitions common in wide, deep-seated, crustal-scale shear zones, where such transitions may occur over kilometres and involve several clearly mappable textural types. The introduction of two objectively defined field mapping terms, "homoclastic" and "heteroclastic," describing the macroscopic grain-size distribution within the porphyroclast population provides descriptive flexibility without changing the matrix–porphyroclast basis of the established classification. This allows the description of textural paths other than protomylonite → mylonite → ultramylonite and facilitates the consideration of textural paths in terms of strain partitioning between the constituent grains of the deforming aggregate, rather than as a simple function of finite strain.

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