Abstract

Localized, intense shearing of a metadolerite dike has produced a distinctly layered mylonite from originally homogeneous rock. Observations indicate that during mylonitization pyroxenes were more ductile than plagioclase, and that layering resulted from the different behavior of these minerals under shearing stress. The normal metadolerite has 58 per cent salic, and 42 per cent femic minerals. An analyzed dark layer of the mylonite has 60 per cent femic minerals, whereas an analyzed light layer shows 86 per cent salic minerals. Pneumatolytic alteration accompanied mylonitization but did not produce the layering. The layered mylonite illustrates that metamorphic differentiation may result from mechanical deformation only, but this probably occurs rarely. However, shearing and partial mylonitization may be more widespread and may produce privileged directions, planes, and layers for mimetic recrystallization and growth during subsequent metamorphism. Thus lineated, striped, and layered ortho-amphibolites may be formed from originally homogeneous rocks of basaltic composition.

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