Crenulation cleavage differentiation: implications of solution-deposition processes

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Abstract

Metamorphic differentiation associated with cleavage development in crenulated anisotropic rock fabrics is commonly due to a redistribution of mineral phases within a certain volume of the fabric. Such a volume redistribution can be explained by solution transfer of soluble minerals from sites of high chemical potential, fold limbs, to sites of low chemical potential, fold hinges. The processes involved are dissolution, diffusional transfer via grain-boundaries and redeposition. The driving force for the diffusion, differences in chemical potential, is relatable to stress and fabric variations around the microfolds. The rate of transfer is influenced by the initial solubility of the mineral grains, the kinetics of grain-boundary diffusion, the nature of grain contacts and the microfold wavelength. The sense of migration of dissolved species is dependent on spatial variations in the magnitudes of normal stress and mean stress combined with grain shape and grain orientation changes around the microfolds.

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      Citation Excerpt :

      It is also possible that a similar fold of low amplitude and small decrease of volume in the limbs increases the amplitude by flattening, thus maintaining the similar form. Microfolds (crenulation), commonly associated with cleavage lamellae (crenulation cleavage), are a typical fabric of folded incompetent rocks (mainly slates or phyllites) with a previous strong planar mechanical anisotropy (e.g. slaty cleavage) (Cosgrove, 1976; Gray, 1977, 1979; Gray and Durney, 1979). The microfolds can be symmetrical or asymmetrical, and usually have a geometry close to that of a similar fold (e.g. Ramsay and Huber, 1987, p. 436).

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