On volume change and mass transport during the development of crenulation cleavage

Abstract

A comparison of the bulk chemistry between isolated zones of crenulation cleavage and the surrounding relatively homogeneous slates from three localities (Boscastle, SW England; Moselle Valley, Germany; Lac de Roselend, French Alps) demonstrates that marked bulk volume change is not a prerequisite for the development of crenulation cleavage. X-ray powder diffraction results also demonstrate that there is no significant mineralogical change between crenulated and uncrenulated zones for the samples studied. Mineralogical differentiation and mass transfer takes place on a local scale between the limb and hinge domains of individual crenulation folds and of larger scale folds in layering, but can occur without significant overall volume change on the scale of crenulated zones containing many microlithon and cleavage domains. Relative volume changes between crenulated and uncrenulated regions do not exceed ca 5% at Boscastle and the majority of samples from the Moselle Valley. At one locality in the Moselle Valley, however, a volume loss of around 35–40% occurred from the crenulated zone, produced almost entirely by loss of SiO 2 (as quartz) from the system. Isolated parallel-sided bands of crenulation cleavage, such as the examples from Boscastle and the Moselle Valley, are geometrically constrained to represent some combination of volume change and heterogeneous simple shear, superimposed upon a background homogeneous strain. In examples with minimal volume changes, the heterogeneous strain component must be one of simple shear.