ON VOLUME CHANGE AND MASS-TRANSPORT DURING THE DEVELOPMENT OF CRENULATION CLEAVAGE

A comparison of the bulk chemistry between isolated zones of crenulati on cleavage and the surrounding relatively homogeneous slates from thr ee 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 significa nt mineralogical change between crenulated and uncrenulated zones for the samples studied. Mineralogical differentiation and mass transfer t akes place on a local scale between the limb and hinge domains of indi vidual crenulation folds and of larger scale folds in layering, but ca n occur without significant overall volume change on the scale of cren ulated zones containing many microlithon and cleavage domains. Relativ e volume changes between crenulated and uncrenulated regions do not ex ceed ca 5% at Boscastle and the majority of samples from the Moselle V alley. At one locality in the Moselle Valley, however, a volume loss o f around 35-40% occurred from the crenulated zone, produced almost ent irely by loss of SiO2 (as quartz) from the system. Isolated parallel-s ided bands of crenulation cleavage, such as the examples from Boscastl e and the Moselle Valley, are geometrically constrained to represent s ome combination of volume change and heterogeneous simple shear, super imposed upon a background homogeneous strain. In examples with minimal volume changes, the heterogeneous strain component must be one of sim ple shear.