A microstructural and microchemical study of cleavage lamellae in a slate

The results of a detailed microstructural and microchemical study of a slate from the Chewton area, Victoria, Australia are reported in which it was found that cleavage developed from microcrenulations. Phyllosilicates in P-domains differed in their chemistry from those in Q-domains and were concentrated in the P-domains by localized metamorphic reactions which were aided by mechanical rotation and solution processes. Wide, dark cleavage lamellae, often taken as evidence for the passive concentration of insoluble minerals, appear to have had a similar origin. The P-domains exhibited two types of microstructure, one which was consistent with annealing and the other with deformation within the domain. Cataclasites had formed along some P-domains, suggesting that they had become preferential sites for late phase brittle deformation.