Relationship of concretions and chlorite-muscovite porphyroblasts to the development of domainal cleavage in low-grade metamorphic deformed rocks from north-central Wales, Great Britain

Abstract

Spaced or domainal cleavage is widespread in deformed rocks of low metamorphic grade. This study presents evidence on the origin of spaced cleavage in deformed pelites from Silurian turbidite sequences in north-central Wales. The variable development of cleavage folia is related to the presence of concretions, which served to concentrate strain effects of well-developed cleavage folia in the matrix in some zones and to produce ‘strain shadows’ (few or no cleavage folia) in others. Compositional differences between matrix, narrow transition zones, and concretions also influenced the development of cleavage folia. Chlorite-muscovite porphyroblast growth was initiated before cleavage formation, but its further growth and development took place during cleavage formation. The relationship of cleavage to the concretions and to the chlorite-muscovite porphyroblasts indicates that cleavage developed in a matrix that must have been in an advanced state of lithification (anchimetamorphic to lowermost greenschist facies of metamorphism) and not during soft-sediment deformation.