Discordant relationship between degree of very low-grade metamorphism and the development of slaty cleavage

Definitive comparisons of data compiled on the grade of incipient metamorphism associated with the appearance of slaty cleavage in different areas are impeded by the lack of uniform characterization of the slaty cleavage. However, the grade associated with the appearance of a well-developed, closely spaced, continuous slaty cleavage is not uniform: though low- to mid-anchimetamorphic in many areas, it ranges from high diagenetic to 'epimetamorphic'.This discordant relationship may in part reflect differences in lithology of the preexisting rocks, taking place at lower grades in pelites and marls than in greywackes. It may also possibly reflect differences of fabric and preservation of pre-deformational illite 'crystallinities' during subsequent regional deformation and cleavage formation, for instance due to lack of synkinematic recrystallization of phyllosilicates in the initial stages of formation of crenulation cleavage in pelites with a strong pre-existing layer-parallel fabric. Another explanation for the discordance is believed to lie in the different time relations between deformation and metamorphism in the various terrains: in many cases the assumed syncrystalline nature of the deformation has been insufficiently substantiated. More information - specifically including TEM/SEM studies on the deformation of the newly formed phyllosilicates - are needed to interpret the nature of the discordance adequately.