Development of slaty cleavage and degree of very‐low‐grade metamorphism: a review

Abstract

Abstract Illite crystallinity (IC) and other indicators of the grade of very‐low‐grade metamorphism associated with the appearance of various stages of slaty cleavage in phyllosilicate‐rich rocks have been compiled from a wide variety of terranes. IC values have been converted to a Kübler‐equivalent standard scale, but the diverse characterizations of the cleavage fabrics in published descriptions do not always allow an unequivocal identification of equivalent stages of cleavage development.Nevertheless, there exists a distinct relationship between grade and the appearance of various stages of cleavage development.(1) Indications of incipient slaty cleavage, such as S0–S1 pencil structure, appearance of primary (S1) crenulation cleavage and of closely‐spaced cleavage without parallel fabric in the microlithons, is associated with a wide range of mostly medium‐ and high‐grade diagenetic IC values.(2) The appearance of smooth cleavage with a strong parallel fabric in the microlithons and/or quartz–mica ‘beards’and the chlorite–mica stacks shortened at a high angle to (001), and of irregular cleavage in sandy beds is associated with a much narrower range of predominantly low‐ and medium‐grade anchimetamorphic grades (rarely high‐grade diagenetic). The first appearance of these stages of cleavage development with higher grades can often be related to post‐kinematic magmatic heating, polymetamorphism (pre‐cleavage metamorphism), or ‘static’recrystallization without cleavage formation, for example in low‐strain zones.There exists a relationship between finite strain, fabric and metamorphic grade in mudstones and slates; in coarser clastic rocks the same finite strain–fabric relationship occurs at appreciably higher grades. A relationship between finite strain in carbonate rocks and IC in the nearby rocks has been reported from the Helvetic zone of the Swiss Alps.The earlier stages of cleavage formation are associated with little improvement in IC; the narrow range of IC associated with smooth cleavage is concluded to represent recrystallization and grain growth concurrent with cleavage formation.