Abstract Measuring clay mineral “crystallinity” by the illite “Kübler” or chlorite “Árkai” index has long been used to determine the metamorphic grade of pelitic metasedimentary rocks by recognition of the anchizone: a transitional state of regional metamorphism lying between diagenetic and low-grade metamorphic conditions. Originally defined by B. Kübler on the basis of clay mineral paragenesis, the application of the anchizone as a metamorphic tool has remained ambiguous partly due to inconsistencies in the standardization of the method and partly due to the lack of reliable correlations with well accepted metamorphic zones assemblages. In this study, 54 pelitic samples were analyzed from localities of defined mineral facies from the well-studied rocks of the South Island of New Zealand. Standardized Kübler and Árkai indices are presented for this sample suite using the widely applied standardized Crystallinity Index Scale (CIS) of measurement, and employing the new Frey-Kübler equivalent upper and lower boundary limits of the anchizone at 0.32 and 0.52°2θ. The results presented indicate that 76–78% of both Kübler index and Árkai index values correspond with the expected mineral facies when the epizone is taken to correspond to both the greenschist (chlorite-zone) and pumpellyite-actinolite facies, the anchizone to the prehnite-pumpellyite facies and the diagenetic zone to the zeolite facies. This good correspondence between Kübler and Árkai index values and metamorphic mineral zones confirms that clay “crystallinity” indices provide a useful method for mapping regional grades of low temperature metamorphism and the general state of cleavage development. Its effectiveness is, however, dependent on appropriate standardization to an internationally recognized scale of clay mineral “crystallinity” measurement, and implementation of true Kübler-equivalent anchizone boundary limits. The consistent variations between illite and chlorite “crystallinities” observed for the studied pelites of southern New Zealand, as defined on Kübler - Árkai index correlation plots, is suggested to reflect variations in the activities of K⁺, Fe²⁺ and Mg²⁺ in the metamorphic pore fluid.

Highlights • The Kübler and Arkai indices correlate well with metamorphic mineral zones of New Zealand. • Pelites of the pumpellyite-actinolite facies are best assigned to the epizone. • Correlations of Kübler and Arkai indices may provide information on K⁺, Fe²⁺ and Mg² activity.