Deformed conglomerates and textural zones in the Otago Schists, South Island, New Zealand

Abstract

The prehnite-pumpellyite to greenschist facies rocks of the classic low-grade Otago schist terrane are subdivided into a number of textural zones based on the development of slaty cleavage and the appearance of segregation lamination in metagreywackes. In this study, conglomerates from fifteen localities have been studied in order to investigate the relationship between ductile strain and the sequence of textural zones. Strain measurements on lithic pebbles show a clear correlation between strain and textural zone, ranging from around 40% shortening in TZ IIA with the development of slaty cleavage to more than 70% in TZ IIIA corresponding to the appearence of segregation lamination. The much lower and more prolate strains of the quartzose pebbles compared to the corresponding lithic clasts may be explained by differential strain partitioning between clast and matrix. Microstructural evidence, including well-developed strain shadows and solution seams, indicates that solution-transfer was the dominant deformation mechanism. The development of quartz segregation laminae in TZ III is due to three related processes: the development of mica-rich solution seams demarcating quartzo-feldspathic microlithons; the development of amalgamated and recrystallised strain shadow overgrowths on clastic grains; and the formation, rotation and disruption of early formed veins. The large increase in strain from TZ II to TZ III is responsible both for increased segregation and for the rotation and flattening of segregation features into the foliation, thus producing a segregation lamination. Large strains developed by solution-transfer during the low-grade metamorphism of greywackes will lead inevitably to segregated schists.