The initiation and development of metamorphic foliation in the Otago Schist, Part 1: competitive oriented growth of white mica

Abstract

The 3D shape, size and orientation data for white mica grains sampled along two transects of increasing metamorphic grade in the Otago Schist, New Zealand, reveal that metamorphic foliation, as defined by mica shape-preferred orientation (SPO), developed rapidly at sub-greenschist facies conditions early in the deformation history. The onset of penetrative strain metamorphism is marked by the rapid elimination of poorly oriented large clastic mica in favour of numerous new smaller grains of contrasting composition, higher aspect ratios and a strong preferred orientation. The metamorphic mica is blade shaped with long axes defining the linear aspect of the foliation and intermediate axes a partial girdle about the lineation. Once initiated, foliation progressively intensified by an increase in the aspect ratio, size and alignment of grains, although highest grade samples within the chlorite zone record a decrease in aspect ratio and reduction in SPO strength despite continued increase in grain size. These trends are interpreted in terms of progressive competitive anisotropic growth of blade-shaped grains so that the fastest growth directions and blade lengths tend to parallel the extension direction during deformation. The competitive nature of mica growth is indicated by the progressive increase in size and resultant decrease in number of metamorphic mica with increasing grade, from c. 1000 relatively small mica grains per square millimetre of thin section at lower grades, to c. 100 relatively large grains per square millimetre in higher grade samples. Reversal of SPO intensity and grain aspect ratio trends in higher grade samples may reflect a reduction in the strain rate or reduction in the deviatoric component of the stress field.

The initiation and development of metamorphic foliation in the Otago Schist, Part 2: evidence from quartz grain‐shape data

“…This study follows from an earlier paper (Stallard & Shelley, 2005) that interpreted the development of the white mica SPO in the Otago Schist as the result of competitive anisotropic growth of blade‐shaped grains parallel to the extension direction during deformation.…”

“…In both transects, dissolution–precipitation creep modified quartz grain shapes so that they eventually attained axial ratios of 1:2:4, comparable with the axial ratios of 1.9:4.75:9 recorded by a deformed conglomerate in the Lake Hawea transect (Norris & Bishop, 1990). The accompanying white micas have axial ratios of 1:7:13 through similar processes of selective grain growth and removal during progressive strain (Stallard & Shelley, 2005). The general structure of the very low‐grade rocks, with mica‐rich solution seams anastomosing around eye‐shaped areas dominated by rigid quartz clasts (Figs 8 & 9), indicates strain partitioning with higher strains in mica‐rich domains.…”

“…In sections oriented parallel to lineation, quartz long axes are generally strongly aligned, and together with other minerals, especially mica (Stallard & Shelley, 2005), define foliation. The distribution of grain orientations resembles a normal distribution (Fig.…”

“…, 1990; Yassaghi et al. , 2000; Stallard & Shelley, 2005), and SPO studies are important in understanding the history of structural development of metamorphic rocks as well as the deformation mechanisms by which strain was accumulated (e.g. Shelley, 1995; Mulchrone, 2002).…”

“…Rivers & Fyson, 1977; Yassaghi et al. , 2000; Stallard & Shelley, 2005). Studies that include data for SPO in 3D are also limited in number (e.g.…”