Textural zones in the Haast Schist - a reappraisal

Field subdivision of schist using textural appearance is valuable as a means to readily identify postmetamorphic faults and to subdivide monotonous schist, at 1:50 000 scale or smaller. Recent regional mapping in the Haast Schist of South Island, New Zealand, has revealed ambiguities and shortcomings in the existing field-based systems of textural subdivision. We propose a revised textural zonation scheme that is broadly compatible with the previous Hutton-Turner and Bishop systems, yet overcomes their deficiencies. The main criteria for identifying textural zones (TZ) are white mica grain size and foliation development. For field use, the main features are: (I)restriction to first generation penetrative textures and fabrics; (2) clarification of the definition and usage of segregation to better distinguish TZIIB from III schist; and (3) grouping of TZIIIB and IV rocks because of problems of protolith identification and quartz veining. The revised system is applicable to both sandstone and mudstone protoliths.