Textural zones in the Haast Schist—a reappraisal

Abstract

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: (1) 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.

The QMAP 1:250 000 Geological Map of New Zealand project

“…Mineral metamorphic grade changes are indicated by the presence or absence of key minerals or mineral assemblages but textural metamorphic zones, a semi-quantitative measure of the degree of cleavage, foliation and metamorphic segregation development, are considerably easier to apply and map in the field. Field-based textural metamorphic grade criteria were developed by Bishop (1974) and improved upon by Turnbull et al (2001) and applied to the Otago and Alpine schists. The complex greywacke-schist transition in the Southern Alps, north Otago and south Otago has also been mapped using textural zones (Turnbull 2000;Forsyth 2001;Cox & Barrell 2007;Rattenbury et al 2010).…”

“…Alpine Schist has an additional younger metamorphism phase, with further deformation resulting from movement at the AustraliaÁPacific plate boundary. Mapping of textural metamorphic zones using revised criteria (Turnbull et al 2001) and broad lithological associations of pelitic and psammitic greyschist and greenschist have revealed complex interactions of Mesozoic and Cenozoic structures and metamorphic fabrics. Through the Haast area, old fold structures tighten to the north and new foliation fabrics become more pervasive.…”

Structure of the Blue Lake Fault Zone, Otago Schist, New Zealand

“…The increase in metamorphic grade on the northeastern margin of the belt is accompanied by progressive metamorphic recrystallisation and foliation development, and this gradient has been subdivided for regional mapping purposes into textural zones (Bishop 1972;Turnbull et al 2001). Prehnite-pumpellyite facies greywackes and associated argillites (Textural Zone 1, TZ 1) are unfoliated and sedimentary textures associated with turbiditic origins are preserved with little recrystallisation other than in the sedimentary matrix (Mackinnon 1983).…”

“…Lower greenschist facies schists (TZ 3) are almost pervasively recrystallised to metamorphic quartz, albite, muscovite, chlorite, epidote, titanite and calcite. TZ 3 schists are pervasively foliated, and locally tightly folded with incipient development of a second foliation parallel to fold axial surfaces (Turnbull et al 2001; MacKenzie & Craw 2005). TZ 4 schists, not discussed in this paper, are more recrystallised and coarser grained upper greenschist facies metamorphic rocks (Turnbull et al 2001).…”

Structural and lithological controls on gold mineralisation at Oturehua on the northeastern margin of the Otago Schist, New Zealand

“…2) (Mortimer 1993(Mortimer , 2000. Schist foliation and textural reconstitution become more developed from pumpellyiteactinolite facies rocks through to upper greenschist facies, in approximate parallelism with metamorphic grade increase (Turnbull et al 2001). Textural reconstitution is complete and foliation is pervasive in greenschist facies rocks.…”