Plagioclase and quartz preferred orientations in a low-grade schist: the roles of primary growth and plastic deformation

Abstract

Albite in a lower greenschist facies Haast Schist from New Zealand has a maximum of poles to [001] parallel to the elongation direction; [010] forms a girdle about grain length. Plastic deformation with slip dominantly on (010)[001] is rejected as the orienting mechanism because: (1) the relationship of crystal axes to grain length would require multiple slip systems incompatible with the lack of rotation of albite twin planes; (2) euhedral terminations and other textural features indicate primary crystallization; (3) albites in quartzose layers are less elongate than those in sheet silicate-rich layers. The albite fabric is ascribed to growth anisotropy and competition in response to stress-driven solution and growth during formation of metamorphic segregation layering. This explanation is compatible with the common observation that plagioclase is brittle at low metamorphic grades. Associated weak quartz c-axis fabrics are ascribed to plastic deformation, and this deformation enhanced a dimensional preferred orientation already established by primary growth.