QUARTZ C-AXES PARALLEL TO STRETCHING DIRECTIONS IN VERY LOW-GRADE METAMORPHIC ROCKS

Quartz preferred orientations with c-axes parallel to stretching direc tions are widespread in slates, phyllites and very low-grade schists o f the South Island, New Zealand. In general, these rocks lack evidence for plastic deformation of quartz and often preserve the original ela stic quartz which naturally tends to be elongate parallel to the c-axi s. The c-axis preferred orientation can be explained, therefore, by me chanical rotation of elastic grains during deformation so that long ax es became parallel to the stretching direction. Plastic deformation do es, however, play a role in some very quartz-rich layers which display a preferred orientation due to basal slip with c-axes at high angles to the stretching direction. Solution transfer is important in enhanci ng the contrasts between pelitic and quartz-rich lithologies, but its role in producing c-axis preferred orientations by competitive anisotr opic growth or solution seems important only in slickenfibres and in f ibrous quartz veins of shear zones where quartz displays very strong p referred orientations with c-axes parallel to the local stretching dir ection. In contrast, the non-fibrous quartz pressure shadows in some m uscovite-rich layers display a c-axis preferred orientation similar to adjacent quartz-rich sand layers, and it probably results from nuclea tion of pressure-shadow quartz on pre-existing mechanically oriented q uartz clasts.