Quartz c-axes parallel to stretching directions in very low-grade metamorphic rocks

Abstract

Quartz preferred orientations with c-axes parallel to stretching directions are widespread in slates, phyllites and very low-grade schists of the South Island, New Zealand. In general, these rocks lack evidence for plastic deformation of quartz and often preserve the original clastic quartz which naturally tends to be elongate parallel to the c-axis. The c-axis preferred orientation can be explained, therefore, by mechanical rotation of clastic grains during deformation so that long axes became parallel to the stretching direction. Plastic deformation does, 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 enhancing the contrasts between pelitic and quartz-rich lithologies, but its role in producing c-axis preferred orientations by competitive anisotropic growth or solution seems important only in slickenfibres and in fibrous quartz veins of shear zones where quartz displays very strong preferred orientations with c-axes parallel to the local stretching direction. In contrast, the non-fibrous quartz pressure shadows in some muscovite-rich layers display a c-axis preferred orientation similar to adjacent quartz-rich sand layers, and it probably results from nucleation of pressure-shadow quartz on pre-existing mechanically oriented quartz clasts.