The deformation of quartz in a granite

Publisher Website
Google Scholar
Abstract
The deformation structures in a quartz porphyroclast are described in detail and the sequence of their formation is established. During the final stages of plastic deformation, the crystal structure was distorted by undulatory bending resulting in undulose extinction, and by the development of oblique surfaces of optical discontinuity lying parallel to the axix of undulatory bending and about 40° to the optic axis. The optic axis of the crystal has been rotated into parallelism with the oblique surfaces of optical discontinuity along a recrystallised zone, and the axis of rotation approximately coincides with the axis of undulatory bending. It is postulated that the reorientation of the crystal structure was accomplished by the intragranular rotation within needle‐like units elongated parallel to the axis of undulatory bending, and that this was initiated by movements along the oblique surfaces of discontinuity in response to external stresses. Similar structures with signs of reorientation of the optic axis have been observed in many pophryroclasts with very different orientations and it is suggested that the postulated mechanism of reorientation might also explain the commonly observed (ac) girdle in the quartz orientation patterns of polycrystalline tectonites.Subsequent recrystallisation in place and later primary recrystallisation from nuclei released local strain in the most distorted parts of the porphyroclast and resulted in the formation of numerous minor grains. The minor grains formed by primary recrystallisation have a pronounced preferred orientation which is unrelated to the orientation of the parent crystals and it appears that this developed without any appreciable movement of the fabric.

This publication has 9 references indexed in Scilit: