MICROSTRUCTURAL EVOLUTION IN QUARTZITES EXPERIMENTALLY DEFORMED TO HIGH SHEAR STRAIN
At low strains (g<2.5, rexl<50%) the bulk CPOs in all regimes are dominated by the porphyroclasts and are quite similar. Basal <a> slip rotates c axes into two peripheral maxima, one normal to the shear plane and the other parallel to s1.
At higher strains the bulk CPOs are increasingly dominated by the rexl grains, and distinctly different patterns develop. In regime 2 (subgrain rotation rexl) a strong peripheral maximum inclined ~75° to the shear plane (in the sense of shear) is developed at g=7, with an elongation toward the center of the pole figure, consistent with dominantly basal <a> slip. However in regime 3 (higher T, grain boundary migration rexl) a partial girdle is developed at g=5-6, with strong maxima at Y and ~35-40° to Y, consistent with prism <a> and rhomb <a> slip. Elongate CPO domains are larger than the original grain size, indicating development by growth and coalescence.
We find that the partial girdle CPO, which is common in quartzites naturally sheared at upper greenschist grade, requires (a) grain boundary migration rexl (it does not form in samples rexl by subgrain rotation) and (b) high shear strain (it is not seen in 100% rexl but lower strain axial compression samples or in rexl grains of lower shear strain samples). Grains originally favored for basal <a> slip rotate either into a semi-stable soft orientation (c axes normal to shear plane) or into a hard orientation where further slip is impossible (c axes parallel to s1). In contrast, grains originally favored for prism <a> and rhomb <a> slip deform easily without c axis rotation. After moderate strain (g~3) they have lower strain energy than grains originally deformed by basal <a> slip, thus grain boundary migration rexl favors them to consume the other orientations and come to dominate the CPO.