Microstructures and c-axis fabrics indicative of quartz dissolution in sheared quartzites and phyllonites

Abstract

Quartzites sheared in low-metamorphic-grade shear zones that extend along the contact with migmatitic-gneissic complexes in the Quadrilátere Ferífero region (southeastern Brazil), show quartz microstructures indicating deformation by dissolution. Dissolution occurred adjacent to the foliation planes, preferentially in quartz porphyroclasts oriented with their (0001) planes parallel to the foliation. Dissolution worked as an efficient mechanism to remove porphyroclast edges, smoothing foliations and enabling strain accommodation. Both dissolution and crystal plasticity were competitive, crystallographically controlled processes, operating in different domains of the evolving microstructure. Dissolution occurred along the foliation planes while isovolume crystal plasticity (and accompanying recrystallization) operated essentially in the quartzose domains between folia. Phyllonites formed in the most strained domains of the shear zones. Fine-grained quartz grains in phyllonites show uncommon c-axis fabrics (c-axis oriented at low angles to the stretching lineation) which were interpreted to form either by direct reprecipitation or by residual concentration of grains unfavourably orientated to undergo dissolution. Domainal c-axis fabrics reflect a deformation partitioning path with increasing participation of solution transfer (strengthening c-axis maxima at low angles with the stretching lineation) at the expense of crystal-plastic processes during progressive phyllonitization.