MICROSTRUCTURES AND C-AXIS FABRICS INDICATIVE OF QUARTZ DISSOLUTION IN SHEARED QUARTZITES AND PHYLLONITES

Quartzites sheared in low-metamorphic-grade shear zones that extend al ong the contact with migmatitic-gneissic complexes in the Quadrilatero Ferifero region (southeastern Brazil), show quartz microstructures in dicating deformation by dissolution. Dissolution occurred adjacent to the foliation planes, preferentially in quartz porphyroclasts oriented with their (0001) planes parallel to the foliation. Dissolution worke d as an efficient mechanism to remove porphyroclast edges, smoothing f oliations and enabling strain accommodation. Both dissolution and crys tal plasticity were competitive, crystallographically controlled proce sses, operating in different domains of the evolving microstructure. D issolution 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 ph yllonites show uncommon c-axis fabrics (c-axes oriented at low angles to the stretching lineation) which were interpreted to form either by direct reprecipitation or by residual concentration of grains unfavour ably orientated to undergo dissolution. Domainal c-axis fabrics reflec t a deformation partitioning path with increasing participation of sol ution transfer (strengthening c-axis maxima at low angles with the str etching lineation) at the expense of crystal-plastic processes during progressive phyllonitization.