Jf. Hippertt, MICROSTRUCTURES AND C-AXIS FABRICS INDICATIVE OF QUARTZ DISSOLUTION IN SHEARED QUARTZITES AND PHYLLONITES, Tectonophysics, 229(3-4), 1994, pp. 141-163
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.