THE CONTROL OF RECRYSTALLIZATION ON THE CRYSTALLOGRAPHIC PREFERRED ORIENTATION OF NATURALLY DEFORMED QUARTZ AND FELDSPAR, RIBEIRA BELT (SE BRAZIL)

The rheology of quartz and feldspar exerts a major control on the evolution of the continental crust. At high temperature and strain conditions these phases commonly deform by dislocation creep. However, dynamic recrystallization, which promotes grain-size reduction, may obliterate pre-existing fabrics or induce a switch to grain-size sensitive deformation mechanisms, affecting the crustal rheology. Since studies relating dynamic recrystallization and crystallographic preferred orientation (CPO) are still limited, we performed a detailed microstructural investigation of those minerals in granulite facies mylonites from the Além Paraíba-Pádua shear zone (southeastern Brazil).

Quartz and feldspar CPOs are consistently weak, probably due the intense recrystallization, which favors diffusion creep and grain boundary sliding; deformation mechanisms capable of weakening pre-existing fabrics.

Feldspar occurs as porphyroclasts or forming recrystallized domains of relatively strain-free grains. Its CPO is difficult to interpret, due a very dispersive distribution of axes. However, in some samples, one may suggest the activation of the (001) <100> and (010) <100> slip systems, suggesting that dynamic recrystallization only partially erased the former CPO.

Three types of quartz CPOs of three distinct degrees of recrystallization that relate to three different states of strain, are observed. Quartz grains from weakly recrystallized mylonites have [0001] close to the stretching lineation and prisms {m} parallel to the pole of foliation, suggesting activation of the prism <c> slip system, and a minor effect of dynamic recrystallization.

Quartz from moderately recrystallized mylonites shows [0001] relatively close to the lineation, suggesting a tendency of activation of the prism <c> slip system, and a moderate effect of dynamic recrystallization and diffusion processes.

In strongly recrystallized mylonites quartz CPO suggests deformation through the activation of multiples slip systems, such as prism <a>, rhomb <a> and basal <a>, and a major effect of dynamic recrystallization.

These observations indicate that increasing recrystallization, related to an increase in strain, is the most important factor controlling the rheology of quartz and feldspar during high temperature deformation.