BREAKDOWN OF FELDSPAR, VOLUME GAIN AND LATERAL MASS-TRANSFER DURING MYLONITIZATION OF GRANITOID IN A LOW METAMORPHIC GRADE SHEAR ZONE

In a low metamorphic grade shear zone from Quadrilatero Ferrifero (sou theastern Brazil), granitoid rocks have been transformed into phylloni tes and mylonites via activation of crystal-plastic processes in quart z and fluid-assisted reaction-softening in feldspars. Quartz deformed by basal <a> and prism <a> slip, with concurrent subgrain rotation rec rystallization. Breakdown of matrix plagioclase (An(22-35)) via mica-p roducing softening reactions required cationic exchanges between Ca, N a and K, forming replacement perthites in the K-feldspar megacrysts, w hich occupy around 60% of the original rock volume. Subsequent breakdo wn of the perthitic plagioclase (An(12-15)) led to disruption of the m egacrysts, enabling strain accommodation that localized the mylonite a nd phyllonite zones. Domainal volume and mass balance calculations bas ed on major and minor elements indicate only small positive changes in mass and volume (around 15%) in the whole system. However, accentuate d differences (ranging from mass and volume losses of 25 and 35% at th e margins, to gains of 85 and 95% in the centre, respectively) exist b etween the individual longitudinal subzones occupied by the different tectonite types. This scenario indicates that deformation proceeded as a nearly isochemical process, although with lateral mobility of major elements (principally Si, Al and K) within the shear zone. These resu lts are different from those previously reported for other granitic my lonites, most of them indicating volume losses in compressional tecton ic settings. It is suggested that the extensional tectonic framework a ttributed to the Moeda-Bonfim shear zone (where mass/volume losses are in principle not required) enhanced the access of fluid and consequen t transport of components into the shear zone (principally Fe, Si and K), producing the small gains of mass and volume observed. (C) 1998 El sevier Science Ltd.