Breakdown of feldspar, volume gain and lateral mass transfer during mylonitization of granitoid in a low metamorphic grade shear zone

Abstract

In a low metamorphic grade shear zone from Quadrilátero Ferrífero (southeastern Brazil), granitoid rocks have been transformed into phyllonites and mylonites via activation of crystal-plastic processes in quartz and fluid-assisted reaction-softening in feldspars. Quartz deformed by basal 〈a〉 and prism 〈a〉 slip, with concurrent subgrain rotation recrystallization. Breakdown of matrix plagioclase (An22−35) via mica-producing softening reactions required cationic exchanges between Ca, Na and K, forming replacement perthites in the K-feldspar megacrysts, which occupy around 60% of the original rock volume. Subsequent breakdown of the perthitic plagioclase (An12–15) led to disruption of the megacrysts, enabling strain accommodation that localized the mylonite and phyllonite zones. Domainal volume and mass balance calculations based on major and minor elements indicate only small positive changes in mass and volume (around 15%) in the whole system. However, accentuated differences (ranging from mass and volume losses of 25 and 35% at the margins, to gains of 85 and 95% in the centre, respectively) exist between 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 results are different from those previously reported for other granitic mylonites, most of them indicating volume losses in compressional tectonic settings. It is suggested that the extensional tectonic framework attributed to the Moeda-Bonfim shear zone (where mass/volume losses are in principle not required) enhanced the access of fluid and consequent transport of components into the shear zone (principally Fe, Si and K), producing the small gains of mass and volume observed.