Grain-size reduction and foliation development in a deformed granitoid Batholith

The predominantly peraluminous Wologorong Batholith in eastern New South Wales, Australia, has undergone heterogeneous deformation. It always has at least one gneissic foliation, but in strongly deformed zones it has two foliations. One of these is a gneissic foliation (S₁) consisting of relatively discontinuous, fine-grained, micaceous and feldspathic folia anastomosing between feldspar relics and elongate aggregates of recrystallized quartz. The other foliation (S₂) is more continuous than S₁, and the folia are fine-grained and commonly laminated, as in mylonites. The two foliations could have formed siimultaneously or sequentially during a single deformation. S₂ formed oblique (up to 30°) to S₁, in order to accommodate an imposed shear component of the strain in strongly deformed parts of the batholith. S₂ folia appear to have initiated on suitably oriented parts of anastomosing S₁ folia. Continued deformation locally reduced the angle between S₁ and S₂ and resulted in the development of mylonite zones. Grain-size reduction by recrystallization and/or neocrystallization occurred in all minerals during the deformation, but quartz appears to have attained a steady-state grain-size of around 0.15 mm, which was not reduced during more intense deformation. Deformation mechanisms are difficult to determine, but microfracturing associated with offsetting and microboudinage, intragranular slip, and solution-transfer of material may have operated in the earlier stages of deformation, and grain-boundary sliding and/or solution-transfer of material may have dominated the later stages. The material transfer may have been associated with chemical and mineralogical changes that accompanied neocrystallization.