Fluid-controlled fabric development across a brittle-ductile shear zone: an example from a Wyangala Batholith, Australia

Abstract:

This study focuses on the rheology and mineral reactions in a brittle-ductile shear zone within the Wyangala Batholith, SE Australia. In the shear zone, a coarse-grained two feldspar-quartz-biotite granite is transformed into a fine-grained quartz-muscovite mylonite.
The samples from shear zone margins display cataclasis of feldspar and crystal-plastic deformation of quartz. Quartz accommodates most of the deformation and is extensively recrystallized showing distinct crystallographic preferred orientations (CPO). Feldspar-to-muscovite, biotite-to-muscovite and albitization reactions occur locally at fracture and porphyroclast margins. However, the bulk rock composition shows very little change in respect to the wall rock composition.
In the shear zone centre, quartz occurs as large, weakly deformed porphyroclasts, in sizes similar to that in the wall rock, suggesting that it has undergone little deformation. Feldspars and biotite are almost completely reacted to muscovite, which is arranged in a fine grained interconnected network. Muscovite has accommodated most of the strain. Muscovite rich layers contain significant amounts of fine grained intermixed quartz with random CPO. XRF bulk rock analysis suggests a significant increase in SiO2 and depletion in NaO content compared with the wall rock composition.
We suggest that the high and low strain fabrics represent markedly different scenarios and cannot be interpreted as a simple sequential development with respect to strain. The large quartz clasts in the central parts of the shear zone have never undergone the structural development displayed by the low strain domain. We interpret that the fabrics in the shear zone centre have formed due to fluid influx probably along an initially brittle fracture where hydration reactions dramatically changed the rheological property of the rock. Subsequent ductile deformation focussed in the newly produced muscovite-quartz layers, thus original quartz grains did not recrystallize. In contrast, in the shear zone margins the amount of fluid and reactions was limited therefore deformation was taken up mainly by the relatively weak original igneous quartz.
The studied shear zone exemplifies the role of fluids and fluid-induced reactions on contrasting rheological behaviour at local scales.