Abstract
Abstract. This study focuses on physiochemical processes occurring in a brittle–ductile shear zone at both fluid-present and fluid-limited conditions. In the studied shear zone (Wyangala, SE Australia), a coarse-grained two-feldspar–quartz–biotite granite is transformed into a medium-grained orthogneiss at the shear zone margins and a fine-grained quartz–muscovite phyllonite in the central parts. The orthogneiss displays cataclasis of feldspar and crystal-plastic deformation of quartz. Quartz accommodates most of the deformation and is extensively recrystallized, showing distinct crystallographic preferred orientation (CPO). Feldspar-to-muscovite, biotite-to-muscovite and albitization reactions occur locally at porphyroclasts' fracture surfaces and margins. However, the bulk rock composition shows very little change in respect to the wall rock composition. In contrast, 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 matrix. Muscovite-rich layers contain significant amounts of fine-grained intermixed quartz with random CPO. These domains are interpreted to have accommodated most of the strain. Bulk rock chemistry data show a significant increase in SiO2 and depletion in NaO content compared to the wall rock composition. We suggest that the high- and low-strain microstructures in the shear zone represent markedly different scenarios and cannot be interpreted as a simple sequential development with respect to strain. Instead, we propose that the microstructural and mineralogical changes in the shear zone centre arise from a local metasomatic alteration around a brittle precursor. When the weaker fine-grained microstructure is established, the further flow is controlled by transient porosity created at (i) grain boundaries in fine-grained areas deforming by grain boundary sliding (GBS) and (ii) transient dilatancy sites at porphyroclast–matrix boundaries. Here a growth of secondary quartz occurs from incoming fluid, resulting in significant changes in bulk composition and eventually rheological hardening due to the precipitation-related increase in the mode and grain size of quartz. In contrast, within the shear zone margins the amount of fluid influx and associated reactions is limited; here deformation mainly proceeds by dynamic recrystallization of the igneous quartz grains. The studied shear zone exemplifies the role of syn-deformational fluids and fluid-induced reactions on the dominance of deformation processes and subsequent contrasting rheological behaviour at micron to metre scale.
Highlights
The brittle–ductile transition zone (BDTZ) represents the strongest part of the Earth’s crust (Kohlstedt et al, 1995), the main seismogenic layer (e.g. Sibson, 1982; Scholz, 2007), and is a major source and transport region for ore-forming fluids (e.g. Kolb et al, 2004)
BDTZ is defined as a transitional layer between the pressure-dependent brittle rheology of the upper crust and thermally activated viscous creep in the lower crust
The fluid flow was highly localized in narrow central parts of the shear zone due to a cataclastic precursor and rapid metasomatic reactions, which created necessary porosity for fluid infiltration
Summary
The brittle–ductile transition zone (BDTZ) represents the strongest part of the Earth’s crust (Kohlstedt et al, 1995), the main seismogenic layer (e.g. Sibson, 1982; Scholz, 2007), and is a major source and transport region for ore-forming fluids (e.g. Kolb et al, 2004). The brittle–ductile transition zone (BDTZ) represents the strongest part of the Earth’s crust (Kohlstedt et al, 1995), the main seismogenic layer Kolb et al, 2004) It is the leastunderstood part of the continental crust, where the rheological strength estimates and assumptions of rock deformation mechanisms vary widely. The strength of BDTZ is often estimated using power-law rheology of quartz, which is the weakest and most abundant phase in granitic assemblages. Experimental data demonstrate that power-law creep in quartz can be activated at temperatures as low as 300 ◦C, while feldspar, being another abundant mineral in granitoids, has a high frictional strength up to temperatures of 500 ◦C (Passchier and Trouw, 2005)
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