MICROSTRUCTURAL AND TEXTURAL ANALYSIS OF GRANULITES IN THE MOUNT HAY BLOCK, CENTRAL AUSTRALIA: IMPLICATIONS FOR THE RHEOLOGY OF NATURALLY DEFORMED POLYPHASE LOWER CRUSTAL MATERIALS (Invited Presentation)

We present the results of integrated microstructural analysis and electron backscatter diffraction (EBSD) textural analysis from exhumed lower crustal granulites in the Mount Hay block of central Australia to provide estimates of the rheology and deformation conditions in heterogeneous polyphase lower crustal materials. Forty-two samples were chosen from distinct structural and compositional domains in interlayered mafic and felsic lithologies containing mixtures of anorthite, pyroxene, and quartz. Full thin-section maps were produced using EBSD methods to characterize crystallographic textures and other quantifiable aspects of the rock microstructure in all constituent phases. EBSD analysis reveals the presence of strong crystallographic preferred orientations (CPO) in nearly all constituent phases, suggesting deformation dominated by dislocation creep. Differential stresses during deformation are calculated using grain size piezometry for all major phases, and range between 34-54 MPa for quartz within monophase layers, and 53-103 MPa and 10-41 MPa for plagioclase and orthopyroxene, respectively. Two-pyroxene geothermometry constrains deformation temperatures to ca. 780-810 °C. Based on the determined CPO patterns, stress, and temperature, we quantify strain rates and effective viscosities of all major phases through application of monophase flow laws. Monophase strain rates range from 2.1 x 10^-12 s^-1 to 1.6 x 10^-11 s^-1 for quartz, 4.7 x 10^-15 s^-1 to 2.5 x 10^-13 s^-1 for plagioclase feldspar, 1.6 x 10^-18 s^-1 to 1.6 x 10^-16 s^-1 for enstatite, and 5.7 x 10^-16 s^-1 to 1.0 x 10^-14 s^-1 for diopside. The determined flow law values used for monophase calculations were subsequently applied to two different models – the Minimized Power Geometric model of Huet et al. (2014) and the Asymptotic Expansion Homogenization (AEH) method of Cook (2006) – to calculate a bulk aggregate viscosity of the polyphase material. At a strain rate of 10^-14 s^-1, polyphase effective viscosities for the samples range from 3.07 x 10²⁰ to 2.74 x 10²¹ Pa·s. We find that the bulk viscosity of heterogeneous, granulitic lower crust in the Mount Hay region lies between that of monophase plagioclase and monophase quartz, and varies as a function of composition. These results are consistent with past modeling studies and geophysical estimates.