Rheology of Felsic Granulite at High Temperature and High Pressure

Rheological properties of continental lower crust (CLC) are essential for understanding many geodynamical processes in the Earth. Here we performed a series of deformation experiments on synthetic felsic granulite (65% plagioclase + 15% quartz + 9% clinopyroxene + 6% orthopyroxene + 5% amphibole) using a 5 GPa modified Griggs-type deformation apparatus at 827-927 degrees C and 1 GPa. All experiments have reached a steady-state creep under conditions of constant pressure, temperature, and strain rate. Our data yield the following flow law: (epsilon) over bar = 10(-4.4 +/- 0.2) MPa (-4.2)s(-1)sigma(4.2 +/- 0.1)exp(-260 +/- 30 kJ / mol/RTO), where (epsilon) over bar is in s(-1), sigma in MPa, and T in Kelvin. Microstructural observations show that plagioclase and pyroxene in deformed samples develop noticeable intracrystalline plasticity and shape preferred orientation (SPO). Electron backscatter diffraction (EBSD) measurements demonstrate more significant crystallographic preferred orientations (CPOs) of pyroxene and plagioclase in deformed granulites compared to those in hot-pressed ones. By contrast, quartz develops nearly random fabrics probably due to the simultaneous activation of multiple slip systems during deformation. These features indicate that dislocation creep dominates the deformation of these minerals and felsic granulite, consistent with the obtained stress exponent. Our data imply that a CLC consisting of felsic granulite is weaker rheologically compared to the quartz-dominated upper crust and the olivine-dominated uppermost mantle, which supports the 'jelly sandwich' model for the strength of continental lithosphere. Additionally, extrapolations of flow laws of our felsic granulite and formerly-reported mafic granulite strongly favor the hypothesis of delamination-induced decratonization of the North China Craton.