Strength variation and deformational behavior in anisotropic granitic mylonites under high-temperature and -pressure conditions – An experimental study

Abstract

We performed deformation experiments on foliated granitic mylonites under high-temperature and -pressure conditions. To investigate the effects of pre-existing fabric properties on the rheology of the rocks, these experiments were carried out at different compression directions 30°, 45°, and 60° relative to the foliation, at temperatures of 600–850 °C, under confining pressures of 800–1200 MPa, within a strain rate range of 1 × 10−4/S – 2.5 × 10−6/S. The results of the experiments show that the deformation of three group samples is in the semi-brittle region at temperatures between 600 and 700 °C, and that the deformation of the samples transforms to plastic deformation by power-law creep with the stress exponent n = 3 ± 0.3 at temperatures between 800 and 850 °C. In the semi-brittle region, the mechanical data show that strength reaches its minimum value at an angle of 30° between the compression direction and the original foliation. In the plastic deformation regime, strength reaches its minimum value at an angle of 45° between the foliation and the orientation of the maximum principal stress. The strength with angles between 30° and 60° is lower than that of the compression direction perpendicular to foliation and the compression direction parallel to foliation. Microstructure analysis based on optical and electron microscopy of the deformation microstructures showed plastic deformation of aggregates of biotite and quartz at 800–850 °C. This deformation was extensive and formed new foliation. Quartz c-axis fabrics analysis by EBSD show that at temperatures of 600–700 °C, the c-axis fabric patterns could have been formed by the dominant activity of basal<a> slip, similar with the starting granitic mylonite samples, but the dominant slip systems have been changed and transformed from basal <a> slip to rhomb <a> slip and prism<a> slip at temperature of 800 °C and 850 °C. Microfractures were developed in hornblende and feldspar grains with local plastic deformation. Dehydration reaction was observed in grain rims of hornblende and biotite, where new fine-grained hornblende and biotite crystals grew, accompanied by partial melting. This was followed by experimental deformation and replacement of the original foliation of the samples. The mechanical microstructure data show that there is a significant effect of fabric on the strength of rock, but almost no effect on brittle-plastic transition and deformation mechanism.