Petrophysical properties of a granite-protomylonite-ultramylonite sequence: insight from the Monte Grighini shear zone, central Sardinia, Italy

The Monte Grighini intrusive complex (central-west Sardinia) has been affected by a 1.5 km wide late-Variscan shear zone; microstructures and deformation fabrics indicate that high strain domains resulted in sharp transitions among protomylonite to ultramylonite. A representative geological cross section of the shear zone has been sampled, as well as samples from outside the mylonitic belt for use as slightly deformed protolith reference samples. Physical and mechanical analyses were performed on each group in order to evaluate a possible correlation between petrophysical properties (density, porosity and mechanical strength) and the degree of mylonitic deformation. Ultramylonites are generally characterized by a higher solid-phase density (2.82–2.85 g/cm3) than less deformed protomylonites and mylonites (2.73–2.79 g/cm3), testifying to the formation of high-density minerals with an increase in deformation. Conversely, bulk density tends to decrease as deformation increases (from 2.52–2.57 g/cm3 to 2.47–2.48 g/cm3) due to the total porosity increase occurring during mylonitization. The mean values of total porosity range from 6.33 to 9.27 % in protomylonites and mylonites and from 12.01 to 12.85 % in ultramylonites. The relatively high value of porosity in more deformed ultramylonites is the result of the sum of closed micropores and open pores produced by different processes. Mechanical strength measurements show a noticeable anisotropy, with higher strength values measured applying the load perpendicular to foliation (Z axis) and lower values obtained parallel to it (X,Y axes). In the Monte Grighini mylonite the anisotropy is due to the distribution of pores and to the preferred orientation of new minerals, as highlighted by the correlation between scalar physical properties (solid, real and bulk densities, porosity) and vector properties (resistance to punching or uniaxial compressive strengths). The results show that the degree of deformation of mylonitic rocks can be characterized by petrophysical properties, in order to shed light on the tectonic processes experienced at depth.