The role of intragranular fracturing on grain size reduction in feldspar during mylonitization

Measurements of the size, shape and orientation of K-feldspar and plagioclase grains within a granitic mylonite in the Kashio shear zone, central Honshu, Japan, revealed that grain size was the most effective parameter controlling development of intragranular fracturing during mylonitization. Intragranular fractures are dominantly located near the centre of each grain and form preferentially along the (001) cleavage. The grain size distribution of both feldspars is log-normal with an average of 760 μm. The proportion of fractured grains expands dramatically to 100% as the grain size increases, whereas few feldspar grains smaller than 300 μm have been fractured during the mylonitization, indicating that the fractures have preferentially developed in coarser grains. Furthermore, the shape and shape-preferred orientation of feldspar grains also affects the development of fractures. The proportion of fractured grains not only increases up to 20% towards the extension direction, but also rises slightly as the aspect ratio of feldspar grains increases. It is, therefore, suggested that feldspar grains lying perpendicular to the extension direction may resist fracturing regardless of grain size, unless their shape-preferred orientations became sub-parallel to the extension direction due to rotation or modification of their grain shapes during mylonitization. The development of intragranular fractures could be statistically explained in terms of a fibre-loading model, although individual fractures may result from various fracture mechanisms.