Lattice- and shape-preferred orientation of orthopyroxene porphyroclasts in peridotites: an application of two-dimensional numerical modeling

Orthopyroxene porphyroclasts in peridotite mylonites are elongated to a variable extent and this variation depends on the crystallographic orientation of grains. We simulated the development of lattice- and shape-preferred orientation of orthopyroxene grains in peridotite using a purely geometrical two-dimensional model. In this model, under the given matrix deformation, orthopyroxene grains are assumed to deform and rotate so as to minimize the difference in displacement between the matrix and grains. In addition, the deformation of an orthopyroxene grain is limited to dislocation glide on the unique slip system (100)[001]. The results show that glide-plane orientation (θ) of grains with a large aspect ratio (R) become separated into two orientations during progressive matrix deformation ranging from simple shear to pure shear. In addition, the degree of asymmetry in R–θ distribution increases with the degree of non-coaxiality of matrix deformation. In contrast, the long-axis orientation (φ) of grains becomes concentrated around the elongation axis of matrix deformation. A natural example from the Horoman peridotite complex, northern Japan, shows an R–θ–φ relation consistent with matrix deformation by progressive simple shear. Thus, the R–θ–φ relations of orthopyroxene porphyroclasts may be useful for the estimation of strain magnitude as well as the degree of non-coaxiality of deformed peridotites.