Microstructure and seismic properties of amphibole-rich rocks from the deep crust in southern Tibet

Abstract

The crust of southern Tibet shows strong azimuthal and radial seismic anisotropies. However, their origin has remained ambiguous. Here we characterized the composition, microstructure, and seismic properties of amphibole-rich deep-crust xenoliths and exhumed rocks from a deep-crust terrane in southern Tibet. The xenoliths show typical magmatic textures and have amphiboles with homogenous chemical composition, low intracrystalline deformation, but strong crystallographic preferred orientations (CPOs). We interpret these CPOs as products of rotation of non-equant amphibole crystals in a deforming magma. The exhumed amphibolites display evidence for amphibole deformation by dislocation creep. Plagioclase in all samples shows nearly random CPOs. The calculated seismic properties of the studied amphibolites indicate that amphibole is likely a major contributor to the deep-crustal seismic anisotropies observed in southern Tibet. However, these anisotropies do not necessarily record solid-state flow. They may also result from magmatic flow in large synkinematic mafic plutons.