Résumé
The Hangay region in Mongolia displays a high-elevation, low relief topography, underlain by a low velocity zone in the upper mantle at depths between 100 to 200 km beneath central Mongolia. The relation between this active intra-continental uplift and the mantle dynamics are, however, still poorly understood. To constrain the deformation and thermal evolution and estimate the seismic properties of the mantle lithosphere beneath the Hangay, we analyzed the microstructures and crystal preferred orientations (CPO) of olivine, pyroxenes, and hydrous phases, when present, of 40 mantle xenoliths carried up by the Shavaryn-Tsaram, Zala, and Haer volcanics, which erupted between 8 and <1 m.y. along the northeastern boundary of the dome. All xenoliths are medium to coarse-grained spinel-lherzolites. Some xenoliths contain rare amphibole, phlogopite and silicate glass. Coarse-granular, highly annealed microstructures predominate. They are characterized by equiaxed polygonal olivine crystals with rare, widely-spaced subgrains and either sinuous or straight grain boundaries and coarse irregularly-shaped pyroxenes. Tabular microstructures, marked by a weak shape preferred orientation of olivine crystals flattened normal to [010], but clear of internal deformation features (except for rare subgrains), are also common. Porphyroclastic microstructures recording a recent deformation related to the surface uplift are not present among the studied xenoliths. The annealed microstructures are associated with well-developed olivine and pyroxenes CPO, typical of deformation under high temperature, moderate pressure, dry conditions. This association indicates that ductile deformation was followed by static recrystallization, which annealed the deformation microstructure but preserved the CPO and, hence, the anisotropy of physical properties in the Hangay mantle lithosphere. This static recrystallization might be due to a long quiescence episode since the last deformation episode or to heating of the mantle lithosphere; the Mesozoic ages of the last tectonic episode recorded in the crust favor the second hypothesis. Seismic properties calculated based on the modal compositions, CPO, and equilibration temperatures of the peridotites show typical upper mantle seismic anisotropy patterns, with fast polarization of S-waves and propagation of P- and Rayleigh waves parallel to the flow direction. Maximum P-waves and S-waves polarization anisotropies attain 10% and 7%, respectively. The compositional heterogeneity of the peridotites in the three volcanic sites results in variations in seismic velocities, but at a scale that cannot be sampled by seismological studies. Comparison between xenolith seismic properties to preexisting and ongoing seismological studies in the region will help constraining the large-scale structure and deformation in the Hangay mantle lithosphere.