Plastic Deformation and Seismic Properties in Fore-arc Mantles: A Petrofabric Analysis of the Yushigou Harzburgites, North Qilian Suture Zone, NW China

Abstract:

The fore-arc mantle above a subducting slab is a unique site where complex partial melting, melt/fluid–rock interaction, and deformation of mantle rocks occur. To constrain these processes, we analyzed the deformation microstructures, crystal preferred orientations (CPO), and water content in natural harzburgites that occur as exhumed massifs in the North Qilian suture zone, NW China. These harzburgites are very fresh, and have mineral assemblages of olivine (~81‒87 vol.%), orthopyroxene (~11‒17 vol.%), clinopyroxene (~1‒2 vol. %), and spinel (~1 vol.%). Detailed analyses of mineral textures, CPO patterns, and rotation axis distributions suggested that the plastic deformation of olivine and pyroxene were accommodated by activating a series of slip systems of dislocation. The olivine (A-/D-type fabric) showed dominant (010)[100] and/or (001)[100] slip systems, as well as other minor [100]-glide, {0kl}[100], and [001]-glide slip systems. The orthopyroxene showed dominant (100)[001] and subordinate (010)[001] slip systems, with minor (100)[010], (100)[0vw] slip systems. The water content was extremely low in the orthopyroxene (38‒44 wt. ppm), equilibrated olivine (4‒7 wt. ppm), and bulk-rock samples (9‒14 wt. ppm). Integrated with the previously reported refractory mineral and whole-rock compositions (Song et al., 2009), as well as the estimated low pressure (~1‒2 GPa), high temperature (~1100‒1300 °C), low stress (~1‒4 MPa), and water-poor conditions of deformation, it is concluded that these harzburgites represent a remnant of a fossil fore-arc lithospheric mantle which was probably both formed and deformed in a young and warm fore-arc mantle setting (i.e. infant subduction zone). Based on these results, a refined schematic model of olivine fabric distributions in subduction zones was proposed. In this model, the opposing polarizing directions of A-/D-type olivine fabrics (prevalent in the fore-arc lithospheric mantle) with other underlying anisotropic sources in a mantle wedge (e.g., B-type olivine fabrics) may weaken the trench-parallel fast S-wave anisotropy contributed by the deformed fore-arc mantle, and thus provide an alternative explanation for the short or nearly null delay times of local shear-wave splitting that have been detected in some fore-arc regions.