The origin and geophysical implications of a weak C-type olivine fabric in the Xugou ultrahigh pressure garnet peridotite

Abstract

We report here a comprehensive investigation of mineral chemistry, water content and deformation features (including the crystallographic preferred orientation and dislocation substructures of olivine) of the Xugou garnet peridotite from the southern Sulu ultrahigh pressure (UHP) terrane. Our results suggest: (1) the Xugou garnet peridotite was equilibrated and deformed at high pressure (4.8–6.6 GPa) and relatively low temperature (810–930 °C) conditions. (2) In contrast to the pronounced shape-preferred orientations of olivine, pyroxene and garnet, electron backscattered diffraction (EBSD) measurements show a weak C-type fabric in olivine, a strong (100)[001] fabric in clinopyroxene and orthopyroxene and a nearly random fabric in garnet, respectively. The deformation mechanism map suggests a dominant deformation mechanism of dislocation-accommodated grain boundary sliding (DisGBS) in olivine. (3) The trivial amount of water in olivine (15±10 ppm), orthopyroxene (21±17 ppm), clinopyroxene (64±48 ppm) and garnet (dry) correspond to a water-poor deformation condition. We proposed that the C-type olivine fabric was generated by a dominant DisGBS with some contribution from diffusion creep at elevated pressures. Calculated seismic properties of the peridotite show the fastest P-wave velocity and the fast polarization direction of S-waves are oriented normal to the foliation plane and perpendicular to the lineation in the foliation plane, indicating that the C-type olivine fabric could also be a candidate to explain the observed strike-parallel faster S-wave polarization near trenches and in collisional orogenic belts.