Type-C olivine fabric in the Zhimafang garnet peridotite of the southern Sulu ultrahigh- pressure metamorphic terrane: Formation conditions and tectonic implications
南苏鲁芝麻房石榴石橄榄岩中橄榄石的“C”类组构及其形成条件探讨

Upper mantle seismic anisotropy is mainly attributed to the lattice-preferred orientation ( LPO) of olivine, which is regarded as an indicator of different dynamic processes. Five types of olivine fabric have been identified for olivine aggregates in simple shear deformation experiments under various water contents and stresses. Using the electron backscatter diffraction (EBSD) technique, we measured the LPOs of olivine porphyroclasts in the Zhimafang garnet peridotite of the southern Sulu ultrahigh-pressure metamorphic terrane. Although these olivine grains have experienced different degrees of deformation, all of them have distinctive type-C fabric in which 100] axes is concentrated subnormal to the foliation and 001] axes subparallel to the lineation. Our data do not show any correlation between the types of olivine fabric and degrees of deformation. This observation together with the differences in the shear direction inferred from olivine fabric from that from quartz fabricformed during exhumation demonstrate that the type-C olivine fabric did not result from deformation associated with the exhumation phases of the Sulu UHP rocks. In stead, the type-C olivine fabric was formed in a subduction zone under high water activity, as suggested by water contents of olivine crystals and the temperature and pressure conditions of the Zhimafang garnet peridotite. These results suggest that during subduction of the Yangtze plate, the Zhimafang garnet peridotite was captured from the overlying mantle wedge and then experienced ultrahigh-pressure metamorphism as the Yangtze continental block was subducted to mantle depths.