Evolution of Olivine Fabrics During Deep Subduction and Exhumation of Continental Crust: Insights From the Yinggelisayi Garnet Lherzolite, South Altyn, NW China

The different olivine fabrics in ultramafic rocks have been widely used to discuss past tectonic settings, given that the olivine fabrics vary with pressure, temperature and water content. However, there are no research related to whether and how the olivine fabrics transform at different metamorphic stages in a natural rock during the process of deep subduction and exhumation. Yinggelisayi garnet lherzolites from South Altyn have experienced deep continental subduction and exhumation. The garnet lherzolites contain well-preserved residual protolith minerals, and near-peak (M-1), granulite-facies retrograde (M-2), and amphibolite-facies retrograde (M-3) metamorphic mineral assemblages. Olivine grains in M-1 formed at P-T conditions of 2.52-3.08 GPa, 1,095-1,136 degrees C and low water contents (183-213 ppm H/Si), and showed [010] axes sub-normal to the foliation and [001] axes subparallel to the lineation, which is characteristic of B-type fabric ((010)[001]). Olivine grains in M-2 formed at P-T conditions of 1.31-1.80 GPa, 851-893 degrees C and also low water contents (93-139 ppm H/Si), and exhibited [010] axes sub-normal to the foliation and [100] axes subparallel to the lineation, which is characteristic of A-type fabric ((010)[100]). These observations suggest that olivine fabrics in high pressure-ultrahigh pressure metamorphosed ultramafic rocks are different in the near-peak and retrograde metamorphic stages, and also that the olivine fabrics can be transformed during deep continental subduction and exhumation. Therefore, the dispersed or no clear olivine fabric is probably caused by multi-stage deformation and metamorphism, and the distinct olivine fabrics can also be used as a clue to identify geological processes and better understand metamorphism and deformation during subduction and exhumation.