Evolution of olivine fabrics during deep subduction and exhumation of
continental crust: Insights from the Yinggelisayi garnet lherzolite,
South Altyn, NW China
Abstract
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
researches that 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 (M1), granulite-facies
retrograde (M2), and amphibolite-facies retrograde
(M3) metamorphic mineral assemblages. Olivine grains in
M1 formed at P–T conditions of 2.52–3.08 GPa,
1095–1136°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 M2 formed at
P–T conditions of 1.31–1.80 GPa, 851–893°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 HP-UHP 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 probably caused by multi-stage deformation and metamorphism, and
the distinct olivine fabrics can be used as a clue to identify
geological processes and better understand metamorphism and deformation
during subduction and exhumation.