Deformation microstructures of mantle peridotite from Luobusha ophiolite,Tibet , China and its geological implication

Mantle peridotite is one of the main compositions of Luobusa ophiolite. According to its microstructures characteristics, we can analyze the characteristics of deformation, define rheological parameters, and investigate the deformation history of mantle peridotite. In this paper, we chose lherzolite and harzburgite with weak serpentinization degree as research objiects. Analysis of the dislocation configurations of olivine by oxidation decoration technique show that dislocation microstructures are dominant linear type free dislocation which usually forms at low temperature, and also include some high temperature dislocation, such as dislocation bows, dislocation loops and dislocation wall. The results propose that Luobusha mantle peridotite deformation is dominated by low temperature plastic flow, and some localized regions experience high temperature plastic flow. Based on the statistical analysis, free dislocation density of lherzolite is 4.422×107/cm2, while that of harzburgite is 9.137×107/cm2. Applying an olivine dislocation density piezometer, we determine that the stress on lherzolite and harzburgite during this deformation event were 65MPa and 93MPa. A-type lattice-preferred orientation (LPO) of olivine from all samples is observed by Electron Backscatter Diffraction (EBSD), which is conventional in shallow mantle. The depth is inconsistent with that indicated by diamond and coesite.