Microstructure of Finero phlogopite peridotite, indicating deformation history during exhuming from the upper mantle

Finero phlogopite peridotite is considered to be emplaced into the lower continental crust with shear deformation due to plate divergence in Paleozoic time. Moreover, it is demonstrated that it was metasomatized in the mantle wedge during the emplacement. We studied the detail of deformation history of the Finero phlogopite peridotite during the exhumation from the upper mantle through microstructural observation mainly with optical and electron microscopy, namely SEM/EBSD and TEM.

The summary of our microstructural observation is described as follows. The peridotite was originally deformed by dislocation creep with [100](010) olivine slip system, which is a general one for the upper mantle. The porphyroclastic texture was formed during this process. Subsequently, while deformation by dislocation creep was still proceeding, the active slip system of olivine changed to [100]{0kl}, which is known as a dominant system at higher stress condition. The change of slip system was probably caused by temperature reduction during ascent of the peridotite. Then, the fluid infiltration triggered the brittle deformation of the peridotite. The complex undulatory extinction observed in olivine grains due to high dislocation tangling should be created during this process under extremely high stress condition. The recrystallization of these olivine grains is characteristic. The driving force of the recrystallization might be the elastic strain energy introduced by high dislocation tangling. These recrystallized olivine grains make LPO with [100](001), which is a dominant system under wet condition. These facts suggest that the brittle deformation of the peridotite caused by the fluid infiltration occurred at plastic regime.