[S-MP25] Deformed rocks, Metamorphic rocks and Tectonics

[SMP25-P10] Microstructural characteristics of ultra-high pressure metamorphic rocks from the Dabie-Shan, eastern China

*Katsuyoshi Michibayashi¹, Kouhei Asano¹ (1.Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University)

Keywords:Ultrahigh pressure metamorphic rock, Deformation microstructure, Rheology

Ultrahigh-pressure (UHP) metamorphic rocks with their origin of continental material, which have formerly been subducted to mantle depths more than 80 km, are located in the Dabie-Shan, China. UHP rocks often record geodynamic processes during a series of subduction and exhumation. Deformation microstructures of UHP rocks at peak metamorphic conditions provide important information about rheological behavior of deeply subducted crustal material within subduction channel. However, it is pretty rare to preserve deformation microstructures of UHP rocks at peak metamorphic conditions due to subsequent retrograde metamorphism during exhumation. Here, we present deformation microstructures and crystallographic-preferred orientations (CPOs) of UHP rocks from the Dabie-Shan in order to study rheological behavior of deeply subducted continental material at UHP conditions. We investigated three mafic-ultramafic rocks: eclogite (garnet-omphacite-quartz), garnet-clinopyroxenite (garnet-Ca-pyroxene) and garnet peridotite (olivine-garnet-Ca-pyroxene) and one felsic rock: jadeite-quartzite (quartz-jadeite-garnet). Whereas the mafic-ultramafic rocks show distinctly high-strained microstructures such as intense elongated foliations with strong grain size reduction, the felsic rock preserves relatively low-strained microstructures such as weakly foliated granular texture, indicating that the jadeite-quartzite have been less deformed than mafic-ultramafic rocks. We argue that coesite occurrence after quartz in the felsic rock at the UHP condition can explain the deformation contrast between the mafic-ultramafic and the felsic rock. The presence of coesite with higher strength than quartz may result in increasing the bulk strength of felsic rocks, leading to strain localization in mafic-ultramafic rocks. EBSD analyses of the jadeite-quartzite revealed that the intracrystalline strains have been accumulated only within quartz grains, resulting in dense but irregular grain boundary formation, in contrast to the other minerals maintaining coarse grains with serrated grain boundaries. The formation of shear zones associated with strain localization at HP/UHP conditions can induce the detachment of subducted crustal material from subducting lithosphere, which is one of the necessary processes for exhumation of UHP rocks. We propose that strain localization in mafic-ultramafic rocks may trigger the exhumation of UHP rocks. Therefore, coesite could significantly influence rheological behavior of crustal material subducted to the depths where coesite is stable at UHP condition.