Structure and composition of mantle peridotites at the boundary with crustal complexes of ophiolites in the Syumkeu massif,Polar Urals

Intense viscous-ductile deformations with multiorder flow folds and thin banding have been established in lherzolite and harzburgite of the Syumkeu massif 1.0–1.5 km below the boundary with crustal complexes. Intense shear deformation of mantle restites is traced along the entire boundary zone. The mineral composition of lherzolite and harzburgite in this zone occupies a transitional position between peridotite restites and olivine websterite from the lower part of the banded dunite-wehrlite-pyroxenite-gabbro complex. This implies that the mantle rocks from the crust-mantle transition zone were substantially transformed under transpressional intense shear stress settings along with a high-temperature ductile flow of mantle restites interacting with the supplied melt at a depth of more than 10 km. This type of transition zones differs from those known elsewhere in the Urals and supplements our knowledge on modes of mantle restite juxtaposition with crustal plutonic rocks.