Ophiolites of the southern Uralides adjacent to the East European continental margin

Abstract

In the southern Urals, the allochthonous ophiolite belt at the junction between the Uralides and the East European continental margin includes lherzolite (Kraka, Nurali, Mindyak, etc.) and harzburgite, or lherzolite-harzburgite (Khabarny, Kempersay, etc.) massifs. Over a distance of 300 km, all the lherzolite massifs are uniform in their structure and composition. They show similar patterns of plastic deformation in the mantle sequence, and degree of geochemical depletion growing toward the sharp and steep contact with the transitional olivine-clinopyroxene rocks, above which the layered gabbro unit is missing. The former unit is intruded here by amphibole gabbro and gabbro-diorites. In contrast, the harzburgite massifs display a more complex style of deformation in the mantle sequence, and are affected by multistage magmatic depletion and impregnation; the contact with olivine-clinopyroxene-plagioclase rocks is less sharp, and the transition zone is gradual; the upper sequence includes layered and isotropic gabbros, sheeted dykes, and MORB-type lavas. The allochthonous lherzolite massifs are associated with clastic units typical of continental slope facies, and in the root zone, they occur as a chain of tectonic slices. Cherty sediments intercalated with lavas of harzburgite sequences are of oceanic origin. All these differences suggest that the lherzolite ophiolite sequences were formed during the rift events with poorly manifested magmatic activity, which affected the passive continental margin, whereas harzburgite sequences illustrate spreading events in marginal basins with normal oceanic crust. The synchronism of Early-Middle Ordovician rifting and spreading stages in the southern Urals seems to indicate that basin opening was prograding northward along the continental margin. During Silurian-Devonian imbrication, ophiolite terranes of both types were intruded by basic magmas. According to available UPb age determination, this occurred at about 400 Ma.