OPHIOLITES OF THE SOUTHERN URALIDES ADJACENT TO THE EAST EUROPEAN CONTINENTAL-MARGIN

In the southern Urals, the allochthonous ophiolite belt at the junctio n between the Uralides and the East European continental margin includ es lherzolite (Kraka, Nurali, Mindyak, etc.) and harzburgite, or lherz olite-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 towar d the sharp and steep contact with the transitional olivine-clinopyrox ene rocks, above which the layered gabbro unit is missing. The former unit is intruded here by amphibole gabbro and gabbro-diorites. In cont rast, the harzburgite massifs display a more complex style of deformat ion in the mantle sequence, and are affected by multistage magmatic de pletion and impregnation; the contact with olivine-clinopyroxene-plagi oclase rocks is less sharp, and the transition zone is gradual; the up per sequence includes layered and isotropic gabbros, sheeted dykes, ac id MORE-type lavas, The allochthonous Iherzolite massifs are associate d with elastic units typical of continental slope facies, and in the r oot zone, they occur as a chain of tectonic slices, Cherty sediments i ntercalated with lavas of harzburgite sequences are of oceanic origin. All these differences suggest that the Iherzolite ophiolite sequences were formed during the rift events with poorly manifested magmatic ac tivity, which affected the passive continental margin, whereas harzbur gite sequences illustrate spreading events in marginal basins with nor mal oceanic crust. The synchronism of Early-Middle Ordovician rifting and spreading stages in the southern Urals seems to indicate that basi n opening was prograding northward along the continental margin. Durin g Silurian-Devonian imbrication, ophiolite terranes of both types were intruded by basic magmas. According to available U-Pb age determinati on, this occurred at about 400 Ma.