Abstract
The Urals contain a 2000 km belt of mafic-ultramafic bodies. The Sm Nd and Rb Sr systematics of two of these bodies, the Kempersai Massif in the South Ural Mountains and the Voykar-Syninsky Ophiolite Complex in the Polar Ural Mountains have been examined. These data confirm the hypothesis that these bodies represent fragments of pre-collision oceanic crust and establish constraints on the nature and timing of events in the Uralian Orogeny. Two Kempersai gabbros define Sm Nd internal isochrons of 397 ± 20My and 396 ± 33My with ε Nd(T) = +8.7 ∓ 0.6 and +8.4 ∓ 1.3, respectively. Whole rock samples of pillow basalt, diabase, gabbros, troctolite, and a metasediment give Sm Nd values which lie on this isochron indicating that these rocks are genetically related and have an igneous crystallization age of 397 My. Whole rock samples of Voykar-Syninsky diabase, gabbros, and clinopyroxenite give Sm Nd values which lie on or within ∼ 1 ε-unit of this isochron indicating an age and ε Nd(T) virtually identical to those of Kempersai. ε Nd(T) for the Kempersai and Voykar-Syninsky mafic samples range from +7.3 to +9.0 with an average value of +8.4. This indicates that the Urals ophiolites are derived from an ancient depleted mantle source and are most plausibly pieces of the oceanic crust and lithosphere. The fact that a metasediment has the same ε Nd(397 My) as the other samples indicates derivation from an oceanic source with negligible continental input. ε Nd(T) for the massifs is ∼ 1.5 ε-units lower than the average for modern MORBs. This may be due to the differential evolution of the MORB source over the past 397 My and in conjunction with data for other ophiolites and Mesozoic MORB suggests that over the past 750 My the source for MORB has evolved at a rate less than or equal to its rate of evolution averaged over the age of the earth. Initial 87Sr 86Sr ratios are highly variable ranging from ε Sr(T) = −25.2 for a gabbro to +70.3 for a highly serpentinized harzburgite. This reflects the effects of seawater alteration which is particularly strong on ultrabasic rocks. We conclude that the long belt of mafic-ultramafic rocks in the Urals, which includes the Kempersai and Voykar-Syninsky Massifs, represents segments of Siluro-Devonian oceanic crust. Our igneous age for Kempersai in conjunction with other age constraints suggest that these segments of oceanic crust formed at least 80 My before the collision that produced the Urals.
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