The age and emplacement of obducted oceanic crust in the Urals from SmNd and RbSr systematics

R. Lawrence Edwards, G. J. Wasserburg

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Abstract

The Urals contain a 2000 km belt of mafic-ultramafic bodies. The SmNd and RbSr 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 SmNd internal isochrons of 397 ± 20 My and 396 ± 33 My with ε{lunate}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 SmNd 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 SmNd values which lie on or within ∼ 1 ε{lunate}-unit of this isochron indicating an age and ε{lunate}Nd(T) virtually identical to those of Kempersai. ε{lunate}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{lunate}Nd(397 My) as the other samples indicates derivation from an oceanic source with negligible continental input. ε{lunate}Nd(T) for the massifs is ∼ 1.5 ε{lunate}-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{lunate}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.

Original languageEnglish (US)
Pages (from-to)389-404
Number of pages16
JournalEarth and Planetary Science Letters
Volume72
Issue number4
DOIs
StatePublished - Mar 1985

Bibliographical note

Funding Information:
Kempersai in the fall of 1979 as a part of bilateral scientific exchange. Our knowledge of the southernmost Urals was greatly enhanced by discussions and intellectual discourse with our colleagues A.A. Efimov, N.V. Puchkov, V.R. Schmeliov, N.A. Paneyakh, S.F. Sobolev, S.F. Karpenko, A.A. Marakushev and Galina N. Savelyeva. We hope that this modest effort may provide a token of thanks for the extraordinary opportunity in the field. Robert G. Coleman served as traveling companion and hammer thrower as well as omnipotent ophiolist. He continued to provide valuable discussion on his area of expertise as well as reference material and hospitality to the senior author. N.J (no period) Page generously provided well documented samples which he had collected from the Voykar-Syninsky Ophiolite Complex, shared with us his first-hand knowledge of the geology of the area, and provided reference material that was otherwise unobtainable. This project has benefited from numerous discussions with H.F. Shaw and D.A. Papanastassiou. We thank H.H. Ngo for unselfishly and patiently sharing his time and expertise on chemical procedures. This work has been supported by NSF grant No. EAR83-12882 and an NSF Graduate Fellowship to R.L.E. The functioning laboratory in which this work was carried out was supported by NASA grant No. NAG9-43. Contribution No. 4016 (460).

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