The ophiolites of the Polar Urals: new age and petrologic data, and conclusions.

D.N. Remizov*, S.I. Grigoriev**

*A.P.Karpinsky Russian Geological Research Institute, St.-Petersburg, Russia;**St. Petersburg State University, St.-Petersburg, Russia

dnr1957@yandex.ru

 

Ophiolites of the Polar Urals were studied in the course of the geological mapping in scale 1:200 000. The works were located in the southern part of the Polar Urals including the largest Voykarsky (Voykar) ultramafic Massive. The Voykar Massiv is included in the ophiolite complex together with the Kershor dunite-verlite-clinopyroxenite-gabbro complex (DVKG) and the Lagortayu sheet dikes complex (SDK) [1 and others].

Geological situation. Paleooceanic complexes of the Polar Urals, including ophiolite allochthonous, are thrusted over the passive margin of the East European paleocontinent during the Late Paleozoic collision. From the north-west to the south-east they are represented by the Dzelayu ultramafic-metagabbro tectonic plates. Further to the east one can observe the Voykar ultramafites. The Eastern framing of ultramafites is composed by the DVKG complex, which includes a powerful tectonic zone with SDK fragments. Next is the tectonic plate, composed by the tonalite-trondemite series (TT) Kokpela, which is built on by of the volcanic-sedimentary rocks of the Early Devonian island arc on the east.

Isotope dating. To date, the dating of zircon derived from dunite is 585±6 Ма [2] and Re-Os figures of rocks from the Voykar Massif are 600 and 536±17 Ma [3], that is very close to the dating of 578 ± 11 Ma obtained earlier for the gabbro-norites of the Dzelayu metamorphosed gabbronorite ultramafic-metagabbroic massive [4]. We obtained concordant dating of single grains of zircon from three samples of homogeneous Kershor gabbro, which demonstrates an excellent convergence at about 450 Ma [5]. 444.1±6.5 dating Ma was obtained for zircons from plagiogranites forming part of the SDK Lagortayu. These data do not allow combining the complexes mentioned above into a single ophiolite association.

Mineralogy and geochemistry. The Voykar Massif predominantly consists of the low ferriferous (F'= 8-9%) dunites, gartzburgites and a small number of genetically problematic lherzolite. Their restite nature is obvious for the present time.

DVKG rocks have homogeneous mineral composition (Ol, OPx, CPx and Pl in various proportions). Plagioclase, where it is present represents the extreme anorthite, rock-forming minerals represent not zoned for the large number of microprobe studies. Most of the rocks are metamorphosed to the greenschist facies. Dunites attributable to this complex, have increased (up to 18%) iron content. The composition of gabbro resembles the cotectic one under P = 10-11 kbar pressures of [6].

Geochemical parameters of all the rocks are characterized by extreme low content of the rare-earth elements, which are on the chondrite level, and for ultramafic rocks - an order of magnitude below that level. It is noteworthy that the contents of LREE in the ultramafic rocks of the Voykar Massive as well as in the DVKG rooks are similar that excludes the assumption about the fusion of the melt initial for DVKG from the ultramafites Voykar.

The presence of Nb-Ta negative anomaly in gabbro DVKG indicates their supersubduction nature, and very depletion REE indicates the depletion of the source.

Geodynamic reconstruction. In our view, this situation can be explained by the model of passive asthenospheric window (passive slab window) [7]. The initial oceanic crust had been forming at least since the late Neoproterozoic (more than 600 Ma). Early island arcs or oceanic plateaus (Dzelayu) formed at the end of Neoproterozoic. The new intraoceanic island arc had been forming in Cambrian, and the collision of this arc with the Dzelayu thickened crust happend at about 450 Ma. As a result of this event subduction zone was jammed, followed by isolation of the slab. The passive window produced a low-energy Kershor cotectic melt, which solidified inside of supersubduction mantle wedge and did not produce volcanic products ("Plutonic gabbro" by G.B. Fershtater). In another scenario DVKG are cumulates of early boninite or picrite melt of the subduction zone laying. Subduction continued and was forming SDK Lagortayu. Volcanic products of this phase of Voykar have not preserved, but they have preserved northward and southward. At the end of Late Silurian(?) - Early Devonian new intraoceanic arc had been forming, which produced a large amount of tonalite-trondjemites of the Kokpela and Voykar’s volcanic formations. The Neoproterozoic-Ordovician stage abyssal rocks (Dzelayu, ultramafites of Voykar and DVKG of Kershor with Lagortayu SDK) were tectonically combined with shallow and superficial formations of Early Devonian Voykar island arc during the Late Paleozoic Ural collision.

 

1. G. N. Savelieva. Gabbro–Ultramafic Complexes in the Urals and Their Equivalents in the Modern Oceanic Crust. Nauka, Moscow, 1987, 246 p. [in Russian].

2. G. N. Savelieva, P. V. Suslov, A. N. Larionov. Vendian Tectono-Magmatic Events in Mantle Ophiolitic Complexes of the Polar Urals: U–Pb Dating of Zircon from Chromitite // Geotectonics, 2007, Vol. 41, No. 2, pp. 105–113.

3. V. G. Batanova, G. Brügmann, Savelieva G.N., Sobolev A.V. Application of Re–Os isotope system to dating of mantle processes: evidence from ophiolites // Yekaterinburg: IGG RAS, 2009, pp. 77-80.

4. D.N. Remizov, V.L. Pease. The Dzela complex, Polar Urals, Russia: a Neoproterozoic island arc // GSL Memoirs. 2004. Vol. 30. P. 107–123.

5. D.N. Remizov, S.I. Grigoriev, S.U. Petrov, et al. The new data of the Kershor gabbros age, Polar Ural // Doklady RAS (in print).

6. D.N. Remizov. The Polar Urals island arc system (petrology and evolution of the deep zones). Yekaterinburg, 2004, 221 p. [in Russian].

7. D.N. Remizov, E. V. Khain, A.A. Fedotova. The slab windows conception in connection with the characteristics of the construction and magmatic events on the Southern Siberia and the Polar Urals // Irkutsk, 2004. V.2. 69-72 pp. [in Russian].