Provenance Area and U-Pb Age of Detrital Zircons from the Upper Proterozoic Deposits of the Polar Urals

Conducted isotopic-geochemical study of the Upper Proterozoic rocks of the Nyarovey series was provided from the Polar Urals. U-Pb LA-SF-ICP-MS dating of detrital zircons from metasandstone the lower part of the series showed that the eroded substrate was dominated by Proterozoic rocks. Analysis of the obtained dating allowed us to conclude that the final stages of sedimentation Nyarovey series are limited to the age of 660 million years ago. According to the contents of indicator rare elements in the meta-sedimentary rocks of the Nyarovey series, it was established that in the provenance area the acid rocks, as well as the formation of the basic and average composition, prevailed. The obtained results indicate the inaccuracy of the selection of the Nyarovey series as the Middle Riphean stratum and suggest that in the Upper Precambrian section of the Northern Urals region the Middle Riphean deposits, if present, have an extremely limited distribution. The obtained results indicate that the Nyarovey series is distinguished as the Middle Riphean of the strontium. They give reason to believe that in the Upper Precambrian section of the Northern Urals of the region, the Middle Riphean of sediment, if present, has extremely limited distribution.


Introduction
In the Timan-Northern Urals region, unlike the southern regions of the Urals, there is no complete Riphean section in the Upper Proterozoic deposits. In recent years, it has been established that the Lower Riphean stratones isolated in stratigraphic schemes do not really belong to the Lower Riphean. They belong to the Lower Precambrian crystalline base [1]. The earlier Middle Riphean deposits refer to the Upper Riphean [2]. The problem of the age of the basal deposits of the Timan-Northern Urals region remains, since many supposedly Middle Riphean stratones do not have geochronological substantiation yet.
The article specifies the age of the Nyarovey series ( Figure 1). Based on the first results of mass U-Pb dating of detrital zircons from the Upper Proterozoic rocks of the Kharbey anticlinorium of the Polar Urals. The Nyarovey series is composed of metamorphosed to the level of the greenschist facies by volcanic-sedimentary rocks. The series is conditionally referred to the Middle Riphean. [3]. An analysis of the age dating of zircons and geochemical features of meta-sedimentary rocks made it possible to establish the expected sources of demolition. structural-facies zone; 5 -terrigenous-carbonate complexes (Є3-C) of the Eletsk structural-facies zone; 6 -plate complex (Mz); 7 -dunite-harzburgite complex (O1-2); 8 -gabbroid complex (O3-S1); 9 -own diorite-plagiogranite complex (S2-D1); 10 -volcanogenic complex: basalts, andesite basalts, rhyolites, tuffs: 11 -geological boundaries: a -thrust boundary of Lemv and Eletsk structural-facies zones, b -boundaries of complexes, suites; 12 -The main Urals deep fault (overthrust); 13 -other faults: a -thrusts and faults, b -steeply falling faults, 14 -the place of sampling.

Methods
The contents of the indicator rare elements La, Sc, Th, Co, Hf, Ce, Cr and V were used for reconstruction of the demolition sources for meta-terrigenous rocks of the Nyarovey series. Their quantity in the rocks may indicate the features of the composition of the eroded substance [4]. The concentration of rare and dispersed elements in rocks was determined by acid decomposition of the initial samples and further analysis with the help of the sector mass spectrometer with ionization in inductively coupled plasma (FS HR ICP-MS) Element2 at the Institute of Geology and Geochemistry of AN Zavaritsky in Yekaterinburg. To determine the lower boundary of the formation of the Nyarovey series, and also to establish the age of eroded rocks of the substrate, isotope studies of terrigenous zircons from the metasandstone U-Pb LA-SF-ICP-MS were carried out by the method. This method is implemented on the basis of a single-collector magnetocenter mass spectrometer with inductively coupled plasma Element XR and an apparatus for laser ablation of UP-213 in the GIS SO RAN [5].
Age values of terrigenous zircons correspond to the age of the eroded material. The youngest dating of allotigenic zircon shows the maximum age of the beginning of accumulation of sedimentary material. Terrigenous zircons were selected from the rocks of the lower part of the series. The rock is represented by chlorite-muscovite-quartz schist. The primary composition of the schist corresponds to the ferruginous sandstone. 100 terrigenous zircon grains represented by all three morphological types were analysed, approximately in the same proportions in which they are present in the rock.   Similar zircons are typical for rocks of gneiss-migmatitic and other polymetamorphic complexes of the Urals, including the Polar Ural ones. They are represented by similar age populations (but not ovate, newly formed) [6]. The obtained data on the age of zircons allow us to conclude that the dominant role in the formation of deposits of the Nyarovey series belonged to the products of erosion of massifs of metamorphosed rocks of the East European craton. Their fragments are probably the polymetamorphic complexes of the Urals.

Results and discussions
To determine the possible lower age limit of the deposits of the Nyarovey series, the most important are the youngest dating of zircons. The youngest age value is 595.2 million years, hence the accumulation of sedimentary deposits of the Nyarovey series did not occur earlier than in the Vendian. However, the Vendian age is obtained only one grain of mineral. It contradicts the available geological data (the occurrence of deposits of the Miniseyshor series is lower than the limestones of the Nemuregan series with microphytolites of the IV complex of Riphean). This dating should probably be excluded from consideration. More realistic is the restriction of the lower age interval of the Nyarovey series, 660 million years ago, the end of the Late Riphean. This age corresponds to the dating of five grains of zircons (660. 1

Conclusions
The main conclusions U-Pb dating of detrital zircons and geochemical investigation of metasedimentary rocks of the Nyarovey series of the Polar Urals are performed. They made it possible to establish that in the feeding province for the terrigenous formations the predominantly acidic, basic and middle breeds were mainly Proterozoic. It is likely that it could be closely located polymetamorphic complexes.
The obtained U-Pb dating also indicates an inaccuracy in the selection of the Nyarovey series as a Middle Riphean stratum. Most likely, the entire section of the series refers to the upper Riphean. The final stages of the accumulation are limited to 660 million years.
The results of the conducted studies do not entirely remove the question of the lower age boundary of the North Ural Upper Precambrian. They give grounds to believe that in the Upper-Cambrian section of the region the Middle Riphean deposits, if, but have extremely limited distribution.