THE RIPHEAN MAGMATISM PRECEDING THE OPENING OF URALIAN PALEOOCEAN : GEOCHEMISTRY , ISOTOPES , AGE , AND GEODYNAMIC IMPLICATIONS

The rocks from different stages of the geodynamic evolution have been preserved in the Urals. In its geologic history, the least studied is the transition period between continental rifting and the beginning of oceanic spreading. This article presents the geochemical data on the Sr-, Nd-isotopes, zircon U-Pb (SHRIMP) ages for the MesoNeoproterozoic igneous rocks and associated ores from the Bashkir meganticlinorium (BMA) on the Urals western slope. A Large Igneous Province (LIP) formed there as a result of mantle plume activity during the Middle Riphean (1380–1350 Ma). Later on (1200–1100 Ma), short-term rifting took place, as evidenced by the Nazyam graben, which was followed by the complete break-up of the continental crust. For magmatic rocks in the age range of 1750–1200 Ma, the evolition of chemical composition OIB-type → E-MORB →N-MORB is observed. The εNd(t) values for the igneous rocks and the associated BMA ores vary from negative (–6) to positive ones (+5), and thus give evidence of the lithosphere mantle depletion with time. These facts and the Sr-isotope ratios for the magmatic rocks from the subsequent evolution stages confirm that the oceanic basin to the east of the East European platform started to open at the end of the Middle Riphean. For the Vendian-Cambrian, some traces of orogenes (Timanian stage) are observed. The development of the Uralian Paleozoic ocean started in the Ordovican and continued up to the Late CarboniferousPermian.


INTRODUCTION
The Uralian Mobile Belt is one of a few geologic structures on the Earth where the rocks from the different stages of the geodynamic evolution have been preserved.The Urals is composed of the rocks representing all the geodynamic settings, from continental rifting (the edge of the East European Platform, EEP) and opening of the oceanic basin in the Late Meso-Proterozoic (the area to the east of EEP) to the Carboniferous-Permian collision.
The time of the oceanic basin opening at the EEP edge and the factors initiating that process have been discussed in many papers.The literature overview shows that intraplate rifting, subduction, accretion and collision occurred at the EEP eastern margin in the Precambrian [Maslov et al., 1997;Kuznetsov et al., 2007;Nosova et al., 2009;Samygin et al., 2010;Puchkov, 2013;Ivanov et al., 2014], and different parts of that extended zone did not develop quite synchronously.
The main structure comprising various magmatic formations under study is the Bashkir Meganticlinorium (BMA) (Figures 1, 2, and 3).It is located at the borderline between the Urals and EEP.In our study, we analyzed the composition, age, isotope characteristics and geodynamic settings of the BMA Pre-Cambrian igneous rocks, and tried to determine the nature of magmatism in the early evolutionary stages of the structure, the time of origin and duration of the paleoocean.
For describing the age of the Uralian formations, we use the terms "the Riphean" and "the Vendian".The Lower and Middle Riphean correspond to Meso-Proterozoic; the Late and Uppermost Riphean (760-600 Ma) [Puchkov, 2010;Krasnobaev et al., 2012] refer to the Early and Middle Neoproterozoic; and the Vendian corresponds to the Late Neoproterozoic.

BRIEF GEOLOGICAL DESCRIPTION OF THE BASHKIR MEGANTICLINORIUM
The Bashkir Meganticlinorium (BMA) is located on the western slope of the Urals.It is a part of the Central Uralian megazone (Fig. 1).In the west, it borders the West Uralian megazone and the Preuralian Foredeep at the EEP eastern edge.In the east and southeast, it adjoins (from the north to the south) the Ufaley block, the Magnitogorsk megazone, and the Uraltau anticline structure [Puchkov, 2010].The western and eastern Fig. 1.Tectonic scheme of the Urals, after [Puchkov, 2010].
zones of BMA differ in the stage of metamorphic alteration of rocks.The Zuratkul-Karatash (ZK) fault is the borderline between these zones.
The Vendian deposits lie on the Riphean ones with an unconformity.The Vendian deposits are represented by the Asha group (terrigenous rocks), subdivided into seven formations.The interval of their generation is 618-547.6Ma [Grazhdankin et al., 2011;Levashova et al., 2013].
The Paleozoic deposits are developed in the sinclines (Yuryuzan, and Tyrlyan) in the eastern BMA.On the BMA western slope, the Paleozoic deposits overlie the Vendian sequences with a parallel unconformity.The Ordovician sandstones and the Silurian shales lie unconformably on the deformed Middle-Late Riphean rocks.
According to the data on the regional stratigraphy, lithology and tectonics, the EEP eastern margin developed as a passive margin during the Riphean -Early Vendian, with several episodes of rifting and subsequent formation of well-known supra-rift depressions.The types of sedimentary basins varied with time.The intracratonic basin developed in the Lower and Middle Riphean and was transformed in the Late Riphean into a pericratonic basin, from the western slope of the Urals through the Timan ridge to the Kola Peninsula [Maslov et al., 1997].In the Late Vendian, there was a deep foreland basin filled with molassa [Puchkov, 2013] from the beginning of the Timan activity in the east.According to the recent datings of detritic zircons [Kuznetsov et al., 2014], the detritic material began to transport from the Pre-Uralian-Timan orogen to the Mezen' sedimentary basin only in the Early Cambrian.It means the start of the Timan orogenic process.Before that event, the Timan edge of the Baltica continent developed as a passive margin.
In the Early Paleozoic, there was a shelf basin in the east of EEP.The positions of the sedimentary sequences (from the Ordovician to Devonian) in the geological section suggest at the basin's extension in the western direction [Puchkov, 2010;etc.].
The BMA magmatic formations are concentrated in its eastern part.The Riphean stage includes the Early Riphean subalkaline basalts of the Ai Fm. (western BMA, 1752±18 Ma) [Krasnobaev et al., 2013;Tevelev et al., 2014], and the Middle Riphean subalkaline rhyolitebasalts of the Mashak Fm (east of BMA, 1380-1350 Ma) [Parnachev, 1981;Puchkov et al., 2013].The swarms of basic dikes in different parts of BMA, the Berdyaush and Akhmerovo massifs, the gabbro-granitoid intrusions of the Kusa group, and the Sibirka trachybasalts are comagmatic with basalts of the Mashak Fm.All these objects are considered below.During the Uppermost Riphean (728-700 Ma), the metabasalts of the Arsha group (Igonino suite), Barangulovo and Mazara gabbro-granite massifs were formed [Kuznetsov, 2009;Krasnobaev et al., 2015].The petrogeochemical features of the Riphean magmatic rocks show their generation in the continental rifting setting at the eastern margin of EEP.
The age of zircons from the ash layer (molassa, Late Vendian) is 548.2±7.6 Ma [Grazhdankin et al., 2011].The age of granite gneisses from the Jurma complex in the east of BMA is 540-510 Ma [Shardakova, 2016].In petrogeochemistry, volcanic rocks correspond to riftogenic series, granitoids to A-type of granites, which have characteristics of both riftogenic and suprasubduction series.In the eastern BMA, there are two small granite massifs of the Paleozoic age -Semibratka and Kialim (314-300 Ma) [Shardakova, 2016].Their datings and petrogeochemistry are close to typical early orogenic series located to the east of the Main Uralian Fault zone and associated with the development of collision in the Urals.
Thus, the BMA geologic structure reflects the features of various geodynamic settings.The Pre-Uralian structural level (Late Precambrian, according to [Puchkov, 2013]) reflects the evolution of the continental margin with the episodes of rifting, and the Uralian structural level (Paleozoic) demonstrates subsequent processes, from the further opening of the ocean to its closure and the generation of the collision orogen.
Below we describe the types of magmatic associations which characterize the geodynamic settings of the early stages of the Riphean-Vendian ocean opening.

RESEARCH METHODS
The analyses for major and trace elements were performed at Center Geoanalitik and the Laboratory of Physical and Chemical Methods of Research at the Institute of Geology and Geochemistry, Uralian Branch of RAS (Ekaterinburg).
The concentrations of major elements were determined by X-ray spectroscopy XRF (VRA-30) (analysts N.P. Berseneva and G.S. Neupokoeva).The Fe2O3 and Na2O contents and loss-on-ignition values were determined by the wet chemistry technique.Total iron is presented as FeO.
The analytical measurements of rare and rare-earth elements were conducted using an ElmerELAN 9000 ICP-MS spectrometer (analyst D.V. Kiseleva).The analytical errors were 2 % for concentrations over 100 ppm and 5 % for concentrations below 10 ppm.The Sm and Nd concentrations and isotope compositions were analyzed by isotopic dilution mass spectrometry.The analytical errors of determination of 147 Sm/ 144 Nd and 143 Nd/ 144 Nd ratios were 0.2 % (±2σ) and 0.003 % (±2σ), respectively.The certified international standards (La Jolla and BCR-2) were used to evaluate the quality of the data obtained.The Sm-Nd isotope data were processed using the Isoplot/Ex ver.2.49 software to reveal the Sm-Nd isotopic evolution [Ludwig, 2001].
Single zircon grains were analyzed at the Center of Isotopic Researches, A.P. Karpinsky Russian Geological Research Institute (VSEGEI, St-Petersburg) by analysts D.V. Matukov and E.V. Lepekhina.The U-Pb analysis of zircon grains was performed on a SHRIMP-II ion microprobe following the standard procedure [Williams, 1998].

RESULTS. CHEMICAL COMPOSITION AND GEOCHRONOLOGY OF THE RIPHEAN IGNEOUS COMPLEXES OF BMA
The data on the geology, chemical compositions and ages of most of the Precambrian igneous complexes of the BMA are shown in Figures 4 and 5, and Tables 1, 2, and 3. First, we briefly describe the well-known objects (Berdyaush and Akhmerovo massifs, basalts of the Ai Fm.) to show a full picture of magmatic events.Then the results of our investigation of the rocks of the Kusa-Kopan group of intrusions, the Sibirka deposit, the Nazyam sequence, and the sill-and-dike swarms are presented in detail.
Early Riphean stage.Basalts of the Ai formation are the most ancient igneous rocks of this stage in the study area; 1752±18 Ma [Krasnobaev et al., 2013].Their chemical composition indicate [Parnachev, 1981;Ernst et al., 2006;etc.]relatively high alkalinity of rocks, which are characterized by К2О 3-6 %, TiO2 2-3 %, P2O5 up to 0.70 % contents, as well as a sharp prevalence of light rare-earth elements (LREE) over heavy rare-earth elements (HREE) (see Table 1).According to the contents and ratios of trace elements, the trachybasalts of the Ai Fm. are close to ocean island basalts (OIB-type).
Middle Riphean stage.The Berdyaush granite massif.The Berdyaush massif is located near the eponymous railway station, to the east of the Kusa-Kopan cluster of intrusions.From the east, the massif is bounded by a fault zone; in the west, it intruded the Lower Riphean metasediments of the Satka Fm.The core of the Berdyaush massif is represented by syenodiorites and syenites containing gabbro xenoliths, as well as by the bodies of nepheline syenite; a peripheral zone of the massif is composed of granosyenites and rapakivi granites.The latter are related to A-granites formed during extension.The Berdyaush pluton is composed of typical intraplate rift-related series of the BMA.It is described in detail by many authors [Popov et al., 2002;Krasnobaev et al., 2011;Puchkov, 2013;Shardakova, 2016;Ronkin et al., 2016;etc.].
The gabbroids of the Berdyaush massif are very specific.According to the contents of HFSE (Nb, Ta, Y, Hf, and Zr), La/Yb, Nb/Zr, and Y/Zr ratios (Figs 5, and 6), they are similar to subalkaline dikes cutting the Lower Proterozoic rocks of the Alexandrovsk-Akhtensk metamorphic complex (see below).Apparently, such rocks are related to a more enriched mantle source.
The following ages were obtained for different rocks of the Berdyaush pluton: 1388 Ma (gabbroids), 1372 Ma (quartz syenite porphyry), 1388-1354 Ma (rapakivi granites), and 1373-1368 Ma (nepheline syenite) [Krasnobaev et al., 2011;Ronkin et al., 2006;etc.].These datings lie in the age range of formation of the Kusa-Kopan gabbro-granite complex and determine the lower age boundary of the Middle Riphean in BMA.The εNd value for rapakivi granites ranges from -5.0 to -7.3 and indicates a significant role of the crustal component in a melt source [Larin, 2011].As for gabbro, the presence of depleted mantle material (ISr=0.7032) in the substrate is suggested [Gorozhanin, 1998].
The Akhmerovo granite massif is located in the eastern edge of BMA among the Lower Riphean metasedimentary rocks and confined to the core of the Beloretsk structure.The Akhmerovo granites have high contents of FeO and TiO2, the total REE >300 ppm, La/Yb 5-11, and Eu/Eu* <1.These features and the distribution of other trace elements (Sr, Nb, and Zr) are similar to those of rhyolites of the Mashak Fm. and granites of the Berdyaush pluton, indicating a withinplate setting.The geology details and the chemical composition are described in [Puchkov, 2012;Shardakova, 2016;etc.].

The Kusa-Kopan group of gabbro-granite intrusions.
This group of intrusions occurs as a 2-5 km wide NE-striking zone that is traced for a distance of about 80 km in the BMA northeastern part to the west and then to the south-west from the town of Zlatoust in the Southern Urals.
The results of chemical analysis for the Kusa-Kopan group are given in Table 2. Gabbroids are characterized by a smaller SiO2 content in contrast to all other Middle Riphean mafic rocks.This is reflected in slightly higher MgO and CaO contents, low contents of K2O, Rb, as well as most of highly charged elements, such as Nb,Ta,Zr,Hf,Y (Figs. 4,5).A specific group of rocks is represented by ilmenite-bearing gabbro-norites penetrated by borehole no. 2 located to the east of the Kusa deposit.This type of gabbro-norite is characterized by high contents of Nb and Ta, while, according to the concentrations of other incompatible elements, it is close to other gabbroids of the Kusa-Kopan group.A high V content in all the gabbro is due to accumulation of V-rich  П р и м е ч а н и е. 1-7 -базальты: 1-2 -айская свита, 3-5 -машакская свита, 6-7 -месторождение Сибирка; 8-9 -назямские амфиболиты (метабазальты).* -все железо приведено в форме Fe2O3.
magnetite and titanomagnetite ores.According to [Fershtater et al., 2001[Fershtater et al., , 2005;;etc.],this is an evidence of a high oxygen fugacity during their formation.The granites of the Ryabinovka and Gubenka massifs have high content of Fe, Ti, total REE, and deep negative Eu-anomaly.The geochemistry of these granites is consistent with a continental rift setting.
The gabbro-granite massifs of the Kusa-Kopan group were formed at different depth levels.In the north, the emplacement of Kusa and Gubenka massifs occurred under conditions of abyssal depth facies at the pressures of 6-8 kbar and more.In the south, the total and fluid pressure during formation of less deep gabbro and granitoid intrusions (Kopan and Ryabinovka mas-sifs) was reduced to 1-3 kbar.A total difference in the formation depth between gabbro-granite intrusions in the north and south of the Kuvash graben and the ZK fault is up to ~20 km [Fershtater et al., 2001[Fershtater et al., , 2005]].
T a b l e 3. Contents of major (wt %) and trace (ppm) elements in the representative samples of rocks from the mafic sill-and-dike complex of BMA Т а б л и ц а 3. The Kusa-Kopan group of intrusions has been recently studied in great detail using the isotope method [Kholodnov et al., 2006[Kholodnov et al., , 2010;;Kholodnov, Shagalov, 2012;etc.].The isotopic age data obtained with U-Pb, Sm-Nd, and Rb-Sr methods indicate that the ages of ore-bearing gabbroids of the Kusa-Kopan group and the Gubenka and Ryabinovka granite massifs overlying them in the east, lie in the same range of 1385-1395 Ma.The Sm-Nd age of gabbro-norites of the Kusa deposit is 1388±63 Ma; that of vein-like massive magnetite-ilmenite ores of this deposit is 1392±130 Ma.Similar Sm-Nd zircon ages were obtained for gabbro-norites of the southern Kopan massif and granitoids of their comagmatic Ryabinovka massif (1385±25 Ma); anorthosite from rhythmically layered unit of ore-bearing rocks of the Medvedevka deposit yielded an age of 1379±8 Ma.The Rb-Sr isotopic age data obtained for granitoids of the Ryabinovka and Gubenka massifs are as follows: the Ryabinovka massif (1394 Ma, 87 Sr/ 86 Sri=0.705485±0.000034);the Gubenka massif (1388.5 Ma, 87 Sr/ 86 Sri=0.70570±0.00012).Thus, owing to the variety of isotope methods used, we were able to reliably establish that gabbroids, granitoids, and the ores of the Kusa-Kopan complex formed in a similar age range, namely, at the beginning of the Middle Riphean.A few granite porphyry dikes intruded the Medvedevka deposit a bit later: 1353±16 Ma [Kholodnov, Shagalov, 2012].
Sill-and-dike complex.The study of the isotope systematics of sill-and-dike swarms, widely developed in BMA, is problematic due to a lack of age data.Currently, there are only a few reliable isotope age datings.The U-Pb baddeleyite age of dolerites of the Main Dike of the Bakal ore field age is 1385.3±1.4Ma [Ernst et al., 2006].Picrobasalts and picrites of the sill-like bodies of the Taratash block and picrobasalts from the exocontact zone of the Kusa intrusive massif have similar age datings [Nosova et al., 2012].In terms of geochronology, these subvolcanic bodies were formed synchronously with other Middle Riphean intrusive and volcanic formations of BMA.
In addition, there are younger dolerite dikes in different parts of BMA.For example, a swarm of sublatitudinal dikes crossing stratified gabbroid intrusions of the Kusa-Kopan ore-bearing complex, elongated along the ZK fault.Dikes in the southern shallow massifs (Kopan and Matkal) have chilled contact zones.They are usually not metamorphosed and represented by fine-grained equigranular, porphyritic rocks with ophitic texture.In the deep northern Kusa intrusion, late dikes have no chilled zones; they often occur in endocontact zones and foliated and tectonically deformed gabbro.This gives evidence that those dikes intruded after one of the phases of tectonic deformation.Among the granite-gneisses of the Gubenka massif, sheets of orthoamphibolites, concordant with the surrounding rocks, are present.All of these bodies are boudinaged to varying extents.Dikes in the Berdyaush massif are represented by amphiboledolerite.They cut intrusive rocks and enclosing deformed dolomites of the Satka Fm. and have chilled contacts with them.
The mafic rocks of the sill-dike swarms have similarities and differences in composition.A common feature of all these groups of mafic rocks is higher contents of Fe, V and Ti (Table 3), which are considerably higher than their average contents in traps and basalts of continental rifts.According to the content of potassium and incoherent trace elements (Figs. 4,5), they are very different and subdivided into three groups.
(III) The latest sublatitudinal dolerite dikes crosscutting the intrusive rocks of the Kusa-Kopan intrusion are characterized by a low alkalinity, low contents of K2O (0.20-0.30wt %), and Rb, Cs, Ba, P, Nb, La, and Ce (see Table 3), the lowest La/Yb ratio (2-4) at high K/Rb ratio .On spidergrams (Fig. 6) the latest dikes have sharp minimums of K, Rb, Sr, and Zr, and positive anomalies of Sc, V, Li, and Ba.On the chondrite-normalized REE distribution diagram (Fig. 6), the points of dikes of this group form a unified field with metavolcanics of the Nazyam sequence.In contrast to the other Middle Riphean rocks, they are characterized by the high HREE contents against the background of low LREE contents.
There are the following isotope datings.The age of dolerite sill at the outskirts of the Kusa town is 1360±9 Ma (Ar-Ar method) [Puchkov, 2012].According to [Nosova et al., 2012], the low-Ti mafic rocks (picrites and dolerites) formed in BMA somewhat later (1320 Ma) than the high-Ti mafic rocks (1385 Ma).In addition, there is the only Sm-Nd isochron age dating (1291±67 Ma) obtained for picrites of the Ishlya complex (i.e. the central part of the BMA) [Sazonova et al., 2011].
Thus, the above isotopic age data suggest that the gabbro-granitoid intrusions and several crosscutting dikes formed almost synchronously with the bimodal basalt-rhyolite volcanism at the Mashak complex.The magnesite deposits formed in the Lower Riphean deposits of the Satka Fm. (1380±14 Ma, U-Pb method) [Ovchinnikova et al., 2014].All these igneous and metasomatic rocks recorded episodes of rifting during the Mashak event in their composition and textural-structural features [Maslov et al., 1997].
Trachybasalts and andesitic basalts of the Sibirka deposit.The Sibirka rare metal deposit is located on the left bank of the Satka River, near the Sibirka village (Chelyabinsk Region).Despite the deposit containing the mineable contents of Nb, Ta, Zr, Th, and Mo, it is assigned to a noncommercial type due to poor ore washability.It is confined to a small circular volcanic structure in a feathering fault associated with the ZK fault.The ore-bearing rocks are feldspathic, feldspathoid-feldspathic, and carbonatite metasomatic rocks developed after basaltic trachyandesites.Sometimes, the latter look like eruptive breccia and contain xenoliths of felsic rocks which are similar in mineral and chemical composition to granites of the Ryabinovka massif, located north-east, also in the fault zone.The central part of the volcanic structure is occupied by more siliceous rocks, namely biotite-microcline syenites (often albitized, hematitized, and silicified) which are attributed to the end members of magmatic series.
Alkaline metasomatic rocks contain biotite, aegerine, alkaline amphibole, and hematite; there is a metasomatic zoning in the distribution of minerals.The finegrained rare-metal mineralization (columbite, pyrochlore, Nb-aeschynite, molybdenite, thorite, etc.) is associated with the metasomatic rocks resulting from hydrothermal-metasomatic alteration of volcanic rocks which intruded the sedimentary rocks of the Bakal and Satka Formations.The data on the composition, structure, and mineralogy of ores are given in [Zoloev et al., 2004].
The Sibirka trachybasalts and basalts are ascribed to a specific petrogeochemical type of the BMA rocks.They are characterized by the highest contents of alkalis (K and Na), Ti, P and some trace elements (Nb, Ta, U, Th, TR, Zr) (see Table 1).The cause of these petrochemical features is probably geochemical specifics of the parental magma, complex mechanism of generation and subsequent metasomatic processes.
We have obtained new isotope-geochronological data characterizing the early development period of the Sibirka deposit.The concordant age of one of the zircon associations extracted from the amphibole-biotite granite xenolith in trachybasaltic eruptive breccia is 1354±7 Ma [Shagalov et al., 2014] that corresponds to Fig. 6.MORB-and chondrite-normalized [Sun, 1982] REEpatterns for the Mesoproterozoic rocks from BMA.
the terminal stage of the Middle Riphean granitoid magmatism (dike cutting the Medvedevka gabbro massif) within the ZK fault.
The Sm-Nd-isotope systematics (Table 4) is characterized by a number of erochronous dependencies.One of them is characteristic of trachybasaltic volcanics and some types of ore-bearing metasomatic rocks with a general age 1337±150 Ma, 143 Nd/ 144 Nd=0.51111± ±0.00011, MSWD=39.Trachyandesites have the highest εNd values (+4…+4.9)indicating their source within the depleted mantle.The highest εNd value of syenite is slightly lower (+1.6…+2.8).The model Nd ages of the magmatic sources are in the range of 1316-1857 Ma.
In general, the beginning of formation of the Sibirka polychronous deposit (1337 Ma) is in agreement (within an error) with the Rb-Sr dating (1323±53 Ma) from [Nosova et al., 2009] for trachybasalts of this structure.
Amphibolites (metabasalts) of the Nazyam sequence.These volcanic rocks are the youngest ones cropping out in the northern Kuvash volcanic area.Until now, this sequence has not been included into any formation due to its unclear stratigraphic position.Amphybolites form a zone in the eastern part of this area separated by a tectonic fault zone from the rocks of the apical part of the Gubenka granite massif, 3 km to the NNE of the Zlatoust city.The Nazyam sequence was named after the submeridional Nazyam (Nazma) Ridge extending in the north of the town of Zlatoust (more than 10 km in length), where metavolcanics of this type were first described.
The thickness of metavolcanics in some areas of the sequence is 0.5-1.0km.To the east, metavolcanics are tectonically overthrusted by the Middle Riphean quartzites and schists of the Taganay Fm.
Amphibolites have a granolepidoblastic texture with the parallel orientation of elongated minerals and are made of almost entirely saussuritized plagioclase, altered amphibole, and epidote, as well as accessory minerals (apatite, leucoxene, and magnetite).The relicts of the structure and the mineral association suggest that the primary rocks corresponded to basalts.The Nazyam metabasalts, the youngest rocks of this stage, are characterized by the lowest K content (0.10-0.20 % K2O) and associated trace elements (Rb, Cs, Ba, Sr, P, Nb, La, Ce, etc.) and low La/Yb ratio (2-3).
The K-Ar age of K-poor amphibole varying in the range of 1155-1254 Ma (Table 5) was obtained for the samples of the Nazyam metavolcanics.The dating of 1254 Ma may show the late age limit of the Nazyam metavolcanogenic sequence formation, as well as the age of epidote-amphibolite facies metamorphism.The age of the primary volcanogenic substrate can be more ancient, probably in the range of about 1250-1300 Ma.Acording to our Sm-Nd isotopic data (εNd +3.7…+5.5)(Table 6), the mantle component part in the source was significant.

DISCUSSION. INTERPRETATION OF GEOCHEMICAL AND ISOTOPE DATA, AND GEODYNAMIC SETTINGS
The compositions of Middle Riphean igneous rocks and dike swarms in BMA, as well as the most typical intra-continental rift systems worldwide are charac-  teristic of the chemical composition evolution of rocks: from early more alkaline and subalkaline ones to essentially depleted in K and non-coherent lithophylic rare elements ( see Tables 1 and 3).Separate sites of alkaline rocks still persist in the study area (Sibirka, Berdyaush).
In the plots of Nb/Yb versus Th/Yb (Fig. 7), the above-described rock associations form a major field, which is elongated along the mantle trend.Some of them are more alkaline rocks (the Sibirka trachybasalts, the Berdyaush mafic rocks, and dikes in the Aleksandrovsk-Akhtensk block) that are similar to the enriched rocks such as OIB-type.The other rocks (basalts of the Mashak Fm., gabbro-norites of the Kusa-Kopan complex, late dikes in gabbro intrusions, dikes in rocks of the Satka Fm.) are attributed to E-MORB.The Nazyam amphibolites occupy the field between E-MORB and N-MORB.All the spectra of above-described BMA rocks in the Th/Yb-Nb/Yb coordinates closely match the rocks of the East African Rift and the Red Sea Rift (see Fig. 7) [Rogers, 1993;Volker et al., 1997;Barrat et al., 1998;etc.].
Having analyzed the published data and our isotopic and geochemical data on the Middle Riphean igneous rocks of BMA, we suggest that magmatic (mantle and crustal) sources varying in the composition could have been involved in the rift magmatism.
The area of the lowest εNd values is marked by the positions of the points (Fig. 8, a) of granite-rapakivi from the Berdyaush massif and dikes in the Aleksandrovsk-Akhtensk block (from -5.0 to -7.3).For comparison, the figure shows the field of sedimentary carbonate of the Satka Fm. (1550 Ma) containing typical riftogenic Satka magnesite deposits (1400 Ma) [Krupenin et al., 2016].
The lower right-hand part of the diagram (see Fig. 8, a) shows the position of the rocks of the Taratash complex (Archean) [Popov et al., 2002], which could be a part of the Kuvash-Mashak structure basement.
The massive magnetite-ilmenite ores of the Kusa deposit and the gabbro-norites of the Kopan massif from that structure are characterized by the lowest negative εNd values (-1.1 and -2.4, respectively).We note that the latter ones, together with the granites of the Ryabinovka and Gubenka massifs, have a higher initial ratio of 87 Sr/ 86 Sr (0.7050-0.7060).Probably, the mantle source for the rocks of the Kusa-Kopan group was metasomatically enriched by the crustal component.The volcanic members of the Middle Riphean sequence (basalts of the Mashak Fm.) also belong to the derivatives of poorly enriched mantle source (εNd +0.6…+0.8).The εNd values in the syenitoids of the Sibirka deposit (about +2), the gabbro xenoliths in the granites of the Berdyaush massif (εNd +4…+4.9)T a b l e 5. K-Ar ages of hornblende (amphibole) from metabasalts of the Nazyam mountains* Т а б л и ц а 5. indicate the participation of a depleted mantle component in the source.Maximal εNd values (+3.7…+5.5)are revealed for the Nazyam metabasalts.Probably, the extension of the continental crust was the most intense in the northern area of the Kuvash-Mashak rift structure.The small Nazyam rift was formed, its bottom descended to a considerable depth, and almost complete break-up of the crust took place (Fig. 8, b).The mature pre-Riphean continental crust was transformed into a "suboceanic" type during the Lower and Middle Riphean.This process was illustrated in [Karsten et al., 1997] based on the analysis of the geochemistry of volcanic rocks of the Mashak Fm.These facts allow us to draw an analogy with the geodynamic conditions immediately preceding and accompanying the opening of rift structures and the beginning of spreading, for example, of the Afar branch of the East African Rift system.For comparison, Fig. 7 shows the points of the average composition of igneous rocks of the Red Sea Rift and the Gulf of Aden.In our case, there is no full section of the oceanic crust (or its fragments), as in the structures noted above.The existence of the Nazyam rift-graben was not very long.Apparently, the structure closed in the range of 1250-1150 Ma, possibly during one of the early stages of the Grenville activity; the nature of the latter in the Urals was discussed in [Krupenin, 2004;Maslov et al., 2014].Starting from about 1200 Ma, from the east of EEP, the primary "rift-spreading" structures opened, and, subsequently, a large Riphean-Vendian oceanic basin was formed.
The synchronous involvement of different magmatic sources in the Middle Riphean riftogenic magmatism in BMA, the enriched mantle, the significantly depleted mantle (or with the crustal substance) seem to give evidence of penetration of a mantle plume into the lower part of the continental lithosphere, accompanied by partial melting and contaminationof the crustal substrate.The enrichment of mafic rocks with Fe and Ti, HFSE, the presence in their composition of picrites (as well as dolerites) in sill-dike complexes, is an additional indicator that the Riphean riftogenic mafic rocks of BMA are related to the mantle plume.
Our data support the idea of R. Ernst [Ernst, 2014] and V. Puchkov [Puchkov, 2013] about the joint activity of plume-and plate-tectonic processes on the edge of the Columbia supercontinent.The rocks of the Middle Riphean sill-and-dike swarms and theintrusive and volcanic complexes of BMA formed a typical LIP due to the plume influence.It is known [Barrat et al., 1998;Puchkov, 2013;Ernst, 2014;Puchkov, 2016;etc.]that some igneous rocks in South America, Africa and China have a similar genesis and datings.So the members of this LIP are globally widespread.The plume initiated the beginning of intraplate rifting and, subsequently, the break-up of the continental crust (Nazyam rift, see Fig. 8, b).
For comparison, the εNd values for different in composition and genesis gabbro-ultramafic complexes of the eastern paleoceanic sector of the Urals, mainly from the Main Uralian Fault zone, are shown in the left corner of Fig. 8 (according to the literature).Then, the volcanics and granitoids of the western paleocontinental slope of the Northern, Middle and Southern Urals Fig. 7. Plots of Nb/Yb versus Th/Yb [Pearce, 2008] for magmatic rocks of BMA.
The range of 1250-700 Ma was a prolonged and relatively amagmatic period in the development of the western continental margins of the Rifean-Vendian oceanic basin (see Fig. 8).There is a single dating in this time interval.This is the age of the metadolerite from the sill (1079±41 Ma [Petrov et al., 2014] intruded by quartzite sandstones of the Isherim Fm. (Late Riphean).This metadolerite is considered to be derived from substantially depleted mantle (εNd +6.57).
Magmatic rocks of this age are unknown on the eastern slope of the Urals either.Probably, this gap corresponds to the stage of the most active opening of the Riphean-Vendian oceanic basin.Apparently, magmatism was mainly concentrated in the structures of the Middle Oceanic Ridge, and its traces hardly remained due to subsequent subduction of the oceanic crust.
The isotope-geochemical (εNd-T) and metallogenic evolution of intrusive magmatism in the Uralian Mobile Belt in the age range of 1400-250 Ma is schematically shown in Fig. 9.This scheme was compiled using the I (right-hand side of this figure) -massifs, Ti-Fe-V and rare-earth metal deposits of BMA; II -lherzolite complexes of the Southern Urals (Nurali, Mindyak) and mafic rocks of the Kvarkush anticline; III -massifs and Fe-V-Ti-Pt deposits of the Uralian Platinum Belt; IV -ophiolitic massifs (Cr, Pt); V -island-arc associations (Cu, Zn, Au, Pb); VI -continental-margin massifs and deposits (Cu-Mo, Au-W, Fe-Ti-V), VII -late orogenic massifs and ore-bearing complexes (Mo-W, Be-Ta-Li, etc.).DM -depleted mantle evolution trend.Green diamonds show the positions of the Nyazam metabasalts.
At the Riphean stage of the geodynamic evolution of the western slope of the Urals, several successive stages of rifting destruction took place at the eastern margin of EEP.The lithospheric mantle derivatives were depleted (Fig. 9, right-hand side; field I).As a result, there is a gradual increase of the εNd values (-2…+7), and a decrease in the primary 87 Sr/ 86 Sr ratio from 0.7060 to 0.7030-0.7025.As seen from Fig. 9 (left-hand side), the depletion of lithospheric mantle derivatives continued until the Early Paleozoic (Fields II-IV, the grey arrow).
Paleozoic intrusive magmatism of the Urals in the range of 440-250 Ma is mainly attributed to subduction and collision.This period is characterized by a different evolution pattern of isotope-geochemical parameters.The εNd-values decrease from +8 to -14, and increase in the 87 Sr/ 86 Sr primary ratio from 0.7025 to 0.710 and above.This evolution is characterized by discontinuous changes of island arc magmatism (440-360 Ma; field V), mantle-crustal magmatism of marginal-continental type (360-290 Ma; field VI), and the late collisional crustal and mantle-crustal granite magmatism (290-250 Ma; field VII).
The subsequent motion of the lithospheric plates and transformation of the continental margins resulted in a change of the isotopic-geochemical parameters of the igneous rocks and related [Khanchuk et al., 2016].These settings in the Southern Urals caused formation of such specific magmatic complexes as, for example, the Early Carboniferous Magnitogorsk gabbro-granite and the Early Permian Stepninsk monzodiorite-granite series.Their associated skarn-magnitite ores of the Magnitogorsk and Kachar deposits have higher εNd values (+5…+6, field VI).This trend is shown by the Fe arrow in Fig. 9.
Regular changes in the isotopic and geochemical parameters during the closure of the Uralian paleocean and the formation of the continental crust of the Uralian orogen were accompanied by synchronous changes in the composition of endogenous mineralization as follows: Cu-pyrite, Au-Cu-porphyry and Cu-Mo porphyry mineralization (island-arc stage)  skarn magnetite and Au-sulfide-quartz (with scheelite) mineralization (continental-margin and transform-collision stages  Mo-W and rare-metal mineralization (Be, Ta, Mo, Li, etc.) resulted from the late collisional mantle-crustal and crustal granite magmatism.
The general trend of the isotopic-geochemical evolution of magmatism at different time intervals can indicate the inheritance of the processes of oceanic generation in the Riphean-Vendian and the subsequent Paleozoic stage of evolution in the study area.

CONCLUSIONS
In the Lower-Middle Riphean, the eastern margin of EEP (i.e. the western paleocontinental sector of the Urals) underwent several successive stages of intraplate rifting.The riftogenic activity reached its maximum in the Middle Riphean, when the LIP was formed.Its genesis may be related to the mantle plume.The latter initiated the process of opening of the Nazyam rift (1250-1150 Ma) followed by the complete break-up of the continental crust.
During the Middle Riphean, the igneous rocks of BMA varied from E-MORB and OIB to the rocks similar to N-MORB.The εNd values vary from negative (-6) for the derivatives of the mature continental crust to positive (+4…+5 and above) for the Nazyam metabasalts, and thus give evidence of the lithosphere mantle depletion with time.Apparently, the relatively amagmatic period (1250-700 Ma) indicates that the period of opening and existence of the Riphean-Vendian ocean was prolonged.Acording to the data on the Northern and Polar Urals rocks, the closure of an oceanic basin (Protouralian or Timanian ocean) took place at the Late Vendian -Early Cambrian [Puchkov, 2010;Kuznetsov et al., 2014].Later on, the Uralian paleoocean was opened, and its evolution lasted from the Ordovician to the Permian and completed due to collision processes.
T a b l e 1. Contents of major (wt %) and trace (ppm) elements in the representative samples of basalts from BMA Т а б л и ц а 1.
T a b l e 4. Sm-Nd isotopic compositions of the rocks sampled from the Sibirka deposit and the adjacent intrusive complexes Т а б л и ц а 4.

Fig. 9 .
Fig. 9.The evolution of εNd isotope ratios during magmatic and ore-forming processes in the Middle and Southern Urals in the age interval of 1440-240 Ma.

Content of major (wt %) and trace (ppm) elements in the representative samples of gabbro and granites from BMA
T a b l e 2.

K-Ar возраст амфибола из метабазальтов Назямских гор*
The sample was collected from the Chernaya Mountain, 8.5 km to the west of the Zlatoust city, on the watershed near the Zlatoust-Magnitka road.The analysis was conducted at Centre Geoanalitik, IGG UB RAS(analysts A.I. Stepanov, and B.A. Kaleganov).