Lithos xxx (2012) xxx–xxx

LITHOS-02821; No of Pages 16

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The ca. 1380 Ma Mashak igneous event of the Southern Urals

Victor N. Puchkov a,⁎, Svetlana V. Bogdanova b, Richard E. Ernst c,d, Vjacheslav I. Kozlov a,1,Arthur A. Krasnobaev e, Ulf Söderlund b, Michael T.D. Wingate g,h,Alexander V. Postnikov f, Nina D. Sergeeva a

a Institute of Geology, Ufimian Scientific Centre, Russian Academy of Science, Russiab Department of Earth and Ecosystem Sciences, Lund University, Swedenc Ernst Geosciences, Ottawa, Canadad Department of Earth Sciences, Carleton University, Ottawa, Canadae Institute of Geology and Geochemistry, Uralian Department of Russian Academy of Sciences, Ekaterinburg, Russiaf Gubkin Russian State University of Oil and Gas, Moscow, Russiag Geological Survey of Western Australia, 100 Plain St., East Perth, WA 6004, Australiah Department of Applied Geology, Curtin University, Kent St., Bentley, WA 6102, Australia

⁎ Corresponding author at: K. Marx St. 16/2, Institute oTel.: +7 9173442601, +7 347 2447807; fax: +7 347 2

E-mail addresses: puchkv@ufaras.ru (V.N. Puchkov),(S.V. Bogdanova), richard.ernst@ernstgeosciences.com (krasnobaev@igg.uran.ru (A.A. Krasnobaev), Ulf.Soderlunmichael.wingate@dmp.wa.gov.au (M.T.D. Wingate), apo(A.V. Postnikov).URL: http://ig@ufaras.ru (N.D. Sergeeva

1 Deceased.

0024-4937/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.lithos.2012.08.021

Please cite this article as: Puchkov, V.N., et a10.1016/j.lithos.2012.08.021

a b s t r a c t

a r t i c l e i n f o

Article history:Received 8 December 2011Accepted 29 August 2012Available online xxxx

Keywords:UralsVolgo-UraliaMesoproterozoicMashak eventU–Pb datingZircons

A review of the geochronology, geochemistry and distribution of the 1380 Ma Mashak Large Igneous Province(LIP) of the eastern margin of the East European craton indicates a potential link to a major breakup stage ofthe Mesoproterozoic supercontinent Columbia (Nuna), link to a major stratigraphic boundary (Lower–MiddleRiphean), and economic significance for hydrocarbons and metallogeny. Specifically, the Mashak event likelyhas much greater extent than previously realized. Two U–Pb baddeleyite (ID TIMS) age determinations on dol-erite sills obtained from borehole (Menzelinsk–Aktanysh-183) confirm the western extent of the Mashak eventinto the crystalline basement of the East European Craton (1382±2 Ma) and into the overlying Lower Ripheansediments (1391±2 Ma), and the imprecise ages reported elsewhere indicate the possible extension into theTiman region, with an overall areal extent of more than 500,000 km2 (LIP scale). It has tholeiitic compositionsand is associated with breakup on the eastern margin of the craton – in addition, precise SHRIMP zircon agesof 1386±5 Ma and 1386±6 Ma (this paper) provide confirmation of previous approximate 1380–1383 Mazircon age determination of the same formation, and suggest an age of ca. 1.4 Ga for the Lower/Middle Ripheanboundary which was formerly considered to be 1350±10 Ma. Contemporaneous magmatic rocks in the north-eastern Greenland part of Laurentia (Zig‐Zag Dal andMidsommerso formations) and Siberia (Chieress dykes andother dolerites) together with theMashak event are suggested to be fragments of a single huge LIP and to corre-spond to breakup stage of the Columbia (Nuna) supercontinent. The Mashak LIP also has some significance, atleast in Volgo-Uralia, for hydrocarbons and metallogeny.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

1.1. Objectives and geological background

TheEast European craton is an important “puzzle piece” in the effortsto reconstruct the proposed ca. 1.7–1.4 Ga Columbia (Nuna) and ca. 1.0–0.7 Ga Rodinia supercontinents (e.g., Li et al., 2008;Meert, 2012 and ref-erences therein). The craton consists of three crustal blocks that came

f Geology, 450 077 Ufa, Russia.730368.Svebog@yandex.ruR.E. Ernst),d@geol.lu.se (U. Söderlund),stnikov@mtu-net.ru).

rights reserved.

l., The ca. 1380 Ma Mashak i

together in the Paleoproterozoic: Fennoscandia (including the Kareliancraton), Sarmatia, and Volgo-Uralia (Fig. 1) (e.g. Bogdanova et al., 2008).

Of great importance in the history of Volgo-Uralia and the closelyconnected western part of the Southern Urals, is the ca. 1380 MaMashak magmatic event. This event is represented by a NNE-trendingbelt of volcanic rocks in the Southern Urals, interpreted as a rift complex(e.g., Ernst et al., 2006; Parnachev, 1981; Puchkov, 2000). It is associatedwith the Kama Belsk aulacogen (rift), and also includes a NW-trendingdyke swarm (Fig. 1) potentially related to this event on the basis of K–Arages (Postnikov, 1976). Another, smaller aulacogen – Sernovodsk–Abdulino – originated in Mashak time and branches from the Kama–Belsk aulacogen in a sublatitudinal (E-W) direction. The Mashak eventhas been best characterized in the Bashkirian anticlinorium of theSouthern Urals, wherein basement rocks are exposed (Fig. 2). Theevent divides the Riphean succession into two major sequences sepa-rated by a significant regression and break in platformal sedimentation

gneous event of the Southern Urals, Lithos (2012), http://dx.doi.org/

Fig. 1. Regional geology of the eastern part of the East European craton (Baltica) showing its three components: Fennoscandia, Volga-Uralia, and Sarmatia (insert). The basemap is afterBogdanova et al. (2008), to which has been added the location of the drill hole (marked by the blue circle and labeledM-A-183) and dykes of NW trendwith possible ages between 1600and 1000 Ma (Postnikov, 1976). Rifts, aulacogens, basins, and internal parts of passive margins are labeled as follows: K–B = Kama–Belsk aulacogen, В = Bashkirian anticlinorium(S. Urals), S–A = Sernovodsk–Abdulino aulacogen, Pa = Pachelma aulacogen, MR = Mid-Russian aulacogen, K–D = Kandalaksha–Dvina graben, Mez = Mezen rifts.

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(Puchkov, 2010; Fig. 3). The subdivisions Lower, Middle and UpperRiphean correspond broadly to the use of Stenian (1000–1200 Ma),Ectasian (1200–1400 Ma, and Calymmian (1400–1600 Ma) inGradstein et al. (2004) and Ogg et al. (2008). A detailed comparisonof the Riphean classifications with this alternative classification isprovided in Puchkov (2010).

Here we wish to review the characteristics of this Mashakmagmatic event and associated rifting, metallogeny, and potentiallinks with coeval magmatism on other blocks.

2. Mashak event

2.1. General stratigraphy

The NNE‐trending belt of Middle Riphean volcanic andvolcano-sedimentary rocks of the Mashak Formation is parallel tothe typical Uralian strike (20–30 degrees NE), and is more than270 km long and 1–12 km wide (points 1–3, 6–8 and 12 in theFig. 2). The Mashak Formation, up to 3000 m thick, thins out and

Please cite this article as: Puchkov, V.N., et al., The ca. 1380 Ma Mashak i10.1016/j.lithos.2012.08.021

completely disappears at a distance of 10–20 km tо the west,suggesting a buried shoulder of a deep NNE-trending graben; thecoarse grain size of many sedimentary rocks and geochemistryof the volcanics support this suggestion. The Formation in itsstratotype (Mashak Range) and hypostratotype (Bolshoi ShatakRange) is represented by basalts, rhyodacites, rhyolites, rhyoliteand basalt tuffs, tuff breccias and terrigenous rocks: conglomerateswith quartzitic pebbles and boulders, quartzites, siltstones andblack shales; volcanic rocks predominate in the lower part butare also present in the middle parts of the section. Rhyolitesoccur 400–600 m above the base of the section (Fig. 4). A moredetailed description of the type section follows below. In thenorthern part of the Bashkirian anticlinorium (Fig. 2, points 8 and12) analogs of the Mashak Formation are metamorphosed togreenschist and amphibolite facies and are known as the KuvashFormation. The most comprehensive section of the Mashak Forma-tion is found in the No. 3 Karagas borehole (Fig. 2, point 3). Theborehole penetrates basic metavolcanic rocks of extrusive andvolcanoclastic facies.

gneous event of the Southern Urals, Lithos (2012), http://dx.doi.org/

Fig. 2. Geological map of the Bashkirian anticlinorium, a basement window in the Urals. Note the distribution of Middle Riphean units. 1— sample 898, rhyolite, Shatak range;2 — sample 125, rhyodacite, Shatak range; 3 — No3 Karagas borehole, basalts; 4 — Akhmerovo granite, 5 — Beloretsk eclogite, 6 — sample 109, Dungansungan mtn., Mashakrange; 7 — sample 323, rhyolite, Kuzyelga river, Mashak range; 8 — sample 906, rhyolite, Berezyak river, Kuvash complex; 9 — Main Bakal dolerite dyke; 10 — Berdyaushrapakivi granite and associated rocks; 11 — Kusa–Kopan gabbro intrusion, 12 — Ryabinovo granites, 13 — Kusa dolerite sill, 14 — Maly Navysh mtn., subvolcanic daciteporphyrite.

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There are many different schemes of subdivision of the Formationinto Subformations, and we prefer that of Kozlov et al. (2007 andreferences therein) who divided it into three Subformations: Lower,Middle and Upper.

Please cite this article as: Puchkov, V.N., et al., The ca. 1380 Ma Mashak i10.1016/j.lithos.2012.08.021

In the Shatak Range, the Lower Subformation overlies black andgray quartz siltstones and sandstones of the Jusha Formation of theupper part of the Lower Riphean. In some places there is an angularunconformity at the bottom of the Mashak Formation, though it

gneous event of the Southern Urals, Lithos (2012), http://dx.doi.org/

Fig. 3. Chronostratigraphic correlations of the Riphean and Vendian (Meso- and Neoproterozoic) sedimentary complexes of the Volgo-Uralian area and the Urals. Locations are as follows: theSouthern Urals correspond to the Bashkirian anticlinorium (Fig. 2), and Sernovodsk–Abdulino and Kama–Belsk aulacogens are located a few hundred km to the west and northwest, respec-tively (Fig. 1). Compiled byV.N. Puchkov andV.I. Kozlov (Puchkov, 2010): Stratigraphic units in the platform: Lower Riphean, Kyrpinsk series: R1pk— Prikamian subseries; Oryebash subseries:R1kl—Kaltasinian formation (three subformations); R1kb—Kabakovo formation;Middle Riphean, Serafimovo series. Formations: R2nd—Nadezhdino, R2tk— Tukaevo, R2—Olkhovka, R2us—Usa. Upper Riphean Abdulino series; Formations: R3ln— Leonidovo, R3pr— Priyutovo, R3sn— Shikhan, R3lz— Leuza. Formations in the Southern Urals. Lower Riphean: R1ai— Ai, R1st— Satka,R1bk — Bakal; Middle Riphean: R2ms — Mashak, R2zg — Zigalga, R2zk — Zigazino-Komarov, R2av — Avzyan; Upper Riphean: R3zl — Zilmerdak, R3kt — Katav, R3in — Inzer, R3mn — Minyar,R3uk— Uk, R3kr— Krivoluk. The Uppermost (Latest) Riphean— R4ar— Arshinian. V as — Vendian, Asha series. Numbers in circles — seismic reflectors.

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may be a result of tectonic deformations localized along this boundaryultilizing it as a plane of weakness. The base of the Formation is repre-sented by coarse-grained, polymictic sandstones with layers of poorlysorted conglomerates of a variable thickness. The succession from silt-stones to sandstones and then conglomerates is regressive. The firstflow of green, chloritized basalts appears at 60–70 m above the baseof the Formation. Columnar jointing is characteristic in some places.The next three flows, 15–25 m thick, are divided and overlain by con-glomerates, 15–20 m thick, with quartz sandstone pebbles. Higher up,a thick (230 m) layer of rhyodacites appears; the rocks have an aphyricbase and large porphyritic crystals of feldspar. These rhyodacites aretraced along the whole belt of outcrops of the formation, extendingfrom the Shatak to the Kuvash ridges, for a distance of almost 200 km.The rhyolites are overlain by the next flow of grayish-green, fine-grained metabasalt, with epidote and chlorite, partly amygdaloidaland 200 m thick. The upper 375 m of the Lower Mashak Subformationare poorly exposed, and represented by carbonaceous shales, quartzsiltstones, thin-grained polymictic sandstones. The total thickness ofthe Lower Mashak Subformation is 800–1000 m.

The Middle Subformation in its lower part is representedby uneven alternation of poorly exposed polymictic sandstones,metabasalts, quartz sandstones, tuff sandstones and siltstones, withan average thickness of about 700 m. The contact with the LowerMashak is unexposed, but elsewhere a transitional contact isdescribed. Above this part of the section, a layer of conglomerates,30 m thick, is well exposed. The pebbles are composed of quartz

Please cite this article as: Puchkov, V.N., et al., The ca. 1380 Ma Mashak i10.1016/j.lithos.2012.08.021

sandstones, with a matrix of polymictic sandstones. The conglomer-ates grade upward to a 45 m-thick sandstone unit. The sandstonesare overlain (with no visible contact between them) by flows(#6 and #7), represented by aphyric to fine-grained, partly amygda-loidal basalts, unevenly chloritized, and locally with epidote, 70 mthick. They are overlain by more than 50 m of fine-grained polymicticsandstones, with layers of siltstones, and subordinate tuff sandstones;a layer of sandstones with rare pebbles is at 10 m above the base ofthe layer. The layer with rare pebbles marks the upper part of theSubformation. The Middle Mashak Subformation has a total thicknessof 900 m. Basalt layers #6 and #7 are the uppermost volcanics in theentire Mashak Formation.

The Upper Mashak Subformation is represented by an unevenalternation of polymictic and quartz sandstones and siltstones, carbona-ceous shales, layers of tuff siltstones and (rare) limestones, and limydolomites. The total thickness is 1100–1200 m. The upper boundaryof the formation is marked by quick transition to the thick-layeredlight-gray to white quartzite sandstones of the Zigalga Formation. Thetotal thickness of the Mashak Formation is here 3000–3100 m (Fig. 4).In summary, (Kozlov et al., 2007), the general features of the MashakFormation are the following:

1) It is represented by sedimentary, volcanosedimentary and volcanicrocks.

2) Coarse-grained rocks play a conspicuous role; conglomeratesmark the interval close to the base of the Formation.

gneous event of the Southern Urals, Lithos (2012), http://dx.doi.org/

Fig. 4. Schematic stratigraphic column of the Mashak Formation, Middle Riphean of the Southern Urals (after V.I. Kozlov).

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3) Some intervals of the terrigenous successions have a regressivecharacter: siltstones and shales at their bases are followed upwardby sandstones and then conglomerates.

4) Detrital fragments of Lower Riphean rocks are present in theconglomerates; higher up in the section, they are substitutedby fragments of underlying units of the Mashak Formation.

Please cite this article as: Puchkov, V.N., et al., The ca. 1380 Ma Mashak i10.1016/j.lithos.2012.08.021

5) Facial changes of the Formation are expressed in relative variations inthe proportion of volcanogenic and coarse-grained sedimentary rocks.

6) In the terrigenous rocks, which overlie the volcanics, theadmixture of volcanogenic material is gradually reduced upward,leading to a complete disappearance by the upper third of theUpper Subformation.

gneous event of the Southern Urals, Lithos (2012), http://dx.doi.org/

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7) The variation in the thickness of the Formation in differentsections (ranging from 1000 to 3100 m) depends mainly on thethickness of volcanogenic and coarse-grained sedimentary rocks.

8) The total thickness of the volcanic rocks in the Lower, Middle andUpper Mashak formation is, up to 650, 550, and 0 m, respectively,for a total cumulative maximum thickness of up to 1200 m.

The general time of formation of theMashak succession is difficult tocalculate, because the ages of volcanic rocks other than the rhyolites(lower Subformation) are poorly constrained.

2.2. Geochemistry

The geochemistry of the volcanics in the Mashak units is summa-rized from Ernst et al. (2006), and a selection of plots from the mainvolcanic sequence (Bolshoi Shatak Range and Karagas drillhole) isshown in Fig. 5. Additional chemistry of the dated rhyolites, from ShataktoKuvash area (Krasnobaev et al., submitted for publication) is added toFig. 5 as K.

The majority of the Mashak volcanics are represented by low- tomedium- K, mainly tholeiitic basalts, while acid volcanic rocks arerecorded only in the lower parts of the formation and comprise10–15% of the total thickness of volcanic rocks. They are representedby rhyolites, trachy-rhyolites and dacites. The volcanic suite is charac-terized by a negative Sr anomaly, and negative K and Ba anomalies,though this is probably a consequence of the widespread low grademetamorphism. REE patterns are moderately enriched chondritenormalized LREE/HREE, La/Ybn=2 to 4, and there is a weakly negativeTa–Nb anomaly. With respect to geochemical fingerprinting, theMashak magmatic events have low La/Nb ratio ranging from 0.8–1.5,low to moderate Th/Tan (=2 to 3) and La/Ybn ratios (mainly =3.3 to6), La/Smn (=2 to 4) and low Gd/Ybn ratios (=1.5 to 1.9). In terms oftectonic setting they plot aswithin-plate and in terms ofmantle sourcesthey plot as Depletedwith significant addition of Enriched and Recycledsources (see Condie, 2003). More felsic samples have a distinctlydifferent pattern consisting of negative K, Ba, Rb, Sr, P and Ti anomalies(Ernst et al., 2006); there is a difference between mafic and felsic units,as noted previously byKarsten et al. (1997). Felsic samples have numer-ous elemental anomalies (e.g. strongly negative Ti, P and Sr) that are notpresent (apart from aminor negative Sr anomaly) in themafic samples.At the same time we know that both felsic and mafic magmatism arevery similar in age. Therefore, the origin of felsic component is probablydue to crustalmelting under the influence of ascending basalticmagma.

The ‘grain’ of the Mashak formation is parallel to the present Urals.The Mashak formation sediments (including basal conglomerates)and mafic volcanic rocks define a rift setting. The setting appears tobe Within Plate, but could also be back arc basin. There is no subduc-tion signature. A plume source is plausible.

Themain objectives of this research are the acquisition of precise in-formation on the age of the Mashak volcanics and associated intrusiverocks, elaboration of the distribution of magmatic rocks of the Mashakevent, and global correlation of the event in support of constrainingcontinental reconstructions.

2.3. Previous geochronology

Until recently, the age of Mashak volcanic rocks, situated at the bot-tom of the Middle Riphean in the standard Proterozoic section of theSouthern Urals, was based on Rb–Sr (whole-rock, 1346±41 Ma) andimprecise U–Pb (zircon, “classic” multi-grain method, 1350±30 Ma)determinations (isotopic ages discussed in this paper are quoted with2σ or 95% confidence intervals, unless noted otherwise). On this basisthe age of theMashak Formation and the base of the Yurmatinian serieswere constrained to 1348±30 Ma (Krasnobaev et al., 1985). This datewas for a long time accepted as the lower boundary of the MiddleRiphean. A similar Pb-evaporation 207Pb/206Pb single zircon date of ca.

Please cite this article as: Puchkov, V.N., et al., The ca. 1380 Ma Mashak i10.1016/j.lithos.2012.08.021

1350 Ma was obtained for an eclogite facies mafic dyke (Fig. 2, point 5in the Beloretzk metamorphic complex, Glasmacher et al., 2001).

In 2006–2008, new geochronology of the volcanic rocks was pro-duced at VSEGEI, St. Petersburg, using SIMS SHRIMP method, and thespread of these ages was broad. The new dates either supported theabove-mentioned reference date, or gave older values (1337±19,1347±15, 1357±42, 1366±6, 1366±10, 1370±16 and also,1436±14, 1465±22, 1478±22, 1482±14, 1494±14, 1508±19,1528±43, 1538±12) (Field trip guide on the Proterozoic of theSouthern Urals, 2006; Krasnobaev et al., 2008a; Ronkin et al., 2007).These dates were very difficult to reconcile with well-founded tradi-tional geology-based ideas and chronometry data concerning a timingof magmatism, which was thought to be short, and led to criticisms ofthe method itself (Maslov and Ronkin, 2008), and indicated thatadditional dating was required.

The first reliable dates for Mashak volcanism (Fig. 6) were obtainedfor zircons of two samples (rhyolite and rhyodacite, Fig. 2, points 1 and7) located ca. 300 m above the basal conglomerates of the MashakFormation. The zircons were analyzed by the CA-IDTIMS methodand yielded virtually identical results of 1381.1±0.7 Ma (weightedmean 207Pb/206 Pb date; MSWD=0.7) and 1380.2±0.5 Ma (weightedmean 206Pb/238U date; MSWD=1.4) (Fig. 6), (Puchkov et al., 2009). Аnew series of zircon samples was again dated using SHRIMP atVSEGEI, in 2010. This time, the process of choosing the crystals andpoints within them for analysis was conducted more carefully, and itwas found (Krasnobaev et al., in press) that their volcanic parts can besorted out reliably, and the ages of the volcanic zircons in differentsamples are synchronous (1383±3 Ma on average). The igneouszircons or their fragments can be easily recognized by the presence ofmelt inclusions (Fig. 7). Inherited zircons are also present, and theseare recognized by characteristic zonal structure, reflecting equilibratedconditions of zircon growth, comparable to abyssal conditions ofgranitoids and these crystals experienced some resorption. They yieldages of about 1600 Ma (Fig. 8D, sample 125). The age of the Mashakvolcanism (1383±3 Ma, Fig. 8E) was determined as a mean valuefrom four samples of rhyolites and dacites (see also Fig. 2, points 1, 2,7 and 8), obtained from 400 m above the base of the Lower MashakSubformation (Fig. 4). Admitting the rate of sedimentation to be about25 m/Ma (a rather arbitrary estimate), the age of the base of theMiddleRiphean (Yurmatinian) and the time of an initial (non-volcanic) riftingwith the deposition of the first layers of conglomerates could be about1400 Ma (Krasnobaev et al., submitted for publication).

Mashak magmatism was not restricted to volcanic rocks of theMashak Formation preserved in the Bashkirian anticlinorium. LowerRiphean rocks to the west (and to a lesser degree to the east) of theMashak belt are intruded by dolerite and dacite dykes and also gabbroand granite massifs. Accurate ages have been obtained recently fromBerdyaush intrusive suite rocks, cutting the Satka Formation (Fig. 2,point 10), mainly by SHRIMP methods (Ronkin et al., 2005). Numerousdates acquired by SHRIMP and ID-TIMSmethods confirm that themainpulses of the Berdyaush Pluton occurred at ca. 1370 Ma although ayounger suite of nepheline syenite dykes cutting the intrusion yieldeda Neoproterozoic age (792±70 Ma according to SHRIMP zircon analy-ses) (Krasnobaev et al. (2011). Regarding the Berdyaush suite, a dateof 1395±20 Ma was obtained from a gabbro (probably a xenolith froma co-genetic intrusion), 1372±12 Ma from a quartz syenite–diorite,1369±13 Ma from a rapakivi granite and 1373±21 and 1368.4±6.2 Ma from a nepheline syenite (Ronkin et al., 2005; Sindern et al.,2003). A similar SHRIMP date of 1369±6 Ma was obtained for othersamples of the same granites (Field trip guide on the Proterozoic ofthe Southern Urals, 2006). Ronkin et al. (2007) added two moreSHRIMP dates of 1389±28 Ma for gabbro and 1370±4.6 Ma forrapakivi granites. Other large intrusions thought to be comagmatic,were also dated by U–Pb SHRIMP. The Kusa–Коpan gabbro intrusion(Fig. 2, site 11) was dated at 1385±25 Ma, and Ryabinovsk granites, incontact with this gabbro (site 12), as 1386±40 Ma (Krasnobaev et al.,

gneous event of the Southern Urals, Lithos (2012), http://dx.doi.org/

Fig. 5. Geochemical diagrams for volcanics of the Mashak Formation: (A) Zr/TiO2 vs Nb/Y classification diagram of Winchester and Floyd (1977) as revised by Pearce (1996);(B) TiO2 vs Mg#; (C) Zr vs Ti/100 vs Y*3 ternary classification diagram after Pearce and Cann (1973); (D) Ti vs V after Shervais (1982); (E) Log Zr/Nb vs Nb/Th after Condie,1997, 2003. DEP (depleted), REC (recycled), EN (enriched). OIB is ocean island basalt, PM is primitive mantle, UC is upper crust. NMORB is normal mid1ocean ridge basalt.OPB is ocean plateau basalts, MORB is mid1ocean ridge basalt, ArcB is arc basalt; (F) Log Th/Ta vs log La/Yb after Condie (2003). Labels as in Fig. 5e; (G) Log Nb/Y vs log Zr/Y(after Fitton et al., 1997 as modified by Condie, 2003). (H and I) Spidergram normalized using chondrite normalization of Thompson et al. (1983); labels as in Fig. 5e; (J) Gd/Yb,versus La/Sm; (K) chondrite normalized. (A)–(J) data after Ernst et al. (2006; 20 mafic samples from sites 18, 19 and 24; 3 felsic samples from site 14), (K) data (16 samples,basic and felsic, the latter from sites 323,898 and 906) after Krasnobaev et al. (submitted for publication).

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Please cite this article as: Puchkov, V.N., et al., The ca. 1380 Ma Mashak igneous event of the Southern Urals, Lithos (2012), http://dx.doi.org/10.1016/j.lithos.2012.08.021

Fig. 6. Concordia diagram and summaries of CA-IDTIMS isotopic data, for samples К-323 and К-808 (Puchkov et al., 2009).

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2006). Single zircons from the Akhmerovo granites and eclogites in thecore of the Beloretsk dome (Fig. 2, points 4 and 5), situated to the eastof the Mashak belt, were dated by IDTIMS as Mashak-related, althoughthe results were partly misinterpreted (Glasmacher et al., 2001): theseauthors considered that the emplacement age was ca. 900 Ma, andthat ca. 1350 Ma zircons of Mashak age were inherited. Subsequently,SHRIMP work on the Akhmerovo granites confirmed an emplacementage of 1381±23 Ma (Krasnobaev et al., 2008b). A 40Ar–39Ar age of1360±9 Ma was acquired for a sill near the town of Kusa; this sill isthe largest of many, that intrude the Lower Riphean Satka formationin this area (Ernst et al., 2008a) (Fig. 2, point 13). The most precisedate for the Mashak intrusive event is 1385.3±1.4 Ma (U–Pbbaddeleyite; Ernst et al., 2006) on the Bakal dolerite dyke (Fig. 2, point9), which had previously been dated at 1360±35 Ma (Rb–Sr)(Ellmies et al., 2000).

The extent of Mashak magmatism in the Bashkirian anticlinoriumhad been expanded by dating of some subvolcanic (?) rocks, previ-ously regarded as volcanics of the Lower Riphean Ai Formation atthe periphery of the Taratash uplift of the crystalline basement. Inparticular, a SHRIMP zircon age of 1400±10 Ma was reported fromsome volcanics of this area (Ronkin and Lepikhina, 2009). Somewhatearlier, in 2006, we determined a SHRIMP zircon age of 1362±17 Mafor a porphyritic dacite body from Maly Navysh Mountain (Fig. 2,point 14), surrounded by terrigenous rocks of the Ai Formation. Wealso dated Paleozoic dykes from this area (Puchkov et al., 2011).These facts show that the area around and within the Taratashblock experienced multiple magmatic events.

Volcanic rocks of the same age cannot be traced to the Middle Uralsbecause the Middle Riphean sections are not exposed there. However,in the Timan region, further northwest of the Urals, “Mashak” dateswere obtained for rocks within the Palyu-21 borehole at the NE slopeof the Timan uplift (point 4 in Fig. 13), although the presence of theMiddle Riphean sections has not been confirmed by stratigraphy.Diorites yielded a Rb–Sr date of 1360±31 Ma (Andreichev andLitvinenko, 2007). Recently, V.L. Andreichev (personal communication)determined a Sm–Nd mineral isochron (Pl+Bt+Amph) for the samediorites of 1369±59 Ma. A “Mashak” age is also suggested by K–Arages of 1375–1330 Ma for amphiboles fromMiddle Timanian dolerites(Andreichev and Litvinenko, 2007).

3. New U–Pb geochronology

New U–Pb ages are obtained from two areas from the southernUrals and from the Volga-Uralian platform. The former providetighter constraints on the age duration of the volcanic portion ofthe event while the latter ages confirm the extent of the event tothe west.

Please cite this article as: Puchkov, V.N., et al., The ca. 1380 Ma Mashak i10.1016/j.lithos.2012.08.021

3.1. Samples from the Southern Urals

A previous age of 1465±22 Ma, determined in 2006 by theVSEGEI lab for a dacite–rhyolite sample P125 (=K125), from theLower Mashak Formation of Matveev Zalavok area in the BolshoiShatak Range (see point 2 in Fig. 2), needed to be verified, becauseit seemed to be too old and there was a suspicion that some olderinherited zircons had been dated. A second sample, P109, from theDungansungan rhyolite (“liparite”) at Dungansungan mountain(also the Lower Mashak), situated to the east of the town ofMezhgorye (see point 6 in Fig. 2), had not previously been analyzed.These two samples represented opportunities to clarify the olderage limit of the Middle Riphean (Yurmatinian).

Zircons from both samples were separated at Lund University usingthe technique of Söderlund and Johansson (2002), and analyzed usingthe SHRIMP II ion microprobes at the John de Laeter Centre for MassSpectrometry at Curtin University in Perth, Australia. Detailed analyticalprocedures are described by Wingate and Kirkland (2010). An initialsession (12 December 2010) was cut short owing to instrumentproblems, and the analyses were continued during a second session(22 January 2011). Owing to an apparent U/Pb calibration shift duringthe second session, data from this session were reduced in two parts(sessions 2 and 3 in Tables 1 and 2). U/Pb ratios and dates were deter-mined by calibration against the known 238U/206Pb ratio of the BR266zircon standard (559 Ma, 903 ppm 238U; Stern, 2001). Data werereduced using the recent versions of SQUID and Isoplot softwareof Ludwig (2003, 2009). Common-Pb corrections were applied toall analyses using the measured 204Pb/206Pb and contemporaneouscommon-Pb isotopic compositions determined according to the modelof Stacey and Kramers (1975). Details of calibration and reproducibilityuncertainties are included with the data tables.

The characteristics of zircons in the two samples are essentiallyidentical (Fig. 10A and B). The crystals are colorless to dark brown,anhedral to euhedral, equant to elongate, and up to 300 mm long.In cathodoluminescence images, concentric growth zoning is ubiqui-tous, and many zircons display lobate embayments indicative of mag-matic resorption. The inner zones of some crystals are significantlymore enriched in uranium than the outer zones. Irregular to sphericalinclusions of other minerals are common.

3.1.1. Analytical results

3.1.1.1. P109 (Dungansungan rhyolite, Mashak Formation). Twenty-fiveanalyses were conducted of the 24 zircons. Of these, twenty-three con-cordant to slightly discordant analyses of 23 zircons yield a weightedmean 207Pb/206Pb date of 1386±5 Ma (MSWD=0.62), interpreted asthe age of magmatic crystallization of the rhyolite (Fig. 10A). Oneanalysis (B11.1), which is 16% reversely discordant, indicates a high

gneous event of the Southern Urals, Lithos (2012), http://dx.doi.org/

Fig. 7. Cathodoluminescence image for zircon sample 125, with U and Th concentrations (ppm); T — age (Ma).

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proportion (>1%) of common 206Pb, and yields a 207Pb/206Pb date of1241±78 Ma (1σ), and is excluded from the main calculation. Oneanalysis (B1.1) of a zoned zircon core yields a 207Pb/206Pb date of1428±14 Ma (1σ), interpreted as the age of an inherited crystal.Including this result, which is 2.87 σ above the mean, yields aweighted mean date of 1387±5 Ma (n=24, MSWD=0.96), which isnot significantly different from our preferred age of 1386±5 Ma, asdiscussed above.

3.1.1.2. P125 (Shatak dacite–rhyolite, Mashak Formation). Twenty-fouranalyses were conducted of the 23 zircons. Of these, twenty-one essen-tially concordant analyses of 20 zircons yield a weighted mean 207Pb/206Pb date of 1386±6 Ma (MSWD=1.5), interpreted as the ageof magmatic crystallization of the dacite–rhyolite (Fig. 9B). Oneanalysis (B1.1) is excluded because it yields a 207Pb/206Pb date of

Please cite this article as: Puchkov, V.N., et al., The ca. 1380 Ma Mashak i10.1016/j.lithos.2012.08.021

1357±9 Ma (1σ), which is 3.0 σ below the mean. Two analyses(A2.1, A5.1) yield 207Pb/206Pb dates (1σ) of 1556±9 and 1474±27 Ma, interpreted as the ages of inherited components.

There is no apparent difference in the ages of the two samples,or between fractions A and B (c.f. online Tables 1 and 2) of eachsample. Combining the results for the two samples yields a weightedmean 207Pb/206Pb date of 1386±3 Ma (n=44, MSWD=1.03) for theLower Mashak Subformation magmatism (Fig. 11). In most crystalsanalyzed, there is no systematic or significant difference in agebetween inner, high-U zones and outer low-U zones. However, aminority of zircons do contain older cores, some of which are atleast as old as c. 1556 Ma. Therefore the analyses by the Australianlaboratory of sample P125 are almost identical to the most recentresult of 1385±15 Ma from the VSEGEI laboratory (Section 1.2;compare Figs. 10B and 8D). The only difference is that the Australian

gneous event of the Southern Urals, Lithos (2012), http://dx.doi.org/

Fig. 8. U–Pb ages of zircons of Mashak volcanic rocks (Krasnobaev et al., submitted for publication). А. Sample 323. Т=1387±12 Ma (n=9; MSWD=0.22; Р=0.64). B. Sample898. Т =1390±15 Ma (n=8; MSWD=0.24; Р=0.63). C. Sample 906. Т3=1382±11 Ma (n=8; MSWD=0.06; Р=0.64). D. Sample 125. Т1=1385±15 Ma (n=7; MSWD=1.5; Р=0.22). Т2=1597±27 Ma (n=2; MSWD=0.06; Р=0.82). Т3=536±11 Ma (n=2; MSWD=0.21; Р=0.64). E. Samples: 323, 898, 906, 125. Т=1383±3.0 Ma (n=31;MSWD=0.96; Р=0.33). Р — probability.

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SHRIMP result is more precise because it includes more analyticalpoints. This verification, particularly in view of the limitations of theprevious results (Section 1.2), makes us now very confident in thepresent results.

Unfortunately all the reliable isotope dates are confined to therhyolite interval of the Lower Mashak Subformation, and thereforewe cannot make a good estimation of a time interval and velocity ofaccumulation of the Mashak Formation as a whole, nor of an age forthe Middle Mashak subformation.

3.2. Samples from the Volga-Uralia platform area

Until now, none of themanydykes and sills found in the eastern partof the East European craton (EEC) had a precise age; previous impreciseK–Ar determinations ranged from 1.6 to 1.0 Ga (e.g., Postnikov, 1976).

Please cite this article as: Puchkov, V.N., et al., The ca. 1380 Ma Mashak i10.1016/j.lithos.2012.08.021

However, with new dating, the areal extent of the Mashak igneousevent is expanded to include not only the Lower Riphean in theBashkirian anticlinorium and Beloretsk dome, but also rocks withinthe EEC platform much further to the west. A precise U–Pb baddeleyitedate (this study) was obtained for a >100 m thick dolerite sill recov-ered from a drill hole at a depth of 2.2–2.3 km into the sedimentarybasin to the west of the Ural Mountains in Volga-Uralia. This resultand a matching companion age on a dolerite intruding the underlyingbasement at a depth of about 2590 m in the same drill hole (Figs. 1and 12) suggest that the Mashak magmatic rocks were emplacedwidely within the Volga-Uralia sedimentary basin. This adds furthersupport to the possibility that the widespread rifting and associatedmagmatism in the EEC at 1.4–1.2 Ga (e.g., Bogdanova et al., 2008)mainly belong to the 1390–1380 Ma Mashak event. The sedimentarybasin hosting the newly dated sill has oil and gas potential and the

gneous event of the Southern Urals, Lithos (2012), http://dx.doi.org/

Fig. 9. А. Cathodoluminescence image of representative zircons from sample P109(fraction A). Numbered circles indicate the approximate positions of the analysis sites.B. Cathodoluminescence image of representative zircons from sample P125 (fraction A).Numbered circles indicate the approximate positions of the analysis sites.

Fig. 10. А. U–Pb analytical data for zircons from sample P109: Dungansungan rhyolite,Mashak Formation. Dashed ellipses indicate analyses not included in the calculation ofthe weighted mean 207Pb*/206Pb* date, which is quoted with 95% confidence limits.B. U–Pb analytical data for zircons from sample P125: Shatak dacite–rhyolite, MashakFormation. Dashed ellipses indicate analyses not included in the calculation of the

207 206

11V.N. Puchkov et al. / Lithos xxx (2012) xxx–xxx

chance that a ca. 1390–1380 Ma thermal pulse has contributed to thehydrocarbon evolution is discussed later.

weighted mean Pb*/ Pb* date, which is quoted with 95% confidence limits.

3.2.1. Analytical resultsA deep drill hole, Menzelino–Aktanysh 183, penetrated more than

2.2 km of sedimentary rocks of the Paleozoic platform cover, theaulacogenic Lower Riphean Kyrpin series, and into the underlyingbasement (Figs. 1, 3, 12). A dolerite intrusion (probably a sill) wasidentified within Mesoproterozoic sediments at depths between2180 and 2300 m. Sample 183-1A was collected from the upperpart of the sill, at a depth interval of 2222.2–2230.4 m. A secondsample, 183‐5, was obtained from another dolerite intrudingbasement rocks at 2591.0–2593.6 m depth in the same drill hole.

Baddeleyites were separated from these two samples using thetechnique outlined in Söderlund and Johansson (2002). Analyticalwork, IDTIMS mass spectrometry and data reduction follow theprotocol by Nilsson et al. (in press), with isotopic data shown inTable 3. Three analyses of 6 to 10 baddeleyite grains in each fractionextracted from sample 183-1a plot close to concordia (all are lessthan 1% discordant), with one analysis plotting slightly aboveconcordia (Fig. 13A). Regression yields an upper intercept age of1391±2 Ma. Three fractions of baddeleyite extracted from sample183-5a are variably discordant (up to 8%) and yield an upper inter-cept age of 1383±2 Ma (Fig. 13B). The intercept ages are interpretedto date crystallization of basaltic magma for these two samples.

Please cite this article as: Puchkov, V.N., et al., The ca. 1380 Ma Mashak i10.1016/j.lithos.2012.08.021

4. Discussion

4.1. Link between Mashak event and breakup

The new dating adds to the areal extent of the 1385 Ma Mashakevent and confirms that it is a major event in the east of the EastEuropean craton, probably associated with its breakup from otherblocks. However it is not a direct evidence for the time of formationof a continental margin and new oceanic crust. The plagiogranites ofthe Timanian ophiolites in the Polar Urals have dates in the range734 to 670 Ma (Scarrow at al., 2001; Kuznetsov, 2009), but we cannotrule out that some ophiolites are older than that. We know also thatalong the Timan part of the craton the Riphean sediments demon-strate a facial transition from shallow-water shelf sediments in thesouth-west to more deep-water, basinal in the north-east (Puchkov,2010). But these sediments are thought to be Upper Riphean(younger than 1000 Ma), while the presence of the Middle Ripheanis suggested, but not proven there. Therefore the present data admita link between the Mashak event and breakup along the easternmargin of the East-European craton, but do not prove the link.

gneous event of the Southern Urals, Lithos (2012), http://dx.doi.org/

Fig. 11. Drill core log for the Menzelino–Aktanysh 183 drill hole. Diagram provided by A. Puchkov (2010). Asterisks indicate the locations of cores sampled for age determination ofdolerite sills (Menz–Akt 183-1a and Menz–Akt 183-5a).

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4.2. Reconstruction implications

The state of our understanding of Precambrian supercontinentreconstructions is very much at a preliminary stage with large uncer-tainties in the positioning of most blocks in the various proposedsupercontinents (e.g., Bleeker, 2003; Li et al., 2008; Meert, 2012).Matching of ages of LIP magmatism between crustal blocks is provingto be a useful new tool for constraining Precambrian Supercontinentreconstructions (e.g., Bleeker and Ernst, 2006; and other papers inthis special issue; www.supercontinet.org). Our focus in the presentpaper is not global reconstructions but we discuss the specific impli-cations of the Mashak LIP profiled herein for our understanding of theposition of the EEC in the Columbia (Nuna) supercontinent.

4.2.1. Age matches on other crustal blocksIn addition to the widespread distribution of the ca. 1385 Ma

Mashak magmatism in the EEC (Southern and Middle Urals andalso in the Volgo-Uralia region), intraplate magmatism of this age isalso recognized in other crustal blocks: western Laurentia (HartRiver sills and Salmon River Arch sills), in northern Greenland(Midsommersø sills and Zig-Zag Dal volcanic rocks), in the Anabar

Please cite this article as: Puchkov, V.N., et al., The ca. 1380 Ma Mashak i10.1016/j.lithos.2012.08.021

Shield of Siberia (Chieress dykes and other dolerites), in Antarctica(Vestfold Hills dykes), in the Congo (Congo–Tanzania) craton, andalso in southern Kalahari craton, mainly as a series of alkaline andcarbonatitic intrusions, but probably also including the tholeiitic toalkaline imprecise dated Pilanesberg dyke swarm (Ernst et al., 2008b,and references therein). Geographically, these units (mainly doleritesand basalts) may be grouped into two distinct (and probably indepen-dent) regional magmatic nodes/clusters located on opposite sides ofLaurentia (Ernst et al., 2008b). Western Laurentia Node: Events inAntarctica, and in the Kalahari cratons are located close to westernLaurentia in some Columbia (Nuna) reconstructions (i.e., near thenode comprising theHart River and SalmonRiver Arch sills) (see reviewof Columbia (Nuna) reconstructions in Meert, 2012). NortheasternLaurentia Node: The units in northern Greenland (NE Laurentia) canbe linked with the Mashak units in the southeastern East Europeancraton, and also with the magmatism in northern Siberia. The otherca. 1385 Ma events in the Congo craton and Kalahari craton cannotyet be linked to either of these 1385 Ma nodes. In summary, 1385 Maintraplate magmatism is present on at least seven blocks, and clustersinto at least two separate concentrations of magmatism (nodes), onopposite sides of Laurentia (Ernst et al., 2008b).

gneous event of the Southern Urals, Lithos (2012), http://dx.doi.org/

Fig. 12. A, B. U–Pb concordia diagrams for samples Menz–Akt 183-1a (A) and Menz–Akt183-5a (B). The age for each sample is determined as the upper intercept of threefractions.

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4.2.2. Reconstruction links with Laurentia and SiberiaWe focus on the Northeastern Laurentia Node that involves the

EEC and Mashak event. As noted above, volcanic rocks and dykesof the same age are also present in the northern Greenland portionof Laurentia (Upton et al., 2005) and also in the northern andpossibly eastern parts of Siberia (Ernst et al., 2000a,b; Khudoley etal., 2007). A new continental reconstruction (Evans and Mitchell,2011) suggested close and long-lived ties (1.9 to 1.2 Ga) betweenthree continents: Laurentia, Baltica and Siberia, making a tight clus-ter in the core of a hypothetical Columbia (Nuna) supercontinent(Fig. 13), which is supported by paleomagnetic data. However, itshould be noted that an alternative interpretation of the paleomag-netic data suggests a gap between Siberia and northern Laurentia atleast at ca. 1000 Ma and perhaps during the Mesoproterozoic (e.g.,Lubnina, 2009; Pisarevsky et al., 2008). According to the Evans andMitchell (2011) reconstruction, the boundaries of these three fu-ture continents inherited the position of two rifts. The first, withan age of ca.1385 Ma, situated between Siberia on one side andBaltica and Greenland on another; the second, dated at 1270 Ma,is located between Siberia and Laurentia.

Please cite this article as: Puchkov, V.N., et al., The ca. 1380 Ma Mashak i10.1016/j.lithos.2012.08.021

4.3. Economic implications

The Bakal group of siderite deposits, hosted in the Lower RipheanBakal Formation (carbonates and shales), close to the western wall ofthe Mashak graben, is the world's largest iron formation of this type.The formation of the Bakal siderite deposits is related to tectonic eventsin the lateMiddle Riphean (Krupenin, 2004), and therefore can possiblybe linked to the thermal effects of theMiddle RipheanMashak event de-scribed herein. The same author also notes that the 1385 Ma Bakal dykeinitiated brucitization of magnesite in the Bakal Formation, which indi-cates a pre-Mashak age for the huge Satka stratiformmagnesite miner-alization hosted by the Satka Lower Riphean Formation in the samearea. The conglomerates at the base of the Mashak Formation at theShatak Range contain precious metal mineralization, concentrated inhematite and magnetite fractions (up to 10 g/t Au, 2 g/t PGE, 5 g/t Ag;Kovalev and Vysotsky, 2006). Dating of the Mashak Formation alsoprovides a more precise constraint on the mineralization age ofdynamometamorphic black shales containing new types of preciousand rare metals, and U–Th-REE ore mineralization in the higher levelsof the formation (e.g., Kovalev et al., 2009; Kovalev, personal communi-cation in 2012).

There is extensive oil production in the Phanerozoic sedimentarybasin west of the Urals in Volga-Uralia (Lozin, 2002). Dating of themajor stratigraphic boundary in the Riphean stratotype in the Uralsis of regional importance and provides insights into the history ofthe Volgo-Uralian hydrocarbon basin (e.g., Lozin, 2002). The correla-tion of the Uralian section with boreholes in the basin shows thatthe Lower/Middle Riphean boundary was certainly one of the mostimportant (Fig. 3). It corresponds to a major stratigraphic break andprobably to an episode of volcanism in the Nadezhdino Formationat the beginning of the Mashak time. The formation is representedby alternating shale, feldspar-quartz siltstone, and sandstone withrare dolomite; it has irregular distribution, and probably filled somesmall grabens.

Arguments advocating correlation of theNadezhdino Formationwiththe Mashak Formation have been put forward in many publications.Especially important are the results of the 1-Vostochnoaskinskaya deepborehole (Kozlov et al., 2007). As for the volcanic rocks, they werereported at this level in many papers (Dymkin et al., 1956; Lomot,1954; Solontsov, 1959). However, the age of the Nadezhdino Formationas well as the presence of volcanic rocks within it, is still a subject ofdiscussion. Unfortunately, the drill cores containing volcanic rocks havebeen lost. However, if the interpretation is correct, we can suggest thatthe Mashak event affected an even larger area of the East Europeanplatform and the heat associated with the event was responsible forhydrocarbon maturation during the early-to middle Mesoproterozoic,affecting the thick Lower Riphean sedimentary rocks. By that time, theBashkirian anticlinorium itself was an integral part of the Volga-Uralbasin, which was later folded (as an effect of the Timanian and Uralianorogenies). The Paleozoic oils west of the Urals exhibit geochemicalfingerprints of the Devonian black shales, suggesting no directcompositional influence from the 1390–1380 Ma event. However, theProterozoic basin in the platform has only been studied very poorly,and we cannot exclude the presence of hydrocarbon deposits.

4.4. Implications for mass extinction and global stratigraphy

The themeofmass extinctions iswidely discussed in geological liter-ature, especially in the last three decades, starting with a statisticalstudy of Raup and Sepkoski (1982) and with abundant literaturesince. The link between Large Igneous Provinces (LIPs) andmass extinc-tions,with orwithout the participation of catastrophic bolide impacts, iswell established (e.g., Jin et al., 2000; Courtillot and Renne, 2003;Whiteand Saunders, 2005; Schoene, et al., 2010; and many others). Howeverthe theme ofmass extinctionswasmostly restricted to five Phanerozoicevents (late Ordovician, Late Devonian, Permian–Triassic, Late Triassic,

gneous event of the Southern Urals, Lithos (2012), http://dx.doi.org/

Fig. 13. Reconstruction of the core of the Columbia (Nuna) supercontinent at 1740–1270 Ma (modified from Evans and Mitchell, 2011). 1, 2 — Rifts of the initial stages of Nunabreakup: 1 – the first stage about 1380 Ma; 2 – the second stage about 1270 Ma. The numbers in the scheme indicate magmatic formations of the first stage: 1—Mashak Formation(this paper) , 2 — Menzelino–Aktanysh sills (this paper); 3 — Anabar dyke (Ernst et al., 2000a,b) 4 — Palyu-21 borehole (Andreichev and Litvinenko, 2007), 5 — Midsommersø sillsand Zig-Zag-Dal volcanics (Upton et al., 2005), 6— Victoria Land dykes (Evans andMitchell, 2011), 7— Hart River sill (Thorkelson et al., 2005), 8— Salmon River arch sills (Ernst andBleeker, 2010).

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and Cretaceous–Paleogene). The organic world of the Archean–Proterozoic (except the Ediacaran) is represented by microfossils(brown and green algae, sulfur and cyanobacterias, fungi etc.),united arbitrarily under a name of Acritarcha (Fensome et al.,1990). The pace of their morphological evolution seems to bevery slow, which may be explained by their great ability to adjustto environment changes (Stanevich, 2011). All the more interest-ing is to try to look for a link between the newly-established LIPand history of the Proterozoic organic world.

Given the strong correlation between the Mashak magmatic eventand the Lower/Middle Riphean boundary it is worth consideringwhether the Mashak event is responsible for the biotic changes atthis boundary. The shapes of stromatolites as well the morphologyof microfossils changed in the Middle Riphean (Avzyan Formation),as compared to the Lower Riphean (Satka and Bakal Formations).Unfortunately neither stromatolites nor microfossils are found inthe Mashak Formation between them (Field trip guide on theProterozoic of the Southern Urals, 2006). At a more general scale,Lower and Middle Riphean fossils are thought to correspond to а sin-gle major assemblage of Proterozoic biota – Anabarian (1.65–1.2 Ga).But the boundary between the Early (An1) and Late (An2) Anabarianintervals of biotic evolution is roughly constrained between 1.35 and1.45 Ga (Sergeev et al., 2010). This boundary coincides well with thedate of the Middle Riphean lower boundary (1.4 Ga) found by us anddiscussed in Section 3, and therefore we may suppose a positivecorrelation.

We suggest that the precise dating of theMashak LIP aswell as othervolcanic events (Ai and Arshinian Formations) of the Riphean succes-sion of the Southern Urals (one of the best Meso-Neoproterozoicsuccessions in the world) will be helpful in defining a natural breakupin the International stratigraphic scheme. In the International Strati-graphic Chart (Gradstein et al., 2004, Ogg et al., 2008), approved bythe International Commission on Stratigraphy, the Mesoproterozoic(1000 to 1600 Ma), is divided into Systems/Periods of equal length:Stenian (1000–1200 Ma), Ecstasian (1200–1400 Ma) and Calymmian(1400–1600 Ma). Such a division does not follow the general principlesof stratigraphy, whereby boundaries should be defined on the basis ofreal stratigraphic sections and events to create a “natural” stratigraphicscale (e.g. Bleeker, 2004).

Please cite this article as: Puchkov, V.N., et al., The ca. 1380 Ma Mashak i10.1016/j.lithos.2012.08.021

As for the real importance of the Mashak dating, it is clear that itdoes not confine exactly the boundary of the Lower andMiddle Ripheanof the current scheme. But there is no argument against drawing amajor boundary in the Mesoproterozoic at the base of the Mashak rhy-olites. It is a normal practice in the Phanerozoic stratigraphy – “nailing”a GSSP in a rather monotonous section, avoiding transgressive bound-aries and possible lacunes.

Generally speaking, LIPs are very widely distributed and short-livedevents in the Earth's history, and their use for establishment of strati-graphic boundaries is just a matter of time.

5. Conclusions

1. Precise SHRIMP U–Pb zircon ages of 1386±5 and 1386±6 Ma(Section 3) were obtained for two samples of dacite–rhyolitefrom the Mashak Formation. These data are in good accordancewith zircon age determinations of the same formation, obtainedby CA-IDTIMS in Boise University, USA and SHRIMP II VSEGEI(St. Petersburg) (Krasnobaev et al., submitted for publication;Puchkov et al., 2009).

2. The Mashak volcanic rocks are developed at the base of the MiddleRiphean and form a long band close to the axis of the Bashkiriananticlinorium. On both sides of this band, the dykes and largerintrusions of contrasting (mafic and felsic) compositions andapproximately the same age cut the Lower Riphean rocks, makingthe area of the Mashak igneous event much larger than the extentof the Mashak Formation itself. In addition to these data, two U–Pbage determinations were made at the University of Lund, Sweden,on baddeleyite from dolerite sills intruded into the crystallinebasement of the East European Craton (1382.0±2.2 Ma) andoverlying Lower Riphean sediments (1391.2±2.3 Ma) in theMenzelinsk–Aktanysh borehole (reported herein, Section 2.2.1).These results suggest that the Mashak LIP extends over a signifi-cantly larger area than previously realized, most parts of whichare buried under younger sediments. These complexes may alsoextend to Cis-Polar and Polar areas of the Urals but they are poorlystudied there.

3. The Mashak event can be reconstructed with coeval magmaticrocks in northeastern Greenland (Zig‐Zag Dal volcanics and

gneous event of the Southern Urals, Lithos (2012), http://dx.doi.org/

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Midsommersø sills) and Siberia (Chieress dyke and other dolerites).We suppose that the ca. 1380–1385 Ma Mashak event is a part of asingle LIP (including equivalent magmatism in Siberia and NELaurentia) and corresponds to a breakup stage of the Columbia(Nuna) supercontinent.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.lithos.2012.08.021.

Acknowledgements

This is publication no. 15 of the “LIPs— Supercontinent ReconstructionProject (www.supercontinent.org)” which is sponsored by an industryconsortium. W. Bleeker, K. Chamberlain, D. Evans, P. Hollings andM. Santosh are thanked for their comments. SHRIMP U–Pb analyseswere conducted in part using the SHRIMP II ion microprobes at the Johnde Laeter Centre of Mass Spectrometry at Curtin University, in Perth,Australia. MTD Wingate publishes with permission of the executiveDirector of the Geological Survey of Western Australia. This paper isdedicated to the blessed memory of our co-author and colleague,Vyacheslav Kozlov, who was an unsurpassed expert on the stratigraphyof the Bashkirian anticlinorium and a good man.

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