Volgo-Uralia: The first U-Pb, Lu-Hf and Sm-Nd isotopic evidence of preserved Paleoarchean crust

VOLGO-URALIA: THE FIRST U-Pb, Lu-Hf AND Sm-Nd ISOTOPICEVIDENCE OF PRESERVED PALEOARCHEAN CRUST

SVETLANA V. BOGDANOVA*, BERT DE WAELE**, ELENA V. BIBIKOVA***,ELENA A. BELOUSOVA§, ALEXANDER V. POSTNIKOV§§,

ANNA A. FEDOTOVA***, and LUBOV’ P. POPOVA§§

ABSTRACT. The crustal segment Volgo-Uralia is the least known part of the EastEuropean Craton. Its crystalline crust is hidden beneath a thick Neoproterozoic toPhanerozoic cover but disclosed by thousands of drill holes. In conjunction with therecent “Tatseis” reflection seismic profile, we conducted the first isotopic study of theBakaly granitoid block in eastern Volgo-Uralia, which represents a subsurface sectionof the layered upper-middle crust. The study included whole-rock Sm-Nd and ion-probe zircon U-Th-Pb (SIMS) and Lu-Hf (LA-ICPMS) analyses of granitoids from sevendrill cores. The Bakaly block was also targeted because its rocks have never been subjectedto granulite facies metamorphism, making it possible to date pristine, pre-metamorphiczircon. Our study showed that the four principal suites of granitoids in the Bakaly block aredifferent in age, each corresponding to a particular stage of Archean crustal evolutionbetween 3.3 and 2.6 Ga. The Tashliar monzonitic suite, belonging to an alkaline seriesyielded zircon ages of 3.3 and 3.2 Ga, which are the oldest ages yet found in Volgo-Uralia.The �Hf(T) values of the dated zircon and the �Nd(T) values of their host rocks indicatethat a Paleo- to Eoarchean protolith with model TDM ages up to 3.8 Ga had been involved inthe formation of the Tashliar melts. Three Neoarchean rock suites, one comprising quartzdioritic and tonalitic gneisses (the Bak 1), another K-rich granodiorites, granites andmigmatites (the Bak 2), and the third monzonitic granitoids (the Aktanysh suite) wereformed sequentially between 2.72 and 2.60 Ga. The 2.72 Ga Bak 1 suite is chemicallydiverse. It includes granitoids of the TTG type related to slab/subduction melts as well asrocks formed by the re-melting of older crust with whole-rock Nd TDM and Hf TDM modelages of 3.4 to 3.2 Ga. The 2.69 to 2.65 Ga Bak 2 suite was probably associated with a majorcollisional event, which defined the stacked structure of the Archean crust in Volgo-Uraliaand its seismic layering. Our data suggest that the Bak 2 melts originated partly fromjuvenile sources with �Hf(T) zircon values up to �4.8, as well as mixed crustal and juvenilemantle materials. Some crustal contamination of the melts appears to have occurred asevidenced by incorporated xenocrystic zircon. The chemical compositions of Bak 2granitoids from the different plutons, their zircon �Hf values, and the Hf- and Nd TDM agesall mirror a heterogeneous, collisional, crustal structure. During post-collisional extensionat 2.6 Ga, the intrusion of Aktanysh monzonitic granitoids took place. These rocks alsobear evidence of a long crustal pre-history with Nd and Hf TDM model ages of 3.3 to 3.5Ga. The Aktanysh rocks are coeval with the Tuymazy gabbro-norite-anorthosite intrusions,which are widely distributed along post-collisional shear zones in the Bakaly block. Theycould have provided the heat necessary to melt the crust at this stage. Altogether, theisotopic evidence suggests several episodes of crustal growth and recycling possiblyreaching back to 3.6 and 3.8 Ga. Metamorphic zircon rims show that the Archean crust inthe Bakaly block were subjected to several tectonothermal overprints in the Paleoprotero-zoic between 2.4 and 1.9 Ga ago.

Key words: East European Craton, Volgo-Uralia, Paleoarchean, granitoids,zircon, geochronology

* Department of Earth and Ecosystems, Lund University, Solvegatan 12, 22362 Lund, Sweden;[email protected]

** SRK Consulting, 10 Richardson Street, West Perth WA 6005, Australia; [email protected]*** Vernadsky Institute of Geochemistry and Analytical Chemistry, RAS, Kosygin street 19, Moscow,

Russia; [email protected]§ GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie Univer-

sity, Sydney NSW 2109, Australia; [email protected]§§ Gubkin State University of Oil and Gas, Leninsky pr. 65, 117296 Moscow, Russia, [email protected]

[American Journal of Science, Vol. 310, December, 2010, P. 1345–1383, DOI 10.2475/10.2010.06]

1345

introductionThe distribution of Archean cores within continents of various ages has attracted

attention particularly with regard to the geodynamic modeling of craton formation,craton survival and the influence of Archean cratons on plate motions (Abbott andothers, 2000; Artemieva and Mooney, 2001; King, 2005; Condie and Kroner, 2008).Moreover, Archean crust is of substantial interest in diamond and gold exploration(Richardson and others, 1984; Griffin and others, 2003). A recent world-wide inven-tory of �35 Archean cratons has demonstrated that their correlation and reconstruc-tion within Archean global palaeogeography are still ambiguous, even though severalattempts have been made (Bleeker, 2003, and references therein). This is not only dueto poor preservation of Archean crust through tectonic detachment and recycling. Infact, many Archean cratons have simply not yet been recognized or are poorlyunderstood because of the absence of adequate age data.

The crustal segment Volgo-Uralia, which makes up one third of the East EuropeanCraton (fig. 1), has long been considered Archean, based mostly on general consider-ations and comparisons of crustal lithologies with those in the shields supported by afew U-Pb zircon TIMS and Rb-Sr ages (Bogdanova and others, 1979; Bibikova andothers, 1984, 1994; Postnikov, ms, 2002). These have disclosed several late Archeanand Paleoproterozoic crustal events between �2.8 and 1.8 Ga, but the earliestevolution of this continental block remained uncertain.

Here, we present U-Th-Pb and Lu-Hf zircon evidence that the long multistageformation of Volgo-Uralian crust commenced at the latest in the Paleoarchean, �3.3to 3.2 Ga ago, with whole-rock geochemical and Sm-Nd data even suggesting possibleEoarchean protoliths.

general geologyAmong the three crustal segments of the East European Craton (fig. 1, inset),

Volgo-Uralia is the least known because of its almost complete burial beneath a thicksedimentary cover of Neoproterozoic to Phanerozoic age. The only exposed fragmentsof the crystalline basement are small blocks in the adjacent Paleozoic Uralides(Sindern and others, 2005). Knowledge of this basement is therefore largely based ongeophysical data and drill cores, which are particularly numerous because of the highoil and gas potential of the region. Some ten of the deep drill cores have sampled thecrystalline basement down at depths up to three and more kilometers, providingvaluable information on rock relationships and abundances (Bogdanova, 1986; Musli-mov and Lapinskaya, 1996; Postnikov, ms, 2002; Bogdanova and others, 2005).

Volgo-Uralia is generally a high-grade terrain, where 2.7 Ga granulite and amphi-bolite facies rocks (fig. 1) appear to prevail, and compose supracrustal and plutonicinfracrustal belts. In addition, there also exist Archean “greenstone-belt” sequenceswith komatiitic metavolcanics intruded by �2.8 Ga granitoids. All these rock beltsoutline a fold-and-thrust structure of late Archean crust formed as a result of collisionat �2.7 Ga (Bogdanova, 1986; Postnikov, ms, 2002). Associated NE-SW to ENE-WSWtrending zones of strong shearing, which accommodate �2.6 Ga gabbroic anorthositicto monzogranitic intrusions are most probably related to post-collisional geodynamics.These shear zones appear to have been reactivated in the Paleoproterozoic, when largeparts of the Volgo-Uralian crust and upper mantle were subjected to several crustalremelting events, metamorphism and deformation. Characteristically, large-scale (250-350 km across) “domes” featuring circular patterns of magnetic and gravity anomaliesappear to have been superimposed on the Archean structural pattern (Bogdanova,1986). Metamorphic circular zoning and remnants of a Paleoproterozoic metasedimentary-metavolcanic cover, as well as mafic and �2.1 Ga granitoid intrusions, have beendisclosed by drilling within the “domes.” The new seismic reflection transect “Tatseis”

1346 S. V. Bogdanova and others—Volgo-Uralia: The first U-Pb, Lu-Hf

across Volgo-Uralia (Trofimov, 2006) demonstrates a strongly stacked Archean toPaleoproterozoic crust, which has been affected by mantle underplating and diapirismwithin one of the “domes” in northwestern Volgo-Uralia. These data suggest thatmantle upwelling beneath the Volgo-Uralian lithosphere might have caused this largescale “doming” of the Archean crust in the Paleoproterozoic. However, a combinedeffect of regional NE- to NNE-striking fold-and-thrust structures and transverse strike-slip faults can still not be ruled out entirely as an explanation of some of these

Fig. 1. Map of the western Bakaly block and adjacent areas based on deep drilling and geophysical data(modified after Bogdanova, 1986 and Muslimov and Lapinskaya, 1996). The letters mark the principal Bak 2granitic plutons: MZ—Menzelinsk, MS—Muslumovo, MT—Mustafino, UR—Ural’sky and ZA—Zainsk.Seismic transect line is shown on map as a gray line labeled Tatseis. The numbers along the transect lineindicate profiling intervals in kilometers. The three-segment subdivision of the East European Craton shownin the bottom corner is after Bogdanova (1993); the open square indicates the site of the Bakaly block. At thebottom is shown a fragment of the seismic reflection transect “Tatseis” (modified after Trofimov, 2006).

1347and Sm-Nd isotopic evidence of preserved Paleoarchean crust

geophysical features. The latest deformation and metamorphic reworking occurredbetween 1.9 and 1.8 Ga (Muslimov and Lapinskaya, 1996; Bogdanova and others, 2005;Sindern and others, 2005). Numerous K-Ar ages of amphibole and biotite from variousrocks define several peaks of uplift history and thermal overprinting at �2.5 to 2.2, 2.0to 1.9 and 1.8 to 1.7 Ga. (Kratz, 1979; Bogdanova, 1986, 1993).

Large areas of Paleoproterozoic turbiditic metapelitic mica schists, siltstones,sandstones and carbonaceous shales occur along the southwestern margin of Volgo-Uralia. Their metamorphism and associated anatectic granitic magmatism have oc-curred between 2.05 and 2.00 Ga. This is regarded as the time of the collision betweenVolgo-Uralia and Sarmatian terranes (Shchipansky and others, 2007; Bibikova andothers, 2009). Subsequently, Proterozoic rifting severely complicated the margins andinteriors of Volgo-Uralia between 1.4 and 0.6 Ga (Bogdanova and others, 2008; Peaseand others, 2008).

Because of this multistage evolution, which was marked by several high-grade,particularly granulite facies, metamorphic events both in the Archean and in thePaleoproterozoic, the pristine ages of the pre-metamorphic rocks of the Volgo-Uraliancrust are difficult to determine. In the present study, we focus on granitoids andgneisses in the so-called “Bakaly” block in eastern Volgo-Uralia (fig. 1), where metamor-phism did not exceed amphibolite facies, and there is a good chance that igneouszircon have been preserved.

7 cm

Fig. 2. Photograph of the dated migmatite Tashliar 26. Note that the leucosome is syntectonic, andcrystallized during shearing.


the bakaly granitoid blockThe 130 by 150 km large Bakaly block is represented by a mosaic of mostly

negative gravity and magnetic anomalies. Data from nearly 800 deep drill cores suggestthat this pattern is due to the presence of massive and gneissic granitoids of TTG(� tonalite-trondhjemite-granodiorite) type (the Bak 1 suite), intruded by minor(70 to 200 km2) bodies of K-rich tonalite, monzogranites, and granites with associatedmigmatites (the Bak 2 suite). In many deep drill cores, the Bak 1 granitoids form up to20 m thick sheet-like bodies separated by more than 50 m thick zones made up of Bak 2rocks. The host rocks occupy a minor volume only and are represented by relativelyfine grained amphibolites, amphibole-biotite gneisses and sporadic high-Al garnetgneisses (fig. 1). Previous U-Pb zircon TIMS dating has yielded rather similar ages forthe Bak 1 and Bak 2 granitoids. These are 2700�48 Ma for a K-feldspar granite of theBak 2 suite and 2698�12, 2697�31 and 2593�31 Ma for the geologically older Bak 1granitoids (Bibikova and others, 1994).

The tectonic grain of the Bakaly block is rather complicated at its southwesternmargin, where Archean supracrustal rocks and 2.6 Ga gabbro-norites and anorthositesfeature NE-trending thrusting structures. Here, the shear/thrust- and reverse faults dipgently toward the northwest and are evident in the “Tatseis” seismic images (fig. 1,profile). Flat-lying foliation is characteristic of the Bakaly granitoids in most drill cores.In the Proterozoic and later, numerous normal and strike-slip faults were superim-posed upon the NE-striking deformation zones (fig. 1). Paleoproterozoic metasedimen-tary rocks with 1.9 Ga high-grade granites and leucosomes overlie the Bakaly grani-toids. K-Ar ages of biotite and amphibole of the Bakaly granitoids ranging from �2.5 to1.65 Ga also indicate several events of Paleoproterozoic thermal overprinting andtectonic reworking of Archean crust (Kratz, 1979; Bogdanova, 1986). A number ofmafic dikes cut the Bakaly block, especially along its eastern boundary with theMesoproterozoic Kama-Belsk aulacogen (Bogdanova and others, 2008).

The monzonitic granitoids disclosed by several drill cores within the Tashliar area(fig. 1) have lately attracted particular attention due to their deformation andmigmatization in the proximity of the Bak 2 granitic intrusions. Preliminary geochrono-

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Fig. 3. Chemical classification of the Bakaly granitoids: A: An-Ab-Or feldspar normative diagram; B:K-Na-Ca diagram showing trondhjemitic differentiation (Td) and calc-alkaline (CA) trends (after Barkerand Arth, 1976). Sources: 70 chemical analyses from the archives of the Department of Lithology, theGubkin State University of Oil and Gas, Moscow, were used together with 27 new ISP analyses (table 1).Symbols: upright crosses—Bak 1 suite, solid gray circles—Bak 2 suite, solid black circle-vein granite Tashliar26 L.


logical work has suggested that they can be older than 3.0 Ga. This sets apart theTashliar rocks from the Bak 1 and 2 suites as well as from unmigmatized monzoniticgranitoids (the Aktanysh suite) associated with the 2.6 Ga gabbro-anorthosite intru-sions.

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These observations indicate a complex magmatic evolution of the Bakaly block inthe Archean. To assess this evolution, samples for geochronological and isotopic workwere selected from four Bakaly block granitoid suites. These are:

(1) the Tashliar suite of partly migmatized monzodiorites and quartz monzoniticgranitoids,

(2) the Bak 1 suite of quartz diorite, tonalite (�trondhjemites) and granite,(3) the Bak 2 suite of granodiorite, monzogranite and granite that has intruded the

Bak-1 granitoids and forms leucosomes in migmatized granitoids of both theTashliar and Bak 1 suites, and

(4) the Aktanysh suite of monzonite and monzonitic granite associated withNE-trending shear zones and 2.6 Ga gabbro-norite-anorthosite intrusions.

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Fig. 4 (continued)


Petrography and Chemical Characteristics of the Granitoid SuitesThe Bakaly granitoids have been deformed and recrystallized to various extent.

Most of the granitoids are foliated gneisses, and some are migmatites (fig. 2).Metamorphism and attendant migmatization rarely exceeded the amphibolite facies.Igneous textures and mineral assemblages are preserved well enough to assess also themodal compositions. Rock classification and grouping are, therefore, largely based ontheir chemistry (figs. 3, 4, 5, and 6). Petrographical characteristics of the dated samplesin this study and analytical specifics are presented in Appendices 1 and 2, respectively.

The Tashliar suite.—Among the granitoids of the Bakaly block, monzonitic typesare rare and have previously been thought to be exotic, possibly late Archean or evenPaleoproterozoic in age. Detailed examination of drill cores during this study has,however, shown that monzonitic rocks occur in the separate Tashliar block boundedby several fault sets. The Tashliar monzonitic granitoids are deformed and migmatizedin the vicinity of the Bak-2 pluton (ZA in fig. 1). They are medium- to fine grained andcontain quartz (�15-20%), andesine (45-60%), microperthitic K-feldspar (15-30%)and biotite, and have high abundances of allanite, apatite and zircon. The studiedsamples are: (1) a metatextite with monzonitic mesosome (Tashliar 26 D) and graniticleucosome (Tashliar 26 L), that occurs 1 to 2 cm wide veins (fig. 2), and (2) amonzodiorite (Suleyevo 585). The Tashliar rocks differ from the other Bakaly grani-toids in their major and trace element contents (figs. 3, 4 and 5; table 1). Both analyzedrocks are probably members of a single alkalic suite. They have SiO2 between 53 to 62weight percent, #Mg between 43 and 20, and elevated contents of K2O (K2O/Na2O �1.5), Ba (up to 3800 ppm), Rb (up to 180 ppm) and Zr, Hf, Th and Y (fig. 4). Traceelement and, particularly, REE patterns demonstrate consistency of rock compositionsand trends characteristic of magmatic series rocks (figs. 4, 5 and 6). These monzoniticrocks have the highest abundances of REE among all the Bakaly granitoids, reaching�1000 ppm in total (table 1), and fractionated REE, particularly HREE (Gd/Yb � 8),patterns with slightly negative Eu (Eu/Eu* � 2.3) anomalies (fig. 6, table 1).

The Bakaly 1 and 2 suites.—The dominant granitoids of the Bakaly block belong totonalite-trondhjemite-granodiorite�granite (TTG) magmatic suites, resembling Ar-chean TTG lithologies. Cross-cutting contacts and chemical compositions distinguishtwo suites in the Bakaly block (Bogdanova, 1986; Popova and Postnikov, 1992). Theearlier one (Bak 1) comprises quartz diorite, tonalite, and rare trondhjemite andgranodiorite, while the later (Bak 2) consists of granodiorites, monzogranite andsyenegranite. The granitoids of both suites are mostly coarse- to medium-grained,variously deformed and foliated, and partly recrystallized. The Bak 1 rocks oftencompose the paleosomes of migmatites, while plagiogranitic to strictly granitic leuco-somes are typically related to the intrusions of the Bak 2 suite (fig. 1).

Based on the least deformed samples, the Bakaly granitoids were originally coarse-to medium-grained, equigranular or seriate. They are both amphibole- and biotite-bearing, and the total concentrations of these minerals are commonly between 10 and30 percent. Biotite replaces amphibole, especially in zones of dynamic recrystallization.High quartz contents reaching 40 percent are probably due to deformation and theredistribution of quartz along weakness zones. Characteristic accessories are up to 1mm large crystals of allanite, sometimes rimmed by epidote, titanite, apatite, rutile,and Ti-magnetite.

Rock compositions of both Bakaly suites, calculated using their normative feld-spars, are plotted in figure 3A. Despite some overlap between the two suites, it isobvious that they follow two different trends (fig. 3B), the one mostly trondhjemitic(Bak 1), the other mostly calc-alkaline (Bak 2). In both, the SiO2 contents rangebetween �59 and 72 percent, all oxides except K2O correlate negatively with SiO2 (fig.


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Fig. 5. Multi-element diagrams for the Bakaly granitoids: thick lines denote the dated samples. (A)Tashliar Suite, (B) Bak 1 Suite, (C) Bak 2 Suite, (D) Aktanysh Suite, Tash�Tashliar, Sul�Suleyevo, Ural�Ural’skaya, TI-T�Tylanchi-Tamak, M-A�Menzelino-Aktanysh, Must�Mustafino, Sab�Sabanchino

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Fig. 6. REE patterns of the Bakaly granitoids: thick lines denote the dated samples. (A) Tashliar Suite,(B) Bak 1 Suite, (C) Bak 2 Suite, (D) Aktanysh Suite. Tash�Tashliar, Sul�Suleyevo, Abbreviations same as infigure 5.


Table 1

Chemical analyses of granitoids from the Bakaly block. Bold style marks dated rocksRock suites Tashliar suite Bak 1 suite wt. % Sul 585 Tash 26D Ural 40002 Urt 752-17 Mus 41 Urt 752-8 M-A 26 Sab 2261 SiO2 52.66 61.59 59.05 59.4 60.28 63.66 64.05 66.6TiO2 0.81 0.51 0.96 0.95 0.66 0.37 0.44 0.45Al2O3 18.03 16.15 16.6 17.08 14.13 18.01 16.16 16.35Fe2O3 8.12 7.81 9.09 6.75 8.66 3.84 4.38 4.48MnO 0.08 0.07 0.1 0.06 0.12 0.05 0.04 0.03MgO 5.61 1.73 2.13 3 4.46 1.33 1.75 1.36CaO 1 2.31 4.8 3.83 3.72 4.87 4.48 3.78Na2O 3.74 3.61 4.64 4.02 3.76 4.63 4.1 4.64K2O 5.36 5.48 2.07 2.96 2.43 1.65 2.02 1.55P2O5 0.7 0.37 0.48 0.45 0.03 0.06 0.25 0.11LOI 3.1 -0.1 -0.1 1.1 1.5 1.2 2.1 0.5Sum 99.26 99.54 99.82 99.61 99.77 99.67 99.77 99.84ppm Ba 3796 2400 852.9 496.6 550 507.1 636 529Hf 10.9 7.6 8 8.9 2.4 2.8 6.5 4.6Nb 9.7 10.9 8.3 8.6 10.1 3.5 4.9 3Rb 104.3 180.6 85 152.5 92.5 51 76.2 61.1Sr 213 441 807 323 195 352 696 349Ta 0.4 0.4 0.4 0.7 0.4 0.4 0.1 0.1Th 28.9 32.9 18.4 22.4 0.4 3.1 2.4 7.7U 1.5 1.2 2.3 2 1.5 0.9 0.7 0.4V 61 44 101 99 119 46 57 44Zr 450 292 317 355 62 109 242 158Y 26.8 17.6 16 9.6 23.7 8.3 7.8 3.7La 243.5 121 93.9 122 10.5 11.5 15.6 35.8Ce 476.5 236.4 188.1 206.1 27.9 20.3 29.1 58.5Nd 185.6 93.3 73.8 76.4 14.4 9.1 13.3 14.5Sm 23.5 13.21 9.7 9.3 3.81 1.8 2.73 1.84Eu 3.78 1.73 2.26 1.18 0.68 0.71 0.84 0.56Gd 14.42 8.86 5.56 4.48 4.03 1.66 2.31 1.11Tb 1.48 0.93 0.78 0.56 0.74 0.27 0.33 0.17Dy 6.74 4.16 3.39 1.96 4.06 1.44 1.6 0.78Ho 1 0.6 0.52 0.34 0.84 0.29 0.27 0.14Er 2.45 1.52 1.46 0.76 2.37 0.77 0.77 0.36Tm 0.26 0.14 0.18 0.12 0.36 0.12 0.11 0.05Yb 1.5 0.9 1.29 0.77 1.96 0.81 0.7 0.31Lu 0.2 0.12 0.19 0.12 0.29 0.12 0.1 0.04Ni 23 17 18 4 46 5 22 12Cr 55 48 34 41 185 7 48 14REE tot 1012 509 401 445 75 51 71 119(La/Yb)N 69 56 45 112 3 10 14 69(La/Sm)N 5.9 6.7 6.2 8.5 1.8 4.1 3.7 12.6(Gd/Yb)N 8.1 8.0 4 5 2 2 3 3Eu/Eu* 1.31 2.25 2.2 1.3 0.8 1.2 1.2 1.2#Mg3 43 20 21 33 36 28 31 25K2O/Na2O 1.4 1.5 0.4 0.7 0.6 0.4 0.5 0.3Nb/Ta 24 27 21 12 25 9 49 21Sr/Y 7.9 25.0 50.5 33.7 8.2 42.4 89.3 94.2

M-A�Menzelino-Aktanysh, Mus�Muslimovo, Must�Mustafino, Sab�Sabanchino, Sul�Suleyevo,Tash�Tashliar, Tl-T�Tlyanchi-Tamak, Ural�Ural’skaya, Urt�Urat’minskaya. Samples in bold were dated,and drill hole localites are shown on figure 1.


Table 1

(continued)Rock suites Bak 1 suite wt. % Tash 27 Mus 43 M-A 183-4 M-A 99 M-A 126 Sul 582 Sul 53 Mus 34 SiO2 67.04 67.33 67.5 68.81 69 71.82 66.89 71.29TiO2 0.52 0.46 0.37 0.3 0.36 0.23 0.35 0.33Al2O3 15.07 15.68 15.19 15.26 14.69 15.12 16.57 13.72Fe2O3 5.36 4.81 3.07 5 4.3 3.06 4.25 4.63MnO 0.06 0.03 0.05 0.04 0.04 0.02 0.05 0.03MgO 1.7 1.67 1.64 0.93 1.96 0.58 1.08 1.15CaO 3.5 2.8 3.38 2.34 2.46 3.08 3.17 1.96Na2O 4.19 4.43 4.28 4.48 4.03 4.39 5.47 3.17K2O 1.66 1.08 2.63 2.31 2.46 1.26 1.26 2.43P2O5 0.14 0.13 0.22 0.08 0.05 0.06 0.09 0.04LOI 0.6 1.4 1.2 0.4 0.4 0.2 0.7 1Sum 99.82 99.87 99.54 99.96 99.77 99.81 99.89 99.78ppm Ba 493 215 924.6 542.2 880 611 179 620Hf 3.1 5.5 3 4.1 5.7 3.6 2.1 4.2Nb 3.7 5.6 5.9 4.2 5.6 1.3 8 7.1Rb 59.2 39.4 74.2 61.9 70.8 28.2 60.6 88Sr 345 220 895 328 316 502 299 372Ta 0.1 0.2 0.4 0.2 0.2 0.1 0.3 0.6Th 19.8 8.9 13.4 5.9 7.6 3.2 2.6 15U 0.6 1 2.1 0.3 0.9 0.3 1.1 3.9V 49 52 57 26 49 19 29 54Zr 104 190 124 167 152 146 70 147Y 8.4 7.3 9.6 2.3 1.7 1.2 5.3 6.3La 56.3 28.5 68.8 22 15.7 13.4 9.8 39.5Ce 110.2 56.6 131.3 38.2 27 23.5 16.5 85.4Nd 33.2 19.2 53.5 12.4 8.6 6.3 5.4 32.3Sm 4.22 2.63 6.3 1.3 1.02 0.84 1.14 5.21Eu 0.77 0.55 1.62 0.55 0.53 0.68 0.61 1.23Gd 2.69 1.78 3.47 0.79 0.6 0.4 1.09 3.42Tb 0.39 0.3 0.45 0.1 0.08 0.06 0.19 0.41Dy 1.85 1.36 1.82 0.35 0.28 0.19 1.1 1.59Ho 0.31 0.25 0.33 0.08 0.06 0.04 0.18 0.27Er 0.63 0.72 0.82 0.22 0.18 0.08 0.53 0.54Tm 0.1 0.08 0.11 <.05 0.03 0.01 0.08 0.07Yb 0.48 0.55 0.76 0.21 0.2 0.1 0.42 0.47Lu 0.07 0.08 0.11 0.03 0.04 0.01 0.06 0.06Ni 19 22 3 13 79 9 9 30Cr 34 21 14 41 143 14 21 55REE tot 222 118 284 80 57 48 39 179(La/Yb)N 62 28 59 70 61 117 13 51(La/Sm)N 8.6 7.0 7.1 10.9 9.9 10.3 5.5 4.9(Gd/Yb)N 5 3 4 3 2 3 2 6Eu/Eu* 1.1 0.9 2.0 1.4 1.5 2.4 1.3 1.5#Mg3 26 28 37 17 34 17 22 22K2O/Na2O 0.4 0.2 0.6 0.5 0.6 0.3 0.2 0.8Nb/Ta 37 28 30 21 28 13 27 12Sr/Y 41.1 30.1 93.2 142.6 185.6 418.2 56.5 59.0



Table 1

(continued)

Rock suites Bak 1 suite

Bak 2 suite Aktanysh suite

wt. % Mus 66 Must 37 M-A 183-11 Sul 611 Urt 799-2 Tash 26L Tl-T 684 M-A 9 SiO2 71.91 69.45 69.83 71.55 71.88 72.22 56.05 69.69TiO2 0.21 0.21 0.22 0.23 0.24 0.07 0.72 0.43Al2O3 10.44 15.29 15.27 14.08 13.96 13.18 15.68 12.77Fe2O3 5.45 5.4 2.1 3.29 1.93 4.31 9.77 7.49MnO 0.12 0.04 0.02 0.03 0.02 0.03 0.14 0.07MgO 2.88 0.63 0.61 0.58 0.7 0.33 3.9 0.68CaO 4.28 2.44 1.85 2.28 1.57 1.15 5.26 1.32Na2O 2.42 5.3 3.31 3.47 3.57 2.84 4.04 3.2K2O 1.44 1.07 4.81 3.66 4.08 5.76 3.2 4.27P2O5 0.02 0.08 0.06 0.07 0.06 0.04 0.41 0.11LOI 0.6 0.1 1 0.5 1.5 -0.3 0.5 -0.3Sum 99.83 100.02 99.08 99.75 99.51 99.6 99.68 99.74ppmBa 639 115 3273 1254 1543 2868 1008 2098Hf 2.5 3.8 6.4 3.8 3.9 2.5 5.1 10.8Nb 3.5 5.4 4.8 3.7 4.4 2.3 8.6 15.9Rb 31 51 146 86.3 97.1 121.7 100.2 116.5Sr 138 266 728 342 233 404 774 162Ta 0.3 0.5 0.6 0.2 0.3 0.1 0.6 0.7Th 2.8 34.8 48 7 16.9 10.9 14.1 5.2U 0.9 2 5 0.5 3.1 1.4 3.9 2.2V 47 19 23 24 20 <8 155 19Zr 76 129 228 121 147 84 177 420Y 17.3 5 14 6.9 7.7 2.6 21.7 31.1La 12.6 72.5 173.7 27.2 45.5 30.4 68.1 26.6Ce 29.6 136.1 283.5 49.2 79.8 49.4 154.1 58.2Nd 15.3 44.2 91.1 18.7 28.1 17.5 69.2 33.5Sm 2.9 5.9 9.5 3.13 3.4 2.05 9.7 6.7Eu 0.82 0.71 1.43 0.93 0.56 1 2.24 1.16Gd 2.95 3.29 4.69 2.44 2.16 1.49 5.85 6.12Tb 0.51 0.4 0.69 0.34 0.32 0.12 0.87 1.07Dy 2.94 1.27 3.04 1.52 1.4 0.59 4.07 5.6Ho 0.59 0.17 0.46 0.25 0.24 0.06 0.69 1.05Er 1.74 0.35 1.2 0.59 0.76 0.19 2.06 3.23Tm 0.29 <.05 0.17 0.07 0.11 <0.01 0.31 0.47Yb 1.74 0.23 0.92 0.47 0.75 <0.05 2.05 3.09Lu 0.24 0.03 0.14 0.05 0.13 <0.01 0.3 0.42Ni 18 12 2 8 2 11 29 12Cr 212 41 7 14 1 34 41 41REE tot 76 279 598 110 171 108 338 155(La/Yb)N 5 145 101 32 42 112 23 6(La/Sm)N 2.8 8 12 6 9 10 5 2(Gd/Yb)N 1 12 4 4 2 25 3 2Eu/Eu* 1.1 0.9 1.5 1.3 0.9 1.8 2.09 1.06#Mg3 37 11 24 16 29 8 31 9K2O/Na2O 0.6 0.2 1.5 1.1 1.1 2.0 0.8 1.3Nb/Ta 12 11 8 19 15 23 14 23Sr/Y 8.0 53.2 52.0 49.6 30.2 155.2 35.6 5.2



Table 1

(continued)

Rock suites Bak 11 Bak 21 Averagewt. % n=178 n=154 TTG2

SiO2 65.82 70.99 68.8TiO2 0.48 0.19 0.38Al2O3 15 14.36 15.5Fe2O3 5.27 3.41 3.24MnO 0.06 0.03 0.05MgO 1.9 0.57 1.25CaO 3.72 1.86 3.15Na2O 4.16 3.70 4.67K2O 1.89 3.88 1.92P2O5 0.16 0.06 0.14ppm n=15 n=5Ba 539 1811 713Hf 4.3 4.1 3.95Nb 5.6 4.1 7.1Rb 68.0 101.0 65Sr 406 394 489Ta 0.3 0.3 0.78Th 8.8 23.5 6.73U 1.4 2.4 1.46V 59 22 47.9Zr 159 142 156Y 10 7 11.5La 38 70 31.1Ce 72 120 57.8Nd 27.1 39.9 22.7Sm 3.92 4.80 3.49Eu 0.94 0.93 0.91Gd 2.62 2.81 2.43Tb 0.39 0.37 0.33Dy 1.84 1.56 1.66Ho 0.34 0.24Er 0.90 0.62 0.79Tm 0.14 0.12Yb 0.81 0.59 0.66Lu 0.12 0.09 0.13Ni 20 7 18.7Cr 57 19 37.8REE tot 156 253(La/Yb)N 46 89 30(La/Sm)N

(Gd/Yb)N

Eu/Eu*#Mg3 28 18 43K2O/Na2O 0.5 1.2 0.4Nb/Ta 22 15 9Sr/Y 88 86 51

Analyses were conducted at ACME Laboratories Ltd., Canada. Analytical specifics are presented inAppendix 2.

1 Averages of oxides in the Bak 1 and Bak 2 suites are from Muslimov and Lapinskaya, 1996.2 Average of Archean TTG is from E. Martin and others, 2008.3 #Mg as MgO/100�(FeO�MgO) wt. %.


4). The Bak 1 suite is mostly magnesian (#Mg 17-38), while the Bak 2 granitoidscontain relatively more FeO. This correlates with Ni and Cr contents, which are 20 and57 ppm in Bak 1, and 7 and 19 (Bak 2), respectively. Characteristically, the Bak 1 suiteis high-Al, with Al2O3 mostly above 15 percent, and has higher CaO and Na2O than theBak 2 (fig. 4). In contrast, K2O contents and K2O/Na2O are much higher in the latter(table 1). The large Bak 2 intrusions like Menzelinsk and Zainsk (MZ and ZA in fig. 1)show somewhat distinct crystallization trends from granodiorites to granite (fig. 4)suggesting fractional crystallization of different parent melts and sources (Popova andPostnikov, 1992).

The Bakaly rocks of both suites are mainly moderate to low in Sr (138 to 441 ppm)with rare deviations to Sr abundances up to 895 ppm in the Bak 1 suite, depending onplagioclase contents. The variations of Th, Y, Zr, Hf and LREE, and their relative peaks(fig. 5) appear to be connected closely to the abundance of zircon, titanite, apatite andallanite. In some samples these reach 2 percent. The Bakaly granitoids are rich in REE,particularly in LREE, but rather poor in HREE (fig. 6). The (La/Yb)N ratios in the Bak1 suite vary between 3 and 117 at (La/Sm)N � 2 to 13 and (Gd/Yb)N � 2 to 5. The lowconcentrations of YbN and Y provide evidence that garnet was present in the meltsource (Wyllie and Wolf, 1993; Rapp and Watson, 1995). Some of the Bak 1 samplesfeature Eu enrichment (Eu/Eu* up to 2.4) and stronger fractionation, while others arerather depleted in Eu (fig. 6B). This can be an effect of melt fractionation with highlycumulative plagioclase in some fractionates and more evolved compositions in others.The REE variations in the Bak 2 granitoids (fig. 6C) resemble those in the Bak 1 suite,but show steeper patterns with (Gd/Yb)N between 2 and 25, and (La/Sm)N � 6 to 12.The leucosome (Tashliar 26L) shows the highest REE fractionation and extremely lowconcentrations of Yb and Lu (fig. 6C).

The Aktanysh suite.—There exist several small intrusions of quartz-monzonitic,quartz-syenitic to granitic compositions in this suite. Most have not yet been studiedextensively, but the two selected samples (Tlyanchi-Tamak 684 and Menzelino-Aktanysh 9) appear distinct in their mineral and chemical compositions (table 1,Appendix 1). Both contain andesine, microperthitic K-feldspar, Na-rich amphibole,annitic biotite and large crystals of titanite. Quartz contents are �10 percent. SampleTlyanchi-Tamak 684 is a deformed, partly recrystallized, medium-grained quartzmonzodiorite, while sample Menzelino-Aktanysh 9 is a well preserved medium-grainedmonzogranite.

The Aktanysh granitoids have markedly high abundances of TiO2, HFSE (Nb, Ta,Zr and Hf), Y, and total REE (table 1). These characteristics define their specific traceelement and REE patterns (figs. 6D and 7D). The REE patterns in both rocks are lessfractionated and feature lower LREE and higher HREE concentrations than in themonzonitic granitoids of the Tashliar suite (fig. 6A). The (La/Yb)N ratios are low(6 and 23) in the Aktanysh rocks, as well as (La/Sm)N (�2-5) and (Gd/Yb)N (2-3)ratios at YbN contents 12 and 18 (table 1). This suggests a shallower, non-garnetbearing melt residue. The variable trace element patterns (fig. 5D) imply somewhatdifferent melt composition and crystallization history of the two studied Aktanyshsamples.

geochronology and isotope composition

We have selected several samples from each of the Bakaly granitoid suites todetermine their U-Pb zircon age using SIMS (Secondary Ion Mass Spectrometry), andLu-Hf isotopic composition of the dated zircon by laser ablation microprobe ICP-MS.Additionally, several of these rock samples were selected for Sm-Nd isotopic analysis.The description of the methods is given in Appendix 2.


U-Pb Ages and Lu-Hf Isotopic Composition of ZirconThe Tashliar suite.—Zircon from the migmatite Tashliar 26 was separated from the

mesosome (26D) and the leucosome vein (26L), but cathodoluminescence (CL)imaging of these phases does not appear to show significantly different zirconpopulations (fig. 7). This may be due to inefficient separation of mesosome andleucosome, but could also indicate that the melt fraction incorporated significantzircon from the mesosome. The zircon from this sample ranges in size from 50 to 150�m and has length to width ratios between 1:1 and 3:1. The grains appear rounded,interpreted to reflect some magmatic abrasion or partial dissolution in a melt phase.The zircon grains are characterized by core and rim domains, both recording oscilla-tory zoning patterns, but this zoning is very faint in some rims (fig. 7). The cores aregenerally darker in CL than the rims. Fifty-one analyses were conducted, twenty-threein the first session on zircon separated from the whole rock, ten each on zirconseparated from the mesosome (26D) and leucosome (26L) portions of the rock in asecond session, eight from the mesosome in a third and another eight in a fourthsession (table 2). Proportions of non-radiogenic Pb were small, with the highest valueof f206 (proportion of non-radiogenic 206Pb in total 206Pb) recorded in a zircon fromthe leucosome (26L-2.1, 1.4%), which also has a high U content of 1479 ppm. U andTh contents are variable, in the ranges of 137 to 2853 ppm and 36 to 1409 ppm,respectively, with Th/U ratios between 0.06 and 0.69. No significant systematicdifference in U and Th content or Th/U ratio is apparent between the zircon from the

26 26

585

6

6rpt9r 9c

10

3.1

2.11.21.1

Fig. 7. Selected CL-images of dated zircon from the Tashliar migmatite (sample Tashliar 26) andmonzodiorite (sample Suleyevo 585). The circles mark the analytical sites and the numbers refer to datashown in table 2; sample numbers are in upper left-hand corner.


Table 2

U-Pb SHRIMP analyses of zircon from granitoids of the Bakaly block

U Th Spot S (ppm) (ppm)

232Th 238U

f206

(%)

238U 206Pb*

1σσσσ abs err

207Pb* 206Pb*

1σσσσ error

206Pb* 238U

age (Ma)

1σσσσ abs err

207Pb* 206Pb*

age (Ma)

C %

Sample Tashliar 26 : 26 - whole rock; 26D - mesosome; 26L - leucosome 26-1 1 0.1 198 63 0.33 1.7546 ± 0.0285 0.25061 ± 0.00180 2907 ± 39 3189± 11 90 26-2 1 0.0 838 380 0.47 1.6667 ± 0.0265 0.25078 ± 0.00067 3030 ± 39 3190± 4 95 26-3 1 0.0 1038 518 0.52 2.2898 ± 0.0351 0.23676 ± 0.00136 2336 ± 30 3098± 9 67 26-4 1 0.0 827 357 0.45 1.7600 ± 0.0273 0.23056 ± 0.00079 2900 ± 37 3056± 6 95 26-5 1 0.0 621 250 0.42 1.7168 ± 0.0264 0.24578 ± 0.00064 2959 ± 37 3158± 4 93 26-6 1 0.0 2853 1409 0.51 1.9331 ± 0.0292 0.18751 ± 0.00032 2688 ± 34 2720± 3 99 26-6rpt 1 0.1 475 80 0.17 2.0719 ± 0.0320 0.18729 ± 0.00061 2539 ± 33 2718± 5 93 26-7 1 0.2 929 409 0.46 2.0703 ± 0.0361 0.23188 ± 0.00067 2540 ± 37 3065± 5 79 26-8 1 0.1 944 353 0.39 2.4922 ± 0.0383 0.22657 ± 0.00143 2175 ± 29 3028± 10 61 26-9r 1 0.1 188 69 0.38 1.5847 ± 0.0272 0.27068 ± 0.00319 3154 ± 44 3310± 18 95 26-9c 1 0.5 1026 614 0.62 2.6738 ± 0.0412 0.23470 ± 0.00203 2048 ± 27 3084± 14 49 26-10 1 0.0 891 473 0.55 1.7820 ± 0.0273 0.23806 ± 0.00167 2872 ± 36 3107± 11 92 26-11r 1 0.3 274 62 0.23 1.8271 ± 0.0335 0.23229 ± 0.00185 2814 ± 42 3068± 13 91 26-12 1 0.2 525 256 0.50 2.1317 ± 0.0391 0.22653 ± 0.00315 2480 ± 38 3028± 22 78 26-13 1 0.0 903 605 0.69 1.6293 ± 0.0249 0.25501 ± 0.00042 3085 ± 38 3216± 3 96 26-14 1 0.1 329 99 0.31 1.7327 ± 0.0607 0.23032 ± 0.00085 2937 ± 84 3054± 6 96 26-14c 1 0.0 1371 788 0.59 1.8157 ± 0.0624 0.21150 ± 0.00044 2828 ± 80 2917± 3 97 26-15 1 0.0 1167 664 0.59 1.9536 ± 0.0673 0.24130 ± 0.00083 2665 ± 76 3129± 5 83 26-16 1 0.0 860 442 0.53 1.5274 ± 0.0527 0.24343 ± 0.00187 3247 ± 90 3143± 12 103 26-17 1 0.0 1097 709 0.67 1.7133 ± 0.0589 0.22776 ± 0.00034 2964 ± 83 3036± 2 98 26-18 1 0.0 979 472 0.50 1.6082 ± 0.0568 0.25026 ± 0.00038 3117 ± 89 3186± 2 98 26-19 1 - 415 52 0.13 1.5710 ± 0.0544 0.25699 ± 0.00080 3175 ± 89 3228± 5 98 26-20 1 0.0 1441 966 0.69 1.6896 ± 0.0581 0.23244 ± 0.00050 2997 ± 84 3069± 3 98 26D-1.1 3 0.0 259 78 0.31 1.5618 ± 0.0195 0.25815 ± 0.00069 3190 ± 32 3235± 4 99 26D-2.1 3 0.0 442 97 0.23 1.6396 ± 0.0140 0.24774 ± 0.00051 3070 ± 21 3170± 3 97 26D-3.1 3 - 248 92 0.38 1.5337 ± 0.0142 0.26144 ± 0.00213 3236 ± 24 3255± 13 99 26D-4.1 3 0.1 153 68 0.46 1.6126 ± 0.0154 0.25668 ± 0.00097 3110 ± 24 3226± 6 96 26D-5.1 3 0.0 765 349 0.47 1.7344 ± 0.0143 0.24940 ± 0.00049 2935 ± 20 3181± 3 92 26D-6.1 3 0.1 260 86 0.34 1.8223 ± 0.0166 0.22962 ± 0.00152 2820 ± 21 3049± 11 92 26D-7.1 3 0.1 609 36 0.06 2.9211 ± 0.0247 0.11933 ± 0.00042 1898 ± 14 1946± 6 98 26D-8.1 3 0.2 151 66 0.45 1.5608 ± 0.0157 0.25969 ± 0.00123 3192 ± 26 3245± 7 98 26D-9.1 3 0.0 137 64 0.48 1.5329 ± 0.0145 0.26210 ± 0.00087 3237 ± 25 3259± 5 99 26D-10.1 3 0.0 196 83 0.44 1.5536 ± 0.0143 0.25525 ± 0.00080 3203 ± 24 3218± 5 100 26L-1.1 3 - 410 114 0.29 1.9489 ± 0.0168 0.18838 ± 0.00084 2670 ± 19 2728± 7 98 26L-2.1 3 1.4 1479 737 0.51 1.8788 ± 0.0154 0.18113 ± 0.00086 2751 ± 19 2663± 8 103 26L-3.1 3 0.9 279 94 0.35 1.9372 ± 0.0171 0.24197 ± 0.00255 2683 ± 20 3133± 17 86 26L-4.1 3 0.2 303 86 0.29 2.4651 ± 0.0222 0.17466 ± 0.00094 2195 ± 17 2603± 9 84 26L-5.1 3 0.0 271 69 0.26 1.9432 ± 0.0171 0.18391 ± 0.00058 2676 ± 20 2688± 5 100 26L-6.1 3 0.0 832 352 0.44 1.7274 ± 0.0204 0.24722 ± 0.00145 2944 ± 28 3167± 9 93 26L-7.1 3 0.0 155 70 0.47 1.5861 ± 0.0200 0.25739 ± 0.00126 3151 ± 32 3231± 8 98 26L-8.1 3 0.0 342 216 0.65 1.8783 ± 0.0390 0.18602 ± 0.00211 2752 ± 47 2707± 19 102 26L-9.1 3 0.0 848 429 0.52 1.5991 ± 0.0132 0.26080 ± 0.00080 3131 ± 21 3252± 5 96 26L-10.1 3 0.0 421 110 0.27 1.9181 ± 0.0162 0.18619 ± 0.00045 2705 ± 19 2709± 4 100 26-3.1.1 4 0.1 402 150 0.39 1.7150 ± 0.0199 0.25651 ± 0.00124 2961.3 ± 27.5 3225± 8 91 26-3.2.1 4 0.4 169 64 0.39 1.6241 ± 0.0206 0.257448± 0.00225 3093 ± 31.2 3231± 14 96 26-3.4.1 4 0.0 236 85 0.37 1.9540 ± 0.0236 0.286974± 0.00900 2664.2 ± 26.4 3401± 49 72 26-3.9.1 4 0.2 186 55 0.30 1.6832 ± 0.0269 0.266389± 0.00174 3006 ± 38.5 3285± 10 91 26-3.3.1 4 0.0 535 179 0.35 1.8715 ± 0.0217 0.253086± 0.00357 2759.7 ± 26 3204± 22 84 26-3.5.1 4 0.6 315 142 0.47 2.3499 ± 0.0309 0.247415± 0.00134 2285.6 ± 25.3 3168± 9 61 26-3.6.1 4 0.2 218 83 0.39 1.3949 ± 0.0181 0.255243± 0.00102 3484.4 ± 34.8 3218± 6 108 26-3.9.1 4 0.2 186 55 0.30 1.6832 ± 0.0269 0.266389± 0.00174 3006 ± 38.5 3285± 10 91 Sample Suleyevo 585 585-1 2 0.0 314 149 0.49 1.5855 ± 0.0550 0.26532 ± 0.00170 3152 ± 88 3279± 10 96 585-2 2 0.0 172 174 1.05 1.5512 ± 0.0540 0.26031 ± 0.00086 3207 ± 90 3249± 5 99 585-3 2 0.0 309 183 0.61 1.6151 ± 0.0558 0.25279 ± 0.00061 3107 ± 87 3202± 4 97 585-4 2 0.0 175 138 0.81 1.3945 ± 0.0488 0.26395 ± 0.00113 3485 ± 96 3270± 7 106


Table 2

(continued)U Th Spot S

(ppm) (ppm)

232Th238U

f206

(%)

238U206Pb*

1σσσσ abs err

207Pb*206Pb*

1σσσσ error

206Pb*238U

age (Ma)

1σσσσabs err

207Pb*206Pb*

age (Ma)

C %

Sample Suleyevo 585585-5 2 0.0 100 117 1.21 1.5245 ± 0.0537 0.26378 ± 0.00114 3251 ± 92 3269± 7 99 585-6 2 0.0 149 169 1.18 1.4591 ± 0.0510 0.26063 ± 0.00240 3365 ± 94 3251± 15 103 585-7 2 0.0 162 180 1.15 1.5464 ± 0.0538 0.26505 ± 0.00078 3215 ± 90 3277± 5 98 585-8c 2 - 231 313 1.40 1.6663 ± 0.0586 0.26177 ± 0.00069 3030 ± 87 3257± 4 93 585-8r 2 0.0 126 115 0.95 1.5434 ± 0.0538 0.25963 ± 0.00089 3220 ± 90 3244± 5 99 585-9 2 0.0 204 239 1.21 1.5734 ± 0.0548 0.26657 ± 0.00083 3172 ± 89 3286± 5 96 585-10 2 0.0 202 203 1.04 1.5783 ± 0.0550 0.26098 ± 0.00083 3164 ± 89 3253± 5 97 585-11 2 0.1 554 33 0.06 2.1142 ± 0.0732 0.18453 ± 0.00049 2497 ± 73 2694± 4 92 585-1.1 3 0.0 226 103 0.47 1.5278 ± 0.0191 0.26717 ± 0.00075 3239 ± 23 3279± 4 99 585-2.1 3 0.0 150 179 1.23 1.5284 ± 0.0197 0.26140 ± 0.00092 3244 ± 25 3253± 6 100 585-3.1 3 0.1 135 282 2.16 1.5358 ± 0.0226 0.35707 ± 0.00139 2815 ± 23 3156± 9 89 585-4.1 3 0.0 212 160 0.78 1.5685 ± 0.0201 0.28555 ± 0.00082 3092 ± 23 3258± 5 95 585-5.1 3 - 96 80 0.86 1.5629 ± 0.0208 0.26603 ± 0.00106 3185 ± 26 3277± 6 97 585-6.1 3 0.0 777 378 0.50 1.5896 ± 0.0198 0.25624 ± 0.00038 3144 ± 23 3221± 2 98 585-7.1 3 0.0 217 100 0.48 1.4865 ± 0.0186 0.26630 ± 0.00089 3316 ± 24 3284± 5 101 585-8.1 3 0.1 176 94 0.55 1.6605 ± 0.0213 0.27224 ± 0.00131 3012 ± 23 3275± 8 92 585-9.1 3 0.0 287 132 0.47 1.5304 ± 0.0188 0.26379 ± 0.00098 3241 ± 23 3268± 6 99 585-10.1 3 - 170 80 0.49 1.5679 ± 0.0202 0.26290 ± 0.00120 3177 ± 24 3259± 7 97 585.2.1 4 0.0 195 81 0.43 1.6224 ± 0.0325 0.27349 ± 0.00439 3095 ± 49 3326± 25 93 585.1.2 4 2.1 901 670 0.77 3.9162 ± 0.0554 0.22330 ± 0.00145 1466 ± 19 3005± 10 -5 585.3.1 4 1.5 1082 680 0.65 2.4475 ± 0.0282 0.23840 ± 0.00130 2208 ± 22 3109± 9 59 585.4.1 4 0.0 372 478 1.33 3.5158 ± 0.0433 0.26792 ± 0.00312 1614 ± 18 3294± 18 -4 585.5.1 4 0.4 741 369 0.51 1.8981 ± 0.0221 0.25394 ± 0.00076 2728 ± 26 3210± 5 82 585.6.1 4 0.3 816 402 0.51 2.0148 ± 0.0382 0.24418 ± 0.00127 2598 ± 41 3147± 8 79 585.7.1 4 0.0 657 270 0.42 1.6103 ± 0.0184 0.27465 ± 0.00116 3114 ± 28 3333± 7 93 585.8.1 4 0.0 203 656 3.34 2.3899 ± 0.0369 0.27235 ± 0.00523 2253 ± 29 3320± 30 53 585.9.1 4 0.1 1047 593 0.58 1.6201 ± 0.0182 0.25208 ± 0.00150 3099 ± 28 3198± 9 97 585.10.1 4 0.0 693 318 0.47 1.5849 ± 0.0180 0.24765 ± 0.00065 3153 ± 28 3170± 4 99 585.12.1 4 0.1 860 515 0.62 1.9187 ± 0.0222 0.25607 ± 0.00117 2704 ± 25 3223± 7 81 585.13.1 4 0.0 120 119 1.02 1.5656 ± 0.0205 0.27154 ± 0.00239 3184 ± 33 3315± 14 96 Sample Ural'skaya 4000240002-1 1 0.0 1141 565 0.51 1.9513 ± 0.0304 0.18718 ± 0.00074 2667 ± 32 2715± 7 98 40002-2 1 0.0 849 1921 2.34 1.9419 ± 0.0323 0.18513 ± 0.00036 2678 ± 32 2724± 3 98 40002-3 1 0.0 504 725 1.49 1.9277 ± 0.0332 0.18579 ± 0.00046 2694 ± 33 2714± 4 99 40002-4 1 0.0 717 461 0.67 2.2028 ± 0.0304 0.18715 ± 0.00125 2413 ± 30 2652± 11 90 40002-5 1 1.0 265 131 0.51 1.9932 ± 0.0302 0.18539 ± 0.00203 2621 ± 33 2646± 19 99 40002-6 1 0.0 341 505 1.53 1.9384 ± 0.0296 0.18758 ± 0.00053 2682 ± 33 2723± 5 98 40002-7 1 0.0 1134 2419 2.20 1.9422 ± 0.0323 0.18618 ± 0.00047 2677 ± 32 2720± 4 98 40002-8 1 0.0 722 364 0.52 1.9514 ± 0.0288 0.18749 ± 0.00037 2667 ± 35 2713± 3 98 40002-9 1 0.0 967 1737 1.86 2.1653 ± 0.0292 0.18590 ± 0.00037 2448 ± 30 2703± 3 90 40002-10 1 0.1 742 2289 3.19 2.5484 ± 0.0307 0.18645 ± 0.00051 2134 ± 27 2688± 5 74 Sample Mustafino 3737-1 1 2.4 227 112 0.51 1.9757 ± 0.0352 0.18719 ± 0.01180 2640 ± 39 2718± 104 97 37-2 1 0.0 246 74 0.31 1.9854 ± 0.0448 0.18345 ± 0.00156 2630 ± 49 2684± 14 98 37-3 1 1.6 353 116 0.34 2.4780 ± 0.0383 0.18332 ± 0.00301 2185 ± 29 2683± 27 77 37-4 1 0.3 267 23 0.09 1.9008 ± 0.0290 0.18408 ± 0.00353 2725 ± 34 2690± 32 101 37-5 1 0.3 630 3 0.00 2.5511 ± 0.0944 0.14118 ± 0.00124 2132 ± 67 2242± 15 95 37-6 1 0.6 518 177 0.35 2.0314 ± 0.0390 0.18141 ± 0.00107 2581 ± 41 2666± 10 97 37-7 1 0.1 160 61 0.39 1.4044 ± 0.0363 0.31468 ± 0.00908 3466 ± 69 3544± 44 98 37-8 1 0.1 917 136 0.15 7.7695 ± 0.1303 0.15439 ± 0.00089 780 ± 12 2395± 10 -107 37-9 1 0.6 154 73 0.49 1.9059 ± 0.0302 0.18430 ± 0.00183 2719 ± 35 2692± 16 101 37-10 1 0.3 579 147 0.26 2.1467 ± 0.0331 0.17968 ± 0.00077 2465 ± 32 2650± 7 93


leucosome and mesosome. Taken together, the data appear to form two clusters, bothaffected by trends defined by recent Pb-loss. The older cluster is dominated by zirconseparated from the mesosome, but also incorporates zircon from whole-rock andleucosome. Nine analyses plot on concordia and define a weighted mean 207Pb/206Pbage of 3237�11 Ma (fig. 8). The younger cluster is defined by six concordant datapoints collected from zircon from the leucosome, and defines a weighted mean207Pb/206Pb age of 2710�19 Ma (fig. 8). One data point (26D-7.1), collected from arim domain on a zircon from the mesosome, recorded a low Th/U ratio of 0.06, anddefines a concordant 207Pb/206Pb age of 1946�6 Ma. This analysis may indicate ametamorphic event at that time, although more data would be required to verify this.We interpret the crystallization age of the protolith to be 3237�11 Ma, as recorded bythe mesosome, while migmatization took place at 2710�19 Ma as shown by the age ofzircon from the leucosome.

Table 2

(continued)U Th Spot S

(ppm) (ppm)

232Th238U

f206

(%)

238U206Pb*

1σσσσ abs err

207Pb*206Pb*

1σσσσ error

206Pb*238U

age (Ma)

1σσσσabs err

207Pb*206Pb*

age (Ma)

C %

Sample Tumenyak 50 50-1 1 0.2 405 196 0.50 2.0518 ± 0.0311 0.17874 ± 0.00105 2559 ± 32 2641± 10 97 50-2 1 0.0 297 103 0.36 2.1443 ± 0.0331 0.18003 ± 0.00103 2468 ± 32 2653± 10 92 50-3 1 4.7 516 229 0.46 2.3470 ± 0.1100 0.17587 ± 0.00736 2288 ± 90 2614± 70 86 50-4 1 1.6 412 233 0.58 2.2549 ± 0.0339 0.17604 ± 0.00236 2366 ± 30 2616± 22 89 50-5 1 0.2 346 62 0.19 2.0285 ± 0.0309 0.17864 ± 0.00070 2584 ± 32 2640± 7 98 50-6 1 6.1 716 260 0.38 2.4282 ± 0.0629 0.17277 ± 0.02443 2223 ± 49 2585± 236 84 50-7 1 3.1 275 100 0.38 2.1170 ± 0.0326 0.17793 ± 0.00399 2494 ± 32 2634± 37 94 50-8 1 0.1 155 38 0.25 1.9812 ± 0.0368 0.18121 ± 0.00086 2634 ± 40 2664± 8 99 Sample Tlyanchi-Tamak 684 684-1c 1 0.1 174 74 0.44 1.6959 ± 0.0301 0.22736 ± 0.00320 2988 ± 42 3034± 23 98 684-2 1 0.1 297 70 0.24 2.3211 ± 0.0415 0.15726 ± 0.00060 2309 ± 35 2426± 7 95 684-3r 1 0.6 326 63 0.20 2.1024 ± 0.0323 0.16779 ± 0.00202 2508 ± 32 2536± 20 99 684-4 1 0.1 269 220 0.85 1.5827 ± 0.0257 0.23347 ± 0.00128 3157 ± 41 3076± 9 103 684-5r 1 0.2 208 69 0.34 2.0735 ± 0.0347 0.17161 ± 0.00162 2537 ± 35 2573± 16 99 684-6 1 0.3 216 83 0.40 2.0575 ± 0.0363 0.17735 ± 0.00141 2554 ± 37 2628± 13 97 684-7 1 0.2 365 94 0.27 2.0386 ± 0.0319 0.17325 ± 0.00116 2573 ± 33 2589± 11 99 684-8r 1 0.2 271 55 0.21 1.9559 ± 0.0299 0.17859 ± 0.00116 2662 ± 33 2640± 11 101 684-9 1 0.0 394 68 0.18 1.9878 ± 0.0302 0.17230 ± 0.00190 2627 ± 33 2580± 18 102 684-10 1 0.0 1105 113 0.11 2.1979 ± 0.0323 0.17368 ± 0.00034 2417 ± 30 2593± 3 93 684-11 1 - 545 129 0.24 2.0323 ± 0.0442 0.17871 ± 0.00087 2580 ± 46 2641± 8 98 684-12 1 0.0 627 55 0.09 2.0557 ± 0.0397 0.18046 ± 0.00300 2555 ± 41 2657± 28 96 684-13 1 0.1 264 58 0.23 2.0746 ± 0.0319 0.17460 ± 0.00175 2536 ± 32 2602± 17 97 684-14 1 - 354 84 0.25 2.4700 ± 0.0379 0.14457 ± 0.00127 2191 ± 29 2283± 15 96 Sample Menzelino-Aktanysh 99-1 1 0.2 300 191 0.66 3.1512 ± 0.0552 0.16925 ± 0.00082 1777 ± 27 2550± 8 56 9-2 1 0.1 255 164 0.66 2.3300 ± 0.0355 0.17213 ± 0.00069 2302 ± 30 2578± 7 88 9-3 1 0.2 289 193 0.69 2.2883 ± 0.0347 0.17153 ± 0.00074 2337 ± 30 2573± 7 90 9-4 1 0.5 198 68 0.35 2.1532 ± 0.0333 0.17363 ± 0.00125 2459 ± 32 2593± 12 95 9-5 1 0.0 266 158 0.62 2.0290 ± 0.0311 0.17509 ± 0.00082 2583 ± 33 2607± 8 99 9-6 1 0.4 269 184 0.71 2.5960 ± 0.0400 0.16800 ± 0.00095 2101 ± 28 2538± 10 79 9-7 1 0.1 216 89 0.43 2.0326 ± 0.0311 0.17375 ± 0.00105 2579 ± 33 2594± 10 99 9-8 1 0.0 246 212 0.89 2.2226 ± 0.0338 0.17234 ± 0.00064 2395 ± 30 2581± 6 92

f206�the proportion of common 206Pb in the total 206Pb; * All ratios and ages are corrected for commonPb using measured 204Pb and composition appropriate to the age of the zircon (Stacey and Kramers, 1975);C�concordance (%).


Zircon grains from sample Suleyevo 585 range in size from 100 to 150 �m andhave aspect ratios between 1:1 and 2:1. Like sample 26, the crystals appear to besomewhat rounded, interpreted to reflect some chemical abrasion in the melt phase.CL images indicate complex zoning patterns, with quite a few zircon showing core andrim domains, both recording oscillatory zoning (fig. 7). In most cases the core shows alower CL response than the rim. Twenty-two analyses were conducted during twoseparate sessions and indicate very low f206 values (table 2). U and Th contents are

2690

2650

Tashliar 26

2730

Tashliar 26 (leucosome)2710±19 Ma

432

0

207

206

Pb/

Pb

age

(Ma)

3280

Tashliar 26 (mesosome)3237±11 Ma

26D (mesosome)

26L (leucosome)

Error crosses at 2σ

Error bars at 2σ Error bars at 2σ

0.12

0.16

0.20

0.24

0.28

0.32

1.4 1.8 2.2 2.6 3.0

238U/206Pb

207P

b/20

6P

b

2000

2800

360026 (whole rock)

26 (session 4)

207

206

Pb/

Pb

age

(Ma)

Fig. 8. Concordia diagram for zircon from the Tashliar migmatite (sample Tashliar 26), together withdata used to calculate the weighted mean of the two concordant populations.


variable, in the ranges 96 to 777 ppm and 33 to 378 ppm respectively, with Th/U ratiosbetween 0.47 and 2.16, but one analysis records an extremely low ratio of 0.06 (585-11).The majority of data plot in a tight cluster, with eighteen concordant analyses defininga weighted mean 207Pb/206Pb age of 3266�7 Ma (fig. 9). Analysis 585-11, whichshowed the anomalously low Th/U ratio of 0.06, records a 207Pb/206Pb age of 2694�9Ma, but this value can only be taken as a minimum age estimate as the data are only 92

3230

3290

207 206Pb/ Pb age (Ma)


Suleyevo 5853266±7 Ma

0.18

0.20

0.22

0.24

0.26

0.28

1.5 2.5 3.5 4.5

238U/206Pb

207P

b/20

6P

b

2600

3000

3400

Fig. 9. Concordia diagram for zircon from the Tashliar monzodiorite (sample Suleyevo 585), togetherwith data used to calculate the weighted mean of 3266�7 Ma.


percent concordant. A number of analyses, though discordant, indicate potentiallyolder inherited components up to �3.4 Ga (table 2, fig. 9). In conclusion, we assignthe age of 3266�7 Ma to the crystallization of the protolith, while the single analysis ofa low-Th/U zircon with age 2694�9 Ma may record a metamorphic overprint at thattime, as it is close to the age of the vein leucosome from the migmatite sample Tashliar 26.

The zircon populations from both the 3266 Ma monzodiorite (Suleyevo 585) andthe 3237 Ma mesosome of the migmatite (Tashliar 26) have a narrow range of εHf (T)values varying from around �2.5 to �4, indicating a moderately evolved source. Themodel ages suggest that the age of the protolith is at least 3.45 Ga and could be as old as3.8 Ga (see crustal TDM model age in table 3), if the modeling is done using the Lu/Hfcomposition of the average continental crust of 0.015. This is more comparable to theNd TDM ages for these rocks (table 4). The older population (3285-3326 Ma) fromthese 2 samples (585 & 26) shows moderately positive εHf (T) values, but their modelages are similar to the younger population (3170-3231 Ma). This may imply that thehost rocks for both zircon populations derived from the same protolith or similarsource.

The Bak 1 suite.—Zircon from sample Ural’skaya 40002 ranges in size from 100 to250 �m and has length to width ratios between 2:1 and 3:1. The grains are somewhatrounded owing to magmatic abrasion. CL imagery reveals broad, washed out sectorzoning, possibly indicating some solid-state recrystallization (fig. 10). Ten zircon grainswere analyzed during a single session and, with the exception of one analysis, recordvery low f206 values (table 2). U and Th are quite high, in the ranges 265 to 1141 ppmand 131 to 2419 ppm, respectively, with relatively high Th/U ratios between 0.51 and3.19. Six concordant data points record a weighted mean 207Pb/206Pb age of 2718�5Ma, taken to be the best estimate for the crystallization age of the quartz diorite(fig. 11). One zircon (40002-5) recorded a younger concordant 207Pb/206Pb age of2646�38 Ma, but this zircon also recorded f206 � 1 percent. Nevertheless, we considerit plausible that this analysis records some neocrystallization of zircon at that time.

Zircon from sample Ural’skaya 40002, using its crystallization age of 2718 Ma,shows a small range of Hf TDM crustal ages between 3.2 and 3.4 Ga (fig. 12, table 3)indicating a quite homogeneous protolith of at least �3.2 Ga age. εHf (T) values in therange 0.2 to �2.5 indicate a small involvement of older material during zirconcrystallization.

The Bak 2 suite.—Zircon grains from sample Mustafino 37 range in size from 100 to200 �m, and have aspect ratios between 1:1 and 3:1. The crystals are euhedral, withwell-defined bipyramidal terminations. CL imagery reveals concentric zoning patterns,with mottled appearance interpreted to reflect some solid-state recrystallization (fig. 10).Some grains clearly display core and rim domains, both showing oscillatory patterns.Ten analyses were conducted during a single session and record variable f206 values upto 2.4. U and Th are in the ranges 154 to 917 ppm and 3 to 177 ppm, respectively,yielding Th/U ratios between 0.00 and 0.51 (table 2). A core (37-7) gave a concordant207Pb/206Pb age of 3544�88 Ma, interpreted as xenocrystic. A concordant cluster of sixdata allows the calculation of a weighted mean 207Pb/206Pb age of 2663�18 Ma,interpreted to represent the crystallization age of the tonalite rock (fig. 13). An analysisof a zircon rim with an extremely low Th/U ratio (37-5) corresponds to a concordant207Pb/206Pb age of �2.24 Ga, which could record a metamorphic event.

Zircon grains extracted from sample Tumenyak 50 are from 150 to 300 �m in sizeand have relatively high aspect ratios between 2:1 and 4:1. The crystals are subhedral inshape, with clearly defined subrounded bi-pyramidal terminations (fig. 10). CL imag-ery reveals prominent concentric zoning, with invasive zones of low CL-responsesuggesting solid-state recrystallization. Eight analyses were conducted during a single


Tab

le3

Lu-

Hf

data

for

date

dzi

rcon

from

gran

itoid

sof

the

Bak

aly

bloc

k17

6 Lu

λ λa : B

liche

rt-T

oft

and

othe

rs (

1997

)Sc

here

r an

d ot

hers

(20

01)

Biz

zarr

o an

d ot

hers

(20

03)

Ana

lysi

s

NA

ge

(Ma)

176 H

f17

7 Hf

1 SE

17

6 Lu

177 H

f

176 Y

b17

7 Hf

Hf i

ε Hf

1 SE

TD

M

(Ga)

TD

MC

rust

al

(Ga)

Hf i

ε Hf

1 SE

T

DM

(Ga)

TD

MC

rust

al

(Ga)

Hf i

ε Hf

1 SE

TD

M

(Ga)

TD

MC

rust

al

(Ga)

Sam

ple:

Tas

hlia

r 26

Tas

hlia

r su

ite

26.3

-1

3225

0.

2807

020.

0000

090.

0006

30.

026

0.28

0662

0.8

0.3

3.38

3.50

0.28

0663

-1.8

0.

3 3.

503.

700.

2806

602.

9 0.

3 3.

293.

3426

.3-2

32

310.

2806

89

0.00

0010

0.00

089

0.03

6 0.

2806

31-0

.2

0.3

3.42

3.56

0.28

0633

-2.7

0.

3 3.

543.

760.

2806

301.

9 0.

3 3.

333.

4026

.3-3

32

040.

2806

72

0.00

0010

0.00

081

0.03

3 0.

2806

21-1

.2

0.3

3.44

3.60

0.28

0622

-3.7

0.

3 3.

563.

810.

2806

190.

9 0.

3 3.

343.

4526

.3-9

32

850.

2807

18

0.00

0012

0.00

038

0.01

9 0.

2806

933.

4 0.

4 3.

343.

380.

2806

940.

7 0.

4 3.

463.

580.

2806

935.

5 0.

4 3.

253.

23Sa

mpl

e: S

uley

evo

585

585-

2-1

3326

0.28

0732

0.

0000

100.

0007

70.

040

0.28

0681

3.9

0.4

3.36

3.38

0.28

0682

1.2

0.4

3.47

3.58

0.28

0679

6.1

0.4

3.27

3.23

585-

5-1

3210

0.28

0686

0.

0000

110.

0007

50.

033

0.28

0638

-0.4

0.

4 3.

413.

560.

2806

40-3

.0

0.4

3.53

3.76

0.28

0637

1.7

0.4

3.32

3.40

585-

9-1

3198

0.28

0744

0.

0000

110.

0010

00.

053

0.28

0680

0.8

0.4

3.36

3.48

0.28

0683

-1.7

0.

4 3.

483.

670.

2806

792.

9 0.

4 3.

273.

3258

5-10

-1

3170

0.28

0727

0.

0000

150.

0007

90.

038

0.28

0677

0.0

0.5

3.36

3.50

0.28

0679

-2.5

0.

5 3.

483.

700.

2806

762.

1 0.

5 3.

273.

3558

5-12

-1

3223

0.28

0758

0.

0000

120.

0008

20.

042

0.28

0705

2.3

0.4

3.33

3.40

0.28

0707

-0.3

0.

4 3.

443.

600.

2807

044.

4 0.

4 3.

243.

2558

5-13

-1C

33

150.

2806

88

0.00

0020

0.00

049

0.02

9 0.

2806

562.

8 0.

7 3.

393.

440.

2806

570.

1 0.

7 3.

513.

650.

2806

554.

9 0.

7 3.

303.

2958

5-13

-1R

33

150.

2807

85

0.00

0019

0.00

094

0.06

9 0.

2807

235.

1 0.

7 3.

303.

300.

2807

252.

5 0.

7 3.

423.

490.

2807

217.

3 0.

7 3.

213.

14Sa

mpl

e: U

ral’

skay

a 40

002

Bak

1 s

uite

4000

2-01

27

180.

2811

410.

0000

130.

0019

30.

100

0.28

1037

1.9

0.5

2.91

3.04

0.28

1040

-0.2

0.

5 3.

013.

210.

2810

343.

6 0.

5 2.

842.

9140

002-

02

2718

0.28

0997

0.

0000

110.

0002

80.

009

0.28

0982

-0.1

0.

4 2.

983.

160.

2809

82-2

.3

0.4

3.08

3.34

0.28

0981

1.7

0.4

2.90

3.03

4000

2-03

27

180.

2811

54

0.00

0010

0.00

198

0.09

7 0.

2810

482.

3 0.

4 2.

903.

020.

2810

510.

2 0.

4 3.

003.

180.

2810

454.

0 0.

4 2.

822.

8940

002-

04

2718

0.28

1101

0.

0000

110.

0016

30.

074

0.28

1013

1.1

0.4

2.94

3.09

0.28

1016

-1.0

0.

4 3.

053.

260.

2810

112.

8 0.

4 2.

862.

9640

002-

05

2718

0.28

1066

0.

0000

120.

0016

80.

082

0.28

0975

-0.3

0.

4 2.

993.

180.

2809

79-2

.4

0.4

3.10

3.35

0.28

0973

1.4

0.4

2.91

3.04

4000

2-06

27

180.

2811

14

0.00

0011

0.00

170

0.07

9 0.

2810

221.

4 0.

4 2.

933.

070.

2810

25-0

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0.4

3.03

3.24

0.28

1020

3.1

0.4

2.85

2.94

4000

2-07

27

180.

2810

65

0.00

0011

0.00

122

0.05

7 0.

2809

990.

6 0.

4 2.

963.

120.

2810

02-1

.6

0.4

3.06

3.29

0.28

0998

2.3

0.4

2.88

2.99

4000

2-08

27

180.

2811

72

0.00

0015

0.00

282

0.12

8 0.

2810

201.

3 0.

5 2.

943.

080.

2810

25-0

.7

0.5

3.04

3.24

0.28

1016

2.9

0.5

2.86

2.95

4000

2-09

27

180.

2809

91

0.00

0008

0.00

031

0.01

0 0.

2809

74-0

.3

0.3

2.99

3.18

0.28

0975

-2.5

0.

3 3.

093.

350.

2809

741.

4 0.

3 2.

913.

0440

002-

10

2718

0.28

1049

0.

0000

110.

0012

50.

056

0.28

0982

-0.1

0.

4 2.

983.

160.

2809

84-2

.2

0.4

3.09

3.33

0.28

0980

1.7

0.4

2.90

3.03

Sam

ple:

Mus

tafi

no 3

7

Bak

2 s

uite

37-0

1 26

630.

2811

930.

0000

140.

0007

70.

040

0.28

1152

4.7

0.5

2.76

2.83

0.28

1154

2.6

0.5

2.85

2.99

0.28

1151

6.4

0.5

2.68

2.70

37-0

2 26

630.

2812

51

0.00

0014

0.00

163

0.06

9 0.

2811

655.

1 0.

5 2.

742.

800.

2811

683.

1 0.

5 2.

842.

950.

2811

636.

8 0.

5 2.

672.

6837

-03

2663

0.28

1202

0.

0000

130.

0007

50.

037

0.28

1163

5.0

0.5

2.74

2.80

0.28

1164

2.9

0.5

2.84

2.96

0.28

1162

6.8

0.5

2.67

2.68

37-0

4 26

630.

2811

74

0.00

0014

0.00

061

0.02

8 0.

2811

424.

3 0.

5 2.

772.

850.

2811

432.

2 0.

5 2.

873.

010.

2811

416.

0 0.

5 2.

702.

7237

-05

2663

0.28

1164

0.

0000

170.

0005

90.

026

0.28

1133

4.0

0.6

2.78

2.87

0.28

1134

1.9

0.6

2.88

3.03

0.28

1132

5.7

0.6

2.71

2.74

37-0

6 26

630.

2811

15

0.00

0016

0.00

084

0.04

2 0.

2810

711.

8 0.

6 2.

863.

010.

2810

72-0

.3

0.6

2.96

3.17

0.28

1069

3.5

0.6

2.79

2.88

37-0

7 26

630.

2811

50

0.00

0011

0.00

062

0.02

8 0.

2811

173.

4 0.

4 2.

802.

900.

2811

181.

3 0.

4 2.

903.

070.

2811

165.

1 0.

4 2.

732.

7837

-08

2663

0.28

1135

0.

0000

150.

0005

40.

025

0.28

1107

3.0

0.5

2.82

2.93

0.28

1108

0.9

0.5

2.91

3.09

0.28

1106

4.8

0.5

2.74

2.80

37-0

9 26

630.

2811

55

0.00

0012

0.00

074

0.03

5 0.

2811

163.

4 0.

4 2.

802.

910.

2811

181.

3 0.

4 2.

903.

070.

2811

155.

1 0.

4 2.

732.

7837

-10

2663

0.28

1274

0.

0000

180.

0011

40.

047

0.28

1214

6.9

0.6

2.68

2.69

0.28

1216

4.8

0.6

2.77

2.84

0.28

1212

8.6

0.6

2.61

2.57


Tab

le3

(con

tinue

d)17

6 Lu

λλa : B

liche

rt-T

oft

and

othe

rs (

1997

)Sc

here

r an

d ot

hers

(20

01)

Biz

zarr

o an

d ot

hers

(20

03)

Ana

lysi

s

NA

ge

(Ma)

176 H

f17

7 Hf

1 SE

17

6 Lu

177 H

f

176 Y

b17

7 Hf

Hf i

ε Hf

1 SE

TD

M

(Ga)

TD

MC

rust

al

(Ga)

Hf i

ε Hf

1 SE

T

DM

(Ga)

TD

MC

rust

al

(Ga)

Hf i

ε Hf

1 SE

TD

M

(Ga)

TD

MC

rust

al

(Ga)

Sam

ple:

Tum

enya

k50

50-0

1 26

480.

2811

07

0.00

0018

0.00

125

0.06

1 0.

2810

420.

4 0.

6 2.

913.

080.

2810

44-1

.7

0.6

3.01

3.25

0.28

1040

2.1

0.6

2.83

2.95

50-0

2 26

480.

2811

98

0.00

0032

0.00

237

0.10

3 0.

2810

741.

5 1.

1 2.

873.

010.

2810

78-0

.5

1.1

2.97

3.17

0.28

1070

3.1

1.1

2.79

2.89

50-0

3 26

480.

2811

36

0.00

0015

0.00

111

0.05

4 0.

2810

781.

7 0.

5 2.

863.

000.

2810

80-0

.4

0.5

2.96

3.17

0.28

1076

3.4

0.5

2.78

2.87

50-0

426

480.

2805

81

0.00

0015

0.00

102

0.05

1 0.

2805

28-1

7.9

0.5

3.57

4.20

0.28

0529

-20.

00.

5 3.

704.

400.

2805

26-1

6.2

0.5

3.48

4.04

50-0

526

480.

2811

14

0.00

0013

0.00

127

0.06

0 0.

2810

470.

6 0.

5 2.

903.

070.

2810

50-1

.5

0.5

3.00

3.23

0.28

1046

2.3

0.5

2.82

2.94

50-0

626

480.

2807

92

0.00

0025

0.00

154

0.06

8 0.

2807

11-1

1.4

0.9

3.34

3.80

0.28

0714

-13.

40.

9 3.

463.

990.

2807

09-9

.7

0.9

3.26

3.65

50-0

726

480.

2810

75

0.00

0009

0.00

076

0.03

4 0.

2810

350.

2 0.

3 2.

913.

090.

2810

37-1

.9

0.3

3.01

3.26

0.28

1034

1.9

0.3

2.83

2.96

50-0

826

480.

2810

81

0.00

0015

0.00

119

0.04

8 0.

2810

19-0

.4

0.5

2.94

3.13

0.28

1021

-2.5

0.

5 3.

043.

300.

2810

171.

2 0.

5 2.

863.

0050

-09

2648

0.28

1186

0.

0000

150.

0016

30.

076

0.28

1100

2.5

0.5

2.83

2.95

0.28

1103

0.4

0.5

2.93

3.11

0.28

1098

4.1

0.5

2.75

2.83

50-1

026

480.

2811

15

0.00

0015

0.00

139

0.06

3 0.

2810

420.

4 0.

5 2.

913.

080.

2810

45-1

.7

0.5

3.01

3.25

0.28

1040

2.1

0.5

2.83

2.95

Sam

ple:

Men

zelin

o-A

ktan

ysh

9

A

ktan

ysh

suite

9-

0126

000.

2811

240.

0000

130.

0013

80.

067

0.28

1053

-0.4

0.5

2.89

3.09

0.28

1056

-2.4

0.5

2.99

3.25

0.28

1051

1.3

0.5

2.82

2.96

9-02

2600

0.28

1063

0.

0000

100.

0008

70.

043

0.28

1018

-1.6

0.

4 2.

933.

170.

2810

20-3

.7

0.4

3.04

3.34

0.28

1017

0.1

0.4

2.86

3.03

9-03

2600

0.28

1039

0.

0000

090.

0012

30.

070

0.28

0976

-3.1

0.

3 2.

993.

260.

2809

78-5

.1

0.3

3.10

3.43

0.28

0974

-1.5

0.

3 2.

913.

139-

0426

000.

2810

46

0.00

0008

0.00

166

0.08

8 0.

2809

61-3

.7

0.3

3.02

3.29

0.28

0964

-5.7

0.

3 3.

123.

460.

2809

58-2

.0

0.3

2.94

3.16

9-05

2600

0.28

1087

0.

0000

100.

0014

00.

072

0.28

1015

-1.7

0.

3 2.

943.

170.

2810

18-3

.7

0.3

3.05

3.34

0.28

1013

-0.1

0.

3 2.

863.

049-

0626

000.

2811

01

0.00

0009

0.00

138

0.07

2 0.

2810

30-1

.2

0.3

2.92

3.14

0.28

1032

-3.2

0.

3 3.

033.

310.

2810

280.

4 0.

3 2.

853.

019-

0726

000.

2810

56

0.00

0010

0.00

134

0.06

6 0.

2809

87-2

.7

0.4

2.98

3.23

0.28

0989

-4.7

0.

4 3.

083.

400.

2809

85-1

.1

0.4

2.90

3.10

9-08

2600

0.28

1127

0.

0000

270.

0042

10.

240

0.28

0910

-5.5

1.

0 3.

123.

400.

2809

18-7

.3

1.0

3.23

3.57

0.28

0904

-4.0

1.

0 3.

043.

279-

0926

000.

2811

08

0.00

0014

0.00

186

0.10

5 0.

2810

12-1

.8

0.5

2.95

3.18

0.28

1015

-3.8

0.

5 3.

053.

350.

2810

09-0

.2

0.5

2.87

3.05

9-10

2600

0.28

1081

0.

0000

110.

0014

60.

080

0.28

1006

-2.1

0.

4 2.

963.

190.

2810

09-4

.1

0.4

3.06

3.36

0.28

1004

-0.4

0.

4 2.

883.

06

aIn

itia

lHf,

εHfa

nd

Mod

elag

esca

lcul

atio

nsd

one

usin

gdi

ffer

ent1

76L

ude

cay

con

stan

ts(l

)re

port

edin

liter

atur

e.A

llfi

gure

swer

edo

ne

wit

hva

lues

calc

ulat

edus

ing

Sch

erer

and

oth

ers

(200

1)co

nst

ant.

Hf i�

17

6H

f/177H

frat

io�

(176L

u/177H

frat

io�(E

XP(

176L

u�207Pb

/206Pb

rati

o/10

00)

�1)

).


session and indicated relatively high f206 values up to 6.1 percent (table 2). U and Thare between 155 to 716 ppm and 38 to 260 ppm, respectively, yielding Th/U ratiosbetween 0.19 and 0.58. The data define a clear Pb-loss pattern on the concordiadiagram (fig. 14) in agreement with both U and Th contents. The three mostconcordant points correspond to a weighted mean 207Pb/206Pb age of 2648�33 Ma,while a regression of all data gives an upper intercept age of 2659�18 Ma and a poorlydefined lower intercept of 549�490 Ma. The upper intercept and 207Pb/206Pb age arewithin error, and we take the weighted mean 207Pb/206Pb age of the concordantpoints, 2648�33 Ma, as the best estimate for the crystallization age of the garnetgranite.

The Hf TDM crustal model ages from the 2663 Ma tonalite (sample Mustafino 37)vary between 2.9 and 3.2 Ga. They show higher values of εHf (T) up to �4.8 comparedto any other sample reported in this study (fig. 12), thus indicating a higher contribu-tion of juvenile material during magma generation. These Hf-isotope data agree with axenocrystic origin of a single zircon of 3544 Ma age recorded in this sample (table 3).The Hf TDM crustal model ages of zircon from the 2648 Ma garnet-bearing granite(sample Tumenyak 50) scatter between 4.4 and 3.1 Ga. The main cluster of zirconfrom this sample plots near the CHUR reference line implying a moderately evolvedsource. However, two grains from this garnet-bearing granite have noticeably lowerεHf(T) values (ca �13 and �20) and, thus, much older Hf TDM crustal model ages(3.99 Ga and 4.4 Ga, respectively). We do not have U-Pb data for these two grains withanomalously εHf(T) values, and consequently, our interpretation is somewhat problem-atical: if these two have the same U-Pb age as the main population (for example, 2648Ma), then it may suggest mixing with an enriched source and remelting of ancientcrust of at least 3.0 to 3.7 Ga old. Otherwise, these anomalous values could be mucholder inherited xenocrystic grains.

The Aktanysh suite.—Zircon in sample Tlyanchi-Tamak 684 ranges in size from 50to 150 �m and has length to width ratios between 1:1 and 2:1. The grains are rounded,interpreted to reflect significant chemical abrasion in the melt phase. CL imageryindicates that most of zircon grains are complex, although several single-domain grainsare present. The complex zircons reveal prominent oscillatory zoning in the cores,overgrown by a wide medium-response homogenous rim domain (fig. 15). Carefulinspection of the rims, and single sector zircon with similar response, shows very faintconcentric zoning suggesting the rims and single-domain zircon have crystallized frommagmatic melts. Fourteen analyses were conducted during a single session on bothcore and rim domains and single-domain grains (table 2). The chemical character ofcore and rim domains is not significantly different. Overall, f206 values are low, up to0.62 percent, and U and Th in the ranges 174 to 1105 ppm and 55 to 220 ppm,respectively. Th/U ratios are between 0.09 and 0.85, with the low values recorded in

Table 4

Sm-Nd whole rock analyses of granitoids of the Bakaly blockConcentration,

ppm Isotopic ratios ±2σ Model ages Locality of the drill

core, sample Rock

Sm Nd 147Sm/144Nd 143Nd/144Nd TDM1 TDM2

Zircon age (T),

Ga

ε Nd (T)

Suleyevo, 585 monzodiorite 33 278 0.0716 0.510225±4 3.1 3.5 2.7 -3.6 Tashliar, 26-4D mesosome 12.2 92.4 0.0800 0.510265±4 3.3 3.7 2.7 -5.8

Ural’skaya, 40002-1 quartz diorite 8.4 59.2 0.0855 0.510592±9 3.0 3.3 2.7 -1.3 Tlyanchi-Tamak

684-4 quartz

monzonite 9.9 66.9 0.0899 0.510676±5 3.0 3.3 2.6 -1.2


both core and rim domains. Two grains define concordant 207Pb/206Pb ages of3034�44 (684-1c) and 3076�17 Ma (684-4) and are interpreted to represent xenocrys-tic zircon (fig. 16). A concordant cluster, made up of eight data points, and comprisingboth core and rim analyses, yields a weighted mean 207Pb/206Pb age of 2619�24 Ma,

40002

5

40002

37

5

37

7

37 50

8

50 50

100 micron

2

5

7

Fig. 10. Selected CL-images of dated zircon from the Bak 1 and Bak 2 granitoids. Circles mark theanalytical sites and the associated numbers refer to data shown in table 2. Sample numbers are in topleft-hand corner.


taken to represent the crystallization age of the quartz monzonite. There are also twoslightly younger, yet discordant, data points (table 2), but because of their discordantnature no geological meaning is ascribed to these analyses.

0.176

0.180

0.184

0.188

1.9 2.1 2.3 2.5 2.7

238U/206Pb

207P

b/20

6P

b

2600

2680


Ural’skaya 400022718±5 Ma

2705

2735


5

Fig. 11. Concordia diagram for zircon from the Bak 1 quartz diorite (sample Ural’skaya 40002). Insetshows data used to calculate the weighted mean age of 2718�5 Ma.

-20

-15

-10

-5

0

5

10

15

2500 2600 2700 2800 2900 3000 3100 3200 3300 3400 3500

Age (Ma)

Menzelino-Aktanysh-9

Tumenyak-50

Mustafino-37

Ural’skaya-40002

Suleyevo-585

Tashliar-26

DM

CHUR

Hf

0

2

4

6

8

10

12

2,5 3,0 3,5 4,0 MoreHf T DM crustal age, Ga

Fre

qu

en

cy

,n

ε

n=51

Fig. 12. εHf—diagram for dated zircon from granitoids of the Bakaly block. The histogram shows a widedistribution of Archean crustal ages with a major peak at �3.3–3.5 Ga (data from table 3).


In sample Menzelino-Aktanysh 9 zircon varies in size from 100 to 250 �m, andcrystals have length to width ratios between 2:1 and 3:1. The grains are euhedral, butshow some signs of rounding, interpreted to reflect some chemical abrasion in themagma chamber. CL imagery reveals prominent oscillatory zoning patterns, indicativeof magmatic crystallization (fig. 15). Eight zircon grains were analyzed during a singlesession and have low f206 values below 0.5 percent. U and Th are tightly constrained, inthe ranges 198 to 300 ppm and 68 to 212 ppm, respectively, with Th/U in the range0.35 to 0.89 (table 2). The data plot along a clearly defined Pb-loss trend with an upperintercept age of 2606�15 Ma and a lower intercept age of 498�160 Ma (fig. 17). Usingonly the three most concordant points, a weighted mean 207Pb/206Pb age of 2600�11Ma can be calculated, within error of the upper intercept age. We take the weightedmean age of the three concordant points as the best age estimate for the crystallizationof this monzogranite.

The Hf TDM ages for sample Menzelino-Aktanysh 9 vary between 3.57 and 3.31 Ga,similar to the source ages of the Bak 1 granitoids (sample 40002). Zircons from sampleMenzelino-Aktanysh 9 are characterized by negative εHf values with some as low as �7(table 3, fig. 12). These data indicate that the magma was derived largely fromremelting of �3.3 Ga crust, probably with the incorporation of older (up to �3.6 Ga)crustal material. The xenocrystic zircon from the 2.6 Ga sample Tlyanchi-Tamak 684 of�3.1 to 3.0 Ga age supports this interpretation.

Sm-Nd Whole Rock DataIn some samples, from which zircon was dated, we investigated the Sm-Nd isotopic

system (table 4). The 147Sm/144Nd ratios in these rocks do not exceed the mean values

0.12

0.16

0.20

0.24

0.28

0.32

0.36

2 4 6 8

238U/206Pb

207P

b/20

6P

b

1500

2500

3500

7

5


2550

2750


Mustafino 372663±18 Ma

Fig. 13. Concordia diagram for zircon from the Bak 2 tonalite (sample Mustafino 37). Inset shows dataused to calculate the weighted mean age of 2663�18 Ma.


for continental crust and by the single-stage model of DePaolo (1981), the protolithseparation from a mantle source must have occurred between 3.0 and 3.3 Ga. However,these rocks have obviously undergone metamorphism and migmatization, and thepristine U-Pb isotopic systems of the zircon, and probably also the Sm-Nd whole-rockisotopic systems, may have been disturbed. In these cases, therefore, the two-stagemodel of Liew and Hofmann (1988) appears more favorable. Since the principalArchean metamorphic event in Volgo-Uralia and in the Bakaly block took place at�2.7 Ga, we find that the 147Sm/144Nd isotopic ratios correspond to the mean valuesfor 3.3 to 3.7 Ga continental crust. This suggests that a Paleoarchean crust had beenrecycled during the Archean evolution of Volgo-Uralia.

discussion

Age of the Bakaly Granitoid Suites and Their SourcesOur geochronological data indicate that the granitoid rocks in the Bakaly block

were formed by a sequence of magmatic events between 3.3 and 2.6 Ga. The oldestU-Pb zircon crystallization ages of 3266�7 Ma and 3237�11 Ma derive from twodifferent samples of the Tashliar monzonitic rocks. The variation of the εHf(T) valuesof their zircon between �2.5 and �4 (table 3) and the εNd(T) values between �3.6 and�5.8 (table 4) suggests the involvement of much older, up to 3.8 Ga, source rocks.

As shown by this study, the TTG-like Bak 1 suite was emplaced at 2718�5 Ma,which is slightly older but still close to previously obtained 2698�12 Ma and 2697�31Ma U-Pb ages of zircon from two other rocks of this suite (Bibikova and others, 1994).Even though limited to only one sample, the εHf(T) values of �0.2 to �2.5 of thezircon and the whole-rock εNd(T) value of �1.3 suggest a possible �3.4 to 3.2 Gasource for the quartz diorite from the Ural’skaya 40002 core.

The age of the Bak 2 granodioritic-granitic suite is indicated by ages of 2663�18Ma and 2648�33 Ma, but the leucosome vein in migmatite sample Tashliar 26 (fig. 2)

0.14

0.16

0.18

0.20

0.22

0.24

2.1 2.3 2.5

238U/206Pb

207P

b/20

6P

b

2200

2600U I. .=2659±18 Ma


Tumenyak 50 2648±33 Ma

2675


2655

2635

Fig. 14. Concordia diagram for zircon from the Bak 2 garnet granite (sample Tumenyak 50). Insetshows data used to calculate the weighted mean age of 2648�33 Ma.


is 2710�19 Ma and thus somewhat older. A metamorphic-rim zircon age of 2694�9 Mathat overgrows 3266 Ma magmatic zircon was found in the Suleyevo 585 monzodiorite(table 3). Considering that the dated Tashliar rocks occur in immediate contact withthe Bak 2 Zainsk intrusion (fig. 1), we interpret the Tashliar 26 leucosome and theSuleyevo 585 metamorphic zircon as products of the Bak 2 event. It follows that its firstpulse took place as early as �2690 Ma ago. The Hf isotopic characteristics of zirconfrom the two dated Bak 2 samples show that these had significantly different sourcecompositions and ages (tables 3 and 5). The biotite tonalite sample Mustafino 37, withεHf(T) zircon values varying between �4.8 and �0.3, originated from a fairly juvenileprotolith with Hf TDM crustal ages of 3.2 to 2.8 Ga. Most analyzed zircon from garnetgranite sample Tumenyak 50 have εHf(T) values ranging between �0.43 and �2.50,which corresponds to Hf TDM crustal ages of 3.3 to 3.1 Ga (table 3). However, twozircon grains with εHf(T) values of �13 and �20 in this S-type granite may indicateinvolvement of an enriched source and remelting of at least 3.0 to 3.7 Ga old crust, anda possible participation of Eoarchean to Paleoarchean materials in the melt source.This may also apply to the Mustafino 37 tonalite, which contains a 3544 Ma xenocrysticzircon (table 2).

Aktanysh monzonitic granitoids from two intrusions that terminated the Archeanmagmatic evolution in the Bakaly block have crystallization ages of 2619�29 and2600�11 Ma (table 5). We suggest that the latter is the best estimate for this magmaticevent, since it derives from the little-deformed Menzelino-Aktanysh 9 well core. Theobtained ages confirm that the Aktanysh monzonitic rocks are coeval with the �2620Ma old Tuymazy gabbro-anorthosites (table 5). The Hf TDM model ages of zircon fromthe 2600 Ma monzogranite range between 3.57 and 3.25 Ga, which is higher than the

684

8R

684

14

684 91c 9

1110

7

6

Fig. 15. Selected CL-images of dated zircons from the Aktanysh monzonitic granitoids. The circles markthe analytical sites and the numbers refer to data shown in table 2: sample numbers are in upper left-handcorner.


source ages of 3.4 to 3.2 Ga for the Bak 1 quartz diorite from drill core Ural’skaya40002 (fig. 1, table 3). The other Aktanysh granite (sample Tlyanchi-Tamak 684) ismore juvenile with an εNd(T) value of �1.2 and Nd TDM model age of �3.3 Ga (table4), and notably contains xenocrystic zircon of �3076 and 3034 Ma in age. Thesedifferences between the two Aktanysh intrusions agree well with their positions in twodifferent blocks of stacked Neoarchean crust. The Menzelino-Aktanysh 9 intrusion isclose to the sites of the oldest crustal sources for both the Tashliar and the Bak 1 rocks(fig. 1).

The Archean crustal evolution of the Bakaly block was completed at �2.6 Ga bythe post-collisional bimodal magmatism that generated the Tuymazy and Aktanyshgabbro-anorthositic to monzonitic intrusions. Paleoproterozoic reworking did notsubstantially affect the Archean crust in the Bakaly block. It was restricted to fault zonesand troughs filled by Paleoproterozoic metasediments and metavolcanics (fig. 1). Afew zircon grains in samples Tashliar 26, Mustafino 37 and Tlyanchi-Tamak 684 havemetamorphic rims with 207Pb/206Pb ages of 2426, 2242 and 1946 Ma (table 2). Thiscould have been caused by recurrent thermal input into the Archean crust duringperiods of extension and mantle underplating in the early Paleoproterozoic.

Implications for Archean Crustal Evolution in Volgo-UraliaThe integration of zircon age with Sm-Nd and Hf isotopic data allows a discussion

of the development of Volgo-Uralia in the Archean. Important aspects relate to theTashliar monzonites and the Bak 1 and Bak 2 suites, which appear to characterizeparticular geodynamic stages of crust formation.

The Tashliar rocks and the problem of mature crust at 3.3 to 3.2 Ga.—Somewhatunexpectedly against the background of previous views, this study has indicated that

0.14

0.16

0.18

0.20

0.22

0.24

1.5 1.7 1.9 2.1 2.3 2.5 2.7

238U/206Pb

207P

b/20

6P

b

2400

2800

3r2

14

1c4


Tlyanchi-Tamak 6842619±24 Ma

2680


2640

Fig. 16. Concordia diagram for zircon from the Aktanysh quartz-monzonitic granite (sample Tlyanchi-Tamak 684). Inset shows data used to calculate the weighted mean age of 2619�24 Ma.


the Tashliar monzonitic suite is the oldest hitherto recognized major coherent rockunit in Volgo-Uralia. Still older ages are model ages or relate to single xenocrysticzircon or parts of zircon grains. The 3.3–3.2 Ga Tashliar quartz monzonite (sample 26,mesosome) and monzodiorite (Suleyevo 585) both preserve their pristine igneousminerals and textures (Appendix 1), although one has been somewhat deformed andmigmatized (sample Tashliar 26), and the other hydrothermally affected (Suleyevo585). Both feature quite similar multi-element diagrams and REE patterns characteris-tic of alkalic igneous rocks (figs. 4 to 6). Like many sub-alkaline and alkaline granitoids,they are rich in Ti, P, and total REE (509-1012 ppm) at (La/Yb)N of 56-69 (table 1).

The high alkalinity of the Tashliar suite is remarkable and appears to imply that anevolved continental crust existed in Volgo-Uralia already 3.3 to 3.2 Ga ago. If so, wemay speculate that the Sm-Nd isotope compositions and the Hf isotopic characteristicsof the dated zircon, as well as the model Nd- and Hf ages, demonstrate recycling ofPaleo- and even Eoarchean crust with crustal TDM model ages up to 3.8 Ga. If this wasthe case, melt production processes could have included partial involvement ofenriched crust by dehydration melting or melting in the presence of water as shown byexperiments on the origin of Archean K-rich granitoids (Lopez and others, 2006, andreferences therein). The xenocrystic 3544 Ma zircon found in the 2.7 Ga Mustafinotonalite indeed indicates that Paleoarchean rocks could have been distributed widelyamong the Neoarchean granitoids of the Bakaly block. This is supported additionallyby the 3.8 to 3.2 Ga Hf model ages from several Bakaly sites (fig. 1, table 5).

The origin of the 3.3 to 3.2 Ga Tashliar monzonitic rocks can be explained by meltextraction from an enriched mantle/lower crustal source, which interacted with thethen existing crust of Volgo-Uralia. Mantle plume-crust interaction in the Eoarcheanand Paleoarchean has been proposed by several researchers using various approaches(Griffin and others, 2003; Rollinson, 2007; Van Kranendonk and others, 2007; Condieand Kroner, 2008). Thus, despite the limited data, we may raise the question whethermelting of an Eoarchean or Paleoarchean TTG crust in conjunction with mantle-

2620


2600

2580

0.167

0.169

0.171

0.173

0.175

0.177

2.0 2.4 2.8 3.2

238U/206Pb

207P

b/20

6P

b

2540

2580

2620

2σ error crosses

Menzelino-Aktanysh 92600±11 Ma

Fig. 17. Concordia diagram for zircon from the Aktanysh monzogranite (sample Menzelino-Aktanysh9). Inset shows data used to calculate the weighted mean age of 2600�11 Ma.


Tab

le5

Geo

chro

nolo

gyof

the

Bak

aly

bloc

kan

dH

fis

otop

icch

arac

teri

stic

sof

date

dzi

rcon

Z

irco

nW

hole

rock

Site

of

drill

ing

Dri

ll co

re

Dep

th, m

R

ock

type

U

-Pb

age,

Ma

M

etho

d*ε H

f (T

) H

f T

DM

crus

tal

Ga

ε Nd

(T)

Nd

TD

M

Ga

Ref

eren

ce

Tas

hlia

r su

ite

Tash

liar

26

16

93.0

-169

4.1

Mon

zoni

tic m

esos

ome

3237

+/-1

1 SI

MS

-3.7

to +

0.7

3.81

- 3

.58

-5.8

3.

7 th

is s

tudy

Su

leye

vo

585

1665

.2-1

670.

4 M

onzo

dior

ite32

66+/

-7

SIM

S -3

to +

2.5

3.

76 -

3.4

9-3

.6

3.5

this

stu

dy

Bak

1 s

uite

U

ral’s

kaya

4000

2 17

20-1

727

Qua

rtz

dior

ite27

18+/

-5

SIM

S -2

.5 to

+0.

2 3.

35 -

3.1

8-1

.3

3.3

this

stu

dy

Ura

l’ska

ya

4000

2 17

20-1

727

Qua

rtz

dior

ite

2593

+/-3

1 T

IMS

Bib

ikov

a an

d ot

hers

, 199

4 M

enze

lino-

Akt

anys

h26

1786

.5-1

806.

5Q

uart

z di

orite

2698

+/12

TIM

SB

ibik

ova

and

othe

rs, 1

994

Sa

banc

hino

2261

20

00-2

038

Bio

tite

tona

lite

2697

+/-3

1 T

IMS

Bak

2 s

uite

Ta

shlia

r

26

1693

.0-1

694.

1 V

ein

leuc

osom

e

2710

+/-1

9SI

MS

this

stu

dyM

usta

fino

37

1752

-175

4B

iotit

e to

nalit

e26

63+/

-18

SIM

S 3544+/-88**

-0.3

to +

4.8

3.17

- 2

.84

this

stu

dy

Tum

enya

k 50

17

63-1

765

Gar

net-

bear

ing

gran

ite

2648

+/-3

3 SI

MS

-1.7

to +

0.4

(-20

, -13

.4)

3.30

- 3

.11

(4.4

0; 3

.99)

this

stu

dy

Tum

enya

k 50

17

63-1

765

Gar

net-

bear

ing

gran

ite

2700

+/-4

8 T

IMS

Bib

ikov

a an

d ot

hers

, 199

4 A

ktan

ysh

suite

Tly

anch

i-Ta

mak

684

1920

-192

4Q

uart

z m

onzo

dior

ite26

19+/

-24

3076+/-17**

3034+/-44**

SIM

S-1

.23.

3th

is s

tudy

Men

zelin

o-A

ktan

ysh

918

02.9

-180

5.9

Mon

zogr

anite

2600

+/-1

1 SI

MS

-7.3

to -

2.4

3.46

- 3

.25

T

uym

azy

suite

B

avly

2001

1 30

06-3

140

Leu

coga

bbro

-nor

ite

2622

+/-8

T

IMS

Post

niko

v, m

s, 2

002

Azn

akae

vo s

uite

U

ral’s

kaya

40

014

1763

-176

8 G

arne

t bea

ring

gra

n ite

1898

+/-1

0T

IMS

Bib

ikov

a an

dot

hers

, 199

4

*SI

MS�

Seco

nda

ryIo

nM

ass

Spec

trom

etry

;TIM

S�T

her

mal

Ion

izat

ion

Mas

sSp

ectr

omet

ry;*

*D

ata

for

xen

ocry

stic

zirc

ons.


plume activity could have caused the formation of the 3.3 to 3.2 Ga rocks inVolgo-Uralia in a manner similar to that discussed for the Pilbara craton (Smithies andothers, 2009). Alternatively, the Tashliar rocks may represent alkalic (shoshonitic)melts derived due to mantle upwelling facilitated by post-collisional extension ofthickened crust (for example Bonin and others, 1998). Also in this case, a mature crustmust have existed in Volgo-Uralia when the Tashliar suite was intruded.

Diversity within the 2.72 Ga Bak 1 suite.—While the modal and normative composi-tions of some rocks traditionally referred to the Bak 1 suite (fig. 3) resemble those oftonalite-trondhjemite-granodiorite (TTG) suites typical of Archean cratons (Martin,1994; Condie, 2005; Martin and others, 2005; Foley, 2008), chemical variation issubstantial and there are numerous instances where the analyses do not plot in TTGfields and do not follow trondhjemitic trends of melt evolution (fig. 3). Like Neo-archean TTG, the Bak 1 granitoids are highly aluminous (Al2O3 � 15%) but usuallycontain more CaO than common TTG (fig. 3, table 1). They have variable rather highconcentrations of K2O and high K2O/Na2O ratios that reach 0.8 at #Mg up to 38.Compared with average TTG, the Bak 1 granitoids are higher in Nb, Nb/Ta, Ba, Th,Sr/Y and LREE (fig. 5, table 1). Also, the Bak 1 rocks have (La/Yb)N ratios higher thancommon Archean TTG, which indicates stronger REE fractionation.

Among Bak 1 rocks with #Mg of 22-37 at SiO2 contents between 67 and 72 percent,some have high Ni and Cr at rather low Zr and Sr levels (table 1), and satisfy thetypology of sanukitoid to Closepet-type granitoids (Moyen and others, 2003). Rockswith such compositions normally relate to late- and post-collisional tectonics (Martinand others, 2005). At present, this particular group of rocks has still not been datedand its relationship with the rest of the Bak 1 suite is unclear.

All these similarities and deviations of Bak 1 compositions relative to ArcheanTTG suggest that the Bak 1 suite comprises granitoids crystallized from a wide range ofmelts in different tectonic settings. Some resemble TTG suites produced by slab/mantle-wedge melting (Martin and others, 2005), while, for instance, the 2.72 Ga Bak 1 rockfrom the Ural’skaya 40002 drill hole indicates participation of a 3.4 to 3.2 Ga meltsource (table 5), implying that partial melting of much older materials was important.The 3.3 to 3.2 Tashliar rocks could have been such an older melt source, which wouldalso explain why some Bak 1 rocks are enriched in LILE and LREE. The latter rock typehas recently been assigned to “transitional” TTG rocks characteristic of multiplyre-melted Archean crust (Champion and Smithies, 2007).

Additional isotopic and geochemical work on the Volgo-Uralian rocks will berequired to clarify the diversity of Bak 1 origins and assess whether its rocks should bere-grouped into several suites.

The Bak 2 suite and a 2.69 to 2.65 Ga collisional event.—A change of tectonic regimeappears to have taken place at 2.69 to 2.65 Ga when the voluminous Bak 2 suite wasformed. The Bak 2 rocks are leucocratic granitoids and migmatites rich in K-feldspar.They are ubiquitous throughout the Bakaly block, culminating in a number of fairlyhomogeneous plutons (fig. 1), which contain diatextites as well as evolved igneousgranitoids. The latter apparently represent zones of maximum accumulation of Bak 2melts. The migmatites, which are very common in the Bak 2 suite, comprise leuco-somes of various types, ranging from in-situ and in-source segregations to veinleucosomes by Sawyer’s (2008) classification. Similar to the veins in the Tashliar 26migmatite (fig. 2), the latter are particularly common in the vicinity of the majorgranite plutons (fig. 1). The greatest thickness of the individual zones of migmatiza-tion exceeds 50 meters. As seen in many drill cores, migmatization was associated withshearing. It would appear that the structure of the highly stacked Archean crustcontrolled the siting of Bak 2 granite plutons and zones of migmatization and


deformation. These occur at different crustal levels along thrusts/reverse faultsdipping gently northwest (fig. 1, profile).

The various Bak 2 intrusions differ with regard to their chemical compositions.Mostly, their granitoids are either magnesian and metaluminous or ferroan andperaluminous (Bogdanova, 1986; Popova and Postnikov, 1992). Their zircon features arange of εHf values and Hf TDM model ages (table 3). This suggests that rather juvenilesources as well as mixed crustal and juvenile materials participated in the formation ofthe Bak 2 melts. An example is sample Mustafino 37 with Hf crustal model ages of 3.1to 2.8 Ga and zircon εHf values up to �4.8, but this rock also contains a xenocrysticzircon with an age of 3544 Ma (table 2). Another example may be the garnet-bearinggranite in the Tumenyak 50 core. Apart from a principal zircon population with εHfvalues of �0.4 to �2.5 and Hf TDM model ages of 3.3 to 3.1 Ga, this granite containsextremely “enriched” zircon with εHf values of �20 and �13 (table 3). Since theTumenyak drill hole is located close to extensive metasedimentary rock sequences (fig.1) we cannot, however, exclude contamination of the granitic melt by detrital zirconwith Hf TDM model ages up to 4.4 Ga.

Thus, the compositional differences between the various Bak 2 plutons maymirror a heterogeneous structure of the source regions where various crustal com-plexes were stacked tectonically during a collisional stage before serving as meltsources for the Bak 2 K-rich intrusions. Mafic magmatism at the bottom of the crust canhave contributed to the heating necessary for melt production. This mechanism canhave been still more active during the emplacement of the 2.6 Aktanysh monzonites,which are coeval with the Tuymazy gabbro-anothosites that were formed in conjunc-tion with post-collisional extension of the thickened Neoarchean crust. However, thezircon of the monzonites feature relatively low εHf values between �2.4 and �7.3, andHf TDM crustal ages of 3.3 to 3.6 Ga (table 3), which may reflect admixture of oldercrustal materials in the central part of the Bakaly block.

Altogether, the Bakaly granitoid magmatism thus comprised several events at 3.3to 3.2, 2.72 to 2.70, 2.69 to 2.65 and 2.60 Ga. However, the Nd TDM rock ages, the HfTDM ages of the dated zircon, and the presence of xenocrystic zircon all indicate thatthe Bakaly crust contains crustal components as old as 3.8 to 3.6 Ga, and also 3.4 to 3.2and 3.2 to 3.0 Ga (fig. 12, histogram). These mark several episodes of accretionarygrowth and recycling of the crust. During a period of collisional orogeny lasting �40Ma between 2.69 and 2.65 Ga, the Neoarchean Bak 2 granitoids built up much of thecrust. This probably caused the differentiation and pronounced layering of the crust inVolgo-Uralia revealed by the Tatseis seismic transect.

acknowledgmentsSB thanks the Gledden Foundation at the University of Western Australia, which

provided funding for the SHRIMP dating of the Volgo-Uralian rocks. The financialcontribution of the Swedish Science Research Council to the SB project is acknowledged.The help of Z.-X. Li with the conducting of some of the SHRIMP analyses is muchappreciated. Graham Begg generously supported the Hf isotope studies of zircon by E.Belousova. U-Pb SHRIMP analyses were conducted on the Perth Consortium SHRIMPfacilities at the John de Laeter Centre for Mass Spectrometry at Curtin University.

The authors are grateful to Vicky Pease and another paper reviewer and the GuestEditors for their thorough comments and suggestions, which have significantly im-proved the paper contents and style. Roland Gorbatschev is acknowledged for hiscritical review and help in improving the manuscript.

This is a contribution to the special issues in honor of Alfred Kroner, whose workalways inspires Precambrian researchers.


Appendix 1

Description of Rocks Selected for Age Determinations

The Tashliar suite.—Sample Tashliar 26D represents the fine-grained mesosome of metatextite consist-ing of andesine—50%, quartz—15%, microperthitic K-feldspar—15%, and biotite—up to 20%. Highcontents of allanite, apatite and zircon, mostly associated with biotite are characteristic. Plagioclase (An30-32) is often subhedral, but has corroded and rounded grain shapes with thin films of K-feldspar. Biotiteflakes outline the individual plagioclase grains or their aggregates. Polycrystalline quartz occurs togetherwith microperthitic microcline in separate zones of intense recrystallization. These are marked by finesubgrained structure, complex mineral interrelations, and pronounced myrmekitization. Since the rock stillpreserves pre-migmatitic, igneous structure it can be classified as a quartz monzonite, as also its chemistryshows.

Sample Suleyevo 585 is a medium-grained monzodiorite with well defined equigranular igneous texture. Itconsists of about 60% plagioclase (An 34-36), 20% biotite, 15% micropertitic K-feldspar, including fine euhedralcrystals of plagioclase, and 5 percent intergranular quartz. Numerous well-shaped crystals of zircon, apatite andallanite are associated with biotite. Apatite contains micro-inclusions of zircon. Plagioclase is albitized, andchrorite in part replaces biotite. Unlike sample Tashliar 26 D, deformation in this rock is only indicated bysomewhat irregular grain boundaries and undulose extinction of quartz.

The Bak 1 suite.—Sample Ural’skaya 40002 derives from medium- to fine-grained, seriate quartz diorite,containing up to 30% amphibole plus biotite, up to 60% andesine, ca. 5 to 10% quartz and small amounts ofK-feldspar. Apatite, titanite and magnetite are accessory. Zircon is located within plagioclase and at contactswith amphibole and biotite. This rock is partly deformed and recrystallized, showing a foliation marked bybiotite flakes.

Two previously dated granitoids from the Bak 1 suite (samples Sabanchino 2261 and Menzelino-Aktanysh 26, fig. 1, table 5) are coarse leucocratic tonalites. The first is equigranular, while the other isseriate to porphyritic. They are weakly deformed biotite tonalites with euhedral or subhedral igneousminerals. Similar to all other Bakaly granitoids, they contain accessory allanite, apatite, zircon and titanite.The modal compositions of these rocks are: Sabanchino 2261 (oligoclase An26-27—55-60%, K-feldspar—0-5%,quartz—25-30%, biotite—5-10%, muscovite—up to 5%); Menzelino-Aktanysh 26 (andesine An34-36—30-35%,K-feldspar—0-15%, quartz—20-25%, biotite—10-15, amphibole—10-15%).

The Bak 2 suite.—Sample Tashliar 26L is of the leucosome vein in the monzonitic metatextite Tasliar26. It contains 30 to 35% plagioclase, 35% K-feldspar, 35% quartz, and �1% biotite and is fine- to medium-grained, deformed and partly recrystallized. It preserves, however, subhedral grains of feldspar and quartz,which indicate igneous crystallization. The leucome appears to follow the tectonic fabric of the migmatite(fig. 2), indicating syntectonic emplacement.

Sample Mustafino 37 is a coarse-grained, equigranular biotite tonalite (oligoclase—60-65%, bio-tite—ca 15%, quartz—�10%). Accessories are apatite, allanite, titanite and magnetite. Zircon is included inplagioclase. Muscovite, epidote and titanite are located along the feldspar boundaries in thin zones ofdeformation where subgrained aggregates of quartz and feldspar occur. Deformation is mostly evidenced bypolycrystalline quartz with undulose and “blocky” extinction.

Sample Tumenyak 50 is a medium-grained, seriate, garnet-bearing granite. It is rich in microperthiticK-feldspar with inclusions of euhedral plagioclase (up to 50%), oligoclase (�25%) and quartz (25-30%) witha small amount of garnet and biotite. Accessory minerals are apatite, zircon and monazite.

The Aktanysh suite.—Sample Tlyanchi-Tamak 684 is a weakly deformed, recrystallized, medium-grainedquartz monzodiorite. It contains andesine (An 36-38)—50 to 55%, microperthitic K-feldspar—10 to 15%,Na-rich, microscopically “bluish” amphibole—10 to 20%, annitic biotite—10 to 15% and large crystals oftitanite. Quartz contents are less than 10%. Apatite, zircon and allanite are accessory.

Sample Menzelino-Aktanysh 9 is a well-preserved, medium-grained seriate, monzogranite that containsmicroperthitic K-feldspar forming up to 5 mm crystals (up to 15%) set in a matrix of 1 to 1.5 mm grains ofeuhedral plagioclase (An 30-32) occupying 60 to 65% of the rock. Amphibole—5 to 10%, biotite—5 to 10%,titanite and magnetite occur in-between feldspars and quartz (up to 25%). Apatite, zircon and allanite arenotable accessory minerals. Deformation appears weak and is marked by slightly undulose extinction inquartz with rare myrmekite.

Appendix 2

Analytical Methods

Chemical analyses.—The rock selection for chemical analyses was made with careful evaluation. Allselected core samples were checked microscopically and are fresh and unweathered. We avoided strictmylonites and stromatic migmatites, in which the separation of paleosomes/mesosomes from leucosomes isdifficult to make. We were able to identify rock types, which is possible due to abundant drill cores of theBakaly rocks, some up to 400 m deep.


The chemical analyses presented in table 2 were carried out at ACME Analytical Laboratories, Ltd.,Canada (http://acmelab.com). Total abundances of the major oxides and several minor elements arereported on a 0.2g sample analysed by ICP emission spectrometry following a lithium metaborate/tetraborate fusion and dilute nitric acid digestion. Loss on ignition (LOI) is by weight difference afterignition at 1000 °C. Total carbon and sulphur analysis by Leco are included. Detection limits are 0.01 mass.percent for major oxides, LOI and C, and 0.02 mass. percent for Cr2O3 and S. Rare earth and refractoryelements are determined by ICP mass spectrometry following a lithium metaborate/tetraborate fusion andnitric acid digestion of a 0.2g sample. In addition a separate 0.5g split was digested in aqua regia and analysedby ICP MS to obtain the precious and base metals. Detection limits for trace elements (table 2) are (in ppm):0.01 (Tb, Tm and Lu), 0.02 (Eu and Ho), 0.03 (Er), 0.05 (Sm, Gd and Yb), 0.1 (Hf, Nb, Rb, Ta, U, Y, Zr, Laand Ce), 0.2 (Th), 0.3 (Nd), 0.5 (Ga and Sr) and 1 (Ba).

For better rock characterization, we have also used in classification diagrams (figs. 3 to 6) the chemicaldatabase for major element compositions from the archives of the Department of Lithology of the GubkinState University of Oil and Gas in Moscow, Russia. The comparison of chemical analyses of the same samplefrom the archive database and those carried out at ACME Laboratories has shown similar values.

U-Pb isotope analyses.—Zircon was separated using standard heavy liquid and magnetic separatingtechniques. The zircons were hand-picked under a binocular microscope and mounted, together with asuitable natural zircon standard, into epoxy resin mounts. The mount was polished to expose the grainsmid-section, and imaged on optical and scanning electron microscopes (SEM), the latter fitted with bothback scatter electron (BSE) and cathodoluminescence (CL) detector. The mount was then thoroughlycleaned to minimize surface contaminants (mainly lead) using organic and inorganic solvents and thencoated with a thin coat of gold to impart surface conductivity. The mount was placed in the Sensitive HighResolution Ion MicroProbe (SHRIMP II) sample lock and pumped to high vacuum 24 hours prior to theanalytical session to allow degassing. Analysis techniques follow those described by De Waele and others,2009. CZ3, Temora-2 and BR266 standards (Pidgeon and others, 1994; Stern, 2001; Black and others, 2003)were used for calibration of U and Pb/U ratios in three different sessions (details given in table 3). With theexception of samples 26 and 585, the calibration errors on the standard were not included in the data, butare reported in the table. For samples 26 and 585, which were analyzed during more than one session, theerrors on the standard were added in quadrature to the Pb/U ratios and ages. Common Pb (non-radiogenicPb) was corrected using the measured 204Pb value. Surface contamination was reduced by rastering time of 2to 3 minutes prior to analysis, and corrections applied a composition of common Pb appropriate to the ageof the zircon, calculated after Stacey and Kramers (1975). All analyses are reported at 1 confidence level intable 2, but error bars in diagrams and weighted mean ages in the text are at the 2 level. Data were reducedusing the Squid 1.06 plug-in for Excel© (Ludwig, 2001b) and ages calculated and plotted using Isoplot 3.00plugin for Excel© (Ludwig, 2001a).

Hf-isotope analyses.—Hf-isotope analyses were carried out in-situ using a New Wave/Merchantek UP-213laser-ablation microprobe, attached to a Nu Plasma multi-collector ICPMS. The analyses were carried outwith a beam diameter of ca 55 �m and a 5 Hz repetition rate. This resulted in total Hf signals of 1–6 10�11

A, depending on conditions and the Hf contents. Typical ablation times were 100 to 120 seconds, resulting inpits 40 to 60 �m deep. He carrier gas transported the ablated sample from the laser-ablation cell via a mixingchamber to the ICPMS torch.

Interference of 176Lu on 176Hf was corrected by measuring the intensity of the interference-free 175Luisotope and using 176Lu/175Lu � 0.02669 (DeBievre and Taylor, 1993) to calculate 176Lu/177Hf. Similarly,the interference of 176Yb on 176Hf has been corrected by measuring the interference-free 172Yb isotope andusing 176Yb/172Yb to calculate 176Yb/177Hf. The appropriate value of 176Yb/172Yb was determined by spikingthe JMC475 Hf standard with Yb, and finding the value of 176Yb/172Yb (0.58669) that gives the correct ratioof 176Hf/177Hf as obtained on the pure Hf solution. Analyses of standard zircons (Griffin and others, 2000)illustrate the precision and accuracy obtainable on the 176Hf/177Hf ratio, despite the severe corrections on176Hf. The typical 2SE precision on the 176Hf/177Hf ratios presented here is �0.00002, equivalent to 0.7 εHfunit. The accuracy and precision of the method are discussed in further detail by Griffin and others (2000)and detailed discussions regarding the overlap corrections for 176Lu and 176Yb were provided by Griffin andothers (2006), Griffin and others (2007), and by Pearson and others (2008).

To calculate model ages (TDM) based on a depleted-mantle source, we have adopted a model with(176Hf/177Hf)i � 0.279718 at 4.56 Ga and 176Lu/177Hf � 0.0384; this produces a present-day value of176Hf/177Hf (0.28325), similar to that of average MORB (Griffin and others, 2000). TDM ages, which arecalculated using the measured 176Lu/177Hf of the zircon, can only give a minimum age for the sourcematerial of the magma from which the zircon crystallized. Therefore we have also calculated, for each zircon,a “crustal” model age (TDM

C), which assumes that its parental magma was produced from an averagecontinental crust (176Lu/177Hf � 0.015; Geochemical Earth Reference Model database, http://www.earthref.org/) that was derived from a depleted mantle.

For the calculation of εHf values, we have adopted the chondritic values of Blichert-Toft and Albarede(1997). There are several proposed values for the decay constant of 176Lu, including: 1.93 10�11 yr�1

(Blichert-Toft and Albarede, 1997); 1.865 10�11 yr�1 (Scherer and others, 2001); and 1.983 10�11 yr�1


(Bizzarro and others, 2003); calculations using all three are provided in the data table 3. Values of εHf (T)and the model ages used in figure 12 have been calculated using the decay constant proposed by Scherer andothers (2001).

Sm-Nd analyses.—Nd isotope data were obtained using conventional methods at the isotope laboratoryat the Vernadsky Institute of Geochemistry and Analytical Chemistry, the Russian Academy of Sciences,Moscow. 20 to 30 mg sample together with a mixed 150Nd�149Sm tracer was dissolved in HF�HNO3 (5:1) insealed teflon beakers at 200°C. After decomposition the solution was evaporated and the precipitate wastransformed into chloride form. Sm and Nd were separated in two steps by ion exchange chromatography.At the first stage a REE enriched fraction was obtained using DOWEX 50W-X8 cation exchange resin; thesecond stage, Sm and Nd were separated by HDEHP reagent. Total blanks were 0.03 ng for Sm and 0.1 ng forNd. The isotope composition of Sm and Nd was measured using a multicollector Triton mass-spectrometer.The concentrations of Sm and Nd were determined by isotope dilution. The precision was 0.1 percent for147Sm/144Nd. The measured 143Nd/144Nd ratio was normalized to 148Nd/144Nd � 0.241572, correspondingto 146Nd/144Nd � 0.7219. Present day isotopic ratios for uniform chondritic reservoir used for εNd(T) valuecalculations were: 143Nd/144Nd�0.512638, 147Sm/144Nd�0.1967. The model ages (TDM) were calculatedusing the following ratios for the depleted mantle: 143Nd/144Nd�0.513151, 147Sm/144Nd�0.212. Calcula-tion of the model Nd age (TDM2) according to the two-stage model (Liew and Hofmann, 1988) was based onthe assumption that the source had a typical continental crust ratio of 147Sm/144Nd�0.132 until 2.7 Ga.

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Volgo-Uralia: The first U-Pb, Lu-Hf and Sm-Nd isotopic evidence of preserved Paleoarchean crust

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