The Sr Isotopic Characterization and Pb–Pb Age of Carbonate Rocks from the Satka Formation, the...

120

ISSN 0869-5938, Stratigraphy and Geological Correlation, 2008, Vol. 16, No. 2, pp. 120–137. © Pleiades Publishing, Ltd., 2008.Original Russian Text © A.B. Kuznetsov, G.V. Ovchinnikova, M.A. Semikhatov, I.M. Gorokhov, O.K. Kaurova, M.T. Krupenin, I.M. Vasil’eva, B.M. Gorokhovskii, A.V. Maslov,2008, published in Stratigrafiya. Geologicheskaya Korrelyatsiya, 2008, Vol. 16, No. 2, pp. 16–34.

INTRODUCTION

The capability of carbonate rocks and minerals toretain the Sr isotopic characteristics of sedimentationbasins permitted determination of the

87

Sr/

86

Sr

ratiovariations in the Phanerozoic oceans and to correlatethem with the known geological events of that time(Peterman et al., 1970; Hodell et al., 1990; Veizer et al.,1999). In the last decades, scientific interest to the LateRiphean glaciations and fundamental changes in bio-logic evolution recognizable across the Vendian–Cam-brian boundary stimulated an intense study of theUpper Riphean and Vendian carbonate deposits, andthis extended to a considerable extent the database onisotopic characteristics of the Late Precambrian seawa-ter (Derry et al., 1992; Gorokhov et al., 1995; Brasieret al., 1996; Shields, 1999; Walter et al., 2000;Melezhik et al., 2001; Thomas et al., 2004; Nogueiraet al., 2007; and references therein). New data facili-tated the reconstruction of geodynamic environment,the assessment of composition and erosion intensity ofthe Earth crust, and correlation of the Precambrian

marine sediments. The Sr-isotopic database on theLower Riphean is much less representative so far, con-taining a few analytical results and restricting ourknowledge about the

87

Sr/

86

Sr

variations in the EarlyRiphean seawater and their correlation with importanttectonic and biologic events, which took place at theEarly–Late Proterozoic boundary time (Karlstorm et al.,2001; Sarangi et al., 2004). The deficiency of necessaryinformation is in part a consequence of the lack of thecarbonate formations, which have an adequate isotopic-geochronological characterization and avoided epige-netic recrystallization.

The type sedimentary successions of the Ripheanhave accumulated in majority on the platforms and donot alternate usually with volcanogenic rocks, the com-mon bearers of the isotopic-geochronological informa-tion. Consequently, sedimentary geochronometers areof prime significance in such a situation, for instancethe carbonate rocks, which can be potentially dated bythe U–Pb (Pb–Pb) method. As it has been shown inmany works, the Pb–Pb ages of the Precambrian marine

The Sr Isotopic Characterization and Pb–Pb Age of Carbonate Rocks from the Satka Formation,

the Lower Riphean Burzyan Group of the Southern Urals

A. B. Kuznetsov

a

, G. V. Ovchinnikova

a

, M. A. Semikhatov

b

, I. M. Gorokhov

a

, O. K. Kaurova

a

, M. T. Krupenin

c

, I. M. Vasil’eva

a

, B. M. Gorokhovskii

a

, and A. V. Maslov

c

a

Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, St. Petersburg, Russia

b

Geological Institute, Russian Academy of Sciences, Moscow, Russia

c

Institute of Geology and Geochemistry, Uralian Division, Russian Academy of Sciences, Yekaterinburg, Russia

Received June 5, 2007

Abstract

—The Rb–Sr and U–Pb systematics are studied in carbonate deposits of the Satka and Suran forma-tions corresponding to middle horizons of the Lower Riphean Burzyan Group in the Taratash and Yamantauanticlinoria, respectively, the southern Urals. The least altered rock samples retaining the

87

Sr/

86

Sr

ratio of sed-imentation basin have been selected for analysis using the original method of leaching the secondary carbonatephases and based on strict geochemical criteria of the retentivity (Mn/Sr < 0.2, Fe/Sr < 5 and Mg/Ca < 0.024).The stepwise dissolution in 0.5 N HBr has been used to enrich samples in the primary carbonate phase beforethe Pb–Pb dating. Three (L-4 to L-6) of seven consecutive carbonate fractions obtained by the step-wise leach-ing are most enriched in the primary carbonate (in terms of the U–Pb systematics). In the

206

Pb/

204

Pb–

207

Pb/

204

Pb

diagram, data points of these fractions plot along an isochron determining age of

1550

±

30

Ma(MSWD = 0.7) for the upper member of the Satka Formation. The initial

87

Sr/

86

Sr

ratio in the least altered lime-stones of this formation is within the range of 0.70460–0.70480. Generalization of the Sr isotopic data pub-lished for the Riphean carbonates from different continents showed that 1650–1350 Ma ago the

87

Sr/

86

Sr

ratioin the world ocean was low, slightly ranging from 0.70456 to 0.70494 and suggesting the prevalent impact ofmantle flux.

DOI:

10.1134/S0869593808020020

Key words

:

87

Sr/

86

Sr

ratio in oceans, Pb–Pb age of carbonate rocks, Lower Riphean, Burzyan Group, southernUrals.

STRATIGRAPHY AND GEOLOGICAL CORRELATION

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THE Sr ISOTOPIC CHARACTERIZATION AND Pb–Pb AGE OF CARBONATE ROCKS 121

carbonates characterize reliably (within limits of ana-lytical uncertainty) the accumulation time of sedimen-tary formations (Moorbath et al., 1987; Jahn and Cuv-ellier, 1994; Ovchinnikova et al., 1995, 1998, 2001;Babinski et al., 1995; 1999; Frei et al., 1997; White-house and Russell, 1997; Folling et al., 2000; Bolharet al., 2002).

Being obtained simultaneously, the Sr-chemostrati-graphic characterization of carbonate deposits and theU–Pb (Pb–Pb) isotopic-geochronological informationabout their accumulation time open new perspectivesfor getting insight into the

87

Sr/

86

Sr

variations in theProterozoic world ocean. Data of this kind recentlyobtained for carbonate formations of the Riphean typesuccessions in the Urals and Siberia have been used todefine more precisely the age constraints of type sec-tions and added new important details to formerlyknown trends of the

87

Sr/

86

Sr

ratio variations in theMiddle and Late Riphean seawater (Gorokhov et al.,1995; Ovchinnikova et al., 1995, 1998, 2000, 2001;Kuznetsov et al., 1997, 2003a, 2003b, 2006; Semi-khatov et al., 2000, 2002). Based on these and otherpublished results, a new correlation chart for the UpperRiphean key sections in Arctic Canada, Spitsbergen,East Siberia and the southern Urals has been suggested,several time spans of predominantly mantle Sr flux intothe Late Riphean oceans have been determined, andspecific geodynamic situation of the Grenvillian orog-eny not accompanied by the

87

Sr/

86

Sr

ratio increase inseawater has been shown.

The effectiveness of the Sr-chemostratigraphic andU–Pb (Pb–Pb) geochronological methods is muchdependent on the quality of the analyzed materials. Ourexperience of studying the carbonate deposits in Siberia(Gorokhov et al., 1995; Ovchinnikova et al., 1995,2001; Semikhatov et al., 2000, 2002) and the Urals(Kuznetsov et al., 1997, 2003b; Ovchinnikova et al.,1998, 2000) showed that successful results can beobtained, when (1) the retentivity of Rb–Sr and U–Pbisotopic systems in the rocks is geochemicallyassessed, and (2) the selective dissolution is used inorder to remove secondary carbonate phases fromselected samples and to enrich them in primary carbon-ate material. Using this approach, we established thatlimestones are the best rocks, which yield the most reli-able Sr-chemostratigraphic and Pb–Pb isotopic-geo-chronological information. To evaluate the postsedi-mentary alterations in the rocks under investigation, itis also reasonable to study thick concurrent carbonatesuccessions situated in different structures of oneregion. An appropriate example is the Lower Ripheanstratotype, i.e., the Burzyan Group of volcanogenic-sedimentary rocks in the southern Urals (

The RipheanStratotype…

, 1983). Deposits of the group that is ofconsiderable stratigraphic range are studied well interms of lithology, facies transitions, and paleontology,being altered in some zones just under conditions ofdeep catagenesis (Anfimov, 1997; Maslov et al., 2001).The reliable Sr-chemostratigraphic and Pb–Pb geo-

chronological characterization of the Bakal Formationcrowning the group has been obtained recently withdue regard for the strict methodical approach to select-ing and processing the samples (Kuznetsov et al.,2003a, 2005). On the other hand, the middle interval ofthe group that is composed predominantly of carbonaterocks of the Satka and Suran formations remains inad-equately studied.

In this work, our objectives were as follows: (1) todescribe the methodical approach to simultaneousstudy of the Sr chemostratigraphy and the Pb–Pb isoto-pic geochronology of carbonate rocks, (2) to determineage of middle horizons in the Burzyan Group, theLower Riphean stratotype, (3) to present new data onthe

87

Sr/

86

Sr

ratio in the Lower Riphean deposits, (4) togeneralize secular trend of the

87

Sr/

86

Sr

variations in theEarly Riphean seawater based on the review of Sr-iso-topic data published for the key sections in the southernUrals, Siberia, India, and North America.

LITHOSTRATIGRAPHY AND CHRONOMETRIC CONSTRAINTS OF THE BURZYAN GROUP

Volcanogenic-sedimentary deposits of the BurzyanGroup (up to 8 km thick), the Lower Riphean stratotype(

The Riphean Stratotype…

, 1983; Kozlov et al., 1989),are exposed in the Taratash and Yamantau anticlinoriaof the Bashkirian meganticlinorium (Fig. 1). In theTaratash anticlinorium, deposits of the group discor-dantly overlie the Archean–Lower Proterozoicgneisses, crystalline schists, quartzites, and theTaratash granitoids, which intruded these rocks. In thisstructure, the Burzyan Group is divided in three units:the Ai Formation of volcanogenic–siliciclastic deposits(3000–3500 m), the Satka Formation of predominantlycarbonate rocks (1700–3500 m), and the Bakal Forma-tion of carbonate and clayey sediments (1400–1600 m).These formations conformably overlie one another, andthe latter is overlain with angular unconformity byquartzitic sandstones of the Middle Riphean ZigalgaFormation. In the Yamantau anticlinorium, strati-graphic equivalents of the above subdivisions are theBolshoi Inzer Formation of siliciclastic rocks (2200 m),the Suran Formation of carbonate–siliciclastic deposits(1000–2800 m), and the Yusha Formation of sandstonesand shales (600–1000 m) (

The Riphean Stratotype…

,1983; Kozlov et al., 1989; Maslov et al., 2001). Thebase of the Bolshoi Inzer Formation is unexposed,whereas the Yusha Formation is overlain with angularunconformity by volcanogenic–siliciclastic deposits ofthe Mashak Formation, the basal unit of the MiddleRiphean stratotype, which grade upward into quartziticsandstones of the Zigalga Formation. On the west of theTaratash anticline orium, deposits of the BurzyanGroup are altered under conditions of deep catagenesisand reveal in places the magnesite and siderite mineral-ization, the result of metasomatic impact (Anfimov,1997, 2007; Krupenin, 1999; Maslov et al., 2001). Theextent of secondary alterations in the rocks of the

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KUZNETSOV et al.

Yamantau anticlinorium corresponds to the level ofdeep metagenesis, whereas their transformation underconditions of the greenschist metamorphic facies isobservable near the Main Uralian Fault (Anfimov,1997; Maslov et al., 2001).

In Taratash anticlinorium, middle horizons of theBurzyan Group are composed predominantly of car-bonate rocks (Satka Formation), proportion of which islower in the Yamantau anticlinorium, where they arereplaced in part by siliciclastic deposits (Suran Forma-tion). Carbonate sediments of the Satka and Suran for-mations accumulated in shallow-water, moderatelyactive settings of open shelf (Maslov et al., 2001).Influx of fine-grained siliciclastic material into theSatka paleobasin was perceptible at the initial and mid-dle stages of the basin evolution. Sedimentation of thattime resulted in deposition of sediments characteristicof contrasting hydrodynamic settings, i.e., of the calm-water (below the fair weather and storm wave base) andintertidal (shallow-water coastal settings and small car-bonate shoals) zones. At the late stage, siliciclasticinflux into the Satka paleobasin became significantlyreduced because of the provenance peneplanation orflooding. Carbonate sediments of that stage accumu-lated predominantly between the fair weather and stormwave bases. The same trends of sedimentation and

development were characteristic of the Suran paleoba-sin except for a somewhat higher influx of siliciclasticmaterial.

Five lower-rank subdivisions of the Satka Formationare the lower and upper Kusa (700–900 and 800–900 m, respectively), Polovinka (160–180 m), lowerand upper Satka subformations (300–450 and up to1200 m, respectively); the Polovinka Subformationsolely is composed of siliciclastic rocks (Fig. 2).

The most complete sections of two lower and twoupper subformations are exposed in vicinity of theKusa and Satka towns respectively. The lower KusaSubformation includes three members. Prevailingrocks of the lower (170 m) and upper (590 m) membersof the subformation are clayey dolomites, whereas themiddle one (230 m) is composed mostly of dolomitizedlimestones. In terms of lithology, rocks of this subfor-mation correspond to massive or horizontally beddedcarbonates with interbeds of gross-laminated and stro-matolitic varieties intercalated with lenses of intraclastsand thin interlayers of dark, slightly carbonaceousclayey shales and siltstones. At the subformation base,there are horizons of chert and phosphorite nodules. Inthe upper Kusa Subformation, the basal 13-m-thickpacket of variegate siltstones and shales is overlain bydolomitic marls (180 m) containing packets of stroma-tolitic and oncolitic varieties and thin chert interlayers.Near the base and top, dolomitic rocks are intercalatedwith thin members of flaggy clayey limestones. Thelower Satka Subformation consists of gray sandy dolo-mites with interlayers of shales and dolomitized lime-stones. The terminal subformation of the Satka Forma-tion is divided into three units of lower rank: theKamennogorka Member of clayey dolomites (130–160 m), the Karagai Member of dolomites (750 m), andthe Kazym Member of limestones (30 to 140 m). Thelast two members are contaminated by siliciclasticmaterial to the minimum extent (3% in average) ascompared to all other units of the Satka Formation. TheKaragai Member composed predominantly of darkgray thin-bedded dolomites encloses thick (5–20 m)beds and lenses of carbonate breccia. Slab-shaped mag-nesite bodies in its lower part crosscut original sedi-mentary structures of carbonate rocks (Anfimov, 2007).Interlayers of synsedimentary carbonate breccia andclayey dolomitic shales are confined to the base of theKazym Member largely composed of gray laminatedfine-grained limestones. Thickness of this member cor-responding to 32 m in outcrops of the Mt. Kazy-movskaya increases up to 140 m in borehole sections(Sidorenkov, 1964).

The Suran Formation (1000–2800 m) that is strati-graphic equivalent in the Yamantau anticlinorium of theSatka Formation is divided into the Minyak, Berdagul,Angastak, Serdauk, and Lapyshta subformations. Thelower and upper of these subformations are of carbon-ate composition, whereas the others are represented bysiliciclastic rocks with thin carbonate interlayers

100 km

Belaya R.

60° E

Inzer

Satka Zlatoust

Kusa

Min’yar

Ufa

55°N

Zilim

R.

Sim

R.

Ufa

R.

Yuryuzan R.

1

2

3

4 Beloretsk

Verkh. Avzyan

Magnitogorsk

3

1 2 3 4 5 6

Fig. 1.

Geological structure of Bashkirian meganticlino-rium and localities of studied sections of the BurzyanGroup: (1) Taratash Complex of the Archean–Lower Prot-erozoic, (2) Lower Riphean, (3) Middle Riphean, (4) UpperRiphean, (5) Vendian, (6) localities of the studied sections;encircled numbers in the figure denote the Kusa (1), Satka(2), Kartalinskaya Zapan (3), and Ismakaevo (4) sections.


Vol. 16

No. 2

2008


Gro

up

Form

atio

n

Subf

orm

atio

n

Mem

ber

Form

atio

n

Subf

orm

atio

n

Taratashanticlinorium

Yamantauanticlinorium

1385 ± 1.4

1430 ± 30

1372 ± 16

Yur

m.

Z.-

K.

Zg.

Bak

alup

per

Z.-

K.

Zg.

Mas

hak

low

erM

akar

ov

Yus

ha

Bu

rz

ya

nSa

tka

Sura

n

Ai

B. I

nz.

500

0

1389 ± 28, 1368 ± 6

St5-

3St

5-2

St5-

1

St5

St4

St3

St2

St1

Sr5

Sr4

Sr3

Sr2

Sr1

1615 ± 45

1615±45

1430±30

2-21, UC-79UC-78

UC-73, UC-742-16, 2-18

UC-712-32-1

UC-55UC-54UC-53

UC-52UC-51UC-50

ST6-45

ST6-44

K3C-22K3C-18K3C-16

K3C-12K3C-9

K3C-2

UC-40UC-33

UC-37UC-28

UC-27UC-26UC-20

UC-15

UC-6

UC-2I2-3

I2-13

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

Fig. 2.

Lithostratigraphy of the Burzyan Group upper part in the Taratash and Yamantau anticlinoria with indicated sampling levels:(1) fine-grained, (2) stromatolitic, and (3) clayey limestones; (4) crystalline, (5) stromatolitic, and (6) silty dolomites; (7) dolomitewith phosphorite lenses; (8) low-carbonaceous shale; (9) siltstone; (10) sandstone; (11) gravelstone, conglomerate; (12) basic vol-canics; (13) silicic volcanics; (14) gabbro-diabase; (15) rapakivi granite; (16) nepheline syenite; (17) stratigraphic unconformity;(18) U–Pb zircon age of igneous rock, Ma; (19) Pb–Pb age of limestone, Ma. Abbreviations: (Yurm.) Yurmata Group; (B. Inz.)Bolshoi Inzer, (Zg.) Zigalga, and (Z.-K.) Zigazino-Komarovo formations. Indices of subformations: (St1) lower Kusa, (St2) upperKusa, (St3) Polovinka, (St4) lower Satka, (St5) upper Satka, (Sr1) Minyak, (Sr2) Berdagul, (Sr3) Angastak, (Sr4) Serdauk, (Sr5)Lapyshta. Indices of members of the upper Satka Subformation: (St5-1) Kamennogorka, (St5-2) Karagai, (St5-3) Kazym.

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KUZNETSOV et al.

(Fig. 2). The carbonate succession of the Minyak Sub-formation is most thick near the Ismakaevo Settlementin the Bolshoi Avzyan River valley, and that of theLapyshta Subformation is thickest near the settlementof Kartalinskaya Zapan in the same valley. We studiedthese successions, as they include, in distinction fromthe others, not only dolomites, but also limestones (

TheRiphean Stratotype…

, 1983; Maslov et al., 2001). TheMinyak Subformation (400 m) begins with a thick (20–80 m) member of silty limestones having fine cross-lamination and overlain by gray fine-grained dolomiteswith interlayers of siltstones and low-carbonaceousshales. The Lapyshta Subformation (~500 m) is com-posed of light gray medium- to coarse-grained lime-stones with clayey shale and sandstone interlayers.

The minimal dates known for the pre-Burzyan crys-talline rocks of the Taratash Complex constrain themaximum age limit of the Burzyan Group. The Rb–Srage of

1800

±

2.6

Ma is obtained for biotite-plagioclaseamphibolite formed in the course of retrograde meta-morphism of the pre-Burzyan rocks (Sindern et al.,2005). In the Taratash anticlinorium, the minimum ageof the group can be determined based on the U–Pb dateof baddeleyite (

1385

±

1.4

Ma, Ernst et al., 2006) andthe Rb–Sr date of biotite (

1360

±

35

Ma, El’mis et al.,2000) from gabbro-diabase of the Main Dyke crosscut-ting deposits of the Bakal Formation. In the Yamantauanticlinorium, the group minimum age is constrainedby the U–Pb SHRIMP-II dates of zircons from dacitesoccurring in the unconformably overlying Mashak For-mation of the Middle Riphean (

1370

±

16

Ma, Ronkinet al., 2007). The U–Pb dates of zircons from igneousrocks of the Berdyaush massif intruding deposits of theSatka Formation are within a narrow range from

1389

±

28

to

1368

±

6

Ma (Ronkin et al., 2005, 2007), beingconsistent with the formation time of the Main Dykeand the Mashak volcanics. More strictly, the age limitsof middle horizons in the Burzyan Group are con-strained by dates known for volcanogenic and sedimen-tary rocks exposed in the Taratash anticlinorium. TheU–Pb zircon age of

1615

±

45

Ma (the upper concordiaintercept, Krasnobaev et al., 1992) has been estimatedfor trachybasalts from middle part of the Ai Formation,and the Pb–Pb age of limestones from the middle Ber-ezovaya Member of the Bakal Formation correspondsto

1430

±

30

Ma (Kuznetsov et al, 2003a, 2005). Con-sequently, accumulation of the Burzyan Group middlehorizons lasted long from 1615 to 1430 Ma.

ANALYTICAL TECHNIQUE

Sampling carbonate rocks for chemostratigraphicand geochronological investigation, we selected mostlylimestones and dolomites with low content of silicatefraction from members, which are remote in sectionsfrom siliciclastic rocks, because the initial isotopic-geochemical systems of carbonates could be changedin the rocks during their recrystallization and/or inter-action with ground and meteoric waters. Difference in

chemical composition between potential epigeneticsolutions and seawater leads to precipitation of second-ary carbonate phases enriched, as a rule, in Mn, Fe, andPb, but depleted in

87

Sr as compared with initial marinesediments (Brand and Veizer, 1980; Banner and Han-son, 1990; Chaudhuri and Clauer, 1993; Gorokhov et al.,1995; Kuznetsov et al., 2003b). Consequently, determi-nation of minor element concentrations in limestonesand dolomites is, in addition to petrographic examina-tion, an important part of laboratory work aimed atselecting the least altered carbonate rocks.

Samples of carbonate rocks lacking visible indica-tions of secondary alteration have been split in twoparts, one for petrographic study, the other one forchemical and isotopic analyses. Carbonate samples dis-solved in 1 N HCl were used to determine Ca and Mgcontents by wet chemical method and to measure Mnand Fe concentrations by atomic absorption techniquein the analytical laboratory of Geological Institute RAS(Table 1).

The Rb–Sr systematics has been studied using thestepwise dissolution of powdered material (about100 mg). The procedure included preliminary treat-ment of material with 1N ammonium acetate(

NH

4

OAc

) under room temperature and subsequentdissolution of residue in 10% acetic acid (Gorokhovet al., 1995). The isotope dilution technique with mixed

87

Rb +

84

Sr

spike was used to determine Rb and Sr con-centrations in both fractions on the MI 1320 single-col-lector mass spectrometer. Separation of Sr from solu-tions for determining the

87

Sr/

86

Sr

ratio was carried outusing the ion-exchange method with the Dowex

AG50Wx8

(200–400 mesh) resin, and 2.5 N HCl aseluent. The Sr isotope composition was determined instatic acquisition mode on the multi-collector Triton T1mass spectrometer. The normalized to

86

Sr/

88

Sr

=0.1194 laboratory average

87

Sr/

86

Sr

ratio of

0.710262

±

0.000005

(2σ, n = 34) and 0.709191 ± 0.000005 (2σ,n = 17) were determined for the NIST SRM 987 andUSGS EN-1 standard carbonates, respectively, duringthe period of data gathering.

Fraction dissolved in ammonium acetate (AMA)corresponds to the late epigenetic generation (or gener-ations) of carbonate minerals, whereas fraction dis-solved in acetic acid (ACA) is considerably enriched inprimary carbonate material. Percentage of AMA frac-tion is higher in limestones than in dolomites, beingequal in average to 5.4 and 2.8%, respectively (Table 2).This proportion reflects difference between calcite anddolomite solubility in ammonium acetate (Gorokhovet al., 1995; Kuznetsov et al., 2003b). The Rb concen-tration in AMA fractions of the Burzyan limestones is10 times higher in average than in ACA fractions,whereas in the case of dolomites it is 28 times higher.The Sr concentration is approximately equal in bothfractions of limestones, being 1.5 times higher in AMAthan in ACA fraction of dolomites. This means thatdolomites contain certain amount of secondary calcite

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 16 No. 2 2008


Table 1. Concentration of minor elements, percentage and mineral composition of silicate fraction in carbonate rocks ofthe Satka and Suran formations

Sample no.

Sampling level1 Rock2 SF3, % Mg, % Mn,

ppmFe,

ppmSr,

ppm Mg/Ca Mn/Sr Fe/Sr Minerals of SF4

Satka Formation (Kazym Member, upper Satka Subformation), Mt. Kazymovskaya

UC-79 22 L 3.5 0.1 31 790 2450 0.003 0.01 0.3 Q

2-21 21 L 0.8 0.2 14 280 2740 0.005 0.01 0.1 –

UC-78 20 L 0.9 0.1 11 250 2750 0.003 0.00 0.1 –

2-18 19 L 1.7 0.2 20 460 2180 0.005 0.01 0.2 –

2-16 15 L 2.2 0.2 18 700 2340 0.006 0.01 0.3 Q

UC-74 8 L 1.1 0.2 21 420 1680 0.005 0.01 0.3 –

UC-73 7 L 3.1 0.1 22 730 1490 0.003 0.01 0.5 Q

Satka Formation (Karagai Member, upper Satka Subformation), Mt. Kazymovskaya

UC-71 640 D 1.3 12.4 45 1200 24.6 0.589 1.8 48.8 Q

2-3 635 D 0.7 12.9 85 740 66.4 0.599 1.3 11.1 –

2-1 630 D 2.4 12.8 63 560 24.8 0.594 2.5 22.6 Q

Satka Formation (Karagai Member, upper Satka Subformation), Satka surroundings

UC-55 350 D 3.8 13.4 135 710 34.2 0.637 4.0 20.8 Q

UC-54 340 D 4.7 13.2 125 2500 32.8 0.625 3.8 76.2 Q

UC-53 330 D 15.5 12.7 160 5750 36.4 0.626 4.4 158.0 Q, Fsp, Mc, Chl

UC-52 320 D 5.3 12.3 142 3450 38.1 0.614 3.7 90.6 Q, Fsp

UC-51 210 D 2.3 12.7 57 550 37.3 0.614 1.5 14.7 Q

UC-50 205 D 1.0 12.5 73 630 39.3 0.614 1.9 16.0 Q

ST6-45 10 D 1.0 12.7 70 600 44.2 0.610 1.6 13.6 Q

Satka Formation (Kamennogorka Member, upper Satka Subformation), Satka surroundings

ST6-44 –10 D 9.6 12.9 150 800 51.0 0.610 2.9 15.7 Q, Fsp, Mc, Chl

Satka Formation (upper Kusa Subformation), Kusa surroundings

UC-40 1075 D 42.5 10.6 52 1130 50.7 0.627 1.0 22.3 Q

UC-37 1030 D 10.4 12.6 76 1410 31.2 0.634 2.4 45.2 Q, Fsp

UC-33 1070 L 10.0 1.1 132 1430 69.9 0.028 1.9 20.5 Q, Fsp

Satka Formation (lower Kusa Subformation), Kusa surroundings

UC-20s 830 D 13.0 12.3 165 3890 21.9 0.615 7.5 178 Mc, Chl, Q, Fsp

UC-15 290 L 13.9 0.1 205 1790 56.8 0.004 3.6 31.5 Q, Fsp, Mc, Chl

UC-6 175 L 16.3 0.6 110 1260 85.4 0.016 1.3 14.8 Mc, Chl, Q, Fsp

UC-2 45 D 68.7 7.2 60 1490 37.4 0.646 1.6 39.8 Q

Suran Formation (Lapyshta Subformation), Kartalinskaya Zapan surroundings

K3C-22 450 L 11.4 0.1 80 1530 343 0.004 0.24 4.5 Q, Mc, Fsp

K3C-18 345 L 4.6 0.3 114 1250 368 0.008 0.31 3.4 Q, Fsp, Mc

K3C-16 325 L 6.0 0.1 120 1040 359 0.003 0.33 2.9 Q

K3C-12 185 L 3.5 0.1 125 1230 344 0.003 0.36 3.6 Q

K3C-9 134 L 7.8 0.7 80 1060 350 0.017 0.23 3.0 Mc, Q, Fsp

K3C-2 15 L 8.6 0.4 130 1170 320 0.011 0.41 3.7 Mc, Q, Fsp

Suran Formation (Minyak Subformation), Ismakaevo surroundings

I2-3 25 L 16.6 2.8 350 6010 182 0.075 1.92 33.0 Q, Fsp, Mc

I2-13 20 L 4.9 1.2 160 1490 700 0.029 0.23 2.1 Q, Fsp

Notes: 1 sampling level in meters above the base of carbonate succession exposed in the respective section or below the top in the case ofthe Kamennogorka Member; 2 rock types: (L) limestone, (D) dolomite; 3 silicate fraction (SF); 4 minerals in order of decreasingabundance: (Q) quartz, (Mc) mica, (Fsp) feldspars, (Chl) chlorite; (–) undetermined.

126


KUZNETSOV et al. Table 2. Rb–Sr analytical data for soluble carbonate fractions of samples from the Satka and Suran formations

Sampleno. Rock1 Carbonate

fraction2

Fractioncontent in sample, %

Rb, ppm Sr, ppm 87Rb/86Sr87Sr/86Sr,measured

87Sr/86Sr,initial3

Satka Formation (Kazym Member, upper Satka Subformation), Mt. Kazymovskaya

UC-79 L ACA – 0.58 2450 0.0005 0.70473 0.70472

2-21 L ACA – 0.10 2740 0.0001 0.70465 0.70465

UC-78 L AMA 4.9 0.36 4310 0.0002 0.70468 0.70467

ACA 94.8 0.03 2670 0.0001 0.70460 0.70460

2-18 L AMA 4.4 2.80 2585 0.0032 0.70497 0.70490

ACA 93.3 0.12 2180 0.0002 0.70470 0.70470

2-16 L AMA 4.0 0.73 2356 0.0009 0.70483 0.70481

ACA 95.7 0.12 2340 0.0002 0.70466 0.70466

UC-74 L ACA – 0.04 1675 0.0001 0.70468 0.70468

UC-73 L AMA 4.2 2.32 1530 0.0044 0.70501 0.70491

ACA 93.3 0.54 1490 0.0011 0.70482 0.70480

Satka Formation (Karagai Member, upper Satka Subformation), Mt. Kazymovskaya

UC-71 D ACA – 0.07 24.6 0.0083 0.70849 0.70830

2-3 D ACA – 0.14 66.4 0.0062 0.70900 0.70886

2-1 D AMA 2.6 1.58 28.0 0.1652 0.70839 0.70471

ACA 95.0 0.16 24.8 0.0189 0.70798 0.70756

Satka Formation (Karagai Member, upper Satka Subformation), Satka surroundings

UC-55 D ACA – 0.31 34.2 0.0068 0.70936 0.70921

UC-54 D ACA – 0.13 62.8 0.0061 0.71604 0.71591

UC-53 D AMA 3.6 4.18 64.2 0.1907 0.73393 0.72969

ACA 80.9 0.32 56.4 0.0166 0.73079 0.73042

UC-52 D ACA – 0.46 38.1 0.0069 0.71932 0.71917

UC-51 D ACA – 0.06 37.3 0.0048 0.70743 0.70732

UC-50 D AMA 3.5 6.47 134 0.1415 0.71267 0.70952

ACA 93.8 0.06 36.9 0.0048 0.70793 0.70782

ST6-45 D ACA – 0.30 44.2 0.0199 0.70659 0.70616

Satka Formation (Kamennogorka Member, upper Satka Subformation), Satka surroundings

ST6-44 D ACA – 0.60 51.0 0.0345 0.71481 0.71404

Satka Formation (upper Kusa Subformation), Kusa surroundings

UC-40 D AMA 2.9 1.41 12.6 – – –

ACA 56.1 0.43 52.6 0.0238 0.71978 0.71933

UC-37 D AMA 2.4 2.42 35.3 0.2008 0.71624 0.71235

ACA 87.4 0.18 31.1 0.0168 0.71479 0.71447

UC-33 L AMA 5.6 1.44 92.4 0.0456 0.72016 0.71928

ACA 85.8 0.05 68.4 0.0021 0.70976 0.70972



or low-Mg dolomite. Difference between 87Sr/86Srratios measured in AMA and ACA fractions(∆87Sr/86Sr) corresponds to 0.0001–0.0104 for lime-stones and to 0.0014–0.0199 in dolomites (Table 2).For limestones and dolomites, this parameter correlatespositively with Mn/Sr (r = 0.78, n = 20) and Fe/Sr (r =0.68, n = 20) ratios, thus evidencing non-cogenetic sta-tus of the fractions and enrichment of secondary AMAphases in Mn, Fe and radiogenic 87Sr after epigeneticrecrystallization. In case of dolomites, correlationbetween ∆87Sr/86Sr and Mg/Ca ratios is negative (r =–0.95, n = 7), implying that calcite or low-Mg dolomitepresent in these rocks are secondary epigenetic miner-als. In pertinent sections of this work, we consider only

those 87Sr/86Sr ratios, which are determined in the ACAfractions enriched in initial carbonate material.

Selecting the least altered limestone samples, whichretain the 87Sr/86Sr signature of sedimentation media,we used as before the severe geochemical criteria oftheir retentivity: Mn/Sr < 0.2, Fe/Sr < 5.0, Mg/Ca <0.024 and δ18é > –10‰ (Kuznetsov et al., 1997, 2003a,2003b; Semikhatov et al., 2002). Critical values of thesame ratios for dolomites used to be determined pereach lithostratigraphic unit individually, and only ifcorrelation between the Mn/Sr and Fe/Sr values, on theone hand, and the 87Sr/86Sr ratio, on the other, has beenestablished. Dolomites of the Burzyan Group do notrepresent an opportune case (Fig. 3). The samegeochemical criteria and critical 87Sr/86Sr value less

Table 2. (Contd.)

Sampleno. Rock1 Carbonate

fraction2

Fractioncontent in sample, %

Rb, ppm Sr, ppm 87Rb/86Sr87Sr/86Sr,measured

87Sr/86Sr,initial3

Satka Formation (lower Kusa Subformation), Kusa surroundings

UC-20 D AMA 3.2 8.35 26.0 0.9405 0.74267 0.72447

ACA 87.3 0.28 21.7 0.0375 0.72275 0.72202

UC-15 L AMA 4.4 3.04 39.8 0.2237 0.73227 0.72794

ACA 83.3 0.18 57.7 0.0090 0.72999 0.72982

UC-6 L AMA 5.3 5.86 97.8 0.1755 0.72517 0.72177

ACA 78.7 0.29 84.6 0.0101 0.71852 0.71832

UC-2 D AMA 2.2 4.81 57.1 0.2467 0.71278 0.70801

ACA 32.0 0.26 36.0 0.0209 0.71276 0.71236

Suran Formation (Lapyshta Subformation), Kartalinskaya Zapan surroundings

K3C-22 L AMA 8.9 0.68 262 0.0076 0.70864 0.70847

ACA 77.6 0.49 350 0.0041 0.70852 0.70843

K3C-18 L AMA 7.3 0.28 225 0.0037 0.70678 0.70670

ACA 89.9 0.15 380 0.0012 0.70663 0.70660

K3C-16 L AMA 8.9 0.38 285 0.0039 0.70690 0.70681

ACA 86.3 0.92 367 0.0073 0.70679 0.70663

K3C-12 L AMA 8.8 0.28 149 0.0054 0.70638 0.70626

ACA 88.6 0.13 364 0.0030 0.70627 0.70625

K3C-9 L AMA 4.6 4.74 322 0.0431 0.70773 0.70677

ACA 88.5 0.19 353 0.0016 0.70773 0.70768

K3C-2 L AMA 6.5 0.35 180 0.0056 0.70823 0.70811

ACA 86.8 0.69 330 0.0061 0.70793 0.70779

Suran Formation (Minyak Subformation), Ismakaevo surroundings

I-2-3 L ACA – 0.40 182 0.0064 0.71104 0.71090

I-2-13 L ACA – 0.30 700 0.0013 0.70587 0.70584

Notes: 1 rock types, (L) limestone, (D) dolomite; 2 fraction types, (AMA) carbonate material dissolved in 1 N NH4OAc, (ACA) carbonatematerial leached in 10% CH3COOH from residue after sample treatment in 1 N NH4OAc; 3 age value of 1550 Ma has been usedto calculate the 87Sr/86Sr initial ratios for carbonate rocks if the Satka and Suran formations; (–) undetermined.

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KUZNETSOV et al.

than 0.001 (Ovchinnikova et al., 1998, 2001; Kuz-netsov et al., 2005) have been used to select preliminarythe least altered limestone samples for the U–Pb dating.Nevertheless, even the “best” samples well satisfyingthe above criteria and suitable for studying the Srchemostratigraphy can be altered in terms of the U–Pbsystematics, because the Pb concentration in carbon-ates is much lower than in potential epigenetic fluids.Accordingly, our approach to investigation of the U–Pb

systematics in Proterozoic carbonates included addi-tional procedures aimed at getting enrichment of sam-ples in the primary carbonate material. First, the pre-liminary leaching with NH4OAc, HCl or HBr solutionsof different concentration facilitates removal of 2 to25% of the secondary Pb from carbonates (Ovchinnik-ova et al., 1995, 1998, 2000, 2001; Frei et al., 1997;Whitehouse and Russell, 1997; Babinski et al., 1999).However, this procedure is not always securing separa-

Table 3. U–Pb characteristics of carbonate fractions obtained by step-wise leaching of limestone samples from the KazymMember of the Satka Formation

Sample no.

Carbonatefraction

Fraction content in the rock, % Pb, ppm U, ppm 238U/234Pb 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb

2-21 L-4–L-6 – 0.68 – – 26.878 16.480 37.400

2-18 L-4–L-6 – 1.47 6.73 328 27.134 16.510 38.200

2-16 L-4–L-6 – 1.03 1.49 104 26.849 16.475 38.363

UC-78 L-1 3.1 – – – 24.919 16.220 37.902

L-2 12.2 – – – 29.874 16.748 37.585

L-3 17.9 – – – 32.250 16.946 37.571

L-4 32.6 – – – 33.011 17.073 37.474L-5 15.1 – – – 35.059 17.257 37.502L-6 12.1 – – – 36.384 17.411 37.571L-7 6.4 – – – 32.362 17.024 37.476

ISR 0.6 – – – 19.844 15.865 37.237

UC-73 L-4–L-6 – 0.80 – – 39.561 17.702 39.701

Notes: Fractions are obtained by leaching samples in 0.5 N HBr as described in the text (section Investigation Procedure); shown in bolditalic are isotopic ratios in fractions characterizing, as is established, the least altered (“primary”) carbonate material; isotopic ratiosof lowermost row characterize the combined carbonate fractions L-4 to L-6 obtained at the fourth to sixth stages of the sampleleaching; (–) undetermined.

0.705

0 1

87Sr/86Sr

Mn/Sr

0.700

0.710

0.715

0.720

0.725

2 3 4 5

A

1 2 3

Mn/Sr = 0.2

0.705

0 20Fe/Sr

0.700

0.710

0.715

0.720

0.725

40 60 80 100

B

Fe/Sr = 5

Fig. 3. Correlation diagrams 87Sr/86Sr–Mn/Sr (A) and 87Sr/86Sr–Fe/Sr (B) for rocks of the Burzyan Group: (1) limestones and(2) dolomites of the Satka Formation; (3) limestones of the Suran Formation.



tion of carbonate fraction unaltered in terms of theU−Pb systematics, especially when samples have beencollected close to zones of epigenetic and metasomaticrecrystallization. We find it necessary, therefore, toapply differential dissolution of a pilot sample in orderto reveal its fractions most enriched in primary carbon-ate material (Ovchinnikova et al., 2000; Kaurova et al.,2001). Such an approach helps to figure out an optimaldissolution procedure for all the samples selected foranalysis, which is depending on conditions and inten-sity of the U–Pb system resetting during postsedimen-tary alterations.

In this work, differential (stepwise) dissolution oflimestones in 0.5 N HBr consisted of 7 stages each ter-minated with centrifuging and drying of insoluble resi-due. At the first stage, powdered aliquot of sample wastreated in 0.5 N HBr during 30 min to obtain fractiondesignated L-1. This fraction contains about 3% of thesample carbonate constituent (Table 3), which corre-sponds approximately to the percentage of carbonatethat can be leached from limestone with 1 N NH4OAc(Tables 2 and 3). Residue after the first leaching wastreated by the same reagent during 60 min, and about 12to 14 % of carbonate substance entered the solution atthis stage (fraction L-2). Each of the fractions L-3, L-4,L-5, and L-6 obtained at the next four stages contained12 to 33% of carbonate material present originally inthe sample. The last fraction L-7 contained the rest ofcarbonate components (~6%). The residue insoluble inHBr (ISR fraction) was decomposed in autoclave withmixed HF and HNO3 under T = 220°ë. One aliquot ofeach fraction was used for measuring the Pb isotopecomposition, the other one for determination of U andPb concentrations by isotope dilution technique withmixed 235U + 208Pb spike. The Pb extraction from 0.5 NHBr has been performed using the ion exchangeresin Bio-Rad 1 × 8 (100–200 mesh) as described byManhes et al. (1978), and the UTEVA SPEK resinhas been used to extract U from 2 N HNO3 with0.01 N HNO3 as eluent.

The U and Pb isotope compositions were analyzedin static mode on a Finnigan MAT-261 multicollectormass spectrometer with Re-filaments. The measured Pbisotopic ratios were corrected for fractionation factor of0.13% per atomic mass unit, which is estimated basedon repeated determinations of Pb isotope compositionin the NBS SRM 982 standard. Reproducibility of Uand Pb concentrations measured in the BCR-1 standardcorresponds respectively to ±0.5% (2σ) and ±1% (2σ),and the procedure blank in the course of this study was0.01 ng for U and 0.13 ng for Pb. The blank for Pb iso-topic ratios was as follows: 206Pb/204Pb = 17.945,207Pb/204Pb = 15.324, and 208Pb/204Pb = 37.159. Thecomputational algorithms by Ludwig, (1989, 1998)have been used to assess uncertainties and correlationof U–Pb data (PBDAT program) and to calculate isoch-ron parameters (program Isoplot/Ex. Version 1.00).

RESULTS

Geochemical and Rb–Sr Characteristics

Satka Formation. Among 25 samples from this for-mation, which have been studied, 10 correspond incomposition to limestones and 15 to dolomites. Sevenof limestone samples are from the terminal KazymMember, two from the middle member of the lowerKusa Subformation, and one from the base of the latter(Fig. 2). Eleven dolomite samples have been collectedfrom the upper Satka Subformation, whereas both Kusasubformations are characterized by two samples pereach. Rocks of the lower Satka Subformation have notbeen studied, because it is poorly exposed and rich insiliciclastic interlayers.

Limestones. All the limestone samples from theKazym Member retained the original sedimentary layer-ing evident from alternation of thin fine-grained laminae.These samples are composed of calcite (Mg < 0.2%) andcontain insignificant admixture of detrital quartz (0.8–3.5%). Characteristic of limestones are low Mn (11–31 ppm) and Fe (250–790 ppm) concentrations buthigh Sr concentration (1490–2750 ppm). In concentra-tion range of Mn and Fe, the Kazym limestones arecomparable with recent carbonate sediments, whereasthe Sr concentration in these rocks is higher than in thelow-Mg calcite of marine genesis (Peterman et al.,1970; Hodell et al., 1990; Veizer et al., 1999). Amongthe Lower Riphean carbonate deposits, only limestonesof the Newland Formation of the Belt Supergroupshowed a similarly high Sr content (1500–3900 ppm,Hall and Veizer, 1996). Taking into consideration thatmarine carbonates become depleted in Sr during thepost-sedimentary recrystallization (Brand and Veizer,1980), it is reasonable to think that carbonate sedimentsof the Kazym Member originally contained aragonite.The Mn/Sr and Fe/Sr ratios in the Kazym limestonesare not higher than 0.01 and 0.1, respectively (Fig. 3).Consequently, these rocks satisfy the geochemical cri-teria of retentivity, and the 87Sr/86Sr initial ratio rangingin them from 0.70460 to 0.70480 was most likely char-acteristic of seawater in the Satka paleobasin (Table 2).

Clayey varieties of limestones from the lower (sam-ples UC-6 and UC-15) and upper (Sample UC-33)Kusa subformations are more enriched (10.0–13.9%) insiliciclastic material (illite, chlorite, quartz, feldspars;Table 1). Being composed of microcrystalline to fine-grained calcite (Mg/Ca 0.004–0.028), they also retainfine sedimentary lamination. As compared to lime-stones of the Kazym Member, these rocks are relativelyenriched in Mn (110–132 ppm) and Fe (1260–1790 ppm), being considerably depleted in Sr (56.8–85.4 ppm). The high 87Sr/86Sr ratio typical of the desig-nated samples is within the range of 0.70976–0.72999.The isotopic-geochemical characteristics of clayeylimestones suggest resetting of their Rb–Sr systems,which entrapped the radiogenic 87Sr from illite andfeldspars during epigenesis.

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KUZNETSOV et al.

Dolomites of the Satka Formation are fine- tomedium-grained rocks in general, and their coarse-grained varieties occur in lower part of the KaragaiMember (samples ST6-45 and UC-50). The Mg/Caratio in most samples (0.610–0.646) is higher than instoichiometric dolomite, being decreased to 0.589–0.599 near the Karagai Member top 10 to 15 m belowthe Kazym Member base. Percentage of siliciclasticadmixture ranges widely from 0.7 to 68.7% (Table 1).In dolomites of two Kusa subformations, content of thismaterial is over 10.4% (illite, chlorite, quartz, and feld-spars), whereas dolomites of the upper Satka Subfor-mation are contaminated predominantly by quartz(commonly <5.3%) most concentrated in samples ST6-44 (9.7%) and UC-53 (15.5%). In the Satka dolomites,the Sr concentration is low (21.9–62.8 ppm), whileminor element concentrations (Mn = 45–165 ppm; Fe =550–5750 ppm) and 87Sr/86Sr ratio (0.70616–0.73042)are positively correlated with abundance of siliciclasticadmixture. These data suggest partial exchange ofminor elements and radiogenic 87Sr between silicateand carbonate components of the rocks in the course ofburial diagenesis. An indirect evidence in favor of thisinference is the fact that the minimum 87Sr/86Sr ratio(0.70616) in dolomite sample ST6-45 from the KaragaiMember is significantly higher than in the least alteredlimestones of the Kazym Member (0.70460–0.70480).Consequently, dolomites of the Satka Formation do notcharacterize the Sr isotope composition in seawater ofthe respective paleobasin.

Rocks of the Suran Formation have been sampledin two limestone members only: at the base of the Min-yak Subformation in the Ismakaevo section and in theLapyshta Subformation of the Kartalinskaya Zapansection (Fig. 2). Limestones of the Minyak Formationcontain a minor amount of dolomite (Mg = 1.2–2.8%),whereas crystalline limestones of the Lapyshta Forma-tion are composed of calcite (Mg < 0.7%). Content ofsiliciclastic fraction (illite, quartz, and feldspars) in theSuran limestones ranges from 4.9 to 16.6%. Concentra-tion variations of Mn, Fe, and Sr in the Minyak lime-stones (160–350, 1490–6010, and 192–700 ppm,respectively) are more significant than in limestones ofthe Lapyshta Subformation (80–130, 1060–1530, and320–368 ppm; Table 1). All the samples from the SuranFormation contain more Mn and Fe than the leastaltered limestones of the Satka Formation, being com-paratively depleted in Sr at the same time. The 87Sr/86Srratio varies broadly from 0.70587 to 0.71104 in lime-stones of the Minyak Subformation and over narrowerlimits from 0.70627 to 0.70852 in the Lapyshta Subfor-mation (Table 2). The Mn/Sr ratio in the Suran lime-stones (0.23–1.92) is higher than critical value separat-ing rocks with the disturbed Rb–Sr systems from lime-stones retaining primary carbonate material. The lowδ18é values (–15.3 to –11.6‰) also point to essentialepigenetic recrystallization of limestones in the SuranFormation. Hence, none of the samples from this for-mation is suitable for determining the 87Sr/86Sr ratio inseawater of the Suran paleobasin, although conceivablyit was not higher than 0.70584.

U–Pb Systematics

The U–Pb (Pb–Pb) systems are much more sensitiveto influence of post-sedimentary alterations than theRb–Sr system. Accordingly, even the “best” carbonatesamples, which satisfy the accepted severe geochemi-cal criteria and retain the Sr-isotopic signature of sedi-mentation media, need in additional removal of second-ary carbonate generations. In our collection, only sam-ples from the Kazym member turned out to beappropriate in this respect.

Among the “best” limestones, we selected SampleUC-78 with lowest 87Sr/86Sr ratio (0.70460) for thepilot step-wise dissolution. The 206Pb/204Pb ratio growsfrom 24.919 in fraction L-1 of this sample to 36.384 infraction L-6 and then declines to 32.362 in fraction L-7(Fig. 4). The 208Pb/204Pb value in fraction L-1 is higherthan in the others: 37.902 versus 37.474–37.585(Table 3). Accordingly, carbonate material is heteroge-neous even in the least altered limestone samplesselected based on the severe geochemical criteria. Thelow 206Pb/204Pb ratio in fractions L-1, L-2, L-3 and high208Pb/204Pb value in fraction L-1 suggest that these frac-tions contain allogenic Pb that is a result of surficialcontamination, on the one hand, and of the epigeneticalteration, on the other (Ovchinnikova et al., 2001).Fraction L-7 contains a minor impurity of Pb derived

0 20 40 60 80 100Dissolved material, %

40

35

30

25

20

15

206Pb/204Pb

L-1

L-2

L-3 L-4L-5

L-6

L-7

Limestone, Sample UC-78from the Satka Formation

1 2 3

Fig. 4. Levels of 206Pb/204Pb ratio in carbonate fractions oflimestone sample UC-78 from the Satka Formation asdependent on amount of dissolved material: (1) fractionsrepresenting unaltered carbonate material, (2) fractionsenriched in primary carbonate material, (3) siliciclastic res-idue insoluble in 0.5 N HBr.



from the siliciclastic admixture (in ISR fraction206Pb/204Pb = 19.844, 208Pb/204Pb = 37.237), as the206Pb/204Pb ratio is lowered in this fraction. Hence, onlyfractions L-4, L-5, and L-6, obtained at intermediatestages of dissolution and containing about 60% of orig-inal carbonate material, can be regarded as unaltered interms of the U–Pb systematics. In addition to the pilotsample, we subjected to step-wise dissolution the otherfour samples of “best” limestones from the Satka For-mation. When characterizing the U–Pb systematics ofprimary carbonates of the rocks, we considered sum-mary isotopic data on fractions L-4, L-5, and L-6.

In 207Pb/204Pb–206Pb/204Pb diagram, data points ofintermediate fractions (L-4, L-5, and L-6 altogether) offour aforementioned samples and of fractions L-5 andL-6 of Sample UC-78 determine a straight regressionline with a slope corresponding to the age of 1550 ±30 Ma (MSWD = 0.7). Data on fraction L-4 of SampleUC-78 are ignored when plotting the isochron becauseof a high uncertainty of respective Pb isotopic ratios.Absence of correlation between the data points in206Pb/204Pb–1/204Pb diagram and low MSWD value con-firm that the regression line is an isochron. In208Pb/204Pb–206Pb/204Pb diagram, there is also no correla-tion of data points that is consistent with the extremelylow 232Th/238U ratio in seawater and presumably evi-dences against entrapment of Th (or 208Pb) by carbon-ates after their deposition. Consequently, straight linein 207Pb/204Pb–206Pb/204Pb diagram has geochronologi-cal sense, and the estimated age value of 1550 ± 30 Macharacterizes time of deposition (or early diagenesis)for limestones of the Satka Formation. The data point ofinsoluble residue (ISR fraction) is displaced relative tothe isochron, and this is evidence against the Pb redis-tribution between carbonate and siliciclastic compo-nents of limestones at the time of diagenesis (Fig. 5).

Thus, limestones of the Satka Formation areyounger than basaltic volcanics of the underlying AiFormation (1615 ± 45 Ma, U–Pb zircon date, upperconcordia intercept, Krasnobaev et al., 1992) and olderthan limestones of the overlying Bakal Formation(1430 ± 30 Ma, Pb–Pb date for limestones from theBerezovaya Member, Kuznetsov et al., 2003a, 2005).Besides, rocks of the Berdyaush massif intruded intothe Satka deposits are much younger than the Kazymlimestones. The U–Pb zircon dates obtained for gabbro,rapakivi granite and nepheline syenite of the massifrange from 1389 ± 28 to 1370 ± 5 Ma (Ronkin et al.,2005, 2007).

VARIATIONS OF 87Sr/86Sr AND 238U/204Pb (µ) RATIOS

IN THE BURZYAN PALEOBASIN AND EARLY RIPHEAN SEAWATER

The Sr isotope composition in seawater of theBurzyan paleobasin can be evaluated based on the87Sr/86Sr values characterizing the least altered lime-stones of the Kazym member in the Satka Formation

(Table 2) and of the Berezovaya Member in the BakalFormation (Kuznetsov et al., 2003a, 2005). Limestonesamples from both formations satisfy severe geochem-ical criteria of the retentivity (Mn/Sr < 0.2, Fe/Sr < 5and Mg/Ca < 0.024) and are analyzed after the proce-dure of enrichment in primary carbonate material,being suitable at the same time for the correct determi-nation of their Pb–Pb isochron age. The last opportu-nity is important for getting without geochronologicalinterpolations the age constraints of pertinent segmentsof the 87Sr/86Sr variation curve characterizing the EarlyRiphean oceans. Thus, we regard the Sr isotope compo-sition in the studied samples as correctly exemplifying,in terms of the methodical approach, the 87Sr/86Sr ratioin seawater, from which carbonate sediments of theSatka and Bakal formations precipitated respectively1550 and 1430 Ma ago (Fig. 6).

Because of the low Rb (< 0.58 ppm) but high Sr con-centration in the least altered Burzyan limestones,increment of radiogenic 87Sr in the rocks owing todecay of radioactive 87Rb is negligible. The respectivecorrection of the measured 87Sr/86Sr ratios is as high as0.00002 for one sample only, being equal to 0.00001for three samples and practically close to zero for theothers. Taking this into account, we figured out that the

10 20 4030 50206Pb/204Pb

19

18

17

16

15

207Pb/204Pb

1 2 3 4

5 6 7S-K

S-K

Limestones of theSatka Formation,

T = 1550 ± 30 Ma,MSWD = 0.7

Fig. 5. Diagram 207Pb/204Pb–206Pb/204Pb for limestonesfrom the Kazym member of the Satka Formation: (1–5) car-bonate fractions L-4 (1), L-5 (2), L-6 (3), and sum of frac-tions L-4 to L-6 (4); (5) residue insoluble in 0.5 N HBr,(6) field of Pb isotopic ratios characterizing rocks of theTaratash metamorphic complex 1550–1340 Ma ago(unpublished data of G.V. Ovchinnikova and A.M. Larin),(7) model curve by Stacey and Kramers for the terrestriallead of the Earth.

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KUZNETSOV et al.

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0

1 2 3

Fig. 6. Variations of 87Sr/86Sr ratio in carbonate samples from the Burzyan Group: (1) limestones satisfying critical geochemicalratios Mn/Sr < 0.2, Fe/Sr < 5 and Mg/Ca < 0.024; (2) limestones unsatisfying these criteria; (3) dolomites (abbreviations and sym-bols as in Fig. 2).



87Sr/86Sr initial ratio varies from 0.70460 to 0.70480 inlimestones of the Kazym Member of the Satka Forma-tion (Table 2) and from 0.70456 to 0.70481 in the Ber-ezovaya Member of the Bakal Formation (Kuznetsovet al., 2003a, 2005). The horizontal trend of data pointsin the 208Pb/204Pb–206Pb/204Pb is characteristic of unal-tered marine carbonates (Ovchinnikova et al., 1995;Kuznetsov et al., 2005) and points to accumulation ofthe Satka and Bakal sediments in paleobasins, whichcommunicated with open ocean.

Hence, the 87Sr/86Sr initial ratio in seawater of theBurzyan paleobasin was practically unchangeable from1550 ± 30 Ma (0.70460–0.70480) to 1430 ± 30 Ma(0.70456–0.70481) (Fig. 7). In addition to these results,the data characterizing the 87Sr/86Sr variations in theEarly Riphean seawater may be inferable from resultspublished for globally spaced carbonate formations ofthat time, which have been studied in Siberia (Pok-rovskii and Vinogradov, 1991; Gorokhov et al., 1995),North America (Hall and Veizer, 1996), and centralIndia (Ray et al., 2003). Most rocks of these formationsare reliably coordinated with the chronometric scale,and their Sr-isotopic characteristics are obtained usingthe procedure of preliminary leaching.

An important contribution to our knowledge of Srisotope composition in the Riphean ocean representdata on the Lower Riphean limestones of the SemriGroup exposed in southeastern part of the Vindhyanbasin of central India (Ray et al., 2003). In the Son Val-ley, this group includes the Kajrahat Formation in itslower part and the Rohtas Formation in the upper one,which correspond to limestone successions 200 and500 m thick respectively. The Kajrahat Formation isconformably overlain by the Deonar Formation ofsiliciclastic rocks with tuff interlayers dated at 1631 ± 5and 1628 ± 8 Ma by the U–Pb zircon method (Rayet al., 2002; Rasmussen et al., 2002). The Pb–Pb age oflimestones from the Rohtas Formation corresponds to1599 ± 48 Ma (Sarangi et al., 2004), being concordantto the U–Pb dates of 1602 ± 10 and 1599 ± 8 Maobtained for zircons from volcaniclastic sediments atthe formation top (Rasmussen et al., 2002). The Sr iso-topic composition has been studied in 11 limestonesamples from the Kajrahat Formation and 8 limestonesamples from the Rohtas Formation (Ray et al., 2003).All the samples have been preliminary leached in 1 Nsolution of acetic acid. In four samples from the Kajra-hat Formation, which satisfy geochemical criteria(Mn/Sr < 0.2) accepted in our work, the 87Sr/86Sr ratioranges from 0.70460 to 0.70494, whereas only onesample from the Rohtas Formation with 87Sr/86Sr =0.70479 (Fig. 7) can be regarded as the least altered.Somewhat higher values of 0.70481 and 0.70496 havebeen established for two other samples (Mn/Sr = 0.23)from this formation.

As for the Sr isotopic characteristics of the LowerRiphean successions in Siberia, they are known fordolomites from lower part of the Kyutingda Formation

of the Olenek Uplift (Gorokhov et al., 1995) and fromthe Kotuikan Formation of the Anabar massif (Pok-rovskii and Vinogradov, 1991). The Kyutingda Forma-tion is estimated to be 1480 Ma old according to stro-matolite-based correlation with upper part of the Ust-Il’ya Formation of the Anabar massif (Semikhatov andSerebryakov, 1983). The Rb–Sr date of 1483 ± 5 Maobtained for glauconite from the Ust-Il’ya depositsdetermines time of their early diagenesis (Gorokhovet al., 1991). Because of insignificant amount of silici-clastic fraction (< 1%) and low Mn (250 ppm) and Fe(610 ppm) concentrations in dolomite sample from theKyutingda Formation, the 87Sr/86Sr initial ratio in ACAfraction of these rock (0.70465) can be regarded as agood approximation to the Sr isotopic composition inseawater of that time (Gorokhov et al., 1995). Pok-rovskii and Vinogradov (1991) published the 87Sr/86Srmeasured ratios (10 values ranging from 0.70501 to0.71045) for dolomites of the Kotuikan Formation

3 2

15

4678

9

10

11

1213

900 1000 14001200 1600Ma

0.704

87Sr/86Sr

1 2 3 4

1700

0.705

0.706

0.707

Riphean

Late Middle EarlyPR1

Fig. 7. Variations of 87Sr/86Sr ratio in the Early Ripheanseawater: (1) limestones with Mn/Sr < 0.2 and Fe/Sr < 5,(2) limestones with Mn/Sr > 0.2; (3) dolomites; (4) barite.Numbers in the figure denote the following stratigraphicsubdivisions: (1) Semri Group, India (Ray et al., 2003);(2) Satka Formation, southern Urals (this work); (3) Kyut-ingda Formation, Olenek Uplift (Gorokhov et al., 1995);(4) Newland Formation, Belt Supergroup, North America(Hall and Veizer, 1996); (5) Bakal Formation, southernUrals (Kuznetsov et al., 2003a, 2005); (6) Kotuikan Forma-tion, Anabar Uplift (Pokrovskii and Vinogradov, 1991);(7) Yusmastakh Formation, Anabar Uplift (Pokrovskii andVinogradov, 1991); (8) Debengda Formation, Olenek Uplift(Gorokhov et al., 1995); (9) Society Cliff Formation, NorthCanada (Kah et al., 2001); (10) Malga Formation, Uchur–Maya region of Siberia (Semikhatov et al., 2002);(11) Sukhaya Tunguska Formation, Turukhansk Uplift(Gorokhov et al., 1995); (12) Neryuen Formation, Uchur–Maya region (Semikhatov et al., 2002); (13) Burovaya For-mation, Turukhansk Uplift (Gorokhov et al., 1995).

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(about 1420 Ma old) resting on the Ust-Il’ya deposits(Semikhatov and Serebryakov, 1983) and discordantlyoverlain by the Yusmastakh Formation of the MiddleRiphean (1280–1270 Ma, Gorokhov et al., 2001). Asgeochemical characterization of these dolomites hasnot been presented, we think that only two lowest87Sr/86Sr values (0.70478 and 0.70501) established intwo samples from the formation upper part, which arecomparatively depleted in Rb (<0.25 ppm in the bulkcarbonate constituent), can approach the ratio in theEarly Riphean seawater (Fig. 7).

The 87Sr/86Sr values measured in 25 carbonate sam-ples from the Belt Supergroup range broadly from0.70484 to 0.74491 (Hall and Veizer, 1996), but onlytwo limestone samples from the Newland Formation inthe supergroup lower part have the lowest Mn/Sr ratios(0.27 and 0.40) and also low 87Sr/86Sr values corre-sponding to 0.70484 and 0.70514. The former one issimilar to the 87Sr/86Sr ratio (0.70473 and 0.70485)established for two samples of syngenetic barite fromthe same formation (Strauss, 1993). All three lowestvalues can probably be used for constraining the upperlimit of 87Sr/86Sr ratios in the Early Riphean seawater(Fig. 7). It is also remarkable that the U–Pb dates of zir-cons from synsedimentary sills of the Prichard Forma-tion, a stratigraphic equivalent of the Newland Forma-tion (Ross and Villeneuve, 2003, and referencestherein), vary between 1468 ± 3 and 1457 ± 2 Ma (Searset al., 1998). Consequently, carbonate sediments of theNewland Formation accumulated ca. 1460 Ma ago.

Thus, the 87Sr/86Sr ratio variations in the Early Riph-ean (1650–1400 Ma) seawater were within the narrowrange of 0.70456–0.70494 (Fig. 7). Judging from theother available data on the Sr isotope composition inthe Riphean oceans, similarly low 87Sr/86Sr ratiosremained characteristic of seawater until commence-ment of the main (Elzevirian) phase of the Grenvillianorogeny 1250 Ma ago (see review in Semikhatov et al.,2002). In general, data on the Sr isotope composition inthe Early–initial Middle Riphean carbonates suggestpredominantly mantle source of the Sr flux into oceansat the eve of the Grenvillian orogenic cycle.

An important information about local provenancesinterrelated with the Burzyan paleobasin can beobtained from analysis of the Pb isotope composition incarbonate sedimentary rocks. The Pb residence time inocean is shorter than stirring cycle of oceanic watermass (Jahn and Cuvallier, 1994) that is why the Pb iso-tope composition in seawater varies with time underinfluence of local factors and composition of rockscomplexes in provenances. The Pb isotopic ratios and238U/204Pb (µ) value characterize proportion of “man-tle” and “crustal” rocks subjected to erosion in prove-nances. Based on the model by Stacey and Kramers(1975), we found the isotopic composition of Pb in theearly diagenetic fluids, which influenced limestones ofthe Kazym Member in the Satka Formation (1550 ±30 Ma) and of the Berezovaya Member in the overlying

Bakal Formation (1430 ± 30 Ma) and figured out theappropriate µ values (= 238U/204Pb). The 206Pb/204Pb and207Pb/204Pb ratios in the fluid for the Kazym limestoneswere equal respectively to 16.44 and 15.42; the sameratios for limestones of the Berezovaya member wereequal to 16.55 and 15.52. Calculated µ values of 10.09and 10.15 are higher a little than µ = 9.74 assessed byStacey and Kramers for the average terrestrial lead ofthe Earth. These results suggest that the Burzyan basinof carbonate sedimentation was under influence ofprovenance composed of the Archean or Early Protero-zoic upper crustal rocks. Model values of Pb isotopicratios in early diagenetic fluids are close to those char-acteristic of rocks of the Taratash Metamorphic Com-plex (unpublished data of G.V. Ovchinnikova andA.M. Larin) during deposition of the Satka and Bakallimestones 1550–1430 Ma ago (Fig. 5). Hence, in termsof the U–Pb systematics, carbonate sediments of theSatka and Bakal formations were at the stage of earlydiagenesis in the isotopic equilibrium with materialeroded from the Taratash Complex. Being close to eachother, µ values of diagenetic fluids, which interactedwith carbonate sediments of both formations, implythat the Taratash Complex represented local prove-nance for the Burzyan paleobasin during the Satka andBakal time spans. The REE distribution and εNd(í)values in all siliciclastic formations of the BurzyanGroup also show that provenance of sedimentary mate-rial remained unchanged in the Early Riphean (Maslovet al., 2004).

CONCLUSIONS

A series of least altered limestone samples suitablefor assessment of the Sr isotopic composition in seawa-ter has been distinguished after geochemical examina-tion of carbonate rocks of the Satka and Suran forma-tions. These are carbonate samples with Mn/Sr < 0.2,Fe/Sr < 5.0, and Mg/Ca < 0.024 from the upper KazymMember of the Satka Formation of the Taratash anticli-norium. Preliminary leaching of samples with 1 Nammonium acetate (NH4OAc) has been used to getsample fractions enriched (in terms of the Rb–Sr sys-tematics) in primary carbonate material. Carbonatefractions the least altered in terms of the U–Pb system-atics have been obtained using the optimal, speciallytested procedure of step-wise dissolution of the leastaltered samples in HBr.

The Pb–Pb age of limestones from the Satka Forma-tion (1550 ± 30 Ma) clarifying geochronology of theBurzyan Group middle part is consistent with the otherisotopic-geochronological dates known for the LowerRiphean stratotype. At present, the isotopic datesexactly are most efficient for constraining the age limitsof the Early Riphean Erathem, the basal one in chronos-tratigraphic scale of Upper Precambrian authorized inRussia.

The 87Sr/86Sr ratio in carbonate sediments of theSatka paleobasin ranged from 0.70460 to 0.70480.



Generalization of Sr-isotopic data known for the Riph-ean deposits suggest influx of the predominantly juve-nile material into the World Ocean and interconnectedseas 1650–1350 Ma ago. Low and insignificantly vari-able 87Sr/86Sr ratios (0.70456–0.70494) characteristicof the Early Riphean seawater differed just a little fromthose of the late Early Proterozoic oceans. The 87Sr/86Srratio increase in seawater commenced ca. 1250–1200 Ma ago in connection with the Elzevirian stage ofthe Grenvillian orogeny (Semikhatov et al., 2002).

ACKNOWLEDGMENTS

We are grateful to Yu.A. Shukolyukov for criticalcomments used to improve the manuscript and toI.V. Kislova, G.V. Konstantinova, E.P. Kutyavin,N.N. Mel’nikov, and T.L. Turchenko for their assis-tance in analytical work. The work was supported bythe Priority program no. 18 of the Presidium RAS, theProgram of fundamental investigations no. 8 of theEarthscience Division RAS and by the Russian Foun-dation for Basic Research, projects 05-05-65290, 05-05-65329, 06-05-64592, and 07-05-01107.

Reviewers Yu.A. Shukolyukov and E.V. Bibikova

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The Sr Isotopic Characterization and Pb–Pb Age of Carbonate Rocks from the Satka Formation, the...

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