Geochemistry of the Mesoproterozoic Lakhanda shales in southeastern Yakutia, Russia: implications for mineralogical and provenance control, and recycling

Precambrian Research 104 (2000) 77–93

Geochemistry of the Mesoproterozoic Lakhanda shales insoutheastern Yakutia, Russia: implications for mineralogical

and provenance control, and recycling

Robert L. Cullers a,*, Victor N. Podkovyrov b

a Department of Geology, Kansas State Uni6ersity, 108 Thompson Hall, Manhattan, KS 66506-3201, USAb Institute of Precambrian Geology and Geochronology RAS, St Petersburg, 199034, Russia

Received 18 May 1999; accepted 24 May 2000

Abstract

Shales of the Lakhanda Group of Late Mezoproterozoic age (1050–1000 Ma) from the southeastern Siberiancraton in Russia have been analyzed for major elements and a number of trace elements, including the REE’s. Shalesalong the Maya River formed as platform sediments in a deeper shelf facies, whereas, shales along the Belaya Riverformed in more active and open environments of an upper shelf carbonate ramp. The log of most elementalcompositions to Al2O3 ratios are the same in the Maya and Belaya River samples, suggesting a similar source rockcomposition for rocks in the two areas. The log of SiO2, MgO, Na2O, K2O, Rb, Ba, and Ni to Al2O3 ratios aresignificantly higher and the log of TiO2 to Al2O3 ratios are significantly lower in shales from the Belaya River thanthe Maya River sections. The CIA (chemical index of alteration) is thus significantly lower in the Belaya shales thanthe Maya shales, suggesting less weathering of the in the Belaya shales than the Maya shales. The ICVs (Index ofCompositional Variability=Fe2O3+K2O+Na2O+CaO+MgO+TiO2/Al2O3) of the Lakhanda shales are lessthan 1, suggesting that they are compositionally mature and were likely dominated by recycling. Several samples haveICV\1, suggesting some first cycle input. The low K2O/Al2O3 ratios of these shales suggest that minimal first cyclealkali feldspar was present in the initial source. Most shales of the Lakhanda plot parallel and along the A–K linein A–CN–K plots suggestive of intense chemical weathering (high CIA) and do not indicate any clear-cut evidenceof K-metasomatism or direct weathering back to the original source. If K-metasomatism produced these rocks, thenthey could have formed from tonalites to basalts. If weathering produced these rocks then they could have beenproduced from varied amounts of mostly granodiorite to granite. Elemental ratios critical of provenance (La/Sc,La/Cr, La/Co, Th/Sc, Th/Cr, Th/Co, and Eu/Eu*) are not significantly different between the Maya River and BelayaRiver shales, and the ratios are similar to fine-fractions derived from the weathering of mostly granitoids and notbasic rocks. The Eu/Eu*, Th/Sc and low K2O/Al2O3 ratios of most shales suggest weathering from mostly agranodiorite source rather than a granite source, consistent with a source from old upper continental crust. Somesamples at the bottom of the Belaya River section contain very low Eu/Eu* (0.35), suggesting significant input of first

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* Corresponding author. Fax: +1-785-532-5159.E-mail address: [email protected] (R.L. Cullers).

0301-9268/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.

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R.L. Cullers, V.N. Podko6yro6 / Precambrian Research 104 (2000) 77–9378

cycle detritus from highly differentiated granitoids similar to those from the Aldan Shield. © 2000 Elsevier ScienceB.V. All rights reserved.

Keywords: Proterozoic; Shales; Trace elements; Rare earth elements; Provenance

1. Introduction

The mineralogical and chemical composition offine-grained sedimentary rocks are commonlyused as a sensitive indicator of provenance andweathering conditions and only in a few cases as atool of tectonic setting (Ronov and Migdisov,1971; Cullers et al., 1975, 1979; Taylor andMcLennan, 1985, 1991; Bhatia and Crook, 1986;Roser and Korsch, 1986, 1988; Ronov et al.,1990; Cullers, 1994b; Cox and Lowe, 1995; Cox etal., 1995; Nesbitt et al., 1996; Cullers and Berend-sen, 1998; Cullers, 2000). On a global scale, mu-drock chemistry reflects the average compositionof continental crust (Taylor and McLennan,1985). Most mudrocks, however, form in re-stricted basin environments in specific tectonicsettings that reflect the composition of the sourcerocks (Cox and Lowe, 1995). Elements concen-trated in basic rocks (e.g. Sc, Cr, Co) and ele-ments concentrated in silicic rocks (La, Th, REE),REE patterns, and Eu-anomaly size have beenused for provenance and tectonic determinationsof mudrocks (Cullers, 1994b; Mongelli et al.,1996). Of course these signatures of the sourcerock may be modified by weathering, hydraulicsorting and diagenesis (Cullers et al., 1987;Condie et al., 1995; Nesbitt et al., 1996).

Although few studies deal with effects of basi-nal tectonic settings controlling the chemical com-position of mudrocks, it is generally assumed thatin more stable and evolved intracratonic settingsmudrocks are more homogenized and representthe average composition of continental crust inthe region (Ronov et al., 1974; Bhatia, 1985;Taylor and McLennan, 1985; Cox and Lowe,1995). Recent studies of the influence of grain-sizeand transportation distance in a given tectonicenvironment on the chemical composition of sedi-ments show that some major element and traceelement concentrations and ratios, including REEpatterns and negative Eu-anomaly size, are similar

to the source rock in mudrocks compared withthe more variable chemical composition of sand-stones in the same sedimentary sequences (Cullerset al., 1975; Cullers, 1988, 1994a,b; Mongelli etal., 1996). Thus, mudrock compositions providemore information not only about weathering con-ditions and sediment recycling, but also regionaltectonic settings compared with the more variablecomposition of associated sandstones (Bhatia,1983; Bhatia and Crook, 1986; Roser and Korsch,1988; Sochava et al., 1994; Cullers, 1995).

The specific aims of this paper are as follows:(1) to examine secular variations in mudrock com-position of a single sedimentary unit (theLakhanda Group, 1.05–1.01 Ga) on the edge of amature craton and (2) to examine the effect of theinput of the composition of the sediment as aresult of the changing tectonic evolution.

2. Geology

2.1. Possible source rocks

The study area is located in the Uchur–MayaRegion on the southeastern edge of the Siberianplatform (Figs. 1 and 2) in which platform sedi-mentation occurred during the Riphean–Vendian(Meso- to Neoproterozoic, 1.60–0.54 Ga)(Semikhatov and Serebrykov, 1983; Semikhatov,1991). The underlying basement complex in theAldan Shield include strongly metamorphosedand deformed Archean to Early Proterozoic mag-matic and supracrustal volcaniclastic units of theUchur block to the west and the Batomga blockto the southeast. Supracrustal shales andmetagraywackes predominate in the Uchur block(Neelov et al., 1971; Dook et al., 1986). TheUchur block probably formed during accretion ofseveral arc-related terrains onto the eastern edgeof the Archean Siberian craton (Glebovitsky andDrugova, 1993; Kotov et al., 1995; Kovach et al.,1996).

R.L. Cullers, V.N. Podko6yro6 / Precambrian Research 104 (2000) 77–93 79

The Batomga granite–greenstone block consistsof granulite gneisses associated with a varietymetamorphosed ultrabasic to silicic igneous rocks,and younger bimodal metavolcanics, shales, calc-silicate rocks with gabbro and granite intrusions(Larin, 1997). The latest pre-Riphean magmaticevent in the Batomga block is the Ulkan volcano-plutonic anorogenic (A-type) rapakivi complex(1721–1703 Ma) (Larin, 1997).

The proportion of rock types in the easternAldan Shield varies across structural domains, butgranitic rocks (low- and moderate-K, with smoothREE-spectra with a small, negative Eu-anomaly)predominate with smaller amounts of mafic andmetasedimentary rocks. The Proterozoic sedi-ments include both immature, arc-related wackes(lower REE content than the granites with smallor no Eu anomalies) and more evolved cratonic-type mudrocks (moderately high REE contentand a negative Eu-anomaly close to the average ofmost Phanerozoic shales (Kovach et al., 1999).Some rare trachyrhyolites and subalkaline gran-ites contain high REE content, marked LREEenrichment (LaN=327–539, [La/Yb]N=10.1–11.7) and a large negative Eu-anomaly (Eu/Eu*=0.11–0.15) (Larin, 1997).

2.2. Sedimentation history

The Riphean (Mezo- to Neoproterozoic) andVendian (1.65–0.54 Ga) sediments of the Uchur–Maya region unconformably cover the Archean–Proterozoic basement of the Aldan Shield. Thesesediments form the two main tectonic provinces.One provenance is the mature continental blockof the Uchur–Maya plate, and the other prove-nance is the marginal trough of the Yudoma–Maya region (Saleeby, 1981; Semikhatov andSerebrykov, 1983; Semikhatov, 1991; Khudoley etal., 2000).

The Riphean of the Uchur–Maya basin con-tains sediment sampled in this study; it is a trans-gressive sedimentary series. The total thickness ofthese strata in the Uchur–Maya basin is 4–4.5km, and it increases in thickness to 11–12 km tothe east in the Yudoma–Maya trough(Semikhatov and Serebrykov, 1983; Semikhatov,1991; Khudoley et al., 2000).

Fig. 1. Index map of the area in which samples were collectedalong the southeastern edge of the Siberian platform (heavyoutlined area).

Fig. 2. Location of the sections from which samples werecollected along the Maya River (platform sediment; sections77, 78, and Lh) and the Belaya River (near source; sections 51and 52).


Fig. 3. Stratigraphic section along the Maya River with themain lithologies identified.

record for the Lakhanda carbonate sequences dis-play a marked shift to heavy d13Ccarb values (from+4 to +6 in PDB scale) (Podkovyrov and Vino-graadov, 1996) that is similar to that of othersimilar Mesoproterozoic successions of that age.They also contain low 87Sr/86Sr ratios (0.70519–0.70566) that probably reflect platform marinesedimentation with an input of ocean water en-riched in a juvenile Sr isotopic component(Semikhatov et al., 1998).

2.3. Lakhanda Group

The Lakhanda Group was sampled in thisstudy. The Lakhanda Group carbonates are in-terbedded with multi-colored shales and minorsiltstones–sandstones that were deposited in epi-cratonic shallow marine and upper shelf environ-ments. They form a gently north- andnortheast-dipping rock sequence (400–550 mthick) in central to eastern parts of the Mayabasin, and they thicken (Fig. 3) eastwardly (1000–1200 m) in thrust-faulted and folded successionsin the Yudoma–Maya trough (Sklyrov, 1981;Semikhatov and Serebrykov, 1983; Khudoley etal., 2000). In the Maya sections, an erosionalunconformity with remnants of an ancient iron-rich, kaolinite weathered crust precedes the Mayaplatform sequence. Samples of this sequence weresampled along the Maya River (Figs. 2 and 3).Here the sequence is subdivided into two forma-tions: the Neruen Formation in the lower portionand Ignikan Formation in the upper portion(Semikhatov and Serebrykov, 1983). In theNeruen Formation (60–65% terrigenous rocks,30–35% carbonaceous), fine-grained terrigenoussediments are abundant in the lowermost unit(45–50 m) and upper unit (110–125 m). Thesetwo units are separated by interbedded multi-col-ored stromatolitic and clastic limestones anddolomitic limestones (70–80 m).

The lower unit of the Neruen Formation (Fig.3, section 77) includes iron-rich, kaolinite-bearingcalcareous shales with rare siltstones, dolomiticshales and siderite beds or concretions at the base.The overlying sequence within the lower unit (Fig.3, section 77) consists of varigated shales withminor stromatolitic dolomites, quartz sandstones,

The upper sequences of the Riphean include theLakhanda and the Ui Groups sediments that havetraditionally been attributed to Upper Riphean(960–750 Ma) (Semikhatov and Serebrykov,1983). However, more recent U�Pb ages of maficsills cutting the Lakhanda and Ui sediments giveages of 1010–965 Ma so that their age must beolder than 1.0 Ga (Rainberd et al., 1998). Thus,these units must really be of Mezoproterozoic(Middle Riphean) age. The observed isotopic


siltstones and siderite concretions. These beds ofthe lower part of the Neruen Formation weredeposited in a transgressive, high energy marinesequence, most likely tidal flat to distal sublitoral,near the provenance (Semikhatov and Sere-brykov, 1983). These beds were enriched in heavyminerals, especially zircon.

The upper terrigenous unit of the Neruen For-mation in the central Maya sections (Fig. 3, sec-tion 78) contains varigated shales with minorlaminae of siltstones, rare sandstones, sideritelenses, and stromatolitic limestones (Semikhatovand Serebrykov, 1983). The mudstones are inter-preted to have formed in low-energy, partlyanoxic, outer shoal environments (Semikhatovand Serebrykov, 1983). The source rock of claymaterial was probably, as with the lower unit, theadjacent Aldan Shield.

The Belaya River section is the one that wasstudied in the Uchur–Maya trough. It is locatedin the north-east part of the region, along theBelaya River on the eastern flank of the Gornos-takh Anticline (Figs. 1 and 4). The section (sec-tion 52) is composed of mostly platformcarbonate rocks (up to 82% in the Neruen and95–97% in the Ignikan Formations), and minorshales and siltstones, and sandstones (Semikhatovand Serebrykov, 1983; Podkovyrov and Vino-graadov, 1996; Semikhatov et al., 1998). The se-quence is divided into seven transgressive–regressive carbonate–shale units. The limestonesand dolomites were formed in a marine shoal insubtidal and tidal environments. Black thinly lam-inated shales in lower units represent predomi-nantly low-energy subtidal, distal ramp and openmarine environments, whereas, multi-coloredshales with thin horizontal and low-angle cross-bedding, small-scale flazer lamination and smallsymmetrical ripple marks in upper units weredeposited in low-energy tidal, lagoonal and, prob-ably, supratidal settings.

3. Sampling and methods

Representative samples (200–400 g) were ob-tained from the Maya River and Belaya River atreference sections (77,78, 51,52; Figs. 3 and 4).

The samples were collected at 5–15 meter inter-vals in thin-bedded and more heterogeneous se-quences and at 30–35 meter intervals inthick-bedded and more homogenous sequences.Each sample was cut in half. One half was usedfor the preparation of thin sections or epoxy-based polished sections for microprobe analysis,and the other half was used for other chemicalanalyses. A total of twenty samples were ana-lyzed. The major elements were analyzed by X-rayfluorescence and partly by standard wet-chemicalmethods in the Central Chemical Laboratory,NW Geological Centre, St Petersburg, Russiaalong with USGS standard rocks. The total Fecontent is reported as FeO. The precision of SiO2

and Al2O3 are better than 95%, and that of FeO(total), TiO2, MgO, CaO, Na2O are better than98–10%; P2O5 and MnO are better than 12–15%. The concentrations of V, Cr, Co, Ni, Rb, Sr,Zr, Ba and Pb were analyzed by X-ray fluores-cence in IGGD RAS, St Petersburg using USGSstandards. The precision of most trace elementsare better than 8%, but that of Co, Ba, Rb and Pbare 10–12%. The elements Fe, Na, La, Ce, Nd,Sm, Eu, Tb, Yb, Lu, Co, Sc, Hf, Th, Ba and Rbwere analyzed by neutron activation at KansasUniversity (after Gordon et al., 1968; Jacobs etal., 1977). The precision of all trace elements butYb and Lu are better than 5%, and the precisionof the Yb and Lu are better than 7%. The valuesof standard rocks are periodically analyzed andcompared with average values (e.g. Cullers et al.,1985, 1987).

4. Results

4.1. Mineralogy

The B1 mm fractions were separated fromseven samples (51–38, 52–9, 52–117, 77–3, 77–6,77–12 and Lh-13) and analyzed by X-ray diffrac-tion (T.L. Turchenko, analyst) in order to corre-late the mineralogy and chemistry. The samplescontain mostly illite–muscovite, and remnants ofmixed layer illite–smectite, chlorite, kaolinite, py-rophyllite, quartz, calcite, and feldspars. In theBelaya River section of the Yudoma–Maya


trough, sample 52–117 consists mainly of 2M1

muscovite-type minerals (95–97%) with minorchlorite, quartz and pyrophyllite, and they reflect

the high degree of diagenetic alteration among theLakhanda shales. Samples 52–9 and 51–38 con-tain progressively decreasing amounts of minerals

Fig. 4. Stratigraphic section along the Belaya River with the main lithologies identified.


Tab

le1

Mud

ston

esof

the

Bel

aya

Riv

eran

dM

aya

Riv

erre

gion

s

Mud

ston

esfr

omth

eB

elay

aR

iver

Are

a—

near

sour

ce 52–1

152

–31

52–3

452

–44

52–4

652

–59

52–8

052

–85

52–1

03E

lem

ent

52–1

51–3

852

–9

62.1

65.2

SiO

257

.256

.061

.557

.360

.262

.859

.649

.354

.157

.021

.321

.318

.720

.423

.418

.522

.322

.320

.3A

l 2O

322

.623

.418

.91.

271.

291.

191.

31.

111.

271.

451.

281.

181.

141.

181.

25T

iO2

10.5

9.64

6.59

3.86

14.0

3.66

4.94

5.73

2.21

9.27

Tot

alF

eO7.

6714

.40.

010.

010.

010.

010.

010.

010.

010.

010.

010.

01M

nO0.

010.

071.

311.

950.

560.

641.

722.

21.

682.

051.

481.

631.

91.

59M

gO0.

130.

750.

380.

220.

310.

540.

41.

110.

280.

60.

20.

13C

aO0.

280.

370.

281.

130.

40.

180.

380.

87N

a 2O

0.44

0.38

1.02

1.11

4.10

3.23

2.17

2.61

2.21

5.54

4.29

6.21

4.28

5.22

K2O

4.99

3.54

0.12

0.08

0.02

0.10

0.08

0.07

0.02

0.09

0.12

P2O

50.

020.

120.

114.

263.

464.

923.

873.

963.

155.

594.

224.

073.

795.

045.

03L

OI

99.5

599

.48

99.6

299

.46

99.4

899

.599

.54

99.5

699

.57

99.5

99.4

99.8

Sum

155

162

9718

413

420

422

024

9R

b16

514

622

316

657

7056

1874

16Sr

165

3425

6791

7835

045

024

255

323

344

342

824

935

237

9B

a50

847

225

420

026

930

024

928

337

026

529

519

518

222

3Z

r13

760

115

100

9313

210

186

119

6019

214

4V

16.2

6.5

38.1

11.8

15.7

4.7

20.7

8.2

Co

16.5

933

14.2

2515

Ni

3620

2738

1731

2129

3730

28.2

31.0

16.8

16.8

20.5

16.6

28.5

27.4

25.8

23.6

Th

17.3

21.9

9.5

7.4

9.8

15.1

6.8

6.7

8.5

6.5

8.4

8.8

7.5

8.0

Hf

2.5

2.9

1.6

1.9

2.1

1.8

Ta

1.9

22.

12.

32.

22.

612

714

083

7210

095

153

148

64C

r12

414

091

27.5

16.8

18.3

26.2

16.8

14.7

21.8

18.6

31.7

24.7

32.7

27.3

Sc66

.456

.960

.273

.442

.246

.758

.632

.579

.261

.184

.673

.4L

a11

815

110

582

.812

365

.712

517

015

613

5C

e11

812

053

.840

.648

.4–

34.8

29.4

5024

.157

.654

64.6

54.6

Nd

10.1

6.61

10.2

18.7

6.59

4.57

11.0

3.9

8.73

11.0

13.0

11.5

Sm1.

972.

951.

370.

751.

730.

591.

820.

86E

u2.

032.

271.

981.

041.

410.

961.

722.

60.

920.

611.

420.

530.

991.

411.

561.

33T

b6.

668.

324.

063.

474.

792.

576.

735.

474.

62Y

b6.

006.

395.

980.

950.

680.

91.

210.

590.

520.

690.

390.

810.

881.

000.

88L

u0.

588

0.49

10.

587

0.51

80.

659

0.53

0.52

0.50

0.35

0.6

0.59

0.61

Eu/

Eu*

3.29

2.80

2.51

3.18

2.69

1.75

2.41

2.50

La/

Sc2.

692.

592.

473.

393.

7211

.29

1.11

3.96

3.73

6.91

La/

Co

9.66

4.01

1.85

9.40

4.45

3.21

0.47

0.52

0.51

0.65

0.59

0.34

0.43

0.54

0.60

0.59

La/

Cr

0.89

0.67

2.21

2.85

2.41

4.89

1.17

1.73

1.54

1.91

2.55

2.91

2.92

1.98

La/

Ni

1.04

1.03

1.54

1.18

1.00

1.14

0.94

0.89

0.86

0.89

0.79

0.86

Th/

Sc1.

744.

770.

441.

421.

313.

531.

383.

34T

h/C

o1.

432.

870.

661.

22


Tab

le1

(Con

tinu

ed)

Mud

ston

esfr

omth

eB

elay

aR

iver

Are

a—

near

sour

ce 52–1

152

–31

52–3

452

–44

52–4

652

–59

52–8

052

–85

52–1

0352

–1E

lem

ent

52–9

51–3

8

0.19

0.19

0.22

0.22

0.20

0.23

0.21

0.17

0.19

Th/

Cr

0.24

0.27

0.18

1.13

2.07

0.47

0.62

0.54

0.98

0.95

0.88

Th/

Ni

0.64

0.89

1.04

0.87

0.95

0.95

0.95

0.88

0.94

CIW

0.89

0.86

0.92

0.87

0.96

0.96

0.96

0.86

0.85

0.85

0.70

0.80

0.67

0.84

0.77

0.83

0.81

CIA

0.70

0.70

0.72

1.08

0.52

0.46

1.02

0.91

0.67

1.07

0.60

1.02

0.91

0.81

ICV

0.11

K2O

/Al 2

O3

0.13

0.28

0.13

0.29

0.20

0.36

0.21

0.29

0.16

0.20

0.16

Mud

ston

esfr

omth

eM

aya

Riv

erar

ea—

plat

form

mud

ston

es

Lh-

877

–12

77–1

077

–852

–119

77–7

78–3

77–6

77–3

Ele

men

t78

–278

–1L

h-13

56.8

60.8

57.0

44.3

41.2

54.9

55.1

57.9

63.7

SiO

252

.643

.758

.727

.320

.923

.422

.325

.622

.220

.626

.326

.024

.419

.123

.7A

l 2O

3

1.48

1.38

1.40

1.46

1.74

1.50

1.34

1.89

1.50

1.81

1.33

1.62

TiO

2

4.55

2.21

2.23

19.3

23.2

2.36

2.58

2.79

Tot

alF

eO9.

6720

.12.

772.

080.

040.

010.

020.

020.

040.

02M

nO0.

020.

010.

010.

180.

030.

010.

990.

920.

470.

420.

410.

550.

720.

891.

150.

72M

gO1.

750.

590.

210.

190.

210.

280.

410.

410.

280.

280.

140.

340.

380.

3C

aO0.

140.

280.

110.

10.

080.

080.

090.

550.

080.

230.

410.

17N

a 2O

4.75

4.67

2.21

2.01

2.00

2.69

2.98

4.08

K2O

2.92

3.00

3.10

5.54

P2O

50.

090.

100.

070.

040.

020.

040.

030.

060.

050.

020.

060.

077.

137.

199.

3610

.310

.410

.78.

915.

7010

.27.

72L

OI

3.66

7.54

99.5

99.5

999

.510

099

.210

199

.610

099

.299

.54

99.6

99.5

Sum

151

172

193

172

110

8510

511

916

314

112

813

6R

b16

712

391

4745

8311

014

4Sr

5242

5364

245

324

225

161

144

207

Ba

370

492

439

324

352

329

300

306

388

260

236

409

300

308

200

200

Zr

310

300

100

9580

143

202

162

142

217

182

150

100

100

V8.

89.

410

.913

.45.

221

.125

.93.

68.

118

.432

.925

.0C

o20

3229

174

2415

40N

i15

1015

4219

.921

.624

.422

.321

.916

.221

.421

.120

.425

.817

.024

.1T

h7.

87.

59.

96.

46.

810

.17.

58.

58.

8H

f10

.17.

212

.42.

11.

61.

62.

22.

32.

11.

82.

52.

12.

21.

71.

9T

a11

485

121

106

115

8795

121

103

103

7696

Cr

25.4

22.6

22.9

22.2

21.7

21.7

22.2

26.1

Sc26

.421

.825

.422

.949

.452

.667

.862

.162

.442

.646

.157

.553

.663

.545

.267

.6L

a16

112

112

381

.989

.111

496

.410

813

8C

e11

412

792

.564

.749

.750

.731

.935

.342

.9N

d45

.640

.845

.638

.747

.338

.012

.89.

198.

715.

786.

487.

397.

688.

79Sm

8.39

7.39

8.00

6.48


Tab

le1

(Con

tinu

ed)

Mud

ston

esfr

omth

eM

aya

Riv

erar

ea—

plat

form

mud

ston

es

78–1

Lh-

13L

h-8

77–1

277

–10

77–8

77–7

77–6

77–3

78–3

Ele

men

t78

–252

–119

1.66

1.14

2.48

1.55

1.50

1.06

1.22

1.35

1.47

Eu

1.35

0.65

1.79

1.64

1.21

1.28

0.92

1.07

1.19

0.83

1.14

Tb

1.53

1.61

1.49

0.70

5.89

5.34

5.90

Yb

3.88

3.85

4.47

5.56

4.71

7.5

7.18

7.17

4.09

0.87

0.81

0.91

0.6

0.68

0.84

0.65

0.71

1.14

1.08

Lu

0.59

1.12

0.52

0.35

60.

660.

560.

550.

560.

580.

560.

560.

511

0.67

0.60

Eu/

Eu*

2.23

2.30

2.67

2.75

2.72

1.92

2.12

2.65

2.05

2.50

2.07

2.56

La/

Sc6.

224.

6312

.02.

021.

7816

.05.

616.

62L

a/C

o2.

71.

373.

451

5.60

0.56

0.59

0.54

0.49

La/

Cr

0.49

0.62

0.48

0.52

0.61

70.

590.

70.

433.

391.

942.

152.

5111

.52.

43.

291.

341.

25L

a/N

i4.

514.

524.

233

0.90

0.94

0.96

0.99

0.96

0.73

0.99

0.97

0.78

1.01

60.

780.

91T

h/Sc

2.26

2.30

2.24

1.66

4.21

0.77

0.83

5.86

2.52

1.40

0.52

0.96

Th/

Co

0.20

0.21

0.19

0.19

0.23

0.17

0.17

0.20

Th/

Cr

0.25

0.22

0.25

0.25

1.22

0.70

0.76

0.95

5.35

0.88

0.51

Th/

Ni

0.51

1.72

1.7

1.61

1.33

0.98

0.97

0.97

0.96

0.97

0.95

0.98

0.99

0.93

0.97

CIW

0.96

0.96

10.

880.

750.

800.

800.

890.

880.

880.

860.

840.

849

0.81

0.86

CIA

ICV

0.57

0.69

0.51

0.32

0.88

1.06

0.37

0.42

0.41

1.24

0.62

0.34

0.22

0.23

0.09

0.10

0.11

0.11

0.17

0.12

K2O

/Al 2

O3

0.29

0.14

0.13

0.17


Table 2A comparison of the elemental concentrations of the Belayaand Maya Formations

Belaya element orElement Maya element orratioratio

53.196.5SiO2 58.994.423.792.421.191.7Al2O3

TiO2 1.25390.095 1.5490.198.2998.07.2794.09Total FeO

0.03590.047MnO 0.01590.0170.7690.281.5790.49MgO

0.4090.28CaO 0.2890.090.1890.150.5590.35Na2O

4.191.3K2O 3.291.0P2O5 0.08190.038 0.05390.023

8.5491.654.2390.71LOI140933Rb 1759428994363939Sr

3969106Ba 284989288965261953Zr

110936V 14294415.299.3Co 15.791021.3910.828.398.5Ni21.192.9Th 22.895.18.491.88.692.2Hf

2.190.4Ta 2.090.3102914109931Cr

23.195.8Sc 23.491.8La 55.399.360.6915

113923122928Ce4599Nd 46912

8.391.89.494.0Sm1.5190.37Eu 1.5490.731.2590.271.2490.56Tb5.591.3Yb 5.391.6

0.8490.190.7890.22Lu0.53190.094Eu/Eu* 0.57590.048

2.3690.312.6690.44La/Sc5.6094.36La/Co 5.3093.10.5490.070.5790.13La/Cr

2.391.0La/Ni 3.692.7.9090.101.090.2Th/Sc

2.091.3Th/Co 2.191.60.2190.03Th/Cr 0.2190.03

1.4491.310.8990.41Th/Ni

contain progressively decreasing amounts of min-erals as follows: illite (with 5–10% of expandedI–S phase, I=0.45–1.5)\kaolinite\chlorite\quartz\ feldspar9pyrophyllite, glauconite andhematite.

4.2. Geochemistry

The elemental concentrations and averages ofthe Lakhanda shales are given in Tables 1 and 2,respectively, and the average elemental concentra-tions are compared with the average of post-Archean shales (Taylor and McLennan, 1985) inFig. 5. The average of the Lakhanda shales aresignificantly higher in Al2O3, TiO2, Zr, Th, Hf, Sc,and the REE concentrations and lower in SiO2,MgO, CaO, Na2O, P2O5, Sr, Ba, and Ni concen-trations than corresponding elemental concentra-tions in the PAAS. The Lakhanda shales and thePAAS have similar concentrations of FeO (total),MnO, K2O, LOI, Rb, V, Co, Ta, and Cr.

The enrichment of immobile elements likeAl2O3 and depletion in mobile elements like MgO,CaO, Na2O, and Sr results in fairly high chemical

Fig. 5. Selected elemental concentrations of the average of allsamples are compared with those of the PAAS (post Archeanaverage shale values from Taylor and McLennan, 1985). Ex-cept for the REE the ratios are plotted in increasing concen-tration relative to the PAAS. Only selected REE are plotted inorder of decreasing atomic number (some of the REE like Ceand Lu plot similarly to adjacent REE). Error bars are esti-mates based on one standard deviation of the Lakhandavalues for each element as no standard deviation is reportedfor the PAAS.

as follows: dioctahedral illite–muscovite with alow index of crystallinity (I=0.20–0.36)\\Mg–Fe chlorite\quartz: feldspars. The sam-ples from the Maya River section have a lesserdegree of diagenetic transformation of primaryclay minerals than do those from the BelayaRiver. Samples 77–3, 77–6, 77–12 and Lh-13


Fig. 6. The SiO2 versus Al2O3 concentrations of shales fromthe Belaya River (open triangle) and Maya River (closedtriangle) are plotted relative to the idealized composition ofthe observed minerals. Much of the variation in compositionmay be accounted for by variation in quartz and clay miner-als-muscovite. The three samples with high FeO total and lowSiO2 are skewed in the direction of hematite, opaque mineralsand Fe-rich chlorite compositions. The FeO total vs. Al2O3

concentrations of shales from the Belaya and Maya Rivers areplotted relative to the composition of the observed minerals(same symbols as in Fig. 6a). Again much of the variation incomposition may be accounted for by variation in quartz andclay minerals-muscovite. The Fe-rich samples are againskewed toward the hematite, magnetite, and Fe-rich chloritecompositions.

primary and recycled clay material (Cabanis andLecolle, 1989; Cullers, 1994b, 1995). Ancientweathering profiles are observed in some parts ofthe Lakhanda sequence. These profiles containmostly kaolinite with lesser hematite and chlorite(Semikhatov and Serebrykov, 1983). The B2 mmfractions of samples in this study, however, weremainly illite–muscovite with smaller amounts ofkaolinite and chlorite. Elemental plots are alsoconsistent with these phases, but the whole rocksamples have variations in major element compo-sitions that suggests a significant amount ofquartz and iron oxides may be present (e.g. Fig.6).

Also the high concentrations of Zr, Th, REE,Hf, and Th relative to the PAAS in the Lakhandashales could be due to the concentration of certainaccessory minerals (zircon, monazite, ilmenite, ru-tile). For example, the observed correlations be-tween Zr, Hf, and TiO2 tends to support thispossibility. The correlation of Sc and Cr withAl2O3 and lesser correlation of Th and the REEwith Al2O3 suggests these elements may also beincluded in the clay minerals of the Lakhanda.

There are poor or no trends in the elementalcompositions with time. The best of these correla-tions is a slow increase in Na2O in both the MayaRiver and Belaya River samples with time.

5. Discussion

5.1. Comparison of the composition of samplesfrom the Belaya and Maya Ri6ers

The composition between the samples from theBelaya and Maya Rivers are compared by usingthe Student t-test of the log10 of the elementalratios relative to Al2O3. Statistically comparingthe log10 of elemental ratios avoids the constantsum problem that insures that there must be somecorrelations of elements since they must add to100 percent. Comparing the log10 of the elementalratios converts the constant sum data with contin-uous variables that range to infinity so the datamay be compared using parametric tests such asthe Student t-test (Cardenas et al., 1996).

index of weathering values (CIA=0.67–0.89;Table 1), especially shales from the Maya Riverarea. These chemical characteristics are consistentwith formation in stable platform environmentswith intense chemical weathering of mostly silicicsource rocks (Ronov and Migdisov, 1971;Sochava et al., 1994) with possible mixing of


The log of the ratios of TiO2 to Al2O3 issignificantly higher and those of SiO2, MgO,Na2O, K2O, Rb, Ba, and Ni to Al2O3 ratios arelower in the Maya River than the Belaya Riversections (Fig. 7). Other elemental ratios betweenthe two areas are statistically the same thus sug-gesting similar source rock compositions for theBelaya and Maya River areas. The differences inthe composition of samples along the Belaya andMaya Rivers that may reflect the degree of weath-ering, proximity to the source, or sedimentarysorting processes. For instance, the CIA (chemicalindex of alteration) is thus significantly lower inthe Belaya shales than the Maya shales, suggest-ing much less weathering of the material in theBelaya shales than the material in the Mayashales.

5.2. Source rock composition — major elements

Also shales along the Maya River suggest thatthey were formed as platform sediments in adeeper shelf facies (epicratonic and restrictedmarine basin), whereas, those along the BelayaRiver probably formed in more open and activeenvironments of an upper shelf carbonate rampthat could have occurred closer to the source(Semikhatov and Serebrykov, 1983).

The present major element composition of mu-drocks or shales reflect changes through time,including the changes due to diagenesis and meta-morphism (Cox et al., 1995). The present chemicalcomposition can be used to suggest the originaldetrital mineralogy of the shales (Cox et al.,1995).

Fig. 7. The ratios of the log of the elemental concentration to Al2O3 ratios in the Belaya relative to the Maya shales are compared.


Fig. 8. Samples of the Lakhanda shale are plotted in anA–CN–K diagram. Samples have no tendency to project backto source rock compositions either parallel to the A–CN line(implying weathering changes; the lighter lines) or perpendicu-lar to the heavier A–K line (implying K-metasomatism; theheavier line). G, granite; Gn, granodiorite; T, tonalite.

average ICV=0.62 (range=0.32–1.24), suggest-ing that most shales were compositionally matureand were likely dominated by recycling. The fewshales with ICVs greater than 1, however, suggestthat there may be periodic input of first cyclesediment in both sample sets.

Also K2O/Al2O3 ratios may suggest how muchalkali feldspar vs. plagioclase and clay mineralsmay have been present in the original shales (Coxet al., 1995). In order from high to low values, theK2O/Al2O3 ratios of minerals are alkali feldspars(�0.4–1), illite (�0.3), other clay minerals (�nearly 0) (Cox et al., 1995). Shales with ratios ofK2O/Al2O3 greater than 0.5 suggest a significantquantity of alkali feldspar relative to other mineralsin the original shale; those with ratios of K2O/Al2O3 less than 0.4 suggests minimal alkali feldsparin the original shale (Cox et al., 1995). The Belayashales of the Lakhanda Formation have an averageratio of K2O/Al2O3=0.22 (range=0.11–0.29) andthe Maya shales have an average ratio of K2O/A2O3 of 0.15 (range=0.10–0.23), suggesting min-imal alkali feldspar relative to other minerals in theoriginal shale.

Another approach to the composition of theoriginal source rock is to plot molar ratios ofAl2O3–CaO+Na2O–K2O in A–CN–K diagramsin order to potentially separate compositionalchanges of shales and coexisting sandstones relatedto chemical weathering, transportation, diagenesis-metamorphism, and source composition (Fedo etal., 1995, 1997a,b). The CaO included in carbonateor apatite is not included in the chemical plots. TheA–CN–K diagams are useful in that the averagesource rock composition and metasomatic effectscan be inferred especially if a wide range ofcompositions of shales and sandstones are availableto be plotted. In such a system, plots of shales andsandstones due to weathering trends plot parallelto the A–CN boundary, and they extract back toa plagioclase–alkali feldspar horizontal line of thesource composition (Fig. 8) unless metasomatismaffects the rocks. The K-metasomatism of kaoliniteweathered rocks can produce illitic rocks withpoints plotted at right angles to the A–K side ofthe diagram (Fig. 8). The Lakhanda Formation,however, contains only shales and no sandstones sothat plots in the A–CN–K diagrams produces no

One approach is to use the Index of Composi-tional Variability (ICV=Fe2O3+K2O+Na2O+CaO+MgO+TiO2/Al2O3) and the ratio ofK2O/Al2O3 (Cox et al., 1995). Non-clay mineralshave a higher ratio of the major cations to Al2O3

than clay minerals so the non-clay minerals have ahigher ICV. For example, the ICV decreases in theorder of pyroxene and amphibole (�10–100),biotite (�8), alkali feldspar (�0.8–1), plagioclase(�0.6), muscovite and illite (�0.3), montmoril-lionite (�0.15–0.3), and kaolinite (�0.03–0.05)(Cox et al., 1995). Thus, immature shales with ahigh percent of non-clay silicate minerals willcontain ICV values of greater than one. Theseshales are often found in tectonically active settingsin first cycle deposits (Van de Kamp and Leake,1985). In contrast, more mature mudrocks withmostly clay minerals ought to have lower ICVvalues of less than one (Cox et al., 1995). Suchshales ought to form in cratons of quiesecentenvironments (Weaver, 1989). They have also beenfound, however, in some first cycle material thatwas intensely weathered (Barshad, 1966). TheLakhanda shales at the Belaya River contain anaverage ICV=0.81 (range=0.46–1.08) and thosefrom the Maya River platform shales contain an


Table 3The range of elemental ratios of shales in this study are compared with those of fine-fractions derived from silicic and basic sourcerocks

Belaya River Maya River PAASbRange of fine-fractions from Range of fine-fractions from sililicsililic sourcesa sourcesa— platform— near source

0.36–0.67 0.32–0.83 0.70–1.02 0.66Eu/Eu* 0.35–0.661.92–2.75 0.70–27.71.75–3.39 0.40–1.1La/Sc 2.4

0.79–1.54Th/Sc 0.73–1.02 0.64–18.1 0.05–0.4 0.911.11–11.3La/Co 1.37–12.0 1.4–22.4 – 1.65

0.52–5.68 0.30–7.50.44–4.77 –Th/Co 0.630.17–0.27Th/Cr 0.067–4.00.17–0.25 0.002–0.045 0.13

a From a summary in Cullers (2000).b From Taylor and McLennan (1985).

clear trend back to the source composition orindication of metasomatism. Most shales of theLakhanda plot parallel and along the A–K linesuggestive of intense chemical weathering (highCIA). If K-metasomatism produced these rocks,then they could have formed from tonalites tobasalts. This interpretation is not consistent withother trace element characteristics discussed in thenext section (e.g. Eu/Eu*, Th/Sc, and REE pat-terns). If weathering produced these rocks thenthey could have been produced from variedamounts of mostly granodiorite to granite. Thelow K2O/Al2O3 ratios of these shales suggests thatthe amount of granite in the source may have beenminimal unless the original K2O was removedfrom the system by other processes. This interpre-tation is more consistent with the trace elementcomposition of these rocks as discussed below.

5.3. Source rock composition — trace elements

Elemental ratios critical of provenance (La/Sc,La/Cr, La/Co, Th/Sc, Th/Cr, Th/Co, and Eu/Eu*)are not significantly different between the Mayaand Belaya River samples. Moreover, these ratiosand the size of the negative Eu-anomaly size arefairly similar to platform sediment or fine-grainedHolocene sediment that has been interpreted tohave been derived from silicic source rocks such asgranodiorite to granite rather than basic rocks(Table 3). The higher La or Th relative to Cr, Co,or Ni ratios and the more negative Eu-anomaly ofmost of the Lakhanda shales relative to sediment

averages like the PAAS, however, suggests that theLakhanda shales may on the average be derivedfrom somewhat more differentiated granitoidsthan those that make up the PAAS.

In addition, the low K2O/Al2O3 ratios, the A–CN–K plots along with the moderately negativeEu anomalies, Th/Sc ratios, and La–Th–Sc plots(Fig. 9) of the Lakhanda shales are most consis-tent with mostly a granodiorite rather than agranite source. For example, a Holocene source inthe Wet Mountains, USA, is composed of mostlygranodiorite with minor granite, tonalite, and ba-sic rocks (B15%). Fine-grained stream sediment

Fig. 9. Lakhanda shales plot in a fairly narrow region in aLa–Th–Sc plot in the granodiorite field rather than basalt orgranite.


draining this area is producing the same range ofnegative Eu-anomaly size and Th/Sc ratios as theLakhanda shales thus supporting a dominatelygranodiorite source (Cullers et al., 1987; Cullers,1994a). The Th/Sc ratios and the negative Eu-anomaly size are also in the range of values forold upper continental crust although the Eu-anomaly size is somewhat more negative than isusually observed (McLennan et al., 1993).

Samples 52–103 and 52–119 in the lower por-tion of the Belaya River samples contain lowerEu/Eu* values of 0.35 than the other samples sothey could have been derived from a source withmore granite with large negative Eu anomalies.The only presently observed differentiated gran-ites are those found in the Proterozoic Ulkancomplex of the eastern Aldan Shield (e.g. Eu/Eu*=0.11–0.15) in addition to the more com-mon less differentiated granitoids and olderrecycled mudrocks derived from them. No U�Pbdetrital zircons have been analyzed but sandstonesthroughout the Riphean have U�Pb ages of zir-cons which are almost entirely Proterozoic in age(Khudoley et al., 2000).

6. Summary

The average concentrations of the Lakhandashales are significantly higher in Al2O3, TiO2, Zr,Th, Hf, Sc, and the REE and are significantlylower in SiO2, MgO, CaO, Na2O, P2O5, Sr, Ba,and Ni than corresponding elemental concentra-tions in the PAAS; other elemental concentrationsare the same. The enrichment in the immobileelements like Al2O3 and TiO2 and depletion inmobile elements like MgO, CaO, Na2O, and Srresults in a high CIA (0.67–0.89) in the Lakandashales, suggesting fairly intense chemicalweathering.

Most of the chemical variation of the majorelements may be explained by varied mixing ofthe observed quartz, Fe-rich minerals (hematite,magnetite), and clay minerals (kaolinite, illite,chlorite).

Much of the variation in composition of sometrace elements can be related to variation in acces-sory minerals. For example, correlations of Th,

Hf, and the REE and less correlation to the majorelements could be explained by heavy mineralvariations such as zircon and columbite.

The log of the concentration of SiO2, MgO,Na2O, K2O, Rb, Ba, and Ni to Al2O3 ratios aresignificantly higher and the log of the concentra-tion of TiO2 to Al2O3 ratio is significantly lower inthe Belaya River shales than from the Maya Rivershales. Most of the elemental to Al2O3 ratios arethe same for the Belaya River and Maya Rivershales, suggesting a similar provenance for thesamples. Differences in degree of weathering,proximity to the source, or sedimentary sortingprocesses could have produced some of the differ-ences in chemical composition.

The low ICVs (most B1) of the Lakhandashales suggest that they are compositionally ma-ture and were likely dominated by recycling al-though several samples have ICVs\1, suggestingsome first cycle input. The low K2O/Al2O3 ratiosof these shales suggest that minimal first cyclealkali feldspar was present in the initial source.

Most shales of the Lakhanda plot parallel andalong the A–K line suggestive of intense chemicalweathering (high CIA) and do not indicate anyclear-cut evidence of K-metasomatism or directweathering back to the original source. If K-meta-somatism produced these rocks, then they couldhave formed from tonalites to basalts. If weather-ing produced these rocks then they could havebeen produced from varied amounts of mostlygranodiorite to granite.

Elemental ratios critical of provenance (La/Sc,La/Cr, La/Co, Th/Sc, Th/Cr, Th/Co, and Eu/Eu*) are not significantly different on the averagebetween the Belaya River and Maya River shales,and the ratios are within the range of fine sedi-ment derived from silicic source rocks rather thanbasic rocks. The Eu/Eu*, Th/Sc, La–Th–Sc plotsand low K2O/Al2O3 ratios of these shales suggestweathering from mostly granodiorite sourcerather than a granite source. The Th/Sc ratios andthe negative Eu-anomaly size are also in the rangeof values for old upper continental crust althoughthe Eu-anomaly size is somewhat more negativethan is usually observed. A few of the Lakhandashales at the bottom of the Belaya River sectionhave the largest negative Eu anomalies (Eu/


Eu*=0.35), suggesting significant input from theweathering of more highly differentiated grani-toids similar to those in the Aldan Shield.

Acknowledgements

We thank the crew of the Kansas State Univer-sity for irradiating our samples and the Depart-ment of Mechanical-Nuclear Engineering for theuse of their counting equipment for neutron acti-vation analyses.

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Geochemistry of the Mesoproterozoic Lakhanda shales in southeastern Yakutia, Russia: implications for mineralogical and provenance control, and recycling