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Precambrian Research 104 (2000) 77 – 93

Geochemistry of the Mesoproterozoic Lakhanda shales in

southeastern Yakutia, Russia: implications for mineralogical

and provenance control, and recycling

Robert L. Cullers

a,

_{*, Victor N. Podkovyrov}

b

a_{Department of Geology}_,_{Kansas State Uni}

6ersity,108 Thompson Hall,Manhattan,KS 66506-3201, USA b_{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 Siberian craton in Russia have been analyzed for major elements and a number of trace elements, including the REE’s. Shales along the Maya River formed as platform sediments in a deeper shelf facies, whereas, shales along the Belaya River formed in more active and open environments of an upper shelf carbonate ramp. The log of most elemental compositions to Al2O3ratios are the same in the Maya and Belaya River samples, suggesting a similar source rock

composition for rocks in the two areas. The log of SiO2, MgO, Na2O, K2O, Rb, Ba, and Ni to Al2O3 ratios are

significantly higher and the log of TiO2to Al2O3ratios are significantly lower in shales from the Belaya River than

the Maya River sections. The CIA (chemical index of alteration) is thus significantly lower in the Belaya shales than the Maya shales, suggesting less weathering of the in the Belaya shales than the Maya shales. The ICVs (Index of Compositional Variability=Fe2O3+K2O+Na2O+CaO+MgO+TiO2/Al2O3) of the Lakhanda shales are less

than 1, suggesting that they are compositionally mature and were likely dominated by recycling. Several samples have ICV\1, suggesting some first cycle input. The low K2O/Al2O3ratios of these shales suggest that minimal first cycle

alkali feldspar was present in the initial source. Most shales of the Lakhanda plot parallel and along the A – K line in A – CN – K plots suggestive of intense chemical weathering (high CIA) and do not indicate any clear-cut evidence of K-metasomatism or direct weathering back to the original source. If K-metasomatism produced these rocks, then they could have formed from tonalites to basalts. If weathering produced these rocks then they could have been produced 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 Belaya River shales, and the ratios are similar to fine-fractions derived from the weathering of mostly granitoids and not basic rocks. The Eu/Eu*, Th/Sc and low K2O/Al2O3 ratios of most shales suggest weathering from mostly a

granodiorite source rather than a granite source, consistent with a source from old upper continental crust. Some samples 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).

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R.L.Cullers,V.N.Podko6yro6/Precambrian Research104 (2000) 77 – 93 78

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

1. Introduction

The mineralogical and chemical composition of fine-grained sedimentary rocks are commonly used as a sensitive indicator of provenance and weathering conditions and only in a few cases as a tool of tectonic setting (Ronov and Migdisov, 1971; Cullers et al., 1975, 1979; Taylor and McLennan, 1985, 1991; Bhatia and Crook, 1986; Roser and Korsch, 1986, 1988; Ronov et al., 1990; Cullers, 1994b; Cox and Lowe, 1995; Cox et al., 1995; Nesbitt et al., 1996; Cullers and Berend-sen, 1998; Cullers, 2000). On a global scale, mu-drock chemistry reflects the average composition of continental crust (Taylor and McLennan, 1985). Most mudrocks, however, form in re-stricted basin environments in specific tectonic settings that reflect the composition of the source rocks (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 been used for provenance and tectonic determinations of mudrocks (Cullers, 1994b; Mongelli et al., 1996). Of course these signatures of the source rock may be modified by weathering, hydraulic sorting 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 that in more stable and evolved intracratonic settings mudrocks are more homogenized and represent the average composition of continental crust in the region (Ronov et al., 1974; Bhatia, 1985; Taylor and McLennan, 1985; Cox and Lowe, 1995). Recent studies of the influence of grain-size and transportation distance in a given tectonic environment on the chemical composition of sedi-ments show that some major element and trace element concentrations and ratios, including REE patterns and negative Eu-anomaly size, are similar

to the source rock in mudrocks compared with the more variable chemical composition of sand-stones in the same sedimentary sequences (Cullers et al., 1975; Cullers, 1988, 1994a,b; Mongelli et al., 1996). Thus, mudrock compositions provide more information not only about weathering con-ditions and sediment recycling, but also regional tectonic settings compared with the more variable composition 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 (the

Lakhanda Group, 1.05 – 1.01 Ga) on the edge of a mature craton and (2) to examine the effect of the input of the composition of the sediment as a result of the changing tectonic evolution.

2. Geology

2.1. Possible source rocks

The study area is located in the Uchur – Maya Region on the southeastern edge of the Siberian platform (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 the Aldan Shield include strongly metamorphosed and deformed Archean to Early Proterozoic mag-matic and supracrustal volcaniclastic units of the Uchur block to the west and the Batomga block

to the southeast. Supracrustal shales and

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The Batomga granite – greenstone block consists of granulite gneisses associated with a variety metamorphosed ultrabasic to silicic igneous rocks, and younger bimodal metavolcanics, shales, calc-silicate rocks with gabbro and granite intrusions (Larin, 1997). The latest pre-Riphean magmatic event 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 eastern Aldan Shield varies across structural domains, but granitic rocks (low- and moderate-K, with smooth REE-spectra with a small, negative Eu-anomaly) predominate with smaller amounts of mafic and metasedimentary rocks. The Proterozoic sedi-ments include both immature, arc-related wackes (lower REE content than the granites with small or no Eu anomalies) and more evolved cratonic-type mudrocks (moderately high REE content and a negative Eu-anomaly close to the average of most Phanerozoic shales (Kovach et al., 1999). Some rare trachyrhyolites and subalkaline gran-ites contain high REE content, marked LREE enrichment (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) and Vendian (1.65 – 0.54 Ga) sediments of the Uchur – Maya region unconformably cover the Archean – Proterozoic basement of the Aldan Shield. These sediments form the two main tectonic provinces. One provenance is the mature continental block of the Uchur – Maya plate, and the other prove-nance is the marginal trough of the Yudoma – Maya region (Saleeby, 1981; Semikhatov and Serebrykov, 1983; Semikhatov, 1991; Khudoley et al., 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 of these strata in the Uchur – Maya basin is 4 – 4.5 km, and it increases in thickness to 11 – 12 km to

the 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 collected along the southeastern edge of the Siberian platform (heavy outlined area).

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

record for the Lakhanda carbonate sequences dis-play a marked shift to heavyd13_C

carbvalues (from

+4 to +6 in PDB scale) (Podkovyrov and

Vino-graadov, 1996) that is similar to that of other similar Mesoproterozoic successions of that age. They also contain low 87_Sr_/86_{Sr ratios (0.70519 –}

0.70566) that probably reflect platform marine sedimentation 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 this study. The Lakhanda Group carbonates are in-terbedded with multi-colored shales and minor siltstones – sandstones that were deposited in epi-cratonic shallow marine and upper shelf

environ-ments. They form a gently north- and

northeast-dipping rock sequence (400 – 550 m thick) in central to eastern parts of the Maya basin, and they thicken (Fig. 3) eastwardly (1000 – 1200 m) in thrust-faulted and folded successions in the Yudoma – Maya trough (Sklyrov, 1981; Semikhatov and Serebrykov, 1983; Khudoley et al., 2000). In the Maya sections, an erosional unconformity with remnants of an ancient iron-rich, kaolinite weathered crust precedes the Maya platform sequence. Samples of this sequence were sampled along the Maya River (Figs. 2 and 3). Here the sequence is subdivided into two forma-tions: the Neruen Formation in the lower portion and Ignikan Formation in the upper portion (Semikhatov and Serebrykov, 1983). In the Neruen Formation (60 – 65% terrigenous rocks, 30 – 35% carbonaceous), fine-grained terrigenous sediments are abundant in the lowermost unit (45 – 50 m) and upper unit (110 – 125 m). These two units are separated by interbedded multi-col-ored stromatolitic and clastic limestones and dolomitic limestones (70 – 80 m).

The lower unit of the Neruen Formation (Fig. 3, section 77) includes iron-rich, kaolinite-bearing calcareous shales with rare siltstones, dolomitic shales and siderite beds or concretions at the base. The overlying sequence within the lower unit (Fig. 3, section 77) consists of varigated shales with minor stromatolitic dolomites, quartz sandstones, The upper sequences of the Riphean include the

Lakhanda and the Ui Groups sediments that have traditionally been attributed to Upper Riphean (960 – 750 Ma) (Semikhatov and Serebrykov,

1983). However, more recent UPb ages of mafic

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siltstones and siderite concretions. These beds of the lower part of the Neruen Formation were deposited in a transgressive, high energy marine sequence, most likely tidal flat to distal sublitoral, near the provenance (Semikhatov and Sere-brykov, 1983). These beds were enriched in heavy minerals, 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 minor laminae of siltstones, rare sandstones, siderite lenses, and stromatolitic limestones (Semikhatov and Serebrykov, 1983). The mudstones are inter-preted to have formed in low-energy, partly anoxic, outer shoal environments (Semikhatov and Serebrykov, 1983). The source rock of clay material was probably, as with the lower unit, the adjacent Aldan Shield.

The Belaya River section is the one that was studied in the Uchur – Maya trough. It is located in the north-east part of the region, along the Belaya River on the eastern flank of the Gornos-takh Anticline (Figs. 1 and 4). The section

(sec-tion 52) is composed of mostly platform

carbonate rocks (up to 82% in the Neruen and 95 – 97% in the Ignikan Formations), and minor shales and siltstones, and sandstones (Semikhatov and Serebrykov, 1983; Podkovyrov and Vino-graadov, 1996; Semikhatov et al., 1998). The se-quence is divided into seven transgressive – regressive carbonate – shale units. The limestones and dolomites were formed in a marine shoal in subtidal and tidal environments. Black thinly lam-inated shales in lower units represent predomi-nantly low-energy subtidal, distal ramp and open

marine environments, whereas, multi-colored

shales with thin horizontal and low-angle cross-bedding, small-scale flazer lamination and small symmetrical ripple marks in upper units were deposited 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 at reference 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 in

thick-bedded and more homogenous sequences. Each sample was cut in half. One half was used for the preparation of thin sections or epoxy-based polished sections for microprobe analysis, and the other half was used for other chemical analyses. A total of twenty samples were ana-lyzed. The major elements were analyzed by X-ray fluorescence and partly by standard wet-chemical methods in the Central Chemical Laboratory, NW Geological Centre, St Petersburg, Russia along with USGS standard rocks. The total Fe content is reported as FeO. The precision of SiO2

and Al2O3are better than 95%, and that of FeO

(total), TiO2, MgO, CaO, Na2O are better than

98 – 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 USGS standards. The precision of most trace elements are better than 8%, but that of Co, Ba, Rb and Pb are 10 – 12%. The elements Fe, Na, La, Ce, Nd, Sm, Eu, Tb, Yb, Lu, Co, Sc, Hf, Th, Ba and Rb were analyzed by neutron activation at Kansas University (after Gordon et al., 1968; Jacobs et al., 1977). The precision of all trace elements but Yb and Lu are better than 5%, and the precision of the Yb and Lu are better than 7%. The values of standard rocks are periodically analyzed and compared with average values (e.g. Cullers et al., 1985, 1987).

4. Results

4.1. Mineralogy

The B1 mm fractions were separated from

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trough, sample 52 – 117 consists mainly of 2M1

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

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

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R

Mudstones of the Belaya River and Maya River regions

Mudstones from the Belaya River Area — near source

52–11 52–31 52–34 52–44 52–46 52–59 52–80 52–85 52–103

Element 51–38 52–1 52–9

62.1 65.2

SiO2 56.0 59.6 49.3 54.1 57.0 57.2 61.5 57.3 60.2 62.8

21.3 21.3 18.7 20.4 23.4 18.5

22.3 22.3

10.5 9.64 6.59 3.86 14.0 3.66 4.94 5.73 2.21

9.27

Total FeO 7.67 14.4

0.01 0.01 0.01 0.01 0.01 0.01

0.01 0.01

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

P2O5 0.11 0.12 0.02

4.26

3.46 3.79 5.04 5.03 4.92 3.87 3.96 3.15 5.59 4.22 4.07

LOI

99.55

99.48 99.5 99.4 99.8 99.62 99.46 99.48 99.5 99.54 99.56 99.57

Sum

350 450 242 553 233 443

428 249

352 379

Ba 508 472

254

200 195 182 223 269 300 249 283 370 265 295

Zr

28.2 31.0 16.8 16.8 20.5 16.6

28.5 27.4

16.8 24.7 32.7 27.3 18.3 26.2 16.8 14.7 21.8 18.6 31.7

Sc

66.4

56.9 61.1 84.6 73.4 60.2 73.4 42.2 46.7 58.6 32.5 79.2

La

118 151 105 82.8 123 65.7

125 170

1.97 2.95 1.37 0.75 1.73 0.59

1.82 0.86

Eu 1.04 1.98 2.27 2.03

1.41

0.96 1.41 1.56 1.33 1.72 2.6 0.92 0.61 1.42 0.53 0.99

Tb

6.66 8.32 4.06 3.47 4.79 2.57

6.73 5.47

0.491 0.6 0.59 0.61 0.587 0.518 0.659 0.53 0.52 0.50 0.35

Eu/Eu*

3.29 2.80 2.51 3.18 2.69 1.75

2.41 2.50

La/Sc 3.39 2.47 2.59 2.69

3.72 11.29 1.11 3.96 3.73 6.91

La/Co 4.01 1.85 9.40 4.45 3.21 9.66

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 2.91 2.92 1.98 2.41 4.89 1.17 1.73 1.54 1.91 2.55

La/Ni

1.04

1.03 0.89 0.79 0.86 1.54 1.18 1.00 1.14 0.94 0.89 0.86

Th/Sc

1.74 4.77 0.44 1.42 1.31 3.53

1.38 3.34

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R

Mudstones from the Belaya River Area — near source

52–11 52–31 52–34 52–44 52–46 52–59 52–80 52–85 52–103

52–1

Element 51–38 52–9

0.19 0.19 0.22 0.22 0.20 0.23 0.21 0.17 0.19

Th/Cr 0.27 0.24 0.18

1.13 2.07 0.47 0.62 0.54 0.98

0.95 0.88

Th/Ni 0.87 1.04 0.89 0.64

0.95 0.95 0.95 0.88 0.94

CIW 0.86 0.87 0.96 0.96 0.96 0.89 0.92

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 1.02 0.91 0.81 0.52 0.46 1.02 0.91 0.67 1.07 0.60

ICV

0.11

K2O/Al2O3 0.28 0.29 0.16 0.20 0.16 0.13 0.13 0.29 0.20 0.36 0.21

Mudstones from the Maya River area — platform mudstones

Lh-8 77–12 77–10 77–8

52–119 78–3 77–7 77–6 77–3

Element 78–2 78–1 Lh-13

56.8 60.8 57.0 44.3 41.2 54.9

55.1 57.9

63.7

SiO2 58.7 43.7 52.6

27.3

20.9 24.4 19.1 23.7 23.4 22.3 25.6 22.2 20.6 26.3 26.0

Al2O3

1.48

1.38 1.81 1.33 1.62 1.40 1.46 1.74 1.50 1.34 1.89 1.50

TiO2

4.55 2.21 2.23 19.3 23.2 2.36

2.58 2.79

Total FeO 2.08 2.77 20.1 9.67

0.04 0.01 0.02 0.02 0.04 0.02

MnO 0.01 0.01 0.18 0.03 0.01 0.02

0.99 0.92 0.47 0.42 0.41 0.55

0.72 0.89

4.75 4.67 2.21 2.01 2.00 2.69

2.98 4.08

K2O 5.54 3.10 3.00 2.92

P2O5 0.10 0.05 0.02 0.06 0.07 0.09 0.07 0.04 0.02 0.04 0.03 0.06

7.13 7.19 9.36 10.3 10.4 10.7

8.91 5.70

10.2 7.72

LOI 3.66 7.54

99.5

99.59 99.54 99.6 99.5 99.5 100 99.2 101 99.6 100 99.2

Sum

245 324 225 161 144 207

Ba 492 439 324 352 329 370

300 306 388 260 236 409

300 308

21.6 25.8 17.0 24.1 24.4 22.3 21.9 16.2 21.4 21.1 20.4

Th

25.4 22.6 22.9 22.2 21.7 21.7

22.2 26.1

Sc 22.9 25.4 21.8 26.4

49.4

52.6 63.5 45.2 67.6 67.8 62.1 62.4 42.6 46.1 57.5 53.6

La

161 121 123 81.9 89.1 114

96.4 108

138

Ce 114 127 92.5

64.7 49.7 50.7 31.9 35.3 42.9

Nd 40.8 45.6 38.7 47.3 38.0 45.6

12.8 9.19 8.71 5.78 6.48 7.39

7.68 8.79

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77

–

93

85

Table 1 (Continued)

Mudstones from the Maya River area — platform mudstones

78–1 Lh-13 Lh-8 77–12 77–10 77–8 77–7 77–6 77–3

78–3

Element 52–119 78–2

1.66 1.14 2.48 1.55 1.50 1.06 1.22 1.35 1.47

Eu 0.65 1.35 1.79

1.64 1.21 1.28 0.92 1.07 1.19

0.83 1.14

Tb 0.70 1.49 1.61 1.53

5.89 5.34 5.90

Yb 3.85 7.5 7.18 7.17 4.09 3.88 4.47 5.56 4.71

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

0.356 0.511 0.67 0.60 0.66 0.56 0.55 0.56 0.58 0.56 0.56

Eu/Eu*

2.23

2.30 2.50 2.07 2.56 2.67 2.75 2.72 1.92 2.12 2.65 2.05

La/Sc

6.22 4.63 12.0 2.02 1.78 16.0

5.61 6.62

La/Co 5.60 3.451 1.37 2.7

0.56 0.59 0.54 0.49

La/Cr 0.62 0.617 0.59 0.7 0.43 0.49 0.48 0.52

3.39 1.94 2.15 2.51 11.5 2.4

3.29 1.34

1.25

La/Ni 4.233 4.52 4.51

0.90

0.94 1.016 0.78 0.91 0.96 0.99 0.96 0.73 0.99 0.97 0.78

Th/Sc

2.26

2.30 1.40 0.52 0.96 2.24 1.66 4.21 0.77 0.83 5.86 2.52

Th/Co

0.20 0.21 0.19 0.19 0.23 0.17

0.17 0.20

Th/Cr 0.25 0.25 0.22 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 0.961

0.88

0.75 0.849 0.81 0.86 0.80 0.80 0.89 0.88 0.88 0.86 0.84

CIA

ICV 0.69 0.41 1.24 0.62 0.34 0.57 0.51 0.32 0.88 1.06 0.37 0.42

0.22 0.23 0.09 0.10 0.11 0.11 0.17

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Table 2

A comparison of the elemental concentrations of the Belaya and Maya Formations

Belaya element or

Element Maya element or

ratio

TiO2 1.25390.095 1.5490.19 8.2998.0

P2O5 0.08190.038 0.05390.023 8.5491.65

La 60.6915 55.399.3

113923

Eu/Eu* 0.57590.048

2.3690.31

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

quartz\feldspar9pyrophyllite, glauconite and

hematite.

4.2. Geochemistry

The elemental concentrations and averages of the 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) in Fig. 5. The average of the Lakhanda shales are significantly 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 the PAAS have similar concentrations of FeO (total),

MnO, K2O, LOI, Rb, V, Co, Ta, and Cr.

The enrichment of immobile elements like Al2O3and depletion in mobile elements like MgO,

CaO, Na2O, and Sr results in fairly high chemical

Fig. 5. Selected elemental concentrations of the average of all samples are compared with those of the PAAS (post Archean average 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 in order of decreasing atomic number (some of the REE like Ce and Lu plot similarly to adjacent REE). Error bars are esti-mates based on one standard deviation of the Lakhanda values for each element as no standard deviation is reported for the PAAS.

as follows: dioctahedral illite – muscovite with a

low index of crystallinity (I=0.20 – 0.36)\ \

Mg – Fe chlorite\quartz:feldspars. The

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Fig. 6. The SiO2 versus Al2O3 concentrations of shales from the Belaya River (open triangle) and Maya River (closed triangle) are plotted relative to the idealized composition of the observed minerals. Much of the variation in composition may be accounted for by variation in quartz and clay miner-als-muscovite. The three samples with high FeO total and low SiO2are skewed in the direction of hematite, opaque minerals and Fe-rich chlorite compositions. The FeO total vs. Al2O3 concentrations of shales from the Belaya and Maya Rivers are plotted relative to the composition of the observed minerals (same symbols as in Fig. 6a). Again much of the variation in composition may be accounted for by variation in quartz and clay minerals-muscovite. The Fe-rich samples are again skewed toward the hematite, magnetite, and Fe-rich chlorite compositions.

primary and recycled clay material (Cabanis and Lecolle, 1989; Cullers, 1994b, 1995). Ancient weathering profiles are observed in some parts of the Lakhanda sequence. These profiles contain mostly kaolinite with lesser hematite and chlorite

(Semikhatov and Serebrykov, 1983). The B2 mm

fractions of samples in this study, however, were mainly illite – muscovite with smaller amounts of kaolinite and chlorite. Elemental plots are also consistent with these phases, but the whole rock samples have variations in major element compo-sitions that suggests a significant amount of quartz 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 Lakhanda shales could be due to the concentration of certain accessory minerals (zircon, monazite, ilmenite, ru-tile). For example, the observed correlations

be-tween Zr, Hf, and TiO2 tends to support this

possibility. The correlation of Sc and Cr with Al2O3 and lesser correlation of Th and the REE

with Al2O3 suggests these elements may also be

included in the clay minerals of the Lakhanda. There are poor or no trends in the elemental compositions with time. The best of these correla-tions is a slow increase in Na2O in both the Maya

River and Belaya River samples with time.

5. Discussion

5.1. Comparison of the composition of samples

from the Belaya and Maya Ri6ers

The composition between the samples from the Belaya and Maya Rivers are compared by using the Student t-test of the log10 of the elemental

ratios relative to Al2O3. Statistically comparing

the log10 of elemental ratios avoids the constant

sum problem that insures that there must be some correlations of elements since they must add to 100 percent. Comparing the log10of the elemental

ratios converts the constant sum data with contin-uous variables that range to infinity so the data may be compared using parametric tests such as the Student t-test (Cardenas et al., 1996).

index of weathering values (CIA=0.67 – 0.89;

Table 1), especially shales from the Maya River area. These chemical characteristics are consistent with formation in stable platform environments with intense chemical weathering of mostly silicic

source rocks (Ronov and Migdisov, 1971;

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The log of the ratios of TiO2 to Al2O3 is

significantly higher and those of SiO2, MgO,

Na2O, K2O, Rb, Ba, and Ni to Al2O3 ratios are

lower in the Maya River than the Belaya River sections (Fig. 7). Other elemental ratios between the two areas are statistically the same thus sug-gesting similar source rock compositions for the Belaya and Maya River areas. The differences in the composition of samples along the Belaya and Maya Rivers that may reflect the degree of weath-ering, proximity to the source, or sedimentary sorting processes. For instance, the CIA (chemical index of alteration) is thus significantly lower in the Belaya shales than the Maya shales, suggest-ing much less weathersuggest-ing of the material in the Belaya shales than the material in the Maya shales.

5.2. Source rock composition — major elements

Also shales along the Maya River suggest that they were formed as platform sediments in a deeper shelf facies (epicratonic and restricted marine basin), whereas, those along the Belaya River probably formed in more open and active environments of an upper shelf carbonate ramp that 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 chemical composition can be used to suggest the original detrital mineralogy of the shales (Cox et al., 1995).

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Fig. 8. Samples of the Lakhanda shale are plotted in an A – CN – K diagram. Samples have no tendency to project back to 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; the heavier line). G, granite; Gn, granodiorite; T, tonalite.

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

Also K2O/Al2O3 ratios may suggest how much

alkali feldspar vs. plagioclase and clay minerals may have been present in the original shales (Cox et al., 1995). In order from high to low values, the K2O/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 of K2O/Al2O3 greater than 0.5 suggest a significant

quantity of alkali feldspar relative to other minerals in the original shale; those with ratios of K2O/

Al2O3less than 0.4 suggests minimal alkali feldspar

in the original shale (Cox et al., 1995). The Belaya shales of the Lakhanda Formation have an average ratio of K2O/Al2O3=0.22 (range=0.11 – 0.29) and

the Maya shales have an average ratio of K2O/

A2O3of 0.15 (range=0.10 – 0.23), suggesting

min-imal alkali feldspar relative to other minerals in the original shale.

Another approach to the composition of the original source rock is to plot molar ratios of Al2O3– CaO+Na2O – K2O in A – CN – K diagrams

in order to potentially separate compositional changes of shales and coexisting sandstones related to chemical weathering, transportation, diagenesis-metamorphism, and source composition (Fedo et al., 1995, 1997a,b). The CaO included in carbonate or apatite is not included in the chemical plots. The A – CN – K diagams are useful in that the average source rock composition and metasomatic effects can be inferred especially if a wide range of compositions of shales and sandstones are available to be plotted. In such a system, plots of shales and sandstones due to weathering trends plot parallel to the A – CN boundary, and they extract back to a plagioclase – alkali feldspar horizontal line of the source composition (Fig. 8) unless metasomatism affects the rocks. The K-metasomatism of kaolinite weathered rocks can produce illitic rocks with points plotted at right angles to the A – K side of the diagram (Fig. 8). The Lakhanda Formation, however, contains only shales and no sandstones so that 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 of

K2O/Al2O3 (Cox et al., 1995). Non-clay minerals

have a higher ratio of the major cations to Al2O3

than clay minerals so the non-clay minerals have a higher ICV. For example, the ICV decreases in the

order of pyroxene and amphibole (10 – 100),

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Table 3

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

Belaya River Maya River _{Range of fine-fractions from} _{Range of fine-fractions from sililic} PAASb sililic sourcesa _sourcesa

— platform — near source

0.36–0.67 0.32–0.83 0.70–1.02 0.66

Eu/Eu* 0.35–0.66

1.92–2.75 0.70–27.7

1.75–3.39 0.40–1.1

La/Sc 2.4

0.79–1.54

Th/Sc 0.73–1.02 0.64–18.1 0.05–0.4 0.91

1.11–11.3

La/Co 1.37–12.0 1.4–22.4 – 1.65

0.52–5.68 0.30–7.5

0.44–4.77 –

Th/Co 0.63

0.17–0.27

Th/Cr 0.17–0.25 0.067–4.0 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 or indication of metasomatism. Most shales of the Lakhanda plot parallel and along the A – K line suggestive of intense chemical weathering (high CIA). If K-metasomatism produced these rocks, then they could have formed from tonalites to basalts. This interpretation is not consistent with other trace element characteristics discussed in the next section (e.g. Eu/Eu*, Th/Sc, and REE pat-terns). If weathering produced these rocks then they could have been produced from varied amounts of mostly granodiorite to granite. The low K2O/Al2O3ratios of these shales suggests that

the amount of granite in the source may have been

minimal unless the original K2O was removed

from the system by other processes. This interpre-tation is more consistent with the trace element composition 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 Maya and Belaya River samples. Moreover, these ratios and the size of the negative Eu-anomaly size are fairly similar to platform sediment or fine-grained Holocene sediment that has been interpreted to have been derived from silicic source rocks such as granodiorite 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 of most of the Lakhanda shales relative to sediment

averages like the PAAS, however, suggests that the Lakhanda shales may on the average be derived from somewhat more differentiated granitoids than those that make up the PAAS.

In addition, the low K2O/Al2O3 ratios, the A –

CN – K plots along with the moderately negative Eu 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 a granite source. For example, a Holocene source in the Wet Mountains, USA, is composed of mostly granodiorite with minor granite, tonalite, and

ba-sic rocks (B15%). Fine-grained stream sediment

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draining this area is producing the same range of negative Eu-anomaly size and Th/Sc ratios as the Lakhanda shales thus supporting a dominately granodiorite source (Cullers et al., 1987; Cullers,

1994a). The Th/Sc ratios and the negative

Eu-anomaly size are also in the range of values for old upper continental crust although the Eu-anomaly size is somewhat more negative than is usually observed (McLennan et al., 1993).

Samples 52 – 103 and 52 – 119 in the lower por-tion of the Belaya River samples contain lower Eu/Eu* values of 0.35 than the other samples so they could have been derived from a source with more granite with large negative Eu anomalies. The only presently observed differentiated gran-ites are those found in the Proterozoic Ulkan

complex of the eastern Aldan Shield (e.g. Eu/

Eu*=0.11 – 0.15) in addition to the more

com-mon less differentiated granitoids and older

recycled mudrocks derived from them. No UPb

detrital zircons have been analyzed but sandstones

throughout the Riphean have UPb ages of

zir-cons which are almost entirely Proterozoic in age (Khudoley et al., 2000).

6. Summary

The average concentrations of the Lakhanda shales are significantly higher in Al2O3, TiO2, Zr,

Th, Hf, Sc, and the REE and are significantly lower in SiO2, MgO, CaO, Na2O, P2O5, Sr, Ba,

and Ni than corresponding elemental concentra-tions in the PAAS; other elemental concentraconcentra-tions are the same. The enrichment in the immobile elements like Al2O3 and TiO2 and depletion in

mobile elements like MgO, CaO, Na2O, and Sr

results in a high CIA (0.67 – 0.89) in the Lakanda

shales, suggesting fairly intense chemical

weathering.

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

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

Hf, and the REE and less correlation to the major elements could be explained by heavy mineral variations such as zircon and columbite.

The log of the concentration of SiO2, MgO,

Na2O, K2O, Rb, Ba, and Ni to Al2O3 ratios are

significantly higher and the log of the concentra-tion of TiO2to Al2O3ratio is significantly lower in

the Belaya River shales than from the Maya River shales. Most of the elemental to Al2O3 ratios are

the same for the Belaya River and Maya River shales, suggesting a similar provenance for the samples. Differences in degree of weathering, proximity to the source, or sedimentary sorting processes could have produced some of the differ-ences in chemical composition.

The low ICVs (most B1) of the Lakhanda

shales suggest that they are compositionally ma-ture and were likely dominated by recycling

al-though several samples have ICVs\1, suggesting

some first cycle input. The low K2O/Al2O3 ratios

of these shales suggest that minimal first cycle alkali feldspar was present in the initial source.

Most shales of the Lakhanda plot parallel and along the A – K line suggestive of intense chemical weathering (high CIA) and do not indicate any clear-cut evidence of K-metasomatism or direct weathering back to the original source. If K-meta-somatism produced these rocks, then they could have formed from tonalites to basalts. If weather-ing produced these rocks then they could have been produced 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 on the average between the Belaya River and Maya River shales, and the ratios are within the range of fine sedi-ment derived from silicic source rocks rather than basic rocks. The Eu/Eu*, Th/Sc, La – Th – Sc plots and low K2O/Al2O3 ratios of these shales suggest

weathering from mostly granodiorite source

rather than a granite source. The Th/Sc ratios and the negative Eu-anomaly size are also in the range of values for old upper continental crust although the Eu-anomaly size is somewhat more negative than is usually observed. A few of the Lakhanda shales at the bottom of the Belaya River section

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Eu*=0.35), suggesting significant input from the weathering 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 the use of their counting equipment for neutron acti-vation analyses.

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