REE geochemistry of recent clastic sediments from the Kaveri floodplains, southern India:...

PII S0016-7037(01)00636-6

REE geochemistry of recent clastic sediments from the Kaveri floodplains, southern India:Implication to source area weathering and sedimentary processes

PRAMOD SINGH and V. RAJAMANI *School of Environmental Sciences, Jawaharlal Nehru University, New Delhi, 110067, India

(Received July25, 2000;accepted in revised form February28, 2001)

Abstract—The Kaveri River drains through Archean gneissic and charnockitic terranes in south India. Therare earth element (REE) chemistry of the floodplain sediments, along with their major and trace elementcomposition in different size fractions, are discussed. We observe a strong coherence between REE and TiO2contents in the sediments, which is suggestive of strong control by titanite, or its weathered products, on theREE chemistry. REE concentrations also show a general increase with decreasing size of bulk sediments,different size fractions and of light and heavy mineral fractions. This suggests the presence of REE in theFe-oxy-hydroxide surface coatings on clastic grains, formed by the weathering of hornblende and titanite inthe granodioritic to dioritic protoliths. We also interpret the data as showing that REE patterns of clasticsediments, notably Eu anomaly, are affected by the degree of source rock weathering, as well as by the fluvialprocesses of sorting and mineral differentiation. Therefore, for provenance study, sediments having allgranulometric grades, such as the floodplain sediments, are preferable because they most closely represent thesource, particularly when the source region suffered little chemical weathering. In the present case, thesediments were derived from the high standing hills of Archean charnockites because of their recent uplift andphysical denudation.Copyright © 2001 Elsevier Science Ltd

1. INTRODUCTION

Chemical weathering of rocks is a major geological process,which modifies the Earth’s surface and controls geochemicalcycling of elements, thereby controlling global climate as well.An understanding of the processes of rock weathering andsediment formation is essential in the study of the evolutionaryhistory of the Earth, particularly that of the continental crust(Cochran and Berner, 1996; Berner and Cochran, 1998). This inturn requires a sound knowledge of the geochemical behaviourof elements during weathering/soil formation and during thesedimentary redistribution processes under different climaticconditions.

The effect of chemical weathering on trace element distri-bution in the weathered rock products has been reported inseveral geochemical studies of weathering under variable cli-matic conditions (Nesbitt, 1979; Duddy, 1980; Suttner et al.,1981; Cullers et al., 1987, 1988; Middelberg et al., 1988; Braunet al., 1990, 1993; Rollinson, 1993; Vital and Stattegger, 2000).Because rare earth elements (REE) are known to be particularlyuseful in the studies of crustal evolution, greater attention hasbeen paid to understanding the behaviour of this group ofelements during surface geological processes (Taylor andMcLennan, 1985). REE are shown to be mobile during weath-ering, at least within the profile, particularly during the earlystages of weathering (Nesbitt, 1979; Condie, 1991; Sharma andRajamani, 2000a). In a weathering profile, REE are mobilizedunder acidic pH, and as they move and reach the neutral oralkali pH zone, they get adsorbed onto clays and hydroxides orare precipitated (Roaldset, 1973; Nesbitt, 1979). The observa-tions of Nesbitt (1979), Duddy (1980), Banfield and Eggleton(1989), and Sharma and Rajamani, (2000a, 2000b) are partic-

ularly important because they show that REE can be redistrib-uted within a weathering profile, but net losses or gains ofspecific REEs are not observed, that is, they do not leave thesystem.

Martin and Meybek (1979), Goldstein and Jacobson (1988),and Condie (1991) observed that although the REEs in aweathering profile may get fractionated, by the time REE enterthe suspended load of major rivers, they show a uniformshalelike REE pattern, not unlike their average source. How-ever, Sholkovitz (1988) showed that river-borne sedimentscommonly have REE patterns that are strongly depleted inHREE relative to shale. Therefore, the processes responsiblefor the REE chemistry of different deposits laid down by thefluvial system are not well understood. Cullers et al. (1987,1988) studied the chemistry of riverbed sediments produced indifferent climatic conditions and observed that silt and clay sizefractions most closely resemble their source REE patterns butwith higher abundance. These authors also observed that insediments derived from an intensely weathered source, all thesize fractions have negative Eu anomaly, whereas in moder-ately weathered sediments, only the sand fraction shows apositive Eu anomaly. It is important to understand the Eusystematics as the major distinction between Archean andpost-Archean sediments and therefore the nature of theirsources is based on their Eu anomaly (McLennan et al., 1979,1983; Taylor and McLennan, 1985). The relative importance ofthe nature of source rocks vs. the prevalent surface weatheringconditions in the chemistry of clastic sediments needs to beclarified. This calls for more databases on modern clastic sed-iments derived from diverse lithologies and from differentclimatic conditions. Our study deals with alluvial sedimentsderived from weathering of predominantly mantle-derived Ar-chean granulitic rocks under semiarid conditions that are ex-posed in a tropical rainshadow region.

* Author to whom correspondence should be addressed ([email protected]).

Pergamon

Geochimica et Cosmochimica Acta, Vol. 65, No. 18, pp. 3093–3108, 2001Copyright © 2001 Elsevier Science LtdPrinted in the USA. All rights reserved

0016-7037/01 $20.001 .00

3093

In this study, we evaluate the REE systematics in asedimentary system using less frequently discussed flood-plain sediments and their usefulness in weathering andprovenance studies. As part of our geochemical researchof the Kaveri River basin in southern India, we studied theREE geochemistry of floodplain sediments of this importantriver. Earlier we observed that the floodplain sediments aretexturally, mineralogically, and chemically immature andare likely to have been derived from the high hills ofcharnockite by rapid physical erosion in response to neotec-tonic activities (Singh and Rajamani, 2001). We also foundthat the floodplain sediments in this particular situation aregeochemically similar to the source lithologies, unlike thebed or suspended sediments. We are also studying the natureand extent of weathering of catchment-area lithologies andthe behaviour of REE during this weathering process(Sharma and Rajamani, 2000a, 2000b). Here we report theREE data on the floodplain sediments including differentsize fractions and heavy and light mineral fractions. We alsodiscuss the REE behaviour during sedimentary processesand their implications to provenance studies using clasticsediments.

2. GEOLOGY OF THE KAVERI RIVER BASIN

The Kaveri River basin lies between latitude 10°79N to13°289N and longitude 75°289E and 79°529E. At present thecatchment area experiences a semiarid climate and receives 110cm average annual rainfall in less than 3 months, mainly fromthe southwest monsoon winds. The river basin covers an area of87900 km2. The river originates in the Brahmagiri range of theWestern Ghats at an elevation of 1345 m above mean sea leveland travels approximately around 800 km to join the Bay ofBengal (Fig. 1). The initial course of the river is easterly overthe Mysore plateau (average elevation 1000 m). The finalcourse of the river in the state of Tamil Nadu, where it has builtup floodplains and a delta, is also easterly. Between the MysorePlateau and the Tamil Nadu plains, there are a series of blockmountains thought to have been formed after the collision ofthe Indian continent with the Asian continent (Radhakrishna,1993; Valdiya, 1998). In this mountainous tract, the drainagepattern of the river becomes trellis, resulting in an overallsoutherly flow for the main channel. Radhakrishna (1993)suggested that the old stage of the river in the Mysore plateauhas been rejuvenated because of the uplift of the region, and theriver has become antecedent. In the southern lower reaches of

Fig. 1. Geological map of the area showing the Kaveri River, its tributaries, and its prominent rock types in the catchmentarea. Locations of sampling profiles discussed in text includes 1. Singampettai; 4. Cholasiramani; 5. Dalvayapalyam; 6.Sirugmani; 7. Mukambhu; and 8. Anbil. Modified from Raith et al. (1993).

3094 P. Singh and V. Rajamani

the river, two major streams, Bhavani and Amravati, join theKaveri River. These two major tributaries also originate fromthe uplifted regions of Nilgiri, Cardamom, and Anaimalai hills.The two major tributaries and the main channel are thought tofollow major, crustal scale shears known as the Palghat-Kaverigap, separating two continental blocks (Radhakrishna, 1993).After flowing for 600 km with the channel width of nearly 3km, the river starts bifurcating just before Tiruchy, from whereit starts building up its wide delta. Within the delta, the riverbecomes braided.

The main lithologies of the drainage basin include 3300 to2900 Ma TTG-type Peninsular gneiss approximately 2600 to2500 Ma dioritic to tonalite gneiss and their granulite gradecharnockite, granite and minor amounts of Archean supra-crustal rocks. In the upper reaches, the river flows dominantlythrough amphibolite-granulite transition zone gneisses,whereas in the middle and lower reaches, granulitic rocksdominate the river basin; the uplifted and rejuvenated segmentalso includes predominantly charnockites and other granuliticrocks. The gneisses are strongly foliated, migmatised, andextensively sheared. The dominant minerals are plagioclase,quartz, K-feldspar, biotite, and hornblende (Mahabaleswar etal., 1995). The accessory phases include magnetite, apatite,titanite, and zircon. The foliated charnockites derived from thegneisses also have similar mineralogy, except for the replace-ment of hornblende by hypersthene and the appearance ofgarnet. The upland massif charnockites of Anaimalai and Car-damom hills have quartz, K-feldspar, plagioclase, orthopy-roxenes, magnetite, ilmenite, hornblende, and biotite as domi-nant minerals, with zircon and apatite as accessory phases.

The catchment region is crisscrossed by several N-S andE-W striking shear zones and fault systems. It has been sug-gested by Radhakrishna (1993) and Valdiya (1998) that theuplift and formation of several block mountains, such as theNilgiris, Biligirirangan, and the Sheveroy to name a few, arethe result of reactivation of these shear and fault systems due toHimalayan orogeny. However, the timing and the duration ofthe uplift are still not known. The region is also known to beseismically active historically, with recorded earthquakes ofmagnitude 5 on the Richter scale. The presence of block moun-tains with planation surfaces at different elevations suggeststhat the region has been tectonically active since the time ofseparation of India from Gondwanaland (Radhakrishna, 1993;Valdiya, 1998).

Although rocks of the drainage basin are older than 2500 Ma,they must have been first exposed to the surface possibly duringthe Cenozoic, and at places it may even have been as late asQuaternary (Singh and Rajamani, 2001). Samples of floodplainsediments of the river, some of them used in the present study,have given radicarbon ages that are less than 4300 BP (Singh,1999)

3. SAMPLING AND ANALYTICAL METHODS

Samples were collected from 2- to 5-m vertical profiles cut into thefloodplain of variable width (a few hundred meters to a few kilometerson either side) at eight locations along the channel for a distance of 2˜50km, from Singampettai to Anbil (Fig. 1). The profiles studied werelocated 30 to 300 m from the bank of the river at bank full stage andcontained layers recording several past floods. Samples were collectedfrom each bed, one where samples were thin and the rest where they

were thicker. Approximately 2 kg of sediments were collected from ameter-long groove made in the bed. Sampling site 1 is at Singampettai(samples PR1A-PR1F). Three sites further downstream are Cholasira-mani (samples PR2E-2M), Dalvayapalyam (samples PR4A-4F), andSirugamani (samples PR7A-PR7D). The last sampled profile, justbefore entering the delta region, is situated at Mukambhu (samplesPR6C-6F). The last site is located at Anbil (samples PR8E-8H), whichis situated in the delta region on the northern bank of the main channel,Kollidam.

The texture, mineralogy, and major and trace element geochemistryof all the bulk samples are described in Singh and Rajamani (2001).The same sets of samples were used here for the REE analysis of thebulk samples and different size fractions.

Samples for the REE determination were digested with a flux ofNaOH and Na2O2 in nickel crucible at red-hot condition with contin-uous swirling for 15 min. On cooling, the crucibles were filled withwater and kept for 12 h, after which the digested samples was trans-ferred by 6N HCl into a beaker and kept on a hot plate until concen-tration gel of H4SiO4 formed. Silica gel was filtered, and the filtrate inthe filtration flask was transferred into the beaker and dried completelyon a hot plate. The dried residue was picked up by IN-3N HCl andtransferred to centrifuge bottle. To this was added 12 to 14 drops ofphenol red indicator. Then 1:1 ammonia solution was added to precip-itate the trivalent and tetravalent cations, leaving mono and divalentcations in solution. This solution was centrifuged, and the precipitatewas taken out by repeated washing using 6N HCl and transferred to aTeflon beaker and again dried completely. Then the residue was pickedup and passed through HNO3 and HCl cation exchange resin (AG 50WX8) columns to separate the REEs. Using an ICP-AES (Labtam 8440)machine, metal standards, and in-house working rock standards (pre-viously analysed by isotope dilution method using a mass spectrometerat the State University of New York at Stony Brook), we obtained aprecision better than 5% for all REE except in low REE samples (,53chondrite) for which the precision was,10% for Ce and Nd. Concen-trations of Zr on representative samples were analysed by XRF method(Siemens SRS3000) at Wadia Institute of Himalayan Geology, Dehra-dun. The light mineral fractions were separated from the heavy mineralfraction by using bromoform having specific gravity 2.87. Heavymineral and light mineral fractions in four size fractions (.0.125 mm,0.123–0.063 mm, 0.063–0.037 mm, and 0.037–0.002 mm) were sep-arated from several sediment samples for their REE analysis.

4. RESULTS

The concentrations of REE in the Kaveri floodplain sedi-ments and that of a few relevant trace elements, along with claycontents and their mean grain size, are given in Table 1. Otherchemical details of the sediments are given in Singh andRajamani (2001). Table 2 shows the REE concentration of twochannel bed sediment samples, averages for sandy and siltyfloodplain sediments of the river, charnockite and gneisses ofthe catchment area of the river, and those of upper continentalcrust (UCC), post-Archaean Australian shale (PAAS; Taylorand McLennan, 1985), and average shale given by Sholkovitz(1988). The floodplain sediments in the present study have beencategorized into two groups: (1) a silty type with a mean size.4 phi units and (2) a sandy type with a mean grain size,4phi units. The two groups have similar chondrite-normalisedREE patterns with overlapping abundance (Figs. 2 and 3), andwith the average concentration in silty sediments higher thanthat of the average for sandy sediments (Fig. 4). All samples ofsediments show fractionated, parallel to subparallel REE pat-terns, with (Ce/Yb)N ratios ranging between 6.9 and 11.3(Table 1). The light rare earth elements (LREE) show a moreuniform fractionation than do the heavy rare earth elements(HREE) with (Ce/Sm)N ratio ranging between 3.1 and 3.5. TheHREE are variably fractionated with (Gd/Yb)N ratio rangingbetween 1.5 and 2.0. Most samples show consistently negative

3095REE geochemistry of Kaveri Alluvium, south India

Tab

le1.

RE

Eco

mpo

sitio

nof

sele

cted

sedi

men

tsa

mpl

esfr

omth

eK

aver

ifloo

dpla

inan

dot

her

rele

vant

feat

ures

.

PR

1AP

R1B

PR

1CP

R1F

PR

2EP

R2F

PR

2HP

R2K

PR

2LP

R2M

PR

3EP

R4A

PR

4BP

R4C

PR

4DP

R4E

PR

4FP

R6C

PR

6DP

R6E

PR

6FP

R7A

PR

7BP

R7D

PR

7EP

R8E

PR

8H

Cla

y17

1328

4334

166.

79.

56

3028

286.

521

526

536

932

61

111

150

Mz

3.50

3.30

3.80

4.40

4.20

3.13

3.20

2.90

1.40

4.13

3.90

4.40

2.70

4.30

2.40

4.03

2.40

4.23

4.10

4.03

2.26

2.90

3.90

3.20

2.00

5.10

CIA

5453

6159

5554

5652

5257

5851

5652

5950

6953

6752

5559

5856

58T

iO2

1.3

1.4

1.1

1.1

0.7

0.8

0.9

0.7

0.8

1.1

0.8

0.7

0.6

0.8

0.5

0.6

0.5

1.0

1.0

0.9

0.7

0.6

0.7

0.8

0.9

0.7

0.8

Zr

453

528

519

516

334

370

519

375

349

579

465

355

250

375

201

248

345

298

573

311

348

371

356

478

532

325

232

Ce

101

106

104

9574

8290

7094

117

7675

5373

4576

5292

8095

6159

8873

76.7

5592

Nd

40.7

41.6

36.3

30.2

31.7

28.1

29.6

23.1

35.7

42.0

31.3

30.7

21.0

26.0

20.0

31.3

20.7

37.5

32.8

39.8

24.1

23.5

35.2

29.1

32.9

16.3

31.5

Sm

7.1

7.2

7.3

6.9

5.7

5.6

6.1

4.8

7.1

8.3

5.9

5.8

3.9

5.2

3.7

6.0

3.8

6.6

5.6

6.9

4.1

4.1

6.3

5.3

5.6

3.2

6.3

Eu

1.3

1.2

1.5

1.7

1.5

1.4

1.7

1.4

1.8

1.9

1.4

1.6

1.2

1.5

1.0

1.7

1.0

1.6

1.5

1.7

1.0

1.1

1.8

1.5

1.4

0.8

1.8

Gd

6.2

6.0

6.9

7.5

5.3

5.5

6.2

4.9

6.9

8.2

5.7

5.8

3.4

5.1

3.3

5.6

3.4

5.7

4.9

5.7

3.4

3.5

5.5

4.8

4.8

3.2

6.1

Dy

4.9

4.5

4.8

4.7

4.6

3.8

4.3

3.4

5.3

6.3

4.6

4.8

2.5

3.6

2.8

4.5

2.5

4.9

4.1

4.7

2.7

2.6

4.3

4.0

4.2

2.2

4.2

Er

2.9

2.7

3.1

3.3

2.8

2.5

2.8

2.5

3.2

4.0

3.0

3.0

1.6

2.3

1.7

2.7

1.6

2.8

2.4

2.7

1.6

1.6

2.7

2.6

2.7

1.6

2.6

Yb

2.5

2.4

3.0

3.5

2.4

2.4

2.7

2.6

3.2

3.9

2.7

2.8

1.4

2.2

1.5

2.4

1.5

2.5

2.2

2.3

1.4

1.4

2.4

2.4

2.3

1.6

2.5

TR

EE

167

171

167

153

128

131

143

113

157

191

130

130

8811

979

130

8615

413

315

999

9714

612

313

084

147

(Ce/

Yb)

N10

.511

.38.

96.

97.

78.

68.

46.

97.

57.

67.

16.

89.

48.

37.

88.

29.

19.

59.

210

.611

.110

.89.

37.

88.

68.

99.

4(C

e/S

m) N

3.4

3.5

3.4

3.3

3.0

3.5

3.5

3.4

3.1

3.3

3.0

3.1

3.3

3.3

2.9

3.0

3.2

3.3

3.4

3.3

3.5

3.4

3.3

3.3

3.2

4.1

3.5

(Gd/

Yb)

N2.

02.

01.

81.

71.

71.

81.

81.

51.

71.

71.

71.

61.

91.

81.

81.

91.

91.

81.

82.

01.

92.

01.

81.

61.

71.

61.

9(E

u/E

u*) N

0.57

0.56

0.65

0.71

0.82

0.80

0.85

0.88

0.79

0.72

0.76

0.84

0.98

0.90

0.89

0.88

0.86

0.81

0.86

0.83

0.83

0.88

0.92

0.90

0.80

0.79

0.87

Not

es:

RE

E:

rare

eart

hel

emen

t.C

lay

cont

ents

and

TiO

2in

wt.%

;m

ean

size

(Mz)

inf;

Zr

and

RE

Ein

ppm

.


Eu anomaly with (Eu/Eu*)N ratios ranging between 0.8 and0.98. However, some sandy samples with phi values,3 haveno Eu anomaly and have much lower overall REE abundance.The shale-normalised REE patterns of the average silty andsandy sediments are also similar but with significant differ-ences in the normalised abundance (Fig. 5). Both the groups areslightly enriched in LREE, relative to HREE, in the shale-normalised abundance. A significant feature of the shale-nor-malised pattern is the enrichment of Eu and Gd in both groups.The channel bed sediments, compared with floodplain sedi-ments, have a much lower REE abundance and a flatter patternwith only positive Eu anomaly. Thus, to a first approximationthe sediment texture has determined its REE abundance and Euanomaly and its shale-normalised pattern to a minor extent.

4.1. Mineral Control on REE Distribution

Among the several compositional variables of the sediments,only their TiO2 contents show a strong positive correlation withCe, total REE, and to a less extent, Yb (Fig. 6a, 6b, 6c).However, Yb is not coupled strongly enough with Zr (Fig. 6d)to suggest zircon control on HREE. The coherence betweenTiO2 and REE, in particular LREE, (Table 3)is significantenough to consider the control of LREE-enriched titaniferrousminerals on the REE chemistry of the Kaveri River floodplainsediments. The coherence is not readily explained by ilmenite,a major titaniferous mineral, in charnockites, because ilmenitehas commonly HREE-enriched pattern with a chondrite-nor-malised abundance around 1003 (Gotze and Lewis, 1994;Vital and Stattegger, 2000). Other titaniferous minerals such asbiotite and titaniferrous magnetite have low REE content(Gotze and Lewis, 1994; Gromet and Silver, 1983), whichcannot account for the REE in heavy mineral separates (Table5). Thus, among the several Ti-bearing minerals (such as bi-otite, ilmenite, titaniferrous magnetite, and titanite) present inthe sediments, only titanite is known to have high concentra-tions of REE (Basir and Balakrishnan, 1999) as well as TiO2

contents (;40%) and appropriate REE pattern that could ex-plain the Ti-REE coherence in the sediments. Assuming anaverage abundance of REE 5000 ppm in titanite, a variation of1 to 4% titanite content in the sediments could account for theobserved large variation in TiO2 (0.4–1.6 wt.%) and in the total

REE (50–200 ppm). Therefore, we suggest that titanite (aheavy mineral) among the major minerals could have likelycontrolled the abundance of REE in the sediments. We alsonote here that titanite is an abundant minor mineral in themantle-derived, intermediate felsic rocks of the region (Bal-akrishnan and Rajamani, 1987). Other heavy minerals such aszircon, garnet, and apatite have exerted only a minimum influ-ence on the REE chemistry. Despite the large variations in theabundance, the patterns remained somewhat similar in all thesamples. However, in detail the variation in pattern is morebecause of variation in the (Gd/Yb)N ratios than in (Ce/Sm)N

ratios. This is evident from the strong correlation and covari-ance of (Ce/Yb)N and (Gd/Yb)N, as can be seen in Figure 6f.The uniformity of patterns is disturbed somewhat by heavyminerals and ferro-magnesian minerals such as amphibole,pyroxene, and garnet, contributing differentially to individualsamples.

The REE chemistry of the sediments is not strongly corre-lated to their mean grain size and clay contents (Table 3, Fig.6e), suggesting that the textural control on REE chemistry isnot entirely due to size distribution. It appears that there aresome mineralogical changes accompanying textural or otherchanges, which together control the REE chemistry of thesesediments.

4.2. Textural Fractionation of REE in FloodplainSediments

Our observation of REE in different sediment populations,i.e., silty, sandy, and bed load sediments, and the inferredmineralogical control suggest that fluvial processes of sortingaffect REE distribution in the sediments. Similar observationshave also been reported by McLennan (1989), Morey andSetterholm (1997), and Vital and Stattegger (2000). This dis-tribution is likely affected by differential concentration of var-ious REE holding minerals and diluting minerals in differentsize fractions and the variation in the percentage of differentsize fractions in the bulk sediments. To evaluate the control oftexture and mineralogy on sediment geochemistry in greaterdetail, we have analysed major, trace, and REE of five sizefractions, from sand to clay, on several samples (only twosamples are reported here; see Table 4). We also analysed REE

Table 2. REE composition (in ppm) of bed load sediment (KB, VB), PAAS, UCC, along with the averages for charnockite, gneisses, silty bedsediments, and sandy bed sediments.

REE KB VB PAAS Shale UCCAvg

charnockiteAvg.

gneissesAvg.

sandy bedAvg.

silty bedAvg.

bed load

Ce 19.6 12.6 80.0 83.0 64.0 96.5 55.4 65.6 86.6 16.3Nd 8.2 5.5 32.0 38.0 26.0 35.5 20.1 25.0 33.4 6.9Sm 1.4 1.1 5.6 7.5 4.5 6.25 3.3 4.7 6.2 1.3Eu 0.6 0.4 1.1 1.61 0.9 1.35 0.7 1.3 1.6 0.5Gd 1.4 1.0 4.7 6.35 3.8 — — 5.3 6.9 1.2Tb — — — — 0.71 0.25 — — —Dy 1.4 0.8 4.4 5.49 3.5 4.2 — 3.3 4.5 1.1Er 0.9 0.5 2.9 3.75 — 2.25 — 2.2 2.8 0.7Yb 0.8 0.4 2.8 3.51 2.2 1.65 0.5 2.0 2.6 0.6

Note: PAAS: post-Archaean Australian shale; REE: rare earth elements; UCC5 upper continental crust.Data source: PAAS and UCC (Taylor and McLennan, 1985); shale (Sholkovitz, 1988); charnockite (Stahle et al., 1987); gneisses (Mahabaleswar

et. al., 1995). KB and VB are channel bed sediments from Kulithalai and Velur.


in the heavy and light mineral fractions of the above analysedsize fractions in the same two samples (Table 5).

In all samples studied, the coarser sand fractions have lowerand the finer silt, and clay fractions have higher REE abun-dance than the bulk sample (Fig. 7). Interestingly, although thetwo sand fractions have significant differences in abundance,the fine fractions (silt and clay) have commonly overlappingabundance. We also calculate that nearly 70 to 80% of REE inthe bulk sediment is present in the finer fractions (silt and clay),as Cullers et al. (1987, 1988) also found. The shapes of the REEpatterns for different size fractions, however, are strikinglysimilar except for the Eu anomaly (Fig. 7). The sand fractionhas a positive Eu anomaly, whereas the silt and clay fractionshave different extents of negative Eu anomalies. This likely isbecause of limited chemical weathering in the source rocks, as

can be seen in the lower CIA values (calculated after Nesbittand Young, 1982, 1984) for the samples (Table 1). Under theprevalent physical conditions of weathering in the catchment(seasonal rain and largely semiarid climate), mafic mineralsrich in REE seem to break down first and form secondarymineral phases, as evidenced by the ubiquitous red colorationof soil and regolith. This also results in the concentration ofmafic minerals in the finer fractions. Eu enriched feldspars,along with quartz, are preferentially retained in the coarserfractions in the soils. Thus, positive Eu anomaly in the sandfraction, as well as in bed sediments, likely is due to feldsparbecoming a major REE contributing phase. This is also cor-roborated by the X-ray diffraction study of minerals in differentsize fractions, which show a decrease in finer fractions of bothfeldspar, and, in particular, of quartz. Similar development of

Fig. 2. Chondrite-normalised REE distribution diagram for sandy bed sediments from the floodplain of the Kaveri Riverbasin and average for two probable source rocks, charnockite and gneisses . Note: The samples with phi values of less than2.9 (Table 1) have no Eu anomaly and have lower overall REE abundance.


positive Eu anomaly due to feldspar (mainly plagioclase) en-richment in Devonian Baldwin formation of Australia wasfound by Nance and Taylor (1977) and McLennan (1989).Major and trace element chemistry of different size fractions(Table 4) show an increase in the concentrations of Fe, Mg, Ti,Ni, Cr, and Zr (all associated with mafic and heavy mineralphases) in the finer fractions. Again, we find that in the silt sizefractions, abundance of Ti and Zr are relatively high, indicatingconcentrations of heavy minerals. Therefore, in the finer frac-tions, mafic and heavy minerals contribute significantly to theREE budget and mask the influence of feldspar on the Euanomaly of sediments. Under conditions of intense chemicalweathering, feldspars also transform into clays or are reducedin size and thus are lowered in abundance in the coarse sandfraction (Cox and Lowe, 1985; Condie et al. 1995). In suchsituations, heavy minerals in sand impose their REE patterns,resulting in negative Eu anomaly in sands.

The (Ce/Yb)N ratio in different size fractions range between7.0 and 12.2, a feature also seen in the bulk sediments. Thisvariation in (Ce/Yb)N ratios as in the bulk, is largely due to thevariation in (Gd/Yb)N ratios, as can be seen by larger variationin (Gd/Yb)N values ranging between 1.6 and 2.5. On thecontrary the (Ce/Sm)N values vary in the narrow range of 3 to3.7. Thus the observed variation in (Ce/Yb)N ratios is probablydue to the variable accumulation of mafic and heavy mineralsin different size fractions. To evaluate this we have analysedthe REE in heavy and light mineral fractions of different sizefractions, as discussed earlier.

Chondrite-normalised ratios of REE in heavy and light min-eral fractions of various size fractions are plotted in Figure 8.The heavy mineral fractions have much higher REE abundanceand are less fractionated compared with the light mineral frac-tions. They also have negative Eu anomalies in all size frac-tions. There is a rapid increase of REE abundance with de-

Fig. 3. Chondrite-normalised REE plot for silty sediments (.4 phi units) from the floodplain of Kaveri River basin andfor two probable source rocks, charnockite and gneisses.


creasing size, particularly in the light mineral fractions. Therelative contributions of heavy and light minerals to the bulkREE seem to vary with grain size. We estimate from a massbalance consideration that in the sand fraction, heavy mineralscontribute to an extent nearly 50 to 75% to the total REE. In thefiner fractions, however, heavy minerals contribute only to anextent of 24 to 66%. This is despite the fact that the heavyminerals have much higher concentration of REE in the finerfractions. Therefore, we infer that a very large part of REEseems to be present in the silicate minerals and associatedphases in the finer fractions.

There is a five- to sixfold increase in the REE abundancefrom fine sand to fine silt in the light fractions. Mineralogicalchanges associated with fining grain size, such as increase infeldspar:quartz ratio, could hardly account for this magnitudeof increase in REE abundance. More accessory minerals beingpresent as inclusions in the finer fractions is also unlikely, and

we do not expect concentrations of primary REE holdingmineral in the light fractions. We also observe flattening ofREE patterns with increasing abundance, i.e., decreasing grainsize. Based on these observations, we suggest that a large partof the REE in the fine-grained silicate particles could likelyoccur as coatings on grain surfaces. In the heavy mineralfraction as well, there is no discernable change in the mineral-ogy (as seen in X-ray diffractograms) with grain size change.Although the presence of silicate matrix to the heavy mineralsin the coarser fraction could cause some reduction in the REEabundance, the factor of 4 to 5 times variation cannot beexplained by the mineralogical changes. Therefore, as in caseof the light fraction, a major part of the REE in the finer fractionof the heavy minerals could be present as coatings on the largesurface area provided by the fine-grained particles. Fe-Mnoxide coating on the grains in soils and sediments of the studyarea is observed to have very high concentrations of REE (five-

Fig. 4. Chondrite-normalised REE distribution pattern for averages of sandy bed and silty bed floodplain sediments, alongwith those of shale, post-Archaean Australian shale (PAAS), upper continental crust (UCC), and bed load sediments. Note:The patterns for average of various floodplain sediments are comparable to shale, PAAS, and UCC.


Fig. 5. Shale-normalised REE distribution diagram for averages of sandy bed, silty bed, and bed load sediments from theKaveri River.

Fig. 6. Harker variation diagram of TiO2 with respect to (a) Ce, (b) Yb, (c) TREE; and between (d) Zr and Yb, (e) clayand Ce, and (f) (Ce/Yb)N and (Gd/Yb)N ratios.


to sixfold compared with bulk sediment) with patterns similarto the bulk sediment (Aditi Rawat, personal communication).

The control of titanite on the REE abundance of sedimentswas discussed above. This mineral is also known to be suscep-tible to weathering, even less resistant than plagioclase ingranitic rocks (Condie et al., 1995). During weathering, titanitemay either alter to form a new secondary mineral or breakdown to release its constituent elements, such as Ca, Ti, Si, andREE. Of these the Ti and REE are known to be relativelyimmobile and are not expected to leave the system. Therefore,even after the transformation or break down of titanite, thecorrelation will still be maintained between Ti and REE if itwas the major REE contributing mineral in the source grano-

dioritic rocks. The released immobile constituents could bepresent as coatings on grains. Because with decreasing grainsize the total surface area increases, finer fraction can accom-modate more of coatings, irrespective of the nature of minerals(i.e., light or heavy). Condie et al. (1995) also have shown thathornblende in granodioritic rocks is very susceptible to weath-ering, even more so than titanite. Fe hydroxide or oxyhydrox-ides are products of hornblende weathering and are precipitatedas coatings on mineral grains, imparting ubiquitous red color tothe soils and sediments. REE are known to have strong affinityfor oxyhydroxides of Fe (Fleet, 1984; Bau, 1999). Also in thecatchment of the river, Sharma and Rajamani (2000a, 2000b)reported that Fe and REE show similar enrichment in thesaprolites and regoliths developed on charnockites and gneis-ses. Therefore, we note that secondary, amorphous mineralcoatings (such as oxyhydroxides of Fe) on clastic sedimentscould potentially constitute a major host for REE in clasticsediments in addition to heavy minerals and clay minerals.

5. DISCUSSION

Chemical composition of the clastic sediments is the netresult of a number of geological factors. These include, amongother factors, source rocks and the intensity of their chemicalweathering, the rate of sediment supply, and sorting (bothtextural and mineralogical) during transportation and deposi-tion (Piper, 1974; McLennan 1989; Cox and Lowe, 1995). Eachof these factors must be evaluated before drawing conclusions

Table 3. Correlation coefficient among various REE, TiO2, Zr, clay% and mean size (Mz) for selected floodplain sediments of KaveriRiver.

REE TiO2 Zr Clay Mz

Ce 0.84 0.45 0.55 0.49Nd 0.77 0.29 0.49 0.49Sm 0.78 0.39 0.60 0.53Eu 0.33 0.32 0.60 0.61Gd 0.71 0.48 0.63 0.52Dy 0.65 0.38 0.60 0.53Er 0.60 0.57 0.31 0.35Yb 0.60 0.6 0.56 0.45

Note: REE: rare earth element.

Table 4. Major (%), trace and rare earth element (ppm) composition of different size fractions for selected sediment samples (PR3E; PR7E) fromKaveri River floodplain.

Element

PR3E2(0.125–

0.25 mm)

PR3E3(0.063–

0.125 mm)

PR3E4(0.063–

0.037 mm)

PR3E5(0.037–

0.002 mm)PR3E6

(,0.002)

PR7E2(0.125–

0.25 mm)

PR7E3(0.063–

0.125 mm)

PR7E4(0.063–

0.037 mm)

PR7E5(0.037–

0.002 mm)PR7E6

(,0.002)

SiO2 75.6 73.0 70.6 56.7 45.8 75.2 71.3 64.1 61.7 51.2TiO2 0.5 0.7 0.9 1.1 0.5 0.2 0.9 2.8 0.7 0.7Al2O3 12.7 13.5 14.9 16.6 20.4 12.9 13.1 12.9 16.7 19.8FeO 2.6 2.6 3.0 5.3 10.7 1.9 4 9.1 9.5 10.0MnO 0.0 0.1 0.1 0.1 0.2 0 0.1 0.2 0.1 0.2MgO 1.0 1.1 1.1 1.4 1.2 1.1 1.8 2.4 3.6 4.0CaO 3.0 3.4 3.6 3.4 0.9 3.4 4 3.2 3.5 1.5Na2O 2.5 4.2 3.9 2.3 0.5 2.8 2.9 3.1 2.5 0.4K2O 2.2 2.6 2.7 1.9 1.1 2.5 2 1.9 1.7 1.2P2O5 0.1 0.1 0.1 0.1 0.2 0.1 0.1 0.2 0.2 0.1Total 100.2 101.1 100.8 88.9 81.5 100.1 100.2 99.9 100.2 89.0Ni 21 31 25 78 72 40 89 112 250 285Cr 119 128 149 245 334 65 156 340 315 312Ba 560 594 676 663 386 761 721 591 715 207Sr 381 414 463 416 219 490 462 431 298 64.3Zr 207 306 582 1194 147 53 131 810 235 46Y 12 14 20 34 35 9 19 45 29 15Ce 32.1 48.2 85.1 126.0 80.2 27.7 66.3 117.1 103.0 109.0Nd 11.5 16.8 27.5 46.3 34.7 11.6 27.8 45.7 32.0 35.5Sm 2.4 3.4 5.5 9.3 6.4 2.2 5.1 8.8 6.5 6.8Eu 1.0 1.2 1.7 2.2 1.5 1.1 1.5 1.5 1.8 1.6Gd 2.4 3.2 5.6 9.2 6.1 2.0 4.7 8.3 7.0 6.1Dy 1.7 2.2 3.8 7.1 4.9 1.6 4.0 5.4 3.9 4.5Er 1.1 1.4 2.8 4.5 3.1 1.0 2.5 3.9 2.6 2.7Yb 1.1 1.4 2.7 4.6 2.4 0.8 2.4 4.2 2.3 2.3TREE 53.2 77.8 134.8 209.3 139.3 47.8 114.2 195.0 159.1 168.5(Ce/Yb)N 7.6 8.7 8.0 7.1 8.4 8.4 7.0 7.2 11.5 12.2(Ce/Sm)N 3.2 3.3 3.6 3.2 3.0 3.0 3.1 3.1 3.7 3.8(Gd/Yb)N 1.8 1.8 1.7 1.6 2.0 1.9 1.6 1.6 2.5 2.1


Tab

le5.

RE

Eco

mpo

sitio

n*in

heav

ym

iner

alfr

actio

n(H

)an

dlig

htm

iner

alfr

actio

n(L

)of

diffe

rent

size

frac

tions

oftw

obu

lkse

dim

ent

sam

ples

(PR

3Ea

ndP

R7E

).

PR

3E2L

(0.1

25–

0.25

mm

)

PR

3E3L

(0.0

63–

0.12

5m

m)

PR

3E4L

(0.0

63–

0.03

7m

m)

PR

3E5L

(0.0

37–

0.00

2m

m)

PR

3E2H

(0.1

25–

0.25

mm

)

PR

3E3H

(0.0

63–

0.12

5m

m)

PR

3E4H

(0.0

63–

0.03

7m

m)

PR

3E5H

(0.0

37–

0.00

2m

m)

PR

7E2L

(0.1

25–

0.25

mm

)

PR

7E3L

(0.0

63–

0.12

5m

m)

PR

7E4L

(0.0

63–

0.03

7m

m)

PR

7E5L

(0.0

37–

0.00

2m

m)

PR

7E2H

(0.1

25–

0.25

mm

)

PR

7E3H

(0.0

63–

0.12

5m

m)

PR

7E4H

(0.0

63–

0.03

7m

m)

PR

7E5H

(0.0

37–

0.00

2m

m)

Ce

15.7

22.7

45.6

79.7

197.

232

9.3

680.

163

7.6

15.2

17.3

38.1

83.5

127.

121

3.2

522.

311

18.9

Nd

4.8

7.5

15.1

3685

.912

5.2

278.

522

8.9

4.7

5.1

14.7

29.9

6810

0.8

200.

936

1.3

Sm

1.4

1.7

4.2

617

.326

.154

.742

.80.

81.

12.

75.

513

.819

.134

.767

.4E

u0.

500.

800.

901.

202.

904.

1010

.30

6.40

0.78

0.74

0.94

1.05

2.55

3.38

4.78

7.24

Gd

0.9

1.5

2.6

4.7

16.8

25.6

53.9

40.8

0.7

0.9

2.3

4.6

13.3

18.1

30.3

55.6

Dy

0.7

1.1

2.4

5.6

14.1

18.3

47.1

35.4

0.5

0.6

1.8

3.1

11.4

15.6

25.1

38.1

Er

0.4

0.6

1.7

3.9

9.3

12.2

3225

.10.

30.

41.

31.

97.

210

.418

.230

Yb

0.4

0.5

1.8

3.6

9.1

12.2

32.1

26.5

0.3

0.2

0.9

1.7

6.1

9.3

1726

.3

Not

es:

RE

E:

rare

eart

hel

emen

t.*

Val

ues

are

inpp

m.


on the nature of source rocks and tectonics of the region usingthe chemistry of clastic sediments. In this discussion, we firstconsider compositional features of the sediments that may haveresulted from chemical weathering and fluvial process of sort-ing. Then we attempt to evaluate the utility of floodplainsediments in the provenance study and speculate the tectoniccondition of the region that supplied the sediments.

5.1. Influence of Weathering on REE Chemistry

The catchment area of the Kaveri River is characterized bytwo special features: (1) the rain shadow nature of the regionwith only limited southwest monsoonal rains for only 3 monthsin a year and (2) exposure of mid to lower crustal granitic rockssuch as high hills and mountains. These rocks are known to

Fig. 7. Chondrite-normalised REE plot for different size fractions of two selected samples, PR3E and PR7E. Note: Thepatterns for all the size fractions are similar except for the Eu anomaly, which is positive in sand fraction, gradually flattens,and then becomes negative in the silt and clay fractions.


have undergone only limited chemical weathering (Sharma andRajamani 2000a, 2000b). The floodplain sediments of the riverhave mineralogical and geochemical features that are also sug-gestive of only low degree of chemical weathering of thesource rocks (Singh and Rajamani, 2001). The chondrite-nor-malised REE patterns of the river sediments and average shaleare very similar (Fig. 4), whereas the shale-normalised REEpatterns for the river sediments show a small fractionation (Fig.5). Compared with the suspended sediments from many largerivers of the world (Goldstein and Jacobson, 1988; Sholkovitz,1988) these floodplain sediments are much less fractionated inthe shale-normalised plot. This could possibly be related to thelow degree of chemical weathering of the rocks because HREE

are known to be preferentially mobilized during higher degreeof weathering (Nesbitt, 1979; Sholkovitz, 1988). In the shale-normalised plot, Eu in all sediments and Gd only in the flood-plain sediments show relative enrichment (Fig. 5). These fea-tures are not readily understood. Because Eu21 and Gd31 havecompletely half-filled 4f orbital configurations, their behaviourduring weathering could be redox controlled in ways that are asyet poorly understood (Brookins, 1989). Alternatively, it couldbe a result of selective dissolution and reprecipitation of amor-phous apatite during protolith weathering (Ohr et al., 1994). Ingeneral, the geochemical behaviour of REE during rock weath-ering under different physical conditions is not well under-stood.

Fig. 8. Chondrite-normalised plot of REE in heavy minerals (H) and light minerals (L) separated from different sizefractions of the two samples (PR3E and PR7E). 2: fine sand; 3: very fine sand; 4: coarse silt; 5: fine silt.


The different size fractions in a bulk sample have almostsimilar REE patterns but variable abundance and Eu anomalies(Fig. 7). The sand fraction with a lower REE abundance wasseen to have a positive Eu anomaly, whereas the fine silt andclay fractions with a higher abundance have negative Eu anom-aly. This we attribute to the cumulative effect of low degree ofchemical weathering and formation of REE rich secondaryphases occurring as coatings on fine-grained particles. Coarse-grained sand fraction such as the bed load sediments thatpreferentially retain feldspar and quartz relative to mafic min-erals in case of low weathering, as discussed earlier, andaccommodate lower grain coatings shows a positive Eu anom-aly because the effect of feldspars is not completely masked. Infiner grained clastics, however, the feldspar effect is maskedresulting in negative Eu anomalies in the floodplain sediments.

We note here that sand fractions in river sediments areknown to have positive and negative Eu anomalies (Cullers etal., 1987; Cullers, 1988). Under conditions of intense chemicalweathering, feldspars may be almost completely transformed tosecondary clay minerals or at least undergo size reduction (Coxand Lowe, 1995). This results in their incorporation into finerfractions. The sand fraction of sediments derived from suchintensely weathered sources will have insufficient feldspars tomask the effect of heavy minerals on the Eu anomaly. Thisreduction in feldspar contents could lead to a negative Euanomaly in the sand. Therefore, we suggest that the Eu anom-aly in sand, which ultimately gives rise to sandstone, is notalways representative of its source rock characteristics and, ifused at all, the weathering status of sediment sources needs tobe evaluated first.

5.2. Effect of Sedimentary Processes on REEGeochemistry

Once the soil formed over the catchment is taken into astream, fluvial processes of transportation, sorting, and depo-sition affect the chemical characteristics of clastic sediments.Sediments are carried and deposited under different fluvialregimes at different sites. The mineral components being de-posited in different sites will depend on the extent of sourceweathering, the size and hydraulic property of the minerals, andthe energy of the stream carrying them. The fluvial processescan lead to homogenization of sediments near the source (Nes-bitt, 1979) and their differentiation on longer transportation(McLennan, 1989).

In the Kaveri River alluvium, we observe a strong texturaldependency of chemical composition. Concentration of Fe, Mg,Mn, Ni, Cr, and particularly REE increase with decreasinggrain size. In normal flow conditions, the river carries sedi-ments as suspended and bed loads. The suspended load ismostly composed of silt and clay and thus will tend to havehigher concentration of all elements, including REEs, exceptsilica, as seen in Table 4. Similarly, the bed load, composedmainly of sand, has much lower concentration of many ele-ments except for SiO2, Na2O, CaO and K2O (Table 2) becauseof higher contents of quartz and feldspar in them. The flood-plain sediments here, however, have intermediate concentra-tions of all elements including REE. This is because of physicalunsorting at the time of flooding, which leads to the depositionof sediments intermediate in texture between suspended and

bed load sediments. During the time of flood, a part of the bedload sediment under normal flow condition is brought intosuspension because of higher energy conditions. Normal bedload and suspended load sediments get mixed and are depositedon to the floodplain at the time of flooding. This leads to adilution of chemistry of the suspended load due to the incor-poration of quartz- and feldspar-dominated bed load sediments,resulting in a chemistry of floodplain sediments intermediatebetween that of the suspended and bed load. Thus, we see thatfluvial processes lead to chemical differentiation, extremes ofwhich are represented by suspended and bed load sediments. Incontrast, we also find that fluvial processes could lead tochemical homogenization in the sediments deposited over thefloodplain.

5.3. Provenance

The above discussion raises some questions on the utility ofsediment chemistry in interpreting the source rocks. We havediscussed how weathering intensity of source rocks may lead tomodification of their REE patterns, especially of Eu anomaly,and how sorting may lead to modification of the abundance. Itis necessary to look for such a sediment population where theeffect of sediment sorting is minimized so as to keep thesignature of the source from getting modified to a great extent.In the discussion below, we attempt to evaluate the potential offloodplain sediments for provenance studies.

All the samples of floodplain sediments of Kaveri Riverbasin show only a limited variation in their (Ce/Yb)N ratios,which suggests that the sediments were derived from onecommon source; if they were derived from more than onesource, the mixing was very efficient even among differentsizes. Their REE patterns, especially the LREE patterns, areuniform for a distance of 2˜50 km sampled along the river.HREEs show some variation, which may be due to variation inthe abundance of heavy minerals. The two dominant sourcerocks, the gneisses and the charnockite, have very differentREE patterns (Fig. 2). The gneisses are strongly fractionatedwith the average (Ce/Yb)N ratio of 25, whereas the charnock-ites are less strongly fractionated and have (Ce/Yb)N ratio of 10(Table 3). The gneisses are highly depleted in the HREE with(Gd/Yb)N value ranging between 2.5 and 3.2, whereas thecharnockites are less depleted in HREE (Fig. 2). The gneissesalso have positive or no europium anomaly, whereas the char-nockites have negative europium anomaly with Eu/Eu* rangingbetween 0.56 and 0.75.

The floodplain sediments are closely related to the charnoc-kitic source rocks except for the slight HREE enrichment in thesediments, which probably is due to grain coatings derivedfrom mafic minerals and concentrations of heavy minerals. Onaverage the sediments also have higher concentration of Ti, Zr,Ni, and Cr compared with the charnockites. We observe thatfrom the source with a negative Eu anomaly, sands with pos-itive Eu anomaly are derived. To obtain sediments with thepattern of the source, it is essential that all the three components(i.e., the sand, silt, and clay) again are mixed so as to achievethe net concentration and pattern similar to that of the source.This is accomplished during the flood period, as discussed in aprevious section. This can happen only when the extent of


source rock weathering is small enough not to affect signifi-cantly the REE patterns in different size fractions.

The similarity in the chemistry of flood the plain sedimentsto that of the charnockites of the catchment area suggests thatthe latter formed the major source to the sediments, despite thefact that gneisses are more prone to weathering compared withthe charnockites. This is because charnockites in the field,which form high hills, are exposed, whereas gneisses form thelow-lying areas. Because of higher elevation (;2500 mts) andsteeper slopes, the regolith formed over charnockites does notstay there for long and is stripped off immediately by rainwaterdown into the river. In contrast, the soil formed over thegneisses, due to its flat topography, stays for a long time and atmost gets collected into the intermontane basins formed be-tween the surrounding hills. The chemical homogeneity, lowchemical index of alteration, the REE patterns and abundances,and the preservation of primary mineralogy of the source rocksall suggest that the soils in the catchment area suffered littlechemical weathering, were dominantly formed by physicalweathering, and were subjected to rapid erosion (Sharma andRajamani, 2000). The higher elevation of the charnockite hillsand the fresh nature of sediments derived from them, despitethe fact that deep crustal charnockites are;2500 Ma old, implythat they must have been only recently uplifted for denudation.

6. CONCLUSION

We conclude the following from the data and the discussionpresented here:

1. In the Kaveri floodplain sediments derived predominantlyfrom dioritic to granodioritic protoliths, the mineral titaniteor its weathered products seem to have controlled their REEchemistry.

2. A large part of the REE in the clastic sediments could bepresent as surface coatings on grains, in addition to heavyand clay minerals.

3. The REE patterns of clastic sediments are affected by thedegree of source rock weathering and also by the fluvialprocesses of sorting and mineral differentiation. Thus, REEpattern and, in particular, Eu anomaly of sand or sandstoneformed by their diagenesis are not good indicators of prov-enance and should always be studied in association withweathering status to be evaluated from their bulk composi-tion.

4. Physical sorting by fluvial processes may lead to variableaccumulation of various minerals in different size fractions,thus affecting their REE chemistry and patterns. For prov-enance study, sediments having all granulometric gradesshould be preferred because they most closely represent thesource, especially when the source region suffered littlechemical weathering.

5. The floodplains sediments of the Kaveri River were derivedfrom the high standing hills of Archean charnockites be-cause of their recent uplift and denudation.

Acknowledgments— This research was supported by a grant from theCouncil of Scientific and Industrial Research, New Delhi, to VR and bya Research Associate fellowship to PS from the Council of Scientificand Industrial Research, respectively. The authors thank Dr. S. M.McLennan (A.E.), Dr. E. R. Sholkovitz, and two anonymous refereesfor their helpful reviews.

Associate editor:S. McLennan

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