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Geochemistry of Late Miocene Sandstones
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JOURNAL OF SEDIMENTARY RESEARCH, VOL. 74, NO. 2, MARCH, 2004, P. 285–297Copyright q 2004, SEPM (Society for Sedimentary Geology) 1527-1404/04/074-285/$03.00
GEOCHEMISTRY OF SANDSTONES FROM THE UPPER MIOCENE KUDANKULAM FORMATION,SOUTHERN INDIA: IMPLICATIONS FOR PROVENANCE, WEATHERING, AND TECTONIC SETTING
J.S. ARMSTRONG-ALTRIN,1 YONG IL LEE,2 SURENDRA P. VERMA,1 AND S. RAMASAMY3
1 Centro de Investigacion en Energıa, Universidad Nacional Autonoma de Mexico (UNAM), Priv. Xochicalco S/No., Col. Centro, Apartado Postal 34,Temixco, Morelos 62580, Mexico
e-mail: [email protected] and [email protected] School of Earth and Environmental Sciences, Seoul National University, Seoul 151-747, South Korea
3 Department of Geology, University of Madras, Guindy Campus, Chennai 600 025, India
ABSTRACT: Petrographic, major, trace, and rare earth element com-positions of sandstones from the upper Miocene Kudankulam For-mation, Southern India, have been investigated to determine theirprovenance, tectonic setting, and weathering conditions. All sandstonesamples are highly enriched in quartz (Q) but poor in feldspar (F) andlithic fragments (L). The major-element concentrations of these sand-stones reveal the relative homogeneity of their source. Geochemically,the Kudankulam sandstones are classified as arkose, subarkose, lith-arenite, and sublitharenite. The CIA values (chemical index of alter-ation; mean value ; 44.5) for these sandstones and the A–CN–K di-agram suggest their low-weathering nature. Similarly, their Fe2O3* 1MgO (mean ; 2.7), Al2O3/SiO2 (; 0.09), K2O/Na2O (; 2.2) ratiosand TiO2 contents (; 0.3) are consistent with a passive-margin setting.The Eu/Eu* (; 0.5), (La/Lu)cn (; 21), La/Sc (; 5.9), Th/Sc (; 1.9),La/Co (; 5.7), Th/Co (; 1.8), and Cr/Th (; 5.3) ratios support afelsic source for these sandstones. Chondrite-normalized REE patternswith LREE enrichment, flat HREE, and negative Eu anomaly also areattributed to felsic source-rock characteristics for Kudankulam sand-stones. Total REE concentrations of these sandstones reflect the vari-ations in their grain-size fractions. The source rocks are probably iden-tified to be Proterozoic gneisses, charnockites, and granites of the Ker-ala Khondalite Belt, which must have been exposed at least since thelate Miocene. Finally, the unusual Ni enrichment in the Kudankulamsandstones, unaccompanied by a similar enrichment in Cr, Co, and V,may be related to either the presence of pyrite in the sandstones or,more likely, the fractionation of garnet from the source rocks duringtransportation.
INTRODUCTION
Sedimentary rocks are principal sources of information concerning pastconditions on the Earth’s surface. Clastic rocks may preserve detritus fromlong-eroded source rocks and may provide the only available clues to thecomposition and timing of exposure of such source rocks. Geochemistryof sedimentary rocks may complement the petrographic data, especiallywhen the latter are ambiguous. The geochemical composition of sedimen-tary rocks is a complex function of various variables such as source ma-terial, weathering, transportation, physical sorting, and diagenesis (Middle-ton 1960; Piper 1974; Bhatia 1983; McLennan 1989; Cox and Lowe 1995).Examples of using geochemical data from sediments for understanding sed-imentary processes such as weathering, provenance, diagenesis, sorting, andrecycling are increasing in the literature because of the sensitiveness ofsome key trace elements in identifying minor components that are not read-ily recognized petrographically (e.g., Hiscott 1984; Garver et al. 1996).
Several trace elements, such as the rare earth elements (REE; e.g., La,Ce, Nd, Gd, Yb), Y, Th, Zr, Hf, Nb, and Sc are most suited for discrimi-nations of provenance and tectonic setting because of their relatively lowmobility during sedimentary processes and their short residence times inseawater (Holland 1978; Taylor and McLennan 1985). These elementsprobably are transferred quantitatively into clastic sediments during weath-ering and transportation, reflecting the signature of the parent materials,
and hence are expected to be more useful in discriminating tectonic envi-ronments and source-rock compositions than the major elements (Bhatiaand Crook 1986; McLennan 1989; Condie 1993).
Rocks of southern India are older than 2500 Ma, and they are regardedto have been first exposed to the surface possibly during the Tertiary and,at places, as late as the Quaternary (Singh and Rajamani 2001a, 2001b).Geochemical characterisitics of Archean terranes are likely to be substan-tially different than in younger environments. For example, granitic rocksformed during the Archean are more commonly Na- and plagioclase-richgranodiorites–tonalites, whereas during the Phanerozoic they tend to bemore K- and K-feldspar-rich granodiorites–monzonites–granites (Taylorand McLennan 1985; Goodwin 1991). Thus, sedimentary rocks of any age,derived primarily from Precambrian terranes, may be influenced by thesedifferences. On the basis of geochemistry of flood-plain sediments of theCauveri River, southern India (Fig. 1A), Singh and Rajamani (2001a) in-terpreted that exhumation of Archean deep crustal rocks occurred in geo-logically recent times. The purpose of this study is to identify the prove-nance and to test the neotectonic activity in southern India by examininggeochemisty of upper Miocene–Pliocene sandstones cropping out in thesouthern tip of the Indian Peninsula. To know the probable source rock forthe Kudankulam sandstones we compared our data with the gneisses, char-nockites, and granites of Proterozoic Kerala Khondalite Belt, southern India(Chacko et al. 1992; Braun et al. 1996), which is located very near to ourstudy area (see the rectangle marked Kerala Khondalite Belt in Fig. 1A).
In addition, the geochemical features of clastic sedimentary rocks reflectnot only the nature and proportion of detrital components but also thechemical characterisitcs of authigenic minerals formed during diagenesis.Understanding the relative importance of these factors through geochemicaltools extends the knowledge of the processes that produced and affectedrock deposition and lithification (e.g., Fralick and Kronberg 1997 and ref-erences therein). Enriched concentrations of certain trace elements such asNi, Cr, Co, and V are very important for discrimination of provenance andtectonic setting. Some studied Kudankulam sandstones have enriched Nivalues but low contents of Cr, Co, and V. Detrital and/or diagenetic control,and fractionation during transportation, on this peculiar geochemical featurewill be also addressed in this paper.
GEOLOGY AND STRATIGRAPHY OF THE STUDY AREA
The Kudankulam area in Tamil Nadu, South India (Fig. 1A), forms thesouthern extension of the Cauveri River Basin and is limited between Ti-ruchendur in the northeast and Kanniyakumari (Cape Comorin) in the south(Lat. 88 59 3099 N to 88 309 1299 N and Long. 778 309 1099 E to 788 1091299 E).
The lithostratigraphy of the Kudankulam Formation comprises the meta-morphosed Proterozoic and Tertiary sedimentary rocks (Paramasivam andSrinivasan 1980). The older rocks include charnockites, quartzofeldspathicgneiss, and granitic gneiss, whereas the Tertiary strata comprise calcareoussandstone and fossiliferous limestone, which are overlain by subrecent torecent soil cover. The sedimentary rocks include both clastic and carbonaterocks with various faunal contents such as molluscs, bryozoans, foramin-ifers, and fragments of red and blue-green algae (Armstrong Altrin Sam et
286 J.S. ARMSTRONG-ALTRIN ET AL.
FIG. 1.—A) Simplified geological map of Southern India showing the location of the study area (modified after Singh and Rajamani 2001b). The rectangle marked KeralaKhondalite Belt refers to the area of probable source rocks (gneisses, charnockites, and granites of Proterozoic Kerala Khondalite Belt; Chacko et al. 1992; Braun et al.1996) for the Kudankulam sandstones. B) Simplified geological map of the Kudankulam area, showing sample locations.
287GEOCHEMISTRY OF SANDSTONES FROM SOUTHERN INDIA
FIG. 2.—Columnar sections of the Kudankulam Formation arranged in SW to NE direction, showing sample locations. Stratigraphic section names are according to thenearest village (Fig. 1B). Blank crossed space in the Manapaud section represents the area with no exposures.
al. 2001). These Tertiary rocks were deposited directly over the Proterozoicrocks, when a considerable proportion of the land along the coast wasoccupied by the sea as a result of transgression during the Late Tertiary.The contact between the older and Tertiary rocks is rather sinuous, sug-gesting that the Tertiary rocks were deposited on a shallow continentalshelf or shoreline gently dipping towards the sea. The Kudankulam For-mation was correlated with Karaikal beds, of late Miocene to Pliocene age(Ramanathan 1979; Paramasivam and Srinivasan 1980). The last marinetransgression in southern India occurred in the early Miocene, and its in-fluence can be observed in Gujarat in the western part of India, and alsoin Kerala in the southern part (Fig. 1A; Bruckner 1988). On the easternflank of the Cauveri Basin in Sri Lanka, the Jaffna Formation is mostprobably the equivalent formation to the Kudankulam Formation, on thebasis of the fossil assemblages and facies characteristics. The presence ofmicrofossils like Austritrillina howchini and Taberina malabarica placesthe Jaffna Formation in the upper part of the upper Miocene, specificallyin the Burdigalian stage (Sahani 1979; Cooray 1984). Therefore, the Ku-dankulam Formation is either of Burdigalian or of late Miocene–Plioceneage.
Armstrong Altrin Sam and Ramasamy (1997, 1999) discussed the pe-trography, stratigraphy, and depositional history of the Kudankulam For-mation. The authors subdivided this formation into five lithostratigraphicunits: algal limestone, sandy shell limestone, silty clay, clastic limestone,and calcareous sandstone (Fig. 1B). The facies association and the sedimentcomposition show that the Kudankulam carbonates were deposited in ashallow marine nearshore environment (Ramasamy and Armstrong AltrinSam 1998). A preliminary stable-isotope study on the Kudankulam lime-stone reveals that fresh-water circulation played a major role in diagenesis(Armstrong Altrin Sam et al. 2001).
SAMPLING AND METHODS
From eight sections in the Kudankulam area (Fig. 2) fresh rock sampleswere collected from outcrops exposed in stream cuts and road cuts andwere washed thoroughly in distilled water to remove dust contamination.The samples were disaggregated by following the procedure adopted inCox and Lowe (1996). Grain-size analysis was carried out in a Ro-Tapsieve shaker using American Society for Testing and Material (ASTM)sieves ranging from—1.5 f to 4.25 f at 0.50 f intervals for 20 minutes(Folk 1966). Cumulative curves were constructed to calculate the statisticalgrain-size parameters (mean grain size and sorting values) by applying theequations of Folk and Ward (1957). Twenty thin sections were selected fordetailed petrographic study. Four hundred framework grains were countedfrom each thin section. Matrix and cement were not counted. The pointcounts were done using both Gazzi–Dickinson (Gazzi 1966; Dickinson1970) and traditional methods.
Forty-five samples were analyzed for major oxides using an analyticalmethod adopted from Shapiro and Brannock (1962) and Shapiro (1975).Aliquots of 50 mg samples were fused with NaOH in a nickel crucible,and one aliquot, solution A, was prepared for SiO2 and Al2O3 determina-tion. Another aliquot, solution B, was prepared by digesting the sampleswith HF 1 HCl 1 HNO3 acid mixture to estimate Na2O, K2O, Fe2O3*,CaO, MgO, TiO2, and MnO. Total iron, silica, alumina, titania, and man-ganese were determined using a Bausch and Lomb Spectronic 20 spectro-photometer. Calcium and magnesium were determined by a titration meth-od using EDTA with screened calcite and O-cresolpthalein complexionindicator. Sodium and potassium were analyzed by using an Aimil flamephotometer calibrated using standard salt solutions. For the determinationof CaO in the silicate fraction, samples were separately treated with 1M
288 J.S. ARMSTRONG-ALTRIN ET AL.
TABLE 1.—Major-element concentrations in weight percent (wt. %) for sandstones of the Kudankulam Formation along with their mean grain size (MZ) and samplestandard deviation (S) in f units, Chemical index of alteration (CIA, Nesbitt and Young 1982), and Plagioclase index of alteration (PIA, Fedo et al. 1995).
Rock Type
Sample #
Arkose
E4 G2 F8 A2 D4 D2 C2 F5 E9 H8 E14 E3 D3 C6 E8 D1 H15
MZ
SSiO2
TiO2
Al2O3
Fe2O3*MnOMgOCaONa2O
1.021.96
63.980.387.540.550.011.20
13.610.53
1.031.81
46.880.244.121.210.200.75
20.810.78
1.081.76
47.200.334.621.060.030.58
17.711.02
1.261.74
65.800.456.080.580.011.05
13.501.02
1.381.85
60.400.325.910.990.011.65
16.400.56
1.641.78
49.800.255.661.080.031.27
21.500.58
2.001.55
54.200.325.211.100.031.04
18.701.01
2.061.39
52.470.406.121.770.030.78
14.620.40
2.081.40
49.790.144.780.510.020.91
19.920.78
2.251.45
60.720.279.322.310.020.66
13.461.20
2.411.37
56.810.336.981.080.010.56
17.521.05
2.471.20
56.210.265.121.210.020.71
14.500.74
2.631.35
52.740.355.381.530.011.05
16.160.56
2.651.00
53.730.176.970.820.030.64
15.210.39
2.750.90
50.980.366.751.650.030.61
16.581.30
2.860.94
45.330.185.641.360.020.88
24.770.75
3.001.01
62.290.487.000.810.031.21
13.520.47
K2OLOISumCaO*CIAPIAK2O/Al2O3
K2O/Na2OSiO2/Al2O3
Al2O3/SiO2
Fe2O3* 1 MgO
1.5510.0199.363.94
43.7042.180.212.938.490.121.96
2.2022.7599.76
0.7245.3140.15
0.532.82
11.380.092.55
2.1524.9699.66
2.0537.4029.79
0.472.11
10.220.102.20
1.689.50
99.672.79
41.5038.70
0.281.65
10.820.091.81
1.9012.10
100.242.63
43.2340.32
0.323.39
10.220.103.00
2.0918.00
100.262.47
42.3438.43
0.373.608.800.112.86
1.7116.2199.531.77
43.6340.770.331.69
10.400.102.57
1.6521.78
100.022.69
45.4943.850.274.138.570.123.26
1.2621.90
100.011.61
46.1644.790.261.62
10.420.101.82
3.208.75
99.913.04
45.9543.840.342.676.520.153.26
1.5113.7999.641.98
50.0750.070.221.448.140.121.91
1.7819.85
100.401.89
43.7640.700.352.41
10.980.092.38
1.7819.9599.511.73
47.3045.920.333.189.800.103.24
0.8121.45
100.222.89
50.1250.140.122.087.710.131.85
2.7619.1499.891.34
48.6747.670.412.687.550.132.80
2.2518.86
100.041.29
48.3947.240.403.008.040.122.76
1.6012.94
100.352.41
50.4150.540.233.408.900.112.31
Rock Type
Sample #
Arkose
E6 H10 C3 Mean (n 5 20)
Subarkose
E1 E2 F6 B2 B4 A1 H4 H6 F7 H2 C5 G3
MZ
SSiO2
TiO2
Al2O3
Fe2O3*MnOMgOCaONa2O
3.020.96
62.030.269.582.570.030.85
15.281.68
3.130.90
59.240.168.522.060.020.70
13.491.25
3.250.80
41.500.206.040.870.030.75
27.510.98
2.2 6 0.81.36 6 0.38
55 6 70.29 6 0.10
6.4 6 1.51.3 6 0.6
0.02 6 0.010.89 6 0.28
17 6 4.00.84 6 0.33
1.181.95
48.230.393.780.980.011.21
21.981.05
1.201.92
58.100.274.681.060.011.29
19.000.71
1.201.85
42.450.321.980.610.021.30
30.630.56
1.291.86
43.470.331.650.650.011.80
31.630.45
1.471.25
42.500.282.451.050.010.99
30.010.58
1.501.46
55.120.284.141.180.010.72
14.631.68
1.501.08
48.820.243.012.050.020.73
20.021.02
1.571.10
44.620.212.411.100.010.74
29.160.56
1.681.05
56.200.244.890.710.021.20
18.600.65
1.810.97
53.210.273.010.910.020.48
15.211.03
1.891.01
56.700.264.561.360.010.98
16.000.81
2.010.92
48.280.442.980.650.010.44
19.340.30
K2OLOISumCaO*CIAPIAK2O/Al2O3
K2O/Na2OSiO2/Al2O3
Al2O3/SiO2
Fe2O3* 1 MgO
2.735.52
100.502.60
47.8446.930.281.636.470.153.60
3.3211.1299.88
1.6949.4148.99
0.392.666.950.143.11
2.3319.96
100.171.37
47.6946.16
0.392.386.870.152.02
2.0 6 0.616 6 6
99.95 6 0.322.2 6 0.8
45.9 6 3.444 6 5
0.32 6 0.102.6 6 0.88.9 6 1.6
0.12 6 0.212.6 6 0.6
2.2219.6999.54
0.8539.9829.62
0.592.11
12.760.082.74
1.4613.70
100.282.38
39.8136.070.312.06
12.420.082.71
0.7721.42
100.060.52
42.3138.050.391.38
21.440.052.43
0.7819.1099.870.50
39.8232.820.471.73
26.350.043.03
1.0521.29
100.210.50
44.9641.350.431.81
17.350.062.58
2.1719.8699.790.55
40.3832.250.521.29
13.310.082.38
1.9821.8399.720.63
37.7423.490.661.94
16.220.063.57
1.2220.08
100.110.55
42.6436.190.512.18
18.510.052.30
1.5416.20
100.251.68
45.7843.870.312.37
11.490.092.27
2.0124.06
100.210.96
34.8919.520.671.95
17.680.061.83
2.0417.2699.981.97
39.0332.370.452.52
12.430.082.83
0.7027.31
100.451.19
46.6045.550.232.33
16.200.061.49
Rock Type
Sample #
Subarkose
F3 C7 Mean (n 5 14)
Litharenite
G1 H1 F2 E13 G4 H3 B6 C4 F10 Mean (n 5 9)
Sublitharenite
A3 G5
MZ
SSiO2
TiO2
Al2O3
Fe2O3*MnOMgOCaONa2O
2.740.86
55.300.364.251.240.020.71
19.100.86
3.050.81
58.000.334.680.910.030.65
17.300.69
1.7 6 0.61.29 6 0.43
51 6 80.30 6 0.06
3.5 6 1.11.03 6 0.370.02 6 0.010.95 6 0.38
22 6 60.78 6 0.34
1.791.82
53.210.285.712.150.011.01
16.330.84
2.071.80
50.010.294.501.920.021.03
15.911.51
2.261.82
55.080.095.621.380.010.82
18.030.30
2.731.75
53.490.205.621.530.020.93
18.250.63
2.741.26
61.600.306.022.300.020.79
14.500.63
2.841.01
55.990.305.711.710.010.83
14.370.81
2.860.95
54.290.315.951.970.010.51
17.810.40
3.000.91
59.450.277.432.340.020.82
13.500.95
3.000.93
56.960.416.051.370.011.05
13.080.50
2.59 6 0.441.36 6 0.4355.6 6 3.50.27 6 0.09
5.8 6 0.81.8 6 0.4
0.01 6 0.010.87 6 0.1715.8 6 2.00.73 6 0.36
1.261.80
47.370.152.351.610.020.80
19.380.77
2.020.93
49.100.183.131.680.011.41
23.980.54
K2OLOISumCaO*CIAPIAK2O/Al2O3
K2O/Na2OSiO2/Al2O3
Al2O3/SiO2
Fe2O3* 1 MgO
2.2315.8399.901.04
42.6335.710.522.59
13.010.082.32
2.0114.9099.501.14
46.5143.840.432.91
12.390.081.84
1.6 6 0.619.5 6 3.7
99.99 6 0.291.0 6 0.6
41.7 6 3.535 6 8
0.46 6 0.132.08 6 0.4515.8 6 4.20.07 6 0.01
2.4 6 0.5
1.2519.53
100.321.96
47.5546.84
0.221.499.320.113.91
0.7823.5399.50
1.5042.6341.23
0.170.52
11.110.093.88
0.8517.7199.892.83
46.1545.460.152.839.800.102.68
1.0018.54
100.212.31
47.0746.430.181.599.520.113.01
1.8511.7299.732.85
42.2739.250.312.94
10.230.103.51
1.1018.8999.721.84
49.3249.140.191.369.810.103.14
0.9518.29
100.492.51
48.7748.520.162.389.120.113.02
1.2414.02
100.042.79
48.2247.850.171.318.000.133.67
1.2319.2399.892.37
48.3547.910.202.469.420.113.00
1.14 6 0.3217.9 6 3.4
99.98 6 0.322.3 6 0.5
46.7 6 2.645.9 6 3.40.19 6 0.05
1.9 6 0.89.6 6 0.8
0.10 6 0.013.31 6 0.44
0.7526.5199.711.02
37.4033.010.320.97
20.160.053.29
0.9319.10
100.061.16
43.8741.470.301.72
15.690.063.82
CaO* 5 CaO in silicate phase; Fe2O3* 5 Total Fe expressed as Fe2O3.
289GEOCHEMISTRY OF SANDSTONES FROM SOUTHERN INDIA
FIG. 3.—QFL diagram with tectonic fields of Dickinson and Suczek (1979). Q,total quartz (monocrystalline and polycrystalline grains); F, feldspars (plagioclaseand K-feldspars); L, lithic rock fragments (excluding carbonates). Kudankulam sand-stones (20 selected samples) fall entirely within the field of craton-interior sources.
FIG. 4.—Geochemical classification of Kudankulam sandstones using log(SiO2/Al2O3)—log(Fe2O3*/K2 O) diagram (after Herron 1988).
FIG. 5.—K2O/Na2O—SiO2/Al2O3 bivariate plot for the Kudankulam sandstones.1Sandstone samples from this study; average data for comparison are from 2Pettijohnet al. (1972); 3Chacko et al. (1992); 4Braun et al. (1996); 5Condie (1993); 6Taylorand McLennan (1985). UCC 5 upper continental crust.
cold dilute HCl acid before digestion and were analyzed separately. Ourchemical analyses have precisions better than 5% for all elements deter-mined in our samples. The major-element data were recalculated on ananhydrous (LOI-free) basis and adjusted to 100% before using them invarious diagrams.
Twenty-one representative samples were analyzed for trace-element andREE geochemistry at the Korea Basic Science Institute. Trace-element (Ba,Co, Cr, Cu, Ni, Sc, Sr, V, Zn, and Zr) concentrations were determinedusing a Jobin Yvon 138 Ultrace inductively coupled plasma atomic emis-sion spectrometer (ICP-AES). REE and some additional trace elements (Cs,Hf, Nb, Pb, Rb, Th, U, and Y) were analyzed by a VG Elemental PQIIPlus inductively coupled plasma mass spectrometer (ICP-MS) using amethod given by Jarvis (1988). Analytical precision for trace elements andREE is generally better than 5%. United States Geological Standard MAG-1 was used for calibration. Three analyses were made for each sample andaveraged. For REE discussion we used chondrite normalization factors list-ed in Taylor and McLennan (1985).
RESULTS
Petrography
A textural study was carried out for the Kudankulam sandstones to char-acterize grain-size variations. The mean grain size (MZ expressed in funits) of quartz grains of the sandstones ranges from 1.02 f to 3.25 f(Table 1), suggesting that the sand grains are medium to very fine in size.The standard deviation values of sandstones vary from 0.80 f (moderatelysorted) to 1.96 f (poorly sorted; Table 1).
The framework grains are non-undulatory monocrystalline quartz (Qn),undulatory monocrystalline quartz (Qu), polycrystalline quartz (Qp), K-feldspar, plagioclase, and rock fragments. Quartz is the most abundantframework grain in the sandstones, constituting on average 87% of rockvolume. Among quartz grains, Qn is dominant over Qu. Fluid globules andtiny gas bubbles are present in Qn as transport lines, suggesting their ig-neous origin. Qp is a minor constituent and exhibits straight sub-grainboundaries. Heavy minerals such as garnets are rare; ilmenite and magnetite
are common among the opaque grains. All sandstone samples contain mi-nor amounts of feldspar grains (mean ; 10%). Both orthoclase and pla-gioclase feldspars are present, but K-feldspar dominates. The rock frag-ments are comparatively less abundant, and consist of dominantly sedi-mentary rock fragments. The average quartz–feldspar–lithic fragment(QFL) ratio is Q87:F10:L3. Most of the samples are free of matrix. On aQFL diagram (Fig. 3) the Kudankulam sandstones plot in the field of acraton-interior source, indicating that they were derived from igneoussource rocks (Dickinson and Suczek 1979).
Calcite cement occurs in all sandstones. Three types of calcite cementsare observed: micrite, microsparite, and sparry calcite. Calcite cement pro-duced corrosion on detrital grains, particularly in quartz. Sparry calcitecement is interpreted to have been formed by the movement of groundwater saturated with calcium carbonate.
Major Elements
The major-element concentrations of all Kudankulam sandstones are ar-ranged in Table 1 according to rock type and decreasing mean grain size
290 J.S. ARMSTRONG-ALTRIN ET AL.
TABLE 2.—Trace-element concentrations in ppm for sandstones of the Kudankulam Formation.
Rock Type
Sample #
Arkose
E4 A2 D4 D2 C2 H8 D1 E6 H10 C3
Subarkose
E2 F6 B2 B4
Mz
BaCoCrCsCu
1.0230.351.325.880.057.06
1.2638.20
1.7413.73
0.087.08
1.38151.0
1.0421.00
0.0622.70
1.6441.20
3.0114.68
0.0811.20
2.00101.0
2.6410.30
0.2016.40
2.25463.1
2.972.423.96
17.40
2.86170.8
3.0527.980.204.25
3.02143.1
5.3624.870.38
27.55
3.13384.4
5.3624.969.37
62.53
3.25191.1
2.6420.550.267.58
1.20178.0
4.8524.900.66
12.60
1.2036.281.437.820.06
13.25
1.29331.7
1.753.890.08
11.47
1.47101.3
4.8415.000.339.25
HfNbNiPbRbSc
0.400.73
26.822.075.411.82
0.626.01
24.006.027.301.64
0.834.81
106.012.5013.50
4.07
0.291.79
33.6035.30
8.631.97
0.851.47
30.704.37
10.802.03
1.576.66
544.015.83
100.12.83
1.426.08
175.87.68
29.853.92
1.419.27
159.07.61
24.635.19
3.278.71
441.133.02
222.64.22
1.334.45
200.011.5131.074.95
0.631.28
62.407.20
17.005.08
0.360.80
30.203.816.771.55
0.593.34
359.44.27
22.341.70
0.895.05
111.240.0013.381.59
SrThUVYZnZr
87.060.850.197.775.072.824.71
50.202.361.409.738.81
28.404.80
103.05.031.28
29.307.09
16.9031.00
95.35.280.61
13.607.02
13.806.09
120.04.700.81
19.406.83
18.505.73
192.411.873.82
95.717.5231.4227.80
56.459.470.74
24.9812.609.24
35.22
141.215.481.63
121.08.14
32.6726.33
51.3827.824.43
45.0827.7074.1667.13
165.66.180.98
104.33.51
50.5927.89
82.92.820.69
25.903.094.71
10.40
72.350.980.38
11.305.259.783.04
166.03.151.00
13.841.768.268.99
99.04.141.39
26.253.21
17.2518.25
(or increasing f values). Using the geochemical classification diagram ofHerron (1988) the Kudankulam sandstones are classified as arkose, subar-kose, and litharenite, except for two samples that fall in the sublitharenitefield (Fig. 4). This classification is generally consistent with the petrograph-ic data because on a QFL diagram these samples fall in the subarkose andsublitharenite fields (Pettijohn et al. 1972).
Slight enrichment of SiO2 (wt. %) content in litharenite (the mean withone-standard-deviation value being 56 6 4; number of samples n 5 9)and arkose (55 6 7, n 5 20), as compared to subarkose (51 6 8, n 514) and sublitharenite (48.2 6 1.2, n 5 2) can be attributed to the variationof quartz in these sandstones. The average Na2O content for the Kudank-ulam sandstones (arkose 0.84 6 0.33, n 5 20; subarkose 0.78 6 0.34, n5 14; litharenite 0.73 6 0.36, n 5 9; and sublitharenite 0.66 6 0.16, n5 2) is less than 1%. The depletion of Na2O (, 1%) in all groups ofsandstones (Table 1) can be attributed to a relatively smaller amount ofNa-rich plagioclase in them, consistent with the petrographic data. K2O andNa2O contents and their ratios (Table 1) also are consistent with the pet-rographic observations, according to which K-feldspar dominates over pla-gioclase feldspar. Al2O3 content is high in arkose (6.4 6 1.5, n 5 20) andlitharenite (5.9 6 0.8, n 5 9), but decreases in subarkose (3.5 6 1.1, n5 14) and sublitharenite (2.7 6 0.6, n 5 2). Similarly, generally lowconcentrations of Fe2O3* and TiO2 in all Kudankulam sandstones reflectlow abundances of heavy minerals such as Ti-bearing biotite, ilmenite,titanite, and titaniferous magnetite in the analyzed samples. Average K2O/Al2O3 ratios in arkose, subarkose, and sublitharenite are greater than 0.3except in litharenite (; 0.2), indicating that most K2O is present in K-feldspar.
On a K2O/Na2O—SiO2/Al2O3 plot (Fig. 5) arkose and litharenite fromthe Kudankulam Formation are generally similar to the average arkose andlitharenite rocks from the Bradore Formation in Labrador, Canada (Petti-john et al. 1972). The Kudankulam sandstone samples fall away from av-erage values of graywacke (Pettijohn et al. 1972), andesite, basalt 1 ko-matiite (Condie 1993), as well as average UCC (Taylor and McLennan1985). These sandstones are only slightly higher in K2O/Na2O and SiO2/Al2O3 ratios than gneisses, charnockites, and granites of the adjacent sourcearea (Proterozoic Kerala Khondalite Belt of southern India; Chacko et al.1992; Braun et al. 1996). Considering the somewhat mobile nature of thesemajor elements, the above observations suggest that these igneous rockscould be a source for the Kudankulam sandstones.
Trace Elements
Trace-element concentrations of Kudankulam sandstones are reported inTable 2. In comparison with average upper continental crust (UCC) the
concentrations of most trace elements are generally low. The average rel-ative concentration ratios lie between 0.1 and 1, except for Ni, with con-sistently much higher average relative concentration values (; 5.2–8.7),and low values of Rb (; 0.05 for litharenite), Zr (; 0.06–0.07 for subar-kose and sublitharenite), and Hf (; 0.07 for litharenite), in some rocktypes (Fig. 6). In all groups of sandstones, Zr and Hf are somewhat depletedas compared to the other elements, particularly Pb, Y, and U (Fig. 6).
Rare Earth Elements
The results of REE analysis are given in Table 3 and are shown aschondrite-normalized patterns in Figure 7A for arkose and Figure 7B forother rock types. SREE concentrations vary widely in Kudankulam sand-stones (SREE ; 15–148). This wide variation is also characteristic ofindividual rock types, e.g., arkose (; 15–148) and subarkose (; 20–108).All analyzed sandstone samples have SREE abundances less than the av-erage UCC (; 143; Taylor and McLennan 1985) except one arkose sample(C3) with SREE 5 ; 148 (Table 3). All groups of the Kudankulamsandstones show slight LREE-enriched and relatively flat HREE patternswith negative Eu anomaly, except two subarkose samples (B2 and C7) withpractically no Eu anomaly (Table 3; Fig. 7B). These two samples alsocontain higher contents of Sr, considering that both ions (Eu21 and Sr21 )have comparable ionic sizes and thus tend to preferably substitute for Ca21
in plagioclase (Gao and Wedepohl 1995).
DISCUSSION
Tectonic Setting
Roser and Korsch (1986) established a discrimination diagram usinglog(K2O/Na2O) versus SiO2 to determine the tectonic setting of terrigenoussedimentary rocks. These authors used CaO and LOI-free 100% adjusteddata to determine their field boundaries (see Fig. 12b in Roser and Korsch1986). Both parameters (SiO2 and log(K2O/Na2O) values) increase fromvolcanic-arc to active-continental-margin to passive-margin settings (Fig.8A). Because all Kudankulam sandstone samples have a considerableamount of CaO (; 13.1–31.6, 19 6 5, n 5 45), the major-element datawere recalculated to 100% and CaO and LOI-free basis before plottingthem in Figure 8A. This diagram (Fig. 8A) shows a passive-margin settingfor all groups of Kudankulam sandstones.
Discrimination of tectonic settings on the basis of major-element dataalso was proposed by Bhatia (1983); it includes oceanic island arc, conti-nental island arc, active continental margin, and passive margin. Most ofthe Kudankulam sandstone samples fall in the general area of passive-
291GEOCHEMISTRY OF SANDSTONES FROM SOUTHERN INDIA
TABLE 2.—Extended.
Subarkose
H6 F7 C5 F3 C7
Litharenite
G4
Sublith-arenite
G5
1.57106.5
1.8310.410.086.15
1.68286.0
1.438.210.287.43
1.89120.0
1.4522.70
0.069.34
2.74156.0
3.6126.70
0.1026.40
3.02140.0
2.016.980.244.81
2.7498.5
5.0316.00
0.368.65
2.02111.0
3.1528.830.10
14.650.681.54
118.04.67
10.501.95
0.350.75
15.209.35
21.702.01
1.372.07
96.37.03
16.901.53
1.563.51
28.5010.0025.60
3.05
1.039.73
110.03.08
33.701.12
0.4310.50
108.04.256.091.48
0.642.34
118.09.92
17.062.21
69.821.230.51
18.4612.439.704.97
110.01.080.82
16.704.79
23.7019.60
96.33.021.47
32.6011.50
9.485.01
61.104.701.07
22.0015.2010.3025.40
170.01.520.53
45.7018.60
9.0728.00
123.01.150.94
20.106.02
20.7019.30
100.42.631.65
30.759.47
27.3911.77
FIG. 6.—Multi-element normalized diagram for the Kudankulam sandstones, nor-malized against average upper continental crust (Taylor and McLennan 1985), usingthe following values (in ppm): Co 5 10, Ni 5 20, Cr 5 35, V 5 60, Sr 5 350,Rb 5 112, Ba 5 550, Pb 5 20, Zr 5 190, Y 5 22, Nb 5 25, Hf 5 5.8, Th 510.7, and U 5 2.8. Two horizontal lines for rock/upper continental crust values of1 and 0.1 are included for reference.
margin and active-continental-margin fields of the TiO2 versus Fe2O3* 1MgO plot (Fig. 8B), but mostly in the passive-margin field of the Al2O3/SiO2 versus Fe2O3* 1 MgO diagram (Fig. 8C). Low values of Al2O3/SiO2
ratio are an indication of the quartz enrichment in the Kudankulam sand-stones (Fig. 8C).
These discrimination diagrams suggest that the tectonic setting of theCauvery Basin in which Kudankulam sandstones were deposited was thatof a passive margin, which is consistent with general geology of the south-ern Indian Peninsula.
Weathering in the Source Area
Petrographic data show that K-feldspar dominates over plagioclase,which may result from intense weathering in the source area or from dia-genetic alteration. The latter can be ruled out by the presence of abundantcarbonate cement that developed probably during early diagenesis (Arms-trong Altrin Sam et al. 2001). The intensity and duration of weathering insedimentary rocks can be evaluated by examining the relationships amongalkali and alkaline earth elements (Nesbitt and Young 1982, 1996). Feld-spars are by far the most abundant of the reactive minerals. Consequently,the dominant process during chemical weathering of the upper crust is thealteration of feldspars and the neoformation of clay minerals. Duringweathering, calcium, sodium, and potassium are largely removed from feld-spars (Nesbitt et al. 1980). The amount of these elements surviving in thesoil profiles and in the associated sediments is a quantitative index of theintensity of weathering (Fedo et al. 1996; Nesbitt et al. 1997). A goodmeasure of the degree of chemical weathering can be obtained by calcu-lation of the chemical index of alteration (CIA; Nesbitt and Young 1982)using the formula (molecular proportions)
CIA 5 [Al O /(Al O 1 CaO* 1 Na O 1 K O)] 3 1002 3 2 3 2 2
where CaO* is the amount of CaO incorporated in the silicate fraction ofthe rock.
CIA values for the Kudankulam sandstones vary from ; 34.9 to 50.4(44.5 6 3.9, n 5 45; Table 1). For each group of Kudankulam sandstonesCIA varies as follows: arkose (45.9 6 3.4, n 5 20); subarkose (41.7 63.5, n 5 14); litharenite (46.7 6 2.6, n 5 9); and sublitharenite (40.6 64.6, n 5 2). The CIA values for the Kudankulam sandstones also areplotted in Al2O3-(CaO* 1 Na2O)-K2O (A–CN–K) compositional space(molecular proportions) in Figure 9. The compositions of average gneisses,charnockites, and granites from the source area (Chacko et al. 1992; Braun
et al. 1996) and UCC (Taylor and McLennan 1985) are shown, also forcomparison. In the A–CN–K diagram all the Kudankulam sandstones plotclose to the plagioclase K-feldspar line, as well as to the source rocks,suggesting a low degree of chemical weathering of the Kudankulam sand-stones. It is also indicated by their low values of the plagioclase index ofalteration (PIA; Fedo et al. 1995) (41 6 7, n 5 45; Table 1), calculatedby the following equation (molecular proportions): PIA 5 [(Al2O32K2O)/(Al2O3 1 CaO* 1 Na2O2K2O)] 3 100, and are consistent with the CIAvalues.
However, the CIA values (mean ; 44.5) of the Kudankulam sandstonesare still slightly lower than those (close to 50) of upper-continental-crustand Proterozoic rocks (Fig. 9). Thus, the low CIA values of the Kudank-ulam sandstones do not reflect the general chemical weathering conditionsin the source region, which can be inferred from the petrographic obser-vations. This is probably due to the sedimentary sorting effect. Physicalsorting of sediment during transport and deposition leads to concentrationof quartz and feldspar with some heavy minerals in the coarse fraction andof secondary lighter and more weatherable minerals in the suspended-loadsediments (Nath et al. 2000; Singh and Rajamani 2001b; Gu et al. 2002).
Furthermore, there is a significant positive correlation (Fig. 10A) be-tween the CIA and the mean grain size MZ (expressed in f) for the Ku-dankulam sandstones. The linear correlation coefficient (r 5 0.72, n 545) is statistically significant at a very strict significance level of 0.001 (orconfidence level of 99.9%). For individual rock types, this correlation issignificant only for arkose (r 5 0.82, n 5 20) but not for other rock types(subarkose, r 5 0.40, n 5 14; litharenite, r 5 0.39, n 5 9). The increasein CIA with decreasing particle diameter (expressed in millimeters) for thearkoses of the Kudankulam sandstone samples could suggest that the in-tensity of weathering increases from medium to fine or very fine sand.
Th/U in sedimentary rocks is of interest because weathering and recy-cling is expected to result in oxidation and removal of U with a resultantincrease in this ratio. Although highly reduced sedimentary environmentscan have enriched U leading to low Th/U ratios, weathering tends to resultin oxidation of insoluble U41 to soluble U61 with loss of solution andelevation of Th/U ratios (McLennan et al. 1990; McLennan and Taylor1980, 1991). The Th/U ratios in the Kudankulam sandstones range from1.22 to 12.80 (Table 3), with an overall mean value of 4.3 6 3.0 (n 521). Upper crustal igneous rocks have Th/U averaging about 3.8, withconsiderable scatter (Taylor and McLennan 1985; Condie 1993; McLennan2001). Considering the average Th/U ratio of the Kudankulam sandstones
292 J.S. ARMSTRONG-ALTRIN ET AL.
TABLE 3.—Rare-earth-element concentrations in ppm for sandstones of the Kudankulam Formation.
Rock Type
Sample #
Arkose
E4 A2 D4 D2 C2 H8 D1 E6 H10 C3
Subarkose
E2 F6 B2 B4
MZ
LaCePrNdSm
1.023.845.980.802.850.51
1.2611.21
9.630.984.510.91
1.3810.6012.00
1.246.241.08
1.646.93
21.201.074.210.57
2.0012.0018.20
1.638.761.40
2.2515.8332.75
3.2611.40
2.49
2.8622.8544.965.06
18.363.12
3.0225.3652.796.22
22.193.65
3.1327.7056.375.89
19.964.21
3.2537.0753.467.68
29.605.54
1.203.97
13.101.974.320.89
1.204.967.231.023.740.69
1.2912.3920.89
2.227.591.15
1.479.06
16.632.047.331.22
EuGdTbDyErTm
0.080.600.900.530.290.04
0.070.680.080.420.430.06
0.110.830.190.980.450.05
0.110.920.120.820.320.04
0.352.020.301.421.020.15
0.292.500.422.661.750.27
0.552.850.412.261.150.15
0.212.800.361.790.880.11
0.404.250.724.512.930.46
1.196.260.885.032.720.34
0.161.460.231.380.800.13
0.120.810.120.710.430.06
0.340.780.090.440.170.02
0.111.010.130.660.300.04
YbLuSREE(LREE/HREE)§Eu/Eu*Cr/Th
0.240.03
15.427.400.446.92
0.260.03
28.9313.35
0.265.82
0.310.03
33.3310.25
0.344.18
0.280.05
35.9612.63
0.462.78
0.880.10
47.156.740.642.19
1.920.30
73.696.360.350.20
0.960.13
100.9111.410.552.96
0.780.11
115.7015.590.191.61
3.290.50
127.566.510.290.90
2.040.29
147.997.220.623.33
0.700.08
28.094.790.438.83
0.360.05
19.706.660.497.98
0.120.02
45.8126.49
1.041.23
0.260.03
38.2214.570.293.62
Cr/NiCr/VY/NiTh/ScTh/UTh/Co
0.220.760.190.474.470.64
0.571.410.371.441.691.36
0.200.720.071.243.934.84
0.441.080.212.688.661.75
0.340.530.222.325.801.78
0.0040.030.034.193.114.00
0.161.120.072.42
12.803.11
0.160.210.052.989.502.89
0.060.550.066.596.285.19
0.100.200.021.256.312.34
0.400.960.050.564.090.58
0.260.690.170.632.580.69
0.010.280.0051.853.151.80
0.130.570.032.602.980.86
La/YLa/CoLa/Sc(La/Lu)cn†(Gd/Yb)cn†
0.762.912.11
13.292.03
1.276.446.84
38.792.12
1.5010.19
2.6036.68
2.16
0.992.303.52
14.392.66
1.764.555.91
12.461.86
0.905.335.595.481.06
1.817.495.83
18.252.41
3.124.734.89
23.932.91
1.005.176.565.751.05
10.5614.047.49
13.272.49
0.780.820.785.151.69
3.203.473.20
10.301.82
7.297.087.29
64.315.27
5.701.875.70
31.353.15
§ (LREE/HREE) 5 S(La-Sm)/S(Gd-Lu); † Subscript cn refers to chondrite-normalized values.
close to the upper-continental-crust values, it is likely that these sandstoneswere derived from the least weathered source rocks.
Hydraulic Sorting
It is widely accepted that hydraulic sorting can lead to variation in REEconcentrations in sediments with different grain-size fractions and mineralcontents (Cullers et al. 1975; Cullers et al. 1979; Gromet et al. 1984;McLennan 1989). The observed variations in the SREE content for arkose(73 6 47, n 5 10), subarkose (50 6 26, n 5 9), litharenite (51.3), andsublitharenite (56.7) could be either due to weathering or to variation inlithology. During weathering the REEs are relatively immobile, so onlyminor enrichment or loss is expected. However, the LREE and HREE showdifferent types of behavior and may become fractionated (Cullers 1988;Cullers et al. 1997; Condie et al. 1995; Condie et al. 2001). Thus, weinterpret the observed variations in SREE content of the Kudankulam sand-stones to be due to variations in grain-size fractions of these sandstones.Another possibility for the variations in SREE content among the Ku-dankulam sandstones can be related to an influence of a quartz dilutioneffect on abundance of heavy minerals and/or clay. However, we haveobserved petrographically low abundance of clay and heavy minerals inthe Kudankulam sandstones. So the differences in SREE content may bedue to the variations in grain-size fractions (MZ ; 1.02–3.25 f) amongthe samples, which may cause an enrichment or depletion of SREE contentdepending on the actual grain-size values. This interpretation is supportedby significant correlation between SREE versus MZ (Fig. 10B). The linearcorrelation coefficient (r 5 0.92, n 5 21) is statistically significant at thevery strict significance level of 0.001 (or confidence level of 99.9%) forall the Kudankulam sandstones, and also for the individual rock types, e.g.,arkose (r 5 0.98, n 5 10) and subarkose (r 5 0.94, n 5 9). The averageSREE contents in fine grain-size fractions are about 3 to 4 times higherthan those for medium grain-size fractions (Table 3). This strongly suggeststhat the REEs are hosted mainly in fine and very fine grain-size fractionsthan medium grain-size fractions as proposed by Cullers et al. (1979), Cull-ers et al. (1988), Condie (1991), and Mongelli et al. (1996).
Provenance
The high-field-strength elements (HFSE) such as Zr, Nb, Hf, Y, Th, andU are preferentially partitioned into melts during crystallization (Feng andKerrich 1990), and as a result these elements are enriched in felsic ratherthan mafic sources. Additionally, they are thought to reflect provenancecompositions as a consequence of their generally immobile behavior (Tay-lor and McLennan 1985). The slightly higher contents of Nb, Y, U, andTh in the samples with higher SREE probably reflects a control by grain-size fractionation during transport, and may also suggest a contributionfrom a felsic source with high concentration of these elements. The deple-tion of Zr and Hf in all groups of sandstones could be related to the sizevariation (r 5 0.70 for Zr vs. mean grain size Mz, expressed in f units,and r 5 0.67 for Hf vs. Mz; n 5 21) and depletion of heavy-mineralfractions such as zircon in the Kudankulam sandstones.
REE, Th, and Sc are quite useful for inferring crustal compositions,because their distribution is not significantly affected by diagenesis andmetamorphism and is less affected by heavy-mineral fractionation than thatfor elements such as Zr, Hf, and Sn (Cullers et al. 1979; Bhatia and Crook1986; Wronkiewicz and Condie 1987; Cox et al. 1995; McLennan 2001;Mongelli and Dinelli 2001). REE and Th abundances are higher in felsicthan in mafic igneous source rocks and in their weathered products, whereasCo, Sc, and Cr are more concentrated in mafic than in felsic igneous rocksand in their weathered products. Furthermore, ratios such as Eu/Eu*, (La/Lu)cn, La/Sc, Th/Sc, La/Co, Th/Co, and Cr/Th are significantly different inmafic and felsic source rocks and can therefore provide information aboutthe provenance of sedimentary rocks (Cullers et al. 1988; Wronkiewicz andCondie 1989; Condie and Wronkiewicz 1990; Cullers 1994). In our study,the Eu/Eu*, (La/Lu)cn, La/Sc, Th/Sc, La/Co, Th/Co, and Cr/Th values ofthe Kudankulam sandstones are similar to the values for sediments derivedfrom felsic source rocks than those for mafic source rocks (Table 4), sug-gesting that these sandstones probably were derived from felsic sourcerocks.
Furthermore, the relative REE patterns and the size of the Eu anomalyalso have been used to infer sources of sedimentary rocks (Taylor and
293GEOCHEMISTRY OF SANDSTONES FROM SOUTHERN INDIA
TABLE 3.—Extended.
Subarkose
H6 F7 C5 F3 C7
Litharenite
G4
Sublith-arenite
G5
1.5711.4015.152.529.772.00
1.689.27
20.402.148.201.48
1.8913.5018.40
2.7810.10
2.01
2.7420.1025.36
3.7115.70
2.86
3.0521.7040.50
7.0225.30
4.97
2.748.40
23.502.679.202.13
2.0213.2123.342.89
10.651.93
0.472.260.342.051.130.15
0.120.660.100.650.260.03
0.120.750.151.210.310.05
0.322.020.322.781.060.15
1.081.790.352.982.370.31
0.302.630.341.680.920.13
0.421.970.281.630.860.11
0.930.13
46.615.560.678.46
0.160.03
42.9420.960.327.60
0.270.03
48.6915.65
0.257.52
1.010.09
73.078.680.395.68
1.970.27
108.499.130.914.59
0.730.11
51.346.730.39
13.91
0.700.09
56.718.820.65
10.960.090.560.110.632.410.67
0.540.490.320.541.320.76
0.240.700.121.972.052.08
0.941.210.531.544.391.30
0.060.150.171.362.870.76
0.150.800.060.781.220.23
0.240.940.081.191.590.83
5.856.235.859.101.97
4.616.484.61
32.083.34
8.829.318.82
46.722.25
6.595.576.59
23.191.62
19.3810.8019.38
8.340.74
5.681.675.687.932.94
5.984.195.98
15.242.28
FIG. 7.—Chondrite-normalized rare earth element plots for the Kudankulam sand-stones with sample numbers and grain-size values (MZ 5 mean grain size, expressedin f units; Table 1) given next to the sample numbers; Chondrite normalizationvalues are from Taylor and McLennan (1985). A) Arkose rocks; B) other rock types.
McLennan 1985; Wronkiewicz and Condie 1989). Felsic igneous rocksusually contain higher LREE/HREE ratios and negative Eu anomalies, andmafic igneous rocks contain lower LREE/HREE ratios with little or no Euanomalies (Cullers 1994, 2000). Some tonalites or granodiorites derivedfrom eclogite melting may contain very large LREE/HREE ratios with littleor no Eu anomalies (Cullers and Graf 1984). In our study of the Kudank-ulam sandstones, their high LREE/HREE ratio (11 6 6, n 5 21) and asignificant negative Eu anomaly (0.48 6 0.22, n 5 21) support felsicigneous rocks as a possible source (Table 3). This interpretation is in agree-ment with the result of a study on terrigenous sediments of Kudankulamlimestones (Armstrong-Altrin et al. 2003).
Source Rocks
In Figure 11, we used the REE data for the Kudankulam sandstonescompared with those for gneisses, charnockites, and granites of the southernIndian Proterozoic Kerala Khondalite Belt as a tool for determining theirsource rocks (Fig. 1A; Chacko et al. 1992; Braun et al. 1996) as well aswith the UCC (Taylor and McLennan 1985). The general shapes of allREE patterns for the sandstones (Fig. 11A) are similar to the source rocks(Fig. 11B). However, in detail the sandstone samples have somewhat steep-er patterns than the Kerala Khondalite Belt rocks. Clear negative Eu anom-alies are present in most rock types, except for a granite sample. Thissuggests that the Kudankulam sandstones could have been derived by thecontributions from nearby gneisses, charnockites, and granites (KeralaKhondalite Belt source area in Fig. 1A).
Many samples have (Gd/Yb)cn ratios more than 2 (Table 3), suggestingthat these sediments were derived from sources having somewhat depletedheavy rare earth elements whereas others have (Gd/Yb)cn ratios less than2, which suggests that they were derived from less HREE-depleted Archeanor post-Archean sources, or a combination of both. The average ratios ofProterozoic gneisses, charnockites, and granites from the source area (Chac-ko et al. 1992; Braun et al. 1996) also are shown in this plot. These rocktypes have (Gd/Yb)cn and Eu/Eu* ratios overlapping with the Kudankulam
sandstones (Fig. 12), suggesting that the Kerala Khondalite Belt rocks (Fig.1A) could have been the source rocks for the sandstones.
On the basis of the geochemistry of immature flood-plain sediments ofthe Cauvery River, Singh and Rajamani (2001a, 2001b) interpreted theexposed rocks in southern India to have been uplifted and exposed becauseof the stress buildup by the Himalayan Orogeny. However, they stated thatthe time of this uplift might have been anytime from the Cenozoic to aslate as the Quaternary. The present study can be used to constrain that theProterozoic rocks in southern India were already exposed in late Miocenetime and supplied sediments to the Kudankulam area.
Ni Enrichment
The ferromagnesian (or so-called compatible) trace elements Cr, Ni, Co,and V show generally similar behavior in magmatic processes, but theymay be fractionated during weathering (Feng and Kerrich 1990). In thestudied samples, Cr, Co, and V are slightly depleted and Ni is highlyenriched with respect to the average composition of the UCC. This enrich-ment in Ni may suggest some input of mafic materials from the sourceterrane; however, the simultaneous depletion of Cr (16.08 6 8.56, n 521; Table 2), MgO (0.9 6 0.3, n 5 45; Table 1), and Cr/Th (5.3 6 3.6,n 5 21; Table 3) ratio suggests that other factors could have played a rolein concentrating Ni in the sandstones.
High concentrations of Ni (50–130 ppm) and Cr (112–225 ppm) in flood-plain sediments of the Cauvery River (Fig. 1A), were reported and inter-
294 J.S. ARMSTRONG-ALTRIN ET AL.
FIG. 8.—Tectonic-setting discrimination diagrams for the Kudankulam sandstones.The tectonic settings are named in each plot. A) SiO2—(K2O/Na2O) (after Roserand Korsch 1986); B) Fe2O3* 1 MgO—TiO2 (after Bhatia 1983); C) Fe2O3* 1MgO—Al2O3/SiO2 (after Bhatia 1983).
FIG. 9.—CIA ternary diagram, Al2O3 (A)—CaO* 1 Na2O (CN)—K2O (K), afterNesbitt and Young (1982) (CaO* 5 CaO in silicate phase) showing 1Kudankulamsandstones—this study, as well as average compositions of different rock types:2UCC (upper continental crust) from Taylor and McLennan (1985); 3Chacko et al.(1992); 4Braun et al. (1996).
FIG. 10.—Bivariate plots for the Kudankulam sandstones. A) MZ—CIA [MZ 5grain size expressed in f units; CIA (chemical index of alteration) 5 [Al2O3/(Al2O3
1 CaO* 1 Na2O 1 K2O)] 3 100]. B) MZ—(REE)T (total REE content).
preted to be a result of chemical weathering of mafic source rocks (Singhand Rajamani 2001a). However, this interpretation of mafic source rocksis not supported by concentrations of REE and other trace elements in theKudankulam sandstones (Table 4).
Garver et al. (1996) suggested that elevated Cr and Ni abundances (Cr.150 ppm and Ni .100 ppm) and low Cr/Ni ratios (between 1.3 and 1.5)are indicative of ultramafic rocks in the source area of shales, although Cr/Ni ratios were much higher (. 3.0) for sandstones from the same area.
295GEOCHEMISTRY OF SANDSTONES FROM SOUTHERN INDIA
TABLE 4.—Range of elemental ratios of Kudankulam sandstones in this study com-pared to the ratios in similar fractions derived from felsic rocks, mafic rocks, and
upper continental crust.
ElementalRatio
Range of Sandstonesfrom Kudankulam
Formation1 (n 5 21)
Range of Sedimentfrom Felsic
Sources2
Range of Sedimentfrom Mafic
Sources2
UpperContinental
Crust3
Eu/Eu*(La/Lu)cn
La/ScTh/ScLa/CoTh/CoCr/Th
0.19–1.045.15–640.78–19.40.47–6.590.82–14.00.23–5.190.20–13.9
0.40–0.943.00–27.02.50–16.30.84–20.51.80–13.80.67–19.44.00–15.0
0.71–0.951.10–7.000.43–0.860.05–0.220.14–0.380.04–1.40
25–500
0.639.732.210.791.760.637.76
1 This study.2 Cullers (1994, 2000); Cullers and Podkovyrov (2000); Cullers et al. (1988).3 McLennan (2001); Taylor and McLennan (1985).
FIG. 12.—Plot of Eu/Eu* versus (Gd/Yb)cn for the Kudankulam sandstones. Fieldsare after McLennan and Taylor (1991). The average upper continental crust (UCC)and gneisses, charnockites, and granites from the Proterozoic Kerala Khondalite Beltof southern India also are included. 1Kudankulam sandstones, this study; 2Taylorand McLennan (1985); 3Chacko et al. (1992); 4Braun et al. (1996).
FIG. 11.—Chondrite-normalized REE patterns for: A) sandstone samples from thisstudy and, for comparison, average upper continental crust (UCC) (1Kudankulamsandstones, this study; 2Taylor and McLennan 1985); B) gneisses, charnockites, andgranites from the Proterozoic Kerala Khondalite Belt of southern India (3Chacko etal. 1992; 4Braun et al. 1996), which are used here to represent compositions ofsource rocks.
Abundances of chromium and nickel in the Kudankulam sandstones are onaverage 16 ppm and 138 ppm, respectively; Cr/Ni ratios range from ;0.004 to 0.936 (0.25 6 0.22, n 5 21; Table 3), but are mostly below 0.5.Although high Cr and Ni abundances are clearly suggestive of mafic and/or ultramafic provenance, ultramafic lithologies in ophiolite sequences canattain Cr/Ni ratios of 10 or greater (e.g., Jaques et al. 1983) and accordinglythe low Cr/Ni ratios in Kudankulam sandstones are not especially indicative
of an ultramafic provenance. Therefore, the hypothesis proposed by Garveret al. (1996) may not be applicable to the Kudankulam sandstones.
The low Cr/V ratios ; 0.03 to 1.41 (0.66 6 0.37, n 5 21) and thevariable Y/Ni ratios ; 0.005 to 0.533 (0.14 6 0.13, n 5 21; Table 3)indicate that no ophiolite component is present in the Kudankulam sand-stones (e.g., Hiscott 1984). Relatively high Ni contents in the Kudankulamsandstones can be carried by orthopyroxene grains derived from charnock-ites. These orthopyroxene grains may eventually break down to pyrite, amineral that may influence the ferromagnesian budget of sedimentary rocks.In fact, pyrite is a common authigenic phase that may scavenge these el-ements from pore waters after the breakdown of relatively unstable maficminerals such as pyroxene and hornblende (Bock et al. 1998). These au-thors also showed that pyrite in their study had undetectable contents ofCr (, 170 ppm, the detection limit of electron microprobe) and low V (,13 ; 50 ppm) abundances but generally higher Ni abundances (up to 880ppm). Thus, the enriched Ni contents and associated low contents of Crand V may suggest the presence of pyrite in Kudankulam sandstone sam-ples. This should be confirmed in future by microprobe or X-ray analysis.
Yet another possibility, which we consider to be the most likely expla-nation for the Ni positive anomaly in the Kudankulam sandstones (Fig. 6),is fractionation of garnet between the source rocks and these sandstones.Garnet is known to be present in the source rocks (Braun et al. 1996; Braun1999) as well as in coastal sediments from southern India (Ramasamy etal. 1996; Sabeen et al. 2002). Garnet has very high Cr/Ni ratios (; 115;e.g., Glaser et al. 1999) and the partition coefficients for Ni, Cr, V, andCo (e.g., Sisson and Bacon 1992; Torres-Alvarado et al. 2003) differ insuch a way that this mineral can strongly fractionate Ni from the otherelements, as is observed in the Kudankulam sandstones. Thus, it is possiblethat the unusual Ni enrichment, unaccompanied by a similar enrichment inCo, Cr, and V, is due to the fractionation of garnet from the source rocksduring transportation. Fractionation of garnets would also explain the smalldifferences observed in REE patterns (Figs. 11A, B) between the sandstonesamples and source rocks. Similarly, the slight shift of the Kudankulamsandstones towards higher (Gd/Yb)cn values as compared to the sourcerocks (Fig. 12) can also be readily explained by such a fractionation ofgarnet. This fractionation may be related to one or more of several factors,such as hydraulic sorting, mechanical abrasion, diminution of garnet grainsize and isolation from the sand fraction, and chemical weathering (e.g.,
296 J.S. ARMSTRONG-ALTRIN ET AL.
Morton et al. 1994; Nesbitt et al. 1997; Chigira and Oyama 1999; Mortonand Hallsworth 1999). Further research is needed to determine which ofthese processes was the most important for the garnet fractionation in theKudankulam sandstones.
CONCLUSIONS
The K2O and Na2O values and their ratios in Kudankulam sandstonesindicate that K-feldspar dominates over plagioclase feldspar. The tectonic-setting discrimination diagrams support a passive-margin setting for theKudankulam sandstones. The low CIA values (; 44.5) of the Kudankulamsandstones do not reflect the general conditions of chemical weathering inthe source region. This probably is due to the sedimentary sorting effect.Physical sorting of sediment during transport and deposition leads to con-centration of quartz and feldspar with some heavy minerals in the coarsefraction and of secondary lighter and more weatherable minerals in thesuspended-load sediments. The increase in CIA with decreasing grain-sizefractions reveals that the intensity of weathering increases from medium tofine or very fine sand. Eu/Eu*, (La/Lu)cn, La/Sc, Th/Sc, La/Co, Th/Co, andCr/Th ratios and the REE patterns indicate the derivation of these sand-stones from felsic igneous source rocks. The differences of the REE char-acteristics of the Kudankulam sandstones probably do not reflect changesin source composition; the differences can be explained by the variationsin grain-size fractions among these sandstones, suggesting that the REEare hosted mainly in the fine and very fine size fractions than in the mediumsize fractions. We conclude that the source rocks included gneisses, char-nockites, and granites. The present study suggests that the Proterozoic rocksin southern India were already exposed in late Miocene time and suppliedsediments to the Kudankulam area. The Ni enrichment unaccompanied byan increase in Cr, Co, and V in the Kudankulam sandstones calls for newstudies to understand this rare observation.
ACKNOWLEDGMENTS
The authors are thankful to Prof. S.P. Mohan, Head, Department of Geology,University of Madras, for providing certain laboratory facilities through SAP-II andUGC COSIST programs. The first author wishes to express his gratefulness to Rob-ert L. Cullers, P.K. Banerjee, J. Madhavaraju, and S. Srinivasalu for their usefulsuggestions and guidance during the course of this study. We are grateful to thereviewers Giovanni Mongelli and Salvatore Critelli and Editor Mary J. Kraus andAssociate Editor Mark Johnsson for numerous helpful comments to improve ourpaper. Technical editing by John B. Southard is highly appreciated. This researchwas partly supported by Council of Scientific and Industrial Research (CSIR), NewDelhi grant (24/239/98-EMR-II to SR) and Korea Science and Engineering Foun-dation (KOSEF) grant (2000-2-13100-003-5 to YIL).
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Received 8 October 2002; accepted 28 August 2003.
