sandstone geochemistry

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0016-7622/2007-70-2-297/$ 1.00 © GEOL. SOC. INDIA JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.70, August 2007, pp.297-312 Petrography and Geochemistry of Terrigenous Sedimentary Rocks in the Neoproterozoic Rabanpalli Formation, Bhima Basin, Southern India: Implications for Paleoweathering Conditions, Provenance and Source Rock Composition R. NAGARAJAN 1 , J.S. ARMSTRONG-ALTRIN 2* , R. NAGENDRA 1 , J. MADHAVARAJU 3 and J. MOUTTE 4 1 School of Civil Engineering, Sastra University, Thanjavur - 613 402, India 2 Centro de Investigaciones en Ciencias de la Tierra, Universidad Autónoma del Estado de Hidalgo, Ciudad Universitaria, Carretera Pachuca-Tulancingo km. 4.5, Pachuca, Hidalgo, 42184, México 3 Instituto de Geologia, Estacion Regional del Noroeste, Universidad Nacional Autónoma de México, Apart. Postal 1039, Hermosillo, Sónora 83000, México 5 Centre SpiNC, Ecole des Mines, 158 cours Fauriel, F 42023, Sant-Etienne, France * Email: [email protected]; [email protected] Abstract: Petrographic, major, trace, and rare earth element compositions of quartz arenites, arkoses, and siltstones of Neoproterozoic Rabanpalli Formation of Bhima Basin have been investigated to understand the provenance. The quartz arenites, arkoses, and siltstones have large variations in major element concentrations. For example, quartz arenites and arkoses contain the higher SiO 2 (average with one standard deviation being 97±1, 73±2, respectively) and lower Al 2 O 3 (0.95±0.4, 9.6±0.9, respectively) concentrations than siltstones (SiO 2 = 64±4, Al 2 O 3 = 14±1), which is mainly due to the presence of quartz and absence of other Al-bearing minerals in relation with rock types. This is also supported by our petrography, since quartz arenites and arkoses contain significant amount of quartz relative to feldspar and lithic fragments. The observed low CIA values and A-CN-K diagram suggest that the sedimentary rocks of Rabanpalli Formation have undergone K-metasomatism. The Co, Ni, Cr, Ba, Zr, Hf, and Th values are higher in siltstones than quartz arenites and arkoses. The Eu/Eu * , (La/Lu) cn , La/Sc, Th/Sc, Th/Co, Th/Cr, Cr/Th ratios, and Cr, Ni, V, and Sc values strongly suggest that these sediments were mainly derived from the felsic source rocks. This interpretation is also supported by the Th/Sc versus Sc bivariate and La-Th-Sc triangular plots. The rare earth element (REE) patterns of these rocks also support their derivation from felsic source rocks. Further more, these rocks exhibit higher LREE/HREE ratio (8±4) and a significant negative Eu anomaly (0.77±0.16), which indicate the felsic igneous rocks as a possible source rocks. Keywords: Geochemistry, Paleoweathering, Provenance, K-Metasomatism, Sandstone, Bhima Basin, Karnataka. INTRODUCTION The bulk chemical compositions of terrigenous sedimentary rocks are influenced by several factors such as sedimentary provenance, nature of sedimentary processes within the depositional basin, and the kind of dispersal paths that link provenance to the depositional basin e.g. weathering, transportation, physical sorting, and diagenesis (Roser and Korsch, 1986, 1988; McLennan et al. 1990; Eriksson et al. 1992; Weltje and von Eynatten, 2004). However, the bulk chemical compositions of terrigenous sedimentary rocks can be used to identify tectonic environments and provenance characteristics (e.g. Bhatia, 1983; Mongelli et al. 1996; Ugidos et al. 1997; Gotze, 1998; Holail and Moghazi, 1998; Bhat and Ghosh, 2001; Zimmermann and Bahlburg, 2003; Yang et al. 2004). Hence, the study of bulk chemical compositions of terrigenous sedimentary rocks can be used as an effective tool to infer the factors that control sediment characteristics during and after their deposition. In this sense, many studies have contributed to understanding the relationship between chemical composition of terrigenous sedimentary rocks and provenance, weathering and palaeoclimate (e.g. Zhang et al. 1998; Dinelli et al. 1999; Hassan et al. 1999; Lahtinen, 2000; Nath et al. 2000; Mongelli and Dinelli, 2001; Amorosi

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Journal of the Geological Society of India, 2007, v. 70, pp. 297-312

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0016-7622/2007-70-2-297/$ 1.00 © GEOL. SOC. INDIA

JOURNAL GEOLOGICAL SOCIETY OF INDIAVol.70, August 2007, pp.297-312

Petrography and Geochemistry of Terrigenous Sedimentary Rocks inthe Neoproterozoic Rabanpalli Formation, Bhima Basin, Southern

India: Implications for Paleoweathering Conditions,Provenance and Source Rock Composition

R. NAGARAJAN1, J.S. ARMSTRONG-ALTRIN

2*, R. NAGENDRA1, J. MADHAVARAJU

3 and J. MOUTTE4

1School of Civil Engineering, Sastra University, Thanjavur - 613 402, India2Centro de Investigaciones en Ciencias de la Tierra, Universidad Autónoma del Estado de Hidalgo,

Ciudad Universitaria, Carretera Pachuca-Tulancingo km. 4.5, Pachuca, Hidalgo, 42184, México3Instituto de Geologia, Estacion Regional del Noroeste, Universidad Nacional Autónoma de México,

Apart. Postal 1039, Hermosillo, Sónora 83000, México5Centre SpiNC, Ecole des Mines, 158 cours Fauriel, F 42023, Sant-Etienne, France

*Email: [email protected]; [email protected]

Abstract: Petrographic, major, trace, and rare earth element compositions of quartz arenites, arkoses, and siltstones ofNeoproterozoic Rabanpalli Formation of Bhima Basin have been investigated to understand the provenance. The quartzarenites, arkoses, and siltstones have large variations in major element concentrations. For example, quartz arenites andarkoses contain the higher SiO2 (average with one standard deviation being 97±1, 73±2, respectively) and lower Al 2O3

(0.95±0.4, 9.6±0.9, respectively) concentrations than siltstones (SiO2 = 64±4, Al 2O3 = 14±1), which is mainly dueto the presence of quartz and absence of other Al-bearing minerals in relation with rock types. This is also supportedby our petrography, since quartz arenites and arkoses contain significant amount of quartz relative to feldspar andlithic fragments. The observed low CIA values and A-CN-K diagram suggest that the sedimentary rocks of RabanpalliFormation have undergone K-metasomatism.

The Co, Ni, Cr, Ba, Zr, Hf, and Th values are higher in siltstones than quartz arenites and arkoses. The Eu/Eu*,(La/Lu)cn, La/Sc, Th/Sc, Th/Co, Th/Cr, Cr/Th ratios, and Cr, Ni, V, and Sc values strongly suggest that these sedimentswere mainly derived from the felsic source rocks. This interpretation is also supported by the Th/Sc versus Sc bivariateand La-Th-Sc triangular plots. The rare earth element (REE) patterns of these rocks also support their derivation fromfelsic source rocks. Further more, these rocks exhibit higher LREE/HREE ratio (8±4) and a significant negative Euanomaly (0.77±0.16), which indicate the felsic igneous rocks as a possible source rocks.

Keywords: Geochemistry, Paleoweathering, Provenance, K-Metasomatism, Sandstone, Bhima Basin, Karnataka.

INTRODUCTION

The bulk chemical compositions of terrigenoussedimentary rocks are influenced by several factors such assedimentary provenance, nature of sedimentary processeswithin the depositional basin, and the kind of dispersalpaths that link provenance to the depositional basin e.g.weathering, transportation, physical sorting, and diagenesis(Roser and Korsch, 1986, 1988; McLennan et al. 1990;Eriksson et al. 1992; Weltje and von Eynatten, 2004).However, the bulk chemical compositions of terrigenoussedimentary rocks can be used to identify tectonicenvironments and provenance characteristics (e.g. Bhatia,

1983; Mongelli et al. 1996; Ugidos et al. 1997; Gotze, 1998;Holail and Moghazi, 1998; Bhat and Ghosh, 2001;Zimmermann and Bahlburg, 2003; Yang et al. 2004). Hence,the study of bulk chemical compositions of terrigenoussedimentary rocks can be used as an effective tool to inferthe factors that control sediment characteristics duringand after their deposition. In this sense, many studieshave contributed to understanding the relationship betweenchemical composition of terrigenous sedimentary rocks andprovenance, weathering and palaeoclimate (e.g. Zhanget al. 1998; Dinelli et al. 1999; Hassan et al. 1999; Lahtinen,2000; Nath et al. 2000; Mongelli and Dinelli, 2001; Amorosi

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298 R. NAGARAJAN AND OTHERS

et al. 2002; Di Leo, 2002; Lee, 2002; Naqvi et al. 2002;Raza et al. 2002; Armstrong-Altrin et al. 2004; Noda et al.2004).

The variation in the chemical composition of terrigenoussedimentary rocks reflects changes in the mineralogicalcomposition of the sediments due to the effects of weatheringand diagenetic processes (Nesbitt and Young, 1984, 1989;Wandres et al. 2004). Also the spatial and temporal patternsof sedimentation determine changes in the mineralogy andsorting of sediments, which in turn affect their bulkcomposition (Nesbitt et al. 1996; Garcia et al. 2004).Although mineralogically unstable and soluble elements areaffected during weathering, chemically immobile elements(e.g. REE, Th, Cr, Sc) are preserved in detrital sediments,so that they record the chemical signatures of the sourcerocks. Hence these elements and their elemental ratios arehighly useful to determine the provenance characteristicsof sediments. This approach has provided useful results,especially when geological processes have destroyed theoriginal mineralogy (Cullers, 1994a, 1995).

In addition, the chemical approach is a good complementto petrographic analysis of terrigenous sedimentary rocksand the two methods combined are a powerful tool forexamination of provenance and weathering (van de Kampand Leake, 1985; Shao et al. 2001; Cingolani et al. 2003;Le Pera and Arribas, 2004). In the present study, we attemptto evaluate the paleoweathering conditions, provenance,and source rock characteristics of quartz arenites, arkoses,and siltstones of Neoproterozoic Rabanpalli Formation,Bhima basin, using major, trace, and rare earth elementgeochemistry as well as by petrographic analysis. Also thisstudy describes the importance of some ferromagnesiantrace elements to distinguish the felsic, mafic, and/orultramafic source rocks.

GENERAL GEOLOGY

The Bhima Basin, southern India is a NE-SW trendingS-shaped Neoproterozoic, epicratonic, extensional basinformed due to gravity faulting. Total thickness of sedimentis about 300 m extended over an area of 5,000 km2.The sedimentary rocks of Bhima Basin have beendivided into five distinct formations i.e. (i) RabanpalliFormation, (ii) Shahabad Formation, (iii) Halkal Shale,(iv) Katamadevarhalli Formation and (v) Harwal Shale(Janardhana Rao et al. 1975). It comprises an alternatingsequence of terrigenous and carbonate sediments. In theterrigenous unit, fine-grained sediments dominate overcoarse-grained sediments (Kale et al. 1990). The terrigenoussedimentary rocks constituting the lower Bhima Basin is

designated as the Rabanpalli Formation and is well exposedin Adki, Gogi, and Muddebihal areas (Fig.1). The RabanpalliFormation mainly consists of quartz arenites, arkoses,siltstones, and greenish yellow shale. Sedimentation in theBhima Basin started with the deposition of a thinconglomerate but, the conglomerate exposures are veryfew. It contains a considerable amount of angular and sub-angular potash feldspar grains and occasionally pinkgranite clasts. Arkoses are located at the bottom. The arkosesare very fine to medium-grained, showing graded bedding,and consist mostly of angular and sub-angular potashfeldspar grains with minor amounts of sub-rounded, quartzgrains. The siltstones are a transitional member betweenthe arkoses and the overlying greenish yellow shales.Quartz arenites are medium to coarse-grained, showingirregular graded beddings, horizontal laminations, ripplemarks, and cross-laminations.

MATERIALS AND METHODS

Fresh samples were collected from the outcrops and thesamples were washed thoroughly in distilled water to removethe contamination. The samples were disaggregatedfollowing the procedure adopted in Cox and Lowe (1996).Grain size analysis was carried out in a Ro-Tap sieve shakerusing American Society for Testing and Material (ASTM)sieves ranging from –1.5 φ to 4.25 φ at 0.50 φ intervals for20 minutes (Folk, 1966). Cumulative curves wereconstructed to calculate the statistical grain size parameters(MZ: mean grain size) after Folk and Ward (1957).

A detailed petrographic study covering more than25 thin sections were studied. The thin sections weresubjected to Alizarin Red-S stain to confirm the presenceor absence of dolomite and calcite, and potassiumferricyanide to ascertain the presence of ferroan/nonferroancalcite. Friedman’s (1959) organic stain specific for calciteand Katz and Friedman’s (1965) combined organic andinorganic stain specific for iron rich calcite have beenadopted to identify the mineralogical variations. Formodal analysis, four hundred frame work grains werecounted from each thin section. Matrix and cement werenot counted. The point counts were done using bothGazzi-Dickinson (Gazzi, 1966; Dickinson, 1970) andtraditional methods.

Twenty three samples (eight quartz arenites, sevenarkoses, and eight siltstones) were selected for majorand trace elements study. Twelve samples (five quartzarenites, three arkoses, and four siltstones) were selectedfor rare earth elements study. The major, trace, and rare earthelements were analysed using an inductively coupled

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PETROGRAPHY AND GEOCHEMISTRY OF TERRIGENOUS SEDIMENTARY ROCKS IN BHIMA BASIN, KARNATAKA 299

plasma atomic emission spectrometer (ICP-AES - Jobin-Yvon JY138 Ultrace) at the Department of Geochemistry,Ecole des Mines de Saint-Etienne, France. SiO2, Nb, Zr,and Th were analyzed by XRF method on pressedpellets. The analytical precision for trace and REE is betterthan 5%.

PETROGRAPHY

Quartz Ar enites

Quartz arenites mainly consist of well preserved, fine tocoarse-grained quartz (0.51 φ to 1.75 φ; Table 1). The fine-grained quartz grains are angular to sub-angular in shape(Fig.2A). Although dominated by quartz, smaller amountsof rock fragments and potash feldspars are also present.Among the quartz grains, monocrystalline quartz shows bothstraight and undulatory extinction. Polycrystalline quartzexceeds monocrystalline quartz in quartz arenites andmost of the polycrystalline quartz grains consist of morethan three crystals per grain, which exhibit sutured crystalboundaries (Fig.2B). The framework grains show longand concavo-convex contacts. Rock fragments are

predominantly sedimentary and metamorphic. Resistantheavy minerals, zircon and tourmaline are also present inthe quartz arenites.

The quartz arenites show substantial amount of quartzovergrowth (Fig.2C). Two types of cements are encounteredin the quartz arenites, i.e. quartz cement and iron oxidecement. Quartz cement occurs as typical syntaxialovergrowths that make up several percent. Some quartzgrains show both smaller mode (2-3 µm) and larger mode(30-40 µm) overgrowths. In quartz grains with smaller modeof overgrowth, the crystals lack well developed faces andform “blob-like” features as mentioned by Pittman (1972),whereas the crystals having larger mode show smooth andwell formed crystal faces. Iron oxides are dark brown incolour and present in the pore spaces.

The original boundaries of some detrital grains arelost, providing evidence for pressure solution effect. Thisreveals a high degree of compositional maturity, becausethey are mainly composed of resistant quartz. The quartzarenites exhibit bimodal texture with well-rounded grainsfrom about 0.3 to 0.7 mm in diameter, whereas fine-grainedangular grains are generally 0.05 to 0.1 mm in diameter.

Fig.1. Geological map of the Bhima Basin showing the study area. The samples collected from Adki, Gogi, and Muddebihal areas belongto the Rabanpalli Formation.

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300 R. NAGARAJAN AND OTHERS

Such textures have been encountered in numerous upperPrecambrian and lower Paleozoic quartz-rich sandstones(Folk, 1966). The high content of quartz, minor amounts offeldspar and rock fragments, and restricted concentrationof heavy minerals such as tourmaline and zircon suggestthe mineralogical maturity for the quartz arenites. Thisinterpretation is in good agreement to our geochemistryresults.

Arkoses

Arkoses consist of monocrystalline and polycrystallinequartz, and feldspar with minor amounts of biotite andopaque minerals (Fig.2D). The detrital grains are very fineto medium (MZ = 2.0 φ to 3.25 φ; Table 1), sub-angular tosub-rounded in nature, which exhibit long and concavo-convex contacts. Among quartz monocrystalline quartzdominates over polycrystalline quartz. Monocrystalline

Fig.2. Petrographical descriptions of quartz arenites, arkoses, and siltstones of the Rabanpalli Formation (Scale 1 cm = 0.19 mm).(A) Fine-grained sub angular to angular quartz grains in quartz arenites. (B) Polycrystalline quartz grains with sutured crystalboundaries. (C) Quartz overgrowth in quartz arenites. (D) Mono crystalline quartz grains, feldspar (microcline) and rock fragmentsin arkose. (E) Feldspar grain dissolution in arkose. (F) Well-sorted grain supported siltstone.

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quartz exhibits both straight and undulatory extinction.Straight extinction is dominant over undulatory extinction.Polycrystalline quartz grains consist of both 2-3 crystalunits and > 3 crystal units per grain. The grains with2-3 crystal units show straight crystal boundaries, whereasgrains with > 3 crystal units exhibit sutured boundaries.The feldspars are microcline, orthoclase, and minorplagioclase. Feldspar grains shows initial to fifth stage indegree of dissolution (Fig.2E) and are altered to illite. Lithicfragments are quartz and feldspar grains, and other rockclasts. Mica and chlorite are also present within this rockframework. Quartz overgrowths and minor amount of claymatrix are present. Two types of cements are encountered,which are silica and iron oxide cements. Iron oxide cementis in considerable amount.

Siltstones

Siltstones are fairly well-sorted rocks that containapproximately >70% of fine, sub-angular to sub-roundedquartzose grains and some amount of both alkali andplagioclase feldspar grains (Fig.2F; MZ = 4.0 φ to 4.40 φ;Table 1). The quartz grains are mostly monocrystalline,showing straight and undulatory grain boundaries. Grainsare closely packed. The cements are siliceous andferruginous, with significant amount of clay matrix.

Detrital Modes

The average framework grain modes of quartz arenitesand arkoses from Rabanpalli Formation are Q96.4F2.7L0.80

and Q82F15.5L2.5, respectively. Quartz, feldspar, and lithicfragments values are plotted in the QFL diagram (Fig.3;Dickinson and Suczek, 1979) to find out the tectonic settingof the source rocks. All the samples from quartz arenitesand arkoses are fall in the field of cratonic interior, whichclearly indicates that these sedimentary rocks were derivedfrom the igneous source rocks.

GEOCHEMISTR Y

Major (wt. %), trace (ppm), and rare earth element (ppm)concentrations along with the mean grain size values (MZ)of quartz arenites, arkoses, and siltstones of theNeoproterozoic Rabanpalli Formation, Bhima Basin arereported in the Tables 1 and 2.

Major Elements

The CaO content is very low (wt. %; ~0.05-0.97; Table1) in all rock types (quartz arenites, arkoses, and siltstones).The K2O content is higher in arkoses (average with onestandard deviation being 4.60 ± 1.20, n = 7) and siltstones

(3.67 ± 0.71, n = 8) than quartz arenites (0.13 ± 0.08, n =8). This is almost certainly due to the variations in K- feldsparcontent among rock types. The Na2O content is more insiltstones (2.7 ± 0.7) than arkoses (0.27 ± 0.12), and quartzarenites (0.13 ± 0.12; Table 1), which can be attributed tothe greater amount of Na-rich plagioclase and alkalifeldspar in siltstones. Quartz arenites and siltstonesshow low variation in K2O/Na2O ratio (~ 0.5-4.7, ~ 0.99-1.85, respectively) whereas arkoses exhibit high variation(~ 13-28). Similarly, the siltstones have high content ofFe2O3 when compared to quartz arenites and arkoses(Table 1).

Quartz arenites and arkoses show the highest SiO2 andlowest Al 2O3 concentrations than siltstones (Table 1),which is mainly due to the presence of quartz and absenceof other Al-bearing minerals. This is in good agreementwith the petrographic observation, according to that quartzarenites and arkoses contain significant amount of quartzrelative to that of feldspar and lithic fragment. This suggestthat quartz arenites and arkoses were weathered ordiagenetically altered to remove feldspar and lithic fragmentsand thus increasing the relative proportion of quartz relativeto the source rock (Nesbitt et al. 1996). TiO2 content ismore in the siltstones (~ 0.25-0.62) than the quartz arenites(~ 0.01-0.03) and arkoses (~ 0.02-0.09; Table 1). Lowcontent of TiO2 in quartz arenites and arkoses is mainly dueto the negligible amount of phyllosilicates among them

Fig.3. QFL diagram with tectonic fields of Dickinson and Suczek(1979) for quartz arenites and arkoses. Q, total quartz(monocrystalline and polycrystalline grains); F, feldspars(plagioclase and K-feldspars); L, lithic rock fragments(excluding carbonates).

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Table 1. Major (wt. %), trace element (ppm) concentrations, and elemental ratios for quartz arenites, arkoses, and siltstones of the Rabanpalli Formation along with their mean grain size (MZ) in φ units and Chemical index of alteration (CIA; Nesbitt and Young, 1982)

Rock Type Quartz arenites Arkoses Siltstones

Sample # S034 S031 E073 C099 S014 E071 E072 E074 Mean (n = 8) S063 S061 S058 S064 S065 S066 S067 Mean (n = 7) S029 S030 S032 S035 S036 S037 S038 S039Mean (n = 8)

MZ 0.65 1.25 1.75 0.75 0.51 0.63 0.73 0.98 0.91 ± 0.41 2.75 3.25 2.10 2.00 2.50 2.75 2.15 2.5 ± 0.5 4.00 4.13 4.30 4.40 4.20 4.00 4.40 4.00 4.2 ± 0.2

SiO2 96.60 97.00 95.30 97.90 97.00 96.24 97.48 96.43 96.69 ± 0.83 72.00 74.40 75.10 74.69 73.12 69.84 74.28 73.4 ± 1.9 62.30 68.20 64.40 66.20 60.02 58.01 68.00 59.70 63.4 ± 3.9

Al2O3 1.13 1.70 1.00 0.70 0.55 0.75 0.63 1.02 0.95 ± 0.37 10.50 9.98 8.11 9.00 10.29 10.02 9.12 9.6 ± 0.9 14.60 12.20 13.40 11.70 14.90 12.70 13.20 15.20 13.5 ± 1.3

Fe2O3* 0.19 0.18 0.60 0.09 0.11 0.47 0.36 0.20 0.28 ± 0.18 0.41 0.44 0.41 0.45 0.43 0.49 0.41 0.43 ± 0.03 5.88 3.64 6.03 3.93 5.20 6.20 3.10 6.50 5.1 ± 1.3

CaO 0.05 0.07 0.05 0.17 0.07 0.08 0.05 0.09 0.08 ± 0.04 0.50 0.50 0.42 0.65 0.50 0.47 0.48 0.50 ± 0.07 0.71 0.40 0.49 0.36 0.67 0.80 0.36 0.97 0.59 ± 0.23

MgO 0.02 0.03 0.07 0.02 0.03 0.04 0.05 0.03 0.04 ± 0.02 0.19 0.25 0.19 0.20 0.23 0.14 0.18 0.20 ± 0.04 1.05 0.71 1.29 0.64 0.60 1.40 1.20 0.90 0.97 ± 0.31

K2O 0.19 0.27 0.12 0.03 0.05 0.13 0.09 0.17 0.13 ± 0.08 5.88 4.45 3.13 3.57 4.02 6.30 4.79 4.6 ± 1.2 3.26 3.43 3.59 3.22 3.00 5.31 4.01 3.52 3.67 ± 0.71

Na2O 0.12 0.34 0.03 0.02 0.03 0.24 0.06 0.17 0.13 ± 0.12 0.40 0.34 0.11 0.15 0.26 0.29 0.35 0.27 ± 0.12 2.00 3.47 3.16 3.10 1.80 3.60 2.80 1.90 2.7 ± 0.7

MnO 0.003 0.004 0.008 0.002 0.002 0.002 0.003 0.005 0.004 ± 0.002 0.001 0.001 0.001 0.001 0.002 0.001 0.0020.0012 ± 0.0004 0.03 0.02 0.04 0.03 0.002 0.04 0.01 0.02 0.02 ± 0.01

TiO2 0.02 0.03 0.03 0.01 0.02 0.02 0.01 0.03 0.022 ± 0.008 0.09 0.07 0.08 0.06 0.08 0.02 0.07 0.07 ± 0.02 0.40 0.28 0.44 0.41 0.62 0.25 0.30 0.56 0.41 ± 0.13

P2O5 0.02 0.02 0.85 0.02 0.02 0.02 0.53 0.41 0.19 ± 0.32 0.03 0.03 0.02 0.02 0.02 0.03 0.03 0.03 ± 0.01 0.03 0.03 0.01 0.03 0.02 0.03 0.04 0.03 0.03 ± 0.01

LOI 1.24 1.02 1.69 1.11 1.58 1.04 1.01 1.46 1.27 ± 0.27 9.27 8.79 10.01 10.00 9.73 10.60 9.53 9.7 ± 0.6 8.71 7.36 6.27 8.89 11.90 10.90 7.01 10.80 9.0 ± 2.1

Total 99.59 100.7 99.74 100.1 99.46 99.03 100.3 99.25 99.8 ± 0.5 99.26 99.25 97.58 98.79 98.69 98.20 99.25 98.7±0.6 98.90 99.75 99.15 98.15 98.73 99.24 100.0 100.1 98.5 ± 0.6

Sc 0.39 0.50 0.85 0.58 0.29 0.25 0.48 0.35 0.46 ± 0.19 1.51 1.50 2.21 2.02 1.80 2.60 1.90 2.0 ± 0.4 6.38 4.35 6.83 5.54 4.00 5.70 6.10 5.20 5.5 ± 1.0Ga 2.00 2.40 2.80 1.60 2.00 2.50 3.00 2.40 2.3 ± 0.5 9.50 9.80 9.50 10.30 8.40 9.05 11.30 9.7 ± 0.9 15.20 13.70 17.70 14.70 15.60 14.90 13.02 16.00 15 ± 1

V 3.57 3.24 13.30 1.53 6.80 4.70 2.80 3.10 5 ± 4 11.90 14.90 15.00 12.60 14.20 12.20 13.70 13.5 ± 1.3 51.80 35.00 55.10 36.00 45.70 34.10 50.60 52.80 45 ± 9

Cr 6.93 7.41 5.41 15.20 3.80 7.80 4.90 5.30 7.1 ± 3.5 13.90 10.30 10.50 11.70 10.03 12.70 9.40 11.2 ± 1.6 68.20 54.00 73.20 61.90 64.10 66.10 70.50 55.80 64 ± 7

Cu 12.50 9.90 1.18 1.10 4.32 5.00 4.80 1.60 5 ± 4 1.44 1.78 5.01 2.81 2.09 5.03 3.01 3.0 ± 1.5 10.60 6.80 3.72 1.92 4.61 5.20 3.81 8.30 5.6 ± 2.8

Zn 4.04 4.88 9.58 6.86 4.74 5.30 2.20 4.60 5.3 ± 2.2 10.90 12.90 14.30 10.50 11.40 11.00 12.80 12.0 ± 1.4 61.00 35.70 56.30 39.20 40.70 52.90 50.03 37.90 47 ± 10

Co 1.30 1.40 7.40 1.20 4.93 3.80 4.00 2.70 3.3 ± 2.2 10.00 14.60 15.40 16.30 12.80 13.20 14.00 13.8 ± 2.1 47.40 35.00 51.30 37.10 41.70 39.10 43.60 35.70 41 ± 6

Ni 3.27 2.83 11.60 7.90 2.30 2.70 4.00 3.50 4.8 ± 3.3 3.33 4.69 7.87 5.61 4.07 7.02 5.02 5.4 ± 1.6 45.10 11.50 15.20 10.80 13.70 11.30 9.60 12.70 16 ± 12

Rb 7.64 9.55 5.29 1.89 1.57 6.31 7.00 5.20 5.6 ± 2.7 159.0 114.0 87.20 120.0 115.0 98.00 101.0 113 ± 23 166.0 103.0 115.0 105.0 110.0 120.0 102.0 111.0 116 ± 21

Sr 4.40 13.20 43.40 4.09 17.00 12.30 15.05 32.60 18 ± 14 146.0 141.0 47.20 140.0 115.0 98.00 127.0 116 ± 35 102.0 77.80 94.90 94.00 81.00 72.00 91.00 101.0 89 ± 11

Y 2.67 3.40 18.20 1.59 0.74 2.20 0.90 1.90 4 ± 6 10.40 6.17 3.64 7.53 4.90 7.03 3.05 6.1 ± 2.5 8.67 8.39 12.10 16.30 9.30 10.50 11.06 8.90 11.7 ± 2.6

Zr 26.00 38.30 12.00 10.50 1.32 20.40 23.50 13.80 18 ± 11 111.0 45.60 34.20 51.80 40.20 33.70 50.30 52 ± 27 157.0 191.00 309.0 427.0 280.0 310.0 362.0 206.0 280 ± 91

Nb 0.53 0.51 0.70 0.73 0.81 0.62 0.71 0.60 0.65 ± 0.10 1.97 0.84 0.83 0.85 1.02 1.30 0.73 1.1 ± 0.4 4.66 4.03 5.83 5.08 4.03 3.90 5.25 5.60 4.8 ± 0.8

Ba 38.90 682.00 314.00 25.40 276.00 210.00 88.00 127.00 220 ± 215 999.0 791.0 348.0 520.0 862.0 705.0 410.0 662 ± 243 437.0 609.00 647.0 582.0 420.0 510.0 617.0 592.0 552 ± 86

Hf 1.00 1.16 0.67 0.83 0.98 0.66 0.83 0.94 0.9 ± 0.2 3.52 1.05 0.92 1.52 2.48 1.05 3.04 1.9 ± 1.1 3.73 6.18 7.66 10.40 5.30 6.03 8.20 7.04 6.8 ± 2.0

Pb 5.40 3.94 4.82 7.82 8.07 4.00 7.20 4.50 5.7 ± 1.7 23.70 14.90 4.81 10.05 12.83 18.37 17.26 15 ± 6 14.90 6.59 9.09 6.31 7.83 8.2010.74 11.60 9.4 ± 3.0

Th 1.35 2.24 1.61 1.00 0.97 1.20 2.50 0.75 1.45 ± 0.62 11.70 4.42 1.74 3.90 2.70 4.00 5.30 5 ± 3 7.91 9.62 13.30 14.80 10.00 8.30 11.80 9.20 10.6 ± 2.5

U 4.49 4.95 3.55 3.57 4.82 4.20 3.80 4.00 4.2 ± 0.5 6.40 6.08 5.65 5.02 5.70 6.04 5.30 5.7 ± 0.5 5.92 7.51 8.07 6.00 6.40 6.50 8.30 5.20 6.7 ± 1.1

CIA 69.24 63.20 78.57 66.41 70.04 52.41 68.43 61.85 69 ± 8 57.00 61.38 65.17 62.96 64.40 55.13 57.88 60.6 ± 3.9 64.30 54.58 57.33 55.87 66.73 49.17 57.89 63.60 58.7 ± 5.8

K2O/Na2O 1.51 0.80 4.73 1.93 1.55 0.54 1.50 1.00 1.7 ± 1.3 14.55 13.28 27.95 23.80 15.46 21.72 13.67 18.6 ± 5.8 1.63 0.99 1.14 1.04 1.67 1.48 1.43 1.85 1.40 ± 0.32

SiO2/Al 2O3 85.49 57.06 95.49 139.1 177.98 128.32 154.73 94.15 117 ± 40 6.86 7.45 9.26 8.30 7.11 7.00 8.15 7.7 ± 0.9 4.27 5.59 4.81 5.66 4.03 4.57 5.15 3.93 4.75 ± 0.68

K2O/Al2O3 0.16 0.16 0.12 0.04 0.09 0.17 0.14 0.10 0.13 ± 0.05 0.56 0.45 0.39 0.40 0.40 0.63 0.53 0.48 ± 0.09 0.22 0.28 0.27 0.28 0.20 0.42 0.30 0.23 0.28 ± 0.07

Na2O/K2O 0.66 1.26 0.21 0.52 0.65 1.85 0.67 1.00 0.9 ± 0.5 0.07 0.08 0.04 0.04 0.07 0.05 0.07 0.06 ± 0.02 0.61 1.01 0.88 0.96 0.60 0.68 0.70 0.54 0.75 ± 0.18

Fe2O3/K2O 1.04 0.65 4.89 3.10 2.23 3.62 4.00 1.18 2.6 ± 1.6 0.07 0.10 0.13 0.13 0.11 0.08 0.09 0.10 ± 0.02 1.80 1.06 1.68 1.22 1.73 1.17 0.77 1.85 1.4 ± 0.4

Th/Sc 3.47 4.53 1.89 1.72 3.35 4.80 5.21 2.14 3.4 ± 1.4 7.75 2.95 0.79 1.93 1.50 1.54 2.79 2.7 ± 2.3 1.24 2.21 1.95 2.67 2.50 1.46 1.93 1.77 2.0 ± 0.5

Cr/Th 5.13 3.31 3.36 15.20 3.92 6.50 1.96 7.07 5.8 ± 4.2 1.19 2.33 6.03 3.00 3.72 3.18 1.77 3.0 ± 1.6 8.62 5.61 5.50 4.18 6.41 7.96 5.98 6.07 6.3 ± 1.4

Cr/Ni 2.12 2.62 0.47 1.92 1.65 2.89 1.23 1.51 1.8 ± 0.8 4.17 2.20 1.34 2.07 2.46 1.81 1.87 2.3 ± 0.9 1.39 4.70 4.82 5.73 4.68 5.85 7.34 4.39 4.9 ± 1.7

Th/Co 1.04 1.60 0.22 0.83 0.20 0.32 0.63 0.28 0.6 ± 0.5 1.17 0.30 0.11 0.24 0.21 0.30 0.38 0.39 ± 0.35 0.17 0.27 0.26 0.40 0.24 0.21 0.27 0.26 0.26 ± 0.07

Th/Cr 0.19 0.30 0.30 0.07 0.26 0.15 0.51 0.14 0.24 ± 0.14 0.84 0.43 0.17 0.33 0.27 0.32 0.56 0.42 ± 0.23 0.12 0.18 0.18 0.24 0.16 0.13 0.17 0.17 0.17 ± 0.04

Th/U 0.30 0.45 0.45 0.28 0.20 0.29 0.66 0.19 0.35 ± 0.16 1.83 0.73 0.31 0.78 0.47 0.66 1.00 0.8 ± 0.5 1.34 1.28 1.65 2.47 1.56 1.27 1.42 1.77 1.6 ± 0.4

*Total Fe as Fe2O3; n = number of samples

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(Dabard, 1990; Condie et al. 1992). Clear positivecorrelations of K2O with Al 2O3 (r = 0.82, n = 23) andtrace elements such as Ba (r = 0.77), Rb (r = 0.89) and Th(r = 0.55) for all rock types suggest that concentrations ofthese elements are mainly controlled by the clay minerals(McLennan et al. 1983).

Trace Elements

The concentrations of Co, Ni, Cr, Ba, Zr, Hf, and Th arehigher in the siltstones than in the quartz arenites and arkoses(Fig.4). This variation may partially be due to (1) dilutionby quartz in quartz arenites and arkoses relative to siltstonesand (2) higher clay mineral content in siltstones than quartzarenites and arkoses. The depletion of Zr and Hf in quartzarenites and arkoses than siltstones could be related to theamount of heavy minerals (especially zircon) present inthem.

Feldspar is a major host of Ba and Rb in terrigenoussedimentary rocks (Veizer, 1978). In our study, highcorrelation coefficient between Rb-K2O (r = 0.89, n = 23),Rb-Al2O3 (r = 0.93), and Sr-CaO (r = 0.74), for all rocktypes suggest that the distribution of these elements iscontrolled by Rb incorporation into silicate and Sr intocarbonate phases. In addition, a good positive correlationbetween Ba and K2O (r = 0.77, n = 23) suggests that Ba ismainly associated with K-feldspar.

The concentration of U is high in all rock types (~ 3.55-8.30; Table 1). In the study area, the Bhima Basin, thegranitic rocks tends to have high content of U (~ 3.08-20.76, mean 8.18, n = 28; Kumar and Srinivasan, 2002),which could be the reason for the U enrichment than othertrace elements as well as to upper continental crust (UCC,

Table 2. Rare earth element (ppm) concentrations for quartz arenites, arkoses, and siltstones of the Rabanpalli Formation along with their mean grain size (MZ) in φ units

Rock type Quartz arenites Arkoses Siltstones

Sample # S034 S031 E073 C099 S014 Mean (n = 5) S063 S061 S058 Mean (n = 3) S029 S030 S032 S035Mean (n = 4)

MZ 0.65 1.25 1.75 0.75 0.51 0.91 ± 0.41 2.75 3.25 2.10 2.5 ± 0.5 4.00 4.13 4.30 4.40 4.2 ± 0.2

La 5.29 1.71 6.56 2.47 3.74 4 ± 2 25.10 32.90 8.00 22 ± 13 6.94 12.20 17.20 19.10 14 ± 5

Ce 8.30 7.41 8.29 5.81 4.39 7.0 ± 1.7 45.90 58.00 14.90 40 ± 22 11.80 32.10 38.40 54.30 34 ± 18

Pr 1.20 0.90 1.20 1.13 1.00 1.1 ± 0.1 3.70 4.30 1.40 3.1 ± 1.5 1.50 2.30 3.10 3.90 2.7 ± 1.0

Nd 5.72 3.68 6.22 5.20 3.92 5.0 ± 1.0 14.50 20.80 5.08 13 ± 8 7.22 9.34 14.10 17.10 12 ± 5

Sm 1.80 1.20 1.55 1.31 1.40 1.5 ± 0.2 2.60 3.50 1.50 2.5 ± 1.0 1.80 2.00 2.50 3.30 2.4 ± 0.7

Eu 0.47 0.41 0.29 0.32 0.46 0.4 ± 0.1 0.54 0.55 0.52 0.54 ± 0.02 0.38 0.54 0.69 0.86 0.6 ± 0.2

Gd 2.00 1.70 1.30 1.33 1.15 1.5 ± 0.35 2.10 2.50 1.60 2.1 ± 0.5 2.02 1.90 2.50 3.60 2.5 ± 0.8

Tb 0.27 0.25 0.20 0.19 0.13 0.21 ± 0.05 0.32 0.34 0.24 0.3 ± 0.1 0.32 0.29 0.41 0.54 0.4 ± 0.1

Dy 1.17 1.25 0.92 0.87 0.58 0.96 ± 0.27 1.75 1.70 1.30 1.6 ± 0.2 1.90 1.60 2.30 2.90 2.2 ± 0.6

Ho 0.20 0.23 0.15 0.15 0.09 0.16 ± 0.05 0.35 0.32 0.23 0.3 ± 0.1 0.42 0.35 0.49 0.62 0.5 ± 0.1

Er 0.46 0.51 0.31 0.30 0.17 0.4 ± 0.1 0.98 0.77 0.56 0.8 ± 0.2 1.20 0.97 1.40 1.80 1.3 ± 0.4

Tm 0.047 0.056 0.033 0.032 0.015 0.04 ± 0.02 0.13 0.092 0.07 0.10 ± 0.03 0.16 0.13 0.19 0.24 0.2 ± 0.1

Yb 0.26 0.31 0.17 0.17 0.04 0.2 ± 0.1 0.85 0.56 0.41 0.6 ± 0.2 1.14 0.87 1.33 1.63 1.2 ± 0.3

Lu 0.04 0.05 0.025 0.024 0.01 0.03 ± 0.02 0.13 0.08 0.06 0.09 ± 0.04 0.17 0.13 0.19 0.23 0.18 ± 0.04

La/Sc 13.6 3.45 7.72 4.26 12.90 8.4 ± 4.7 16.60 21.90 3.62 14.06 ± 9.42 1.09 2.80 2.52 3.45 2.46 ± 1.00

LREE/HREE 5.02 3.42 7.66 5.20 6.61 5.6 ± 1.6 13.80 18.78 6.91 13 ± 6 4.00 9.28 8.55 8.45 8 ± 2

∑REE 22.23 19.67 27.22 13.32 17.10 22.1 ± 4.8 98.95 126.4 35.87 87 ± 46 36.97 64.72 84.80 110.1 74 ± 31

Eu/Eu* 0.75 0.88 0.61 0.73 1.08 0.81 ± 0.18 0.69 0.54 1.02 0.75 ± 0.25 0.61 0.84 0.84 0.76 0.8 ± 0.1

(La/Lu)cn 13.73 3.55 27.24 10.68 38.83 19 ± 14 20.04 42.69 13.84 25 ± 15 4.24 9.74 9.40 8.62 8 ± 2

(Gd/Yb)cn 6.23 4.44 6.20 6.34 23.30 9 ± 8 2.00 3.62 3.16 3 ± 8 1.44 1.77 1.52 1.79 1.6 ± 0.2

n = number of samples

Fig.4. Multi-element normalized diagram, normalized againstaverage upper continental crust (Taylor and McLennan,1985), using the following values (in ppm): Co = 10,Ni = 20, Cr = 35, V = 60, Sr = 350, Rb = 112, Ba = 550,Pb = 20, Zr = 190, Y = 22, Nb = 25, Hf = 5.8, Th = 10.7,and U = 2.8. Two horizontal lines for rock/upper continentalcrust values of 1 and 0.1 are included for reference.

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304 R. NAGARAJAN AND OTHERS

Taylor and McLennan, 1985; Fig.4). In addition, a positivecorrelation between U and Th (r = 0.79, n = 23; Fig.5) revealsthe characteristic of felsic source rocks.

DISCUSSION

Geochemical Classification

Geochemical classification of terrigenous sedimentaryrocks has been proposed by many authors based on majorelements composition (Pettijohn et al. 1972; Crook, 1974;Blatt et al. 1980; Herron, 1988). Using the indices of SiO2/Al 2O3 and Na2O/K2O ratios, Pettijohn et al. (1972) proposeda classification for terrigenous sands based on a plot of log(Na2O/K2O) versus log (SiO2/Al 2O3). Herron (1988)modified the diagram of Pettijohn et al. (1972) using log(Fe2O3/K2O) along the Y-axis instead of log (Na2O/K2O).The ratio Fe2O3/K2O facilitates arkoses to be moresuccessfully classified, and it is a measure of mineralstability. Thus, in log (Fe2O3/K2O) versus log (SiO2/Al 2O3)plot (Fig.6; Herron, 1988) eight samples plot in the quartzarenite field, seven samples plot in arkose field and theremaining eight samples plot in the wacke field. This plot isin good agreement with our classification based onpetrography.

K2O/Al2O3 ratio of terrigenous sedimentary rocks canbe used as an indicator of the original composition of ancientsediments, because the K2O/Al2O3 ratio for clay mineralsand feldspars are different. K2O/Al2O3 ratios for clayminerals range from 0.0 to 0.3 and for feldspars it rangefrom 0.3 to 0.9 (Cox et al. 1995). In our study, K2O/Al2O3

ratio in siltstones (0.28 ± 0.07, n = 8) and quartz arenites(0.13 ± 0.05, n = 8) indicates the presence of clay minerals

in these rock types. The high K2O/Al2O3 ratio in arkoses(0.5 ± 0.1, n = 7; Table 1) is interpreted to reflect an inputfrom first cycled granitic material as evidenced by thepresence of K-feldspar through petrography study.

Palaeoweathering

Alteration of minerals due to chemical weathering mainlydepends on the intensity and the duration of weathering.The dominant process during weathering of the upper crustis the degradation of feldspars and concomitant formationof clay minerals. During weathering, calcium, sodium, andpotassium largely are removed from feldspars (Nesbitt etal. 1980). The amount of these chemical elements survivingin the soil profiles and in the sediments derived from themis a sensitive index of the intensity of weathering (Nesbitt etal. 1997). A good measure of the degree of chemicalweathering can be obtained by calculating the chemical indexof alteration (CIA; Nesbitt and Young, 1982) using theformula (molecular proportion)

CIA = [Al 2O3/(Al2O3 + CaO* + Na2O + K2O)] × 100

where CaO* is the amount of CaO incorporated in thesilicate fraction of the rock. However, in the samples studied,CaO is very low (~ 0.053–0.709) and there was no objectiveway to distinguish CaO in carbonate from CaO insilicate, so total CaO (Table 1) was used in measuring CIAvalues.

The CIA is a good measure of paleo-weatheringconditions, and it essentially monitors the progressiveweathering of feldspars to clay minerals (Fedo et al. 1995;Armstrong-Altrin et al. 2004). High CIA values (i.e. 76–

Fig.5. Th-U bivariate plot for the samples of the RabanpalliFormation. Note a good positive correlation between Thand U.

Fig.6. Geochemical classification for the samples of the RabanpalliFormation using log(SiO2/Al 2O3) - log(Fe2O3/K2O)diagram (after Herron, 1988).

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100) indicate intensive chemical weathering in the sourceareas whereas low values (i.e. 50 or less) indicateunweathered source areas. In the present study, quartzarenites exhibit wide range of CIA values (~ 52–79;Table 1). Likewise, arkoses (~ 55–65) and siltstones (~ 49–67; Table 1) also show wide variations. The CIA values arealso plotted in Al2O3-(CaO + Na2O)-K2O (A-CN-K; Nesbittand Young, 1982) compositional space in Fig.7 (molecularproportions). In the A-CN-K triangular diagram, all the rocktypes (except one siltstone) plot above the feldspar join line.Quartz arenites and siltstones are scattered in the A-CN-Kdiagram whereas arkoses exhibit definite trend. Generally,quartz arenites should plot away from the feldspar join lineand their trend should approach A-apex, instead of scatteringnear to feldspar join line. Most of the siltstones are plotwell near to the feldspar join and arkoses follow AK line

alter the earlier composition (Glazner, 1988; Nesbitt andYoung, 1989; Sutton and Maynard, 1992; Condie, 1993;Fedo et al. 1997a, 1997b). Potassium metasomatism isparticularly common, which involves conversion of kaolinto illite by reaction with potassium bearing pore waters (Fedoet al. 1995). In sandstones, K-metasomatism can take placein two different ways, 1) aluminous clay minerals (kaoliniteas matrix) converted into illite and/or 2) conversion ofplagioclase to k-feldspar (Fedo et al. 1995). These processesproduce K2O enrichment in the sedimentary rocks, and itmay vary from the weathering trend. Conversion ofkaolinite into illite by K addition results in a CIA value lowerthan the pre-metasomatised rock (Fedo et al. 1995).Conversion of plagioclase to k-feldspar, where authigenick-feldspar replaces plagioclase by K-metasomatism, theCIA values may not change because the process occur molefor mole substitution of K2 for Ca or Na2 (Glazner, 1988).Both these processes may affect the composition ofsedimentary rocks and the extent of these processes canbe identified by petrographic study (Fedo et al. 1995, 1997a,1997b).

In the present study, quartz arenites exhibit low CIAvalues and arkoses show a typical trend towards K-apex.This type of trend is generally found in the sedimentaryrocks that undergone K-metasomatism, by which additionof K to weathered residues (Fedo et al. 1995; See theirFig.1A). This process produces mineralogical changesresults in lowering of CIA values. Hence quartz arenitesplot nearer to the feldspar join rather than displaying theiroriginal chemical maturity (Fig.7). Likewise, siltstones andarkoses also plot nearer to the feldspar join. In the presentstudy, K-metasomatic effect can be identified from thetypical trend of arkoses. Arkoses following the A-K line(Fig.7), which exhibit the addition of K to this rock type. Itis also supported by petrographic study, which shows mostof the arkoses display partially or fully altered plagioclasegrains and it also exhibits the presence of illite as matrixmaterial. Hence the observed low CIA values in thesedimentary rocks of Rabanpalli Formation are mainlydue to the K-metasomatism.

Th/U in terrigenous sedimentary rocks is of interestbecause weathering tends to result in oxidation of insolubleU4+ to soluble U6+ with loss of solution and elevationof Th/U ratios (McLennan and Taylor, 1980, 1991). TheTh/U ratios in the studied samples range from 0.19 to 2.47(Table 1). Upper crustal igneous rocks have Th/U ratiosaveraging about 3.8, with considerable scatter (Taylor andMcLennan, 1985; Condie, 1993; McLennan, 2001). Thesedimentary rocks of Rabanpalli Formation show lowTh/U ratios when compared with upper continental crust

Fig.7. A-CN-K diagram (after Nesbitt and Young, 1982) showing1samples of this study and average composition of 2 uppercontinental crust (UCC; Taylor and McLennan, 1985).A = Al 2O3; CN = CaO* + Na2O; K = K2O (molecularproportion; CaO* = CaO in silicate fraction only);CIA = Chemical Index of Alteration (Nesbitt and Young,1982).

instead of following A-CN line. Apart from this, most of thestudied samples contain considerable amount of K2O thanexpected, and hence it may under gone K-metasomatism.The sedimentary rocks affected by K-metasomatism,generally exhibit low values than the premetasomatisedcomposition (Fedo et al. 1995).

K-Metasomatism

Metasomatic enrichment of potassium to sediments andsedimentary rocks produces mineralogical changes, which

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306 R. NAGARAJAN AND OTHERS

value. The observed low Th/U ratios are mainly due to theelevated concentration of U.

Provenance

Source rock composition is commonly thought to be thedominant factor that controls the composition of sedimentsderived from them (Taylor and McLennan, 1985). However,secondary processes (weathering, transport, diagenesis, etc.)can have an effect on chemical composition (Cullers et al.1987; Wronkiewicz and Condie, 1987), and therefore onebest relies on elements that show little mobility under theexpected geological conditions. Taylor and McLennan(1985) pointed out that such elements should possess verylow partition coefficients between natural waters and uppercrust and short oceanic residence times.

REE, Th, and Sc are quite useful for inferring crustalcompositions, because their distribution is not significantlyaffected by secondary processes such as diagenesis andmetamorphism, and is less affected by heavy mineralfractionation than that for elements such as Zr, Hf, and Sn(Bhatia and Crook, 1986; McLennan, 2001). REE and Thabundances are higher in felsic than mafic igneous sourcerocks and in their weathered products, whereas Co, Sc, V,Ni, and Cr are more concentrated in mafic than felsic igneoussource rocks and their weathered products. In addition, theseelements are relatively immobile during weathering. Theseelements are believed to be transported exclusively in theterrigenous component of sediment and therefore reflect thechemistry of their source rocks (Veizer, 1978; McLennan etal. 1980; Armstrong-Altrin, 2004).

Very high levels of Cr and Ni have been used by manyauthors (e.g., Hiscott, 1984; Wrafter and Graham, 1989) toinfer an ultramafic provenance for sediments. Furthermore,the unusual enrichment of Ni unaccompanied by other

ferromagnesian trace elements is also addressed byArmstrong-Altrin et al. (2004). Garver et al. (1996)suggested that the sediments having elevated concentrationof Cr (> 150 ppm) and Ni (> 100 ppm), high correlationcoefficient of Cr with Ni, and Cr/Ni ratio of ~ 1.4 areindicative of ultramafic source. Higher Cr/Ni ratios probablyindicate mafic source rocks (Garver and Scott, 1995). Inour study, Cr and Ni values, and Cr/Ni ratios arecomparatively higher in siltstones (64 ± 7, 16 ± 12, and5±2, respectively) than quartz arenites (7.1 ± 3.5, 4.8±3.3and 1.8±0.8, respectively) and arkoses (11.2±1.6, 5.4±1.6,and 2.3 ± 0.9, respectively; Table 1), but the values arelower than the sediments derived from ultramafic sourcerocks, except Cr/Ni ratios. The negative correlation ofCr with Ni for arkoses (r = -0.1) and low correlation forquartz arenites (r = 0.3) and siltstones (r = 0.3) imply thatthese sedimentary rocks were derived from the felsic sourcerocks. Likewise, low V (21±19) and Sc (2.6±2.3; Table 1)concentrations are also observed in all the rock types(concentration of V in sediments is about 20 ppm, McCann,1991). Thus the lower values of Cr, Ni, V, and Sc in thequartz arenites, arkoses, and siltstones suggest that thesesediments were mainly derived from the felsic source rocksrather than mafic to ultramafic source rocks.

Furthermore, the ratios such as Eu/Eu*, (La/Lu)cn,La/Sc, Th/Sc, Th/Co, Th/Cr, and Cr/Th are significantlydifferent in mafic and felsic source rocks and may allowconstraints on the provenance of sedimentary rocks(Wronkiewicz and Condie, 1987; Cullers et al. 1988;Wronkiewicz and Condie, 1989, 1990; Cullers, 1994b, 1995;Cox et al. 1995; Armstrong-Altrin et al. 2004). The Eu/Eu*,(La/Lu)cn, La/Sc, Th/Sc, Th/Co, Th/Cr, and Cr/Th ratios(Table 3) of quartz arenites, arkoses, and siltstones ofthe Rabanpalli Formation are compared with those in

Table 3. Range of elemental ratios for quartz arenites, arkoses, and siltstones in this study compared to the ratios in similarfractions derived from felsic rocks, mafic rocks, and upper continental crust

Elemental Range of sandstones and siltstonesRange of sediments from2

Upperratio from the Rabanpalli Formation1

ContinentalQuartz arenites Arkoses Siltstones Crust3

(n = 8) (n = 7) (n = 8)Felsic rocks Mafic rocks

Eu/Eu*4 0.61-1.08 0.54-1.02 0.61-0.83 0.40-0.94 0.71-0.95 0.63

(La/Lu)cn4 3.55-38.83 13.84-42.69 4.24-9.74 3.00-27.0 1.10-7.00 9.73

La/Sc4 3.45-13.60 3.62-21.90 1.09-3.45 2.5-16.3 0.43-0.86 2.21

Th/Sc 1.72-5.21 0.79-7.75 1.24-2.67 0.84-20.5 0.05-0.22 0.79

Th/Co 0.20-1.60 0.11-1.17 0.17-0.40 0.67-19.4 0.04-1.4 0.63

Th/Cr 0.07-0.51 0.17-0.84 0.12-0.24 0.13-2.7 0.018-0.046 0.13

Cr/Th 1.96-15.20 1.19-6.03 4.18-8.62 4.00-15 25-500 7.76

1 This study; 2 Cullers (1994a, 2000); Cullers and Podkovyrov (2000); Cullers et al. (1988); 3 Taylor and McLennan (1985)4 n (number of samples) = 5 for quartz arenites; n = 3 for arkoses; n = 4 for siltstones

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sediments derived from felsic and mafic source rocks(Cullers et al. 1988; Cullers, 1994a, 2000; Cullers andPodkovyrov, 2000, 2002) as well as with upper continentalcrust (UCC; Taylor and McLennan, 1985; Table 3). Thiscomparison points out that the trace elemental ratiosof this study are comparable to the range of sedimentsderived from felsic source rocks rather than mafic sourcerocks.

Th/Sc vs Sc bivariate and La-Th-Sc triangular plotsprovide useful information regarding the source rockscharacteristics (McLennan and Taylor, 1991; Cullers, 2002).The elemental ratio (Th/Sc) and concentrations (Sc, La, Th)of terrigenous rocks of Rabanpalli Formation are plotted inthe Th/Sc vs Sc (Fig.8) and La-Th-Sc (Fig.9) diagrams tofind out the source rocks characteristics. UCC value(McLennan, 2001), Archaean granite, cratonic sandstone,andesite, and basalt + komatiite (Condie, 1993) values areplotted in these two diagrams for comparison. Severalinformations can be made from the Th/Sc vs Sc diagram.Th/Sc ratio is more or less similar in quartz arenites, arkoses,and siltstones, which indicate that the Th/Sc ratio is notaffected by the sorting processes. This information impliesthat Th and Sc are not present in the minerals, which areeasily removed during weathering and/or other sedimentaryprocesses, and Th/Sc ratio can be considered as the one ofthe best indicators of provenance study (Taylor andMcLennan, 1985). Th/Sc ratio, when plotted againstconcentration of Sc that is more sensitive to provenancecomposition than REE (Fedo et al. 1997a).

In the Figures 8 and 9, the quartz arenites, arkoses, andsiltstones are fall near to UCC, Archaean granite, andcratonic sandstone values, which strongly supports that the

studied samples were mainly derived from the felsic sourcerocks rather than the mafic source rocks.

In addition, the relative REE patterns and Eu anomalysize have also been used to infer sources of sedimentaryrocks (Taylor and McLennan, 1985; Wronkiewicz andCondie, 1989). Mafic rocks contain low LREE/HREE ratiosand tend not to contain Eu anomalies, whereas more felsicrocks usually contain higher LREE/HREE ratios andnegative Eu anomalies (Cullers and Graf, 1984). Thedepletion of Eu may be interpreted as shallow, intracrustaldifferentiation, which resulted in Eu-depletion in the uppercontinental crust, associated with the production of graniticrocks (McLennan, 1989). Some Precambrian rocks liketonalite-tronjhemite gneiss (TTG), granodiorite, and quartzdiorite show very large LREE/HREE ratios with positiveEu anomaly and their positive anomaly arises not becauseof enrichment of feldspars but is mainly due to hornblende-melt equilibria (Cullers and Graf, 1984). In the present study,all rock types exhibit higher LREE/HREE ratio (8 ± 4, n =12; Table 2) and a significant negative Eu anomaly (0.77 ±0.16, n = 12; Table 2; Fig.11) indicates the felsic igneousrocks as a possible source rocks.

Discriminant Function Diagram

Discriminant function scores of major element datapermit separation of provenance into four major groups:mafic igneous; intermediate igneous; felsic igneous; andquartzose sedimentary (Roser and Korsch, 1988). In thisdiscrimination diagram (Fig.10), the quartz arenites and

Fig.8. Th/Sc vs Sc bivariate plot for the samples of the RabanpalliFormation. 1 This study, 2 upper continental crust (UCC;McLennan, 2001), and 3 Condie (1993).

Fig.9. La–Th–Sc triangular plot for the samples of the RabanpalliFormation. 1 This study, 2 upper continental crust(McLennan, 2001), and 3 Condie (1993).

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arkoses plot within the quartzose sedimentary provenancefield, and siltstones plot both in the felsic igneous andintermediate igneous provenance fields. This observationclearly indicates the less possibility of the mafic rocks assource rocks for the studied samples of the RabanpalliFormation (Fig.10).

Probable Source Rocks

To know the probable source rocks for the quartzarenites, arkoses, and siltstones of the Rabanpalli Formation,in Fig.11, the average REE data were compared with thoseof Archaean granites (Jayaram et al. 1983; Jayananda et al.1995), and mafic rocks (Khan, 1992; Rao et al. 1999), whichbelongs to the adjacent area (south of the Kaladgi Basin;Fig.1). The chondrite normalize REE plots (Fig.11) ofRabanpalli Formation show LREE enriched and flat HREEpatterns with significant negative Eu anomaly. The shapesof the REE patterns of these rock types are similar to thegranites as well as to upper continental crust (UCC; Taylorand McLennan, 1985). Further more, the rocks of our studyexhibit a clear negative Eu anomaly as similar to the granitesas well as to UCC, but the mafic rocks do not have thenegative Eu anomaly (Fig.11). Thus, we interpreted that allrock types in the present study probably derived from thegranite rocks, which belongs to the adjacent area (south ofthe Kaladgi Basin; Fig.1). Furthermore, quartz arenites and

Fig.10. Discriminant Function diagram for sedimentary provenanceusing major elements (Roser and Korsch, 1988).The discriminant functions are: Discriminant Function 1= (-1.773.TiO2) + (0.607.Al 2O3) + (0.760.Fe2O3) +(-1.500.MgO) + (0.616.CaO) + (0.509.Na2O) +(-1.224.K2O) + (-9.090); Discriminant Function 2= (0.445.TiO2) + (0.070.Al 2O3) + (-0.250.Fe2O3) +(-1.142.MgO) + (0.438.CaO) + (1.475.Na2O) +

(-1.426.K2O) + (-6.861).

Fig.11. Average chondrite-normalized REE patterns for samplesfrom this study and other rock types for comparison.1This study; 2upper continental crust (UCC; Taylorand McLennan, 1985); 3Jayananda et al. (1995); 4Jayaramet al. (1983); 5Khan (1992) and Rao et al. (1999).n = number of samples. Chondrite-normalized valuesare from Taylor and McLennan (1985).

Fig.12. Plot of Eu/Eu* versus (Gd/Yb)cn for the samples of theRabanpalli Formation. Fields are after McLennan andTaylor (1991). 1This study; 2 upper continental crust (UCC;Taylor and McLennan, 1985); 3Jayananda et al. (1995);4Jayaram et al. (1983); 5Khan (1992) and Rao et al. (1999);6Rao et al. (1999). n = number of samples.

arkoses have (Gd/Yb)cn ratios more than 2 and siltstoneshave less than 2 (Table2; Fig.12), suggesting that quartzarenites and arkoses were derived from sources havingsomewhat depleted heavy rare earth elements whereassiltstones were derived from less HREE-depleted Archaeanor post-Archaean sources, or a combination of both. Theaverage ratios of Archaean granites (Jayaram et al. 1983;Jayananda et al. 1995), mafic rocks (Khan, 1992; Rao et al.1999), Proterozoic shales (Rao et al. 1999) from the source

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area are also shown in this plot. The overlapping of thestudied samples with the Archaean granites and Proterozoicshales, suggesting that all rock types in the present studycould have been derived by the contributions from theadjacent area (south of the Kaladgi Basin; Fig.1).

CONCLUSIONS

The rock types were identified as quartz arenites, arkoses,and siltstones using petrography. Other major elementbivariate plots also support our petrographic observations.The sedimentary rocks of the Rabanpalli Formation showlow CIA values and these values were plotted in theA-CN-K diagram in order to find out the paleoweatheringcondition of the source rocks, which reveal that the observedlow CIA values are mainly due to K-metasomatism.

The Cr, Ni, V, and Sc values for all rock types in ourstudy clearly suggest that they were derived from felsicsource rocks rather than mafic and/or ultramafic sourcerocks. The rare earth elements concentration, other traceelement ratios such as Eu/Eu*, (La/Lu)cn, La/Sc, Th/Sc, Th/Co, Th/Cr, and Cr/Th, and the Sc-Th/Sc and La-Th-Scdiagrams of all rock types suggest that these sediments werederived from felsic source rocks rather than mafic source

rocks. This interpretation is in good agreement with themajor element discriminant function diagram. Thus, weinterpreted that all rock types in this study can be derivedfrom felsic source rocks. Furthermore, the REE patterns and(Gd/Yb)cn ratios of different rock types in this study arevery similar to the granite rocks, which belong to the adjacentsource area and we conclude that the granite rocks can bethe possible source rocks.

Acknowledgements: We are grateful to the reviewer Robert L.Cullers for his numerous helpful comments to improve our paper.We would like to thank Prof. S.P. Mohan, Head, Department ofGeology, University of Madras for providing certain laboratoryfacilities through UGC SAP-II, UGC COSIST and DST-FISTprograms. RN wishes to express his sincere thanks to N. RajeswaraRao, V. Ram Mohan, L. Elango, and S. Srinivasalu for theirconstant encouragement during this study. JSA wishes to expresshis gratefulness to Otilio A. Acevedo Sandoval, Enrique CruzChávez, and Kinardo Flores Castro, Centro de Investigaciones enCiencias de la Tierra, Universidad Autónoma del Estado de Hidalgo(UAEH). Financial assistance by SEP-PROMEP (Programa deMejoramiento del Profesorado; Grant No: UAEHGO-PTC-280),SNI–CONACYT (Consejo Nacional de Ciencia y Tecnología),and PII (programa Institucional de Investigación; Grant No:UAEH-DIP-ICBI-AACT-274), Mexico, are highly appreciated.

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(Received: 23 August 2004; Revised form accepted: 27 June 2006)