Sandstone Geochemistry

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 Paleoweathering Conditions, Provenance and Source Rock Composition R. NAGARAJAN1, J.S. ARMSTRONG-ALTRIN2*, R. NAGENDRA1, J. MADHAVARAJU3 and J. MOUTTE4 1

School of Civil Engineering, Sastra University, Thanjavur - 613 402, India 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] 2

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 SiO2 (average with one standard deviation being 97±1, 73±2, respectively) and lower Al2O3 (0.95±0.4, 9.6±0.9, respectively) concentrations than siltstones (SiO2 = 64±4, Al2O3 = 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, 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 b ivariate 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, Din elli, 2001; Amorosi

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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 terrigenous sedimentary rocks reflects changes in the mineralogical composition of the sediments due to the effects of weathering and diagenetic processes (Nesbitt and Young, Young, 1984, 1989; Wandres et al. 2004). Also the spatial and temporal patterns of sedimentation determine changes in the mineralogy and sorting of sediments, which in turn affect their bulk composition (Nesbitt et al. 1996; Garcia et al. 2004). Although mineralogically unstable and soluble elements are affected 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 source rocks. Hence these elements and their elemental ratios are highly useful to determine the provenance characteristics of sediments. This approach has provided useful results, especially when geological processes have destroyed the original mineralogy (Cullers, 1994a, 1995). In addition, the chemical approach is a good complement to petrographic analysis of terrigenous sedimentary rocks and the two methods combined are a powerful tool for examination of provenance and weathering (van de Kamp and Leake, 1985; Shao et al. 2001; Cingolani et al. 2003; Le Pera and Arribas, 2004). In the present study, we attempt to 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 element geochemistry as well as by petrographic analysis. Also this study describes the importance of some ferromagnesian trace elements to distinguish the felsic, mafic, and/or ultramafic source rocks. GENERAL GEOLOGY

The Bhima Basin, southern India is a NE-SW trending S-shaped Neoproterozoic, epicratonic, extensional basin formed due to gravity faulting. Total thickness of sediment is about 300 m extended over an area of 5,000 km 2. The sedimentary rocks of Bhima Basin have been divided into five distinct formations i.e. (i) Rabanpalli Formation, (ii) Shahabad Formation, (iii) Halkal Shale, (iv) Katamadevarhalli Formation and (v) Harwal Shale (Janardhana Rao et al. 1975). It comprises an alternating sequence of terrigenous and carbonate sediments. In the terrigenous unit, fine-grained sediments dominate over coarse-grained sediments (Kale (Kal e et al. 1990). The terrigenous sedimentary rocks constituting the lower Bhima Basin is

designated as the Rabanpalli Formation and is well exposed in Adki, Gogi, and Muddebihal areas (Fig.1). The Rabanpalli Formation mainly consists of quartz arenites, arkoses, siltstones, and greenish yellow shale. Sedimentation in the Bhima Basin started with the deposition of a thin conglomerate but, the conglomerate exposures are very few. It contains a considerable amount of angular and subangular potash feldspar grains and occasionally pink granite clasts. Arkoses are located at the bottom. The arkoses are very fine to medium-grained, showing graded bedding, and consist mostly of angular and sub-angular potash feldspar grains with minor amounts of sub-rounded, quartz grains. The siltstones are a transitional member between the arkoses and the overlying greenish yellow shales. Quartz arenites are medium to coarse-grained, showing irregular graded beddings, horizontal laminations, ripple marks, and cross-laminations. MATERIALS AND METHODS

Fresh samples were collected from the outcrops and the samples were washed thoroughly in distilled water to remove the contamination. The samples were disaggregated following the procedure adopted in Cox and Lowe (1996). Grain size analysis was carried out in a Ro-Tap Ro-Tap sieve shaker using American Society for Testing and Material (ASTM) sieves ranging from –1.5 φ to 4.25 φ at 0.50 φ intervals for 20 minutes (Folk, 1966). Cumulative curves were constructed to calculate the statistical grain size parameters (MZ: mean grain size) after Folk and Ward Ward (1957). A detailed petrographic study covering more than 25 thin sections were studied. The thin sections were subjected to Alizarin Red-S stain to confirm the presence or absence of dolomite and calcite, and potassium ferricyanide to ascertain the presence of ferroan/nonferroan calcite. Friedman’s (1959) organic stain specific for calcite and Katz and Friedman’s (1965) combined organic and inorganic stain specific for iron rich calcite have been adopted to identify the mineralogical variations. For modal analysis, four hundred frame work grains were counted from each thin section. Matrix and cement were not counted. The point counts were done using both Gazzi-Dickinson (Gazzi, 1966; Dickinson, 1970) and traditional methods. Twenty three samples (eight quartz arenites, seven arkoses, and eight siltstones) were selected for major and trace elements study. Twelve samples (five quartz arenites, three arkoses, and four siltstones) were selected for rare earth elements study. s tudy. The major, trace, and rare earth elements were analysed using an inductively coupled JOUR.GEOL.SOC.INDIA, VOL.70, AUGUST 2007

PETROGRAPHY PETROGRAPHY AND GEOCHEMISTR GEOCHEMISTRY Y OF TERRIGENOUS TERRIGENOUS SEDIMENT SEDIMENTAR ARY Y ROCKS IN BHIMA BASIN, BASIN, KARNA KARNATAKA TAKA

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

to the Rabanpalli Formation.

plasma atomic emission spectrometer (ICP-AES - JobinYvon JY138 Ultrace) at the Department of Geochemistry, Ecole des Mines de Saint-Etienne, France. SiO 2, Nb, Zr, and Th were analyzed by XRF method on pressed pellets. The analytical precision for trace and REE is better than 5%. PETROGRAPHY Quartz Arenites

Quartz arenites mainly consist of well preserved, fine to coarse-grained quartz (0.51 φ to 1.75 φ; Table 1). The finegrained quartz grains are angular to sub-angular in shape (Fig.2A). Although dominated by quartz, smaller amounts of rock fragments and potash feldspars are also present. Among the quartz grains, monocrystalline quartz shows both straight and undulatory extinction. Polycrystalline quartz exceeds monocrystalline quartz in quartz arenites and most of the polycrystalline quartz grains consist of more than three crystals per grain, which exhibit sutured crystal boundaries (Fig.2B). The framework grains show long and concavo-convex contacts. Rock fragments are JOUR.GEOL.SOC.INDIA, VOL.70, AUGUST 2007

predominantly sedimentary and metamorphic. Resistant heavy minerals, zircon and tourmaline are also present in the quartz arenites. The quartz arenites show substantial amount of quartz overgrowth (Fig.2C). Two types of cements are encountered in the quartz arenites, i.e. quartz cement and iron oxide cement. Quartz cement occurs as typical syntaxial overgrowths that make up several percent. Some quartz grains show both smaller mode (2-3 µm) and larger mode (30-40 µm) overgrowths. overgrowth s. In quartz grains with smaller mode of overgrowth, the crystals lack well developed faces and form “blob-like” features as mentioned by Pittman (1972), whereas the crystals having larger mode show s how smooth and well formed crystal faces. Iron oxides are dark brown in colour and present in the pore spaces. The original boundaries of some detrital grains are lost, providing evidence for pressure solution effect. This reveals a high degree of compositional maturity, because they are mainly composed of resistant quartz. The quartz arenites exhibit bimodal texture with well-rounded grains from about 0.3 to 0.7 mm in diameter, whereas fine-grained angular grains are generally 0.05 to 0.1 mm in diameter.

300

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 crystal boundaries. (C) Quartz overgrowth in quartz arenites. (D) Mono crystalline quartz grains, feldspar (microcline) and rock fragments in arkose. (E) Feldspar grain dissolution in arkose. (F) Well-sorted Well-sorted grain supported siltstone.

Such textures have been encountered in numerous upper Precambrian and lower Paleozoic quartz-rich sandstones (Folk, 1966). The high content of quartz, minor amounts of feldspar and rock fragments, and restricted concentration of heavy minerals such as tourmaline and zircon suggest the mineralogical maturity for the quartz arenites. This interpretation is in good agreement to our geochemistry results.

Arkoses

Arkoses consist of monocrystalline and polycrystalline quartz, and feldspar with minor amounts of biotite and opaque minerals (Fig.2D). The detrital grains are very fine to medium (MZ = 2.0 φ to 3.25 φ; Table 1), sub-angular to sub-rounded in nature, which exhibit long and concavoconvex contacts. Among quartz monocrystalline quartz dominates over polycrystalline quartz. Monocrystalline JOUR.GEOL.SOC.INDIA, VOL.70, AUGUST 2007


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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 crystal units and > 3 crystal units per grain. The grains with 2-3 crystal units show straight crystal boundaries, whereas grains with > 3 crystal units exhibit sutured boundaries. The feldspars are microcline, orthoclase, and minor plagioclase. Feldspar grains shows initial to fifth stage in degree of dissolution (Fig.2E) and are altered to illite. Lithic fragments are quartz and feldspar grains, and other rock clasts. Mica and chlorite are also present within this rock framework. Quartz overgrowths and minor amount of clay matrix are present. Two types of cements are encountered, which are silica and iron oxide cements. Iron oxide cement is in considerable amount. Siltstones

Siltstones are fairly well-sorted rocks that contain approximately >70% of fine, sub-angular to sub-rounded quartzose grains and some amount of both alkali and plagioclase 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. Grains are closely packed. The cements are siliceous and ferruginous, with significant amount of clay matrix. Detrital Modes

The average framework grain modes of quartz arenites and arkoses from Rabanpalli Formation are Q 96.4F2.7L0.80 and Q82F15.5L2.5, respectively. Quartz, feldspar, and lithic fragments values are plotted in the QFL diagram (Fig.3; Dickinson and Suczek, 1979) to find out the tectonic setting of the source rocks. All the samples from quartz arenites and arkoses are fall in the field of cratonic interior, which clearly indicates that these sedimentary rocks were derived from the igneous source rocks. GEOCHEMISTRY

Major (wt. %), trace (ppm), and rare earth element (ppm) concentrations along with the mean grain size values (M Z) of quartz arenites, arkoses, and siltstones of the Neoproterozoic Rabanpalli Formation, Bhima Basin are reported in the Tables 1 and 2. Major Elements

The CaO content is very low (wt. %; ~0.05-0.97; Table Table 1) in all rock types (quartz arenites, arkoses, and siltstones). The K2O content is higher in arkoses (average with one standard deviation being 4.60 ± 1.20, n = 7) and siltstones JOUR.GEOL.SOC.INDIA, VOL.70, AUGUST 2007

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).

(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- feldspar content among rock types. The Na 2O content is more in siltstones (2.7 ± 0.7) than arkoses (0.27 ± 0.12), and quartz arenites (0.13 ± 0.12; Table 1), which can be attributed to the greater amount of Na-rich plagioclase and alkali feldspar in siltstones. Quartz arenites and siltstones show low variation in K 2O/Na2O ratio (~ 0.5-4.7, ~ 0.991.85, respectively) whereas arkoses exhibit high variation (~ 13-28). Similarly, the siltstones have high content of Fe 2O3 when compared to quartz arenites and arkoses (Table 1). Quartz arenites and arkoses show the highest SiO 2 and lowest Al 2O3 concentrations than siltstones (Table 1), which is mainly due to the presence of quartz and absence of other Al-bearing minerals. This is in good agreement with the petrographic observation, according to that quartz arenites and arkoses contain significant amount of quartz relative to that of feldspar and lithic li thic fragment. This suggest that quartz arenites and arkoses were weathered or diagenetically altered to remove feldspar and lithic fragments and thus increasing the relative proportion of quartz relative to the source rock (Nesbitt et al. 1996). TiO 2 content is more in the siltstones (~ 0.25-0.62) than the quartz arenites (~ 0.01-0.03) and arkoses (~ 0.02-0.09; Table 1). Low content of TiO 2 in quartz arenites and arkoses is mainly due to the negligible amount of phyllosilicates among them


303

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

Rock type Sample #

Quartz arenites S034

Arkoses

Siltstones

S031

E073

C099

S014 Mean ( n = 5)

S063

S061

S058

Mean ( n = 3)

S029

S030

S032

MZ 0.65 1.25 La 5.29 1.71 Ce 8.30 7.41 Pr 1.20 0.90 Nd 5.72 3.68 Sm 1.80 1.20 Eu 0.47 0.41 Gd 2.00 1.70 Tb 0.27 0.25 Dy 1.17 1.25 Ho 0.20 0.23 Er 0.46 0.51 Tm 0.047 0.056 Yb 0.26 0.31 Lu 0.04 0.05 La/Sc 13.6 3.45 LREE/HREE 5.02 3.42 ∑REE 22.23 19.67 * Eu/Eu 0.75 0.88 (La/Lu)cn 13.73 3.55 (Gd/Yb)cn 6.23 4.44

1.75 6.56 8.29 1.20 6.22 1.55 0.29 1.30 0.20 0.92 0.15 0.31 0.033 0.17 0.025 7.72 7.66 27.22 0.61 27.24 6.20

0.75 2.47 5.81 1.13 5.20 1.31 0.32 1.33 0.19 0.87 0.15 0.30 0.032 0.17 0.024 4.26 5.20 13.32 0.73 10.68 6.34

0.51 3.74 4.39 1.00 3.92 1.40 0.46 1.15 0.13 0.58 0.09 0.17 0.015 0.04 0.01 12.90 6.61 17.10 1.08 38.83 23.30

2.75 25.10 45.90 3.70 14.50 2.60 0.54 2.10 0.32 1.75 0.35 0.98 0.13 0.85 0.13 16.60 13.80 98.95 0.69 20.04 2.00

3.25 32.90 58.00 4.30 20.80 3.50 0.55 2.50 0.34 1.70 0.32 0.77 0.092 0.56 0.08 21.90 18.78 126.4 0.54 42.69 3.62

2.10 8.00 14.90 1.40 5.08 1.50 0.52 1.60 0.24 1.30 0.23 0.56 0.07 0.41 0.06 3.62 6.91 35.87 1.02 13.84 3.16

2.5 ± 0.5 22 ± 13 40 ± 22 3.1 ± 1.5 13 ± 8 2.5 ± 1.0 0.54 ± 0.02 2.1 ± 0.5 0.3 ± 0.1 1.6 ± 0.2 0.3 ± 0.1 0.8 ± 0.2 0.10 ± 0.03 0.6 ± 0.2 0.09 ± 0.04 14.06 ± 9.42 13 ± 6 87 ± 46 0.75 ± 0.25 25 ± 15 3±8

4.00 6.94 11.80 1.50 7.22 1.80 0.38 2.02 0.32 1.90 0.42 1.20 0.16 1.14 0.17 1.09 4.00 36.97 0.61 4.24 1.44

4.13 12.20 32.10 2.30 9.34 2.00 0.54 1.90 0.29 1.60 0.35 0.97 0.13 0.87 0.13 2.80 9.28 64.72 0.84 9.74 1.77

4.30 17.20 38.40 3.10 14.10 2.50 0.69 2.50 0.41 2.30 0.49 1.40 0.19 1.33 0.19 2.52 8.55 84.80 0.84 9.40 1.52

0.91 ± 0.41 4±2 7.0 ± 1.7 1.1 ± 0.1 5.0 ± 1.0 1.5 ± 0.2 0.4 ± 0.1 1.5 ± 0.35 0.21 ± 0.05 0.96 ± 0.27 0.16 ± 0.05 0.4 ± 0.1 0.04 ± 0.02 0.2 ± 0.1 0.03 ± 0.02 8.4 ± 4.7 5.6 ± 1.6 22.1 ± 4.8 0.81 ± 0.18 19 ± 14 9±8

S035 Mean ( n = 4) 4.40 19.10 54.30 3.90 17.10 3.30 0.86 3.60 0.54 2.90 0.62 1.80 0.24 1.63 0.23 3.45 8.45 110.1 0.76 8.62 1.79

4.2 ± 0.2 14 ± 5 34 ± 18 2.7 ± 1.0 12 ± 5 2.4 ± 0.7 0.6 ± 0.2 2.5 ± 0.8 0.4 ± 0.1 2.2 ± 0.6 0.5 ± 0.1 1.3 ± 0.4 0.2 ± 0.1 1.2 ± 0.3 0.18 ± 0.04 2.46 ± 1.00 8±2 74 ± 31 0.8 ± 0.1 8±2 1.6 ± 0.2

n = number of samples

(Dabard, 1990; Condie et al. 1992). Clear positive

The concentration of U is high in all rock types (~ 3.55-


303

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

Rock type Sample #

Quartz arenites S034

Arkoses

Siltstones

S031

E073

C099

S014 Mean ( n = 5)

S063

S061

S058

Mean ( n = 3)

S029

S030

S032

MZ 0.65 1.25 La 5.29 1.71 Ce 8.30 7.41 Pr 1.20 0.90 Nd 5.72 3.68 Sm 1.80 1.20 Eu 0.47 0.41 Gd 2.00 1.70 Tb 0.27 0.25 Dy 1.17 1.25 Ho 0.20 0.23 Er 0.46 0.51 Tm 0.047 0.056 Yb 0.26 0.31 Lu 0.04 0.05 La/Sc 13.6 3.45 LREE/HREE 5.02 3.42 ∑REE 22.23 19.67 Eu/Eu* 0.75 0.88 (La/Lu)cn 13.73 3.55 (Gd/Yb)cn 6.23 4.44

1.75 6.56 8.29 1.20 6.22 1.55 0.29 1.30 0.20 0.92 0.15 0.31 0.033 0.17 0.025 7.72 7.66 27.22 0.61 27.24 6.20

0.75 2.47 5.81 1.13 5.20 1.31 0.32 1.33 0.19 0.87 0.15 0.30 0.032 0.17 0.024 4.26 5.20 13.32 0.73 10.68 6.34

0.51 3.74 4.39 1.00 3.92 1.40 0.46 1.15 0.13 0.58 0.09 0.17 0.015 0.04 0.01 12.90 6.61 17.10 1.08 38.83 23.30

2.75 25.10 45.90 3.70 14.50 2.60 0.54 2.10 0.32 1.75 0.35 0.98 0.13 0.85 0.13 16.60 13.80 98.95 0.69 20.04 2.00

3.25 32.90 58.00 4.30 20.80 3.50 0.55 2.50 0.34 1.70 0.32 0.77 0.092 0.56 0.08 21.90 18.78 126.4 0.54 42.69 3.62

2.10 8.00 14.90 1.40 5.08 1.50 0.52 1.60 0.24 1.30 0.23 0.56 0.07 0.41 0.06 3.62 6.91 35.87 1.02 13.84 3.16

2.5 ± 0.5 22 ± 13 40 ± 22 3.1 ± 1.5 13 ± 8 2.5 ± 1.0 0.54 ± 0.02 2.1 ± 0.5 0.3 ± 0.1 1.6 ± 0.2 0.3 ± 0.1 0.8 ± 0.2 0.10 ± 0.03 0.6 ± 0.2 0.09 ± 0.04 14.06 ± 9.42 13 ± 6 87 ± 46 0.75 ± 0.25 25 ± 15 3±8

4.00 6.94 11.80 1.50 7.22 1.80 0.38 2.02 0.32 1.90 0.42 1.20 0.16 1.14 0.17 1.09 4.00 36.97 0.61 4.24 1.44

4.13 12.20 32.10 2.30 9.34 2.00 0.54 1.90 0.29 1.60 0.35 0.97 0.13 0.87 0.13 2.80 9.28 64.72 0.84 9.74 1.77

4.30 17.20 38.40 3.10 14.10 2.50 0.69 2.50 0.41 2.30 0.49 1.40 0.19 1.33 0.19 2.52 8.55 84.80 0.84 9.40 1.52

0.91 ± 0.41 4±2 7.0 ± 1.7 1.1 ± 0.1 5.0 ± 1.0 1.5 ± 0.2 0.4 ± 0.1 1.5 ± 0.35 0.21 ± 0.05 0.96 ± 0.27 0.16 ± 0.05 0.4 ± 0.1 0.04 ± 0.02 0.2 ± 0.1 0.03 ± 0.02 8.4 ± 4.7 5.6 ± 1.6 22.1 ± 4.8 0.81 ± 0.18 19 ± 14 9±8

S035 Mean ( n = 4) 4.40 19.10 54.30 3.90 17.10 3.30 0.86 3.60 0.54 2.90 0.62 1.80 0.24 1.63 0.23 3.45 8.45 110.1 0.76 8.62 1.79

4.2 ± 0.2 14 ± 5 34 ± 18 2.7 ± 1.0 12 ± 5 2.4 ± 0.7 0.6 ± 0.2 2.5 ± 0.8 0.4 ± 0.1 2.2 ± 0.6 0.5 ± 0.1 1.3 ± 0.4 0.2 ± 0.1 1.2 ± 0.3 0.18 ± 0.04 2.46 ± 1.00 8±2 74 ± 31 0.8 ± 0.1 8±2 1.6 ± 0.2

n = number of samples

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

The concentration of U is high in all rock types (~ 3.558.30; Table 1). In the study area, the Bhima Basin, the granitic rocks tends to have high content of of U (~ 3.0820.76, mean 8.18, n = 28; Kumar and Srinivasan, 2002), which could be the reason for the U enrichment than other trace elements as well as to upper continental crust (UCC,

Trace Elements

The concentrations of Co, Ni, Cr, Ba, Zr, Hf, and Th are higher in the siltstones than in the quartz arenites and arkoses (Fig.4). This variation may partially be due to (1) dilution by quartz in quartz arenites and arkoses relative to siltstones and (2) higher clay mineral content in siltstones than quartz arenites and arkoses. The depletion of Zr and Hf in quartz arenites and arkoses than siltstones could be related to the amount of heavy minerals (especially zircon) present in them. Feldspar is a major host of Ba and Rb in terrigenous sedimentary rocks (Veizer, 1978). In our study, high correlation coefficient between Rb-K 2O (r = 0.89, n = 23), Rb-Al2O3 (r = 0.93), and Sr-CaO ( r = 0.74), for all rock types suggest that the distribution of these elements is controlled by Rb incorporation into silicate and Sr into carbonate phases. In addition, a good positive correlation between Ba and K 2O (r = 0.77, n = 23) suggests that Ba is mainly associated with K-feldspar. K-felds par. JOUR.GEOL.SOC.INDIA, VOL.70, AUGUST 2007

Fig.4.

Multi-element normalized diagram, normalized against average 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 Two horizontal lines line s for rock/upper continental continen tal crust values of 1 and 0.1 are included for reference.

304

Fig.5.


Th-U bivariate plot for the samples of the Rabanpalli Formation. Note a good positive correlation between Th and U.

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

Fig.6. Geochemical classification for the samples of the Rabanpalli

Formation using log(SiO 2 /Al2 O 3) - log(Fe 2 O 3 /K 2O) diagram (after Herron, 1988).

in these rock types. The high K 2O/Al2O3 ratio in arkoses (0.5 ± 0.1, n = 7; Table 1) is interpreted to reflect an input from first cycled granitic material as evidenced by the presence of K-feldspar through petrography study. st udy.

Geochemical Classification

Palaeoweathering

Geochemical classification of terrigenous sedimentary rocks has been proposed by many authors based on major elements composition (Pettijohn et al. 1972; Crook, 1974; Blatt et al. 1980; Herron, 1988). Using the indices of SiO 2 / Al2O3 and Na2O/K2O ratios, Pettijohn et al. (1972) proposed a classification for terrigenous sands based on a plot of log (Na 2O/K 2O) versus log (SiO 2 /Al 2O3). Herron (1988) modified the diagram of Pettijohn et al. (1972) using log (Fe2O3 /K2O) along the Y-axis instead of log (Na 2O/K2O). The ratio Fe 2O 3 /K 2O facilitates arkoses to be more successfully classified, and it is a measure of mineral stability. Thus, in log (Fe 2O3 /K2O) versus log (SiO 2 /Al2O3) plot (Fig.6; Herron, 1988) eight samples plot in the quartz arenite field, seven samples plot in arkose field and the remaining eight samples plot in the wacke field. This plot is in good agreement with our classification based on petrography. K2O/Al2O3 ratio of terrigenous sedimentary rocks can be used as an indicator of the original composition composit ion of ancient sediments, because the K 2O/Al2O3 ratio for clay minerals and feldspars are different. K 2O/Al2O3 ratios for clay minerals range from 0.0 to 0.3 and for feldspars it range from 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

Alteration of minerals due to chemical weathering mainly depends on the intensity and the duration of weathering. The dominant process during weathering of the upper crust crus t is the degradation of feldspars and concomitant formation of clay minerals. During weathering, calcium, sodium, and potassium largely are removed from feldspars (Nesbitt et al. 1980). The amount of these chemical elements surviving in the soil profiles and in the sediments derived from them is a sensitive index of the intensity of weathering (Nesbitt et al. 1997). A good measure of the degree of chemical weathering can be obtained by calculating the chemical index of alteration (CIA; Nesbitt and Young, 1982) using the formula (molecular proportion) CIA = [Al2O3 /(Al2O3 + CaO* + Na2O + K2O)] × 100 where CaO * is the amount of CaO incorporated in the silicate fraction of the rock. However, in the samples studied, CaO is very low (~ 0.053–0.709) and there was no objective way to distinguish CaO in carbonate from CaO in silicate, so total CaO (Table 1) was used in measuring CIA values. The CIA is a good measure of paleo-weathering conditions, and it essentially monitors the progressive weathering of feldspars to clay minerals (Fedo et al. 1995; Armstrong-Altrin et al. 2004). High CIA values (i.e. 76– JOUR.GEOL.SOC.INDIA, VOL.70, AUGUST 2007


100) indicate intensive chemical weathering in the source areas whereas low values (i.e. 50 or less) indicate unweathered source areas. In the present study, quartz arenites exhibit wide range of CIA values (~ 52–79; Table Table 1). Likewise, arkoses (~ 55–65) and siltstones (~ 49– 67; Table 1) also show wide variations. The CIA values are also plotted in Al2O3-(CaO + Na 2O)-K2O (A-CN-K; Nesbitt and Young, Young, 1982) compositional space in Fig.7 (molecular proportions). proportions ). In the A-CN-K triangular diagram, diagr am, all the rock types (except one siltstone) plot above the feldspar join line. Quartz arenites and siltstones are scattered in the A-CN-K diagram whereas arkoses exhibit definite trend. Generally, quartz arenites should plot away from the feldspar join line and their trend should approach A-apex, instead of scattering near to feldspar join line. Most of the siltstones are plot well near to the feldspar join and arkoses follow AK line

Fig.7. A-CN-K diagram (after Nesbitt and Young, 1982) showing 1

samples of this study and average composition of 2 upper continental crust (UCC; Taylor and McLennan, 1985). A = Al2O3; CN = CaO * + Na2O; K = K2O (molecular proportion; 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 the studied samples contain considerable amount of K 2O than expected, and hence it may under gone K-metasomatism. The sedimentary rocks affected by K-metasomatism, generally exhibit low values than the premetasomatised composition (Fedo et al. 1995). K-Metasomatism

Metasomatic enrichment of potassium to sediments and sedimentary rocks produces mineralogical changes, which JOUR.GEOL.SOC.INDIA, VOL.70, AUGUST 2007

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alter the earlier composition (Glazner, 1988; Nesbitt and Young, 1989; Sutton and Maynard, 1992; Condie, 1993; Fedo et al. 1997a, 1997b). Potassium metasomatism is particularly common, which involves conversion of kaolin to illite by reaction with potassium bearing pore waters (Fedo et al. 1995). In sandstones, K-metasomatism can take place in two different ways, 1) aluminous clay minerals (kaolinite as matrix) converted into illite and/or 2) conversion of plagioclase to k-feldspar (Fedo et al. 1995). These processes produce K2O enrichment in the sedimentary rocks, and it may vary from the weathering trend. Conversion of kaolinite into illite by K addition results res ults in a CIA value lower than the pre-metasomatised rock (Fedo et al. 1995). Conversion of plagioclase to k-feldspar, where authigenic k-feldspar replaces plagioclase by K-metasomatism, the CIA values may not change because the process occur mole for mole substitution of K2 for Ca or Na 2 (Glazner, 1988). Both these processes may affect the composition of sedimentary rocks and the extent of these processes can be identified by petrographic study (Fedo et al. 1995, 1997a, 1997b). In the present study, quartz arenites exhibit low CIA values and arkoses show a typical trend towards K-apex. This type of trend is generally found in the sedimentary rocks that undergone K-metasomatism, by which addition of K to weathered residues (Fedo et al. 1995; See their Fig.1A). This process produces mineralogical changes results in lowering of CIA values. Hence quartz arenites plot nearer to the feldspar join rather than displaying their original chemical maturity (Fig.7). Likewise, siltstones and arkoses also plot nearer to the feldspar join. In the present study, K-metasomatic effect can be identified from the typical trend of arkoses. Arkoses following the A-K line (Fig.7), which exhibit the addition of K to this rock type. It is also supported by petrographic study, which shows most of the arkoses display partially or fully altered plagioclase grains and it also exhibits the presence of illite as matrix material. Hence the observed low CIA values in the sedimentary rocks of Rabanpalli Formation are mainly due to the K-metasomatism. Th/U in terrigenous sedimentary rocks is of interest because weathering tends to result in oxidation of insoluble ins oluble 4+ 6+ U to soluble U with loss of solution and elevation of Th/U ratios (McLennan and Taylor, 1980, 1991). The Th/U ratios in the studied samples range r ange from 0.19 to 2.47 (Table 1). Upper crustal igneous rocks have Th/U ratios averaging about 3.8, with considerable scatter (Taylor and McLennan, 1985; Condie, 1993; McLennan, 2001). The sedimentary rocks of Rabanpalli Formation show low Th/U ratios when compared with upper continental crust

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value. The observed low Th/U ratios are mainly due to the elevated concentration of U. Provenance

Source rock composition is commonly thought to be the dominant factor that controls the composition of sediments derived from them (Taylor and McLennan, 1985). 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 one best relies on elements that show little mobility under the expected geological conditions. Taylor and McLennan (1985) pointed out that such elements should possess very low partition coefficients between natural waters and upper crust and short oceanic residence times. REE, Th, and Sc are quite useful for inferring crustal compositions, because their distribution is not significantly affected by secondary processes such as diagenesis and metamorphism, and is less affected by heavy mineral fractionation than that for elements such as Zr, Hf, and Sn (Bhatia and Crook, 1986; McLennan, 2001). REE and Th abundances are higher in felsic than mafic igneous source rocks and in their weathered products, whereas Co, Sc, V, Ni, and Cr are more concentrated in mafic than felsic igneous source rocks and their weathered products. In addition, these elements are relatively immobile during weathering. These elements are believed to be transported exclusively in the terrigenous component of sediment and therefore reflect the chemistry of their source rocks (Veizer, (Veizer, 1978; McLennan et al. 1980; Armstrong-Altrin, 2004). Very high levels of Cr and Ni have been used by many authors (e.g., Hiscott, 1984; Wrafter and Graham, 1989) to infer an ultramafic provenance for sediments. Furthermore, the unusual enrichment of Ni unaccompanied by other

Table 3.

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

Elemental ratio

Eu/Eu*4 (La/Lu)cn4 La/Sc4 Th/Sc Th/Co Th/Cr Cr/Th 1 4

ferromagnesian trace elements is also addressed by Armstrong-Altrin et al. (2004). Garver et al. (1996) suggested that the sediments having elevated concentration of Cr (> 150 ppm) and Ni (> 100 ppm), high correlation coefficient of Cr with Ni, and Cr/Ni ratio of ~ 1.4 are indicative of ultramafic source. Higher Cr/Ni ratios probably indicate mafic source rocks (Garver and Scott, 1995). In our study, Cr and Ni values, and Cr/Ni ratios are comparatively higher in siltstones (64 ± 7, 16 ± 12, and 5±2, respectively) than quartz arenites (7.1 ± 3.5, 4.8±3.3 and 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 are lower than the sediments derived from ultramafic source rocks, except Cr/Ni ratios. The negative correlation of Cr with Ni for arkoses ( r = -0.1) and low correlation for quartz arenites ( r = 0.3) and siltstones ( r = 0.3) imply that these sedimentary rocks were derived from the felsic source rocks. 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 the quartz arenites, arkoses, and siltstones suggest that these sediments were mainly derived from the felsic source rocks rather 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 significantly different in mafic and felsic source rocks and may allow constraints 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 of the Rabanpalli Formation are compared with those in

Range of sandstones and siltstones from the Rabanpalli Formation1 Quartz arenites Arkoses Siltstones (n = 8) (n = 7) (n = 8) 0.61-1.08 3.55-38.83 3.45-13.60 1.72-5.21 0.20-1.60 0.07-0.51 1.96-15.20

0.54-1.02 13.84-42.69 3.62-21.90 0.79-7.75 0.11-1.17 0.17-0.84 1.19-6.03

0.61-0.83 4.24-9.74 1.09-3.45 1.24-2.67 0.17-0.40 0.12-0.24 4.18-8.62

Range of sediments from2 Fels Felsic ic rock rockss 0.40-0.94 3.00-27.0 2.5-16.3 0.84-20.5 0.67-19.4 0.13-2.7 4.00-15

Mafi Maficc rock rockss 0.71-0.95 1.10-7.00 0.43-0.86 0.05-0.22 0.04-1.4 0.018-0.046 25-500

Upper Continental Crust3 0 . 63 9 . 73 2 . 21 0 . 79 0 . 63 0 . 13 7 . 76

This study; 2 Cullers (1994a, 2000); Cullers and Podkovyrov (2000); Cullers et al. (1988); 3 Taylor and McLennan (1985) n (number of samples) = 5 for quartz arenites; n = 3 for arkoses; n = 4 for siltstones JOUR.GEOL.SOC.INDIA, VOL.70, AUGUST 2007


sediments derived from felsic and mafic source rocks (Cullers et al. 1988; Cullers, 1994a, 2000; Cullers and Podkovyrov, 2000, 2002) as well as with upper continental crust (UCC; Taylor and McLennan, 1985; Table 3). This comparison points out that the trace elemental ratios of this study are comparable to the range of sediments derived from felsic source rocks rather than mafic source rocks. Th/Sc vs Sc bivariate and La-Th-Sc triangular plots provide useful information regarding the source rocks characteristics (McLennan (M cLennan and Taylor, Taylor, 1991; Cullers, 2002). The elemental ratio (Th/Sc) and concentrations (Sc, La, Th) of terrigenous rocks of Rabanpalli Formation are plotted in the Th/Sc vs Sc (Fig.8) and La-Th-Sc (Fig.9) diagrams to find out the source rocks characteristics. UCC value (McLennan, 2001), Archaean granite, cratonic sandstone, andesite, and basalt + komatiite (Condie, 1993) values are plotted in these two diagrams for comparison. Several informations 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 not affected by the sorting processes. This information implies that Th and Sc are not present in the minerals, which are easily removed during weathering and/or other sedimentary processes, and Th/Sc ratio can be considered as the one of the best indicators of provenance study (Taylor and McLennan, 1985). Th/Sc ratio, when plotted against concentration of Sc that is more sensitive to provenance composition than REE (Fedo et al. 1997a). In the Figures 8 and 9, the quartz arenites, arkoses, and siltstones are fall near to UCC, Archaean granite, and cratonic sandstone values, which strongly supports that the

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Fig.9. La–Th–Sc triangular plot for the samples of the Rabanpalli

Formation. 1 This study, 2 upper continental crust (McLennan, 2001), and 3 Condie (1993).

studied samples were mainly derived from the felsic source rocks rather than the mafic source rocks. In addition, the relative REE patterns and Eu anomaly size have also been used to infer sources of sedimentary rocks (Taylor and McLennan, 1985; Wronkiewicz and Condie, 1989). Mafic rocks contain low LREE/HREE ratios and tend not to contain Eu anomalies, whereas more felsic rocks usually contain higher LREE/HREE ratios and negative Eu anomalies (Cullers and Graf, 1984). The depletion of Eu may be interpreted as shallow, intracrustal differentiation, which resulted in Eu-depletion in the upper continental crust, associated with the production of granitic rocks (McLennan, 1989). Some Precambrian rocks like tonalite-tronjhemite gneiss (TTG), granodiorite, and quartz diorite show very large LREE/HREE ratios with positive Eu anomaly and their positive anomaly arises not because of enrichment of feldspars but is mainly due to hornblendemelt equilibria (Cullers and Graf, 1984). 1 984). In the present study, all rock types exhibit higher LREE/HREE ratio (8 ( 8 ± 4, n = 12; Table Table 2) and a significant negative Eu anomaly (0.77 ± 0.16, n = 12; Table 2; Fig.11) indicates the felsic igneous rocks as a possible source rocks. Discriminant Function Diagram

Fig.8. Th/Sc vs Sc bivariate plot for the samples of the Rabanpalli

Formation. 1 This study, 2 upper continental crust (UCC; McLennan, 2001), and 3 Condie (1993). JOUR.GEOL.SOC.INDIA, VOL.70, AUGUST 2007

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

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Fig.11. Fig.10. Discriminant Function diagram for sedimentary provenance

using major elements (Roser and Korsch, 1988). The discriminant functions are: Discriminant Function 1 = (-1.773. TiO 2) + (0.607 . Al 2O 3) + (0.760 . Fe 2O 3 ) + (-1.500 . MgO) + (0.616 . CaO) + (0.509 . Na 2 O) + (-1.224 . K 2 O) + (-9.090); Discriminant Function 2 = (0.445. TiO 2) + (0.070 .Al 2O 3) + (-0.250 . Fe 2O 3 ) + (-1.142 . MgO) + (0.438 . CaO) + (1.475 . Na 2 O) + (-1.426.K2O) + (-6.861).

arkoses plot within the quartzose sedimentary provenance field, and siltstones plot both in the felsic igneous and intermediate igneous provenance fields. This observation clearly indicates the less possibility of the mafic rocks as source rocks for the studied samples of the Rabanpalli Formation (Fig.10).

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

arkoses have (Gd/Yb) cn ratios more than 2 and siltstones have less than 2 (Table2; Fig.12), suggesting that quartz arenites and arkoses were derived from sources having somewhat depleted heavy rare earth elements whereas siltstones were derived from less HREE-depleted Archaean Archaean or post-Archaean sources, or a combination of both. The average 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

Probable Source Rocks

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

Fig.12.

Plot of Eu/Eu* versus (Gd/Yb)cn for the samples of the Rabanpalli Formation. Fields are after McLennan and Taylor (1991). 1This study; 2 upper continental crust (UCC; Taylor and McLennan, 1985); 3Jayananda et al. (1995); 4 Jayaram et al. (1983); 5Khan (1992) and Rao et al. (1999); 6 Rao et al. (1999). n = number of samples. JOUR.GEOL.SOC.INDIA, VOL.70, AUGUST 2007


area are also shown in this plot. The overlapping of the studied samples with the Archaean granites and Proterozoic shales, suggesting that all rock types in the present study could have been derived by the contributions from the adjacent area (south of the Kaladgi Basin; Fig.1). CONCLUSIONS

The rock types were identified as quartz arenites, arkoses, and siltstones using petrography. Other major element bivariate plots also support our petrographic observations. The sedimentary rocks of the Rabanpalli Formation show low CIA values and these values were plotted in the A-CN-K diagram in order to find out the paleoweathering condition of the source rocks, which reveal that the observed low CIA values are mainly due to K-metasomatism. The Cr, Ni, V, and Sc values for all rock types in our study clearly suggest that they were derived from felsic source rocks rather than mafic and/or ultramafic source rocks. The rare earth elements concentration, other trace element 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-Sc diagrams of all rock types suggest that these sediments were derived from felsic source rocks rather than mafic source

309

rocks. This interpretation is in good agreement with the major element discriminant function diagram. Thus, we interpreted that all rock types in this study can be derived from felsic source rocks. Furthermore, the REE patterns and (Gd/Yb)cn ratios of different rock types in this study are very similar to the granite rocks, which belong to the adjacent source area and we conclude that the granite rocks can be the 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 of Geology, University of Madras for providing certain laboratory facilities through UGC SAP-II, UGC COSIST and DST-FIST programs. RN wishes to express his sincere thanks tha nks to N. Rajeswara Rao, V. Ram Mohan, L. Elango, and S. Srinivasalu for their constant encouragement during this study. JSA wishes to express his gratefulness to Otilio A. Acevedo Sandoval, Enrique Cruz Chávez, and Kinardo Flores Castro, Centro de Investigaciones en Ciencias de la Tierra, Universidad Autónoma del Estado de Hidalgo Hidalg o (UAEH). Financial assistance by SEP-PROMEP (Programa de Mejoramiento 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), UAEH-DIP-ICBI-AACT-274), Mexico, are hig hly appreciated.

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

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Sandstone Geochemistry

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