Journal of Asian Earth Sciences 48 (2012) 160–172
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Geochemistry of Mesoproterozoic sedimentary rocks of upper Vindhyan Group, southeastern Rajasthan and implications for weathering history, composition and tectonic setting of continental crust in the northern part of Indian shield Mahshar Raza ⇑, Abdullah Khan, V.R. Bhardwaj, Sarwar Rais Department of Geology, Aligarh Muslim University, Aligarh 202 002, India
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Article history: Received 26 October 2010 Received in revised form 14 November 2011 Accepted 24 November 2011 Available online 11 January 2012 Keywords: Sediment geochemistry Vindhyan basin Rajasthan Indian shield Purana basin
a b s t r a c t The upper Vindhyan succession of southeastern Rajasthan is divisible into Kaimur, Rewa and Bhander Groups. The major and trace element (including rare earth elements) data of the upper Vindhyan shales and sandstones are investigated to determine the weathering history, composition, and tectonic setting of Mesoproterozoic continental crust. CIA (chemical index of alteration) values, A–CN–K plot (A = Al2O3, CN = CaO + Na2O, K = K2O) and depletion in U, Na2O, CaO, Sr and Ba suggest that the source area experienced moderate to high degree of chemical weathering under warm and humid conditions. Provenance modeling indicates that the Kaimur sandstones are best modeled with a mixture having 40% granitic gneiss, 20% Tonalite–Trondhjemite–Granodiorite (TTG), 20% mafic enclaves and 20% Berach Granite of the Banded Gneissic Complex (BGC). A mixture of 60% granitic gneiss, 20% mafic enclaves and 20% Berach Granite of the BGC can model the Rewa and Bhander Groups. It is suggested that the upper Vindhyan sedimentation commenced at the time of Delhi–Sausar orogeny at about 1100–1000 Ma. The orogenic movements uplifted the parts of old continental crust in the BGC terrain creating positive areas, which exposed older crustal blocks containing TTG as important component. The debris of Kaimur sandstone probably derived from these uplifted blocks. As indicated by Palaeocurrent data, the Rewa and Bhander formations were derived from Bundelkhand Granitic Gneiss Complex (BGGC) occurring to the north of the basin and/ or the Chotanagpur Granitic Gneiss Complex (CGGC) of eastern Indian shield. The derivation of Lower and upper groups of Vindhyan succession from different source terrains of identical composition suggests that at the time of Vindhyan sedimentation, the BGC of Rajasthan, the BGGC of Central India and the CGGC of eastern India had similar lithological composition. This implies that well before the origin of the Vindhyan basin these discrete terrains evolved as a single unit that existed as a Mesoarchaean nucleus in the northern part of the Indian shield. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The Proterozoic Vindhyan basin of Indian shield is a non-linear, large sedimentary basin (Soni et al., 1987), which covers an area of 178,000 km2 (Tandon et al., 1991) and preserves about >4 km thick undeformed continental sedimentary succession. Its basin fill, the Vindhyan Supergroup, is considered as one of the thickest sedimentary successions. Precambrian age, long duration of sedimentation, undeformed nature, vast occupied area and location in front of two prominent orogenic belts are the features, which make this basin unique in the world. In recent years, the Vindhyan Supergroup has attracted a great deal of international attention, particularly in terms of its age and duration (Seilacher et al., 1998; Kumar, 2001; Ray et al., 2002; Ray, 2006; Rasmussen et al.,
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2002; Sarangi et al., 2004; Gregory et al., 2006; Malone et al., 2008). The thick sedimentary column of Vindhyan Supergroup with vast lateral extension may contain important information on the evolution of continental crust, atmosphere and climate. Therefore, the sedimentary succession of the Vindhyan basin is most suitable to undertake geochemical studies. The geochemistry of sedimentary rocks provides important constraints on the composition and evolution of the atmosphere, hydrosphere and continental crust through geological times (Taylor and McLennan, 1985; Condie, 1993). The significance of sediment geochemistry in determining the palaeoclimatic conditions, provenance characteristics and basinal tectonic setting is well established in literature (Bhatia, 1983; Taylor and McLennan, 1985; Jahn and Condie, 1994; Roser and Korsch, 1986; Hofmann, 2005; Osae et al., 2006; Roddaz et al., 2006; Absar et al., 2009; Raza et al., 2010a,b, 2011). Although, various aspects of Vindhyan basin and its succession have been studied for a long time, little attention
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has been paid to geochemistry of sedimentary rocks. In recent years, the sedimentary rocks of Vindhyan Supergroup, occurring on the southern margin of the basin in the Son valley of central India, have been studied for their geochemistry. Paikaray et al., (2008), recognized a granite dominated source terrain for lower Vindhyan shales and partial contribution from a basaltic source for upper Vindhyan shales. On the other hand the trace element and Nd isotopic study of these sedimentary rocks by Chakrabarti et al. (2007), suggests their derivation from an Andean type arc source. In the western part of the basin, the geochemical data are available only on the rocks of Lower Vindhyans (Raza et al., 2002, 2010b). These authors have inferred the derivation of Lower Vindhyan sediments from Archaean Banded Gneissic Complex (BGC) of Rajasthan and its early Proterozoic supracrustals. To date, no geochemical data are available on upper Vindhyan rocks of the western margin. In the present study, we report first account of advance geochemical data of upper Vindhyan shales and sandstones, occurring at western margin of the basin in Rajasthan sector. The aim is to define geochemical characteristics of upper Vindhyan sediments of Rajasthan and to discuss the weathering history, provenance composition, and tectonic setting of the continental crust at the time of their deposition. 2. Geological background 2.1. Geological framework of Vindhyan basin The Vindhyan basin is the largest Proterozoic intracratonic basin that developed in the central part of the Indian shield. The margins of the basin are marked by the presence of an arcuate fold belt comprising Aravalli fold belt in the west and Satpura fold belt in the south. (Fig. 1). These margins are tectonic in nature, characterized by presence of major zones of displacement referred to as Great Boundary Fault Zone (GBFZ) in the west and Central Indian Tectonic Zone (CITZ) in the south. The Vindhyan basin shows characteristics of both an intracratonic basin as well as a foreland basin in front of the Aravalli–Satpura orogenic belt (Raza and Casshyap, 1996; Raza et al., 2009). The dominant lithologies are sandstone, shale, conglomerate and limited carbonate with mafic volcanic products at the base (Prasad, 1984; Raza et al., 2001). The basin-fill, referred to as Vindhyan Supergroup is divisible into Semri, Kaimur, Rewa and Bhander Groups. Due to presence of an unconformity between the Semri and succeeding groups, earlier workers (Soni et al., 1987) have divided the Vindhyan succession into lower and upper Vindhyans. Structural discordance between the lower and upper groups indicates that the upper Vindhyan sedimentation was preceded by a tectonic event (Valdiya, 2010, p. 191). The rocks of Vindhyan Supergroup are exposed in two areas: Son Valley sector in the south and Rajasthan sector in the west. (Fig. 1). The stratigraphic sequence is laterally correlatable and vertically stacked in similar fashion in both of these areas. 2.2. Geological setting of upper Vindhyan Group of Rajasthan In Rajasthan sector, the GBFZ separates the rocks of Vindhyan basin from the Archaean basement, referred to as Banded Gneissic Complex (BGC) and its Palaeoproterozoic supracrustals. In this area, the westernmost limit of the Vindhyan basin is near Chittaurgarh (Fig. 1), where the Archaean Berach Granite and Palaeoproterozoic supracrustals of Aravalli Supergroup juxtapose the sedimentary rocks of Vindhyan Supergroup. The magneto-telluric studies (Gokaran et al., 1995) suggest presence of 1.2 km thick upper Vindhyan rocks and 3 km thick Lower Vindhyan rocks. In this region the Semri Group, comprising the Lower Vindhyan
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succession, is overlain by Kaimur, Rewa and Bhander Groups, of the upper Vindhyan succession. The nature of contact between Semri Group and succeeding formations of the upper Vindhyan Group varies from one area to other. For example in Bundi area, the contact is marked by the presence of lenses of conglomerate. On the other hand, near Chittaurgarh the Kaimur sandstone conformably overlies the rocks of the Lower Vindhyan Group. In upper Vindhyan succession of Rajasthan sector, the proportion of rocks is 34–54% sandstone, 27–48% shales and 18–27% carbonates (Srivastava and Akhtar, 1982). Lithologically, the Kaimur Group consists predominantly of sandstone with minor grit. The overlying Rewa and Bhander Groups contain six formations of shales and four formations of sandstones in addition to two formations of limestone. In the present study, the shale units of the Rewa Group are collectively referred to as Rewa shales, and those of overlying Bhander Group as Bhander shales. Similarly, the sandstone units of the Rewa Group are referred to as Rewa sandstone and those of Bhander Group as Bhander sandstone, the terms, used in type area of Vindhyans of the Son valley and used in international publications. In the Rajasthan sector, the Vindhyan sediments were deposited in environments ranging from fluvial to deep marine (Bhardwaj, 1973; Akhtar, 1978; Bose et al., 2001). Prasad (1984) has worked out a detailed account of geology of the Vindhyan rocks of Rajasthan sector (Fig. 1). 2.3. Age of upper Vindhyan sedimentation The age of Vindhyan sedimentation remained a subject of heated debate for more than 100 years, probably due to the absence of reliable fossil suitable for biostratigraphic dating (Venkatachala et al., 1996). In the past, there has been a consensus that Vindhyan sediments deposited during the period from about 1400 to 550 Ma (Misra, 1969; Crawford and Compston, 1970; Vinogradov et al., 1964). However, after the report of small shelly fossils from lower formations of the basin (Azmi, 1998; Seilacher et al., 1998) the age of Vindhyan sedimentation has attracted international attention. Recently published radiometric dates suggest the initiation of Vindhyan sedimentation at about 1600–1700 (Sarangi et al., 2004; Ray, 2006 and references therein). These results are in agreement with those of Raha and Sastry (1982) and Kumar (2001), who based on stromatolites, assigned an early to middle Raphaian age to lower Vindhyan sediments. The existence of a regional unconformity between lower and upper Vindhyans (Jokhan Ram et al., 1996) and the available geochronological data suggest a younger age for the upper Vindhyans. The kimberlite pipes, which intrude the Semri and Kaimur Groups and are believed to have contributed diamonds to the Rewa conglomerate, have been dated at 1140 + 112 Ma (Paul et al., 1975) and 1067 + 31 Ma (Kumar et al., 1993). The palaeomegnetic pole position and zircon geochronology of upper Vindhyan suggest the age of upper Vindhyans not less than 1075 Ma (Gregory et al., 2006; Malone et al., 2008). Based on available data, the age range of upper Vindhyan sedimentation is considered as 1100–650 Ma (Ray, 2006) or 1000–1070 Ma (Malone et al., 2008) Therefore, the initiation of upper Vindhyan sedimentation coincides with the timing of Delhi–Sausar orogeny (1100–1000 Ma: Deb et al., 1989; Lippot and Hautman, 1994; Valdiya, 2010). 2.4. Sampling and analytical techniques The samples of shales and sandstone formations of upper Vindhyan Supergroup, occurring at different stratigraphic levels, were collected from the area extending from Chittaurgarh in the west to Kota, Bundi, and surrounding areas in the east. Fresh
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M. Raza et al. / Journal of Asian Earth Sciences 48 (2012) 160–172
Fig. 1. Generalized geological map southeastern Rajasthan showing distribution of Vindhyan Supergroup and the sites of collected samples (A). Inlets show the regional outcrop map the Vindhyan basin (B) and its location in India (C). GBF – Great Boundary Fault.
samples from different formations of upper Vindhyan succession were collected from road cuts, riverbeds and quarries. The effect of weathering, veining or open system behavior is not generally observed in these samples. Out of a large number of collected samples, 30 samples of upper Vindhyan shales and 7 samples of associated sandstone units were selected for geochemical analysis. The samples were crushed in a silica pulverizer and analyzed for major and trace elements using standard X-ray fluorescence (XRF) procedures. Analyses were carried out in the geochemical laboratory of Wadia Institute of Himalayan geology, Dehradun. The analytical runs were calibrated using a variety of international standards. Precision for most of the major oxides was always better than 1.5%. The analytical procedure followed was as given by Lucas-Tooth and Pyne (1964). The REE analyses were carried out at National Geophysical Research Institute, Hyderabad by using ICP-MS technique. International rock standards were analyzed along with upper Vindhyan samples. The data on these standers are comparable with those recommended and the precision is better than 10% RSD (Raza et al., 2010a). The petrology of sandstones was
determined using Gazzi–Dickinson point (Dickinson, 1970; Ingersoll et al., 1984).
counting
method
3. Results 3.1. Petrography In terms of petrographic characteristics, the UV sandstones are medium to fine grained and moderately to well sorted in nature. They show high degree of compositional and textural maturity. In general, the grains are rounded to sub-round. They consist of 96–98% quartz, 1–4% feldspar and 1–4% lithic fragments with minor amount of micas and heavy minerals. The quartz fraction is dominated by monocrystalline quartz (92–98%). Rock fragments include chert and quartzite. Abundant quartz relative to feldspar and lithic fragments suggests that the sandstone were significantly weathered or diagenetically altered to remove feldspars and lithic fragments and increase the amount of quartz relative to the source rock (Nesbitt et al., 1996). Fragments of chert and quartzite
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together comprise 2–4% by volume. Scarcity of heavy minerals, small proportions of opaques in total heavies and dominance of rounded grains of most stable minerals such as tourmaline and zircon are the features, which further indicate the compositional maturity of the sandstones. The compositional maturity of sandstones is attributed to deep chemical weathering under tropical conditions and their differentiation into muds and sands (Chandler, 1988). 3.2. Geochemistry The element concentrations, elemental ratios and averages of UV shales and sandstones are given in Table 1. In general the SiO2 concentrations and Th/Sc, Cr/Th, La/Sc, Zr/Sc, La/Sc and Eu/ Eu ratios are higher in sandstones than in shales. In contrast, the TiO2, Al2O3, Fe2O3, MgO, Na2O and K2O concentrations of the sandstones are lower than the shales. This relationship may be partially due to dilution by quartz in the sandstones relatively to the shales or to the higher clay mineral content in the shales. The shales contain 15% Al2O3. On the other hand sandstone (except sample No.40 of Rewa Sandstone) contain >95% SiO2 and less than 1% Al2O3 contents (except one sample of Rewa Sandstone which contains 4% Al2O3). The UV sediments define a linear inverse trend on SiO2–Al2O3 graph (Fig. 2). Sandstones plot at the highest SiO2 and lowest Al2O3 concentrations due to higher contents of quartz. Shales contain the highest Al2O3 and lowest SiO2 concentrations probably due to higher contents of illite, muscovite and chlorite and lower contents of quartz relative to sandstones. In general these sediments can be considered as mixture of quartz and illite end members (Fig. 2). The higher concentrations of TiO2, FeO and MgO in the shales than in the sandstones is probably due to higher chlorite and iron minerals in the shales than in the sandstones. The higher SiO2 and lower Al2O3 and K2O contents in the sandstones relative to shales are possibly due to the higher quartz and lower illite–muscovite in the sandstones relative to shales (Fig. 2). The sandstones display higher SiO2/Al2O3 and low K2O/Na2O ratios relative to associated shales. The relationship between these ratios (Fig. 3) indicates the clay mineral controls on major elements that are diluted by increasing SiO2 contents in associated sandstones. K2O/Al2O3 ratios of UV clastics are low (shales = 0.004–0.04, avg. 0.2; sandstones = 0.02–0.4, avg. 0.2) suggesting minimal alkali feldspar relative to other minerals in the original shales (Cox et al., 1995). The REE patterns of shales and sandstones are broadly similar to those of PAAS (Fig. 4). However, the total REE contents of sandstones (Kaimur 33, Rewa 66 and Bhander 49 ppm) are low due to quartz dilution. No systematic differences in the REE patterns or contents observed among different stratigraphic units or from one unit to another. Both shales and sandstones show strong positive correlation between Zr and TiO2 (r = 0.6), suggesting concentration of certain accessory minerals such as zircon, monazite, ilmenite and rutile. La shows positive correlation with Al2O3 and Zr in shales (r = 0.7, 0.5 respectively) and sandstones (r = 0.8, 0.9 respectively). Yb also show positive correlation with Al2O3 and Zr in shales (r = 0.9, 0.6) and sandstones (r = 0.7, 0.9). These relationships suggest that zircon and clay minerals, both control the REE geochemistry of UV sedimentary rocks. The over all major element compositions of the shale units of the Rewa and Bhander Groups are almost identical. Their average SiO2 contents (67% and 68% respectively) are higher than those of post Archaean Australian shales (PAAS 63%) and average Proterozoic shales (APS 63%) of Condie (1993) but comparable to those of average upper continental crust (AUCC 66%) of Taylor and McLennan, (1985) and Early Proterozoic upper continental crust (EPUCC 67%) of Condie (1993). The Na2O contents of the shales are low (Rewa: 0.04–0.40, avg. 0.2; Bhander: 0.01–0.61,
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avg. 0.3), presumably due to smaller amount of Na-rich plagioclase. The average concentrations of ferromagnesian trace element in shales of Rewa and Bhander Groups are; Ni = 30 and 24 ppm; Cr = 110 and 147 ppm and Co = 16 and 17 ppm respectively. The Rare Earth Element (REE) patterns of UV shales are uniform (Fig. 4) with (La/Yb)N ratio ranging from 8 to 10 and almost flat HREE with (Gd/Yb)N = 1.3–1.6 and moderate Eu-anomalies (Eu/ Eu = 0.55–0.72 ; Fig. 4A). The average SiO2 contents of different units of sandstone are (Kaimur sandstone, 97%, Rewa sandstone 91%, and Kaimur sandstone 96%) higher than that of Proterozoic sandstone (92%) of Condie (1993). Although their K2O and Na2O contents (except Rewa No. 43) are less than 1, the K2O/Na2O ratio shows a large variation (0.17–13.93). Highest values of this ratio are displayed by Rewa sandstone (12.9–13.9 avg. 11) and lowest by Kaimur sandstone (0.17–0.16, avg. = 0.92). The sandstones show high values (>10) of SiO2/Al2O3 ratio (avg. 29–56) suggesting their highly matured nature (McLennan et al., 1993), a feature consistent with our petrological results. In comparison to Proterozoic sandstones (Condie, 1993), the Ni, Cr, Co, and Sc contents of UV sandstones are high but thorium content is comparable. Like associated shales, the REE patterns of UV sandstones (Fig. 4) are parallel. They display prominent Eu-anomalies with Eu/Eu ranging from 0.31 to 1. Only one sample of Rewa sandstone (No. 40) does not show any europium anomaly (Eu = 1.05). Kaimur sandstones have high (La/ Yb)N ratio (avg. 24) in comparison to younger sandstone units (Rewa = 7, Bhander = 13). High (La/Yb)N ratios along with low (
