Geochemistry of a Paleosol Horizon at the Base of the Sausar Group, Central India: Implications on Atmospheric Conditions at the Archeanepaleoproterozoic Boundary

Geoscience Frontiers 7 (2016) 759e773

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Research paper Geochemistry of a paleosol horizon at the base of the Sausar Group, central India: Implications on atmospheric conditions at the ArcheanePaleoproterozoic boundary

Sarada P. Mohanty*, Sangitsarita Nanda

Department of Applied Geology, Indian School of Mines, Dhanbad 826004, India article info abstract

Article history: A paleosol horizon is described from the contact of the Sausar Group (w2400 Ma) and its basement Received 10 April 2015 (Tirodi Gneiss; >2500 Ma) in Central India. Physical evidence of pedogenesis is marked by the devel- Received in revised form opment of stress corrosion cracks, soil peds, corestone weathering and nodular rocks. XRD and SEM-EDX 15 September 2015 data indicate the presence of siderite, ankerite, uraninite, chlorite, alumino-silicate minerals, ilmenite, Accepted 6 October 2015 rutile and magnetite, in addition to quartz, feldspar and mica. The chemical index of alteration, the Available online 4 November 2015 plagioclase index of alteration, and the chemical index of weathering show an increasing trend from parent rock to the paleosol and indicate a moderate trend of weathering. The A-CN-K plot indicates loss Keywords: Paleosol of feldspars, enrichment in Al2O3 and formation of illite. Different major element ratios indicate baseloss Anoxic weathering through hydrolysis, clay formation, leaching of some elements, and more precipitation with good surface Great oxidation event drainage. The paleosol is depleted in HREE in comparison to the parent rock indicating high fluid-rock Paleoproterozoic interaction during weathering. The paleosol samples show flat Ce and Eu anomalies, low SREE, and Sausar Group high (La/Yb)N, indicative of a reducing environment of formation. Reducing condition can also be inferred from the concentration of elements such as V, Co, Cu, Pb, and Zn in the paleosol profile. Although enriched in Fe and Mg, the overall geochemical patterns of the paleosol indicate oxygen deficient conditions in the atmosphere and development by weathering and leaching processes associated with high precipitation and good surface drainage at the time of development of this paleosol during the ArcheanePaleoproterozoic transition. Ó 2015, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

1. Introduction However, the pattern of oxygen rise in the GOE is still in dispute (e.g., Murakami et al., 2011; Lyons et al., 2014). Rising atmospheric The ArcheaneProterozoic transition is marked by many tectonic oxygen was accompanied by CO2 draw down and the widespread and geochemical changes such as orogenic events that formed Huronian Glaciation, which may have produced a “snow ball” Earth megacratons, maﬁc dyke swarms associated with continental break- (Barley et al., 2005). The atmospheric and hydrospheric conditions up, worldwide glaciation, signiﬁcant rise in atmospheric oxygen and during this transition period are inferred from the rocks formed at appearance of life (Barley et al., 2005). Atmospheric conditions were w2500 Ma ago. Precambrian paleosols have long been used to anoxic at w2.5e2.4 Ga, but were followed by a rise of oxygen levels predict ancient atmospheric compositions on the basis of in the atmosphere (the Great Oxidation Event, GOE). Most re- geochemical variations, mineralogical changes, weathering regimes searchers consider the oxygen increase in the atmosphere to have and paleoenvironmental reconstructions (Gay and Grandstaff, 1980; been drastic (e.g., Goldblatt et al., 2006; Bekker and Holland, 2012). Schau and Henderson, 1983; Holland, 1984, 1994; Grandstaff et al., 1986; Kimberley and Grandstaff, 1986; Reimer, 1986; Retallack, 1986a, b; Zbinden et al., 1988; Holland et al., 1989; Maynard, 1992;

* Corresponding author. MacFarlane et al., 1994; Rye and Holland, 1998, 2000; Gall, 1999; E-mail address: [email protected] (S.P. Mohanty). Panahi et al., 2000; Yang and Holland, 2003; Sheldon, 2006a; Peer-review under responsibility of China University of Geosciences (Beijing). Driese et al., 2007; Mitchell and Sheldon, 2009, 2010). http://dx.doi.org/10.1016/j.gsf.2015.10.002 1674-9871/Ó 2015, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 760 S.P. Mohanty, S. Nanda / Geoscience Frontiers 7 (2016) 759e773

We have analysed the geochemical changes associated with a 2. Geological setting paleosol horizon developed by in-situ weathering of Archean granite in Central India (Fig. 1a, b). This paleosol is found at the 2.1. Regional geology contact of the Sausar Group (w2400 Ma) with the basement Tirodi Gneiss (>2500 Ma) (Mohanty, 1993). Re-Os molybdenite ages for The Sausar Belt of the Central Indian Shield is located at the calc-silicate rocks confirm a Paleoproterozoic age for the Sausar southern margin of the Satpura Mountain Belt (Fig. 1a), which was Group of between 2410 and 2450 Ma (Stein et al., 2014). Hence the developed by the collision of the North Indian Block (NIB) and the age of formation of the paleosol can be attributed to the Arche- South Indian Block (SIB) during the Proterozoic (Radhakrishnan aneProterozoic transition period. Paleoproterozoic glaciation has and Ramakrishnan, 1988). The belt comprises two major lithos- also been reported from the Sausar Group (Mohanty, 2006; tratigraphic units: the Tirodi Gneiss (TG) and the Sausar Group (SG), Mohanty et al., 2015). Therefore, the Sausar paleosol profile is and has a curvilinear geometry with a WNWeESE trend in the considered to be important for understanding atmospheric evolu- west, an east-west trend in the centre and an ENEeWSW trend to tion during the ArcheaneProterozoic transition period. the east (Fig. 1b; Straczek et al., 1956; Narayanaswami et al., 1963). The Tirodi Gneiss is an Archean complex of dominantly felsic gneisses with enclaves of metabasic rocks and lenses of high-grade (granulite facies) rocks. The Sausar Group is represented by quartzite, calc-silicate rocks, calcitic and dolomitic marbles, pelitic schist, and Mn-ore horizons of Paleoproterozoic age (Straczek et al., 1956; Narayanaswami et al., 1963; Roy, 1981; Mohanty, 1993, 2010, 2012). The contact of the Tirodi Gneiss and the Sausar Group is marked by the presence of a paleosol horizon and a conglomerate containing pebbles and boulders of the Tirodi Gneiss, indicating an unconformity between the Tirodi Gneiss and the Sausar Group (Mohanty, 1993). Detailed mapping in the Sausar Group indicates that the rocks have undergone four phases of superposed deformations (Mohanty, 1988, 2002, 2003a, b, 2009, 2010, 2012; Bandyopadhyay et al., 1995; Mohanty and Mohanty, 1996; Mohanty et al., 2000; Chattopadhyay et al., 2003a, b). The first deformation in the Sau- sar Group (SD1) is represented by isoclinal folds (SF1) with an axial planar cleavage (S1), whereas the second deformation (SD2)produced upright to steeply inclined isoclinals folds (SF2) with broadly east-west fold axes and an axial planar crenulation cleavage (S2). Open folds with subhorizontal axial planes (SF3) were developed during the third deformation (SD3). The fourth deformation (SD4) gave rise to the development of upright open folds (SF4) with broadly north-south axial traces (Mohanty, 1988, 2002, 2003a, b, 2010; Mohanty and Mohanty, 1996; Mohanty et al., 2000). The Sausar Belt shows Barrovian-type regional metamorphism with increasing grade from south to north and from east to west (Narayanaswami et al., 1963; Bhowmik et al., 1999). Peak metamorphism in this area is constrained at P ¼ w7 kbar and T ¼ w675 C(Brown and Phadke, 1987; Phadke, 1990; Bhowmik et al., 1999). Low-grade greenschist to greenschist-amphibolite facies transition zone metamorphism has been recognised all along the southern periphery of the Sausar belt.

2.2. Age constraints

Geochronologic data from the Sausar Belt are scanty. Sarkar et al. (1986) reported a Rb-Sr whole rock isochron age of 1525 70 Ma and a combined four mineral fraction (biotite, muscovite, plagioclase and K-feldspar) isochron age of 860 25 Ma from the Tirodi Gneiss. They interpreted the 1525 Ma event as the main Sausar metamorphism, leading to partial melting of the basal psammo-pelitic unit, and the younger 860 Ma event as the terminal thermal overprint on the Sausar rocks. The Tirodi Gneiss has yiel- ded a depleted mantle model (Sm/Nd) age between 2325 and 2494 Ma (Mishra et al., 2009). Similar ages are also reported for the equivalent rock units of the basement gneisses, locally known as the Amgaon Gneiss (U-Pb zircon age of 2378 17 and 2432 5 Ma; Ahmad et al., 2009) and the Malanjkhand Granite (2480 3MaU- Figure 1. (a) Generalised geological map of peninsular India. (b) Regional geological Pb zircon age by Panigrahy et al., 2002;2478 9 Ma Re-Os age by map of the western part of the Sausar belt (after Narayanaswami et al., 1963). (c) Geological map of the Mansar area (modiﬁed after Mohanty et al., 2000) showing the Stein et al., 2004). Molybdenite grains in quartz veins intruding the paleosol proﬁle analysed (WP). calc-silicate rocks of the Sausar Group have Re-Os ages between S.P. Mohanty, S. Nanda / Geoscience Frontiers 7 (2016) 759e773 761

2410 and 2450 Ma (Stein et al., 2014). Critical analysis of available quartz and sillimanite. This quartz-muscovite schist horizon is data indicates the first deformation in the Sausar Group to have considered to be a paleosol (Mohanty, 1993). Similar nodular rocks occurred at w2200e2000 Ma, the second deformation accompa- were later identified from the Deolapar and Mansar areas nied by a granulite facies metamorphism at w1600e1550 Ma, and (Chattopadhyay et al., 2003a, b). The nodular rocks of the Deolapar the late (third) deformation at w1000e900 Ma (Mohanty, 1993, area were interpreted to be product of fluid flow through sheared 2010, 2012). On the basis of these data the age of the paleosol is rocks at the contact of a granitic basement (Chattopadhyay et al., considered to be between 2480 and 2450 Ma. 2003a), but Chattopadhyay et al. (2003b) advocated a late stage hydrothermal alteration process for development of these rocks at the border of a granite intruding into the Sausar Group of the 2.3. Local geological setting Mansar area. These interpretations were debated by Mohanty (2003a, b). The paleosol horizon is reported from the Mansar area in the southern part of the Sausar Belt (Fig. 1c). The main litho-units of the Mansar area are biotite granite gneiss, conglomerate-quartzite- 3. Petrography marble, mica schist and marble-quartzite-quartz muscovite schist (Mohanty, 1993). The granite gneiss exposed in the southern and The paleosol horizon is developed at the contact of the Tirodi northwestern part of the Mansar area, was considered as the Gneiss (the basement) and the Sausar Group (the cover rock) basement (Narayanaswami et al., 1963; Rao, 1970; Chakravarty, throughout the Sausar belt. We have analysed a section found in the 1973; Mohanty, 1993). However, the maps published by southern part of the Sausar belt, in the Kandri-Mansar-Wahitola Narayanaswami et al. (1963), Rao (1970) and Chakravarty (1973) areas, where the belt defines a synformal second-generation fold showed these gneisses as an orthogneiss younger than the meta- plunging east (Fig. 1c). Here, the top of the basement granite (Tirodi sediments of the Sausar Group (cf. Fig. 1b). A polymictic conglom- Gneiss) is defined by a zone of nodular rock rich in tabular alumino- eratic horizon (of the Sausar Group) was reported from the contact silicate minerals, calcareous nodules and organic carbon nodules of the biotite gneiss (Mohanty, 1993). The conglomerate contains (Figs. 2 and 3). Extensive development of this weathered zone with pebbles of biotite granites, suggesting that the granite gneiss rep- high alumina content has encouraged the development of several resents the basement (Tirodi Gneiss) of the Sausar Group. In many “brick kilns”. This zone has a light pink to light red colour and has a places, the conglomeratic horizon is separated from the granite stronger magnetic character compared to the adjacent rocks of the gneiss by a thin quartz-muscovite schist with nodular aggregates of basement and cover. The lower part of the profile has stress

Figure 2. Variation in the properties of the paleosol from the bottom to the top of the profile. (a) Stress corrosion cracks (SCC) in the lower part of the soil profile, (b) variation in grain size and colour from bottom to top of the soil profile, (c) corestone weathering, showing alteration around an elliptical block (top left), (d) dispersed nodules on the top of the paleosol, (e) ferruginous alteration along SCC, (f) mica schist with aggregates of sillimanite, biotite and opaques. 762 S.P. Mohanty, S. Nanda / Geoscience Frontiers 7 (2016) 759e773

Figure 3. Mineralogy of the paleosol of the Mansar area studied through scanning electron microscopy (SEM) analysis. (a) Circular nodule of septechlorite (chamosite), (b) rectangular nodule of calcite, (c) laterally linked calcite nodules, (d) ankerite nodules, (e) altered chamosite, (f) septechlorite nodule, (g) euhedral magnetite with inclusion of organic carbon, bordering phengite, (h) euhedral magnetite bordered by phengite with internal cracks adjacent to organic carbon, (i) siderite crystal with internal cracks and clear border, (j) silica nodule with enriched content of Mo and Zr, (k) tuberose phengite, (l) nodule of organic carbon.

corrosion cracks (SCC) and veins (soil peds) with ferruginous al- zircon may represent both original minerals and their meta- terations (Fig. 2a, c, d). Variations in grain size and colour are morphic products; muscovite may be an original mineral as well as observable at short distances (Fig. 2b). The homogeneous granite the metamorphic product of illite; chlorite may represent the showing hypidiomorphic granular texture changes through a zone metamorphic product of chamosite; aluminosilicates are meta- of stress corrosion cracks, corestone weathering, to layered fine- morphic products of illite; and dolomite, uraninite, pyrite and grained rock (Fig. 2a, b, c). The upper part of the layered zone siderite may represent the alteration products developed during shows extensive development of nodules of different composition weathering. (Figs. 2d and 3). The main constituents of the paleosol are quartz, feldspar, sillimanite and mica with minor quantities of calcite, ilmenite and magnetite (Figs. 2e, f and 3). Scanning Electron Mi- 4. Geochemistry croscopy (SEM) with Energy Dispersive X-ray (EDX) analysis of the paleosol confirms the presence of siderite, ankerite, chlorite, nod- Geochemical changes associated with the development of the ules of organic matter, carbonate nodules, silica nodules, and paleosol by in-situ weathering of the granitic basement were magnetite (Fig. 3, Table 1). The preferred orientations of the elon- studied by analysis of samples collected from a profile starting from gated minerals define a schistosity (Fig. 2f). The X-ray diffraction the top of the paleosol towards the basement in the Wahitola area (XRD) pattern of the paleosol sample confirms the presence of (Fig. 1c). Whole rock samples were analysed at the National quartz, orthoclase, plagioclase, muscovite, chlorite, aluminosilicate Geophysical Research Institute (NGRI), Hyderabad, India (Tables 2 (sillimanite/kyanite/andalusite), garnet, staurolite, molybdite, and 3). Trace elements and rare earth elements (REE) were ana- dolomite, magnetite, ilmenite, rutile, zircon, uraninite with siderite, lysed using a Perkin Elmer Sciex ELAN DRC II Inductively Coupled rhodochrosite, and pyrite (Fig. 4). The minerals observed in thin Plasma Mass Spectrometer (ICP-MS), and the major element com- sections and inferred from XRD and SEM analysis have undergone positions of the samples were determined using a Philips MagiX changes during the metamorphism of the original rock and its PRO wavelength dispersive X-ray fluorescence (WD-XRF) spec- paleosol. For example, quartz, K-feldspars, plagioclase, rutile and trometer (PW 2440) at NGRI. International reference standard JG-2 S.P. Mohanty, S. Nanda / Geoscience Frontiers 7 (2016) 759e773 763

Table 1 SEM-EDX analysis of selected mineralsa in paleosol (element concentration in wt.%).

Element a b c d e f g h i j k l C 7.6 59.3 49.3 15.5 26.3 9.3 12.6 6.1 36.5 14.9 5.3 63.0 O 44.0 16.9 15.6 40.7 35.8 45.5 33.3 35.1 23.9 56.6 58.5 23.4 Si 20.1 1.2 5.2 3.3 13.0 17.3 0.7 0.4 4.7 31.7 21.2 5.3 Al 14.3 0.2 2.9 2.0 8.0 10.4 2.4 15.8 0.4 Ti 0.1 0.1 0.0 0.0 0.3 0.0 Fe 5.9 3.1 1.8 61.2 67.9 35.2 2.8 2.6 0.7 Mg 0.6 0.3 0.5 7.2 0.7 0.4 0.4 0.1 0.4 0.3 Mn 13.4 11.0 12.4 7.4 10.6 3.0 Ca 3.8 1.7 13.5 1.3 5.4 1.3 1.2 0.3 2.3 Na 0.3 0.8 0.7 0.5 2.2 1.3 0.5 1.9 0.7 0.6 2.7 K 7.2 1.5 1.2 3.7 0.5 2.3 1.2 5.0 1.5 P 1.0 0.5 1.2 0.3 0.4 S 2.5 0.9 0.5 0.4 0.9 0.5 1.2 0.5 1.1 Cl 0.4 1.5 0.5 1.4 N 14.9 6.8 2.7 Zr 7.1 7.7 3.0 Mo 1.1 Pt 14.6 16.7 Total 100.0 100.0 100.1 100.0 100.0 100.0 99.9 99.9 100.0 99.6 100.0 100.4

a Analyzed minerals labelled ael correspond to those in Fig. 3. of Geological Survey of Japan was used for checking the accuracy of 1989; MacFarlane et al., 1994; Fedo et al., 1995; Panahi et al., ICP-MS and WD-XRF analysis. 2000; Mitchell and Sheldon, 2010). The mobility and immobility The intensity of chemical weathering is controlled mainly by of elements depend upon ionic potential, Eh, pH and redox state. source rock composition, duration of weathering, climatic condi- Cations having an ionic potential below 3 form only weak bonds tions and rates of tectonic uplift of the source region (Wronkiewicz with oxygen; these are preferentially released from their host and Condie, 1987). The distribution of elements within the proﬁle is minerals during weathering. These can be regarded as soluble used to assess the nature and degree of weathering (Nesbitt and cations and are fully hydrated. Cations with higher ionic potential Young, 1982; McLennan, 1989). The relative mobility or immo- form strong bonds with oxygen, and form weathering resistant bility of major and trace elements can be used to infer the surﬁcial oxides, but some of these can form oxyanions, which are soluble environments during the weathering processes (Gay and (Langmuir, 1997). Such cations can form hydrolyzates. Thus, the Grandstaff, 1980; Nesbitt and Young, 1982; Grandstaff et al., 1986; relative mobility can be assessed from the ionic potential, which Kimberley and Grandstaff, 1986; Harnois, 1988; Holland et al., has a direct relationship with the solubility product (Goldschmidt,

Figure 4. XRD pattern of the paleosol indicating possible mineralogy. Alseandalusite/kyanite/sillimanite; Calecalcite; Chlechlorite; Diasediaspore; Doledolomite; Gtegarnet; IIIeillite; IImeilmenite; Magemagnetite; Moemolybdite; Musemuscovite; Oreorthoclase; Plgeplagioclase; Pyepyrite; Qequartz; Rhderhodochrosite; Ruterutile; Sidesiderite; Stestaurolite; Ueuraninite; Zezircon. 764 S.P. Mohanty, S. Nanda / Geoscience Frontiers 7 (2016) 759e773

Table 2 cause depletion of alkali and alkaline earth metals (Ca, Mg, Na and Major element geochemistry of the paleosol developed over the granite of the K). The concentration of these elements surviving in soil proﬁles is Wahitola area (concentration in wt.%). a sensitive index of the intensity of chemical weathering (Nesbitt Sample no. WP-3 WP-4 WP-5 WP-6 WP-7 and Markovics, 1997). Several molecular weathering ratios, such

SiO2 59.05 73.81 73.02 74.72 74.86 as the chemical index of alteration (CIA), hydrolysis, clayeyness, Al2O3 20.38 13.39 14.64 13.73 12.25 leaching, and salinization of both the parent rock and the paleosol Fe2O3 6.85 0.95 1.74 0.98 1.41 samples, can be used to interpret the weathering trend and pale- MnO 0.02 0.01 0.02 0.02 0.03 e MgO 4.43 0.53 0.86 0.64 0.92 oenvironment (Table 4; Figs. 5 7). Samples of paleosol and the host CaO 0.12 0.14 0.11 0.24 0.21 granite were analysed to determine different weathering parame- Na2O 0.59 4.52 1.54 3.12 2.89 ters (Table 5). K2O 6.40 4.77 5.93 5.11 5.69 TiO 0.91 0.13 0.16 0.12 0.10 2 4.1. Gain and loss of elements P2O5 0.05 0.04 0.02 0.09 0.03 Total 98.80 98.29 98.04 98.77 99.02 Sample Paleosol Corestone Moderately Slightly Fresh Some of the major elements (Si, Ca, Na, Mn) show a significant description granite altered granite altered granite granite decrease in concentration in the paleosol compared to the unal- Distance 8.4 18.1 21.6 22.6 24 tered rock. On the other hand, Ti, Al, Fe and Mg show an increase in from top (m) concentration, and K and P show no change in concentration from the parent granite to the paleosol. As the analyses of major elements are expressed in terms of weight percentage, the loss of some components is reflected as gain in the concentration of 1937; Mason and Moore, 1982; Railsback, 2003; Buggle et al., 2011). others. Most geochemical studies of altered rocks assume immo- In general, the elements Al, Ti, and Zr are considered to be chem- bility of Al and other elements are normalized with respect to Al. ically immobile in weathering environments, whereas the alkali We analysed the behaviour of major oxides by normalizing their elements (Na and K) and divalent alkaline earth elements (Ca and value with respect to Al (Table 5a) and Ti (Table 5b). Ratios of Si/Al, Mg) are mobile (Sheldon and Tabor, 2009). The behaviours of Mn Ca/Al, Na/Al, K/Al, P/Al and Mn/Al show decreasing trends, but Ti/Al, and Fe are controlled by the redox state and/or pH. Both precipi- Fe/Al and Mg/Al increase from the fresh rock to the paleosol. The Al/ tation and temperature accelerate chemical weathering in soils and Ti ratio shows a decreasing pattern from the fresh rock to the paleosol, with a relative loss of 82%. The Ti/Al ratio increases sub- stantially (500%) from the fresh rock to the paleosol. Both these ratio indices are affected by the acidification or alkalinity in the Table 3 fi Trace element and REE geochemistry of the paleosol developed over the granite of weathering pro le. Al being more mobile than Ti, the Ti/Al ratio the Wahitola area (concentration in ppm). indicates mobility of Al. In the studied profile, gains of Fe, Mg and Ti relative to Al indicate formation of magnetic soil. The latter con- Elements Sample no. dition is corroborated by the enhanced magnetic property of the WP-3 WP-5 WP-7 paleosol. From these results it is inferred that only Ti was immobile (Paleosol) (Moderately (Fresh granite) altered granite) during soil formation and that all other major elements were lost during the pedogenesis. Variations of major oxides show good Sc 5.0 0.8 1.0 V 47.6 5.8 4.7 correlation with the concentration of Ti (Fig. 6). Ca, Na and Si show Cr 17.2 13.5 18.9 decreased concentrations, but Fe, Mg and Al show an increasing Co 19.2 1.4 2.6 trend with respect to Ti from the unaltered rock to the paleosol. The Ni 6.8 3.7 7.7 rates of change of Ca, Na and Al show exponential patterns and Cu 1.4 0.5 0.5 fi Zn 32.7 11.9 12.3 suggest the dominance of acidi cation process towards the top of Rb 253.9 246.4 285.8 the soil profile. In general, the alteration shows a consistent pattern Sr 31.4 5.5 8.9 from the unaltered rock, through slightly altered and moderately Y 16.6 100.9 69.0 altered rocks to the paleosol. The sample from the corestone granite Zr 95.6 206.3 93.7 shows a geochemical signature similar to that of the slightly altered Nb 16.8 144.7 56.6 Cs 8.3 0.8 0.8 granite. Ba 488.6 59.8 87.7 La 47.3 61.1 62.0 4.2. Trend of weathering Ce 96.3 112.1 122.0 Pr 10.1 10.4 11.8 Nd 38.4 34.7 41.7 The chemical index of alteration (CIA) is a measure of the Sm 7.1 7.7 8.8 breakdown of feldspars and other minerals by the removal of base Eu 1.0 0.1 0.2 cations (mostly Na and Ca) to produce clays (Nesbitt and Young, Gd 5.9 8.1 8.2 1982). With an increase in the degree of weathering, the clay Tb 0.8 1.8 1.4 content and concentration of Al increase, and the proportion of Ca Dy 4.2 15.5 10.8 Ho 0.4 1.9 1.3 and Na decreases. A CIA value of 50 or less represents unweathered Er 1.1 7.0 4.5 rocks; values of 50e60 indicate incipient weathering and pedo- Tm 0.1 1.0 0.6 genesis; 60e80 suggest moderate weathering and pedogenesis; Yb 1.3 10.0 6.3 and 80e100 develop by intense weathering and pedogenesis Lu 0.2 1.5 1.0 Hf 4.6 10.1 6.5 (McLennan, 2001). A triangular plot showing molecular pro- Ta 0.9 9.6 4.2 portions of A-CN-K provides a tool to interpret CIA, possible min- Pb 30.4 25.0 19.3 eral phases and weathering trend (Nesbitt and Young, 1982). In the Th 22.8 66.8 52.2 Sausar paleosol profile, the CIA value for the parent rock is 52% and U 3.6 5.1 3.8 the sample from the top of the paleosol shows a CIA value of 72% Ga 23.2 32.5 27.4 (Figs. 7a and 8a). The trend of weathering matches that of a granite S.P. Mohanty, S. Nanda / Geoscience Frontiers 7 (2016) 759e773 765

Table 4 Weathering indices used for environmental interpretation.

Weathering indices Formula Reference Chemical Index of Alteration (CIA) [Al/(Al þ Ca þ Na þ K)] 100 Nesbitt and Young (1982) Chemical Index of Weathering (CIW) [Al/(Al þ Ca þ Na)] 100 Harnois (1988) Plagioclase Index of Alteration (PIA) [(Al-K)/(Al þ Ca þ Na-K)] 100 Fedo et al. (1995) Clayeyness (C) Al/Si Ruxton (1968); Sheldon and Tabor (2009) Hydrolysis (H) Al/(Ca þ Mg þ K þ Na) Sheldon and Tabor (2009) BasesAl (¼ 1/H) (Ca þ Mg þ K þ Na)/Al Ruxton (1968); Retallack (1999) Baseloss (leaching) (Ca þ Mg þ K þ Na)/Ti Sheldon and Tabor (2009) Leaching (L) Ba/Sr Retallack (2001a,b); Sheldon and Tabor (2009) Acidiﬁcation (provenance) Ti/Al Sheldon (2006a,b) Calciﬁcation (Ca þ Mg)/Al Retallack (2007) Salinization (S) (Na þ K)/Al Retallack (2001a,b); Sheldon and Tabor (2009)

with equal proportions of plagioclase and K-feldspar. Removal of Na to the weathered rock (71% and 94%, respectively; Fig. 7a). The and Ca in solution due to the breakdown of plagioclase in the PIA is mainly based on the chemical decomposition of plagioclase parent granite to form illite can be inferred from the weathering and/or K-feldspar and, hence, is a powerful tool for evaluating the trend. weathering trend for plagioclase and K-feldspar rich rocks In addition to the chemical index of alteration, there are other (Nesbitt and Young, 1984, 1989; Fedo et al., 1995). A triangular weathering indices such as chemical index of weathering (CIW; plot with the molar proportions of (A-K), C and (Na-K) can be Harnois, 1988) and the plagioclase index of alteration (PIA; Fedo used to represent the progressive loss of plagioclase. In the study et al., 1995). These ratios are also based on molar proportions and area, the PIA values for parent rock and weathered rock are 54% the ratios of more mobile to less mobile elements (Table 4). In the and 92%, respectively, and indicate the source rock to be a granite Sausar Belt, the CIW shows an increasing trend from parent rock rich in albite (Figs.7aand8b).

Figure 5. Geochemical variations in the paleosol proﬁle of the Wahitola area.

Figure 6. Variation diagrams of major oxides against TiO2 for the paleosol in the Wahitola area. 766 S.P. Mohanty, S. Nanda / Geoscience Frontiers 7 (2016) 759e773

Figure 7. Variations of weathering indices, REE and trace element concentrations.

We have also analysed our data plotted on an A-K-F diagram to in concentration of Fe and Al with respect to Ti discussed in the infer the behaviour of ferromagnesian minerals (Fig. 8c). In this previous section are substantiated on the A-K-F plot. The signiﬁ- plot, the incipient weathered samples (WP 6, 5 and 4) show loss of cance of this pattern will be discussed in a later section. K and Fe and gain in Al, but the advanced weathered sample (WP-3) Clayeyness is the measure of the amount of clay formation shows substantial gain in Fe with loss of K and Al. The trend of marked by high Al content in clay compared to the original rock weathering indicates substitution of Al by Fe during the formation (Ruxton, 1968; Sheldon and Tabor, 2009). Higher values are indic- of clay. The mobility of Al and the difference in the rate of increase ative of more clay formation with the loss of feldspar and other less resistant minerals (Sheldon and Tabor, 2009). In the Sausar paleosol proﬁle, the clayeyness value for the parent rock is 0.1 and for the Table 5 Weathering indices for the samples of Wahitola area. paleosol is 0.2 (Fig. 7b). The hydrolysis ratio measures the loss of base cations in com- (a) Major elements normalised with Al parison to Al resulting in the formation of clay (Sheldon and Tabor, Sample Si/Al Ti/Al Fe/Al Mg/Al Mn/Al Ca/Al Na/Al K/Al P/Al 2009). This parameter is the reciprocal of the BasesAl ratio, which WP-3 4.92 0.06 0.15 0.55 0.001 0.01 0.05 0.34 0.002 measures the loss of base cations with respect to Al (Ruxton, 1968; WP-4 9.35 0.01 0.03 0.10 0.001 0.02 0.56 0.39 0.002 Retallack, 1999). The hydrolysis ratio for the parent rock in the WP-5 8.46 0.01 0.05 0.15 0.002 0.01 0.17 0.44 0.001 Sausar Belt is 0.90 and for the paleosol it is 1.29 (Fig. 7c). The value WP-6 9.24 0.01 0.03 0.12 0.002 0.03 0.37 0.40 0.005 WP-7 10.37 0.01 0.05 0.19 0.004 0.03 0.39 0.50 0.002 indicates more hydrolysis during weathering and development of the paleosol. The loss of base cations with respect of Ti can be used (b) Major elements normalised with Ti as an alternative to estimate base-loss in areas where Al mobility is Sample Si/Ti Al/Ti Fe/Ti Mg/Ti Mn/Ti Ca/Ti Na/Ti K/Ti P/Ti recorded. The ratio (Ca þ Mg þ K þ Na)/Ti is an index of base-loss WP-3 86.26 17.54 2.60 9.65 0.03 0.19 0.84 5.96 0.03 (Sheldon and Tabor, 2009). For comparison of an unaltered rock and WP-4 754.79 80.68 2.52 8.08 0.09 1.53 44.81 31.11 0.17 its paleosol, the ratio of the difference and the value of the unal- WP-5 606.70 71.68 3.75 10.65 0.14 0.98 12.40 31.42 0.07 WP-6 827.77 89.63 2.82 10.57 0.19 2.85 33.51 36.11 0.42 tered rock normalized to 100 can be expressed as the percentage of WP-7 995.18 95.96 4.86 18.23 0.34 2.99 37.24 48.24 0.17 base-loss. In the study area, 84% base-loss is observed in the pale-

(c) Weathering indices set e I osol with respect to the unaltered granite (Fig. 7f). The leaching parameter uses the behaviour of alkaline earth Sample CIA CIW PIA Hydrolysis Clayeyness Salinisation metals like Ba and Sr and quantifies the amount of leaching during WP-3 71.52 94.49 91.88 1.05 0.20 0.39 the weathering process (Sheldon and Tabor, 2009). Ba and Sr have WP-4 51.02 63.52 51.69 0.94 0.11 0.94 fi WP-5 61.53 84.27 75.05 1.29 0.12 0.61 similar atomic radii and the same molecular af nity but Sr is more WP-6 55.30 71.14 59.55 1.08 0.11 0.78 soluble than Ba (Vinogradov, 1959), and is leached more readily than WP-7 52.03 70.46 54.25 0.90 0.10 0.89 Ba. Values around 10 are indicative of an acidic, well-leached, highly (d) Weathering indices set e II drained paleosol with a long time of development (Retallack, 2001a; Mitchell and Sheldon, 2010). In the Mansar area, significant leaching Sample Calcification Acidification BasesAl Baseloss Fe/Mg Fe þ Mg of the parent granite to form paleosol is inferred from the leaching WP-3 0.56 0.06 0.95 16.63 0.27 0.14 parameter (Ba/Sr), which increases from 9 to 15 (Fig. 7g). Higher WP-4 0.12 0.01 1.06 85.53 0.31 0.02 fi WP-5 0.16 0.01 0.77 55.46 0.35 0.03 values of leaching of weathered rock here indicate signi cant WP-6 0.15 0.01 0.93 83.03 0.27 0.02 leaching and free drainage (cf. Retallack, 2001a; Sheldon and Tabor, WP-7 0.22 0.01 1.11 106.71 0.27 0.03 2009). S.P. Mohanty, S. Nanda / Geoscience Frontiers 7 (2016) 759e773 767

The most commonly used geochemical ratio for predicting provenance of paleosols is the Ti/Al ratio (Sheldon, 2006b; Hamer et al., 2007a; Mitchell and Sheldon, 2009). This ratio provides information about the provenance as well as the acidification/alkalinity during the weathering process. Higher values of the Ti/Al ratio are recorded in in-situ paleosols developed on mafic rocks and low values are found on granitic basements (Maynard, 1992). As these two elements are almost immobile, the ratio may remain constant but in the case of extreme weathering conditions this ratio shows an increasing value because of removal of Al under alkaline conditions (Nesbitt and Young, 1989) as well as acidic conditions (Chadwick et al., 2003). The Ti/Al ratio in general shows wide variations in igneous rocks with different amounts of fractionation. However, analysis of 2485 granites worldwide (cf. Blatt and Tracy, 1997) yields an average Ti/Al ratio of 0.013. In the Sausar paleosol profile, the parent rock has a Ti/Al ratio of 0.01, which is increased to 0.06 in the paleosol. The low value in the area confirms the granitic source and the increased value is indicative of mobility of Al and the development of an alkaline environment. The calcification index, (Ca þ Mg)/Al (Retallack, 2007), quantifies the accumulation of pedogenic calcite and dolomite. In the Sausar paleosol this index increases from 0.22 in the unaltered granite to 0.56 in the paleosol (Fig. 7d). The ratio Ca/(Ca þ Mg) varies from 0.14 to 0.02, indicating availability of more Mg than Ca in the paleosol to form pedogenic calcareous materials. The abun- dance of calcite and ankerite nodules in the paleosol (Fig. 3) corroborates this increasing trend. The parameter also provides information about pH condition during weathering. The occurrence of nodules of silica, calcite, ankerite and organic matter, and the development of siderite and rhodochrosite constrain the pH value to the alkaline range of 7e8. Salinization is the measure of evaporation and precipitation (Sheldon and Tabor, 2009) and water accumulation. The threshold value of 1 (Retallack, 2001a) is indicative of significant salinization. High values of salinization indicate more evaporation compared to precipitation. On the other hand, a low value may indicate a well- drained profile where saline conditions do not develop. In the Sausar Belt, the salinization value for the parent rock is 0.8 and for the paleosol it is 0.3 (Fig. 7c), indicating that high precipitation with less evaporation was prevailing at the time of development of the paleosol profile, or that the profile had well developed drainage.

4.3. Trace element distribution

Trace element geochemistry of paleosols provides valuable information regarding weathering intensity (Kahman et al., 2008), evaluating leaching (Retallack, 1999, 2001a, b; Sheldon, 2006b) and nature of provenance (Sheldon, 2006b; Hamer et al., 2007b; Sheldon and Tabor, 2009). Among the large ion lithophile elements (LILE), Ba is mainly concentrated in mica and K-feldspar, whereas Sr is mainly present in Ca-bearing minerals such as plagioclase, amphibole, pyroxene and carbonate minerals. The Sr content decreases with the destruction of plagioclase. Therefore the Ba/Sr ratio increases with increasing weathering (Nesbitt and Young, 1982). In the Sausar paleosol profile, the concentration of Zr, Sc, Ba and Sr increases from the parent rock to the weathered rock. Significant plagioclase destruction has been inferred from the hydrolysis and clayeyness factor. Therefore, the enriched value of Ba and Sr in the studied paleosol profile can be explained by formation of Ba- and Sr-rich carbonate nodules in the paleosol profile Figure 8. Triangular plots showing the weathering trend of parent rock and paleosol. (Condie et al., 1995), or adsorption onto clay minerals. The (a) A-CN-K plot, (b) AK-C-N plot, and (c) A-K-F plot. The contour lines show the density increased Sr content can be explained by substitution of Sr for Ca. of distribution of data points (in %) per 1% area. Besides these, trace elements like V, Co, Pb, Zn and Cu also show enrichment in the paleosol (Fig. 7i, j). Although Mo content was not analysed, a significant content of Mo was noted in some of the silica 768 S.P. Mohanty, S. Nanda / Geoscience Frontiers 7 (2016) 759e773

Ce and Eu is controlled by the redox state. Eu occurs in two redox þ þ states (as Eu3 and Eu2 ions). The aqueous geochemistry of Eu at þ low temperature is dominated by the Eu3 state, except in reducing/anoxic conditions (Sverjensky, 1984). The complexation 3þ mechanism extends the stability of Eu towards lower f(O2). As a þ þ result, mildly basic condition results in reduction of Eu3 to Eu2 . þ þ Eu2 has a similar size to Ca2 and substitutes for it in reducing environments. In reducing environments with low oxygen fugacity, þ þ þ there will be a dominance of Eu2 and Eu3 /Eu2 will be low (Yangjing and Yongchao, 1997). Ce is another REE with two redox þ þ þ states (Ce3 and Ce4 ). Under alkaline conditions, Ce3 can be þ oxidized by atmospheric oxygen to less soluble Ce4 , which precipitates as cerianite, CeO2, resulting in the development of a pos- itive cerium anomaly in paleosols. However, during anoxic þ weathering Ce3 remains unoxidised and can form Ce-rich rhabdophane, (La, Ce, Nd)PO4 $ nH2O(Utsunomiya et al., 2003). The occurrence of rhabdophane was recorded in the 2500 Ma Pronto paleosol, which does not show a Ce anomaly (Murakami et al., Figure 9. PAAS normalized REE distributions in the paleosol profile of the Wahitola area. 2001). Therefore, the overall content of REE and Ce and Eu anomalies provide important clues on the weathering process. The Post Archean Australian Shale (PAAS; Tayler and McLennan, nodules analysed by SEM-EDX (Fig. 3j). A significant increase in the 1985) normalized REE plots show a HREE depleted value in the V/Cr ratio (0.25 in fresh rock and 2.8 in the paleosol; Fig. 7g) also paleosol of the Mansar area compared to the parent rocks, with an corroborates the reducing environment of soil formation (Mameli enhanced (La/Yb)N ratio of 2.7 compared to 0.7 in the parent rock et al., 2008). (Figs. 7h and 9). The HREEs show slight enrichment in the middle Many of the trace elements show variations in their solubility as part of the profile with respect to the parent rock, but the top of the a function of oxidation and redox conditions in the depositional profile shows HREE depletion. Similar observations were made by environment (Tribovillard et al., 2006). Trace elements such as U, V, Nesbitt and Markovics (1997) for paleosol development over the Zn, Mo, and to some extent Cu, Co and Cr, tend to be less soluble in a Toorongo granite, Australia. The SREE in the paleosol is less reducing environment of deposition (Algeo and Maynard, 2004; (214 ppm) than the parent rock (281 ppm), indicating high fluid Tribovillard et al., 2006). The ratio of U and Th provides valuable rock interaction and dominance of acidic weathering. The parent information about the redox state of the paleosol. Both U and Th are granite shows a distinct negative Eu anomaly (Fig. 9), but the relatively immobile during weathering except under intense paleosol sample does not have this anomaly. As Eu is a redox sen- weathering conditions (Li, 2000). These elements are also mobile sitive element, the value of the Eu anomaly is quantified by under different redox conditions (Sheldon and Tabor, 2009). U determining the change with respect to the two elementspffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi on either becomes mobile in oxidising environments but immobile under side of Eu on the periodic table (i.e. Eu=Eu ¼ Eu= Sm Gd). The reducing conditions. The mobility of Th is enhanced through for- Eu/Eu* value shows a significant increase from parent rock to the mation of soluble anionic complexes of P and organic matter (dis- weathering profile (Eu/Eu* ¼ 0.08 for parent rock and 0.4 for cussed later). The U/Th ratio is slightly higher in the paleosol profile paleosol; Fig. 7h), indicating a reducing environmental condition than the parent rock (Fig. 7h). The overall content of U remains with low pH at the time of soil formation. Flat patterns of LREE near more or less constant, but Th shows decreased concentration in the Ce are observed for both parent and weathered rock. The Ce paleosol. The preservation of U concentration indicates weathering anomaly is also quantified using the equation Ce/Ce*¼(3 Ce)/ under reducing conditions during paleosol formation. The presence (2 La þ Nd). The value of Ce/Ce* in the weathering profile is 1.03, of siderite, pyrite and uraninite corroborate the reducing environ- which is nearly identical to the parent rock Ce/Ce* ratio of 1.04. This ment of the paleosol. The EhepH fence diagram for this association indicates that the total cerium proportion in the weathering profile suggests the pH to be > 7 and the Eh to be < 0. was retained and that Ce did not undergo oxidation, and/or that the weathering condition was not alkaline. The overall REE pattern for all the plots indicates no significant Ce and Eu anomaly. The Sausar 4.4. REE distributions paleosol profile has a high value of Eu/Eu*, a high value of (La/Yb)N and a low value of ƩREE (Fig. 7h) indicative of a reducing envi- The geochemical behaviour of the rare earth elements (REE), ronment (Yangjing and Yongchao, 1997). which include the lanthanides and chemically similar elements Sc and Y, provides information regarding weathering processes. The 5. Discussion lanthanides have deceasing ionic radius from La to Lu and are mostly immobile except Ce and Eu, which have a different valence 5.1. Precambrian paleosols of India state. Stabilization of HREEs is achieved by complexation mechanism in solutions and LREEs are preferentially adsorbed on clays The occurrence of massive sillimanite-corundum and quartz- during weathering (Condie et al., 1995). The solubility of HREE is sillimanite schists has been reported from the Precambrian enhanced at low pH (Nesbitt, 1979), resulting in a change of La/Yb terrain of the Meghalaya plateau, northeastern India (Golani, 1989). or La/Lu ratio and a decrease in total REE content. Under acidic These high-alumina rocks, which show low Fe, Ca, Mg, K and Na, conditions, sorption processes control the REE pattern and result in were interpreted to be paleosols. Paleosol horizons overlying the (La/Lu)N > 1 (where N stands for the PAAS normalized ratio) (Bau, Banded Gneissic Complex (>2500 Ma) at the base of the Aravalli 1991). On the other hand, neutral to mildly basic conditions give Supergroup (2150 Ma) have also been reported from western India rise to complexation mechanisms forming carbonate and hydrox- (Banerjee, 1996), and show perceptible increases in Al, Ti, Si and K, ide complexes of REE and (La/Lu)N < 1(Bau, 1991). The mobility of and loss of Na, Ca and Mg. A reducing environment was proposed S.P. Mohanty, S. Nanda / Geoscience Frontiers 7 (2016) 759e773 769 for these paleosols. Because of diagenetic and metamorphic over- beginning and pattern of oxygen evolution is uncertain. The prints identified from geochemical data, Sreenivas et al. (2001) beginning of the GOE was marked by large-scale global glaciation refrained from interpreting the p(O2) condition of the atmosphere forming a “snowball earth” (Hoffman et al., 1998). Paleoproterozoic during the development of the paleosols of western India. But an glacial deposits are reported from various parts of world, such as oxidizing environment for the formation of these paleosols was the Huronian Supergroup (Ontario) and Snowy Pass Supergroup proposed by Pandit et al. (2008). (Wyoming) in North America (Ojakangas, 1988), the Hamersley A “vertic paleosol” overlying an Archean granite (3328 Ma) of basin in Western Australia (Barley et al., 1997), and the Transvaal the Singhbhum craton in eastern India, was identified near basin in South Africa (Hannah et al., 2004). Evidence of Paleo- Keonjhar, and was inferred to have formed between 2250 and 2000 proterozoic glaciation has also been reported from the Sausar Ma (Bandopadhyay et al., 2010). This paleosol was reported to be Group, overlying the paleosol (Mohanty, 2006; Mohanty et al., depleted in alkali elements, and Si, Fe and Ti. The age of the 2015). Keonjhar paleosol has since been reinterpreted to be between 3290 and 3020 Ma (Mukhopadhyay et al., 2014). Depletion of Ca, Na, Mg, Fe, Mn, Ni, Zn, and Cs normalized with Ti was confirmed from this 5.3. Weathering trends in the Sausar belt paleosol. Prominent negative Ce and Eu anomalies and enrichment of HREEs were also identified. This paleosol is interpreted to be a The Sausar paleosol shows a sharp drop in the concentration of product of oxidative weathering (Mukhopadhyay et al., 2014). mobile elements Si, Ca, Mn and Na, and an increase in the concentration of immobile elements like Al similar to Precambrian paleosols in other parts of the world. The studied paleosol was 5.2. Comparison with Precambrian paleosols of the world derived from a granite with equal proportions of plagioclase and K- feldspars. Chemical weathering under acidic conditions led to the Many paleosol profiles of Precambrian age have been reported destruction of w90% of the plagioclase to form illite. The constant from different parts of the world. We have compared the formation proportions of Mg/(Mg þ Fet) in the parent rock and soil indicate of the Sausar paleosol with some of the paleosols in other parts of that the ferromagnesian minerals did not undergo oxidation. This is the world (Table 6). The Mount Roe paleosols, formed at substantiated by loss of Mn and the constant proportion of Fe/ w2765e2715 Ma, have been reported from the Pilbara craton of (Fe þ Mn) and significant increase in V/Cr ratio in the paleosol Western Australia (MacFarlane et al., 1994; Yang et al., 2002). There compared to the unaltered granite. The acidification index in- are also two well-studied Precambrian paleosol profiles in the Elliot creases towards the top of the profile; at the same time calcification Lake region (the Denison paleosol and the Pronto paleosol). Both also increases. The inconsistency of both of these is due to an in- these paleosols occur below the Huronian Supergroup and devel- crease in Mg content from bottom to top. Further, the occurrence of oped at w2500 Ma (Rye and Holland, 1998). The Denison paleosol pedogenic carbonates in the paleosol indicates that acidification was formed under reducing groundwater conditions with exten- during the weathering process was neutralized by the sufficient sive loss of Fe, Mn, Cu, Ni, Na, Ca, Mg and increase of K content (Gay amount of calcareous materials available, and that the pH was and Grandstaff, 1980). The Pronto paleosol was developed under reduced to neutral to alkaline conditions. oxidising groundwater conditions with the loss of Ca, Na and in- The mobility of elements during the weathering process was crease of Fe, Al, K, Ti, Mg (Gay and Grandstaff, 1980). In both cases, evaluated from the variation diagrams (Figs. 5 and 6). Generally, Al loss of bases indicate moderate to intense leaching conditions and Ti are considered to be immobile during the weathering pro- occurring under a wet or humid climate (Gay and Grandstaff, 1980). cess. In the Sausar paleosol, the Ti/Al ratio shows an increased value From the worldwide distribution of paleosols of Precambrian age, it in the paleosol, indicating mobility of at least one of these two can be concluded that there was oxygen in the Archean atmo- components. A substantial increase in Ti can be interpreted to sphere. In the late Archean and early Paleoproterozoic it was low reflect physical transport of titanates and addition to the weath- and increased to the present atmospheric level. This Great Oxida- ering surface. We analysed these possibilities. Physical addition of tion Event (GOE) took place between 2300 and 1800 Ma (Holland, Ti is not supported for the study area from the analysis of the 1994, 2002; Rye and Holland, 1998). As mentioned earlier, the variation diagram of major oxides with respect to Ti (Fig. 6,

Table 6 Environmental interpretations of different Precambrian paleosols

Area Host rock Age (Ma) Element distribution Atmospheric condition Mount Roe paleosol, Western Australia Developed on w2750 Ti, Zr, Al, Th, Nb were immobile during Formed under either oxygen (at the base of Fortescue Group) basalt weathering; extensive loss of Na, Ca, Si; no absent or very low concentration significant Ce and Eu anomaly (MacFarlane et al., (MacFarlane et al., 1994; 1994; Yang et al., 2002) Yang et al., 2002) Denison paleosol, Elliot Lake, Ontario, Developed on w2450 Extensive loss of Mn, Cu, Fe, Ni, Na, Ca, Mg and Formed under anoxic Canada (overlain by Huronian Supergroup) greenstone increase of K content (Gay and Grandstaff, 1980) groundwater conditions (Gay and Grandstaff, 1980) Pronto paleosol, Elliot Lake, Ontario, Developed on w2450 Enrichment of Fe, Al, Ti, Si, K and primary loss Formed under oxidising Canada (overlain by Huronian Supergroup) granite of Ca, Mg (Gay and Grandstaff, 1980) groundwater conditions in a wet humid climate (Gay and Grandstaff, 1980) Hekpoort paleosol, Transvaal Developed on 2222 Loss of Al, Ti, Zr, V, Cr from the top of the profile Formed due to strong surface Supergroup, Pretoria Group, South Africa Hekpoort Basalt during weathering and reprecipitation of these weathering with the loss of elements in the lower profile as constituents of base cations (Rye and Holland, Fe rich smectite with intensive loss of Na 2000) and Ca (Rye and Holland, 2000) Flin Flon paleosol, Manitoba, Developed on 1850 Increase of Fe, K, Na, Mn towards the top of Formed under oxidising Canada (overlain by Messi Group) Sea floor altered the profile, significant negative Ce anomaly environment (Holland et al., 1989) mafic rocks (Holland et al., 1989) 770 S.P. Mohanty, S. Nanda / Geoscience Frontiers 7 (2016) 759e773

Table 5b). All the elements shown here have a good correlation with weathering profile and precipitates as secondary Fe-bearing min- Ti (with correlation coefficient, R2 ¼ 0.9). Very good correlations of erals (Sugimori et al., 2008). The increase in concentration of Fe in most of the major elements with Ti indicate chemical leaching as the Sausar paleosol profile may be due to deposition of secondary the dominant process during pedogenesis of the area. Such corre- iron-bearing minerals under low p(O2) conditions. Formation of lations are not expected for a physical addition process. Na, Ca and pyrite and siderite in a reducing environment can also cause an Si show depletion, whereas Fe, Mg and Al show an increasing trend. increase in Fe content. These patterns suggest loss of feldspars to form illite; Fe and Mg Thorium is mostly found in accessory minerals like monazite, released from the ferromagnesian minerals were redeposited as allanite, zircon, titanite, epidote, apatite and magnetite, and also carbonates under alkaline environment. To evaluate mobility of the isomorphous with uraninite (Langmuir and Herman, 1980; Pagel, two elements, it is noted that Ti has a greater ionization potential 1982; Wickleder et al., 2006). The highly refractory nature of than Al. Therefore, Ti is expected to be relatively less mobile than Al. these minerals in a weathering environment makes Th insoluble The mobility of Al and substitution by Fe was identified from the and immobile (Langmuir and Herman, 1980), but depletion of Th in AFM plot of the study area (Fig. 8c). the Sausar paleosol is observed. This can be explained because The two immobile elements, Al and Ti, form insoluble phases at thorium’s tendency to form strong complexes increases its poten- pH values as low as 4 and 2, respectively (Sugitani et al., 1996). tial for transport by inorganic complexes in fresh waters (pH of Under a normal pH range of 4e9, Al and Ti precipitate as hydroxides 4e7) and by organic complexes in organic-rich stream waters, and oxides, respectively, or participate in the structure of clay swamps, soils and sediments with a pH of 7e8(Langmuir and (Panahi et al., 2000). The major sources of Ti in granites are ilmenite Herman, 1980). Although adsorption of Th onto clays, oxy- (a titanate of ferrous iron) and biotite (Force,1991). The alteration of hydroxides and organic matter also increases with increasing pH, ilmenite during weathering to form leucoxene (finely crystalline organic ligands, which facilitate formation of Th complexes, inhibit þ rutile) liberates Fe2 (Farrow and Mossman, 1988). Iron liberated the adsorption and lead to depletion of Th from the system. The þ during pedogenesis may be oxidized to its trivalent state (Fe3 )or overall depleted value of Th in the paleosol indicates transport of Th þ remain in its reduced state (Fe2 ) under reducing conditions. The through the formation of complexes and a possible pH of 6e8. alteration of biotite produces septechlorites, sericite, illite, leucox- The content of Cs in the Sausar paleosol is found to be high ene and pyrite (Schwartz, 1958). During this process Al substitutes compared to the parent rock (Fig. 7i). This contrasts with the þ for Si, Mg and Fe2 . Formation of phengite (/illite) causes replace- geochemical property of Cs, which has a large ionic radius and low þ ment of Al by Mg and Fe2 . The enrichment of Fe and depletion of Al ionic charge, and has the property of forming soluble cation. noted from the AFM diagram (Fig. 8c) of the area supports the Comans and Hockley (1992) observed that in anoxic freshwater þ formation of phengite and septechlorite and replacement of Al by environments cesium is remobilized by ion exchange with NH4 . þ Mg and Fe2 . Loss of K from biotite to form septechlorite also in- The released Cs is adsorbed onto K and Ca saturated illites. This creases the total content of Mg-Fe. Several studies indicate that the sorption is irreversible over longer timescales. The anoxic weath- change in the Fe/Mg ratio in chlorite from parent rocks to paleosol ering conditions, enriched K content and development of illite in is controlled by the level of oxygen in the atmosphere at the time of the paleosol explain the enrichment of Cs in the paleosol by weathering (Gay and Grandstaff, 1980; Rye and Holland, 2000; adsorption to K-rich illite. Utsunomiya et al., 2003). The constant proportion of Fe/Mg in the The variations in geochemical character of the paleosol below parent rock and soil indicates that the ferromagnesian minerals did the Sausar Group indicate leaching of some of the components by not undergo oxidation, but the oxidation state of the atmosphere acidic weathering, solution transport, adsorption mechanisms, and may not be relevant for this. changing complexation in a neutral to alkaline environment. These The occurrence of siderite in the study area can be explained by features indicate that there was periodic fluctuation of pH values in precipitation under a combination of conditions, such as neutral to the weathering profile. Such fluctuations can be explained by alkaline pH, zero sulphide activity, restricted circulation, a low Eh shallow water table conditions or low topography. The topographic (0.25 to 0.35 V) and high concentration of carbon dioxide and high areas and areas above the water table were affected by acidic bicarbonates (cf. Curtis and Spears, 1968; Retallack, 1976). The weathering processes (possibly associated with high CO2 or CH4 intensified magnetic signal in the soil profile can also be explained content in the atmosphere), but low lying areas and areas below the by the development of fine-dispersed magnetite grains from the water table were experiencing periodic water-logging and/or thermal decomposition of siderite (cf. Pan et al., 1999) as follows: alkaline conditions. The REE systematics are not affected by hydrothermal or FeCO3 / FeO þ CO2 metamorphic fluid-rock interaction because of the extremely low REE content of hydrothermal and metamorphic fluids (Bau, 1991). 3FeO þ CO2 / Fe3O4 þ CO The REE pattern of the samples analysed here has a distinct depleted pattern with no Ce and Eu anomalies and HREE depletion. The decomposition of siderite becomes complete at Similar REE signatures are observed in many Precambrian paleosol 450e600 C. The formation of magnetite by soil-forming processes profiles. Therefore, the distinctly different REE patterns of the in areas with no detrital input has been identified in some soils nodular rocks and the parent rocks of the Sausar belt do not support (Maher and Taylor, 1988). the hydrothermal base leaching mechanism proposed by Fe and Ti also show increasing concentration in the paleosol Chattopadhyay et al. (2003a, b) for the nodular schists of the Sausar profile (Fig. 7d, e). Fe generally decreases in paleosols formed under belt. reducing environment (Retallack, 1986b), but in case of the Sausar paleosol profile, the Fe concentration is increasing. As discussed 6. Conclusions above, reducing conditions prevailed during the paleosol formation. Hence, this Fe concentration must be explained another way. From the overall geochemistry of the paleosol profile and parent þ þ In almost all cases, Fe2 dissolved from primary Fe2 -bearing rocks it can be concluded that the paleosol in the Sausar belt has þ minerals gets oxidised to form Fe3 hydroxides under oxic condi- formed due to weathering of the parent granite. This is contrary to 2þ tions. But in the case of low p(O2) conditions, part of the Fe the view that the nodular schists bordering the granite of the released from the dissolution of primary minerals flows out of the Mansar area were formed from hydrothermal alteration S.P. Mohanty, S. Nanda / Geoscience Frontiers 7 (2016) 759e773 771

(Chattopadhyay et al., 2003a, b). From the mineralogy and Comans, R.N.J., Hockley, D.E., 1992. Kinetics of cesium sorption on illite. Geochimica e geochemistry of the fresh granite and the paleosol, and comparison et Cosmochimica Acta 56, 1157 1164. Condie, K.C., Dengate, J., Cullers, R.L., 1995. Behaviour of rare earth elements in the with paleosols from other parts of world, it can be concluded that a paleoweathering profile on granodiorite in the Front Range, Colorado, USA. reducing environment was prevailing at the time of formation of Geochimica et Cosmochimica Acta 59, 279e294. the paleosol. Moderate weathering intensity formed clay minerals Curtis, C.D., Spears, D.A., 1968. The formation of sedimentary iron minerals. Eco- nomic Geology 63, 257e270. of illitic composition. Overall, the Sausar paleosol was developed Driese, S.G., Medaris Jr., L.G., Ren, M., Runkel, A.C., Langford, R.P., 2007. 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