A Paleoproterozoic paleosol horizon in the Lesser Himalaya and its regional implications

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A Paleoproterozoic paleosol horizon in the Lesser Himalaya and its regionalimplications

O.N. Bhargava a, Gurmeet Kaur b,⇑, M. Deb c

a 103, Sector7, Panchkula 134 109, Indiab CAS in Geology, Panjab University, Chandigarh 160 014, Indiac CAS in Geology, University of Delhi, Delhi 110 007, India

a r t i c l e i n f o

Article history:Received 28 November 2009Received in revised form 31 July 2011Accepted 1 August 2011Available online 24 August 2011

Keywords:PaleosolPaleoproterozoicLesser HimalayaGeochemistryPrecambrian metamorphism

a b s t r a c t

A Paleoproterozoic paleosol horizon in the Himachal Himalaya along a basement-cover contact is iden-tified on the basis of an integrated field-petrographic-geochemical studies.

The paleosol horizon is exposed in a road section along the Sutlej River near Karcham. It is representedby a 2–5 m thick sericite schist unit along the contact of the 1866 ± 10 Ma Jeori–Wangtu–Bandal GneissicComplex (JWBGC) and the overlying sericite quartzite of the Manikaran Formation (Rampur Group),which is interstratified with 1800 ± 13 Ma tholeiitic flows in its basal part. The geochemical studies reveala sharp drop in the concentration of SiO2, Fe2O3, MgO, CaO, Na2O and a rise in concentration of Al2O3,TiO2, K2O and P2O5 at the contact of granite gneiss and sericite schist. REE plots of granite gneiss, sericiteschist and quartzite samples of the Manikaran Formation display similarity of pattern, fractionationbetween the LREE and HREE and comparable negative Eu anomaly. The total REE of the sericite schistand sericitic quartzite is lower than those of the granite gneiss.

Based on these studies the sericite schist is inferred to be a metamorphosed alumina-rich soil, whichappears to have formed in a warm and humid climate in a waterlogged terrain of gentle relief, and ispost-1866 Ma and pre-1800 Ma in age.

Apparent gradation from the strongly deformed amphibolite facies JWBC to the sericite schist with dif-fused contact indicates that the JWBGC was already metamorphosed and deformed prior to the develop-ment of the paleosol; thereafter both together with the overlying Manikaran Formation were subjected tolow-grade metamorphism during the Himalayan orogeny. The JWBC is involved in the crystalline thrustsheet and is present throughout the length of the Himalaya. Thus, it is inferred that the Paleoproterozoicmetamorphism was a regional event in the Himalaya at a time when the Indian Plate was part of theNuna Supercontinent.

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1. Introduction

Precambrian paleosols have been discovered and documentedin large numbers in recent years by various workers from differentgeological environments. The interest mainly emanates from thesuggestion (e.g. Holland, 1984; Retallack et al., 1984) that paleosolsmay be used to determine the oxygen content of Precambrianatmosphere and thus provide valuable information on early atmo-spheric evolution, because they formed at the Earth’s surface in di-rect contact with the atmosphere. Apart from establishing a hiatusin the stratigraphy, the identification of paleosols in the Precam-brian sequences, as the present contribution shows, can also haveregional significance in the geological history of an area.

Paleosols are distinct petrologic entities, often very alumina-rich (Reimer, 1986; Wright, 1986). Three classes of criteria havegenerally been used to recognize the paleosols: biological traces,soil horizons and soil structures (Retallack, 1992). However, inareas with strong deformation and metamorphism subsequent tothe formation of the weathering profile, such as those found inmany Precambrian terrains, these criteria are of limited use. Insome instances, Precambrian paleosols are also known to have suf-fered hydrothermal alteration as well. In such cases, study of geo-logic setting combined with petrological and geochemical studiesof the sequence along a profile appears to be the most meaningfulexercise. Such an approach, for example, has been successfullyapplied to the Early Proterozoic Hokkalampi paleosol in northKarelia in eastern Finland, represented by quartz–sericite schist(Marmo, 1992). However, even in this line of study doubts do lin-ger regarding the distinction between the geochemical variationsdue to subareal weathering and those produced by any post-burial

1367-9120/$ - see front matter � 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.jseaes.2011.08.001

⇑ Corresponding author.E-mail address: [email protected] (G. Kaur).

Journal of Asian Earth Sciences 42 (2011) 1371–1380

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hydrothermal alteration along a litho-contact or a zone of shearingduring metamorphism (cf. Palmer et al., 1989). Thus, probably the

best results are obtained with an integrated approach using de-tailed field relationships, petrographic and geochemical signatures.

Fig. 1. Geological map of south-eastern closure of the Larji-Rampur Window; the key map shows regional geological setup of the window.

1372 O.N. Bhargava et al. / Journal of Asian Earth Sciences 42 (2011) 1371–1380


In one of the pioneering studies in peninsular India, Dunn andDey (1942) had suspected that the kyanite–quartz lensoid bodiesoccurring regionally along the interface of the Dhanjori and Chaiba-sa group of rocks, along the Singhbhum shear zone were derived bymetamorphism of aluminous weathering materials. Sharma (1979)had interpreted the pyrophyllite–diaspore association found in sev-eral locations on the Bundelkhand granitic complex as paleosols.Precambrian paleosols have been recorded from the Aravalli hillsnear Udaipur. The Banded Gneissic Complex in this part of Rajasthanhas extensive development of sericite–pyrophyllite associationalong the basement–cover interface, which has been interpretedas a paleosol on the basis of geochemical data by several workers(Roy and Paliwal, 1981; Banerjee, 1996; Sreenivas et al., 2001; Pan-dit et al., 2008). The khondalite sequence in the Eastern Ghat mobilebelt has also been suspected to represent metamorphosed Precam-brian aluminous lateritic soils (Dash et al., 1987) or Precambrianpaleosols (Sreenivas and Srinivasan, 1994; Bandopadhyay et al.,2010). Similarly, Golani (1989) suggested that high Al2O3 soils haveacted as protoliths to the sillimanite deposits of Sonapahar inMeghalaya, northeastern India. Based only on field observation,Bhargava (2000) made a passing reference to the possibility of apaleosol in the Himachal Himalaya, discussed in the present com-munication. Consequently, so far there is no definite and confirmedrecord of any Precambrian paleosol horizon in the Himalaya. Thus,this is the first documentation of various attributes of a Paleoprote-rozoic paleosol horizon along a basement–cover contact in theHimachal Himalaya through the integrated approach mentionedabove. This paleosol horizon has far reaching implications on a re-gional scale in the Himalaya.

2. Geologic and stratigraphic setting

The Lesser Himalayan physiographic zone in the HimachalHimalaya exposes Proterozoic sequences with isolated outliers ofEarly–Middle Cambrian (Tal Group) and Paleocene–Middle Eocene(Subathu Formation) sequences (for detailed stratigraphic se-quence refer Bhargava (2000)). Over these sequences rest the Kulu,Jutogh and Vaikrita thrust sheets comprising the Precambrian crys-talline rocks (Fig. 1, inset location map). Where the erosion hasgone deep enough, as in the Satluj and Beas Valleys, the greenschistfacies rocks have been exposed along the antiformal portions aswindows. One such window known as the Kulu-Rampur Window,surrounded by the Kulu Thrust Sheet exists in the Satluj Valleyaround Rampur in the Himachal Himalaya (Figs. 1 and 2) (Berthel-sen, 1951; Bhargava et al., 1972). In this window the oldest rockunit exposed is a gneissic complex between Jakheri and Karcham(Fig. 1) along the National Highway (NH) 22. This gneissic complexas revealed by the U–Pb zircon age is 1866 ± 10 Ma old (Singh et al.,1994). This age has also been corroborated by Rameshwar et al.(1995). An identical gneissic complex of the same age (Frank et

al., 1977), designated as the Bandal Gneiss, is exposed furthernorthwest. Both these complexes, in view of similar setting andage, have been re-designated as the Jeori–Wangtu–Bandal GnessicComplex (JWBGC) (Bhargava, 1980).

The JWBGC comprises crudely foliated granite in the centralpart, followed by porphyroblastic gneiss and augen to streakygneiss (Fig. 3), biotite schist, concordant bodies of metabasites,kyanite gneiss, hornblende gneiss, biotite–kyanite schist, and locallenses of meta-conglomerate having rounded pebbles of tourma-line granite (rare) and quartz within the porphyroblastic gneissin outer parts. Locally the gneiss is mylonitic. The metamorphismvaries from garnet to kyanite–sillimanite grade of the amphibolitefacies. In complete and undisturbed sections the gneiss apparentlygrades upwards into a thin layer of sericite schist along a diffusedcontact (Fig. 4). This relationship can be best studied along NH 22,near Karcham.

The Manikaran Formation (Rampur Group), comprising quartz-ite and metabasic rocks (tholeiites), overlies the sericite schistalong a sharp contact (Fig. 4). The whole rock analyses of themetabasic rocks, associated with the Manikaran Formation, gavean age of 2510 ± 90 Ma by Sm–Nd method (Bhat and Lefort,1992). Miller et al. (2000), however, termed this age as ‘meaning-less’ as the single zircons from these very metabasalts yielded anevaporation age of 1800 ± 13 Ma. Metabasalts of the same ageand in similar geologic set up are extensively present in the LesserHimalaya (Bhat et al., 1998; Bhargava, 2000; Ramakrishanan andVaidyanadhan, 2008). Near Jakheri, the JWBGC tectonically overliesthe rocks of the Manikaran Formation (Rampur Group) along ahigh angled Jakheri reverse fault (Fig. 1), which is a feature withinthe Rampur Window (Bhargava, 1980). In the Bandal area too, theSW contact of the JWBGC with the Manikaran Formation is in theform of a reverse fault. In tectonically undisturbed sections, the

Fig. 2. Cross section along A–B of Fig. 1, to show local geological setup near Karcham.

Fig. 3. Field photograph of the tectonised granite gneiss, showing quartzo-feldspathic augens.

O.N. Bhargava et al. / Journal of Asian Earth Sciences 42 (2011) 1371–1380 1373


Manikaran Formation rims the gneissic complex with interveningsericite schist, besides occurring as infolded outliers within theJWBGC. Sharma (1977) considered that the Bandal Gneiss (notshown in Fig. 1) is intrusive into the Rampur Group and interpretedoutliers of the Manikaran Formations as roof-pendants. A detailedexamination in both the areas revealed that a sericite schist hori-zon intervenes between the gneisses and the Manikaran Formationin undisturbed sections and no intrusive relationship and contactmetamorphic effect is observed in the latter. The JWBGC fromthe road section near Karcham extends north and south of the Sat-luj River in extremely rugged and thickly forested terrains. In viewof the JWBGC being older than the overlying Manikaran Formationand absence of intrusive relationship, the JWBGC was interpretedas a basement complex (Bhargava, 1980, 2000). It was tentativelysuggested by Bhargava (2000) that the gradationally overlying ser-icite schist is a possible B-horizon of a paleosol over which theManikaran Formation (Rampur Group) was deposited. Since thisinterpretation of the JWBGC as basement and of the sericite schistas a paleosol was entirely based on field data, petrographic-geo-chemical studies, as recorded in the present contribution, were car-ried out on the sericite schist horizon to establish if it represents aweathered surface of the JWBGC.

3. Field studies

The field observations and sampling were conducted in a road-cut (N31�2905900; E78�10049.700) along the Hindustan-Tibet Road(NH 22) near Karcham rest house, on the other side, across the riv-er, where the sericite schist horizon is best exposed between theporphyroblastic and tectonised granite gneiss below (Figs. 2 and4) and the overlying quartzite of the Manikaran Formation. Geolog-ical observations were made and samples collected systematicallyacross the granite gneiss – quartzite interface over a distance of30.25 m (sample locations shown in Figs. 6 and 7 on the litholog,corrected to the true thickness of the profile). A total of nine sam-ples were collected along the road section trending N35�W–S35�E,three from the granite gneiss, five from sericite schist and one fromthe overlying quartzite. Unfortunately, this road section has now

been covered by the retaining wall of the Karcham dam, under con-struction. The field photograph (Fig. 4) is thus along the left bank ofthe river.

The three litho-units studied – granite gneiss, sericite schist,and quartzite, show peneconcordant contacts between themselvesalong a N20�E–S20�W trend with a dip of 55� towards SE. The gran-ite gneiss contains porphyroblasts of feldspars, quartzo-feldspathicaugens and shows development of almandine garnets close to thecontact with sericite schist. It shows strong deformation with thegneissosity thrown up in chevron and tight to isoclinal folds. Suchfolds are absent in the overlying schist.

The sericite schist is pale buff coloured, highly fissile with alter-nating streaks and bands of quartz while the overlying quartzite ismassive with sporadic thin sericitic partings. There is no evidenceof any metasomatic or hydrothermal activity along the contacts ofthese lithologies.

4. Petrography

Thin sections of all the nine samples were studied under themicroscope. The granite gneiss contains, besides the main constit-uents quartz and K-feldspars, both micas – muscovite and biotite,with ubiquitous tourmaline and garnet (Fig. 5a). The coarse tomedium grained quartzo-feldspathic bands alternate with themicaceous bands with strong directional fabric to define the gne-issosity. The trails of mica and quartz inclusions in garnet suggestthat garnet growth was syn- to post tectonic with respect to thepervasive schistosity.

The sericite schist has a very similar mineralogy (Fig. 5b) withvery different proportions of the same medium grained minerals,rare biotite, and with some tourmaline and chlorite, garnet beingconspicuous by its absence. The muscovite in this lithology isclearly primary and not the breakdown product of any earlierphase. It’s strong preferred orientation defines the schistosity ofthis fissile rock.

The massive quartzite of the Manikaran Formation has an over-whelming proportion of medium grained, mostly equant quartzwith fine streaks of sericite (Fig. 5c).

Fig. 4. Field photograph of the JWBGC (granite gneiss), paleosol (sericite schist) and Manikaran Formation (sericite quartzite) along the left bank of the Satluj River. Notediffused contact between the JWBGC and the paleosol and a sharp contact between the paleosol and the quartzite of the Manikaran Formation.



5. Geochemical data

Chemical analyses of all the samples for their major and trace-REE concentrations were carried out by XRF (except the quartzitesample, which was analysed by conventional spectrophotometricmethod in the University of Delhi) and ICP-MS techniques respec-tively, in the geochemical laboratories of NGRI, Hyderabad. Themajor element contents of the granite gneiss, sericite schist andthe overlying sericitic quartzite are presented in Table 1a. The totaliron concentrations in the samples are given as Fe2O3. The traceand rare earth element contents of the same samples are presentedin Table 1b.

The data generated show that SiO2, Al2O3 and K2O account for�90–95% of all the rock types along the profile. Silica is highestin the sericitic quartzite and in its contact with the sericite schist(72.8 wt.%). It is about 70 wt.% in the granite gneiss and decreasesin the sericite schist to 66.5 wt.%. Al2O3 is the highest (23.1 wt.%) inthe sericite schist decreasing to 14 wt.% in the sericitic quartzite. Itis between 15 and 16 wt.% in the granite gneiss. The granite gneisssamples have MgO and CaO contents varying between 0.11 to1.55 wt.% and 1.5 to 2.44 wt.%, respectively. These elements alsoattain their lowest abundance around 0.12 wt.% in the sericiticquartzite or in sericite schists. The Na2O content of the granitegneiss is between 0.59 and 1.17 wt.% while that of K2O is around4.8 wt.%. The Fe2O3 content of the granite gneiss is the highest inthe profile at 3.55 wt.% (avg.). The sericite schist unit has Fe2O3

ranging from 2.25 wt.% near the granite gneiss contact to0.90 wt.% near the quartzite contact. It is as low as 0.5 wt.% in ser-icite quartzite. Because of the low Fe2O3 content (average 1.2 wt.%,as total iron) of the sericite schist, determination of FeO was not at-tempted. Na2O content of sericite schist is between 0.41 and0.2 wt.%, but rises to 3.89 wt.% in the sericitic quartzite. K2O con-tent of sericite schist is the highest in the profile at 5.93 wt.%. Itis thus obvious that all the major elements, Fe, Mg, Ca and Naare in very low abundance in the sericite schist. In the overlyingquartzite the abundance of Fe, Mg, Mn and Ca are also rather low.

Spatial plots of concentration of major elements (Fig. 6a–f)along the lithology reveal the following:

(i) A sharp drop in the concentration of SiO2, Fe2O3, MnO, CaOand MgO is seen right at the contact of granite gneiss andsericite schist.

(ii) Though K2O shows an initial rise in concentration in the seri-cite schist, its concentration clearly drops in these strata alongthe profile and rises again marginally in the sericite quartzite.Na2O on the other hand, decreases somewhat in the sericiteschist and rises markedly in the sericite quartzite.

(iii) An unambiguous rise in the concentration of Al2O3, TiO2 andP2O5 is observed at the contact of granite gneiss and sericiteschist, a feature similar to that noted in many modern, Phan-erozoic and Precambrian soils.

(iv) Fe2O3 content of granite gneiss is 3.8 wt.% decreasing contin-uously to 0.9 wt.% in the uppermost sericite schist in contactwith the massive quartzite which has only 0.54 wt.% Fe2O3.MnO is consistently of very low abundance all through theprofile.

The computed Chemical Index of alteration (CIA after Nesbittand Young, 1982) based on the major element data provides anestimate of the intensity of weathering along the profile. These val-ues (Table 1a) are between 60 and 63 in the granite gneiss, rise to75–79 in the sericite schist and fall to 52 in the sericite quartzite.

The picture of trace element variation along the section is not asclear as the major elements though Rb, Sr and Y show sharp fallswhile Zr, Nb, Cr and to some extent, V show enhancement of their

Fig. 5a. Photomicrograph of the granite gneiss showing alternation of quartzoseand micaceous layers and garnet (black) with internal schistosity of fine sericite.

Fig. 5b. Photomicrograph of the sericite schist showing oriented muscovite bladesin a medium grained quartzose groundmass.

Fig. 5c. Photomicrograph of the quartzite showing an equigranular mosaic ofquartz with fine streaks of sericite defining schistosity.



abundance across the contact of granite gneiss with sericite schist(Fig. 7a–c).

Chondrite-normalised REE plots of the three samples of granite,five of sericite schist and one of quartzite reveal conspicuously sim-ilar patterns (Fig. 8a–c) with similar fractionation between LREE andHREE and comparable negative Eu anomaly. The total REE of the ser-icite schist (between 56.18 and 157.80 ppm, mean = 94.9 ppm) andsericitic quartzite (100.59 ppm), however, is distinctly lower thanthose of the granite gneisses (between 252.06 and 97.86 ppm,mean = 184.23 ppm). These data are in contrast to the generallyfound higher abundance of REE within the paleosols than in theirparent material (cf. Retallack and Mindszenty, 1994), which maybe attributed to the metamorphism of the paleosol.

6. Discussions

6.1. Interpretation of the field, petrologic and geochemical data andage of the paleosol

Weathering is the chemical response of a rock when it is ex-posed to the atmosphere. The nature of the soil thus formed is

dependant on the type of rocks being weathered, the climate, oxy-gen content in the atmosphere and any particular local conditionthat prevailed during the period of weathering. Alkali (Na, K) andalkaline earth (Ca, Mg) metals are universally removed by solutionsfrom present day soil profiles. Depending on pH of the solutions,some silica and alumina may also get removed. Iron and manga-nese, which remain in more than one oxidation state, are very re-dox sensitive and their behaviour is strongly dependant on theprevailing oxygen pressure (Palmer et al., 1989).

A sharp drop in the concentration of SiO2, Fe2O3, CaO, MgO andNa2O in the sericite schist right at its contact with the granitegneiss indicates dissolution/removal of these elements during theweathering of the granite gneiss, which consequently led to risein the concentration of the immobile major elements, Al2O3, TiO2

and P2O5 and trace elements Zr, Nb. The somewhat erratic behav-iour of K2O in the sericite schist unit, though showing an overalldecrease, probably reflects the local heterogeneity within thepaleosol. The initial rise in the K2O content of the paleosol maybe due to subareal weathering that produced clay minerals likemontmorillonite followed by its diagenetic conversion to illite byaddition of potassium as a result of salinization. The removal of

Fig. 6. (a–f) Plots of concentration of (a) SiO2; (b) Al2O3; (c) Fe2O3; (d) MgO, CaO; (e) Na2O, K2O; (f) MnO, TiO2, P2O5, in granite gneiss (JWBGC), sericite schist (paleosol) andsericite quartzite (Manikaran Formation) along the profile.



Ca and Mg in particular indicate that the climate under which thepaleosol formed was at least seasonally wet.

The concentration of iron in Precambrian paleosols, based on itstwo oxidation states, has been used widely to estimate PO2 in

Precambrian atmosphere. Holland et al. (1989) used a plot of theconcentration of total iron against the concentration of aluminiumin the paleosol and its parent rock at Flin Flon to determine the de-gree to which ferrous iron was oxidised and retained as ferric iron

Table 1aMajor element abundance in samples along the profile.

Lithology Gr. Gn Gr.Gn Gr. Gn. Ser.Sch Ser.Sch Ser.Sch Ser.Sch Ser.Sch (Ser) qtziteDistance along profile (in m) 0 m 6 m 24 m 26 m 26 m/0.5 m 27.35 m 29.05 m 30.0 m 30.25 m

Contact at 24.85 mSample no. 7 7A 7B 7C1 7C2 7D 7E 7F 7G

SiO2% 70.17 70.77 69.41 66.91 66.55 71.38 71.84 72.84 72.57TiO2% 0.51 0.46 0.11 0.32 0.37 0.13 0.14 0.1 0.27AI2O3% 15.04 15.94 16.93 21.48 23.1 20.54 19.88 18.97 14.04Fe2O3% 3.8 3.32 2.62 2.25 1.21 0.73 0.89 0.9 0.54MnO% 0.06 0.06 0.13 0.02 0.01 0.01 0.01 0.01 0.02MgO% 1.55 0.11 1.04 0.96 0.73 0.42 0.5 0.45 0.12CaO% 1.5 1.98 2.44 0.37 0.22 0.44 0.18 0.13 0.97Na2O% 0.69 1.17 0.59 0.41 0.23 0.21 0.2 0.21 3.89K20% 4.88 4.84 5.49 5.62 5.93 4.76 4.58 4.32 4.71P2O5% 0.17 0.2 0.19 0.27 0.23 0.37 0.21 0.08 0.29

Sum% 98.37 98.85 98.95 98.61 98.58 98.99 98.43 98.01 97.42

Mol. prop. for calculation of CIAAI2O3% 0.148 0.156 0.166 0.211 0.227 0.201 0.195 0.186 0.138CaO% 0.027 0.035 0.044 0.007 0.004 0.008 0.003 0.002 0.017Na2O% 0.011 0.019 0.010 0.007 0.004 0.003 0.003 0.003 0.063K2O% 0.052 0.051 0.058 0.060 0.063 0.051 0.049 0.046 0.050P2O5% 0.001 0.001 0.001 0.002 0.002 0.003 0.001 0.001 0.0023P2O5 0.004 0.004 0.004 0.006 0.005 0.008 0.004 0.002 0.006CaO⁄ = CaO–3P2O5 0.023 0.031 0.039 0.001 �0.001 0.000 �0.001 0.001 0.011AI2O3 + CaO⁄ + K2O + Na2O 0.234 0.258 0.273 0.278 0.292 0.255 0.246 0.236 0.262

CIA = (AI2O3/AI2O3 + CaO⁄ + Na2O + K2 O) � 100 63.146 60.672 60.747 75.825 77.514 78.878 79.390 78.860 52.632

CIA=Chemical index of Alteration

Table 1bTrace element abundance in samples along the profile.

Sample no. 7 7A 7B 7C1 7C2 7D 7E 7F 7G

Profile distance (m) 0 m 6 m 24 m 26 m 26 m/0.5 m 27.35 m 29.05 m 30.0 m 30.25Sc 3.7 3.9 3.8 4.1 3.8 3.7 3.3 3.7 3.1V 9.4 7.3 3.6 7.2 5.3 3.3 3.3 3.6 4.8Cr 60.2 38.5 24.9 40.3 44.9 31.2 24.7 39.8 29.2Co 5.9 8.9 5.6 2.0 2.5 1.1 1.0 1.1 0.9Ni 27.4 15.9 16.8 16.3 13.5 12.3 8.5 18.9 22.3Cu 0.3 0.5 1.5 1.3 0.6 0.5 0.5 0.5 0.7Zn 39.7 50.9 71.1 248.0 23.4 22.1 20.2 17.4 12.7Ga 21.6 20.5 20.5 33.3 27.6 21.5 21.5 19.7 5.5Rb 298.4 258.9 379.1 405.6 372.7 272.0 262.6 231.6 85.1Sr 49.8 64.9 74.7 14.6 11.1 14.2 15.0 30.8 9.8Zr 4.3 3.7 2.9 6.0 2.8 3.0 2.3 2.9 3.6Nb 15.3 13.5 10.0 24.6 18.2 12.2 14.8 13.3 4.8Cs 11.5 9.5 13.4 9.6 8.1 6.2 5.7 5.4 2.5Ba 694.3 590.0 1410.3 412.8 393.1 230.7 384.6 275.5 155.3Y 20.7 24.7 21.7 15.8 20.8 28.6 10.6 20.1 14.6La 46.0 53.5 19.7 21.4 31.1 11.5 9.7 15.4 22.5Ce 89.1 111.5 39.5 43.5 70.2 25.9 22.9 35.1 44.9Pr 9.6 12.1 4.4 4.7 8.0 3.1 2.7 4.3 4.5Nd 36.6 47.4 16.8 17.1 30.4 13.0 11.2 19.0 17.2Sm 7.3 9.6 4.3 3.4 5.2 3.1 2.6 4.7 3.4Eu 0.8 1.1 0.8 0.5 0.6 0.4 0.4 0.6 0.5Gd 5.5 7.0 3.9 3.0 4.2 3.3 2.1 3.6 2.5Tb 0.8 1.0 0.7 0.5 0.7 0.7 0.4 0.6 0.4Dy 4.1 5.2 4.2 2.9 4.0 5.0 2.3 3.9 2.4Ho 0.4 0.5 0.4 0.3 0.4 0.5 0.2 0.4 0.3Er 1.2 1.4 1.2 0.9 1.3 1.6 0.7 1.2 0.9Tm 0.1 0.1 0.2 0.1 0.2 0.2 0.1 0.1 0.1Yb 1.2 1.3 1.6 1.1 1.4 1.9 0.8 1.3 0.9Lu 0.2 0.2 0.3 0.2 0.2 0.3 0.1 0.2 0.1Hf 0.2 0.2 0.2 0.3 0.2 0.2 0.2 0.2 0.2Ta 1.5 1.3 1.7 4.2 2.4 1.6 3.4 2.6 0.3Pb 34.3 35.1 126.0 77.6 10.3 10.4 10.6 13.1 11.8Th 24.8 29.1 13.3 16.4 22.2 13.5 8.6 17.1 17.5U 2.8 24.6 16.2 5.3 5.8 3.0 6.9 4.8 2.6



in the paleosol. They obtained a positive correlation in the Fe/Al ra-tio with the points close to a line passing through the origin andinterpreted that ferrous iron was quantitatively oxidised to ferriciron during weathering and iron was retained in the paleosol alongwith aluminium. In the present study, the iron (Fe2O3) content ofthe rocks along the profile has been found to be low, between3.8 and 0.9 wt.%, showing a decrease in concentration in the seri-cite schist and a broadly negative correlation with aluminium. Thissuggests a loss of iron from the rocks during weathering which ispossible only in a reducing environment.

Amongst the trace elements, Rb, Sr and Y are depleted in the soilin the course of modern weathering while Nb, Zr, Cr and V becamemore concentrated as these are difficult to dissolve during weath-ering. A similar behaviour for these elements is observed in the

sericite schist unit. Similarity of patterns of LREE–HREE fraction-ation in the granite gneiss and the sericite schist are interpretedto indicate that the sericite schist developed due to the weatheringof the granite gneiss. Finally, the CIA also points to the most in-tense weathering in the profile present in the sericite schist hori-zon. Integration of all the data helps us to infer that the protolithof the sericite schist unit on the granite gneiss of the JWBGC is apaleosol. Similar LREE and HREE patterns in the quartzite of theManikaran Formation suggest that the granite gneiss of JWBGCconstituted its provenance.

From the pattern of dissolution of soluble elements like Ca andMg we infer that the climatic conditions were wet and the loss ofiron in the weathering profile points to a reducing condition of theenvironment. Such reducing conditions are generally found inwaterlogged soil. Only a flat terrain or a terrain with gentle topog-raphy can get water logged. The JWBGC was thus exhumed and ex-posed to weathering for quite some time in a humid climate in aterrain of gentle topography prior to 1800 ± 13 Ma.

Fig. 8. (a–c) Chondrite-normalised REE patterns in (a) granite gneiss (JWBGC)sample nos 7, 7A, 7B, (b) sericite schist (paleosol) sample nos. 7C1, C2, 7D, 7E, 7Fand (c) sericite quartzite (Manikaran Formation) sample no 7G. Normalisationvalues from Nakamura, 1974.

Fig. 7. (a–c) Plots of trace elements (a) Ba, Rb; (b) Sr, Y, Nb, Cs, Zr; (c) Cr, Ni, V, Co ingranite gneiss (JWBGC), sericite schist (paleosol) and sericite quartzite (ManikaranFormation) along the profile.



Since the paleosol occurs above the 1866 ± 10 Ma JWBGC andbelow the Manikaran Formation having 1800 ± 13 Ma interstrati-fied tholeiitic flows, we infer that it formed after 1860 Ma and priorto 1800 Ma in Paleoproterozoic time. Since no terrestrial plants ex-isted at that time, formation of the soil was entirely due tomechanical and chemical weathering brought out by meteoricagencies.

6.2. Interpretation of the significance of contrasting metamorphicfacies of the rocks of JWBGC and the overlying sericite schist

The JWBGC, over which rests the sericitic paleosol shows garnetto kyanite–sillimanite grade of amphibolite facies metamorphismand intense deformation on a regional scale. Had the deformationand metamorphism of the JWBGC and the paleosol taken place to-gether, both the units would have shown similarity of deformationpattern and metamorphic grade. The differences in the metamor-phic assemblage and deformational pattern in the granite gneissand the overlying sericite schist, however, are conspicuous. Yetthe granite gneiss apparently appears to grade into the sericiteschist of the greenschist facies along a diffused contact. This anom-alous situation strongly suggests that the JWBGC was alreadymetamorphosed and deformed prior to the development of thealumina-rich paleosol over which the Manikaran Formation wasdeposited. The entire litho-package comprising the JWBGC, thepaleosol and the overlying Manikaran Formation was later affectedby greenschist facies metamorphism during the Himalayan orog-eny. As a result of this metamorphism no pedogenic fabric and/or protolith mineralogy could be retained in the paleosol when itwas metamorphosed to sericite schist with a strong directionalfabric.

6.3. Age of the metamorphism of the JWBGC and its regionalimplications

The metamorphism and attendant deformation of the JWBGC iscertainly pre-1.8 Ga, possibly of �1.86 Ga age close to the age of itsformation. During this time interval (1.9–1.8 Ga) India is believedto have formed part of the Nuna Supercontinent (Hoffman, 1997;Reddy and Evans, 2009; also named as Columbia, Rogers et al.,2002) and its southern and northern blocks were amalgamated.In this supercontinent the east coast of India was attached to thewestern coast of North America. In Greenland (Garde et al., 2002)and parts of China also (Rogers et al., 2002) mid-crustal partition-ing and oblique convergence and upper amphibolite facies meta-morpism have been recorded during this time-slot. In theHimalayan part too there seems to be a regional convergence lead-ing to thickening and formation of S-type of granites recorded inthe JWBGC (cf. Barbarin, 1999) and associated metamorphism. At�1.8 Ga the Nuna supercontinent fragmented and continents reas-sembled to constitute the Nena Supercontinent (Gower et al.,1990). The ca.1.8 Ga metabasalts of the Lesser Himalaya can there-fore, be related to this period of fragmentation of the Nuna super-continent. The Manikaran basin, was thus a part of the Nenasupercontinent.

The JWBGC basement has been involved in thrusting and conse-quently it occurs as a persistent tectonic marker mainly in the formof mylonite gneiss throughout the Himalaya from Kashmir in theNW to Arunachal in the East, though described by different names(Bhargava, 2000; Dalhausie Granite in J&K and Himachal, Bhatiaand Kanwar, 1973; Gahr/Baragaon Gneiss in Himachal, Sharma,1977 and Bhanot et al., 1978; Munsiari and equivalent gneissesin Uttarkhand, Trivedi et al., 1984; Lingtse Gneiss in Darjeeling,Sinha-Roy and Sen Gupta, 1979 and Paul et al., 1996; Gachang/Bur-khola Gneiss in Bhutan, Dasgupta, 1995 and Sarkar and Dasgupta,1995; gneisses in Bomdi La Crystallines in Arunachal Pradesh,

Kumar, 1997). The above observation on metamorphism anddeformational pattern of the JWBGC vis-à-vis the metamorphosedpaleosol establishes that there was a Paleoproterozoic deforma-tional and metamorphic event in the Himalaya on a regional scale.

7. Conclusions

Geochronological data together with the interstratified natureof tholeiites and the metasedimentary units of the Manikaran For-mation suggest that the JWBGC forms a basement complex.

Existing geochronological information, geochemical data, andfield relationships lead to the interpretation that the sericite schistabove the JWBGC is a paleosol of Paleoproterozoic age, whichformed in humid and partly reducing conditions in a water-loggedterrain.

The JWBGC had undergone a phase of deformation and meta-morphism at 1866 ± 10 Ma prior to pedogenesis.

Acknowledgements

We are thankful to Dr. V. Balaram, Head, Geochemistry divisionof N.G.R.I, Hyderabad for allowing the use of his XRF and ICP-MSfacilities. Thanks are also due to the anonymous reviewer and Prof.Ian Metcalfe for their constructive comments and suggestionswhich considerably improved the manuscript.

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A Paleoproterozoic paleosol horizon in the Lesser Himalaya and its regional implications