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Geochimica et Cosmochimica Acta 98 (2012) 203–227

Effective radium concentration across the Main Central Thrustin the Nepal Himalayas

Frederic Girault a,⇑, Frederic Perrier a, Ananta Prasad Gajurel b, Mukunda Bhattarai c,Bharat Prasad Koirala c, Laurent Bollinger d, Monique Fort e,

Christian France-Lanord f

a Equipe de Geomagnetisme, Institut de Physique du Globe de Paris, Sorbonne Paris Cite, University Paris Diderot,UMR 7154 CNRS,

1 rue Jussieu, F-75005 Paris, Franceb Department of Geology, Tri-Chandra Campus, Tribhuvan University, Ghantaghar, Kathmandu, Nepal

c National Seismological Centre, Department of Mines and Geology, Lainchaur, Kathmandu, Nepald Departement Analyse Surveillance Environnement, CEA, DAM, DIF, F-91297 Arpajon, France

e Universite Paris Diderot, Departement de Geographie, CNRS, UMR Prodig 8586, F-75205 Paris Cedex 13, Francef CRPG-CNRS, Nancy University, BP20 F-54501 Vandoeuvre-les-Nancy, France

Received 24 February 2012; accepted in revised form 1 September 2012; Available online 16 September 2012

Abstract

Effective radium concentration (ECRa) of 622 rock samples from 6 different sites in the Nepal Himalayas was measured inthe laboratory using radon accumulation experiments. These sites, located from Lower Dolpo in Western Nepal to EasternNepal, are divided into 9 transects which cut across the Main Central Thrust zone (MCT zone) separating low-grade meta-morphic Lesser Himalayan Sequence (LHS) units to the south and higher-grade metamorphic Greater Himalayan Sequence(GHS) units to the north. This boundary remains difficult to define and is the subject of numerous debates. ECRa values rangefrom 0.03 ± 0.03 to 251.6 ± 4.0 Bq kg�1, and appear to be representative of the formation and clearly related to the locallithology. For example, for the Upper Trisuli and Langtang Valleys site in Central Nepal, the most studied place with 350available ECRa values, LHS rocks are characterized by a mean value of 5.3 ± 1.3 Bq kg�1 while GHS rocks of FormationsI and II show significantly lower values with a mean value of 0.69 ± 0.11 Bq kg�1, thus leading to a LHS/GHS ECRa ratioof 7.8 ± 2.2. This behavior was systematically confirmed by other transects (ratio of 7.9 ± 2.2 in all other sites), with a thresh-old ECRa value, separating LHS from GHS, of 0.8 Bq kg�1, thus bringing forward a novel method to characterize, within theMCT shear zone, which rocks belong to the GHS and LHS units. In addition, Ulleri augen gneiss, belonging to LHS rocks,occurred in several transects and were characterized by high ECRa values (17.9 ± 4.3 Bq kg�1), easy to distinguish from theGHS gneisses, characterized by low ECRa values at the bottom of the GHS, thus providing a further argument to locate theMCT. The measurement of ECRa data, thus, provides a cost-effective method which can be compared with neodymium iso-topic anomalies or estimates of the peak metamorphic temperature. This study, therefore, shows that the measurements ofECRa provides additional information to discriminate different geological formations, and can be particularly useful in areaswhere geology mapping is not straightforward or still remains controversial.� 2012 Elsevier Ltd. All rights reserved.

0016-7037/$ - see front matter � 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.gca.2012.09.001

⇑ Corresponding author. Tel.: +33 1 83 95 74 30; fax: +33 1 83 9577 09.

E-mail address: girault@ipgp.fr (F. Girault).

1. INTRODUCTION

The Himalayas, the highest range of the planet with2500 km length, result from the collision of the Indian plateand the Eurasian plate, which started during Paleocene toEocene time (60–40 Ma). This range is currently

204 F. Girault et al. / Geochimica et Cosmochimica Acta 98 (2012) 203–227

accommodating shortening by about 20 mm yr�1 along thedecollement ramp at depth, the Main Himalayan Thrust(MHT) (e.g., Avouac, 2003; Yin, 2006). Among other struc-tures dipping north, within the Himalayan imbricatedthrusts wedge forming the forward propagating sequenceof thrusting, is the Main Central Thrust (MCT). Originallydefined as the major structure responsible for the overthru-sting of the large High Himalaya Crystalline sheet over theLesser Himalayas (Heim and Gansser, 1939; Gansser,1964), the hangingwall and the footwall of this thrust sys-tem are derived from two different protoliths (Valdiya,1980). Indeed, this thrust places high-grade metamorphicrocks of the Greater Himalayan Sequence (GHS), locatednorthward, over low-grade metamorphic rocks of the Les-ser Himalayan Sequence (LHS) (e.g., Le Fort, 1975; Arita,1983; Upreti, 1999). However, as several geological charac-teristics of these two units are similar and, in general, asthese units are spatially close to each other, they are notstraightforward to distinguish in the field. Therefore, thelocation of the MCT still remains a delicate and controver-sial subject (e.g., Searle et al., 2008).

The matter is complicated by the fact that this majorthrust is not a discrete plane, but a wide zone characterizedby highly deformed rocks over kilometric thickness and istherefore rather described as a broad shear zone, calledthe MCT zone. Given its importance in the reconstructionof the history of the collision, this thrust zone of the Hima-laya has been the subject of a particularly large amount ofgeological, lithological, thermochronological, geochemicaland structural studies throughout the Himalayas (e.g.,Upreti, 1999). Indeed, various approaches have been per-formed with rock samples from the MCT zone, with a sud-den increase in the number of studies in the 1990s and in thebeginning of 2000s. First, direct geological observations,based mainly on lithological and petrological criteria wereanalyzed (Bordet et al., 1971; Colchen et al., 1986), whilestructural observations were also used, for example defor-mation features and strain criteria (Grasemann et al.,1999; Searle et al., 2008). Several studies based on meta-morphic geology were also carried out (Le Fort, 1975; Pe-cher, 1989). In numerous cases, geochemical methodswere used, such as radiochronology with U, Th and Sm(Harrison et al., 1997; Catlos et al., 2001), radioactive dis-equilibrium with Nd (Parrish and Hodges, 1996; DeCelleset al., 2000), or geothermobarometry of various mineralswith different techniques (Kohn et al., 2001; Bollingeret al., 2004). All these methods are complementary andtheir combination is necessary to be able to unravel thecomplex history and evolution of the MCT shear zone.Nevertheless, ambiguities between GHS and LHS units re-main and further methods could be useful to clarify the de-bate around the MCT zone, with its local variants andcomplexities, and the often delicate point of the exact loca-tion of the MCT.

One physical observation which may be of great interest,to distinguish subtle geological differences, is the ability ofrocks to emit radon gas, and especially isotope radon-222,a radioactive gas with a half-life of about 3.8 days which be-longs to the uranium-238 decay chain and is produced byalpha disintegration of radium-226 (half-life of 1600 yr).

Radium-226 is naturally present in rock or soil materials(Tanner, 1964; Nazaroff, 1992), and consequently, radon-222 production is directly proportional to the activity con-centration of radium-226 (CRa). However, not all radonatoms produced in a rock or soil can reach the pore space,and subsequently be transported farther. The visible frac-tion of radon atoms is called the emanation factor E (Tan-ner, 1964). Hence, the radon source term is the productE � CRa, expressed in Bq kg�1, and referred to as the effec-tive radium concentration (Stoulos et al., 2004). The ema-nation factor E depends on the porous network and onthe way the radium is distributed. Radium atoms locatedat the center of compact grains, for example, are not ableto release accessible radon, and are associated with a lowE, while radium resulting from secondary deposition onthe surface of grains can potentially lead to E close to50%, provided some residual water slows the radon atomsand maintains their path inside the pore space (Barillonet al., 2005; Adler and Perrier, 2009). Thus, the effective ra-dium concentration, which represents the ability of somesample to release radon, depends not only on the chemicalcomposition but also on the details of the sample such as itsmicro-fracturation, mineralogy and history.

Geographical areas with large uranium and radium con-tent have been known for long for producing large radonfluxes (Ielsch et al., 2001). More detailed correlation withgeology, however, while not impossible and meaningless,was considered as difficult (Botset and Weaver, 1932). Mostworkers dealing with radon sources, therefore, have focusedtheir efforts on direct measurements in houses (Duboiset al., 2010). Direct measurements with natural samples inthe laboratory were rare and mostly concerned with poten-tially hazardous building materials and soil samples (Breit-ner et al., 2010; Hassan et al., 2011). These detailedlaboratory studies were useful in examining the physics ofthe radon emanation (Sakoda et al., 2011). Meanwhile,some workers were returning to the geological significanceof the results. For example, pioneering studies of the ra-dium content of rocks (Przylibski, 2004) and the associatedradon content of groundwater (Przylibski, 2000) indicatedthat characteristic values of ECRa for given geological for-mations could be considered, and that variations of E mightbe as important as variations of CRa from formation to for-mation. Further progress on the delicate matter of a poten-tial geological significance of ECRa, however, could only beconsidered when a sufficient number of ECRa data directlydetermined on rock samples would become available.

Recently, we have developed an experimental methodol-ogy allowing the measurement of ECRa with a large numberof samples (Girault and Perrier, 2012a,b). Results obtainedwith 177 Kathmandu Valley sediments were promising,with a remarkable coherence in a given horizon and repro-ducible characteristic values and stratigraphic patterns fordistant soil profiles (Girault et al., 2011a). In addition, aremarkable consistency of the values was also observed inrocks, for example in a gneiss tunnel or across the BasalPenninic Contact in the French Alps (Girault and Perrier,2012b). While ECRa values remained constant within agiven lithology, variations were observed across faults, orwithin particular formations, with promising patterns

F. Girault et al. / Geochimica et Cosmochimica Acta 98 (2012) 203–227 205

indicating, among other factors, an important lithologicalcontrol of ECRa. These results provided initial motivationto attempt a more systematic measurement of ECRa in anarea of subtle geological contrasts.

In this paper, we present the results of a large-scale studyof ECRa of rock samples across the MCT in the NepalHimalayas. The results have been obtained with rock sam-ples collected in the MCT zone of six sites from Eastern Ne-pal to Western Nepal, corresponding to nine transectscrossing the MCT. Before describing the sites in some de-tails, we review the current knowledge on ECRa and we de-scribe our measurement method.

2. THE EFFECTIVE RADIUM CONCENTRATION

2.1. General principles of ECRa

The effective radium-226 concentration ECRa, expressedin Bq kg�1, is the product of the radium-226 activity con-centration (CRa) and the emanation factor (E). This quan-tity corresponds to the potential of a given sample torelease radon-222. Its value depends on the radium-226concentration and its spatial distribution with respect tothe pore space, which controls the value of E (Adler andPerrier, 2009; Sakoda et al., 2011). The distribution of ra-dium in a rock is not a trivial matter. Although homoge-neous radium distribution is reported in radiobarites andvolcanic glasses, the distribution of radium atoms is oftenheterogeneous (e.g., Morawska and Phillips, 1993). Indeed,in a rock sample, as a daughter product of the uranium de-cay chain, first, radium can be present initially in mineralfractures, for example in quartz, feldspar, biotite and horn-blende (e.g., Caruso and Simmons, 1985). Second, radiumcan be adsorbed at grain boundaries or in larger fracturesof the sample, preferentially associated with hydrated oxi-des such as ferrous oxyhydroxides (Ames et al., 1983c;Benes et al., 1984; Flexser et al., 1993) and with clay miner-als (Ames et al., 1983a,b; Baraniak et al., 1999; Hidakaet al., 2007). In the case of large weathering, direct precip-itation of weathered primary minerals may also be able toconcentrate radium (Flexser et al., 1993). Finally, like ura-nium, radium is present in accessory minerals (e.g., Bajoet al., 1983) and is able to replace barium or calcium by iso-morphic substitution in the crystal lattice, due to their smalldifference of effective ionic radii (from 1 to 1.6 A). Some ofthese accessory minerals, such as zircon, sphene, monaziteand acicular apatite are less affected by weathering, unlikeallanite, fluorite and prismatic apatite (e.g., Guthrie andKleeman, 1986). However, in the absence of geologicaltransformation (weathering, metamorphism), intrinsic ra-dium concentration of a sample can be considered constant.

The emanation factor E is the key parameter definingthe probability for a radon atom to reach the pore space.This coefficient, first, takes into account the recoil of the ra-don atom after disintegration and the emanation due to ra-don diffusion in grains (Tanner, 1964). In addition, thetrajectory of radon after the radioactive decay can be af-fected by the geometry and content of the pores. The pres-ence of water decreases radon recoil distance and theoccurrence of previous nuclear tracks facilitates radon es-

cape from the grain. In addition, E depends on the porousnetwork, with enhancement of E with pore volume, specificsurface of grains and grain size (Bossus, 1984; De Martinoet al., 1998; Somlai et al., 2008). Water content affects theemanation factor (Adler and Perrier, 2009; Sakoda et al.,2010), but in general E remains relatively constant for watersaturation between 10% and 60% (Girault and Perrier,2012b). Ambient temperature slightly increases E in areversible manner (Iskandar et al., 2004; Girault andPerrier, 2011), an effect that we attribute to adsorption(Girault and Perrier, 2012b), whereas heating or thermaltreatment at high temperature definitely decreases thefactor E (Garver and Baskaran, 2004; Jobbagy et al.,2009). Finally, pressure tends to enhance radon emission(Holub and Brady, 1981), but this phenomenon has onlybeen observed recently near sample rupture (Tuccimeiet al., 2010). Therefore, the value of E is a complex functionof the radium spatial distribution and the properties of thepore space. In general, E varies from a few per cents to30–40% (Przylibski, 2000; Sakoda et al., 2011). ECRa thusappears as a sensitive quantity characterizing a rock samplein a subtle manner.

2.2. Measurement method of ECRa

As it is not possible to know a priori the emanation fac-tor of a sample, the effective radium concentration ECRa

needs to be measured in the laboratory (Stoulos et al.,2003; Girault et al., 2011a). Our measurement techniqueis a radon emanation method and is described in detail inGirault and Perrier (2012-a). Rocks were sampled withhammer on field, and then crushed immediately during fieldcampaigns or later in the laboratory. Crushing consisted insimply reducing characteristic size of the samples in piecesof 1.5 cm maximum size. Samples have never been driednor heated. Sometimes, raw uncrushed samples were alsoused directly. Similar results were obtained with crushedand uncrushed from the same rock samples (Girault andPerrier, 2012a).

The crushed sample was placed in a hermetically closedcontainer (glass pot or jar with pre-perforated natural rub-ber stopper at its top) and, after an accumulation time vary-ing from 4 to 20 days, air of the pot was sampled with a pre-evacuated Lucas scintillation flask. Radon concentration inthe scintillation flask was then measured by using a photo-multiplier (CALENe, Algade, France), after a time lapseof 3.5 h needed to reach radioactive equilibrium. ECRa ofthe sample was finally calculated using (Girault et al.,2011a; Girault and Perrier, 2012a):

ECRa ¼Va

m

CRn

ð1� e�ktÞ ð1Þ

where Va is the free air volume of the pot (m3), m is thesample mass (kg), CRn is the radon activity concentration(Bq m�3), k is the radon decay constant (2.1 � 10�6 s�1)and t is the accumulation time (s). For each sample, threeto four measurements at different accumulation times wereperformed and their values were averaged.

For uncrushed samples mainly, another method used so-lid-state nuclear track detectors (SSNTD) of two different

206 F. Girault et al. / Geochimica et Cosmochimica Acta 98 (2012) 203–227

types: KODALPHAe LR115 cellulose nitrate films fromDosirad (France) and DPR2e dosimeters from Algade(France). With LR115 films, rock sample was inserted inglass jars with the film fixed on the back of the lid. A sheetof filter paper was installed 4 cm from the film above thesample to define the acceptance volume. DPR2e dosime-ters, the film being installed in a fixed calibrated volume,were directly inserted open in glass jars above the sample.Radon was left accumulating inside the jars during 1.5–2.5 months. Then, mean radon concentration was givenby the manufacturers after their process of etching andtrack counting.

Over a period of three years, we performed more than6000 accumulation experiments (Girault and Perrier,2012a), including numerous systematic checks. Measure-ments with selected samples were repeated with identicalor different experimental conditions, and we were able toevaluate the assignment of the experimental uncertaintiesby measuring the dispersion of the individual measurementsin a statistically significant manner (Girault and Perrier,2012a). In the case of scintillation flasks, our main methodhere, the uncertainty on an individual measurement in-cluded a statistical uncertainty due to the counting rate,which dominates for low values of ECRa, and an uncer-tainty due to a dilution correction during sampling, whichis important for small volumes of the accumulation con-tainers. In addition to these uncertainties, the repetitionof measurements indicated that an additional dispersionneeds to be taken into account, and we adopted a conserva-tive value of 8%, which represents the fluctuations of un-known origin associated with the method. Overall, theexperimental uncertainty ranged from 30% (1r) for ECRa

values smaller than 0.2 Bq kg�1 to 8–10% for ECRa valueslarger than 50 Bq kg�1 (Girault and Perrier, 2012a). Thisexperimental uncertainty was slightly larger when usingfilms instead of scintillation flasks, because of a less remark-able repeatability of identical measurements (Girault andPerrier, 2012a). This punctual experimental uncertaintycharacterizes statistically at one standard deviation the dis-persion of two independent measurements, and is the uncer-tainty quoted in the rest of this paper.

In addition to this punctual uncertainty, all measure-ments are affected in a systematic manner by the absolutecalibration of our sensors, which is given with an uncer-tainty estimated to be of 5%. Thus, when comparing oneparticular value with results given by other authors usingother types of instruments, if need would arise, this system-atic uncertainty, to be fully rigorous, needs to be combinedin quadrature with the given punctual uncertainty to obtainthe total uncertainty of a particular measurement.

3. GEOLOGICAL CONTEXT AND SAMPLING SITES

3.1. General geological context

The thrust belt in the Nepalese Himalayas includes fourmain units defined by their tectonic and stratigraphicalcharacteristics (Guillot, 1999; Upreti, 1999; Avouac, 2003;Yin, 2006; Mascle et al., 2010). From south to north, theseunits, the Sub-Himalaya or Siwalik Group (SSG), the Les-

ser Himalayan Sequence (LHS), the Greater Himalayan Se-quence (GHS) and the Tethyan Sedimentary Sequence(TSS), are separated by large thrusts or faults dippingnorth, such as the Main Frontal Thrust (MFT), the MainBoundary Thrust (MBT), the Main Central Thrust(MCT) and the South Tibetan Detachment System (STDS)(Fig. 1). Currently still active, the MFT separates SSG fromthe Quaternary Terai Sediments (QTS) (Lave and Avouac,2000), whereas the MBT, active since about 12 Ma (e.g.,Meigs et al., 1995; Huyghe et al., 2001), separates SSG fromLHS. The STDS is a major normal fault system separatingthe GHS from the TSS (Yin, 2006). The MCT separatesLHS units from GHS units. These two major shear zonesare considered to have been active from latest Oligoceneto middle Miocene, although recent studies have suggested�15–5 Ma shortening accommodation in the MCT zone(DeCelles et al., 2001; Hodges et al., 2004). Within theLHS units, the Ramgarh Thrust (RT) places the oldest Pro-terozoic units of the LHS over younger LHS rocks and hasbeen observed structurally over the whole Nepal Himalayas(Pearson and DeCelles, 2005).

The SSG units encompass various unmetamorphosedsediments of synorogenic origin, such as mainly sandstonesand mudstones (Stocklin, 1980). The LHS units consist ofrelatively thick sequence (10–15 km) of unmetamorphosedsedimentary to greenschist-grade metasedimentary units(Upreti, 1999). Beginning of depositional age of these units,from 1860 to 1830 Ma, is constrained by repeated occur-rence of intrusive “Ulleri” augen gneiss (Fig. 1), which isdated from 1700 to 1900 Ma throughout the Himalayas(e.g., Kohn et al., 2010), about 1780 Ma at its type localityUlleri (Mid-Western Nepal) and 1880 Ma in the UpperTrisuli Valley (Central Nepal). The LHS units show in-verted metamorphic series, from lower greenschist-gradeto the south to higher isograds allowing garnet growth be-neath the MCT (Arita, 1983). The GHS units includeamphibolite-grade metasedimentary and meta-igneousrocks, but also Miocene leucogranites at their northern part(Le Fort, 1975; Guillot and Le Fort, 1995; Harrison et al.,1997). These units are fairly younger than LHS units, from1100 to 440 Ma for the youngest augen orthogneisses in thenorthern part (Parrish and Hodges, 1996; DeCelles et al.,2000; Gehrels et al., 2003). In this paper, we chose to usethe GHS Formations (Fm.) described initially by Le Fort(1975), namely Fm. III for granitic augen gneisses and leu-cogranites, Fm. II for calcareous gneisses and paragneisses,and Fm. I for kyanite-, sillimanite-, garnet- and biotite-richgneisses. The TSS units include Cambrian to Cretaceoussedimentary rocks in Nepal Himalayas, such as for examplelimestones, sandstones, slates, shales or marls (Garzanti,1999).

3.2. Characteristics of the MCT zone

Historically, first field geologists investigated the shearzone according to structural criteria, and sometimes com-plemented with metamorphical or petrological approaches.These structural criteria incorporated macroscale sliversdescription (Bordet et al., 1971), characteristic strong pla-nar-linear fabric with flat synmetamorphic cleavages and

TIBET

INDIA

NEPAL

LowerDolpo(Fig. 9)

Kali Gandaki and Marsyandi Valleys

(Fig. 8)

Upper Trisuli and Langtang Valleys

(Fig. 2)

Lower Trisuli Valley

(Fig. 7)

Syaule and Kodari

(Fig. 6)

Dharan-Dhankuta Transect

(Fig. 5)

0 100 km

Miocene Leucogranites

Tethyan Sedimentary Sequence (TSS) Greater Himalayan Sequence (GHS)

Lesser Himalayan Crystalline Nappe and related formations

Lesser Himalayan Ulleri Augen Gneiss Lesser Himalayan Sequence (LHS) Subhimalaya Siwalik Group (SSG) Quaternary Terai Sediments (QTS)

MCT

MBT

MFT

Thrusts

80°E 84°E 88°E

30°N

28°N

26°N

Fig. 1. General simplified geological map of Nepal with boxes representing the six different studied sites. MFT, MBT and MCT correspond toMain Frontal Thrust, Main Boundary Thrust and Main Central Thrust, respectively. Geology is drawn after Upreti (1999).

F. Girault et al. / Geochimica et Cosmochimica Acta 98 (2012) 203–227 207

penetrative stretching lineation (e.g., Brunel and Andrieux,1980), preferred orientation of the mineral lattices to quan-tify mode and degree of the deformation within the shearzone (e.g., Bouchez and Pecher, 1976; Kawamitsu and Hay-ashi, 1991), and, later, microstructural kinematic indicatorscharacterizing the shearing (Grasemann et al., 1999). Inaddition, within the MCT zone, discrete structures, suchas thin mylonitic gneisses or brittle structures have beenmapped (e.g., Paudel and Arita, 2000).

Metamorphic and petrologic observations could bringsupplementary information due to the different thermal his-tories of the hangingwall and footwall of the MCT. First,the zonal successions of index minerals, which allowedidentifying originally the inverted metamorphism of theHimalaya, were parallel to the structures. The kyanite iso-grad, mapped within the shear zone in the vicinity of themylonitic gneisses, appeared thus as a convenient markerof the MCT in the field (Bordet et al., 1971; Le Fort,1975; Colchen et al., 1986; Pecher, 1989). Several interestingstudies focusing on the thermobarometry of various miner-als were carried out along transects perpendicular to theMCT (Catlos et al., 2001; Kohn et al., 2001; Kohn, 2008;Martin et al., 2010, 2011; Corrie and Kohn, 2011). More re-cently, a new technique using Raman spectroscopy of car-bonaceous LHS rocks was used in LHS units (Beyssacet al., 2004; Bollinger et al., 2004). Some large differencesbetween LHS and GHS units were observed with peakmetamorphic temperature, ranging from about 650 �C tomore than 800 �C for GHS units, and from 540–520 �C

for the structurally highest LHS units to 330 �C for lowestLHS units. In addition, several chronological studies wereundertaken in the MCT zone, using most often U–Pb andTh–Pb dating of monazite, and more rarely Sm–Nd datingof garnet (Copeland et al., 1991; Harrison et al., 1997; Cat-los et al., 2001; Kohn et al., 2001, 2005, 2010; Martin et al.,2007; Corrie and Kohn, 2011). They all noticed youngerages for the GHS units, in general from about 12 Ma toabout 22 Ma.

In the meantime, the different histories of the protolithsof LHS and GHS units were characterized by their lithol-ogy, modal petrographic data, and differences in neodym-ium (Nd) isotopic composition as well as by thedistribution of their detrital zircon ages. Particularly, Par-rish and Hodges (1996) found that zircons from GHS unitswere dated from late Proterozoic to early Palaeozoic,whereas LHS units were of late Archean to early Protero-zoic age. GHS and LHS units have also been dated from800 to 1700 Ma and 61.9 Ga (DeCelles et al., 2000; Kohnet al., 2010), with lower and upper LHS rocks of Protero-zoic and from Paleozoic to younger ages, respectively.Other studies dealing with neodymium isotopic analysis(eNd) mentioned significant difference between GHS andLHS units, typically �16 and �21.5, respectively, with nooverlapping, in many MCT transects along the Himalayas(France-Lanord and Le Fort, 1988; Parrish and Hodges,1996; DeCelles et al., 2000; Robinson et al., 2001; Martinet al., 2005; Richards et al., 2005; Imayama and Arita,2008).

208 F. Girault et al. / Geochimica et Cosmochimica Acta 98 (2012) 203–227

3.3. Sampling sites

In this paper, a total of 622 rock samples were used todetermine their effective radium concentration (ECRa).Our study focused on the MCT shear zone and, as a guid-ing principle, several perpendicular rock sampling transects,cutting across this thrust for ECRa measurement, were per-formed throughout the Nepal Himalayas. Locations of ourstudied sites are displayed in Fig. 1.

From Eastern Nepal to Western Nepal, six sites, subse-quently divided into nine south–north transects, were inves-tigated. In Eastern Nepal, our study was performed alongthe Dharan–Dhankuta Transect (Fig. 1), a sampling tran-sect easily achievable due to a road in good condition. Atthis site, rock series from four units were encountered:namely QTS, SSG, LHS and GHS. There, LHS units wereseparated by the Munsiari Thrust (MT) which defines theboundary between the Nourpul and the Kuncha formations(Shrestha et al., 1984a; Duvadi et al., 2001).

In Central Nepal, three sites were studied: Syaule andKodari, the Lower Trisuli Valley, and the Upper Trisuliand Langtang Valleys (Fig. 1). At these sites, rock sam-ples were taken from LHS and GHS units only. Whilesampling was relatively straightforward in Syaule andKodari and in Lower Trisuli Valley sites, it appearedmore difficult in the Upper Trisuli and Langtang Valleyssite, where outcrops were sometimes absent or arduous toreach.

In Mid-Western Nepal one site was investigated. TheKali Gandaki and Marsyandi Valleys site allowed us tosample rocks from LHS, GHS and TSS units, in particularalong the Kali Gandaki river. Along the Marsyandi river,by contrast with previous studies which had preferentiallyfollowed the eastern bank of the river (Stocklin and Bhatta-rai, 1981; Shrestha et al., 1984b; Catlos et al., 2001; Bollin-ger et al., 2004), we were able to use the newly constructedroad located on the opposite bank, where outcrops ap-peared extremely fresh and with relatively no weatheringusually induced by monsoon or vegetation growth at theirsurface.

Finally, in the Lower Dolpo site, located in Western Ne-pal, rock sampling was undertaken along pathways of horsecaravans, as well as near hot springs outflows. Sampledrocks belonged to LHS, GHS and TSS units.

At all sites, we placed the MCT at, or near, the locationtypically introduced by Colchen et al. (1986). All geologicalmaps in this paper were thus constructed using materialfrom published studies and from our own observations dur-ing fieldwork.

4. RESULTS

For each investigated site, the ECRa values from rocksamples are displayed overlaying a geological map. More-over, ECRa values, sorted by lithology and geological for-mation, are listed in Table 1, whereas they aresummarized for each site in Table 2. When not specifiedotherwise, all average values are arithmetic means, with er-ror bars corresponding to one sigma standard deviation(68.2% confidence interval).

4.1. Results in the Upper Trisuli and Langtang Valleys

Our most comprehensive investigation was carried outin the Upper Trisuli and Langtang Valleys, Central Nepal(Figs. 1 and 2). Along these valleys, a total of 350 rocksamples were taken from various outcrops, during severalfield campaigns between 2007 and 2011. ECRa results areindicated with a colored scale in Fig. 2. The geologicalmap is a compilation of several studies (Stocklin andBhattarai, 1981; Shrestha et al., 1984b; Macfarlane et al.,1992; Inger and Harris, 1993; Parrish and Hodges, 1996;Takagi et al., 2003; Pearson and DeCelles, 2005; Kohn,2008; Pant, 2010; Paudel, private communication) and isalso complemented by our own fieldwork. The MCT islocated north of the Syabru-Bensi geothermal system(Perrier et al., 2009).

Rock samples are characterized by ECRa valuesranging over 5 orders of magnitude, from 0.03 ± 0.03 to252 ± 4 Bq kg�1. Their distribution is relatively close to alog-normal distribution with a mean of 4.30 ± 0.83 Bq kg�1.Significant distinct distributions of ECRa values can be no-ticed when observing different geological layers. Indeed, assummarized in Table 1, it is for example relatively easy to dis-tinguish the gneiss + quartzite layer with mean ECRa of0.61 ± 0.06 Bq kg�1 from Kuncha Formation occurrenceswith average ECRa values varying from about 1.9 to2.8 Bq kg�1. Some formations appear to be characterizedby large ECRa values. For Syabru-Bensi augen gneiss, meanECRa value of 19.3 ± 7.1 Bq kg�1 is similar to the mean va-lue of 18.9 ± 1.7 Bq kg�1 obtained for Ulleri gneiss(Mid-Western Nepal).

Both sillimanite-augen gneiss and leucogranite havingtourmaline of the Fm. III (Fig. 2) appear also as distinctlarge ECRa anomalies in a context of low ECRa value. Fromthese layers, typical ECRa values are larger than 5 Bq kg�1

and sometimes reach mean value higher than 20 Bq kg�1.Thus, lithology seems to provide reliable signatures in ECRa

values. However, while layers in general show homoge-neous ECRa values, even sometimes spectacular (for exam-ple the gneiss + quartzite layer with a dispersion of33 ± 7%), others such as the sillimanite-augen gneiss forma-tion indicate a larger dispersion of 98 ± 31% (N = 15). Thislast point may be related to difference in melting composi-tion at relatively small spatial scales (e.g., Guillot and LeFort, 1995).

When looking at the entire LHS units and the wholeFms. I and II from GHS units, we are able to see large dif-ferences (Fig. 2 in inset and Table 2). Indeed, LHS units(except the augen gneiss) are characterized by ECRa valuesbetween about 1 and 9 Bq kg�1, whereas ECRa values fromGHS units are significantly lower, typically around0.7 Bq kg�1. Consequently, this one-order-of-magnitudedifference between LHS and GHS appears significant at thislocation.

To better appreciate spatial changes and variations ofECRa across the MCT, two profiles associated with simpli-fied cross-sections are shown, one oriented WSW-ENE(Fig. 3), and the other one oriented SSW-NNE (Fig. 4),with a color code for ECRa results corresponding to therock formation to which they belong (see Fig. 2). In

Table 1Summary of the ECRa data set listed according to each geological formation encountered at the different studied sites. For each geologicalformation, the number of samples, the arithmetic mean value and the min–max range of ECRa are indicated.

Geological formation Figure reference Number ofsamples

ECRa (Bq kg�1):mean ± 1r (min–max)

Quaternary Terai Sediments

Undifferenciated Quaternary sediments Fig. 5 1 8.97 ± 0.54

Sub-Himalaya Siwalik Group

Lower Siwalik Fm. calcareous grey sandstones Fig. 5 1 1.47 ± 0.24Lower–Middle Siwalik Fm. sandstone alternating with grey mudstone Fig. 5 1 4.47 ± 0.43Upper-Middle Siwalik Fm. massive sandstones Fig. 5 2 1.34 ± 0.12 (1.22–1.46)Total Sub-Himalaya Siwalik Group 4 2.16 ± 0.77 (1.22–4.47)

Mahabharat (Nourpul Fm.)

Nourpul Fm. Chiuribas Lower Unit: metasandstone + greenphyllite + shale

Fig. 5 2 1.83 ± 0.45 (1.38–2.28)

Nourpul Fm. Chiuribas Middle Unit: green phyllite + purple shales Fig. 5 2 1.31 ± 0.63 (0.68–1.93)Nourpul Fm. Chiuribas Upper Unit: purple metasandstone + green andpurple shales

Fig. 5 2 1.79 ± 0.80 (0.98–2.59)

Nourpul Fm. Sangure Quartzite Unit: white quartzite + purple and pinkquartzite in bands

Fig. 5 3 0.92 ± 0.46 (0.28–1.80)

Total Mahabharat (Nourpul Fm.) 9 1.40 ± 0.27 (0.28–2.59)

Lesser Himalayan Sequence (LHS)

Kuncha Fm. chlorite-phyllite + metasandstone Fig. 2 3 2.48 ± 0.74 (1.71–3.95)Biotite-schist Fig. 2 7 1.86 ± 0.25 (1.28–2.93)Calc-schist + garnet-schist Fig. 2 38 2.77 ± 0.33 (0.62–10.52)Quartzite + marble + graphitic-schist Fig. 2 50 2.04 ± 0.26 (0.04–7.46)Quartzite + marble + calc-schist Fig. 2 48 2.30 ± 0.46 (0.12–19.23)Kuncha Fm. chloritic phyllites + schists with augen gneisses Fig. 5 1 3.76 ± 0.35Kuncha Fm. phyllite + quartzite (Fagfog Quartzite) Fig. 5 3 1.38 ± 0.62 (0.16–2.15)Kuncha Fm. phyllite (Benighat Slate), quartzite, thin bedded carbonaterocks (Jhiku Carbonates)

Fig. 5 2 2.2 ± 1.0 (1.18–3.18)

Predominantly black schists, phyllite, quartzite, carbonate rocks Fig. 5 3 8.4 ± 1.8 (5.89–12.03)Kuncha Fm. phyllite with garnet (Kodari) Fig. 6 3 5.47 ± 0.77 (4.01–6.62)Kuncha Fm. phyllite with garnet (Syaule) Fig. 6 n.m. n.m.Kuncha Fm. phyllite with quartzite (Fagfog Quartzite) (Kodari) Fig. 6 1 3.16 ± 0.28Kuncha Fm. phyllite with quartzite (Fagfog Quartzite) (Syaule) Fig. 6 1 2.27 ± 0.19Kuncha Fm. phyllite (Benighat Slate), quartzite, thin bedded carbonaterocks (Jhiku Carbonates) (Kodari)

Fig. 6 6 1.92 ± 0.77 (0.10–4.66)

Kuncha Fm. phyllite (Benighat Slate), quartzite, thin bedded carbonaterocks (Jhiku Carbonates) (Syaule)

Fig. 6 4 5.4 ± 4.4 (0.15–18.43)

Dolomites with Dhading Dolomites Fig. 6 2 1.48 ± 0.37 (1.11–1.85)Predominantly black schists, phyllite, quartzite, carbonate rocks Fig. 6 6 2.63 ± 0.80 (0.37–5.58)Kuncha Fm. phyllites, conglomerates, massive quartzite Fig. 7 1 1.67 ± 0.35Slates with calcareous slates and carbonates Fig. 7 25 26.8 ± 4.8 (0.33–72.52)Kuncha Fm. predominantly phyllite Fig. 8 3 2.50 ± 0.58 (1.59–3.59)Kuncha Fm. predominantly sandstone Fig. 8 4 0.65 ± 0.10 (0.37–0.82)Fagfog Quartzite + amphibolite Fig. 8 2 0.66 ± 0.48 (0.18–1.15)Dhading Dolomite + Benighat Slates Fig. 8 2 0.64 ± 0.07 (0.57–0.71)Kuncha Fm. phyllite Fig. 8 1 1.06 ± 0.13Kuncha Fm. phyllite (Benighat Slate), quartzite, thin bedded carbonaterocks (Jhiku Carbonates)

Fig. 8 4 8.2 ± 3.4 (1.58–16.87)

Predominantly sericite-schist + quartzite Fig. 8 2 2.33 ± 1.28 (1.05–3.61)Green and black schist + quartzite Fig. 9 7 0.64 ± 0.20 (0.12–1.53)Micaceous marble + marble + quartzite Fig. 9 12 1.20 ± 0.27 (0.30–3.07)Mica-schist + quartzite + amphibolite Fig. 9 28 2.53 ± 0.87 (0.30–20.28)Mica-schist + garnet-schist Fig. 2 19 3.31 ± 0.52 (0.18–9.07)Garnet-biotite-muscovite-schists + quartzite Fig. 5 4 1.33 ± 0.13 (1.02–1.66)Garnet-schist with chlorite + quartzite Fig. 8 4 0.36 ± 0.12 (0.08–0.55)Black schist Fig. 8 2 0.87 ± 0.44 (0.43–1.30)Garnet-biotite schist with carbonate rocks at upper level (Kodari) Fig. 6 2 1.01 ± 0.42 (0.59–1.43)Garnet-biotite schist with carbonate rocks at upper level (Syaule) Fig. 6 8 4.7 ± 1.5 (1.12–13.98)

F. Girault et al. / Geochimica et Cosmochimica Acta 98 (2012) 203–227 209

Geological formation Figure reference Number ofsamples

ECRa (Bq kg�1):mean ± 1r (min–max)

Total Kuncha Fm. and associated formations 308 4.44 ± 0.56 (0.04–72.52)

Augen gneiss with schist and quartzite (Kodari) Fig. 6 1 5.81 ± 0.13Augen gneiss with schist and quartzite (Syaule) Fig. 6 2 9.3 ± 1.0 (8.29–10.30)Augen gneiss Fig. 2 34 19.3 ± 7.1 (0.23–251.60)Augen gneiss Fig. 9 3 5.8 ± 4.4 (0.83–14.66)Coarse-grained quartz-feldspar-biotite-muscovite-tourmaline Ulleri augengneiss

Fig. 8 17 18.9 ± 1.7 (5.40–30.05)

Total augen gneiss 57 17.9 ± 4.3 (0.23–251.60)

Greater Himalayan Sequence (GHS)

Garnet-schist + quartzite (Fm. I) Fig. 2 27 1.08 ± 0.20 (0.03–4.69)Garnet-gneiss + quartzite (Fm. I) Fig. 2 22 0.39 ± 0.09 (0.03–1.56)Gneiss + garnet-rich mica-schist with small granite intrusions Fig. 5 7 0.32 ± 0.05 (0.20–0.50)Garnet-gneiss with granite intrusions Fig. 5 3 2.24 ± 0.75 (1.28–3.73)Garnet-biotite-gneiss with large granite intrusions Fig. 5 2 0.38 ± 0.11 (0.27–0.50)Predominantly pelitic schist (Kodari) Fig. 6 2 0.32 ± 0.07 (0.25–0.38)Predominantly pelitic schist (Syaule) Fig. 6 4 0.80 ± 0.32 (0.19–1.64)Predominantly psammitic schist and quartzite (Kodari) Fig. 6 7 0.60 ± 0.20 (0.08–1.61)Predominantly psammitic schist and quartzite (Syaule) Fig. 6 2 0.42 ± 0.17 (0.25–0.58)Garnet-kyanite-gneiss, biotite-gneiss (Fm. I) Fig. 8 2 0.23 ± 0.08 (0.15–0.30)Mica-schist + garnet-gneiss with kyanite Fig. 8 10 0.20 ± 0.07 (0.06–0.79)Garnet-gneiss with kyanite and sillimanite Fig. 8 12 0.42 ± 0.17 (0.12–2.24)Garnet-kyanite gneiss + mica-schist Fig. 9 13 0.41 ± 0.07 (0.06–0.88)Green quartzite Fig. 7 1 2.85 ± 0.51Garnet-muscovite-biotite-quartz schists + quartzite Fig. 7 12 2.3 ± 1.2 (0.04–14.67)Schists + quartzites + gneisses Fig. 7 4 3.5 ± 1.2 (0.20–5.75)Total Fm. I 130 0.87 ± 0.14 (0.03–14.67)

Kyanite-garnet-gneiss + kyanite-mica-schist + quartzite (Fm. II) Fig. 2 49 0.65 ± 0.22 (0.04–9.94)Garnet-gneiss (Fm. II) Fig. 2 2 0.81 ± 0.11 (0.66–0.96)Gneiss + quartzite (Fm. II) Fig. 2 13 0.61 ± 0.06 (0.32–0.95)Calcareous and banded gneiss, paragneiss, marble (Fm. II) Fig. 8 5 0.38 ± 0.09 (0.19–0.68)Migmatite (Fm. II) Fig. 2 4 0.28 ± 0.18 (0.08–0.83)Migmatites and layered gneiss Fig. 6 1 0.06 ± 0.03Migmatites + layered gneiss Fig. 9 5 0.23 ± 0.02 (0.18–0.29)Total Fm. II 79 0.58 ± 0.13 (0.04–9.94)

Sillimanite-augen gneiss (Fm. III) Fig. 2 15 3.69 ± 0.94 (0.70–13.77)Leucogranite having tourmaline (Fm. III) Fig. 2 13 22.3 ± 8.4 (0.82–118.61)White granitic gneiss (Fm. III) Fig. 6 1 8.53 ± 0.19White granitic gneiss (Fm. III) Fig. 8 1 43.5 ± 1.2White granitic gneiss + tourmaline (Fm. III) Fig. 9 1 5.91 ± 0.59Total Fm. III 31 13.0 ± 4.0 (0.70–118.61)

Tethyan Sedimentary Sequence (TSS)

Metamorphic carbonates rocks Fig. 9 4 0.16 ± 0.04 (0.12–0.27)Annapurna Yellow Fm. limestone, quartzite, schist Fig. 8 1 0.12 ± 0.05Sombre Fm. black slate and sandstone with dolomite Fig. 8 1 0.59 ± 0.08Tilicho Pass Fm. black slate, phyllite, sandstone Fig. 8 1 3.79 ± 0.34Tilicho Lake Fm. limestone, calcschist, black shale Fig. 8 n.m. n.m.Jomsom Fm. micritic and oolitic limestone, sandstone Fig. 8 2 0.26 ± 0.03 (0.23–0.28)Lumachelle Fm. limestone, shale, sandstone, marl Fig. 8 1 0.29 ± 0.06Spiti Fm. grey shale, limestone, sandstone Fig. 8 1 3.14 ± 0.28Chukh Fm. thick bedded grey sandstone, grey shale Fig. 8 2 1.76 ± 1.13 (0.63–2.90)Muding Fm. thin bedded grey sandstone, grey shale Fig. 8 1 0.39 ± 0.07Total Tethyan Sedimentary Sequence (TSS) 14 0.93 ± 0.35 (0.12–3.79)

Note: n.m., not measured.

210 F. Girault et al. / Geochimica et Cosmochimica Acta 98 (2012) 203–227

addition, this allows observing directly the reliability ofECRa measurements within each considered rock formation(Figs. 3 and 4). Location of cross-sections is indicated inFig. 2. Positions of rock samples located at a closest dis-tance smaller than 2 km from the cross-section profile areprojected along it. This choice provides numerous datapoints in the cross-section to facilitate observation, butsometimes induces some mixing of rock samples from dif-

ferent formations in the most sampled area. ECRa datapoints with relative uncertainties larger than 100% were re-moved to enhance clarity. In Fig. 3, we can easily notice theone-order-of-magnitude ECRa difference between LHSunits and GHS units. The largest difference can even reach2 orders of magnitude from augen gneiss to first GHSoccurrence. From Fm. I to Fm. III, moreover, a slightbut significant increase of about 0.1 to 0.2 Bq kg�1 km�1

Table 2Results of ECRa measurements performed at the six investigated sites throughout the Nepalese Himalayas.

Site Figurereference

Whole sample set Lesser HimalayanSequence (LHS)

Greater HimalayanSequence (GHS)Formations I and II

LHS/GHSECRa ratio

Numberof samples

ECRa

(Bq kg�1)Numberof samples

ECRa

(Bq kg�1)Numberof samples

ECRa

(Bq kg�1)

Mean ± 1r(Min–Max)

Mean ± 1r(Min–Max)

Mean ± 1r(Min–Max)

1. Upper Trisuli andLangtang Valleys

Fig. 2 350 4.30 ± 0.83(0.03–251.60)

199 5.3 ± 1.3(0.04–251.60)

117 0.69 ± 0.11(0.03–9.94)

7.8 ± 2.2

2. Dharan–DhankutaTransect

Fig. 5 39 2.12 ± 0.41(0.16–12.03)

13 3.30 ± 0.93(0.16–12.03)

12 0.81 ± 0.30(0.20–3.73)

4.1 ± 1.9

3. Syaule and Kodari Fig. 6 53 2.93 ± 0.50(0.06–18.43)

36 3.83 ± 0.66(0.10–18.43)

16 0.56 ± 0.12(0.06–1.64)

6.9 ± 1.9

4. Lower Trisuli Valley Fig. 7 43 16.5 ± 3.3(0.04–72.52)

26 25.6 ± 4.6(0.33–72.52)

17 2.61 ± 0.90(0.04–14.67)

9.8 ± 3.8

5. Kali Gandaki andMarsyangdi Valleys

Fig. 8 64 1.87 ± 0.73(0.06–43.52)

24 2.27 ± 0.77(0.08–16.87)

29 0.32 ± 0.08(0.06–2.24)

7.1 ± 2.9

6. Lower Dolpo Fig. 9 73 1.65 ± 0.40(0.06–20.28)

50 2.14 ± 0.56(0.12–20.28)

18 0.36 ± 0.05(0.06–0.88)

5.9 ± 1.8

All sites 622 4.33 ± 0.54(0.03–251.60)

348 5.94 ± 0.88(0.04–251.60)

209 0.76 ± 0.10(0.03–14.67)

7.8 ± 1.6

All sites except 4 579 322 4.35 ± 0.81(0.04–251.60)

192 0.60 ± 0.07(0.03–9.94)

7.3 ± 1.6

All sites except 1 272 149 6.8 ± 1.1(0.08–72.52)

92 0.86 ± 0.19(0.04–14.67)

7.9 ± 2.2

All sites except 1 and 4 229 123 2.78 ± 0.35(0.08–20.28)

75 0.46 ± 0.06(0.06–3.73)

6.1 ± 1.2

F. Girault et al. / Geochimica et Cosmochimica Acta 98 (2012) 203–227 211

is also observed. In Fig. 4, rock samples from the LHS unitsare more numerous, thus leading to a better observation ofthe difference on average between LHS (around2.5 Bq kg�1) and GHS (around 0.5 Bq kg�1) ECRa values.This profile being shorter in distance, more details arenoticeable. Values appear also relatively heterogeneous inthe Syabru-Bensi geothermal system, where campaigns withlarge numbers of samples have been undertaken (Girault,2011). However, on average, mean ECRa values from rocksnear Syabru-Bensi are characterized by a mean ECRa of3.5 ± 0.3 Bq kg�1 (Girault, 2011). In addition, along thisprofile, occurrence of augen gneiss south to the MCT is rel-atively visible, due to ECRa values in general larger than10 Bq kg�1. Typically, augen gneiss, thus, shows an in-crease just before the first GHS occurrence, characterizedby a decrease in ECRa value.

Following these encouraging results, several other sitescutting across the MCT have been studied for their ECRa

of rocks. Their respective ECRa results are always depictedwith the same colored scale, superimposed on the geologicalmap. The distributions of ECRa results from the LHS units(in red) and from Fm. I and II of the GHS units (in blue)are also displayed in inset. Summaries of ECRa results areavailable in Tables 1 and 2.

4.2. Results at the five other sites

4.2.1. The Dharan–Dhankuta Transect

In Eastern Nepal, one transect was performed along theroad linking Dharan to Dhankuta (Figs. 1 and 5). Geology

of this site was mainly based on the work carried out byShrestha et al. (1984a) and Duvadi et al. (2001) and onour own observations. This transect cuts across almost allNepal Himalayan formations (QTS, SSG, LHS and GHS)except TSS. Along this transect, 39 rocks were sampledand analyzed for their ECRa content. Samplings belong totwo surveys, in 2007 and 2010. ECRa values vary from0.16 ± 0.05 to 12.0 ± 0.3 Bq kg�1 and their distributionshows a log-normal behavior with a mean of2.12 ± 0.41 Bq kg�1. Here again, ECRa values are associ-ated with the lithology, with, for example, significant differ-ence between LHS black schist layer and SSG massivesandstones (Table 1). In addition, LHS units are also char-acterized in general by larger ECRa values than GHS units,with averages of 3.3 ± 0.9 and 0.8 ± 0.3 Bq kg�1, respec-tively (inset in Fig. 5). It is possible to observe an increasein the ECRa values of LHS rocks from the MT to theRT. ECRa values from GHS rocks are more scattered. In-deed, while first occurrence of GHS units is characterizedby lower ECRa values around 0.3 Bq kg�1, larger valueswith maximum of 3.7 Bq kg�1 are obtained in the garnet-gneiss and the garnet-biotite-gneiss (Table 1). An explana-tion may be the lenticular granitic intrusions mentionednear Hile village (Fig. 5). Interesting evidence of significantdifferences in ECRa values from the various Siwaliks layers(SSG) can also be pointed out (Table 1).

4.2.2. The Syaule and Kodari transects

In the eastern part of Central Nepal, two transects wereundertaken crossing Syaule village (West) and along the

ECRa > 27 Bq kg-1

9 < ECRa < 273 < ECRa < 91 < ECRa< 30.33 < ECRa < 10.11 < ECRa < 0.33ECRa < 0.11

Upper Trisuli and Langtang Valleys

Fault

RT

MCT

Faul t?

Faul t?

Chlorite-Phyllite + Metasandstone (Kuncha Fm.)Biotite-Schist

Calc-schist + Garnet-SchistQuartzite + Marble + Graphitic-SchistQuartzite + Marble + Calc-schistMica-schist + Garnet-Schist

LHS Augen Gneiss

Garnet-Schist + Quartzite

Garnet-Gneiss + QuartziteFm I

Hot springThrust: Ramgarh Thrust (RT),Main Central Thrust (MCT)

Kyanite-garnet-Gneiss +Kyanite-mica-Schist + QuartziteGarnet-Gneiss

Migmatite

Gneiss + Quartzite

Sillimanite-Augen Gneiss

Leucogranite having tourmaline

Fm II

Fm II

I

LHS

B’

B

A

A’

Fig. 2. Simplified geological map with ECRa results from rock samples of the Upper Trisuli and Langtang Valleys, Central Nepal. LHScorrespond to Lesser Himalayan Sequence and Fms. I–III to the different formations of the Greater Himalayan Sequence (GHS). ECRa resultsare represented on the map with a colored scale. Their distributions within LHS units and Fms. I and II of GHS units are illustrated byhistograms in inset. AA’ and BB’ correspond to geological and ECRa cross-sections presented further in Figs. 3 and 4, respectively. Geology isdrawn after Stocklin and Bhattarai (1981), Shrestha et al. (1984b), Macfarlane et al. (1992), Inger and Harris (1993), Parrish and Hodges(1996), Takagi et al. (2003), Pearson and DeCelles (2005), Kohn (2008), Pant (2010), Paudel (private communication), and our own work.

212 F. Girault et al. / Geochimica et Cosmochimica Acta 98 (2012) 203–227

Bharabise-Kodari road (East) (Figs. 1 and 6). Using thework from Stocklin and Bhattarai (1981), Shrestha et al.(1984b), Duvadi et al. (2005), Rai and Upreti (2006) andfrom our fieldworks in 2000 for Syaule and in 1998 and2009 for Kodari, a simplified geological map was obtainedand is inserted in Fig. 6. Geology is more complex in Sya-ule, with the possibility of another fault between the RTand the MCT.

Results of ECRa from 53 rock samples along these twotransects have been included in Fig. 6. ECRa valuesrange from 0.06 ± 0.03 to 18.4 ± 0.5 Bq kg�1 and theiralmost log-normal distribution has an average of2.93 ± 0.50 Bq kg�1. As already observed in the previoustransects, ECRa values are again related to lithology. Onemay highlight spatially homogeneous ECRa values of0.42 ± 0.17 Bq kg�1 from the psammitic schist layer inSyaule or the phyllite layer (Kuncha Fm.) from Kodaritransect with mean of 5.47 ± 0.77 Bq kg�1 (Table 1). In

addition, ECRa values from LHS are larger than the valuesfrom GHS in both transects (inset in Fig. 6). Indeed, theaverage value of ECRa from GHS units is 0.6 ± 0.1 Bq kg�1

and 3.8 ± 0.7 Bq kg�1 for LHS units (Table 2). In bothtransects also, augen gneiss layer is encountered betweenthe RT and the MCT. As noticed previously in the refer-ence Upper Trisuli and Langtang Valleys area, this layeris characterized by large ECRa values reaching here5.8 ± 0.1 and 9.3 ± 1.0 Bq kg�1 in Kodari and in Syaule,respectively. One white granitic gneiss sample with ECRa

value of 8.5 ± 0.2 Bq kg�1 and located north to the MCTin the Syaule transect (yellow dot, Fig. 6) was interpretedas not being in situ and thus could be attributed to GHSFm. III (see Table 1).

4.2.3. The Lower Trisuli Valley transect

One site in the Lower Trisuli Valley was investigated inthe north-western part of the Kathmandu Valley, Central

?

MCTRT

MCT ZoneWSW ENE

A A’

LHSGHS

Fig. 3. ECRa and geological cross-section oriented WSW-ENE of the Upper Trisuli and Langtang Valleys (AA0 in Fig. 2) versus the distancefrom the Main Central Thrust (MCT). The colored code of the values of ECRa corresponds to the color of geological formations used in thecross-section and defined in Fig. 2. LHS and GHS correspond to Lesser Himalayan Sequence and Greater Himalayan Sequence.

?

MCTRT

SSW NNE

B B’

MCT Zone

LHS GHS

Fig. 4. ECRa and geological cross-section oriented SSW-NNE of the Upper Trisuli and Langtang Valleys (BB0 in Fig. 2) versus the distancefrom the Main Central Thrust (MCT). The colored code of the values of ECRa corresponds to the color of geological formations used in thecross-section and defined in Fig. 2. LHS and GHS correspond to Lesser Himalayan Sequence and Greater Himalayan Sequence.

F. Girault et al. / Geochimica et Cosmochimica Acta 98 (2012) 203–227 213

Undifferenciated Quaternary Sediments Kuncha Fm. chloritic Phyllites + Schists withaugen gneisses

Main Frontal Thrust (MFT) Kuncha Fm. Phyllite + Quartzite (FagfogQuartzite)

Lower Siwalik Fm. Calcareous grey SandstonesKuncha Fm. Phyllite (Benighat Slate),Quartzite, thin bedded Carbonate Rocks (JhikuCarbonates)

Lower-Middle Siwalik Fm. Sandstone alternating withgrey Mudstone

Predominantly Black Schists, Phyllite, Quartzite, Carbonate Rocks

Upper-Middle Siwalik Fm. massive Sandstones Ramgarh Thrust (RT)

Main Boundary Thrust (MBT) Garnet-biotite-muscovite-Schists+ Quartzite

Nourpul Fm. Chiuribas Lower Unit: Metasandstone+ green Phyllite + Shale

Main Central Thrust (MCT)

Nourpul Fm. Chiuribas Middle Unit: green Phyllite+ purple Shales

Gneiss + Garnet-rich Mica-Schist with small granite intrusions

Nourpul Fm. Chiuribas Upper Unit: purple Metasandstone+ green and purple Shale

Garnet-Gneiss with granite intrusions

Nourpul Fm. Sangure Quartzite Unit: white Quartzite+ purple and pink Quartzite in bands

Garnet-Biotite-Gneisswith large granite intrusions

Munsiari Thrust (MT)

ECRa > 27 Bq kg-1

9 < ECRa < 273 < ECRa < 91 < ECRa< 30.33 < ECRa < 10.11 < ECRa < 0.33ECRa < 0.11

Dharan-Dhankuta Transect

MFT

MBT

MT

RT

MCT

C

C’

Fig. 5. Simplified geological map with ECRa results from rock samples of the Dharan–Dhankuta Transect, Eastern Nepal. ECRa results arerepresented on the map with a colored scale. Their distributions within LHS units and Fms. I and II of GHS units are illustrated by ahistogram in inset. CC’ corresponds to ECRa cross-section presented further in Fig. 11. Geology is drawn after Shrestha et al. (1984a), Duvadiet al. (2001), and our own work.

214 F. Girault et al. / Geochimica et Cosmochimica Acta 98 (2012) 203–227

Nepal (Figs. 1 and 7). While this area was previously geo-logically mapped by Stocklin and Bhattarai (1981), Shres-tha et al. (1984b) and Pearson and DeCelles (2005), ourrock sampling work allowed us to refine the locations ofsome layer.

At this site a total of 43 rocks were sampled and theirECRa content was then measured. ECRa values varyfrom 0.04 ± 0.03 to 72.5 ± 1.1 Bq kg�1, with a mean of16.5 ± 3.3 Bq kg�1. Their distribution appears neverthelessscattered due to heterogeneous ECRa values in LHS units(Table 1) and, above all, to an extensive sampling in the slateslayer (Fig. 7). Indeed, such an outcrop, examined in detail forits dispersion in Girault and Perrier (2012b), is characterizedby extremely large ECRa values with a mean of45 ± 5 Bq kg�1. Thus, this induces a larger mean ECRa valuefor LHS units (Table 2). While GHS rocks show also heter-ogeneous ECRa values, varying from 0.04 ± 0.03 to14.7 ± 1.2 Bq kg�1, the difference in ECRa values betweenLHS and GHS remains nonetheless relatively similar to whatwe observed in other transects (inset in Fig. 7). This siteoffered definitely a more contrasted situation with a more dif-ficult interpretation based on ECRa measurements alone.

4.2.4. The Kali Gandaki and Marsyandi Valleys transects

In Mid-Western Nepal, the Kali Gandaki Valley (West)and the Marsyandi Valley (East) were studied (Figs. 1 and

8). Geology along the Kali Gandaki Valley is relatively wellknown, in particular due to fieldworks from Vannay andHodges (1996), Shrestha et al. (1987), Upreti and Yoshida(2005), Pearson and DeCelles (2005) and Searle (2010).Along the Marsyandi Valley, however, refinements andimprovements have been achieved during our study usingthe newly constructed road. The Kali Gandaki Valley ischaracterized by various LHS, GHS and TSS units. Alongthe Marsyandi Valley, we stopped sampling 7 km north tothe assumed MCT location, hence staying in GHS units.

Along both valleys, 31 and 33 rock samples from KaliGandaki and Marsyandi, respectively, were analyzed fortheir ECRa content. ECRa values range from 0.06 ± 0.03to 43.5 ± 1.2 Bq kg�1, with a mean of 1.87 ± 0.73 Bq kg�1.Their distributions were similar to Poisson distributionsdue to, first, the relatively weak ECRa values whatever thelayer and, second, the large number of samples taken fromGHS units along the Marsyandi Valley. The agreementwith lithology appears particularly prominent in LHS andGHS units, but also in TSS layers. Indeed, for example,phyllite from Kuncha Fm., calcareous banded gneiss andparagneiss from Fm. II, as well as limestone from Annap-urna Yellow Fm. or shales from Spiti Fm. are all character-ized by significantly different ECRa values with means of2.5 ± 0.6, 0.38 ± 0.09, 0.12 ± 0.05 and 3.1 ± 0.3 Bq kg�1,respectively (Table 1). Differences in ECRa between LHS

Kuncha Fm. Phyllite with Garnet Ramgarh Thrust (RT) Main Central Thrust (MCT)

Kuncha Fm. Phyllite with Quartzite (Fagfog Quartzite) Garnet-Biotite Schist with Carbonate Rocks at upper level Predominantly Pelitic Schist

Kuncha Fm. Phyllite (Benighat Slate), Quartzite,thin bedded Carbonate Rocks (Jhiku Carbonates)

Augen Gneiss with Schist and Quartzite Predominantly Psammitic Schist and Quartzite

Dolomites with Dhading Dolomites Migmatites and Layered Gneiss

Predominantly Black Schists, Phyllite, Quartzite,Carbonate Rocks

Hot spring

ECRa > 27 Bq kg-1

9 < ECRa < 273 < ECRa < 91 < ECRa< 30.33 < ECRa < 10.11 < ECRa < 0.33ECRa < 0.11

MCT

MCT

RT

RT

Syaule and Kodari

Fault?

D

D’

E

E’

Fig. 6. Simplified geological map with ECRa results from rock samples of the Syaule and Kodari areas, Central Nepal. ECRa results arerepresented on the map with a colored scale. Their distributions within LHS units and Fms. I and II of GHS units are illustrated by ahistogram in inset. DD0 and EE0 correspond to ECRa cross-sections presented further in Fig. 11. Geology is drawn after Stocklin andBhattarai (1981), Shrestha et al. (1984b), Duvadi et al. (2005), Rai and Upreti (2006), and our own work.

F. Girault et al. / Geochimica et Cosmochimica Acta 98 (2012) 203–227 215

units and GHS units (Fms. I and II) are also significant (in-sets in Fig. 8), with relatively low GHS ECRa content of0.32 ± 0.08 Bq kg�1 (Table 2). Along the Kali Gandakitransect, white granitic gneiss from Fm. III is particularlyeasy to recognize due to, as already observed in previoustransects, its large ECRa value higher here than 40 Bq kg�1

(Table 1).

4.2.5. The Lower Dolpo transects

Finally, in Western Nepal, fieldwork was carried out inLower Dolpo (Figs. 1 and 9). This area of particular diffi-cult access, was the subject of few geological studies, suchas Fuchs (1964), Arita et al. (1984), Sharma et al. (1984)and more recently Carosi et al. (2002, 2007). In agreementwith Fuchs (1964) and Arita et al. (1984), respectively, weplaced the MCT near Tarakot and near Kagni. Based onour three fieldworks in 2008, 2009 and 2010, and on theseprevious studies, a simplified geological map of our sitewas produced (Fig. 9). GHS units mapped in the area arethin, typically developed on less than 5 km. These unitsare thrusted over the RT sheet, which is mapped for the firsttime in the area.

At this site, a total of 73 rocks were sampled and ana-lyzed for their ECRa content. Values range from0.06 ± 0.02 to 20.3 ± 1.2 Bq kg�1, and their distribution isalmost log-normal with mean of 1.65 ± 0.40 Bq kg�1. ECRa

is again related to lithology, with relatively similar values of0.64 ± 0.20 and 1.2 ± 0.3 Bq kg�1 in the green and blackschist layer and in the micaceous marble + marble layer,respectively, although both layers have been sampled attwo different locations, separated by about 18 km (Fig. 9and Table 1). In addition, at this site, again, and along bothtransects, one across Kagni village and the other one cross-ing Tarakot perched village, LHS units, relatively markedby heterogeneous ECRa values, show larger values thanGHS units. Indeed, GHS units are characterized by ECRa

of 0.36 ± 0.05 Bq kg�1 on average, which is similar to ECRa

values from GHS in the Kali Gankaki and Marsyandi Val-leys site, but which remains small when compared with theother previous transects (Table 2). Along the Kagni tran-sect (West) one rock sample, a white granitic gneiss + tour-maline rock, located in the first GHS units occurrence(yellow dot, Fig. 9) is interpreted as not being in situ andis thus attributed to GHS Fm. III (see Table 1).

4.3. ECRa versus rock type and representativity

In Figs. 3 and 4, for several formations, values of ECRa

of rock samples remain relatively close to each other. Forsome formations that contain different rock types, such asschist and quartzite, ECRa values are more scattered.Generally, ECRa values are higher in schist samples than

Kuncha Fm. Phyllites, Conglomerates, Massive Quartzite

Main Central Thrust (MCT)Garnet-Muscovite-Biotite-QuartzSchists + Quartzite

Slates with Calcareous Slates and Carbonates Green Quartzite Schists + Quartzites + Gneisses

ECRa > 27 Bq kg-1

9 < ECRa < 273 < ECRa < 91 < ECRa< 30.33 < ECRa < 10.11 < ECRa < 0.33ECRa < 0.11Lower Trisuli Valley

MCT

Fig. 7. Simplified geological map with ECRa results from rock samples of the Lower Trisuli Valley, Central Nepal. ECRa results arerepresented on the map with a colored scale. Their distributions within LHS units and Fms. I and II of GHS units are illustrated by ahistogram in inset. Geology is drawn after Stocklin and Bhattarai (1981), Shrestha et al. (1984b), Pearson and DeCelles (2005), and our ownwork.

216 F. Girault et al. / Geochimica et Cosmochimica Acta 98 (2012) 203–227

in quartzite samples. Nevertheless, simple arithmetic meanof ECRa offers an interesting and sufficient parameter, inmost cases, to distinguish the various formations (Table 1).Moreover, as detailed in Table 1 also, large units encom-passing different formations, such as Kuncha Fm. andassociated formations or augen gneiss in LHS units andFms. I, II and III in GHS units, appear all to have charac-teristic values of ECRa. Indeed, while Kuncha Fm. andassociated formations are characterized by relatively largeECRa values with an average of 4.44 ± 0.56 Bq kg�1, suchvalues remain actually rather heterogeneous at smallspatial-scale and are mainly related to the type of rockanalyzed.

The relation between ECRa and lithology has alreadybeen discussed using our extended dataset of rock samplesof different types from various locations (Girault andPerrier, 2012b). In this paper, as observed in Figs. 2–9,we took sometimes several samples in the same formationto refine its characteristic ECRa value, hence raising theproblem of dispersion of the ECRa results. Representativityof ECRa values at both sample scale and scarp scale appearsto be satisfactory, with typical dispersion of the order of13 ± 3% at sample scale and of 17–61% at scarp scale forLHS rocks (Girault and Perrier, 2012b). To date, 17 rockssampled in this present study, belonging to the Ullerigranitic augen gneiss formation near Ulleri village(Mid-Western Nepal) are characterized by a dispersion of37 ± 7%. Further examples of sample representativity aredescribed in Girault and Perrier (2012b). In outcrops ofgneiss + quartzite or the mica-schist + garnet-schist layers

in the Upper Trisuli and Langtang Valleys site (Central Ne-pal), dispersion is of 33 ± 7% (N = 13) and 69 ± 16%(N = 19), respectively. In general, over the whole data set,the ECRa content of rocks appears representative of a givenlithology at all sites, with little anomalies at outcrop level oreven at the kilometric scale.

4.4. ECRa in LHS and GHS units

In all maps (Figs. 2 and 5–9), the distributions of ECRa

values of samples from LHS units (in red) and from Fms. Iand II of GHS units (in blue) are shown separately. At mostsites, the differences in the range of variation of ECRa valuesbetween LHS units and Fm. I and II of GHS units areclearly remarkable, except at the Lower Trisuli Valley site.Calculated from the cumulated distribution, the median ofthe distributions of ECRa values from LHS range from0.75 ± 0.04 Bq kg�1 in the Kali Gandaki Valley to3.16 ± 0.08 Bq kg�1 in the Syaule and Kodari site. By con-trast, median of the distributions of ECRa values from GHSvary from 0.21 ± 0.01 Bq kg�1 in the Marsyandi Valley to0.50 ± 0.01 Bq kg�1 in the Dharan–Dhankuta Transectsite. Moreover, mean ECRa values of LHS units are, at allsites, larger than mean ECRa values of GHS units (Table 2).This significant difference is thus observed consistently fromEastern to Western Nepalese Himalayas.

In Fig. 10, the distributions of our whole dataset ofECRa values obtained from LHS and GHS units (Fms. Iand II) are shown. To first order, they appear log-normal;overall distributions of ECRa values span 5 and 4 orders

Kali Gandaki ValleyKuncha Fm. Predominantly Phyllite

Kuncha Fm. Predominantly Sandstone

Fagfog Quartzite + AmphiboliteDhading Dolomite + Benighat SlatesRamgarh Thrust (RT)Black SchistMain Central Thrust (MCT)Fm. I Garnet-Kyanite-Gneiss, Biotite-GneissFm. II Calcareous and Banded Gneiss, Paragneiss, MarbleFm. III White Granitic GneissSouth Tibetan Detachment System (STDS)Annapurna Yellow Fm. Limestone, Quartzite, SchistSombre Fm. Black Slate and Sandstone with DolomiteTilicho Pass Fm. Black Slate, Phyllite, SandstoneTilicho Lake Fm. Limestone, Calc-schist, Black ShaleJomsom Fm. Micritic and Oolitic Limestone, SandstoneLumachelle Fm. Limestone, Shale, Sandstone, MarlSpiti Fm. Grey Shale, Limestone, SandstoneChukh Fm. Thick bedded Grey Sandstone, Grey ShaleMuding Fm. Thin bedded Grey Sandstone, Grey ShaleFault

Hot spring

Ulleri Augen GneissCoarse-grained quartz-feldspar-biotite-muscovite-tourmaline Ulleri Augen Gneiss

Marsyandi ValleyKuncha Fm. Phyllite Garnet-Schist with chlorite + QuartziteKuncha Fm. Phyllite (Benighat Slate), Quartzite, thin bedded Carbonate Rocks (Jhiku Carbonates)

Main Central Thrust (MCT)

Predominantly Sericite-Schist + Quartzite

Mica-Schist + Garnet-Gneiss with kyanite

Ramgarh Thrust (RT) Garnet-Gneiss with kyanite and sillimanite

ECRa > 27 Bq kg-1

9 < ECRa < 273 < ECRa < 91 < ECRa< 30.33 < ECRa < 10.11 < ECRa < 0.33ECRa < 0.11

Kali GandakiValley

Marsyandi Valley

MCTRT

STDS

MCT

RT

G’

G

F

F’

Ulleri Augen Gneiss

Fig. 8. Simplified geological map with ECRa results from rock samples of the Kali Gandaki and Marsyandi Valleys, Mid-Western Nepal.Locations of the Ulleri augen gneiss samples mentioned previously are also indicated. ECRa results are represented on the map with a coloredscale. Their distributions within LHS units and Fms. I and II of GHS units are illustrated by histograms in inset. FF’ and GG’ correspond toECRa cross-sections presented further in Fig. 11. Geology is drawn after Vannay and Hodges (1996), Shrestha et al. (1987), Upreti andYoshida (2005), Pearson and DeCelles (2005), Searle (2010), and our own work.

F. Girault et al. / Geochimica et Cosmochimica Acta 98 (2012) 203–227 217

of magnitudes and median values are 1.94 ± 0.01 Bq kg�1

and 0.34 ± 0.01 Bq kg�1 for LHS and GHS, respectively.Minimum and maximum range of ECRa values, which is de-fined by the limit of the 90% probability below and above,is of 0.26–22.2 Bq kg�1 for LHS and 0.08–3.31 Bq kg�1 forGHS. Using cumulative distributions of our whole ECRa

dataset, a characteristic ECRa value of 0.8 Bq kg�1 can bedefined quantitatively, in a statistically meaningful manner,to separate ECRa values of LHS units from ECRa valuesfrom Fms. I and II of GHS units. 77.3% of samples belong-ing to LHS and to Fms. I and II of GHS are located aboveand below this threshold, respectively.

For our six investigated sites, eight south-north cross-sections have been defined. Their orientation and positionare indicated in their respective figure. As performed inFigs. 3 and 4 for the two cross-sections from the referencesite, all seven ECRa data sets from additional sites areshown in Fig. 11 versus distance from the MCT. SSW-NNE profile from this first site is also included in Fig. 11.The Lower Trisuli Valley site cross-section, because of itsspecific context, is not displayed. In the reference cross-sec-tion, samples located at 2 km maximum from the cross-sec-tion are taken into account. In Dolpo, samples as far as1 km from the profiles are plotted in the cross-section.

For the other sites, all samples are incorporated in thecross-sections.

In general, as observed in the previous figures, ECRa val-ues decreased just after the MCT location. This decrease issometimes abrupt, especially in cross-sections from LowerDolpo West (HH0 across Kagni), Upper Trisuli Valley(BB0) and Dharan–Dhankuta (CC0). In other sites, the de-crease is less pronounced, but it remains easily detectable,for example in cross-sections from Lower Dolpo East (II0

across Tarakot), Marsyandi (FF0) and Syaule (DD0). Thethreshold ECRa value of 0.8 Bq kg�1 that we establishedin the previous paragraph to separate LHS from GHS isnot exceeded by ECRa values from GHS units in a majorityof cross-sections, except for the Kali Gandaki (GG0) andKodari (EE0) transects. Indeed, in the GG0 cross-section,ECRa values of LHS units appear particularly low and, inthe EE0 cross-section, large heterogeneities in ECRa valuesfrom GHS units prevent defining a clear separation betweenLHS and GHS units based on ECRa study only. Finally, forall transects, the spatial area where large variations of ECRa

are observed might be a good criterion to estimate thethickness of the MCT zone.

Whatever the site, GHS units are always characterizedby smaller ECRa values than LHS units. For each site we

Fig. 9. Simplified geological map with ECRa results from rock samples of the Lower Dolpo region, Western Nepal. ECRa results arerepresented on the map with a colored scale. Their distributions within LHS units and Fms. I and II of GHS units are illustrated by ahistogram in inset. HH’ and II’ correspond to ECRa cross-sections presented further in Fig. 11. Geology is drawn after Fuchs (1964), Aritaet al. (1984), Sharma et al. (1984), Carosi et al. (2002, 2007), and our own work.

218 F. Girault et al. / Geochimica et Cosmochimica Acta 98 (2012) 203–227

derived the LHS/GHS ECRa ratio. This ratio is close toabout 8, hence indicating almost one-order-of-magnitudedifference. Indeed, ratios range from 4.1 ± 1.9 to9.8 ± 3.8 for Dharan–Dhankuta Transect and LowerTrisuli Valley sites, respectively (Table 2). Overall, whentaking into account the whole ECRa dataset obtainedindependently in LHS and GHS units, the ratio amounts

Fig. 10. Distribution of the whole dataset of ECRa values obtained in theGreater Himalayan Sequence (GHS) units.

to 7.8 ± 1.6. Thus, the measurement of ECRa appearsparticularly significant and relevant to distinguish LHSunits from GHS units, and as a result, to constrain theMCT location. More ambiguous interpretations usingECRa appear, however, at the Lower Trisuli Valley site.This point could be due to the proximity of the Kath-mandu klippe. The differences are more convincing when

Lesser Himalayan Sequence (LHS) units and in Fms. I and II of the

Fig. 11. Eight south-north ECRa cross-sections from our studied sites versus the distance from the Main Central Thrust (MCT). The cross-section plotted in Fig. 4 is also shown (BB’ in the right top part). ECRa values are represented without error bars to enhance clarity.

F. Girault et al. / Geochimica et Cosmochimica Acta 98 (2012) 203–227 219

using the ECRa ratio (Table 2). Moreover, the referencesite shows a ratio of 7.8 ± 2.2. This ratio was extremelysimilar to the ratio obtained from the five other sites of7.9 ± 2.2, or from the four sites (without the Lower Tris-uli Valley site) of 6.1 ± 1.2. By contrast with the thresh-old ECRa value of 0.8 Bq kg�1 separating LHS unitsfrom GHS units, the LHS/GHS ECRa ratio, while it isnot a physically meaningful quantity, appears neverthelessas an interesting statistical index to quantify the differencein ECRa values of these two geological units.

5. DISCUSSION

5.1. Significance of ECRa for geological studies

Different values of ECRa have been obtained within thevarious formations studied at all sites (Table 1). For exam-ple, the Kuncha Fm. and associated formations, as alreadyemphasized, show a characteristic ECRa value of4.4 ± 0.6 Bq kg�1. The comparatively large ECRa contentof this formation, in some place comprising black phyllitesand slates, may be explained by the ability of micas and or-ganic matter to retain radium, an assumption that has beenfirst deduced from soil analysis (Ames et al., 1983b;Greeman and Rose, 1996). Similarly, significant ECRa

values of 9.0 ± 0.5 Bq kg�1 have been obtained for Quater-nary sediments, which contain generally more organic mat-ter (Fig. 5 and Table 1). This value is larger than all ECRa

values from other formations, except augen gneiss of theLHS units and Fm. III of the GHS units. Indeed, Siwalikformation and Nourpul formation give lower ECRa valuesof 2.2 ± 0.8 Bq kg�1 and 1.4 ± 0.3 Bq kg�1, respectively.

Considering lithology, some interesting differences inECRa values have already been pointed out, with for exam-ple lower ECRa value within quartzite than within granite(Girault and Perrier, 2012b). In this study, whatever thegeological unit (Table 1), we notice larger ECRa valuesfor schist, shale, slate, phyllite, augen gneiss, granitic gneiss,leucogranite, and lower ECRa values for quartzite, sand-stone, marble, limestone, carbonate, dolomite, amphibolite,gneiss, pelitic and psammitic schist, paragneiss, migmatite.Thus, to first order, lithology appears as the key factor con-trolling the ECRa value of a rock sample.

The Ulleri augen gneisses deserve here some attention.We noticed large ECRa values for this formation near Ullerivillage (Fig. 8). In addition, at the base of the MCT, alongseveral transects such as Kodari, Syaule, Upper Trisuli Val-ley or Lower Dolpo, this formation is also present,although sometimes more metamorphosed and sheared,but having again large ECRa values (Table 1). In total, thisformation yields mean ECRa value of 17.9 ± 4.3 Bq kg�1.Large ECRa may be due primarily to significant amountof radium decaying from uranium chain and not necessar-ily, in this case, from secondary crystallizations or deposi-tions. Indeed, uranium content was found to be relativelyimportant in these augen gneisses, typically with concentra-tion of 7–27 ppm (Colchen et al., 1986). This anomalous Uconcentration is probably due to the presence of graniticlenses that contain U-rich accessory minerals, such as zir-con and monazite (Kohn et al., 2010).

Similarly, augen gneiss and leucogranite of Fm. III fromthe GHS units are also characterized by high ECRa contentof 13.0 ± 4.0 Bq kg�1 on average (Table 1). These forma-tions are effectively known to contain uranium from 3.0

220 F. Girault et al. / Geochimica et Cosmochimica Acta 98 (2012) 203–227

to 30 ppm in bulk rocks and from 6.1 to 7.8 ppm in tourma-lines from leucogranites (France-Lanord and Le Fort, 1988;Inger and Harris, 1993; Guillot and Le Fort, 1995). Leucog-ranites were the subject of recent numerous analyses in ura-nium content and gave an average concentration of11.5 ± 1.9 ppm (N = 89). By contrast, GHS rocks fromFms. I and II and LHS rocks gave mean uranium concen-tration of 2–5 ppm and of 4.5–6 ppm, respectively. Forexample along the Dharan–Dhankuta Transect (Fig. 5),granitic lenses in GHS units participate to enhance ECRa

values. The two other GHS units (Fms. I and II), definedby gneiss, schist and quartzite of different isograds with typ-ically characteristic mineral such as kyanite and sillimanite,have both low ECRa content of 0.87 ± 0.14 and0.58 ± 0.13 Bq kg�1, respectively (Table 1).

The ECRa technique, due to the relatively small disper-sion of ECRa values within a formation and from one sam-pling site to another in the same formation, but also to itsrepeatability within one sample (Girault and Perrier,2012-b), is sufficiently robust so that, with few samplesonly, it may be possible to easily distinguish two differentlithologies. This is in fact the case at some sites such asthe Kali Gandaki Valley or the Dharan–Dhankuta Tran-sect sites, where no more than 20 rock samples from theMCT zone and the GHS units were analyzed.

5.2. The large-scale ECRa differences between LHS and GHS

units

At this stage, different possible general or theoreticalphysical explanations and/or mechanisms can be proposedto understand the significant ECRa difference between theLHS units and Fms. I and II from the GHS units. We willbase our interpretation, first, on the characteristics of theprotolith and, second, on the conditions of metamorphism.

5.2.1. ECRa and characteristics of protolith

Both deposition and deformation ages may play a role inthe ECRa content of a rock. Age of deposition was sug-gested to be involved in different ECRa signatures from sed-iments terraces, and showed an increase in ECRa value withdeposition age (Girault et al., 2011b). Although this obser-vation dealt with Quaternary sediments, such an age effectmay be possible within rocks. Indeed, GHS units, with low-er ECRa values, have a deposition age younger than LHSrocks, with larger ECRa values. Furthermore, the oldestLHS rocks are encountered between the RT and theMCT and show larger ECRa values. Thus, like for soils,ECRa values seem to increase with the age of depositionof these metamorphic units. Consequently, age of deposi-tion may control both the LHS/GHS ECRa difference andthe slight ECRa enhancement in the LHS units towardsthe MCT.

Moreover, a slight ECRa decrease is observed in the GHS(Fms. I and II) from Eastern Nepal to Western Nepal(Table 2), typically of about 10�3 Bq kg�1 km�1. This trendmay hypothetically be explained by the difference in defor-mation ages between Eastern and Western Himalayas, as-sumed to be related to the first contact between Indian andEurasian plates, at around 51 and 57 Ma, respectively

(Guillot et al., 2008). Using 11 samples from LHS unitsand 26 samples from Fms. I and II of GHS units of differentsites (Copeland et al., 1991; Arita and Ganzawa, 1997; Aritaet al., 1997; Catlos et al., 2001; Bollinger et al., 2004), we ob-tain medians of estimated age of hosted minerals and ECRa

distributions of 5.8 ± 0.5 Ma and 1.06 ± 0.13 Bq kg�1 forLHS and about 6.9 Ma and 0.27 ± 0.02 Bq kg�1 for GHS,respectively. Data of estimated age from GHS units remainnevertheless strongly heterogeneous, with a 90% probabilityrange of 1.7–18.8 Ma. Therefore, more dedicated studies areneeded to discuss the possible relation between ECRa andestimated ages of GHS units more in depth. However, to firstorder, GHS units consistently yield minerals characterizedby older ages than in the LHS units and, as a result, GHSrocks should cool earlier than LHS rocks. Thus, the mineralsin the GHS units likely may have accumulated more radia-tion damage than LHS rocks, facilitating radon escapeand, hence, increasing the ECRa value. These two last obser-vations predict the opposite trend to our observations, butcould be a possible effect on ECRa values in the GHS units.

LHS and GHS units may have had an original protolithwith different U concentration. As evidenced previously,shale, slate, phyllite, and augen gneiss with large ECRa val-ues typically are found in the LHS units, whereas gneiss,pelitic and psammitic schist, and migmatite with lower ECRa

values are found in Fms. I and II of the GHS units. The LHSlithologies may contain more U-bearing primary mineralsor more accessory minerals with larger U content (zircon,sphene) than GHS rocks, or both. Similarly, radium canalso be preferentially adsorbed by LHS rocks than GHSrocks. Indeed, muscovite and biotite are more abundant inLHS rocks and retain more efficiently radium at grainboundaries than quartz (Benes et al., 1984, 1986), which,by contrast, is more present in GHS rocks of Fms. I andII. In addition, graphitic rocks, occurring only in LHS units,tend to have fine grain size, thus enhancing ECRa. There-fore, U concentration, Ra concentration, Ra adsorptionand grain size of the LHS and GHS protoliths may alsobe parameters controlling their respective ECRa values.

To characterize LHS and GHS units, several studies car-ried out in the vicinity of the MCT in the Nepal Himalayasbased their conclusions on neodymium isotopic measure-ments from rock samples (e.g., Imayama and Arita,2008). According to these studies, the MCT may be a dis-crete ductile shear zone which may separate different proto-liths characterized by different isotopic signatures. Whiletentative at this stage, it is particularly interesting to com-pare ECRa results with eNd (0 Ma) results. When available,we collected published isotopic Nd data, mainly fromworks performed in our Upper Trisuli and Langtang Val-leys site (Inger and Harris, 1993; Parrish and Hodges,1996; Pearson, 2002), but also in the Kali Gandaki Valley(Robinson et al., 2001) and in the Marsyandi Valley (Mar-tin et al., 2005). Although the same sample could not bemeasured by both Nd method and our technique, by com-paring coordinates or map locations, we did our best tochoose the nearest sample(s) available in our database.When, for example, we sampled several rocks at the loca-tion of the Nd study, we used weighted averages for ourresults.

Fig. 12. Results of eNd (0 Ma) as a function of ECRa for different sites of the Nepalese Himalayas. Symbol colors correspond to the geologicalunits: LHS, GHS and TSS corresponding to Lesser Himalayan Sequence, Greater Himalayan Sequence and Tethyan Sedimentary Sequence,respectively. Data of eNd are taken from the Upper Trisuli and Langtang Valleys in Central Nepal (Inger and Harris, 1993; Parrish andHodges, 1996; Pearson, 2002) and from the Kali Gandaki Valley (Robinson et al., 2001) and the Marsyandi Valley (Martin et al., 2005) inMid-Western Nepal. Dashed line separates LHS samples from GHS samples. In the insets, the distribution of ECRa values (at the top) and eNd

values (on the right), separately for LHS units and for Fms. I and II of GHS units are shown.

F. Girault et al. / Geochimica et Cosmochimica Acta 98 (2012) 203–227 221

Published results from eNd (0 Ma) are shown versus theassociated ECRa results in Fig. 12, totalizing 41 Nd mea-surements. To first order, LHS rocks (in red) are gatheredin the right bottom part of the graph, mainly due to theirlarge eNd values (median of �24.4 ± 0.03) and their largeECRa values (median of 2.86 ± 0.05 Bq kg�1). The GHSrocks from Fms. I and II (in blue), by contrast, are definedby both smaller eNd (median of �16.0 ± 0.05) and smallerECRa values (median of 0.43 ± 0.03 Bq kg�1). Finally, theTSS rocks appear relatively heterogeneous, both in eNd

and ECRa values. In Fig. 12, while no overlapping for eNd

values is observed (inset on the right), few samples overlapwhen considering ECRa only (inset at the top). However, byusing the linear trend added on the graph as boundary,every sample belonging to LHS and GHS units is efficientlyseparated (Fig. 12). In addition, within GHS units, the twosamples from Fm. III (in purple) show similar eNd valueswith Fms. I and II. By contrast, based on ECRa results only,these two samples are clearly separated from the other GHSunits. Thus, difference between LHS and GHS units ob-tained with Nd isotopic measurements can also be observedusing ECRa measurements. This emphasizes the fact thatthe two units have definitely different characteristics whichmay be related to differences in their protolith origin.Therefore, since both techniques give different information,they may be used simultaneously as complementary meth-ods to characterize the MCT zone.

5.2.2. ECRa and metamorphism

Metamorphic conditions of both LHS and GHS unitsmay have controlled their different ECRa values. As we ar-gued before, some rocks of the LHS units have smallergrain size. In addition, the pressure and temperature towhich rocks have been subjected (P–T paths) can be animportant factor. Indeed, higher metamorphic grades tendto yield coarser grain size rocks. This decreases ECRa andthus metamorphic grade can be a parameter affecting theECRa differences between LHS and GHS. In the following,we will discuss the potential relation between metamorphicP–T paths and ECRa.

Laboratory experiments indicate that temperature largerthan 100–300 �C is able to decrease ECRa importantly (Gar-ver and Baskaran, 2004; Jobbagy et al., 2009) and that thischange appears irreversible. Indeed, temperatures of 200and 600 �C applied to uranium-rich minerals suggesteddecreasing emanation coefficient E due mainly to the nucle-ar tracks annealing in minerals (Garver and Baskaran,2004). At 600 �C, these authors noticed a reduction of Eby 35–85%. Such minerals have a relatively large tempera-ture sensitivity, about 8% �C�1, even at room temperatures(Perrier and Girault, 2012). A red mud bauxite sample,heated from 100 to 1000 �C, showed slight increase of Eof about 25% at 300 �C followed by a progressive decreasereaching 25% at 600 �C and finally 75% at 1000 �C(Jobbagy et al., 2009).

Fig. 13. Results of peak metamorphic temperature as a function of ECRa for different sites of the Nepalese Himalayas. Symbol colorscorrespond to the geological units: LHS and GHS corresponding to Lesser Himalayan Sequence and Greater Himalayan Sequence,respectively. Data of peak metamorphic temperature are taken from the Kodari site (Rolfo et al., 2001; Beyssac et al., 2004; Bollinger et al.,2004) and the Upper Trisuli and Langtang Valleys (Kohn, 2008) in Central Nepal, and from the Kali Gandaki Valley (Vannay and Hodges,1996) and the Marsyandi Valley (Catlos et al., 2001; Beyssac et al., 2004; Bollinger et al., 2004) in Mid-Western Nepal. In the insets, thedistribution of ECRa values (at the top) and peak metamorphic temperature (on the right), separately for LHS units and for Fms. I and II ofGHS units are shown.

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In Fig. 13, peak metamorphic temperatures (peak T) of29 LHS rock samples (in red) and 31 GHS rock samples (inblue) are shown versus our ECRa values. As performed forNd isotopic data, we associated each sample measured forits peak temperature with our spatially closest sample mea-sured for ECRa. When several ECRa values were availableat this location, we applied a weighted average to obtainthe final ECRa value. Data of peak temperature are takenfrom the Kodari site (Rolfo et al., 2001; Beyssac et al.,2004; Bollinger et al., 2004), the Upper Trisuli and Lang-tang Valleys (Kohn, 2008), the Kali Gandaki Valley (Van-nay and Hodges, 1996) and the Marsyandi Valley (Catloset al., 2001; Beyssac et al., 2004; Bollinger et al., 2004).LHS samples are gathered in the right bottom part of thegraph, with lower peak temperature values (median of560.0 ± 0.1 �C) and larger ECRa values (median of2.27 ± 0.10 Bq kg�1). By contrast, in a first approximation,Fms. I and II samples from GHS units are characterized bylarger peak temperature values (median of 665.0 ± 2.5 �C)and lower ECRa values (median of 0.22 ± 0.02 Bq kg�1).However, while we observe a significant overlapping ofpeak temperature between LHS and Fms. I and II ofGHS units (inset on the right), only five ECRa values fromGHS overlap the ECRa values of Fms. I and II (inset at thetop). Moreover, the rock sample from Fm. III can be distin-guished from Fms. I and II using its ECRa values, but it isnot possible with its peak temperature (Fig. 13). As a result,samples with larger peak temperature (Fms. I and II fromGHS) show smaller ECRa values, in agreement with heating

experiments (Garver and Baskaran, 2004; Jobbagy et al.,2009). However, while the GHS rocks belonging to theFm. III were involved in the melting zone during anatexisprocess characterized by temperatures typically between700 and 800 �C or more (Inger and Harris, 1993; Guillotand Le Fort, 1995), their ECRa values remain extremelylarge. Consequently, compared with lithological factor,temperature may only have a second order effect on ECRa.In addition, the ECRa increasing trend observed in the GHSunits in the WSW-ENE cross-section in the Upper Trisuliand Langtang Valleys (Fig. 3) cannot be accounted for bytemperature effect. Nonetheless, one may suggest tempera-ture dependence, with a temperature decrease from 540 to330 �C toward the MCT (Beyssac et al., 2004; Bollingeret al., 2004) to explain the general progressive ECRa in-crease in LHS units toward the MCT (Fig. 4). To conclude,difference between LHS and GHS units obtained with peakmetamorphic temperature can also be observed using ECRa

measurements.Pressure affects radon emanation (e.g., Holub and Brady,

1981; Tuccimei et al., 2010; Mollo et al., 2011), but thisphenomenon is not reversible and, to date, a rock samplehas not been shown to record its memory of deformation intoits ECRa value. Only few results of peak metamorphic pres-sure are available to clearly interpret the relation with ECRa.With peak pressure data from 7 LHS samples and 17 GHSsamples of Fms. I and II (Vannay and Hodges, 1996; Kohn,2008), similar conclusions obtained above with peak temper-ature can be drawn: medians of peak pressure and ECRa

F. Girault et al. / Geochimica et Cosmochimica Acta 98 (2012) 203–227 223

distributions give 750 ± 45 MPa and 1.49 ± 0.33 Bq kg�1

for LHS units and 970 ± 15 MPa and 0.30 ± 0.01 Bq kg�1

for Fms. I and II of GHS units, respectively. Effects on ECRa

of the metamorphic reactions with both associated pressureand temperature changes together are totally unknown.However, minerals that host U naturally or in their smallfractures and U-rich accessory minerals, both present inLHS units as well as in GHS units, have different stabilityin the P–T conditions. For example, considering the acces-sory minerals, a rock that equilibrates at low P–T conditionslike LHS units can contain allanite, whereas a rock thatequilibrates at higher P–T conditions like GHS units canhave monazite (e.g., Catlos et al., 2001). For a given ageand U concentration, these minerals may partition U differ-ently, have different grain sizes, and different susceptibilitiesto radiation damage. In addition, in GHS rocks, highergrades of metamorphism can induce significant loss of ura-nium from accessory minerals or from the crystal structureof other minerals (Dostal and Capedri, 1978).

Other effects indirectly associated with metamorphismmay affect ECRa. For example, faulting may change meanECRa value of an initially unfaulted area. Black schists, withsignificant organic matter content and, thus, higher ECRa,are in general located in the vicinity of faults. This is for exam-ple observed in the Upper Trisuli Valley and Lower Dolposites. In the vicinity of such faults, which may induce poten-tially larger permeability in the surrounding rocks, radonexhalation from rock samples may also be enhanced. Fluidcirculation may also play a role in creating a transient, rela-tively recent, disequilibrium between radium-226 and itssource nuclei from uranium-238, and then create significantdifferences in ECRa after some numbers of radium-226 half-lives. Finally, fluid ascent may trigger ECRa changes in rocksincluding weathering, secondary crystallizations or deposi-tions along fluid pathways before reaching the surface (Cope-land et al., 1991). This interesting point could effectively beenvisaged in the MCT zone where several hot springs are lo-cated (e.g., Evans et al., 2008), as well as natural CO2 gas dis-charge zones (Perrier et al., 2009). Such process may accountfor the relatively large ECRa dispersion observed in the MCTzone, for example in the Upper Trisuli and Langtang Valleysor in the Marsyandi Valley sites. Such fluid can also affectECRa through the weathering of primary U-rich accessoryminerals (Guthrie and Kleeman, 1986) and through the alter-ation of secondary U-bearing minerals and oxides (Carusoand Simmons, 1985). For example, contrary to allanite,which is highly susceptible to chemical weathering, monazite,zircon and sphene are generally unaffected by weathering.

To conclude, at this stage, before convincing interpreta-tions can be proposed, it appears necessary to better definethese various processes and to keep in mind that all theseprocesses are most likely secondary compared with the effectof mineralogy and lithology on ECRa. Thus, further studiesare definitely needed in this domain, which should focus onthese secondary processes at constant rock lithology.

6. CONCLUSION

This systematic study of ECRa using rocks across theMCT throughout the Nepal Himalayas showed that ECRa

values, as primarily related to the lithology, were character-ized by smaller values in the GHS units than in the LHSunits, with a mean ratio of 7.8 ± 1.6, thus facilitating theMCT location. Furthermore, we defined a characteristicthreshold for ECRa values separating LHS units fromFms. I and II of GHS units of 0.8 Bq kg�1. To our knowl-edge, this study, thanks to its large dataset, is the first con-vincing instance of large-scale ECRa coherent patternsacross geological faults. Concerning Fm. III of GHS, wehave only a few samples, indicating larger values of ECRa

for leucogranite. This fact might be of significance to under-stand the underlying mechanisms and the Fm. III might bea fruitful subject for a future detailed investigation.

The general decreasing trend when crossing the MCTobtained in this study could be extended not only to othersites along the Nepal Himalayas, but also throughout theentire range across other thrust faults. For example, theaforementioned ECRa differences in the Siwaliks may bemore carefully tested. It appears important to know quan-titatively the ECRa content of rocks in the Siwalik range, lo-cated near the currently active Himalayan thrust, and, inparticular to assess whether an effect related to fault or frac-ture occurrence is noticeable or not.

With similar aims, it could be interesting to perform suchECRa studies in other major thrust belts. The technique isnot time-consuming when only a first order estimate isneeded. Indeed, in addition to be also cost-effective, withinone week and with one person only, it is possible to obtainfirst results of about 24–45 rock samples by using accumula-tion flasks. Refinements can then be achieved with furthermeasurements. Larger sample set of typically 100–200samples can be analyzed by using SSNTDs within twomonths minimum (Girault and Perrier, 2012a). The system-atic measurement of ECRa in reference geological profilescan potentially be of tremendous help especially in areaswhere geological mapping or other geochemical methodsare ambiguous. More generally, this study opens the per-spective that different protoliths might be distinguishedthanks to effective radium concentration.

Hypotheses and tentative conclusions obtained in thisstudy could be subjected to dedicated experimental tests inthe laboratory, both with natural or artificial samples. Whilethe role of the rock fabric has been examined in thelaboratory by numerous workers (Sakoda et al., 2011), therole of mineralization and chemical evolution needs to beexplored systematically so that more detailed interpretationscan be proposed from the field observations. The radonsource, beyond being a mere health hazard, deserves someattention as it emerges as a parameter able to reveal funda-mental aspects of the chemical evolution of the crust.

ACKNOWLEDGMENTS

The authors thank the Department of Mines and Geology, Kath-mandu, for enthusiastic support over the years. Bishal Nath Upretiand Lalu Paudel are thanked for numerous enlightening discussions.Toya Ghimire and Bharat Pant are thanked for sampling one set ofrock samples from the Upper and Lower Trisuli Valley. BuddhaLama, Dawa Tamang, Nabin Tamang, Gautam Tamang, MingmaTamang, Khem Puri, Niraj Jairu, Pemba Tamang and Hiralal

224 F. Girault et al. / Geochimica et Cosmochimica Acta 98 (2012) 203–227

Tamang are thanked for their assistance during sampling andcrushing. Antoine Pitrou is thanked for improving the colored fig-ures. This paper has been significantly improved thanks to theenthusiastic and detailed careful review of Matt Kohn and oneanonymous reviewer, as well as by the outstanding and construc-tive supervision of the Editor. This is IPGP contribution number3329.

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