Rare earth element geochemistry of the Permo-Carboniferous clastic sedimentary rocks from the Spiti Region，Tethys Himalaya: significance of Eu and Ce anomalies

Javid Ahmad Ganai·Shaik Abdul Rashid

Rare earth element geochemistry of the Permo-Carboniferous clastic sedimentary rocks from the Spiti Region，Tethys Himalaya: significance of Eu and Ce anomalies

Javid Ahmad Ganai·Shaik Abdul Rashid

Siliciclasticsedimentaryrocks，including sandstones and associated shales，from the Permo-Carboniferous Kanawar Group of NW Tethys Himalaya，Spiti Region，India were examined geochemically to monitor the evolutionary changes in the upper continental crust in the Himalaya.The rocks are characterized by consistent rare earth element（REE）patterns with light REE enrichment（LaN/YbN=5.3—28.2）and flat heavy REE patterns.The∑REE values are high（up to 281 ppm）with large negative Eu anomalies（avg.Eu/Eu*=0.57）.The REE characteristics of the sediments are similar to those of post-Archean Australian shales and North American shale composite.La/Th values（avg.2.34）correspond to a relatively felsic composition of the terrestrial igneous rocks standard（La/Th of G-1=2.3）.The evolved felsic composition of the sediments probably relates to widespread acidic activity in the source.The REE patterns and Th/U values seem to have been affected by the sedimentary environment as well as by the provenance.The presence of positive Ce anomalies in some sediments may be the result of post-depositional processes.Moreover，the Permo-Carboniferous sediments indicate that hydraulic sorting，even over short transport distances，is capable of concentrating enough accessory phases to influence REE composition and to develop negative Eu anomalies.High∑REE，La/ Yb，and Th/U contents and large negative Eu anomalies reveal that the sediments were deposited in an oxidizing environment，suggesting the surficial environment became oxidizing around the Carboniferous-Permian boundary in the Indian craton.

Late Paleozoic sediments·Tethys Himalaya· REE geochemistry·Provenance·Paleoclimate·Ce anomalies

1　Introduction

The geochemistry of terrigenous clastic sediments provides information about changes in the composition of the continental crust through time.For the last three decades several studies have been carried out to characterize the composition of the continental crust（Taylor and McLennan 1985，1995；McLennan et al.1993，2001；Condie 1993；Barth et al.2000；Yan et al.2002；Rashid 2005；Garzanti et al.2010；Moosavirad et al.2011）.The examination of trace element abundances in sedimentary rocks has added greatly to our understanding of crustal evolution，with rare earth element（REE）patterns and Th being particularly useful.These elements are transferred almost directly into clastic sedimentary rocks（McLennan et al.1993）.It has also been observed that these elements'distribution is uniform in fine-grained clastic sedimentary rocks and that they are resistant to weathering，diagenesis，and metamorphism（Cullers 1994；McLennan et al.1993；Cullers 2000）.Moreover，the residence times of REEs in seawater are short—50—600 years（McLennan et al.1993，Garzanti et al.2010）—and their concentration in the ocean is exceedingly low，being in the parts per trillion range.

Significantly，fundamental differences in the geochemistry（particularly trace elements，includingREEs）oftypical Archean and post-Archean sedimentary rocks reflect differences in the composition of the upper crust exposed to weathering（Taylor and McLennan 1985）.An important discovery in this regard is that the REE patterns of post-Archeansedimentaryrockshaveshownremarkableconsistency（Taylor and McLennan 1985；Garzanti et al. 2011），and are generally characterized by a significant negative Eu anomaly.In contrast，Archean rocks generally have lower total REE abundances，less fractionated patterns（i.e.，lower La/Yb），and almost invariably lack any significant negative Eu anomaly（McLennan et al.1993）. Condie，（1993）in a review of chemical composition and evolutionofthe upper continentalcrust（UCC），argues thata negative Eu anomaly characterizes both Archean and post-Archean rocks.However，he noticed that the size of the Eu anomaly is certainly greater in post-Archean UCC，a feature that is related to the Eu anomalies in rocks of felsic igneous provenance（BorytaandCondie1990；Condieetal.2001）.It has also been argued that the change in composition of UCC is essentially related to the widespread intrusion of K-rich granitoids into the upper crust at the end of the Archean（Wronkiewicz and Condie 1990；McLennan and Taylor 1991；Condie et al.2001）.Changing REE patterns seen in some early Proterozoic successions represent the gradual unroofingofthisaddedmaterial.Adecreaseintheportionof mafics（Komatiites）and the massive acidic igneous activity marks the end of the Archean and thus defines the Archean-Proterozoic boundary at 2.7—2.5 Ga（McLennan et al.1993；Condie 1993）.These studies have clearly indicated that the compositionoftheexposedArcheancrustwasmoremaficin nature（less enriched in incompatible elements，with small negative Eu anomalies）.Besides REEs，a number of other differences exist between Archean and post-Archean sedimentary compositions.Significant differences in major element composition have been recorded and are consistent with the more felsic upper crust in the post-Archean（McLennan 2001）.The ferro-magnesium trace elements Cr and Ni are depleted while other incompatible elements（e.g.，Th.U）are much higher in post-Archean sedimentary rocks.

J.A.Ganai（?）·S.A.Rashid

Department of Geology，Aligarh Muslim University，Aligarh 202002，India

e-mail:jganai.ganai9@gmail.com

In this paper we examine the REE geochemistry of Permo-Carboniferous sediments from the Kanawar Group，Spiti Region，NW Tethys Himalaya in the context of provenance，weathering effects，and evolutionary changes in UCC in the Himalaya.Since the sedimentary rocks under discussion are from a marine environment，the effects of environmental conditions on red-ox sensitive elements，particularly Eu and Ce，are also discussed.

Fig.1 Geological map of the Spiti Valley（after Bagati 1990）

2　Geological setting

The sedimentary succession of the Tethys Himalaya represents the deformed remnants of the northern continental margin of the Indian subcontinent.The succession，one of the most complete and spectacularly exposed in the world，preserves an excellent stratigraphic record of the Precambrian to Cretaceous period.Although involved in the tertiary Himalayan orogeny，the sedimentary succession experienced little deformation（low-grade metamorphism）and thus preserves the complete record of source rock compositions.The Spiti Region，containing an almost uninterrupted Phanerozoic succession of Tethyan Himalaya，is surrounded by Ladakh and Tibet in the north and north-east and the Higher Himalaya in the south（Fig.1）.The presence of a richly fossiliferous and fairly complete classical Phanerozic sequence has attracted the attention of geoscientists around the globe.Detailed investigations by Bagati（1990），Gaetani and Garzanti（1991），Garzanti et al.（1996），Bhargava and Bassi（1998），Draganits et al.（2002），Myrow et al.（2003），Bhargava（2008），and Sciunnach and Garzanti（2012）have established various stratigraphic units of the Phanerozoic sequence with remarkable precision mainly based on fossil assemblages.Based on extensive investigations by various geoscientists，a broad consensus has been reached to divide the Permo-Carboniferous sequence into two major groups（Table 1）although different opinions exist on subdivisions of these groups into formations and members.The present work is mainly confined to Late Paleozoic（Permo-Carboniferous）terrigenous rocks of the Tethyan sequence of the Spiti area，which includes the Lipak，Po，and Ganmachidam formations of the Kanawar Group（Table 1）.These formations consist of clastic sediments—mostly shales，sandstones，and quartzites.The Permo-Carboniferous sections were investigated in detail in different localities exposed along the Spiti Valley from the Takche-Lossar，Muth-Kuling，Kabzima Nalla，and Lingti-Tabbo sections（Fig.1）and over 40 samples were collected for geochemical analysis.

Table 1 Lithostratigraphic description of Permo-Carboniferous sequences of the Tethys Himalaya，Spiti region（after Bhargava 2008）

3　Analytical techniques

Representative samples were air-dried，coarsely crushed withasteeljawcrusher，andpowderedinanagatemilldown toagrainsizesmallerthan200mesh.Atotalof29samplesof sliciclastic sedimentary rocks were selected for analysis. Trace elements，including REEs and high field strength elements，were analyzed using inductively coupled plasma mass spectrometry（ICP-MS）at the National Geophysical Research Institute（NGRI），Hyderabad，India（Table 2）. Analytical precision for trace elements is well within 5%—10%.The rock solutions were prepared using the technique given by Balaram and Gnaneshwar Rao（2003）.

4　Discussion

4.1REE characteristics in Tethys sediments

The REE patterns of the analyzed samples under discussion are shown in Fig.2.Most of the sediments display patterns similar to typical post-Archean sediments such as the North American shale composite（NASC），post-Archean Australian shales（PAAS）etc.，with light REE（LREE）enrichment［（La/Yb）N=5.34—28.22，avg.=13.53］and fairly flat heavy REE（HREE）patterns.Total REE abundances are variable（∑REEs=171—281 in shales；39—212 in sandstones），which may be related to differing amounts of accessoryminerals.Therelativelylowtotalabundancesseenin some of the siliciclastic sedimentary rocks are probably due todilution by quartz（SiO2＞75%）.Most significant arethe prominent negative Eu anomalies（avg.Eu/Eu*=0.58）whicharetypicalofpost-Archeansediments（Eu/ Eu*=0.65；McLennanetal.1993）.TheHREEs show more uniform fractionation patterns［（Gd/Yb）N=1.18—4.22；avg. 2.22］than the LREEs［（La/Sm）N=2.45—6.08；avg.3.78］. Both positive and negative Ce anomalies are found in these sediments.Fluctuations in the Ce anomaly may provide information regarding transgressive—regressive phases in terrestrially-derived near-shore marine sediments.During a regressive phase，eroded nearshore sediments may be enrichedinCe（OH）4thusincreasingthepositiveCeanomalyin marine sediments，whereas during a transgressive phase，there will be less deposition of Ce（OH）4-enriched sediments transported into deeper waters，resulting in a relative negative anomaly（Liu et al.1988）.

Apart from REEs，other elements were analyzed for comparison with post-Archean crust.McLennan（2001）examined Th and U data in sedimentary rocks as a function of age.Since the U is relatively mobile，the U data are not relevant.However，like REE，Th being insoluble during weathering and sedimentation because of very short residence time in seawater is meaningful.McLennan and Taylor（1980）made an attempt to see the relation between La and Th in sedimentary rocks through time.They found a conspicuous compositional relationship for La/Th and Th/ Yb:La/Th values increase and Th/Yb values decrease toward more mafic compositions.In other words，Archean crustal rocks，being relatively mafic in composition，should show an increase in La/Th and decrease in Th/Yb ratios relative to post-Archean sediments.This observation has been confirmed by McLennan et al.（1990）by analyzing clastic sedimentary rocks of widely varying age.Our results from the Spiti Region are consistent with studies carried out elsewhere.The correlation between La and Th in the sedimentary rocks is shown in Fig.3.All our samples have a La/Th ratio between 2 and 4.Average La/Th and Th/Yb values determined for the Tethys Himalayan black shales are 2.34 and 8，respectively，which corresponds to a relatively felsic composition（La/Th and Th/Yb values of United States Geological Survey igneous rock standard G-2 are 2.3 and 27，respectively，data from table of McLennan et al.1980）.

4.2Paleoweathering conditions of source rocks

Several intensive studies have been carried out to understand the chemical processes of weathering（Nesbitt et al. 1980）and their effects on the chemical composition of clastic sedimentary rocks（Nesbitt and Young 1982）.The degree of weathering on sedimentary rocks can be determined by the chemical index of alteration（CIA）proposed by Nesbitt and Young（1982），using molecular proportions:

where CaO*is the amount of CaO incorporated in the silicate fraction of the rocks.The degree of weathering increases with increasing CIA，although uncertainties exist in interpreting the CIA due to mobility of alkali and alkaline earth elements.The CIA values determined for the Permo-Carboniferous black shales and sandstones of the Sipiti area，Tethys Himalaya（CIA 54—78）（Javid et al. 2014）suggest that the source rocks underwent low-tomoderate weathering during deposition.

4.3Provenance of the sedimentary rocks

Sedimentary provenance studies have focused on the use and limitations of REEs as provenance indicators for more than 30 years（McLennan et al.1993；Cullers 2000；Condie et al.2001；Singh and Rajamani 2001；Lawrence and Kamber 2006）.Although REE mobilization can occur during chemical weathering of bedrock，source bedrock REE signatures are preserved in the weathering profile because there is no net loss of REE abundance（Condieet al.1991；Cullers 2000；Compton et al.2003；Ku¨tterolf et al.2008）.Because of this，REE ratios［e.g.（La/Yb）N，（Gd/Yb）N，（La/Sm）N，Eu/Eu*］of sediments are considered to resemble provenance and are commonly used to determine bulk source composition（Singh and Rajamani 2001；Ku¨tterolf et al.2008；Garzanti et al.2010；Dabard and Loi 2012）.However，large variation among the Permo-Carboniferous sedimentary rocks of the Spiti Region reveals that，despite having the same source，REE concentration can fluctuate during sediment production and transport.In addition to mineralogical control，REE concentrations during sediment production and transport largely depend on processes that operate during sediment deposition such as hydraulic sorting and the quartz dilution effect.Because hydraulic sorting affects minerals that host most of the REEs（e.g.zircon，monazite，allanite，titanite，apatite，xenotime，garnet，etc.），the concentration of these minerals，particularly ultradense minerals，varies with the intensity of hydrodynamic processes（Garzanti et al.2010）.

Table 2 Trace elements including REE concentration （ppm） of siliciclastic sedimentary rocks from Spiti region， Tethys Himalaya， India

Table 2 continued

The emplacement of huge volumes of Proterozoic granites along a length of nearly 2，000 km is considered an important episode of acidic magmatism in the history of crustal growth in the Himalaya and is said to mark the Archean-Proterozoic boundary（Sharma and Rashid 2001）. Proterozoic granites of the Himalaya are confined to a linear belt（Fig.4）and are associated with the same tectono-magmatic event as，but are older than，surrounding volcano-sedimentary sequences.The average REE pattern of the Permo-Carboniferous siliciclastic sedimentary rocks from the Spiti area，Tethys Himalaya matches well with the average REE patterns of Proterozoic granites（Fig.5）such as the Bandal Gneiss，Wangtu Gneiss/Granites，and Bomdila Gneiss.Proterozoic granites may have played a crucial role in the crustal evolution of the Himalaya.The uplift，unroofing，and erosion of these granites may have supplied ample detritus to the nearby Tethyan sedimentary basin and significantly contributed to the changing of crustal composition of the Himalaya towards a more felsic composition during the post-Archean.

Fig.2 Chondrite-normalized REE patterns of the Permo-Carboniferous Spiti sedimentary rocks and average chondrite values are from Sun and McDonough（1989）

Fig.3 La vs Th diagram（after McLennan et al.1980）for the Spiti sedimentary rocks.The Spiti sample La/Th ratios fall in between 2 and 4

From the above discussion，it is understood that the post-Archean Himalayan REE patterns and Th data indicate the most evolved felsic composition of the source rocks.There are two possibilities for the felsic nature of the post-Archean crust.First，the intrusion of K-rich granites at the end of Archean times is widely accepted.We do not have any evidence for a granitic intrusion in the Himalaya older than 1，900 Ma（Sharma and Rashid 2001）.However，some earlier work，based on paleocurrent studies，proposed that the source rocks for these sediments might have been the Aravalli—Delhi mountain chain and Bundelkhan massif（Valdiya 1995）.Na-rich（oldest）and K-rich（youngest）phases（Mondal and Zainuddin 1997）of the Bundelkhand massif in the Archean period（3.3 Ga）might be the source of felsic post-Archean Himalayan sediments.A similar conclusion was made by Naqvi and Hussain（1972），following extensive studies from the southern Indian shield where they observed a change in the composition of the upper crust from mafic to felsic during the Archean-Proterozoic because of the emplacement of acid（sodic）plutons into the early Archean crust.Moreover，a good comparison between the REE patterns of the Permo-Carboniferous siliciclastic sedimentary sequences from the NW Tethys Himalaya and the REE patterns of the granitoids from the Himalaya（Fig.5）further confirm the felsic composition of the source rock.The second possibility to explain the felsic nature of the post-Archean crust is rooted in the sedimentary environment，i.e.oxidizing conditions prevailing during post-Archean times.Based on REE studies of the early Proterozoic sediments，several geoscientists have convincingly argued that the REE patterns（including Eu anomalies），though mostly dependent on their provenance，can also be controlled by fO2and sedimentary environment.They observed that when fO2is low（a reducing environment），the∑ sediments deposited should be characterized by low REE values and a positive Eu anomaly，whereas sediments deposited in oxidizing condition∑s（i.e.，fO2is high）should be characterized by highREE and Eu depletion.From thisdiscussionregardingthereduction（EarlyArchean）—oxidation（post-Archean）mechanism，it appears that the post-Archean clastic sedimentary rocks of NW Tethys Himalaya which are characterized by high∑REE and a strong negative Eu anomaly were deposited in an oxidizing environment.

Fig.4 Geological map（after Sharma 1983）showing the occurrences of Paleoproterozoic granitoids along the lesser Himalaya.Sources of age data:1—Treloar and Rex 1990；2—Zeitler et al.1989；3—Bhanot et al.1988；4—Frank et al.1977；5—Singh et al.1993；6—Raju et al.1982；Trivedi et al.1984；7—Dikshitulu et al.1995；Rao 1998

Fig.5 Average REE pattern of the Permo-Carboniferous siliciclastic sedimentary rocks from the Spiti area，Tethys Himalaya，and Proterozoic granites such as Bandal Gneiss，Wangtu Gneiss/Granites，and Bomdilla Gneiss

Fig.6（Gd/Yb）Nvs.Eu/Eu*diagram of the Spiti Sedimentary rocks compared with Paleoproterozoic Himalayan Granitoids/Gneisses

Fig.7 Plot of Eu/Eu*vs total REE of Permo-Carboniferous siliciclastic sediments from the Spiti Region，Tethys Himalaya

Fig.8 Plot of Ce/Ce*vs total REE of Permo-Carboniferous siliciclastic sediments from the Spiti Region，Tethys Himalaya

4.4Significance of Eu and Ce anomalies

Understanding the origin of Eu depletion relative to the other chondrite-normalized REEs in clastic sedimentary rocks is fundamental to any interpretation of crustal composition and evolution.The most significant observation in this regard is that virtually all post-Archean sedimentary rocks（sandstones，mudstones，and carbonates）are characterized by Eu depletion of comparable magnitude（Taylor and McLennan 1988）.That Eu anomalies seen in post-Archean sedimentary rocks are due to oxidation—reduction processes during weathering or the breakdown of feldspars is unfounded since there is no significant upper crustal reservoir with the complementary Eu enrichment.The only tenable explanation is that the Eu depletion in sedimentary rocks，and hence the UCC，is due to chemical fractionation within the continental crust，related to production of K-rich graniticrockswhichtypicallypossessnegativeEu anomalies.The presence of a Eu anomaly is thus the signature of earlier events in a more reducing igneous environment than now exists in the upper crust.Eu/Eu*is typically≥1.0 for Archean sediments and 0.6—0.7 for post-Archeanshales（McLennanetal.1990）.Archean sedimentary rocks generally show HREE depletion resulting in high GdN/YbNratios，commonly greater than 2.0；whereas GdN/YbNfor post-Archaean rocks they is typically 1.0—2.0（McLennan 1989）.However，this ratio can be sensitive to heavy mineral accumulation.For example，McLennan（1989）noted that even small amounts of zircon within a sediment can lead to GdN/YbN＜1，while the presence of only 0.005%monazite will increase the GdN/ YbNto＞2.The REE characteristics of the sedimentary rocks from the study area are summarized in Fig.6 where Eu/Eu*is plotted against GdN/YbN.There is a spread of average values of GdN/YbNand Eu/Eu*for Paleo-Proterozoic Himalayan Granitic bodies occurring south of the Spiti basin.These include the Bandal Gneiss，Wangtu Gneiss/Granites，and Bomdila Gneiss—all potential source rocks that could have provided sediment to the basins under discussion—indicating that the Permo-Carboniferous sediments may have received inputs from all of the Proterozoic Himalayan Granitic belt（Fig.6）.

Customarily，sediment Eu/Eu*values are thought to be inherited directly from the source rock（Taylor and McLennan 1985；Borges et al.2008；Singh 2010）.Spiti sediments have Eu/Eu*values ranging from 0.8 to as low as 0.48 with an average 0.57（Table 1）.The covariation between negative Eu anomalies，a decrease in grain size，and an increase in total REE（Fig.7）suggest the Eu anomaly could be controlled by the accessory phases in the Permo-Carboniferous sandstones and shale sediments.If the negative Eu anomalies are the result of accessory mineral concentration，the Permo-Carboniferous sediments indicate that hydraulic sorting，even over short transport distances，is capable of concentrating enough accessory phases to influence REE composition and to develop negative Eu anomalies that are typically interpreted to represent more felsic，granitic rocks.

The use of the Ce anomaly was first proposed by Elderfield and Greaves（1982）as a consequence of the change in the ionic state of Ce as a function of oxidation state.Ce has two redox states，III and IV.Unlike other lanthanide elements，which are trivalent（except Eu which can bedivalent），Ce3+can be oxidized to Ce4+under oxidizing conditions.Ce（IV）is insoluble and，under oxidizing conditions，is precipitated as CeO2.Thus，sediments deposited under oxic or anoxic conditions can preserve the geochemical signature of Ce3+or Ce4+，though this signature can be altered，post-depositionally，by late diagenesis and metamorphism.Oxidation of Ce commonly leads to fractionation of Ce from the other REEs.This fractionation generally occurs within pore waters during diagenesis，where REE3+are mobilized fromFe andMn oxyhydroxides and other unstable REE carriers，leaving Ce4+behind.A change in Ce/Ce*during diagenesis has been documented by German and Elderfield（1990）and Murray et al.（1991）. Moreover，changes in the oxygen content of marine systems and a heterogeneous source of REE to sediments have also been shown to affect Ce/Ce*（MacLeod and Irving 1996；Murray et al.1990，1991；Wright et al.1987）.Normal seawater contains anomalously low concentrations of Ce because it is the only REE that can easily be oxidized to its relatively insoluble 4+valence state under normal surface conditions.Marine authigenic precipitates（carbonates，phosphates，and cherts）may retain this seawater Ce anomaly，in which case oxic bottom waters are indicated（Holser 1997；Shields and Stille 2001）.Positive Ce anomalies indicate anoxia in the water column，enhancing Ce（III）concentrations in the sediment（Kato et al.2002；Sholkovitz et al.1992）.Negative Ce anomalies suggest suboxic conditions wherein aqueous Ce（III）is depleted（Kakuwa and Matsumoto 2006）.It is premature to assign quantitative meaning to the absolute values of measured Ce in the whole rock.That is，as yet，a scalar relationship betweentheCeanomalyandEhorthefugacityofoxygen，and，secondarily，a direct relationship between depth of water and the absolute value of the Ce anomaly，is still to be defined.However，for uniform depositional and source conditions，it should be possible to interpret relative change with a range from anoxic（negative）to oxic（positive）values.Murray et al.（1990，1991）have shown that the Ce anomaly depends on depositional setting.Thus，increasing values indicate more oxic conditions，whereas decreasing values indicate more reducing or anoxic conditions.The removal of dissolved Ce3+as an insoluble form of Ce4+preferentially occurs in the upper part of the water column，so oxic seawater shows a negative Ce anomaly，whereas oxic sediments have a less negative to a positive value（Wright et al.1987）.Conversely，in anoxic sediments，Ce is released and the sediments show a negative anomaly.Most of the black shales，Cambrian chert-phosphorite assemblages，and fossil apatite discussed during past years have a positive Ce anomaly associated with anoxic sediments during warmerclimate andtransgressive conditions（Wright et al.1987；Wilde et al.1996；Mazumdar et al.1999）.In other words，the observed Ce anomaly is not limited to marine carbonate，conodonts，and ichthyoliths，while the whole-rock Ce anomaly can be used as an indicator of intensity of anoxia and therefore of eustatic sea level.A positive-trending whole-rock cerium anomaly indicates more oxic conditions or a sea-level fall.In contrast，a negative-trending anomaly indicates more reducing conditions or a sea-level rise（Wilde et al.1996）.

Another possible cause of Ce anomalies is Ce fractionation at some stage in the weathering and sedimentary processes.In the early stages，negative Ce anomalies are noticed in weathering products like secondary hydrous phosphates（Braun et al.1990）and positive Ce anomalies emerge in deeply weathered and lateritic profiles where soluble Ce3+oxidizes to insoluble，stable Ce4+and is hosted in secondary cerianite，Ce（IV）O2（Braun et al.1990；Borges et al.2008）.

The consistent positive cerium anomalies observed in the Spiti sediments（Fig.8）are inconsistent with the anoxic depositional environment of these sediments（particularly black shales）.In an attempt to explain this anomaly in the Spiti sediments，the following possibilities are proposed. The strong correlation between Ce anomalies，total REE，and grain size（Fig.6）suggest that the positive anomalies of an anoxic environment may have developed due to postdepositional processes，such as late diagenesis or metamorphism.Moreover，an excellent correlation of La and Ce to Th（0.9）and their high La+Ce contents，suggest that the Th is probably contained chiefly in heavy mineral monazite that may cause enrichment of Ce in the sediments.Since positive Ce anomalies in the sediments have a strong correlation with the sea level eustatic changes（Wilde et al.1996），a fall in sea level or regressive event at the terminal phase of Carboniferous period in the Spiti area is very well recorded in these sediments.

5　Conclusion

The trace element chemical composition and REE patterns of the Permo-Carboniferous siliciclastic sedimentary rocks of the Kanawar Group，Sipti Region，Tethys Himalaya record compositional changes in provenance.The patterns exhibit LREE enrichment and almost flat HREE trends with large negative Eu anomalies which are typical of post-Archean UCC.The large ion lithophile element，high field strength cation，and REE contents；and La/Th ratios of the siliciclastic sedimentary rocks are consistent with a felsic source rock.Thus，the Permo-Carboniferous siliciclastic sedimentary rocks appear to be derived from sources rich in granitic components，with lesser contributions from basalt，tonalite，and Komatiite.The felsic composition is thought to be related to the emplacement of granitic magma，likely Proterozoic granites of the Himalaya that are confined to alinear belt.The granitic magmatism that took place for a period of around 800 Ma might have contributed ample material for the later-formed sedimentary basins including Lesser Himalayan sequences，Higher Himalayan sedimentary sequences，and Tethys Himalayan sedimentary sequences（present study）and may have become the primary cause for compositional change of UCC in the Himalaya from mafic to relatively felsic during the post-Archean period.The REE characteristics and Th/U ratios of these sedimentary rocks seem to be affected by both the provenance of the sediments and the sedimentary environment. The sedimentary REE characteristics and Th/U contents suggest that the atmosphere was oxic in the post-Archean.

AcknowledgmentsThe authors express their thanks to the Chairman，Department of Geology，Aligarh Muslim University，Aligarh，for providing necessary facilities.We are also thankful to the Department of Science&Technology，New Delhi for supporting this research in the form of Research Grant（SR/S4/ES-422/2009）to SAR. Authors are really thankful to the anonymous reviewers who have reviewed the manuscript and given positive and useful comments.

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16 January 2014/Revised:21 February 2014/Accepted:24 February 2014/Published online:17 February 2015 ?Science Press，Institute of Geochemistry，CAS and Springer-Verlag Berlin Heidelberg 2015