Geochemistry of sedimentary rocks from Permian-Triassic boundary sections of Tethys Himalaya:implications for paleo-weathering，provenance，and tectonic setting

Akhtar R.Mir·V.Balaram·Javid A.Ganai·Shamim A.Dar· A.Keshav Krishna

Geochemistry of sedimentary rocks from Permian-Triassic boundary sections of Tethys Himalaya:implications for paleo-weathering，provenance，and tectonic setting

Akhtar R.Mir1·V.Balaram2·Javid A.Ganai1·Shamim A.Dar3· A.Keshav Krishna2

The geochemical characteristics of two sections-the Permian-Triassic boundary（PTB）Guryul Ravine section，Kashmir Valley，Jammu and Kashmir，India；and the Attargoo section，Spiti Valley，Himachal Pradesh，India-have been studied in the context of provenance，paleo-weathering，and plate tectonic setting. These sections represent the siliciclastic sedimentary sequence from the Tethys Himalaya.The PTB siliciclastic sedimentary sequence in these regions primarily consists of sandstones and shales with variable thickness.Present studied sandstones and shales of both sections had chemical index of alteration values between 65 and 74；such values reveal low-to-moderate degree of chemical weathering.The chemical index of weathering in studied samples ranged from 71 to 94，suggesting a minor K-metasomatism effect on these samples.Plagioclase index of alteration in studied sections ranged from 68 to 92，indicating a moderate degree of weathering of plagioclase feldspars.The provenance discriminant function diagram suggests that the detritus involved in the formation of present studied siliciclastic sedimentary rocks fall in quartzose sedimentary and felsic igneous provenances.These sediments were deposited in a passive continental margin plate tectonic setting according to their location on a Si2O versus K2O/ Na2O tectonic setting diagram.

✉ Javid A.Ganai

jganai.ganai9@gmail.com

1Department of Earth Sciences，University of Kashmir，Srinagar 190006，India

2CSIR-National Geophysical Research Institute，Hyderabad 500007，India

3Department of Geology，S.P.College，Srinagar 190008，India

Geochemistry·Permian-Triassic boundary· Guryul Ravine·Tectonic settings

1　Introduction

The sedimentary succession of the Tethys Himalaya stretching from North India to South Tibet 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 history of Gondwana during most of the Phanerozoic.The global stratotype section and point（GSSP）for the Permian-Triassic boundary（PTB）（e.g.Meishan，Ursula Creek，Guryul Ravine，and Attargoo sections，etc.）have been a focus of researchers all over the world due to some important events like mass extinction，formation of the Neo-Tethys Sea，marine anoxia，increase of atmospheric CO2content，global warming，carbon isotopic excursion，acid rain，etc.（Wignall and Twitchett 1996；Berner 2002；Maruoka et al. 2003；Benton et al.2004；Payne et al.2004；Erwin 2005；Huey and Ward 2005；Brookfield et al.2010；Shen and Lin 2010；Shen et al.2011；Xu and Lin 2014）.Various studies throughout the world have been carried out on clastic sedimentary rocks using major-，trace-，or rare earth element data to understand paleo-weathering，paleoclimate，provenance，tectonic setting，paleoredox conditions，and crustal evolution（Zhang et al.2010；Sun et al.2011；Wani and Mondle 2011；Ganai et al.2014；Mir et al.2015）. Despite the global importance of PTB sections，little attention has been paid to geochemical characteristics of PTB sections of the northwest Himalaya（e.g.Guryul Ravine and Attargoo sections）（notable exceptions include Bhargava 2008；Williams et al.2012；Ganai et al.2014）.However，increasing attention has been paid to the paleontology and stratigraphy of the PTB Guryul section，with field-based work concentrated on assigning ages to the rock sequences and delineating depositional environments（Kapoor 1992，1996；Yin et al.2001；Brookfield et al. 2003，2013）.In the present study，we present geochemical data from the PTB sequences in the Kashmir and Spiti regions，North India，with an aim to understand the paleoweathering，provenance，and tectonic setting of these sections.This work adds to existing knowledge of the PTB Guryul Ravine and Attargoo sections and contributes to efforts to correlate geochemical characteristics among PTB sections of the world.

2　Geology of the study area

Rifting and eruption of middle Permian basalts was followed by separation of blocks of the northern edge of Gondwanaland，birth of the Neo-Tethys ocean，and rapid thermal subsidence of the northern Gondwana margin during Upper Permian and Triassic times.Due to this rapid subsidence，continuous sedimentation across the PTB took place on the northern Gondwanaland shelf（Myrow et al. 2003；Sciunnach and Garzanti 2012）.The Permian-Triassic sedimentation sequence is well-preserved at Guryul Ravine（Kashmir Valley，India）and in the Permian Gungri Formation at Attargoo（Spiti Valley，India）.These locations are considered to have the most complete and wellpreserved PTB type-sections in the world.

The Kashmir Valley occupies an oval shaped basin between the Pir Panjal Range in the southwest and the Great Himalayan Range in the northeast.Permian-Triassic outcrops are exposed at various places in the Kashmir Valley e.g.Guryul Ravine，Phalgam，Ashmukam，etc. Guryul Ravine is considered GSSP for the PTB（Fig.1a）（Kapoor 1996；Yin et al.2001；Brookfield et al. 2003，2010）.The Guryul Ravine section consists of a conformable succession of mixed siliciclastic-carbonate sediments（＞100 m thick）deposited in a deep-shelf or ramp setting（Brookfield et al.2010）.The Permo-Triassic sequences in Kashmir Valley are divided into the Zewan Formation of Upper Permian age and the overlying Lower Triassic Khunamuh Formation（Fig.2a，b）.The Panjal Traps，which are chiefly basaltic in composition with subsidiary association of silicic volcanic rocks，lie at the base of the Guryul Ravine section.The Zewan Formation is subdivided into four members named A，B，C，and D by Nakazawa et al.（1975）.

The top-most 5 meters of the Zewan Formation are composed of fine-to medium-grained layers of sandy limestones or calcareous quartz sandstones interbedded with patchy calcareous shales（Brookfield et al.2003）.The transition from parallel lamination to hummocky crosslamination structures suggests decreasing storm activity through time（Saito 1989；Brookfield et al.2003）.These cross-laminated and bioturbated well-sorted sands with a scarcity of fossils likely originated in a near-shore lagoon with graded storm beds transitioning shoreward to higher energy deposits（Brookfield et al.2003，2010）.The Zewan Formation is overlain by the Khunamuh Formation，which is divided into six members，named as E，F，G，H，I，and J on the basis of variation in carbonate content（Nakazawa et al.1975）.Interbedded dark calcareous shales and argillaceous limestones（about 16 m thick）comprise basal parts of the Khunamuh Formation while upper members consist of shale，thin-bedded lime mudstone，and nodular limestone（Nakazawa et al 1975；Nakazawa and Kapoor 1981；Kapoor 1996；Brookfield et al.2003）.

Spiti is located in the northern part of the Himachal Pradesh，which occupies a central position in the Western Himalaya（Fig.1b）.The section exposed at Attargoo village（Fig.2c，d）is composed of Permian gray-black shales of the Gungri Formation and Triassic limestones of the Mikim Formation.Since the Uppermost Permian strata have not been observed here，the cross-boundary biostratigraphy is yet to be confirmed（Garzanti et al.1998；Williams et al.2012）.An upward progression from gray to black in the Gungri Shale supports the hypothesis that oxygen levels were decreasing toward the end of the Permian（Shukla et al.2002）.The Gungri Formation is mostly composed of black splintery shale and interbedded calcareous and micaceous siltstone（Singh et al.1995）.The silty shale mostly occurs in the base of the Gungri Formation，whereas the upper part contains gray-to-black shale capped unconformably by an iron-rich pebble-sand layer（‘ferruginous layer’）2-8 cm in thickness marking，apparently，the PTB（Singh et al.2004）.Three factors point to the ferruginous layer’s being a sub-aerial exposure surface:its regional extent，and mineralogical and sedimentological characteristics（Bhatt et al.1981；Bhargava 1987；Singh et al.1995；Srikantia and Bhargava 1998；Algeo et al.2007；Williams et al.2012）.However，Himalayan Tethyan stratigraphy reflects a major sporadic transgression which commenced in the Mid-Early Permian and proceeded through the remainder of the Permian（Gardner 1992；Garzanti et al.1996）.

3　Sampling and analytical methods

Fig.1 a General geological map of Guryul Ravine section，Kashmir Region Jammu and Kashmir，India（after Bhat and Bhat 1997）.b General geological map of Attargoo section，Spiti Region，Himachal Pradesh，India（after Shukla et al.2002）

Fig.2 a and b Field photograph showing close view of the Permian-Triassic boundary at Guryul Ravine section，Kashmir region Jammu and Kashmir，India；c and d Field photograph showing Permian-Triassic boundary at Attargoo section，Spiti Region，Himachal Pradesh，India

Stratigraphically well-known Permo-Triassic sections exposed atGuryulRavine（（Fig.2a，b）and Attargoo village（Fig.2c，d）of the Spiti region，Himachal Pradesh，northern India，were sampled formajorelementalanalysis.Weathered surfaceswere trenched about10-15 cminto the rock face and about5-10 cmthick fresh chips from different rock types such as sandstones and shales were taken as samples.The fresh samples were put into polythene bags and placed into sealed bags marked with sample numberand location，and transported to the Laboratory forchemical analysis.Geochemicalanalysis was carried outat the National Geophysical Research Institute（NGRI），Hyderabad，India.Samples were crushed and then powdered to 200 mesh size in an agate mill.Major elements were determinedfrom pressed pellets，which were prepared by using collapsible aluminum cups.These cups were filled with boric acid.About 1 g of powdered rock sample was put on top of the boric acid and the cups were then pressed under a hydraulic press at 20 tonspressure.The sample pelletswere analyzed using a Philips MagiX PRO model PW-2440 wavelength dispersive X-ray fluorescence spectrometer（XRF）coupled with automatic sample changer PW 2540.International rock standards GSR-4 and GSR-5 were used as reference material during major element analysis.The precision and accuracy of the data are well within the international standards and are better than 5%.

4　Geochemistry

The major elemental concentrations of both the Guryul Ravine and Attargoo Permian-Triassic sections are presented in Table 1.SiO2and Al2O3concentrations of sandstones from the Guryul Ravine section ranged from 75 to 81 and 3.8 to 5.5 wt.%，respectively.MgO and Fe2O3contents of sandstones from the Guryul Ravine section varied from 3.9 to 4.07 and 6.9 to 7.2 wt.%，respectively. Alkalis and alkaline earth elements（Na2O，K2O，CaO）are more depleted than other major elements in the sandstone of the Guryul Ravine section.The shales of the Guryul Ravine and Attargoo sections showed SiO2concentration between 55 and 77 and 49 and 65 wt.%，respectively.The lower SiO2content observed in some samples suggests possible hydraulic sorting during sedimentary processes. Al2O3concentration of shales from the Guryul Ravine and Attargoo sections，varied from 11 to 17 and 16 to 20 wt.%，respectively，whereas TiO2ranges of the two sections were from 1.5 to 2.2 and 0.7 to 2 wt.%，respectively，andcontents varied from 0.79 to 10 and 2.9 to 17 wt.% respectively.Collectively，these results reveal the presence of a mixed clay assemblage and detrital phases in the shales.

When compared to Post-Archean Australian Shale（PAAS，after Taylor and McLennan 1985）values of shale samples of both sections are characterized by enrichment of TiO2，MgO，and K2O.Elevated K2O（Fig.3）concentration is attributed to contribution of K-feldspars and weathering of plagioclase feldspars.Enrichment of MgO with respect to PAAS may be attributed to the contribution of these elements from the marine fauna.SiO2and Al2O3are more or less similar to PAAS in the shales of both sections.Further，shale samples showed depletion of Fe2O3and P2O5；however，depletion of these oxides was greater in the case of the Guryul Ravine shales.Depletion of Fe2O3and P2O5in shales indicates the presence of relatively lesser quantities of phyllosilicates，ferromagnesian and apatite minerals，and/or may be attributed to the influence of degree of chemical weathering of the source rocks.

According to Cox et al.（1995），the K2O/Al2O3ratio relates to how much alkali feldspar versus plagioclase and clay minerals may have been present in the original shales. In order from high to low values，the K2O/Al2O3ratio of minerals represents alkali feldspars（0.4-1），illite（0.3），and other clay minerals（nearly 0）.Shales with K2O/Al2O3greater than 0.5 are likely to possess a significant quantity of alkali feldspar relative to other minerals in the original shale，whereas those with K2O/Al2O3less than 0.4 likely possess minimal alkali feldspar in the original shale（Cox et al.1995）.The studied shales of the Guryul Ravine sections have average K2O/Al2O3=0.23（range 0.11-0.35）；shales of the Attargoo section have average K2O/Al2O3=0.26（range 0.25-0.29）.These overall low K2O/Al2O3ratios in the studied shales suggest minimal involvement of alkali feldspar relative to other minerals and strongly support that K2O addition to the present samples has not taken place.

The enrichment of essential rock-forming minerals（if any）and altered products was determined by using the index of compositional variability（ICV）.This parameter is calculated as ICV=（Fe2O3+K2O+Na2O+CaO+ MgO+MnO+TiO2）/Al2O3（Cox et al.1995）.Rockforming minerals such as plagioclase，K-feldspars，amphiboles，and pyroxenes show ICV values of＞0.84，whereas typical alteration products such as kaolinite，illite，and muscovite show values of＜0.84（Cox et al.1995；Cullers 2000）.The ICV values of sandstones and shales of the Guryul Ravine section range from 0.68 to 3.35 with an average of 1.69；shales of the Attargoo section show ICV from 0.68 to 1.62 with an average of 1.06.Hence，in both sections ICV values are greater than 0.84，indicating that the studied samples are enriched in rock-forming minerals. Quartz is more resistant to weathering than feldspars，mafic minerals，and lithics（Roser and Korsch 1986，1999）.Hence，SiO2/Al2O3ratio ofsiliciclastic rocks can be used to indicate sediment maturity.The average values of SiO2/Al2O3ratio in fresh igneous rocks ranges from～3.0（in basic rocks）to～5.0（in acidic rocks）.The high SiO2/Al2O3values found in our analysis（Table 1）indicate relatively high sediment maturity（Roser and Korsch 1999）.

5　Discussion

5.1Paleo-weathering

Fig.3 Post-Archean Australian average Shale normalized major element patterns of the siliciclastic sediments（PAAS values after Taylor and McLennan 1985）

Chemical weathering strongly affects major element geochemistry of siliciclastic sediments，as labile cations（e.g.，Ca2+，Na+，K+）are removed preferentially over more stable components（Al，Ti）（Nesbitt and Young 1982；Fedo et al.1995）.The degree of this weathering of feldspars is often measured relative to unweathered protoliths using the chemical index of alteration（CIA）（Nesbitt and Young 1984）.CIA is defined by the equation:CIA=［Al2O3/（Al2O3+CaO*+Na2O+K2O］×100 where all elements are in molecular proportion，and CaO*represents Ca in the silicate fraction only.As the sample Z6 contains CaO greater than 1 wt.%，there would be a chance of presence of organic content or presence of Ca in carbonates（calcite，dolomite）.It is therefore necessary to correct for the measured CaO content in the samples.Due to non-availability of a direct method to distinguish and quantify the amount of CaO belonging to the silicate and non-silicate fractions，a method proposed by McLennan（1993）has been used to calculate the CaO*in the present study. According to this method，if the number of moles of CaO is less than that of Na2O，then the pure CaO value can be adopted.If the number of moles for CaO is greater than Na2O，CaO*should be assumed to be equivalent to Na2O. CIA values for average shales range from 70 to 75 which reflect a composition of muscovite，illite，and smectite. Intensely weathered rock yields mineral compositions trending toward kaolinite or gibbsite and aCIA approaching 100.CIA of unweathered rock is about 50. The studied sandstone and shales of the Guryul Ravine section have CIA values from 65 to 73；Attargoo section shales have CIA values from 71 to 74，reflecting their composition of muscovite and illite.The weathering trend can be illustrated on the triangular Al2O3，CaO*+Na2O，K2O（A-CN-K）plot，which is useful for evaluating and correcting the effects of K-metasomatism and giving some information of the composition of the fresh source rock（Fedo et al.1995）.On this diagram（Fig.4），studied samples plot towards the A-K line away from the plagioclase-K-feldspar join，indicating low to moderate degree of weathering.Chemical weathering can also be measured by chemical index of weathering（CIW，Harnois 1988）and plagioclase index of alteration（PIA，Fedo et al.1995）. CIW eliminates the effect of K-metasomatism and is calculated as:CIW=Al2O3/（Al2O3+CaO*+Na2O）× 100 where the elements are represented in mole proportion and CaO*represents Ca in the silicate fraction.Most strikingly，unweathered potassic granite has a CIW of 80 and fresh K-feldspar has a CIW of 100，similar to values for residual products of chemical weathering（smectite CIW=80；kaolinite，illite，and gibbsite CIW=100）.The CIW values of Guryul Ravine sandstone and shales range from 71 to 91；the CIW of Attargoo section shales from 91 to 94.This indicates a much reduced K-metasomatism effect on the samples of the study area.PIA reflects weathering of plagioclase feldspars and is defined by the equation:PIA=［（Al2O3-K2O）/（Al2O3+CaO*+Na2OK2O）］×100，where all elements are in molecular proportions and CaO*represents CaO in silicate fractions.The maximum PIA value is 100（kaolinite，gibbsite）and unweathered plagioclase has PIA value of 50.Studied sandstone and shale samples of Guryul Ravine section have PIA values from 68 to 87.However，shale of the Attargoo section has slightly higher PIA values of 88 to 92.Therefore，PIA values in studied samples indicate a moderate degree of weathering of plagioclase feldspars in both the Permo-Triassic sections.

Fig.4 A-CN-K ternary diagram（after Nesbitt and Young 1982）of sandstone and shales from the Permo-Triassic sections of Kashmir and Spiti regions Tethys Himalaya showing weathering trends. A=Al2O3；CN=（CaO*+Na2O）；K=K2O　（all in molar proportions）

5.2Provenance and tectonic setting

Geochemical composition of basin sediments is changed to some extend by various processes such as hydraulicsorting，weathering，and diagenesis.However，strong signatures of the original source terrain are stored in these sediments，thus reflecting the nature of the exposed continental crust.Because of low solubility of Al and Ti oxides and hydroxides at low temperatures，these elements are generally stable in the face of siliciclastic weathering（Sugitani et al.2006）.For this reason，Al/Ti ratios of residual soils is believed to be very close to those of their parent material allowing this ratio to be used by various researchers for discrimination of provenance（e.g.，Hayashi et al.1997；Sugitani et al.2006；Odigi and Amajor 2008；Sun et al.2013；Ganai et al.2014；Mir et al.2015）.Studies have shown that Al2O/TiO2ratios ranging from 3 to 8 indicate mafic igneous rocks，from 8 to 21 intermediate rocks，and from 21 to 70 felsic igneous rocks.In our samples Al2O/TiO2ratio varied from 6 to 20，which indicates an intermediate source（Hayashi et al.1997；Sugitani et al.2006）.McLennan et al.（1980），recognizing the significance of Al and Ti in provenance studies，proposed an Al2O3versus TiO2bivariate discrimination diagram to constrain the provenance of siliciclastic rocks.On this diagram，the siliciclastic sediments of the study areas plot along granitic and 3 granite+1 basalt trend lines indicating felsic provenance（Fig.5）.The major element-based provenance discriminant function diagram of Roser and Korsch（1988）is frequently used by many researchers to identify the provenance of terrigenous sediments（Cullers 2000；Islam et al.2002；Rashid 2005；Dey et al.2009）. This diagram discriminates four major provenance fields: mafic，intermediate，felsic，and quartzose sedimentary. Studied Guryul Ravine section samples fall within the Quartzose sedimentary provenance and those of the Attargoo section fall into the Felsic igneous provenance（Fig.6）.However，for further verification of provenances of the studied samples from both PTB sections，it is recommended to use trace element or isotope data.

Fig.5 Al2O3versus TiO2bivariate diagram（after McLennan et al. 1980）

Several studies have revealed that the chemical compositions of siliciclastic rocks are significantly controlled by plate tectonic settings of their provenances，and consequently siliciclastic rocks from different tectonic settings possess terrain-specific geochemical signatures（Bhatia and Crook 1986；Roser and Korsch 1986；Li et al.2003）. Tectonic settings of provenances of ancient siliciclastic rocks can be inferred from the major element-based discrimination diagrams proposed by Roser and Korsch（1986）by utilizing（K2O/Na2O）versus SiO2.Both SiO2and K2O/Na2O values increase from volcanic arc to active continental margin to passive margin settings；high SiO2and low Na2O in the binary discriminant plots suggestcontinentally-derived sediments.In the SiO2versus K2O/ Na2O diagram，samples from both Permo-Triassic sections（Guryul Ravine and Attargoo）in this study plot in the field of passive continental margin setting（Fig.7）.

Fig.6 Major element provenance discriminant function diagram（after Roser and Korsch 1988）for siliciclastic sediments of Permo-Triassic sections of Kashmir and Spiti regions Tethys Himalaya

Fig.7 K2O/Na2O versus SiO2discrimination diagram （after Roser and Korsch 1986）for siliciclastic sediments of Permo-Triassic sections of Kashmir and Spiti regions Tethys Himalaya

6　Conclusions

As expected，geochemically the sandstones of the Permian-Triassic sectionsofthe NWTethys Himalaya，India are more enriched in SiO2and show significant variations in other major elemental abundances.When compared to UCC，the alkalis and alkaline earth elements（Na2O，K2O，MgO，and CaO）are more depleted than other elements in the samples. The salient geochemical characteristics（such as enrichment in felsic elements and their ratios with depleted mafic components）indicate the Permo-Triassic siliciclastic sedimentary rocks of the Guryul Ravine and Attargoo sections may be derived from a felsic-dominant source i.e.quartzose sedimentary and granitic rocksand may have been deposited in a passive continentalmargin tectonic setting.On the bases of CIA and CIW it is concluded that studied samples of the Permo-Triassic sectionsexperienced low to moderate degree ofchemicalweathering.PIAvalues in studied samples show moderate weathering of plagioclase feldspars.The consistent variation in terms of degree of chemical weathering noticed in the sandstones and shales from the studied PTB sections clearly indicatesclimatic influence in the deposition of the Tethys sediments.

Acknowledgments We are thankfulto the Director，NGRI，Hyderabad for permission to analyze the studied samples.First and third authors pay sincere gratitude to Prof.ShakilA.Romshoo，Head，Department of Earth Sciences，University of Kashmir for providing necessary facilities.Authors pay our sincere thanks to reviewers for their constructive comments and suggestions.

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11 January 2016/Revised:6 February 2016/Accepted:18 April 2016/Published online:20 June 2016

©Science Press，Institute of Geochemistry，CAS and Springer-Verlag Berlin Heidelberg 2016