Mineralogy and geochemistry of trachytic rocks from the Lichi Volcanics,Eastern Himalaya: insights into the Kerguelen mantle plume activity in the Eastern Himalayan Region

Pallabi Basumatary • Deepshikha Borah • Hiredya Chauhan • Tribujjal Prakash •Bibhuti Gogoi

Abstract The Lichi volcanics are a suite of mafic-intermediate-felsic rocks and are considered coeval with the Abor volcanics (～132 Ma) of the Siang window in the Eastern Himalaya.Here,we present the first report of trachytic rocks from the Lichi volcanics,which are exposed in the Ranga valley,along the Kimin-Yazali road section in the Eastern Himalayan Region,Northeast India.The trachytes occur in close association with sandstones of the Gondwana Group of rocks and are characterised based on field,petrographical,and geochemical investigations.These fine-grained trachytes are composed of alkali feldspar,biotite,plagioclase,sodic-amphibole,apatite,illmenite,and titanite.The REE profiles of the evolved trachytic rocks (higher SiO2 content) display fractionated trends.The fractionation of accessory mineral phases,like apatite and titanite,was possibly responsible for the strongly fractionated REE patterns of the evolved samples.The trachytic rocks demonstrate high apatite saturation temperatures of 988 ± 14 °C (1σ,n=8).The Aluminium Saturation Index (＜1.1) and binary discrimination diagrams of these peralkaline trachytes define their affinity with A-type granitoids.Elemental ratios like Y/Nb,Nb/U,and Ce/Pb signify that the Lichi trachytes are differentiated products of mantle-derived ocean island basalts.Trace elemental discrimination diagrams Th/Yb versus Nb/Yb,Y versus Nb,and Y+Nb versus Rb reflect a within-plate tectonic regime for the trachytes.From the results presented in this work,we infer that the development of rifting events during the breakup of eastern Gondwana due to the onset of Kerguelen plume activity further led to underplating of basic magma in lower crustal levels.These parental basaltic magmas underwent fractionation processes forming differentiated trachyandesites and trachytes.Taking into consideration the similarities recorded between the Lichi volcanics and Abor volcanics,this study supports the idea that Kerguelen plume activities resulted in the emplacement of these volcanics in the Eastern Himalayas.

Keywords Peralkaline trachytes ∙Trachyandesites ∙Atype granitoids ∙Apatite saturation temperature ∙Abor volcanics ∙Eastern Gondwana

1 Introduction

The Himalayan orogen,between the Indian and Eurasian plates,is the youngest active continent-continent collisional orogen (Yin and Harrison 2000).Extending from Namcha Barwa in the east to Nanga Parbat in the west,the Himalayan domain is a natural laboratory to study collisional orogeny and plate tectonics (Leech et al.2005).Available age data indicate that the Himalayas have witnessed various episodes of magmatism ranging from the Paleoproterozoic(～2500 Ma)to Late Cenozoic(～46 Ma)(McMahon 1884;Auden 1935;Wadia 1957;Singh 2005,2020 and references therein).The magmatic rocks from the Himalayas represent intermittent distribution of ages entailing various tectonic activities such as rifting,plume,subduction,collision,etc.Understanding the genesis of these magmatic rocks is,thus,essential to comprehending the tectonic scenario within the Himalayan domain (Singh 2020).

The Sikkim,Bhutan,and Arunachal Himalayas constitute the eastern Himalayas,which are less explored as compared to the central and western Himalayas.In Arunachal Himalaya,studies have been conducted to explain the emplacement and petrogenesis of the Abor volcanics,which are exposed in the Siang and Yamne valleys within the Paleocene-Eocene Yinkiong Formation (Acharyya and Saha 2013).The Abor volcanic suite of rocks comprises voluminous mafic rocks (basalt and basaltic andesite) and minor amounts of intermediate to felsic rocks,such as rhyolite,dacite,and andesite (Jain and Thakur 1978;Bhat 1984;Acharyya 1994;Liebke et al.2011;Singh and Singh 2012).The Abor volcanics are considered to be contemporary to the Rajmahal-Sylhet-Shillong-Mikir flood basalts of eastern India and product of Kerguelen mantle plume activity (Singh and Singh 2012;Baral et al.2021;Singh et al.2021a,2021b).Further,the zircon U-Pb ages of～132 Ma for the Abor volcanics explain their plausible relation with the Comei-Bunbury magmatic suite from southeast Tibet and southwest Australia(Singh et al.2020).Another suite of rocks named Lichi volcanics,which are equivalent to the Abor volcanics,has been reported from Arunachal Himalaya(Fig.1a;Shanker et al.1989;Sharma et al.2018,2022).These rocks are exposed in the Lichi village of Ranga valley in Papumpare district of Arunachal Pradesh.

Fig.1 a A simplified geological map of the Indian landmass.The location of the Lichi volcanic suite is marked by the red box.CITZ=Central Indian Tectonic Zone,CGGC=Chotanagpur Granite Gneiss Complex,SPGC=Shillong Plateau Gneissic Complex,SONA=Son-Narmada Lineament,PCSZ=Palghat-Cauvery Shear Zone,BVSs=Bathani volcano-sedimentary sequence,MMB=Mahakoshal Mobile Belt,ADMB=Aravalli-Delhi Mobile Belt,EGMB=Eastern Ghats Mobile Belt,MB=Mahanadi Basin,GB=Godvari Basin,NSMB=North Singhbhum Mobile Belt.b A simplified geological map of the Himalayan mountain range (modified after Liebke et al.2011).HH=Higher Himalaya,IYSZ=Indus-Yarlung suture zone,LH=Lesser Himalaya,MBT=Main Boundary Thrust,MCT=Main Central Thrust and STD=South Tibetan Detachment

Very few reports exist on the mafic and intermediate rocks of the Lichi area(Sharma et al.2018,2022).Sharma et al.(2022) reported that the sub-alkaline basalts and trachyandesites from the suite of Lichi volcanics are a product of a continental extensional regime.It has been speculated that Lichi volcanics might also have resulted due to Kerguelen mantle plume activity (Sharma et al.2022).Research has been conducted on the mafic rocks of Lichi,but there is currently no available report addressing the trachytes from the same region.To address this knowledge gap,the present study directs its focus on the trachytes exposed near the Lichi village of Ranga Valley.Here,we present a petrological study of the trachytes involving field investigations,petrography,mineral chemistry,and geochemistry.This study will provide an understanding of the probable mechanism for magma genesis,the tectonic setting responsible for their emplacement,the characteristics of the source,and its potential association with the Kerguelen plume activity.Our study represents the first appraisal of trachytes exposed in the Lichi area of Arunachal Himalaya,which might further provide insights into the tectonothermal evolution of the Himalayan terrane.

2 Geological setting

The Arunachal Himalaya,which forms the eastern sector of the Eastern Himalaya,extends from West Kameng district of Arunachal Pradesh in the west to Namcha Barwa peak in the east (Fig.1b).This～350 km long folded-thrust belt consists of four major lithotectonic units,viz.,Sub-Himalayan sequence,Lesser Himalayan sequence,Higher Himalayan sequence,and Tibetan Himalaya.These lithotectonic domains are further marked by major faults from south to north: Himalayan Frontal Thrust (HFT),Main Boundary Thrust(MBT),Main Central Thrust(MCT),and Tibetan Detachment System (Gansser 1964;Bhattacharjee and Nandy 2008).The Tibetan Himalaya covering the northwestern part of Arunachal Pradesh comprises Proterozoic metasediments and high-grade schists and gneisses.The Higher Himalayan sequence is located to the south of Tibetan Himalaya and north of Lesser Himalaya.Its southern limit is marked by the MCT and lithologically it comprises Paleoproterozoic high-grade schists and gneisses and Tertiary granite intrusives.The Lesser Himalayan sequence is separated from the Sub-Himalayas to the south by the MBT.Lithologically,the Lesser Himalayan sequence comprises Paleoproterozoic metasediments,sediments belonging to the Gondwana Group,and Lesser Himalayan crystallines.The Sub-Himalayan sequence comprises Siwalik sediments and is followed by alluvial cover of the Brahmaputra River to the south (Kesari 2010).

The Lichi volcanics,named after the Lichi village of Papumpare district in Arunachal Pradesh,is a volcanic unit found within the Lesser Himalayan sequence (Fig.1b;Shankar et al.1989).Brown(1912)referred these volcanics to be a separate volcanic entity from the Abor volcanics of the Siang window.The Lichi volcanics is a suite of felsicintermediate-mafic rocks.They are found in association with quartzites and carbonaceous shales of the Bichom-Bhareli Formation (Sharma et al.2018).The studies conducted on the faunal and palynofloral assemblages of the Bichom and Bhareli Formations have yielded a Lower Permian age for the formations (Dutta et al.1988;Srivastava and Bhattacharya 1996;Mondal et al.2021).Thus,the Bichom-Bhareli Formations are categorised under the Gondwana Group (Mondal et al.2021).The MBT separates these Gondwana sediments along with the associated Lichi volcanics from the micaceous sandstones of the Dafla Formation of the Siwalik Group (Sharma et al.2018).Major lithological components recorded from the Lichi volcanic suite are amygdaloidal basalt,aphanitic basalt,basaltic andesite,and trachy-andesite (Roychoudhury 1984;Sharma et al.2018,2022).

3 Field relationships

The studied volcanic rocks are exposed in the Lichi village of the Ranga valley along the Kimin-Yazali road section in Arunachal Pradesh,India (Fig.2).Through field observations conducted within the study area,it has come to light that the trachytic rocks are present,often in conjunction with the sandstones of the Gondwana Group.The studied volcanics are dark grey,fine-grained,and mostly display amygdaloidal texture (Fig.3a,b).Moreover,the studied samples do not show any evidence of metamorphism.A number of constraints such as thick vegetation cover,high rate of weathering,and inaccessibility of the terrain have made it difficult to distinguish the contact between the sedimentary and volcanic units (Fig.3c,d).Additionally,these limitations have prevented the determination of the precise extent covered by the Lichi volcanics.

Fig.2 Regional geological map of parts of the Eastern Himalaya in Arunachal Pradesh showing the study area within the black dashed box (modified after Misra 2007;Sharma et al.2022)

4 Analytical methods

Compositions of mineral phases were determined using electron probe micro-analyzer (EPMA;Cameca SX-5 instrument) at the Department of Geology,DST-SERB National Facility,Banaras Hindu University,India.Operating conditions were: 15 kV accelerating voltage,10 nA beam current.The electron beam was produced with the help of a LaB6source.The locations of crystals(SP1-TAP,SP2-LiF,SP3-LPET,SP4-LTAP,and SP5-PET) concerning matching wavelength dispersive (WD) spectrometers(SP) were verified using the natural silicate mineral andradite as an internal standard.Natural mineral standards wollastonite,halite,fluorite,corundum,apatite,rhodonite,periclase,orthoclase,rutile,chromite,and hematite provided by CAMECA-AMETEK were utilised for general calibration.For major element oxides,the accuracy of the analysis was within 1% and the error for trace element concentrations ranged from 3% to 5% based on multiple standard re-assessments.Correction of raw data was done following the PAP methodology (Pouchou and Pichoir 1987).

Analysis of major oxides and trace elements was carried out at the Wadia Institute of Himalayan Geology (WIHG,India) using a Bruker S8 Tiger Sequential X-ray Fluorescence(XRF)Spectrometer using pressed pellets made from homogenized rock powder (Saini et al.1998,2000).Operating conditions for major elements were: 20/40 kV,no filter,vacuum path;for trace elements were: 55/60 kV,no filter,vacuum path.The RSD (Relative Standard Deviation) percentage for major oxides is less than 5% and less than 12%for trace elements.The average RSD is less than 2%(Saini et al.2007;Purohit et al.2006).Analysis of rare earth elements (REE) was carried out at WIHG using a PerkinElmer SCIEX quadrupole type ICP-MS (ELAN DRC-e).The sample digestion and solution preparation were performed according to Balaram et al.(1990).Rock standards used to reduce matrix influence were: USGS(BHVO-1,AGV-1,and RGM-1) and GSJ (JG-2).All the samples have RSDs that are better than 10%.In the case of samples displaying amygdaloidal texture,the amygdules were removed manually by chipping before powdering.

5 Results

5.1 Petrography and mineral chemistry

The trachytes are fine-grained in nature and consist of fine groundmass of alkali feldspar and biotite as the major mineral phases,while plagioclase,amphibole,Ti-Fe oxide,apatite,and titanite constitute the subordinate phases(Fig.4a).These trachytes mostly exhibit amygdaloidal texture consisting of secondary minerals like quartz and calcite within the vesicles.Mineral chemical analyses of 8 major and minor mineral phases were carried out to determine their compositions.Compositional analyses of these minerals are given in Supplementary Tables 1,2,3,4,and 5.

The feldspar minerals occur amidst groundmass and the analysed alkali feldspar has a composition of Ab0–24-An0–1Or76–100,while the analysed plagioclase grains have a composition of Ab93–99An0–1Or1–6(Fig.5).The browncoloured analysed biotite grains are classified as Mg-biotite,on the biotite classification diagram (Fig.6;Tischendorf et al.1997).The Si value of Mg-biotite ranges from 6.21 to 6.94 apfu,total Al abundance ranges from 1.52 to 1.90 apfu,and Fe+Mg values ranges from 4.24 to 6.10 apfu.The amphibole grains occur as anhedral grains within the groundmass.The amphiboles plot in the field of riebeckite on the amphibole classification diagram(Fig.7),which classifies them as sodic-amphiboles (Leake et al.1997).The Na content of the amphiboles ranges from 1.48 to 1.99 apfu,Si value ranges from 7.79 to 8.03 apfu,and[Mg/(Mg+Fe2+)] ranges from 0.31 to 0.39.Anhedral specs of Ti-Fe oxides are seen scattered in the groundmass.The FeO(T) values for the Ti-Fe oxides range from 46.98 to 48.08 wt%,while TiO2ranges from 49.24 to 50.17 wt%.The calculated Fe2O3values remain below 6 wt%.In the TiO2-FeO-Fe2O3classification diagram,the Ti-Fe oxides plot close to the ideal composition of ilmenite (Fig.8).Apatite grains in the studied samples occur as discrete mineral phases and exhibit two different types of morphology.Discrete subhedral to euhedral tabular prisms(Fig.4b) as well as aggregates of elongate,acicular apatites (Fig.4c) are seen evenly dispersed within the groundmass.The amygdales are composed of aggregates of anhedral secondary quartz grains and calcite (Fig.4d).

Fig.5 Nomenclature of alkali feldspar and plagioclase occurring in trachytes of the Lichi volcanics

Fig.6 Nomenclature and classification of biotite from trachytes of the Lichi volcanics(after Tischendorf et al.1997)

Fig.7 Nomenclature of amphibole from trachytes of the Lichi volcanics (after Leake et al.1997)

Fig.8 TiO2-FeO-Fe2O3 diagram giving the composition of Ti-Fe oxides from the Lichi trachytes

5.2 Geochemistry

A total of 8 samples of the trachytic rocks collected from the Lichi volcanic suite have been analysed for major,trace,and rare earth element distribution (Supplementary Table 6).These rocks are characterised by relatively high SiO2(60.07–65.51 wt%) and high alkali (Na2O+K2-O=10.68–11.97 wt%) contents.The loss on ignition(LOI) values range from (1.26–2.59 wt%).The magnesian number (Mg#) of these rocks [Mg#=Mg ×100/(Mg+Fe2+)] is relatively high and varies from 48.80 to 57.28.The concentrations of fluid immobile elements are as follows: Ti=18404–25838 ppm,Y=18–25 ppm,Nb=42–49 ppm,V=129–150 ppm,La=8.5–246.5 ppm,Yb=0.40–3.29 ppm,Sm=2.26–34.80 ppm,and Th=17.4–26.1 ppm.While the concentrations of fluid mobile elements are as follows: Rb=155–302 ppm,Sr=745–958 ppm,and Ba=2775–3761 ppm.

The total alkali versus silica (TAS) plot with subdivisions after Le Bas et al.(1986),categorizes these volcanic rocks as trachytes(Fig.9a).It is also evident from the plot that the linear trend shown by the basalts,trachyandesites,and trachytes display a co-magmatic linkage.The trachytes have low Al concentrations relative to those of the basalts and trachyandesites,and define a compact group of data points in the peralkaline field(Fig.9b)in the Al/(Na+K)versus Al/(Ca+Na+K) diagram (after Chappell and White 1992).Thus,the studied samples suggest weak peralkaline character,with their Aluminium Saturation Index (ASI=molar Al2O3/CaO+Na2O+K2O) values ranging from 0.65 to 0.75.All samples plot within the shoshonite field in the K2O (wt%) versus SiO2(wt%)diagram (after Peccerillo and Taylor 1976) because of remarkably high K2O content (Fig.9c).In the FeOT/FeOT+MgO versus SiO2(wt%)variation diagram showing the boundaries between ferroan and magnesian series(afterFrost et al.2001) and tholeiitic and calc-alkaline series(after Miyashiro 1974),the samples depict magnesian and calc-alkaline signatures (Fig.9d).

Fig.9 a Total alkali silica(TAS)plot displaying trachytic composition for the studied rocks of Lichi volcanics(after Le Bas et al.1986)b Al/(Na+K)versus Al/(Ca+Na+K)diagram showing peralkaline nature of the trachytes(after Chappell and White 1992)c K2O(wt%)versus SiO2 (wt%) diagram shows high K2O content for the trachytes (after Peccerillo and Taylor 1976) d FeOT/FeOT+MgO versus SiO2 (wt%)variation diagram showing the boundaries between ferroan and magnesian series (after Frost et al.2001) and tholeiitic and calc-alkaline series(after Miyashiro 1974).Data,marked in red and green,used for comparison are from the basalts and trachyandesites of the Lichi volcanic suite of rocks (after Sharma et al.2022)

The primitive mantle-normalized trace elemental profiles of the trachytes of the Lichi volcanic suite depict significantly depleted patterns for the high field strength elements (HFSE) and display enrichments in large ion lithophile elements (LILE) (Fig.10a).These volcanics exhibit consistent positive anomalies of Rb,Ba,K,Pb,and Zr and negative anomalies of Sr,Nb,and P.Depletion of Sr and P imply plagioclase and apatite separation during the process of fractional crystallization or presence of these residual phases in the source rocks.In the chondrite-normalized REE diagram,the studied rocks are characterized by an overall enriched of light rare earth elements (LREE)([La/Yb]N=7.6–16.9 ppm) and depleted heavy rare earth elements (HREE) trend ([Gd/Yb]N=1.67–3.12 ppm)(Fig.10b).

Fig.10 a Primitive mantle-normalized multi-element patterns b Chondrite-normalized REE plots for the representative trachytic samples.Normalizing values are after Sun and McDonough (1989).Data,marked in red and green,used for comparison are from the basalts and trachyandesites of the Lichi volcanic suite of rocks (after Sharma et al.2022)

6 Discussion

6.1 Apatite saturation thermometry

The element phosphorus has a crucial influence on the evolutionary processes of silicate melt systems (Mysen 1990).It controls the geochemical behaviour of important trace elements such as REEs,Th,U,Sr,and other elements through melt and accessory phosphate phase interactions(Exley and Smith 1982;Green and Watson 1982a;Yurimoto et al.1990).Apatite is a minor but ubiquitous,essential phosphorous-bearing phase in igneous rocks.The ability of magmatic apatite to host elements such as the rare earth elements has been well recognized in earlier literature (Nagasawa 1970;Henderson 1980;Wass et al.1980).This has inspired many experimental studies explaining the geochemical behaviour of apatite during the genesis of mafic,intermediate,and felsic magmas(Watson 1980;Green and Watson 1982a;Watson and Capobianco 1981).The experimental studies on the saturation behaviour of apatite in the 850–1000 °C temperature range by Harrison and Watson (1984) showed that the solubility of phosphorus is a function of silica content and temperature of the melt.Precipitation of apatite in meta-aluminous,peraluminous,and peralkaline silicate melts can be expressed as a function of SiO2and P2O5(Harrison and Watson 1984;Pichavant et al.1992).The apatite crystallisation temperature can be estimated from the relationship between SiO2and P2O5bulk concentrations in the initial melt(Piccoli and Candela 1994;Piccoli et al.1999).Harrison and Watson(1984)put forward a well-established model based on the solubility of apatite for rocks with 45–75 wt% SiO2and ＜10% H2O:

here Dpis the ratio of phosphorus concentration in apatite and melt;SiO2is the weight fraction of silica in the melt and TApis the temperature.In the present study,the estimated saturation temperature of apatite(TAp)in the apatitebearing trachytic rocks of the Lichi volcanic suite ranges from 962 to 1000 °C with an average of 988±14°C(1σ,n=8).

6.2 Magmatic evolution

Sharma et al.(2022) proposed that the mafic melt of Lichi volcanics potentially underwent evolution by fractionating two distinct crystallisation assemblages,plagioclase+clinopyroxene+magnetite and plagioclase+clinopyroxene+orthopyroxene from parent magma of same composition.This process led to a change in the mafic magma composition,ultimately resulting in its evolution into an intermediate magma,later forming trachyandesites.Enrichment of LREE compared to HREE in the studied samples can be attributed to the melting of an enriched LREE mantle source,crystal fractionation,or assimilation of the continental crust(Fig.10b;Moraes et al.2003).It is evident from the chondrite-normalized REE plot that the more evolved trachytic rock samples (with higher SiO2content) display strongly fractionated REE profiles characterised by relatively enriched LREE as compared to HREE.Minor minerals,such as apatite,titanite,zircon,and allanite (for intermediate composition)and monazite and xenotime(for acidic composition)play a major role in controlling the concentrations of some trace elements (e.g.,REEs,U,Th,Y,Zr,and Hf) during fractional crystallization(Dodge and Mays 1972;Condie 1978;Tindle and Pearce 1981).In this study,the fractionation of apatite and titanite (mainly apatite) may be responsible for the strongly fractionated REE trends of the evolved samples.

To understand magma evolution through element partitioning,bivariate plots of major oxides of the studied samples were plotted against SiO2(Fig.11).Observation from the series of binary plots shows moderate to welldefined linear relationships among the basalts,trachyandesites,and trachytes for most of the major oxides.With increasing SiO2,the elements MgO,Fe2O3,CaO,MnO,Al2O3,and P2O5all show negative trends indicating fractionation of olivine,pyroxene,amphibole,plagioclase,and apatite.On the other hand,TiO2and K2O display positive correlations for the studied samples.Furthermore,Na2O displays an increasing trend from basalt to trachyandesite and then its abundance decreases from trachyandesite to trachyte,suggesting the fractionation of sodic amphibole and albite as the magma possibly evolved from trachyandesitic to trachytic.The positive and negative correlations of the various oxides against SiO2indicate that the elements were not mobilised by any alteration processes.Harker plots suggest that the major oxides have not been affected by post-magmatic secondary processes.

Fig.11 Harker bivariate diagrams of a–i major elements versus SiO2.Data,marked in red and green,used for comparison are from the basalts and trachyandesites of the Lichi volcanic suite of rocks (after Sharma et al.2022)

6.3 Genetic type: A-type affinity of the Lichi trachytes

The terms I-and S-type granites by Chappel and White(1974) have paved the way for another distinct group of felsic rocks by Loiselle and Wones (1979) known as ‘‘Atype’’granitoids.These‘‘A-type’’granitic rocks have been substantially studied as they are compositionally and tectonically different(King et al.1997).This distinct group of granitoids was thought to be alkaline in nature and were considered to be products of anorogenic setting (Loiselle and Wones 1979).Moreover,it is also agreed upon that the A-type magmas form at high temperatures (＞900 °C)(Zhao et al.2008).Higher temperatures in the crust would only be plausible in extensional settings,hence,explaining the rare occurrences of A-type magmas(Zhao et al.2008).The origin of A-type magma has been described through four principal mechanisms.Mechanisms involving(i) fractional crystallisation of mafic mantle-derived magmas (Eby 1992),(ii) partial melting of crustal-derived igneous rocks(Patino Douce 1997),(iii)low degree partial melting of residual granites in the lower continental crust(Whalen et al.1987),and(iv)mixing of crustal and mantlederived magmas and ensuing fractional crystallisation(Yang et al.2006),explain the origin of A-type magma.Moreover,mafic rocks may also be accountable for the origin of alkaline A-type rocks (King et al.1997).The trachytic rocks of the study area exhibit high Na2O+K2O(10.68–11.97 wt%) and high crystallisation temperature(Taverage=988 °C) which are distinctive properties of A-type suites (Whalen et al.1987;Bonin 2007).Rocks belonging to the A-type group are characterised by ASI ＜1.1 and also belong to the alkaline and peralkaline series (Jiang et al.2006).The studied trachytes are peralkaline with ASI ranging from 0.65 to 0.75 (＜1.1) further inferring their A-type nature.The FeOT/MgO and K2-O+Na2O/CaO versus Zr+Nb+Ce+Y diagrams(Whalen et al.1987) distinguish the A-type granites from fractionated and orogenic (I,S,and M type) granites.Our samples plot in the field of A-type granitoids further highlighting the A-type nature of the studied trachytes(Fig.12a,b).

Fig.12 a,b FeOT/MgO versus Zr+Nb+Ce+Y and(K2O+Na2O)/CaO versus Zr+Nb+Ce+Y (Whalen et al.1987),binary discrimantion diagrams depicting the A-type natures of the trachytes.FG=fractionated granites (I and S type granite),OGT=orogenic granite types(M,I and S type) c,d Rb/Nb versus Y/Nb binary and Nb-Y-3Ga ternary plot categorising the trachytes as A1 type (Eby 1992)

Eby (1992) further classified the A-type granites into two chemical groups: A1and A2.The first subgroup (A1)represents magma derived from intraplate settings and is,hence,restricted to plumes,hotspots,or continental rifts.Whereas,the A2subgroup consists of magmas having similar chemical affinities to island arcs found in convergent settings.The Yb/Nb ratio is also an important chemical evaluator for A-type rocks.A-type suites with Y/Nb ＞1.2 are said to be derived from sources similar to island arcs or continental margin basalts (Eby 1990).On the other hand,those having Y/Nb ＜1.2 are suggested to have similar chemical sources to ocean island basalts(Eby 1990).The Y/Nb ratios of the Lichi trachytes range from 0.43 to 0.55 and,hence,signify their sources similar to ocean island basalts.Moreover,in the Rb/Nb versus Y/Nbdiagram (Eby 1990) with subdivisions of A1and A2,the studied samples plot in and around the A1field (Fig.12c).Nevertheless,some of the samples fall outside the A1field on the plot,due to an increase in the Rb content within these samples.This increase in Rb content may be attributed to the fractionated nature of the samples.The Y/Nb ratio in the continental crust is 2 or greater;therefore,any crustal interaction would move the ratios out of the A1field.This further suggests that the Lichi trachytes more likely represent differentiated products of mantle-derived mafic magmas similar to intraplate,rift zones,and ocean island magmas (Eby 1992).Similarly,in the Nb-Ce-3Ga triangular diagram (Eby 1992),the trachytes cluster in the field of A1,ruling out the possibility of the rocks being generated in a convergent margin setting (Fig.12d).

6.4 Geotectonic framework

Trace elemental abundances in volcanic rocks can be used to understand the nature of their tectonic environment(Pearce and Cann 1973).An increase in the abundance of more incompatible elements as compared to less incompatible elements,negative anomalies for Nb and P,and characteristic enrichment of LREE relative to HREE supports the emplacement of the studied trachytes in continental rift regime(Weaver and Tarney 1984;Ahmad and Tarney 1991).Previous literature suggested that the Lichi volcanic suite of rocks (basalts and trachyandesites) originated in a continental rift environment (Sharma et al.2022).The high Nb/U (average=20.79) and Ce/Pb (average=10.16) ratios characterising the trachytes signify their affinity to the mantle-derived OIB as compared to those of continental crust ratios (for OIB: Nb/U=47,Ce/Pb=25 ± 5,Hoffman 2003;for continental crust: Nb/U ＜9.7,Ce/Pb ＜5,Rudnick and Gao 2003).Present-day N-MORB,E-MORB,and OIB form a diagonal array at the centre of Th/Yb versus Nb/Yb diagram after Pearce(2008).Alkaline ocean island basalts have higher Nb/Yb compared to tholeiitic ocean island basalts,and plot at the high Nb/Yb end of the array.Almost all the basalt and trachyandesite samples plot within the MORB-OIB array and cluster near the OIB composition showing higher Nb/Yb ratios similar to alkaline ocean island basalts.The studied trachytes together with the basalts and trachyandesites display a distinct within-plate enrichment trend (Fig.13;Pearce 2008).The basaltic-trachyandesitic-trachytic rocks of the Lichi volcanic suite follow the trend parallel to the fractional crystallisation vector.This further verifies the prominent role played by fractional crystallisation processes in the possible derivation of the trachyandesitic and trachytic magmas from parental basaltic magmas.Moreover,the Y versus Nb(Pearce et al.1984)and the Y+Nb versus Rb diagrams (Pearce and Gale 1977),wherein the samples plot exclusively in the‘within plate’field are used(Fig.14a,b).This orientation for most of the trachytes primarily positioned in the within plate field further supports the emplacement of the Lichi trachytes in a continental rift regime.

Fig.14 Tectonic discrimination diagrams for the Lichi trachytes a Y versus Nb diagram(after Pearce et al.1984)b Y+Nb versus Rb diagram(Pearce and Gale 1977) showing signatures of within plate tectonic setting

6.5 Geodynamic implications

Geochemical and geodynamic studies have revealed that mantle plumes are upwelling of silicate material which plays a significant role in material transfer through the Earth’s interior (Griffiths and Campbell 1990;Maruyama et al.2007).The core-mantle boundary or the mantletransition zone is regarded as the boundary layer from where the mantle plumes originate (Morgan 1971;Zhang et al.2020).It has been considered that mantle plume activity plays an important role in causing continental breakup,extensive magmatism,and lithospheric thinning(Ernst and Buchan 2003).The breakup of eastern Gondwana,which consisted of today’s Australia,India,and Antarctica,was believed to be the result of impingement of the Kerguelen plume activity (Srivastava et al.2005;Zhu et al.2007).Numerous mafic and felsic magmatic rocks representing the magmatic response to the activity of the Kerguelen plume were formed in India,the Antarctic,Australia,and the Indian Ocean,specifically during changes in plate motion in early Cretaceous (ca.137–136 Ma;Gibbons et al.2013).The Early Cretaceous Comei-Bunbury large igneous province (136 to 130 Ma) crops out in southeastern Tibet and southwestern Australia.Geochemical studies indicated that the Comei-Bunbury province had originated from an OIB-type mantle source,due to interaction between the northeastern margin of Greater India and the Kerguelen hotspot (Kent et al.1992;Zhu et al.2009).Moreover,source components for the Rajmahal-Sylhet-Shillong-Mikir flood basalts from eastern and northeastern India and Kerguelen plume lavas are considered the same (Kent et al.2002;Ghatak and Basu 2011).

The Abor volcanic suite of rocks is exposed in the northeastern margin of the greater India landmass(Fig.1b).These volcanics are a result of multiple phases of magmatic activities occurring over a period of time possibly due to mantle agitation caused in response to the breakup of Gondwana (Singh 2014).Recorded field evidence,geochemical characteristics,and geochronological evidence suggest that the Abor volcanics exposed in the Siang window of the Arunachal Himalaya originated from the same source of Comei-Bunbury igneous province at ca.132 Ma (Singh et al.2020).This further suggests that the Kerguelen plume activity was responsible for the generation of the Abor volcanic suite of rocks.The Lichi volcanics found within Gondwana sediments are exposed to the west of Abor volcanics in the Papumpare district of Arunachal Pradesh in the Eastern Himalaya.Geochemical attributes of the basalts and trachyandesites of the Lichi volcanics depict that these rocks were emplaced in a continental rift environment.Extensive studies and comparative geochemical analyses conducted on the Lichi volcanic suite of rocks depict their similarity with the Abor volcanics.This,therefore,suggests a co-magmatic linkage between the rocks and further points out that the Lichi volcanics are coeval with the early Cretaceous plume activity products mentioned above (Sharma et al.2022).

In the present study,we investigated some newly reported trachytes from the Lichi volcanic suite.Geochemical analyses suggest that the trachytic lavas were generated in a continental rift extensional regime.These trachytes display A-type affinities which further fall into the A1category.Such a pattern represents fractional crystallisation components of mantle-derived OIBs associated with intraplate or continental rift settings.Mafic magma underplating in the lower crustal levels and further fractionation of the basaltic magma could be the plausible explanation for the generation of the trachytic magma.The mafic material from the plume accumulated below the northeastern edge of the Indian Plate and formed the basaltic rocks of the Lichi volcanics on the surface.Subsequently,some amount of basic magma underwent fractional crystallisation,which led to the formation of differentiated trachyandesitic and trachytic magmas of the Lichi volcanic suite.A schematic model representing the evolution of the Lichi volcanic suite of rocks and its equivalent Abor volcanics is shown in Fig.15.On drawing a comparison between the geochemical attributes shown by the trachytes,basalts,and trachyandesites of the Lichi volcanic suite,we envisage that the trachytic magmas were derived by fractional crystallisation of the plume-generated parental basaltic magma.Thus,our study supports the earlier report that the Kerguelen plume activity was responsible for the generation of the Lichi volcanic suite of rocks.

Fig.15 A schematic model illustrating the Kerguelen plume impinging on the Indian Plate and the evolution of Lichi volcanics and their connection with the Abor volcanics

7 Conclusions

(a) Our study represents the first inclusive investigation of the A-type trachytes from the Lichi volcanic suite of rocks exposed in the Eastern Himalayas.These alkaline rocks,with apatite saturation temperatures ranging from 962 to 1000 °C,were emplaced in an extensional continental rift setting.

(b) Considering the geochemical characteristics of the trachyandesites and trachytes,we suggest these intermediate-felsic rocks to be differentiated products of parental basaltic magmas.

(c) Entrapment of parental basaltic magma in shallow crustal levels to form magma ponds with the development of rifting events led to the differentiation processes forming trachyandesites and trachytes.

(d) From the geochemical signatures shown by the trachytes,basalts,and trachyandesites of the Lichi volcanic suite combined with the earlier published data of the Abor volcanics and Comei-Bunbury igneous province,we support that the Lichi volcanics were generated by Kerguelen mantle plume activities.

Supplementary InformationThe online version contains supplementary material available at https://doi.org/10.1007/s11631-023-00650-6.

AcknowledgementsThe last author acknowledges the DST-SERB grant vide Project No.CRG/2020/002635.The first author acknowledges the CSIR-JRF fellowship No.09/1236(11154)/2021-EMR-I.The second author acknowledges the DST-INSPIRE fellowship No.IF210186.The third author is grateful to the Director,of Wadia Institute of Himalayan Geology,for the necessary help and support.We would like to express our sincere gratitude to two anonymous reviewers for their insightful and constructive comments.We thank Dr.Binbin Wang for the editorial handling of the manuscript.The authors are grateful to Prof.N.V.Chalapathi Rao for EPMA analyses at DST-SERB National Facility,Department of Geology (Center of Advanced Study),Institute of Science,Banaras Hindu University.

Declarations

Conflict of interestThe authors declare that in this present work there is no conflict of interests.