1. Introduction

The Himalayan Mountain is the unique example of the youngest continent to continent collision tectonics. The boundary between the Indian and Eurasian plate is marked by the Indus-Tsangpo Suture zone. To the north of the suture zone, the trans- Himalayan batholith runs almost the entire length of Himalaya from NW, through the Nepal Himalaya and to the Myanmar to the east. The batholith from NW to NE is variously named as Kohistan batholith in Pakistan, Gangdese batholih in south Tibet and Lohit batholith in Arunachal Pradesh[22, 26, 53, 44]. These Trans- Himalayan batholiths of calc-alkaline nature are intrusives on the southern edge of the Eurasian plate in the immediate north of the ITSZ. The batholiths occur in a linear belt for about 2500km and 20-80km wide zone and represent an Andean type magmatism of the Tethyan oceanic crust under Eurasian plate during late Cretaceous to Lower Eocene time[19,47]. Compositional variety of these batholiths ranges from gabbro to granite although biotite and hornblende bearing granodiorite dominates in most part. The batholiths are I-type Cordilleran batholith and probably emplaced during 100-40 Ma, the dominant phase being intruded at 60Ma[21]. Nd, Sr and Pb isotopic compositions of Ladakh and Gangdese batholith indicate predominantly their mantle derivation and U-Pb ages vary between 105 -45 Ma for these batholiths[43]. The magma genesis, age relationships and emplacement history of Ladakh and Gangdese batholiths are widely discussed[3, 8, 9, 10, 38, 51]. However, the Lohit batholith of Arunachal Pradesh did not get much attention and only a few published data are available[46, 40, 26, 15, 16]. The Lohit batholith is located in the eastern part the Arunchal Pradesh at the eastern limb of the eastern syntaxis. The batholith is about 100km in width and has a NW-SE extension of about 250km[35] and extends from Tuting in the upper reaches of Siang river, through upstream segments of Dibang and Lohit river and extends further in the south -east direction (Fig. 1). In the south western part, the batholith is thrusted on the Tuting –Tidding suture zone, which is considered as the continuation of the Indus Tsangpo Suture Zone to the south east direction[47,13, 34, 2, 44, 15,17]. The batholith was studied initially by only a few workers because of the inhospitable terrain and poor communication facilities. Previous studies have thrown some light on the geology and geochemistry of the Lohit batholith[46, 47, 40, 15, 16]. This paper highlights the subduction related magmatism of the Tethyan oceanic crust under western edge of the Burmese plate on the basis of major and REE geochemistry in the Lohit batholith.

Outline geology of Lohit batholith

The eastern Himalayan syntaxis consists of three tectonic units: (1) the Himalayan part in the south west (ii) the Tuting-Tidding Suture Zone (TTSZ) and (iii) the magmatic part in the north east, named as the Lohit batholith (Fig.2). The Lohit batholith is thrusted over the TTSZ along the Lohit Thrust[35]. The TTSZ represent the neotethyan oceanic crust and encompasses various lithologies like actinolite-schist, chlorite–quartz-phyllite, graphite-phyllite, crystalline limestone, garnetiferous amphibolite and dykes and sills of serpentinite. The litho-package of the TTSZ shows NE dipping imbricate faulting and has a thrust contact with the Himalayan belt in the south west[42, 1, 16, 12]. The suture belt has a NW-SE extension and in the NW part, it is traceable as continuous belt in the upstream part of the Dibang river, however, it is again discontinuous in the further NW direction and reappears at Tuting, the upper reaches of the Siang river. The batholith is about 100km in thickness in the Lohit valley; however along the NW direction the thickness is reduced to about 70 km in the Dibang valley and further reduced along the Siang valley. An Andean type of calc -alkaline magmatism is represented by the batholith due to the subduction of the neo -tethyan oceanic crust beneath the Eurasian plate. The timing of the subduction, whether late cretaceous or early Eocene is yet to be constrained.

2. Petrography

Quartz diorites are volumetrically more significant in the Lohit batholith especially in the southwestern part. These are dominated by hornblende, plagioclase, quartz and biotite. The mafic constituents are more than 20 percent of the bulk rock. The quartz diorites show subhedral granular texture with sphene, apatite, zircon and iron oxides occurring as accessory phases. Hornblende dominated varieties are hornblende- biotite diorites. Alteration of hornblende to biotite is observed. The K-feldspar poor varieties grade in to tonalite while the K-feldspar bearing varieties are granodiorites. Gabbro occurs as enclaves consisting of plagioclase and pyroxene phenocrysts. Trondhjemites occur as tabular bodies and composed of Na rich plagioclase, quartz and biotite. Towards the eastern part of Chingawanti, granodiorites are observed, which grade in to granites on the basis of model mineralogy. K-feldspars in these rocks are coarse and frequently show fracturing due to brittle deformation. Biotite, hornblende and opaques constitute the mafic phase. Eastern part of the batholith, both in Lohit and Dibang valleys, is dominated by leucogranites. In the Lohit valley near Yasong and in the Dibang valley in Dambwen, leucogranites are observed to be intruding the earlier quartz diorites. Leucogranites are mainly composed of quartz, K-feldspar, plagioclase, biotite, muscovite, epidote and garnet. Both varieties of K-feldspar are found in leucogranite, however, orthoclase is dominant compared to plagioclase. Mafic minerals constitute <5% of the bulk rock in the leucogranites. The sillimanite schist of the Yasong area in the Lohit valley should represent the host rock lithology. The prismatic sillimanite and biotite flakes define the foliation. The foliation occasionally wraps around garnets. In some leucogranites, quartz appears as equent strain free grains. Among the acid intrusives, dacite and rhyolite dominates, consisting k-feldspar, plagioclase and quartz phenocrysts.

Figure 1. Geological map of Arunachal Pradesh modified from Singh and Chowdhury (1990)[42]. 1. Siwalik Belt 2. Abor volcanics 3.Gondwana belt 4.Dedga Menga Belt. 5. Bomdila Belt 6.Sela Belt 7.Tuting –Tidding Belt 8. leucogranite 9.Mishmi belt 10. Fault 11. Thrust. The extent of the investigation in the two river sections are bordered

Figure 2. Geological map of area (modified after Mishra, 2009[33] ; Gururajan and Chowdhury, 2007)[15]. The investigation is carried out in the Lohit and Dibang valleys beyond Tiding Thrust. Sample locations of rock types are shown for Lohit (starts with L) and Dibang valley (starts with D) as per the Tables 1a-1d

Figure 3. K2O versus SiO2 binary diagram of Lohit batholith using the classification scheme of Peccerillo and Taylor (1976)[37]

3. Geochemistry

3.1. Sampling

Samples of gabbro, quartz diorite, trondhjemite and leucogranites are collected from both of the Lohit and Dibang valleys. In the Lohit valley, starting from Payapani up to Lautool, both gabbro and quartz diorites are collected while trondhjemites are collected from Chingawanti. Quartz diorites are exposed even beyond Walong up to Tilam (Fig.2). Leucogranites are collected 1km north east of Samdul to 1.5km northeast of Yasong. In the Dibang valley, gabbro and quartz diorites are collected from Angolian up to Anini while quartz diorites are exposed up to Dambwen and Mipi and samples are collected from different locations along these two sections (Fig.2). Trondhjemites are collected from Angolian. Leucogranites are exposed from 2km SSW of Dambwen along the Dri river valley and samples are collected along this route up to Dambwen (Fig.2).

3.2. Analytical Techniques

About 50 fresh unweathered samples are ground to make fine powder in a tungsten ball mill grinder. These samples are analysed for major, trace and rare earth elements (REE) at Wadia institute of Himalayan Geology, Dehradun, India. Geochemical analyses were performed using standard XRF techniques, analysed by wavelength dispersive XRF system (Siemens SRS 3000). The calibration of the XRF system was done by matrix correction based on intensities[28]. International standards used are JG1 and JG 2 and quality of the analysis was monitored. Analytical accuracy on XRF is better than 5% for major oxides and 12% for trace elements and precession on maximum observed standard deviation is better than 2%[41]. The set of 50 samples also analysed for REE using ICP-MS (Perkins –Elmer SCIEX) with 0.6 g of the sample following a procedure of acid digestion that involved repeated treatment with HF-HCLO₄ in the ratio 2:1. Dissolved samples in the acid mixture underwent two or three treatments until a clean solution is obtained. The residue is than dissolved in 1N HNO₃ and 100ml solution is than ready for analysis. The relative standard deviation (RSD) for most REE analysed on ICP-MS is better than 10%[24]. Geochemical values for major trace and REE for each individual rock type are given in Tables 1a-d.

3.3. Major Element Geochemistry

Figure 4. A/CNK versus A/NK plots of Lohit garnitoids (Shand 1927)[39]

Geochemical data for the different rock types of Lohit batholith are limited[42,16].Although the Lohit valley is studied in the previous studies, the Dibang valley, specifically its upstream section is not well covered. In the present study both the Lohit and Dibang valleys are exhaustively covered. Samples of gabbro, quartz diorites, trondhjemites, leucogranites, dacites and aplites were selected for geochemical analysis. However, considering the limited occurrences of dacites and aplites, most of the geochemical characterization is based on the other major rock types of the area.

Gabbro, quartz diorites, trondhjemites and leucogranites form the bulk of the composition of the batholith with SiO₂ content showing a considerable range (~40-80 wt%). All samples belong to low to medim K calc-alkaline rocks on a classification diagram[37] (Fig.3). However, a few samples of leucogranite also plot in the high –K alkaline field. In the ASI discrimination diagram the granitoid plot dominantly in the metaluminous field, where as some leucogranites also plot on the I-S line (Fig.4).

The A/CNK (molar ratio of Al₂O₃/ (CaO+Na₂O+K₂O) varies from 0.42 to 1.08. Almost all the samples contain biotite and hornblende and therefore belong to I-type granitic rocks[7]. Major element composition of different rock types are characterized by variable amounts of SiO₂, low to medium contents of K₂O, high content of Al₂O₃ and high Na₂O/K₂O ratios. Plotting together, their TiO₂, MgO, CaO, Al₂O₃, MnO, Fe₂O₃ and P₂O₃ decreases while K₂O and Na₂O increase with increasing silica content. In the total alkali silica diagram (TAS) the composition of the granitoid range from gabbro to granite with dominance in the diorite field (Fig.5). Major element in trondhjemites however gives a different concentration pattern. With the SiO₂ wt% varying from ~64 to 75 wt% , Al₂O₃ is varying within a narrow range of ~15 to 18wt%. These rocks are medium K in nature and Na₂O> 3.30 wt% and Na₂O/ K₂O >2 except sample L59. These medium K rocks are weakly peraluminous with A/CNK value close to 1 and greater than 1 in case of L59. In all of these samples MgO is less than 1. Granites of Yasong area are characterized by very high SiO₂ (65.25-80.12 wt%). Compared to gabbro, quartz diorites and trondhjemites, K₂O value in these granites is high which increase with increasing silica content, whereas MgO, CaO and TiO₂ continually decrease indicate they are weakly peraluminous. The plots are straddling the line dividing I-type and S-type granites.

3.4. Trace Element Geochemistry

The gabbro enclaves show lower concentration of lithophile elements (Rb, Ba, Th, Sr) and REE.

Transitional elements like Ni is higher in concentration while there is a significant depletion in Nb. Incompatiable elements like Ba and Th show slight enrichment and depletion of Rb is significant (Table2a-d). The (La/Yb)_N ratio varies between 0.61 to 8.48 and no significant Eu anomaly is observed[25]. Nb and Zr depletion is evident in the sample versus primitive mantle spidergram (Fig.6a) where as Ta and Hf contents are also low in gabbro (ESM Table 1A).

Figure 5. TAS classification diagram of the granitoids using the classification scheme of (TAS Middlemost Plot)[31]

Figure 6. a-d Mantle normalized spidergram for (a) Gabbro (b) quartz diorite (c) leucogranite (d) trondhjemite

Figure 7. a-d.Chondrite normalized spidergram for (a) gabbro (b) quartz diorite (c) trondhjemite (d) leucogranite

Figure 8. Chondrite normalized La/Yb ratios vs.Yb for Lohit granitoids. Fields of (A) adakite and TTG and (B) calc- alkaline lavas are from Martin (1999)[29]. Symbols as in Figs.3-5

The trace element composition of quartz diorites are characterized by high concentration of Sr and low concentration of Yb (Table 2a-d). In the primitive mantle multi element spidergram there is an increase of Nb, Ba, Rb and Sr in the quartz diorites corresponding to increase in silica (Fig.6a-d). Decrease of Nb with increase in silica and higher concentration of Sr in quartz diorites compared to gabbro is also observed (Figs.6b and 7b). Increase in Ba, Pb, Sr and depletion of Th, Nb and Zr are also observed in the spidergrams. Hf and Ta depletion in quartz diorites compared to gabbro is also observed[48]( (ESM Table). Trondhjemites show a different fractionation pattern and are characterized by high (La/Yb)N ratios (~36-4.61) and low Primitive mantle normalized spidergram indicates that large ion lithophile elements like Rb, Ba, Sr and U and Th are higher in these rocks. Low values of Nb and Zr are evident from the spidergram. The (La/Yb)N (0.92-49.79) with two sample showing exceptionally high values 122.08 and 219.16 also supports the fact that leucogranites may be derived from the crustal melts. Heavy REE (HREE e.g. Yb varies between 0.16 and 0.19) and are therefore remarkably different from other cal alkaline rocks of the area (Figs.6d an7d). The high Sr content (583 – 2634.82ppm) and low Yb (0.16 -1.94 ppm) and low Eu anomaly point towards their geochemical characteristics similar to adakites[19, 23].

3.5. Sr Isotopes

Whole rock Rb _(ppm), Sr _(ppm) , ⁸⁷Rb / ⁸⁶Sr _(atomic) and ⁸⁷Sr/⁸⁶Sr(atomic ) are presented for eight sample of the Lohit batholith in the Table-2 .These analysis were carried out at Institute Instrumentation centre, IIT Roorkee, India using techniques described by[48, 49]. Present day ⁸⁷Sr/⁸⁶Sr shown in the Table.2 indicates that the Sr isotopic ratio in the quartz diorites of the south western part (0.703876±0.0003 sample L45) is increasing considerably in leucogranites (0.714987 ± 0.0001 sample L69). This increase in Sr isotopic ratio can be well correlated with the continual increase in LILE and decrease of HREE and HFSE in the granitoids of Lohit batholith from southwestern to north eastern part.

Figure 9. Ce/Pb versus Pb diagram. The trend showing continuous increase of Pb in OIB, E-MORB and increase in Ce in the granite is superimposed from Gursu, 2008[17]. Symbols as in Figs.3-5

Figure 10. Nb/La versus Ba/Rb diagram illustrating the effects of crustal contamination and subduction metasomatism during the evolution of Lohit batholiths (EMM: Averaged Enriched mantl melts Yang et al 2004[52]; Altunkayanak et al 2010[5]; Kula basalts: Dilek and Altunkayanak (2010)[11]. Symbols as in Figs.3-5

4. Discussion

4.1. REE Patterns

In general various intrusive phases in the Lohit batholith show a gradual LILE enrichment and HREE depletion with the increasing silica content. Gradual enrichment of Ba, Sr, Pb and low contents of Yb, Zr and Nb can be studied from the spidergrams (Figs.6a-d and 7a-d). La/Yb ratio increases from early intrusives of gabbro and quartz diorites to the leucgranites. The depletion of Nb, Zr, Hf and Ta in the mafic enclaves and host quartz diorites indicate that the melt was derived from enriched lithospheric mantle contaminated by ancient crustal material[52, 54]. On the other hand, negative anomalies are shown by Ba, Sr and Nb in leucogranites, which have high values of Rb, Th and La (Fig.6.c). These features are compatible to (i) crustally derived melts (ii) partial melting of acid to intermediate igneous rocks or (iii) melting of immature type of sediments for the formation of leucogranites[50]. Trondhjemites are characterized by high (La/Yb)_N (4.61 to 35.55) (Yb)_N (0.94 to11.43). These characteristics place them in the adakite or TTG fields (Fig.8).

Considering the low Ce/Pb ratio[32] (Fig.9), Pb enrichment in the granitoids may be derived from the oceanic crust by fluids adding to the mantle wedge before melting[30]. This mantle Pb may be transferred to the arc magma source before its melting. The fig. shows the plots of this study which can be compared with the Ce/Pb ratios in the Oceanic basalts and Continental crust[31]. Pb enrichment is therefore similar to the enrichment of Rb or U despite being the fact that Pb is moderately compatible in comparison to Rb and U. On the other hand Ce/Pb –Pb variations indicate that Pb may also be added by magmatic differentiation of intracrustal meting of a TT source that is in the garnet stability field under the mantle wedge[14]. This ratio also supports a two component mixing process between the magma derived from a moderately enriched mantle source and a sedimentary component[18].

Trace element patterns of the analysed samples are comparable to middle-upper continental crust which might have been inherited from variable magma sources. The Nb/La vs Ba/Rb plot as observed from the samples of Lohit batholith is represented in the Fig.10 The vertical trend between the mantle derived melts and continental crust (Kula Basalt and EMM) suggest assimilation and fractional crystalisation (AFC) of the mantle derived magma rather than sole influence of metasomatism of the mantle wedge by the subduction generated fluids[4,5,45].

Table 1. a-d Major, Trace and REE contents for gabbro, quartz diorite, trondhjemite and leucogranite

Table 2. Rb, Sr concentrations and 87Sr/86Sr ratios of selected samples of Lohit batholith

4.2. Magma Source

Geochemical details provide evidences for magma sources for three distinct suit of rocks present in the area: (i) gabbro-quartz diorites (ii) trondhjemites and (iii) leocgranites. In all three suits crustal contamination in subduction environment is depicted by the Nb/La – Ba/Rb plots (Fig.10)[4]. Trace element ratios more specifically the HFSE (e.g. Nb, Ta, Ti) are highly sensitive to varying degrees of partial melting of the source. For subalkaline rocks of the area, the degree of partial melting is expected to be high. Therefore, ratios of the incompatible elements may reflect the source characteristics[3]. However, compared to degree of partial melting in intraoceanic arcs on thin lithosphere, the partial melting is lowest in continental arcs on thick lithosphere[36]. Under this condition, melting of the mantle wedge is triggered by addition of the fluids from the subducting slab, where depletion of the mantle wedge post dates the fluid addition. An interesting implication emerging from the study indicate that the residual garnet in the anatectic events can give rise to appropriate high/ heavy REE and HFSE/ REE fractionation. This is evident as there is no strong Nb depletion in these rocks (Figs.6a-d)[5]. Replenishment of the mantle wedge takes place by flux of the LIL enriched materials from the subducting slab[27]. The gabbro-quartz diorite suit is characterized by LREE enrichment, high content of Al₂O₃, Ba and Sr and low contents of Yb. These features suggestive of a source with residual garnet, amphibole or pyroxene and little olivine and plagioclase (i.e. no Eu anomaly). Partial melting of an enriched source melts with high values of Sr and Ba and LREE especially when the degree of partial melting is low[11,23]. On primitive mantle normalized diagram gabbro and quartz diorites display strong Nb and Zr depletion indicating the fact that partial melting is contaminated by contributions of ancient crustal material combined with low SiO₂ contents (40.34 to 51.96 wt%) and low ⁸⁷ Sr/⁸⁶Sr ratios (0.703). This indicates that these rocks should be derived from a metasomatic lithospheric mantle source[53]. However, in quartz diorites, given the fact that SiO₂ and MgO are negatively correlated, fractional crystalisation of the mantle derived basaltic magma might have taken place. This magma is assimilated at different proportions by lower crustal material. Therefore, gabbro enclaves within quartz diorites should represent the original basaltic parent magma with low initial ⁸⁷Sr/⁸⁶Sr ratios. There is a systematic depletion of TiO₂, Fe₂O₃, MgO and CaO from gabbro to quartz diorites with increasing SiO₂ (Table 1.a-d). This indicates hornblende fractionation. With the strong positive Sr anomalies and insignificant Eu anomalies plagioclase fractionation became insignificant when differentiation progressed to an advanced stage[53]. With higher values of Al₂O₃, Na₂O, P₂O₅ and Rb contents and enriched LREE and HREE, leucogranites are characterized as forming from crustal melts[10]. Leucogranites are also chracterised by high ⁸⁷Sr/ ⁸⁶ Sr ratio (0.714987). Although in the previous model[16], the water pressure condition for occurrence of tourmaline bearing leucogranite was emphasized, in the present study, it is found that the leucogranites in the Yasong and Dambuin area (northeastern part of the batholith) are biotite leucogranites rather than tourmaline leucogranites. This negates the high water pressure condition especially for the formation of the leucogranites. Trondhjemites are high in Zr/Sm and La/Yb (Table -1). This indicates garnet stabilization at the deeper part of the crust, which is responsible for the changes of the composition of the melt along with the high value of Si and gradual decrease of Fe, Mg and Yb values[20]. These melts may also be characterised by more sodic and more aluminous[5]. On the other hand, no negative Eu anomaly in trondjhemite indicate that plagioclase is almost nil in the solid residue to deplete the melt in Sr and Eu. Therefore garnet bearing source may account for the geochemical variability obtained for the batholiths.

5. Conclusions

The majority of the granitoids of Lohit batholith are metaluminous and only a few of them are peraluminous. Therefore these granitoids can well be characterized as I-type granitoids. Major and trace element geochemistry of these granitoids indicate both lithosperic and asthenospheric mantle components where the parental mafic magmatic melts evolve to granodioritic or leucogranitic composition specially towards the eastern part. This also indicate involvement of crustal derived melts in the eastern part. The crustal signature in these rocks suggests simultaneous assimilation of the upper to mid crustal rocks and the fractional crystallisation of the mantle derived meltstowards the thicker continental crust in the eastern part. Involvement of the crustal melt is also reflected in the isotopic characteristics with higher Sr isotopic values in the eastern part.

Mantle melting was initiated by fluid injection from the subducting mantle and as a result the mantle wedge is replenished. Fractional crystalisation of the mantle derived magma assimilated with the lower crustal material provided the source for the gabbro-quartz diorite sequence of the Lohit batholith. The early melt through the thin wedge could not fractionate much and therefore the Sr isotopic ratio in the gabbro -quartz -diorite sequence is very low (0.703), which represents very lowly fractionated mantle composition. In the eastern part of the batholith, the temporal variation in the magma genesis is reflected in the chemical characteristics and in the Sr isotopic ratio. The more fractionated and evolved leucogranite of the eastern part shows a rapid increase in the Sr isotopic initial ratio of 0.714. Higher Sr isotopic and La /Yb ratio, higher value of P and decrease in Nb, Zr, Ta and Yb in the eastern part indicate that the younger phases are derived from a depleted mantle later enriched by the process of metasomatism.

ACKNOWLEDGEMENTS

The author is grateful to Department of Science and Technology, New Delhi, for financial assistance (Project No.ESS/16/242/2005/SIANG-LOHIT/06). The author acknowledges the help received from IIT Roorkee Isotope Geology Lab and analytical lab of Wadia Institute of Himalayan Geology, Dehradun. The author is grateful to anonymous reviewer for the comments, which helped improvement of the manuscript considerably.