Geochemistry of Volcanic Rocks from the Naga Hills Ophiolites, Northeast India and Their Inferred Tectonic Setting

Journal of he Geological Society, London, Vol. 146, 1989, pp. 491-498, 6 figs, 2 tables. Printed in Northern Ireland

Geochemistry of volcanic rocks from the Naga Hills Ophiolites, northeast India and their inferred tectonic setting

S. SENGUPTA', S. K. ACHARYYA',H. J. VANDEN HUL2 & B. CHATTOPADHYAY' Geological Survey of India, Calcutta-700 016, India 'International Institute for Aerospace Survey and Earth Science, Delft, Netherlands

Abstract: Highly disrupted and deformed slices of ophiolitic rocks occurring along a linear belt in Nagalandand Manipur states, NE India,are knownas the NagaHills Ophiolites (NHO). The principal rock types include dunite, harzburgite, lherzolite, wehrlite, pyroxenite and mafic volcanics. The volcanic rocks of NHO have been classified into low-Ti and high-Ti groups with 2 wt.% TiO, as the boundary, which is corroborated by the differences in their immobile trace element and Rare Earth element characteristics. The two groups are probably not cogenetic; their chemical differences reflect variable source composition. Within group variation of incompatible elements such as La/Ta in the high-Ti group may be related to varying degrees of partial melting. The Ti/V ratios and REE distribution pattern of low-Ti group show overlapping Mid-Ocean Ridge Basalt and islandarc-like characteristics, and possibly suggest back-arc basin setting. The high-Ti group, on the other hand, has similarities with within-plate basalts erupted at off-axis sea-mounts and ocean islands. The sea-mounts and ocean islands brought to the subduction zone acted as barriers. Within the imbricate fault slices are components from the oceanic crust which were accreted to the continental margin and which represent clipped off portions from these barriers.

A linear belt of ultramafic and mafic rocks associated with associationwith oceanic pelagic sediments (Agrawal & marine pelagic sediments is exposed in north-eastern India Kacker 1980; Acharyya et al. 1984; GSZ 1986). The pile of along the eastern margin of the Indo-Burmese Range (Fig. ophiolite slices is underlain by argillaceous and flyschoid 1 inset). Geologically, this belt is located at the junction of sediments(Disang Formation) which are exposed tothe theTethyan ophiolite belt of theHimalayas and the west. These have been tentatively dated as Eocene and are ophioliteoccurrences along the Indonesian arc. But poor conformablyoverlain by sandyflyschoid to molassic accessibility, ruggedtopography and thicktropical forest sediments(Barail Group) of Oligoceneage. Part of the hinderedsystematic geological investigation in this terrain Disangsediments immediately bordering the ophiolites is until recently.Recognition of the similarity of the rock olistostromal in facieswith blocks of ophioliticrocks and assemblages of this belt with the ophiolitic assemblage (AGI limestone of MaastrichtianandEocene ages. This 1972)has evoked interest in themsince the late seventies Eocene-Oligocenesequence has been deposited onthe (Sen & Chattopadhyay 1978). It is now known as the Naga distal Indian continental shelf. Hills Ophiolite (NHO). Recent geologicalinvestigation by Tothe east, the composite zone of ophiolite slices is the Geological Survey of India has led to thesynthesis of the separatedalong tectonic a contact from continental regional framework of the NHO belt (Acharyya et al. 1984; metamorphicrocks and weakly metamorphosed middle GSI 1986). Cretaceouscover comprising theNaga Metamorphic It is recognized that oceanic crust preserved as ophiolites Complex and Nimi Formation respectively. They constitute might have been produced in diverse tectonic settings (Coish parts of theoverriding Burmese block. A few klippe of & Church 1979; Coleman 1981; Gass 1982; Schmincke et al. Naga Megamorphics overlie the ophiolite slices and klippe 1983). These originaltectonic settings may be identified of both occasionally overlie the Disang Formation. from the geochemicalcharacters of basicvolcanic rocks. Volcanicrocks of theophiolite suite are closely Only scanty geochemical data are presently available for the associated andinterbedded withradiolarian chert and constituentrock assemblages of theNHO, and new occasional limestone. However, the volume of sediments is geochemical data on the comparatively fresh volcanic rocks small. The coccolith and foraminiferal assemblage recovered of NHO are presented here. The purpose of the study is to fromthe sedimentary rockssuggest Maastrichtian to describethe compositional features of the volcanicrocks Palaeocene age and a shallower than carbonate compensa- and to infer theiroriginal tectonic setting on the basis of the tion depth (Acharyya et al. 1984, 1986; Acharyya & Ghosh relatively immobile elements. 1986). Inaddition, there are some radiolarian cherts with exclusiveradiolaria assemblage indicating deeper marine conditions.These cannotbe precisely datedbut may be Geological setting of NHO broadly Cretaceous in age. The dismembered ophiolites are The NHO consists of a highly dismembered composite zone non-conformablyoverlain by shallow-marine to paralic, of various rocks occumng as tectonic slices. Principal rock immature,ophiolite derived sediments (Phokphur Forma- typesinclude dunite, harzburgite Iherzolite, wehrlite, tion) of Middle Eocene age. pyroxenite and mafic volcanicrocks. Theseoccur in At least two set of folds are visible inthis highly 49 1

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r l II

Fig. 1. Geological map of northern Dlsang Fm.RShelf scdimentr (E) part of the Naga Hills Ophiolite belt. Inset shows locationof the belt. Legend: E, Eocene; K,, K, Early and Late Cretaceous;Pt, Protero- Maficvolconics ( KZ ) zoic. Legend inset:X, Precambrian (Indian shield);V, continental vol- lccccc] Cumulates canics; dots, Tertiary belt; heavy m Serpentinisadultromofics broken line, Late Mesozoic- Unconformablecover Palaeogene Ophiolite belts with rocks on ophiolite(h4ld.E; associated sediments;ITO, Indus

NlmiFormation ( K,) Tsangpo Ophiolite; saw-tooth line, thrust beltof schuppen in Naga m]Naqo m*tomorphics (pt) foothills and Main boundary thrust in Himalayas. tectonized zone and the underlying distal shelf sediments. characterized by the preservation of primary minerals and textures. The earlier of the two is aset of mesoscopic, isoclinal Some are aphanitic with fine microlites of plagioclase. The more overturned folds. Viewed fromsouth to north they are crystallinespecimens areplagioclase-phyric with plagioclase, sinistral in shape and generally non-plunging, butoccasional pyroxene and glass constituting the groundmass. Alteration of glass gentle plunges are recorded either towards NNE or towards iscommon in all the samples. One sample (no. 19) contains SSW. Thelater folds have open, upright geometry. Axial prismatic crystals of glaucophane. traces of these folds can be traced over long distance (Fig. Majorelements were determined by X-Rayfluorescence 1). These are co-axial with earlier folds and are, therefore, spectrometry using a lithium tetraborate fusion technique, and the generallynon-plunging with occasional gentle plunge trace elements Cr, Ni, CO, V, Ba, Rb and Sr by atomic absorption culminations and depressions. They are observed to refold spectrometryin the Mining Department, Delft University of Technology. The remaining trace elements including the Rare Earth the mesoscopic folds and produce hook-shaped interference elements (REE) havebeen determined by Instrumental Neutron structures. Thelarge scale folds have affected the Activationat Inter-University Reactor Institute, Delft. Interna- Eocene-Oligocene shelf sediments, the overriding dismem- tionalbasalt standard BHVO-1 was analysed with the samples. bered ophiolite units, the Middle Eocene ophiolite-derived Analyticalresults for thetwo groups of basaltsare presented in cover rocks and also the Naga Metamorphics. Tables 1 and 2. The full set of analytical data will be stored in the Because of the complexly deformed nature, itis difficult, UK-IGBA data file from which it can be retrieved via the National even in close traverses, to correlate the different ophiolite GeochemicalData Bank of theBritish Geological Survey or the slices. The geological map for the Nagaland part of the belt World Data Center-A, Colorado, USA. (Fig. 1) therefore lacks many essential details and only the major structures and contacts are shown. Major elements Analysis and results Major element abundances are generally similar throughout thesuite, though certain minor differences exist. TiOZ, Sampling and analytical techniques however,shows a significant variation.Based on TiO, Volcanic rocks of this ophiolite assemblage show varying degreesof content the samples can be classified into two groups. Data alterationand metamorphism. Samples selected for analysis are for the low-Ti group (<2 wt.% TiO,) are presented in Table

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Table 1. Chemical composition of volcanic rocks from NHO: low-Ti group. Major elements in weight %; trace element in ppm

~ ~~ ~ 8 9 7 4 2 5 6 3 6 5 2 4 No. 7 Sample 9 1 8 10

SiO, 51.10 48.50 46.70 48.10 48.00 47.40 48.40 43.80 47.80 47.30 TiO, 1.84 1.16 1.06 1.22 1.50 1.32 0.69 1.41 0.81 1.11 AI203 16.66 15.10 16.90 15.70 15.00 15.10 16.40 12.60 15.60 11.00 Fe203 12.25 10.15 11.48 12.44 10.43 10.28 9.86 10.00 8.86 11.88 MgO 3.75 4.70 6.40 7.20 7.00 7.78 8.40 10.60 9.60 13.60 CaO 3.76 10.50 5.50 9.30 9.50 12.55 11.70 10.00 11.90 9.10 Na,O 5.46 4.41 8.35 3.35 7.81 2.73 3.40 7.12 2.43 3.50 W 0.09 1.74 1.45 1.35 0.16 0.04 0.11 0.96 0.10 1.78 L01 4.74 3.10 2.10 1.24 1.OO 2.93 1.30 3.00 2.50 0.90 ~ - ~ ~ - - __ - - - Total 99.65 99.36 100.34 100.10 100.40 100.13 100.26 99.49 99.60 100.17

Sr 250 154 198 143 57 90 13 110 19 27 Rb 0.8 5 22 25 bd bd bd 1.5 bd 7 Ba 58 bd 215 na bd 70 bd bd bd bd Ta 0.26 bd bd bd bd bd 1.11 1.01 bd bd Zr 310 bd bd bd bd 120 bd bd bd bd Hf 2.50 2.98 bd 1.86 2.51 2.00 bd 3.26 2.40 2.41 sc 42.7 41.4 36.6 42.5 38.0 43.4 42.0 33.7 36.0 42.3 Cr 250 165 300 210 215 185 415 215 280 750 CO 77 70 68 55 60 52 65 65 45 55 Ni 108 120 80 235 180 60 200 215 240 650 V na 217 497 302 261 na 277 27 1 276 235 La 15.2 3.13 0.72 4.70 3.28 5.4 1.03 3.79 5.16 2.54 Ce 18.0 7.26 bd 13.5 bd 12.5 bd bd 11.1 bd Sm 9.82 2.65 1.06 3.17 3.58 3.38 1.52 3.54 2.47 2.5 Eu 2.90 1.31 0.61 1.31 1.53 1.30 0.70 1.23 1.02 0.91 Tb 2.50 0.66 bd bd 0.79 1.50 0.57 0.80 0.50 bd Yb 5.70 2.89 1.38 3.23 3.07 2.70 2.83 2.83 2.45 2.38 Lu 1.OO 0.48 0.22 0.55 0.54 0.50 0.37 0.43 0.39 0.45

LOI, Loss on ignition; bd, below detection limit; na, not analysed.

1, and for the high-Ti group (a2wt. % TiO,) in Table 2. bimodal distinctly (Fig. 2); moreover a general correlation Samples arearranged in order of increasingMg/(Mg + exists betweenTiO, abundance and (La + Sm + Yb) Fe2+) ratio in the Tables. content.This suggests that both Ti and REE remained SiO, andTiOz abundances in the low-Tivolcanics immobile and support a Ti-based two-fold classification of (43.8-51.1%,0.69-1.84%) are similar to averageocean the rocks. The Ti/V and Hf/La ratios of NHO samples also floor basalts. Mg/(Mg + Fe”) ratios ranging between 0.54 cluster into twodifferent ranges further corroborating the and 0.72 are also comparable to thoseof ocean floor basalts. two-fold classification. SiO, values (45.3-57.1%) andMg/(Mg + Fe2+)ratios (0.28-0.65) in the high-Ti rocks are similar but with a wider range. K,O values for the samples also have a wide range Trace elements and are mostly higher compared to that in MORB. Ni, Cr, CO and Sc values for both groups of NHO basalts There is no systematiccorrelation either between Ti are broadly comparable with ranges in ocean floor basalts abundance and Mg/(Mg + Fe2+) ratio in the NHO samples, (Basaltic Volcanism Study Project 1981, p. 148). Ni and Cr suggesting thatthe variation does not reflect degree of values for one sample from thelow-Ti group (sample 10) are fractionation, or between other major element abundances significantly higher than the rest and resemble abundances and the Mg index.This lack of inter-elementcorrelation in primitivebasaltic glass. Intwo samples of the high-Ti combined with high values for ‘loss on ignition’, generally group (samples 16 and 18), depleted Cr and Ni values are low Mg/(Mg + Fe2+) ratios and exceptionallyhigh values of accompanied by comparatively lower Mg/(Mg + Fe2+) ratio Na,Oin somesamples, areinterpreted to result from and higher Si02values. Such samples may represent evolved alteration and metamorphism. compositionsresulting from fractional crystallization. No It is generally agreed that Ti remains stable over a wide significant correlation exists betweenabundances of these range of conditions during sea-floor alteration and regional elements and the Mg/(Mg + FeZ+)ratio in other samples of metamorphism,and can be used to characterizetectonic NHO from either group. setting of ophiolitic volcanic rocks (Coish 1977; Coish et al. Certain differences in the trace element characteristicsof 1982; Shervais 1982). Certain other trace elements like V, the twogroup of rocks arebrought out clearly in the Hf, Zr, Th, Ta and the REE are also considered relatively Mid-OceanRidge Basalt (MORB) normalizedplots (Fig. immobileduring alterationand metamorphism (Shervais 3). The low-Ti samples show a flat normalized pattern for 1982; Humphris 1984). It is observedthat the TiO, vs. elements from Ce to Cr, typical of MORB (Fig. 3A). The (La + Sm + Yb)plot of NHO samplesshow distinctly high-Ti samplesshow maximum enrichment of elements

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Table 2. Chemical composition of volcanic rocks from NHO: high-Ti group. Major elements in weight %; trace element in ppm

~ ~~ Sample No. 18 16 14 13 19 17 11 12 15

SiO, 57.10 52.40 47.00 50.30 49.70 45.30 45.70 46.10 47.70 TiO, 2.45 2.11 3.63 2.42 2.29 2.37 2.54 2.18 2.02 AI203 10.48 14.35 15.35 13.03 13.41 16.17 14.01 13.71 13.74 Fe203 12.30 13.04 13.77 12.79 12.05 13.57 12.15 10.72 10.77 MgO 2.16 4.24 5.26 6.11 6.60 8.01 7.72 7.74 8.82 CaO 12.30 5.79 5.88 8.35 4.62 8.36 10.08 11.71 8.75 Na,O 1.32 5.73 4.97 4.73 3.04 1.64 3.04 3.71 3.05 KZ0 0.16 0.55 0.44 0.26 4.55 0.69 0.69 0.51 0.94 L01 1.45 2.00 3.96 2.24 4.38 3.86 3.98 4.52 4.48 ~ ~ ~ ~ ~ --~ - Total 99.72 100.21 100.26 100.23 100.64 99.97 99.91 100.10 100.27

Sr bd 190 430 260 50 bd 100 130 210 Rb 2.1 3.1 7.5 1.7 586.8 3.5 3.5 10.0 Ba 83 182 91 68 412 122 72 77 168 Th 1.8 3.3 5.1 1.1 1.3 3.2 1.2 3.4 3.6 Ta 1.2 1.73 3.85 0.67 0.82 bd 2.46 1.96 1.77 Zr 300 450 1390 160 210 500 330 280 570 Hf 3.8 3.4 6.7 3.5 2.5 4.5 3.4 3.6 3.8 sc 29.8 23.2 28.7 37.9 34.8 29.044.07 32.4 38.8 Cr 40 14 102 115 200 313 145 433 367 CO 51 50 73 61 6692 72 66 74 Ni 41 38 165 76 95 117 89 173 192 V 265 215 308 243 208 272 223 218 215 La 22.3 La 26.5 46.2 15.2 14.8 30.616.6 24.3 26.6 Ce 47 44 91 36 3064 35 51 53 Sm 5.83 5.19 9.31 5.58 4.85 7.456.40 6.09 5.99 Eu 2.26 1.93 3.00 2.21 1.60 2.602.57 2.00 2.18 Tb 1.5 1.1 1.9 1.7 2.5 1.2 1.3 1.4 1.7 Yb 2.51 2.00 3.10 2.70 1.94 2.403.30 1.30 2.00 Lu 1 Lu .o 0.3 0.4 0.5 0.5 0.5 0.3 0.5 0.3

La/Ta 18.58 15.32 12.00 22.69 18.0522.69 12.00 15.32 18.58 La/Ta 15.03- 12.40 12.44

L01 = Loss on ignition, bd = below detection limit

from Thto Zr. Three elements Hf, Sm andTi are also LREE depleted sample has the lowest total REE content, enriched compared to MORB, but degree of enrichment is but there is no straightforward relation between these two less than for elements from Th to Zr (Fig. 3B). Yb and Sc parameters. Samples for which Ce-values are available, have are either depleted or close to MORB. The pattern for this (La/Ce), ratios> 1. (Ce/Yb),ratios for these samples group is identical tothat observed in within-plate-basalts rangebetween 0.57 and 1.05 andthe Eu/Eu* values (WPB),particularly those with alkaline affinity (Pearce between 0.86 and 1.34. 1982). The high-Ti samples with higher total REE abundance showfractionated, consistently LREE-enriched patterns (Fig. 4B). The HREE abundance levels in the two groups Rare Earth elements (REE) arethe same. (Ce/Yb), ratios for these samples range REE data for NHO volcanics are presented as chondrite- between 2.41 and 8.92. There is again nostraightforward normalized curves (Fig. 4). Sample 1, which does not fit into relation between the degree of REE fractionation and total theTi-based classification scheme,has an irregular REE REEcontent. (La/Ce), ratios are greater than 1 and profile withgross REE enrichment (Fig. 4A) and such Eu/Eu* values vary between 0.89 and 1.13. features result fromalteration (Hellman et al. 1979). Two othersamples, 8 and 19, alsohave irregular REE profile. Sample 8 shows HREE enrichment,whereas sample 19, Discussion which is metamorphosed, has anomalously high Tb, Ba and The geochemical characters of NHO volcanics show that the K values. REE plots for the other samples show a regular abundance of Ti,certain trace element ratios and REE pattern and appear to beunaffected by secondary processes. characteristicshave remained stable during alteration and The total REE contents of the low-Ti rocks are several metamorphism in mostsamples, and therefore represent factors lower than those of the high-Ti samples. Normalized primaryigneous values. Based onthese parameters the plots for low-Ti samples (Fig. 4A) are flat withvarying NHO volcanic rocks have been classified into two distinct degrees of LREE depletion or slight enrichment. The most groups. The high-Titype rocks aremore abundant and

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10 ROCK MORB

Y n 40- > + I E v,+ 32- IY D J 24-

l6 - 10

ROCK 12 - IS MORB l1 I 17 13 14 l 1 1 I I I 0 0.8 I .6 2.4 3~2 4 I 11#1 I 0.11 I ' ' ' ' I Ti02 (wt%) Sr K Rb Ba Th Ta CO Zr Hf Sm Ti Yb Sc Cr Fig. 3. MORB normalized trace element patterns in the two Fig. 2. (La + Sm + Yb) vs. TiO, diagram. volcanic groups (normalizing values from Pearce 1982). (A) Low-Ti, (B) High-Ti.

appear to be exclusively developed in the central part of the Tectonic setting and petrogenesis NHO belt. The low-Tirocks occurs within themelange, close to the western and eastern boundary of the NHO. The Ti/V ratiois an useful discriminant for oceanic settings, The largedifferences in chemicalcharacteristics of andhas been used for certain ophiolitic rocks (Shervais samples with comparable Mg/(Mg + Fe2*) ratios from the 1982). In the low-Ti group, three samples have Ti/V ratio twogroups indicate that they cannot be produced by <20,acharacteristic feature of island-arc basalts. Five fractional crystallization from the same melt. This is further remaining samples have ratios ranging between 20 and 50 supported by comparable abundances of Cr, Ni, V and CO which are characteristic of MORB. Such a tendency to show in the two groups. overlappinggeochemical characteristics of MORBand Difference in TiO, concentration observed between the island-arc regimes may be diagnostic of a back-arc setting samplescan beproduced bydifferent degrees of partial (Saunders et al. 1980; Hawkins 1980). REE characteristic of melting from the same source (Church & Choish 1976). In the low-Ti rocks also show similar overlap. Some samples of this model the high-Ti rocks represent lower degree melting this group have LREE depleted flat patterns and (La/Sm), requiring garnet as a residual phase to deplete heavy REE ratios close to 'normal' MORB (Schilling 1971). Remaining (Saunders1984). The low-Tirocks may bemodelled as samplesshow lesser LREE depletionsimilar to island-arc higher degree melts with garnet entering the liquid phase, basalts(Basaltic Volcanism Study Project 1981). Typical thereby increasing the HREE abundance level in the melt. island-arc signatures characterized by depletion in Ce, Hf, However,although low-Ti rocks from NHO havea Sm, Ti, Sc compared to LIL elements (Pearce 1980, 1982), comparativelylower (Ce/Yb), ratiothan in the high-Ti together with low abundance of Ni and Cr (Taylor etal. rocks, the HREE abundance levels in the two groups are 1969) are, however, absent from the low-Ti rocks. Thus the the same. Thus the difference in chemical characteristics of low-Ti rocks are comparable only with present day MORB the two groups do not appear to result from a difference in back-arcand basinbasalts (BABB).Geochemical degree of partial melting. The two groups of NHO volcanics differencesbetween these two settings are neglible and may therefore be genetically unrelated and are believed to impossible to detect in altered samples (Saunderset al. 1980; result from different sources. Hawkins 1980). Duetothe presence of overlapping

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0- 15 1 l

Fig. 5. Ce/Yb vs. Ta/Yb diagram for high-Ti samples. Fields of B basalt types after Pearce (1982): A, within plate basalts; B, MORB; C, Volcanic arc basalts; ALK, alkaline; CA, calcalkaline; SHO, shoshonitic; TH, tholeiitic; TR,transitional.

from off-axis seamounts (Shervais 1982). (La/Ce), ratios for these samples are similar or slightly higher than that in alkali basalts from ocean-islands (Sun & Hasson 1975; Flower et al. 1976). Thus the high-Tiextrusives appear to be within plate lavas erupted at off-axis seamounts or ocean islands. Presence of limestone associated with these volcanics and a shallowmarine condition inferred from their fossil & 10 - assemblage (Acharyya et al. 1986; Acharyya Ghosh 1986) supportthe suggestion and also date this eventas Maastrichtian to Palaeocene. Asindicated above, the differences in geochemical charactersobserved between the two groups of NHO volcanics cannot easily be reconciled with degree of partial melting and probably result from differences in their mantle source. Minor differences also exist among samples within eachgroup. The low-Tirocks have bothLREE-depleted A and-enriched samples and they show crossing REE patterns.Such patterns have been interpreted to reflect 1 - 11 II I I1 La Ce SmEu Tb Yb Lu Fig. 4. Chondrite normalized REE patterns in the two volcanic groups (normalizing values from Haskin et al. 1968). (A) Low-Ti, (B) high-Ti.

MORB-like and arc-like geochemical signatures, we favour a back-arc basin setting for the low-Ti extrusives. MORB normalized trace element plots of samples from high-Ti group (Fig. 3B) closely resemble patterns observed in WPB. TheYb normalizedvalues of Ceand Ta can effectively discriminate between lavas from various oceanic domains (Pearce 1982). Seven of the high-Ti samples lie in the field of WPBand vary from tholeiitic through transitional to alkalinein character (Fig. 5). Withinplate setting forthese rocks mayalso beinferred from their fractionated REE character.The Th-Hf-Tavariation diagramalso successfully discriminates ocean floor basalts from differenttectonic settings (Wood et al. 1979;Wood 1980). Four of the samples plot in the field of alkaline WPB Th V Y V vv'y V Y v with a fifth marginally outside. The three remaining samples Ta plot in the common field for E-type MORB and tholeiitic Fig. 6. Th-Ta-Hf/3 diagram for high-Ti samples. Fields after Wood WPB (Fig. 6). (1980). A, N-type MORB; B, E-type MORB and tholeiitic Ti/V ratiosfor samples of the high-Ti grouprange within-plate basalts; C, alkaline within-plate basalts; D, volcanic arc between 55 and 71, which are characteristic of alkali basalts basalts.

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either variable source chemistry or variation in the melting Dept.Delft University of Technology)for help with the XRF process (Langmuir et al. 1977). In the absence of Ta values analysis; M. DeBruin and co-workers(Inter-university Reactor for samples of this group it is not possible to choose between Institute,Delft) for the INA analysis; D. K. Paul (GSI), M. K. the two alternative explanations. Bose(Presidency College, Calcutta), R. G. Coleman(Stanford In the high-Ti samples La/Ta ratios vary between 12 and University), A. W. Hofmann, (Max Planck Inst. Chemistry, Mainz) 22, with intermediate values between 15 and 18. Variation and anonymous reviewers of The Geological Society, London for in this ratio maybe explained by either involvingsource critically reviewing the manuscript. heterogeneity or by the degree of partial melting (O’Nions Permission to publishthe paper was kindlygranted by the et al. 1977;Sun et al. 1979; Tarney et al. 1980). High-Ti Director General, Geological Survey of India. samples with La/Ta ratios of 12-15 may represent alkaline within-plate basalts produced by very small degree of partial References melting. LREE depletion involved in the process will cause reduction of (Ce/Yb), ratio in melts produced subsequently ACHARWA,S.K. & GHOSH,S. 1986. Geology, mineralOccurrences and from thesource. Tholeiitic within-plate basalts from this metallogenetic aspects of Naga Hills Ophiolite belt, India. In: B. CARTER et al. (eds), Metallogeny of m@c and ultramafic complexes, Regional group with La/Taratios of18-22, and alsohaving lower presentations, Theophrastus Pub. S. A. Athens, 53-75. (Ce/Yb),ratios (Table 2) could therefore, represent such -, SRIVASTAVA,R. K., BHATTACHARYA,S., VENKATARAMANA, GHOSH, P,, melts. S. VIDYADHARAN,K. T. & JENA,S. K. 1984. Geology and tectonic frame Closejuxtaposition of mid-oceanridge/back-arc basin of the NagaHills ophiolites, Northern Indo-Burmese Range, India, Ahstr., 27th International Geological Congress Session, 7, 89-90. basalts and alkaline WPB similar to that noted in NHO has -, ROY, D. K. & MITRA,N. D. 1986. Stratigraphy and Palaeontology of been reported from ophiolites in Newfoundland (Casey et the Naga Hills Ophiolite belt. In: Geology of Nagaland Ophiolite, D. B. al. 1985). The alkalinerocks known as Skinner Cove GHOSHcommemorative Vol. Memoir of the Geological Survey of India, sequenceare interpreted to be within-plate ocean island 119, 64-75. AGRAWAL,0. P. & KACKER,R. N.1980. Nagaland ophiolites, India-a basalts(Searle & Stevens1984) which represent clipped subductionzone ophiolite complex in Tethyan orogenic belt. In: alkalic seamounts accreted to the continental margin (Casey PANAYIOTOU,A. (ed.) Ophiolites. Geological Survey of Cyprus, 454-61. & Dewey1984). Similar alkalic lavas have also been AMERICAN GEOLOGICALINSTITUTE 1972. Penrose field conference on reportedfrom beneath the Semail Ophiolite in Oman ophiolites. Geotimes, 17, 24-5. mountains,where they have been interpreted to have BASALTICVOLCANISM STUDY PROJECI1981. Basaltic Volcankm on the Terrestrial Planets. Pergamon, New York. formedthrough seamounts/ocean islands within marginal CASEY,J. F. & DEWY,J. F. 1984. Initiation of subductionzones along oceanic crust (Robertson 1986). transform and accreting plate boundaries, tripple junction evolution, and Prevalence of ocean-islands or island arc settings among forearc spreading centres: Implicationsfor ophiolite geologyand ophiolites may be related to the emplacement mechanism of obduction. In: GAS, I.G., LIPPARD, S. J. & SHELTON,A. W. (eds), Ophiolites and Oceanic Lithosphere. Geological Society, London, Special theophiolites. It is suggested thataccretion of NHO was Publication, U,269-90. possibly caused by temporary blocking of subduction by the -, ELTHON,D. L., SIROKY,F. X., KARSON, J. A.& SULLIVAN,1985. J. ocean island chain. Imbricate fault slices from the oceanic Geochemical and geological evidence bearingon the origin of the Bay of barrierthen became attached tothe leading edge of the Islandsand Coastal Complex Ophiolites of WesternNewfoundland. overridingcontinental block. The clippedocean island Tectonophysics, 116, 1-40. CHURCH,W. R. & COISH,R. A. 1976. Oceanic versusisland arc origin of volcanics of Maastrichtian-Palaeocene age thusconstitute ophiolites. Earth and Planetary Science Letters, 31, 8-14. an important component of the NHO. COISH,R. A. 1977. Ocean floor metamorphism in the Betts Cove ophiolite, Newfoundland. Contributions to Mineralogy and Petrology, 60, 255-70. - & CHRUCH,W. R. 1979. Igneous geochemistry ofmafic rocks in the Conclusions Betts Cove ophiolite, Newfoundland. Contributions to Mineralogy and Petrology, IQ, 29-39. On the basis of differences in Ti abundance, Ti/V, Hf/La -, HICKEY, R.& FREY,F. A. 1982. Rare-earth element geochemistry of ratios,MORB-normalized trace element plots and REE the Betts Cove ophiolite, Newfoundland:Complexities in ophiolite characteristics, theNHO volcanicrocks may be classified formation. Geochimica et Cosmochimica Acta., 46, 2117-34. intotwo groups. The low-Tirocks have overlapping COLEMAN,R. G. 1981. Tectonic setting of ophiolite obduction in Oman. Journal of Geophysical Researches, 86, 2497-508. MORB-like and island arc-like geochemical signatures, and FLOWER,M,, SCHMINCKE,H. U. & BOWMEN,H. 1976. Rare earth and other we suggest thatthese rocks have been produced in a trace elements inhistoric Azorean lavas. Journal of Volcanology and back-arcbasin. The high-Tivolcanics are products of Geothermal Research, 1, 127-48. seamount/ocean island volcanism during the Maastrichtian- GAS, I. G. 1982. Ophiolites. Scientific American, 247, 122-48. GEOLOGICALSURVEY OF INDIA1986. Geology of Nagaland Ophiolite D. B. Palaeoceneperiod. A similarassociation of back-arc-type GOSH commemorative Vol. Memoir of the Geological Survey of India, and seamount-related basalts has been reported from certain 119. Newfoundland and Oman ophiolites. The alkaline rocks in HASKIN,L. A., HASKIN,M. A. & FREY,F. A. 1968. Relative and absolute theseassociations probably represent clipped seamounts terrestrial abundance of the rare earths. In: AHRENS,L. H. (ed.) Origin and dktribution of elements. Pergamon Press, Oxford, 889-912. accreted onto the leading continental edge. HAWKINS,J. W. 1980.Petrology of back-arcbasins and island arcs: their Certain minor differencesexist among samples from each possiblerole in the origin of ophiolites. In: PANAYIOTOU,A. (ed.), group.For the high-Tisamples it is tentativelysuggested Ophiolites, Geological Survey of Cyprus, 244-54. that the observed variation results from the degreeof partial HELLMAN,P. L., SMITH,R. E. & HENDERSON,P. 1979. The mobility of rare melting.Very small degrees of partialmelting produces earth elements: evidence and implications from selected terrains affected by burial metamorphism. Contributions to Mineralogy and Petrology, 71, alkalic within-platebasalts andsubsequent melts are the 23-44. tholeiitic WPBwith less fractionated REE patternsand HUMPHRIS,S. E. 1984. The mobility of the rare earth elements in the crust. higher La/Ta ratios. In: HENDERSON,P. (ed.), Rare earth element geochemktry, Elsevier, Amsterdam, 317-42. LANGMUIR, C. H., BENDER,F., BENCE,J. A. E., HANSON, G.N. & TAYLOR, The authors acknowledge the help of a number of people in course S. R. 1977.Petrogenesis of hasaltsfrom the FAMOUS area: of the work. We thank them all, particularly Th. Verkroost (Mining Mid-Atlantic Ridge. Earth and Planetary Science Letters, 36, 133-56.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/146/3/491/4897790/gsjgs.146.3.0491.pdf by guest on 30 September 2021 498 S. SENGUPTA ET AL.

ONIONS,R. K. HAMILTON,P. J. & EVENSEN,N.M. 1977. Variations in SEN,S. & CHAITOPADHYAY,B.1978. The Ophiolite belt of north-eastern 143 Nd and "Sr/?Sr ratios in oceanic basalts. Earth and Planetary Science Indiaand associated mineralisation. Proceedings of the 3rd Regional Letters, 34, 13-22. Conference on Geology and Mineral Resources of SEAsia, Asian PEARCE, J. A. 1980.Geochemical evidence for the genesisand eruptive Institute of Technology Bangkok, 281-4. setting of lavas from Tethyan ophiolites. In: PANAYIOTOU,A. (ed.), SHERVAIS,J. W.1982. Ti-V plots and the petrogenesis of modern and Ophiolites, Geological Survey of Cyprus, 261-72. ophiolitic lavas. Earth and Planetary Science Leners, 59, 101-8. - 1982. Trace element characteristics of lavasfrom destructive plate SUN,S. S. & HANSON,G. N. 1975. Origin of Rossisland basinitoids and boundaries. In: THORPE,R. S. (ed.), Andesites. John Wiley, London, limitations upon the heterogeneity of mantle sources for alkali basalts 525-47. and nephelenites. Contributions to Mineralogy and Petrology,52, ROBERTSON,A. H. F. 1986. Geochemical evidence for the origin of Late 77-106. Triassic melange units in the Oman Mountains as a small ocean basin -, NESBIIT, R. W. & SHARASKIN,A. YA. 1979.Geochemical formed by continental rifting. Earth and Planetary Science Letters, ?l, characteristics of mid-oceanridge basalts. Earth and Planetary Science 318-32. Letters, 44, 119-38. SAUNDERS,A. D. 1984. The rare-earth element characteristics of igneous TARNEY,J., WOOD,D. A., SAUNDERS,A. D., CANN,J. R. & VARET,J. 1980. rocks from the oceanic basins. In: HENDERSON,P. (ed.), Rare earth Nature of mantle heterogeneity in the North Atlantic: evidencefrom element geochernbtry. Elsevier, Amsterdam, 205-35. deep seadrilling. Philosophical Transactions of the Royal Society of -, TARNEY,J., MARSH,N. G. & WOOD,D. A. 1980. Ophiolites as Ocean London. Series A, 297, 179-202. crust or marginal basin crust: a geochemical approach. In: PANAYIOTOU, TAYLOR, S. R., KAYE, M., WHITE, A. J. R., DUNCAN,A. R. & EWART, A, A. (ed.), Ophiolites, Geological Survey of Cyprus, 193-204. 1969. Genetic significance of CO, Cr, Ni, Sc and V content of andesites. SCHILLING,J. G. 1971. Sea floor evolution: rare-earth evidence. Philosophical Geochimica et Cosmochimica Acta, 33,275-86. Transactions of the Royal Society of London, Series A, 268, 663-706. WOOD,D. A. 1980. The application of a Th-Hf-Ta diagram to problems of SCHMINCKE,H. U,, RAUTENSCHLEIN, M,, ROBINSON, P. T. & MEHEGAN, M. J. tectonomagmatic classification and to establishing the nature of crustal 1983. Troodos intrusive series of Cyprus: A comparisonwith oceanic contamination of basaltic lavas of the British Tertiary volcanic province. crust. Geology, 11, 405-9. Earth and Planetary Science Letters, 50, 11-30. SEARLE,M. P. & STEVENS,R. K. 1984. Obduction process in ancient, modern -, JORON,J. L. & TREUIL,J. 1979. A re-appraisal of the useof trace and future ophiolites. In: GASS,I. G., LIPPARD,S. J. & SHELTON, A.W. elements to classify and discriminate between magma series erupted in (eds), Ophiolites and Oceanic Lithosphere, Geological Society, London, differenttectonic settings. Earth and Planetary Science Letters, 45, Special Publication 13, 303-19. 326-36.

Received 5 January 1988; revised typescript accepted 9 August 1988.

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