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Tectonophysics 393 (

Age, geochemical and Sr–Nd–Pb isotopic constraints for mantle

source characteristics and petrogenesis of Teru Volcanics,

Northern Kohistan Terrane, Pakistan

S.D. Khana,*, R.J. Sternb, M.I. Mantonb, P. Copelanda, J.I. Kimurac, M.A. Khand

aDepartment of Geosciences, University of Houston, Houston, TX 77204-5007, USAbDepartment of Geosciences, University of Texas at Dallas, Richardson, TX 75083, USA

cDepartment of Geosciences, Shimane University, JapandNational Center of Excellence in Geology, University of Peshawar, Pakistan

Accepted 21 April 2004

Available online 23 September 2004

Abstract

This paper presents new geochemical and geochronology data for the Teru Volcanic Formation (previously known as the

Shamran Volcanics) exposed west of Gilgit in the Kohistan terrane of the Pakistani Himalayas. The Teru Volcanic Formation

ranges from basalt through andesite to rhyolite and has subalkaline and midalkaline affinities. Trace-element compositions and

isotopic characteristics suggest these magmas were formed in a subduction zone setting; isotopic studies also support this

conclusion. It is suggested that these lavas originated from a depleted mantle source, which experienced contamination by

variable subduction components. Model mixing calculations using 87Sr/86Sr and 143Nd/144Nd data suggest that addition of

0.2–0.6% of Indus margin sediments and/or 2–4% of fluids derived from Indus margin sediment can generate the

compositional variation of the Teru Volcanic Formation. Two samples from the Teru Volcanic Formation yielded 40Ar/39Ar

ages of 43.8+0.5 and 32.5+0.4 Ma. These ages make the volcanic rocks of the Teru Volcanic Formation the youngest reported

in the Kohistan terrane. These volcanic rocks unconformably overly the Shunji Pluton, which has a 65 Ma Rb–Sr whole-rock

isochron age. The results of this research suggest that subduction-related volcanism was active until 33 Ma in the India–Asia

collision zone.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Teru Volcanic Formation; Pakistani Himalayas; Kohistan terrane

0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tecto.2004.07.038

* Corresponding author. Tel.: +1 713 743 3411; fax: +1 713

748 7906.

E-mail address: [email protected] (S.D. Khan).

1. Introduction

The events leading to the formation of Himalaya

started in the early Cretaceous time, when India

started drifting northward (Molnar and Tapponier,

1975; Scotese et al., 1988). At that time, the intra-

2004) 263–280

S.D. Khan et al. / Tectonophysics 393 (2004) 263–280264

oceanic Kohistan arc formed over a subduction zone

that dipped beneath the arc either to the south or to the

north (Khan et al., 1997). It is widely accepted that the

drift of India was coeval with the development of the

Kohistan arc, which collided with Asia along the

Shyok Suture sometime between 75 and 95 Ma. The

southern margin of Asia, including the Kohistan arc,

then became an Andean-type convergent margin that

lasted for 20–40 million years, until India collided

with Asia. Thrusting of the Kohistan terrane south-

ward over the north Indian margin along the Main

Mantle Thrust (MMT) was started by 55 Ma (Searle et

al., 1999). On the basis of the Treloar et al. (1989) K/

Ar and 40Ar/39Ar ages, Searle et al. (1999) concluded

that the calc-alkaline volcanism (the Utror and Teru

Volcanic Formations) continued until 55 Ma. These

widely accepted views are inconsistent with our

findings on two samples of relatively young 40Ar/39Ar

ages for the Teru Volcanic Formation, which suggests

that volcanism continued until at least 33 Ma. This

Fig. 1. General geological map of the Kohistan–Ladakh terrane

observation could indicate that the Teru volcanics

were emplaced prior, during, and after the collision of

the Indian plate with Kohistan. This interpretation

conflicts with earlier models that classify these rocks

as pre-collisional. Results giving evidence for the

presence of post-collisional volcanic event(s) in

Kohistan might indicate previously unrecognized

widespread volcanic activity throughout the Hima-

layas after the collision of the Indian plate with Asia.

Similar types of volcanic rocks have been reported in

other parts of Himalayas (e.g., Oligocene age in the

Khardung region (Thakur and Misra, 1984), Paleo-

cene to Oligocene Dras II formations from southern

Ladakh (Honegger, 1983; Reuber, 1989) and Paleo-

cene to Early Eocene Linzizong Formation from the

Lhasa block (Maluski et al., 1983; Coulon et al.,

1986)) (Fig. 1).

In this paper, geochronologic, major and trace

element, and isotopic results from the Teru Volcanic

Formation in the Kohistan terrane of Pakistan are

(modified from Searle and Khan, 1996; Sharma, 1991).

Fig. 2. Geological map Teru area, showing location of samples, Northern Kohistan terrane, Pakistani, Himalayas.

S.D. Khan et al. / Tectonophysics 393 (2004) 263–280 265

presented (Figs. 1 and 2). Located between the Main

Karakoram Thrust (MKT) or Shyok Suture Zone to

the north and the MMT or Indus Suture Zone (ISZ) to

the south, rocks of the Kohistan–Ladakh arc crop out

on both sides of the Nanga Parbat spur, in Pakistan

and India (Fig. 1). The Kohistan–Ladakh Arc resulted

from the northward subduction of the Tethys Ocean

along trenches now marked by the MMT and ISZ,

respectively (Tahirkeli et al., 1979; Honegger, 1983).

The southern part of the Kohistan Arc preserves

ultramafic and mafic rocks (e.g. Jijal Complex,

Kamila Amphibolite, Chilas Complex), whereas the

northern part consists mainly of large granodiorite

plutons, although remnants of volcanic arc and

associated sedimentary formations are also present

(Dietrich et al., 1983; Tahirkeli et al., 1979; Shah and

Shervais, 1999).

2. Geological background

Initial volcanism in the Kohistan terrane is dated to

be pre Albian–Aptian (112 Ma) from the presence of

Orbitolina faunas intercalated with the volcanic rocks

throughout the northern part of Kohistan (Pudsey,

1986) and in the Dras I unit in southern Ladakh

(Dietrich et al., 1983; Reuber, 1989). In Tibet, initial

volcanism has also been dated as mid-Cretaceous,

though lasting until late Cretaceous (110–80 Ma;

Coulon et al., 1986). A second period of volcanism in

the arc has been variously dated as early Tertiary (late

Paleocene to early Eocene) in the Dir Group of

Kohistan (Treloar et al., 1989, Shah and Shervais,

1999), Oligocene in the Khardung Volcanic Sequence

(Thakur and Misra, 1984), Paleocene to Oligocene in

the Dras II Unit of southern Ladakh (Honegger et al.,

1982; Reuber, 1989) and Paleocene to early Eocene in

Tibet (Maluski et al., 1983; Coulon et al., 1986). Early

plutonism in the arc was contemporaneous with the

volcanic activity (103 Ma and continued until �25

Ma; Honegger et al., 1982; Petterson and Windley,

1985) and plutonism has been interpreted to represent

an Andean type continental arc evolution (Petterson

and Windley, 1985).

The age of the Teru Volcanic Formation is

controversial. The only previously published isotopic

results concerning this part of Kohistan were obtained

using 40Ar/39Ar and K/Ar by Treloar et al. (1989).

Sullivan et al. (1993) used the 40Ar/39Ar hornblende

age of 58F1 Ma (Treloar et al., 1989) to assign an

early Eocene age to this volcanic sequence. This age

was based on a basaltic andesite tuff sample, which


was collected near Phander (Fig. 2). Our field studies

failed to find any accessible outcrop near Phander,

suggesting that the dated sample was collected from

debris. The age could belong to any of a number of

volcanic units exposed in this part of Kohistan.

Therefore, the age of the Teru Volcanic Formation

awaits resolution.

For the present study, several samples of the Teru

Volcanic Formation were selected for 40Ar/39Ar

analyses and a sample of the Shunji Pluton was

selected for Rb–Sr age dating. In this paper, results

obtained from two samples of Teru Volcanic For-

mations and a sample of Shunji Batholith is presented.

Samples for geochemical analysis were selected so

that they represent the whole spectrum of the Teru

Volcanic Formation; moreover, special emphasis was

given to freshness of the samples. Samples for major

elements were analyzed using ICP-AES at the

University of Kansas, while powdered samples were

sent for ICP-MS analysis at Shimane University,

Japan. For Nd, Sr and Pb isotopic studies nine

samples were analyzed at the Mass Spectrometry

Laboratory, University of Texas at Dallas.

3. Analytical methods

Analytical procedures for ICP-AES are those of

Clark et al. (1998). Analytical precision for major

elements was generally within 4%. Table 1 gives the

major element compositions for the analyzed samples.

Trace element data were obtained by VG Elemental

PQ3 ICP-MS equipment at Shimane University, Japan

(Table 1), using an isobaric interference correction

and a standard addition techniques (Kimura et al.,

1995). Precisions are better than 3% for element in

ppm levels and better than 10% for sub-ppm levels. Sr

and Nd isotope ratios were measured using a Finnigan

Mat 261 multicollector mass spectrometer at Univer-

sity of Texas at Dallas (Table 2). The availability of

specific bresinsQ, actually crown ethers adsorbed to

inert particulate substrates, has changed the chemical

separations necessary for thermal ionization mass

spectrometry. Strontium was separated on Sr-Spec

resin and Nd on HDEHP column by reversed phase

chromatography. During the course of this work,

values of 0.70806 were obtained for the 87Sr/86Sr ratio

of the Eimer and Amend standard and values of

0.511860 were obtained for 143Nd/144Nd for the La

Jolla standard. Pb isotopes were analyzed at 1350 8Cand corrected for internal fractionation using 0.15%

per amu. Standard NBS-981 yielded a mean206Pb/204Pb=16.944+0.005, 207Pb/204Pb=15.500+

0.010, 208Pb/204Pb=36.743+0.029.

Hornblende was separated from selected samples

of Teru Volcanic Formation using sieving, hand

picking, Franz Magnetic Separator and heavy liquid

separation techniques. Ar isotopes were analyzed

using a Mass Analyser Products (MAP 215-50) mass

spectrometer at the University of Houston. For Rb–Sr

geochronology, whole-rock powder and biotite sepa-

rates were dissolved in a mixture of concentrated HF/

HNO3. The Sr isotopic compositions were determined

on unspiked aliquots and Rb and Sr concentrations

were measured by isotope dilution.

4. Results

4.1. Age of Teru Volcanic Formations

Results for the two samples analyzed for 40Ar/39Ar

shown in Fig. 3(A and B). These samples include

sample 44D and 44B, which give a well-defined

plateau age of 43.8F0.5 and 32.5F0.4 Ma, respec-

tively. These ages makes the Teru Volcanic Formation

the youngest reported volcanics in the Kohistan

terrane. A sample from the Shunji Pluton was dated

by Rb–Sr technique. The Shunji Pluton has a non-

conformable contact with the Teru Volcanic Forma-

tion (Fig. 2). Table 3 displays the Rb–Sr data. Based

on the whole-rock biotite pair, the Shunji Pluton is

dated as 65 Ma, making it part of Stage 2 Kohistan

batholith (Petterson and Windley, 1985).

4.2. Major and trace element data

The chemical data obtained for the Teru Volcanic

Formation are presented in Table 1. In Fig. 4A, the data

are plotted on a Le Maitre et al. (1989) diagram;

samples analyzed range from basalt to rhyolite with

subalkaline to midalkaline affinities. When plotted on

theWinchester and Flyod (1976) classification diagram

(Fig. 4B), some felsic samples plot in silica-poor fields,

suggesting a secondary modification of some major

elements (alkali and silica) in response to alteration

Table 1

Major and trace elements analyses of Teru Volcanic Formation

62 42B 44D 44E 7- 4 71 72 44B LP- 4

SiO2 49.73 49.77 50.18 52.23 53.53 53.74 54.52 55.1 56.5

TiO2 0.86 1.67 0.79 1.23 0.65 1.42 1.49 1.1 0.98

AI2O3 21.04 17.59 19.89 17.76 17.26 16.16 15.78 16.4 17.1

Fe2O3 7.77 10.9 11.48 11.06 9.42 11.73 11.72 8.26 9.23

MnO 0.14 0.2 0.2 0.2 0.14 0.2 0.19 0.13 0.21

MgO 2.43 5.7 5.36 4.16 7.01 3.48 3.66 4.6 4.02

CaO 7.11 9.4 8.47 8.06 9.34 8.69 7.9 6.98 7.72

Na2O 4.06 4.18 2.14 3.7 2.08 3.19 3.53 3.68 3.9

K2o 1.8 0.58 2.09 0.58 1 0.84 1.2 2.46 1.44

P2O5 0.12 0.36 0.13 0.23 0.14 0.27 0.33 0.34 0.21

Sum 95.06 100.35 100.73 99.21 100.57 99.72 100.32 99 101

Sr 647 832 345 308 799 193 644 389 419

Ba 226 182 123 167 388 324 168 207 286

Li 19.3 28.7 15.9 20.2 5.8 13.4 10.1 20.3 47.3

Be 0.61 0.59 0.57 1.17 2.9 1.4 1.42 1.48 1.28

Rb 50.8 9.5 19 33.7 184.4 7 33 59.2 49.1

Y 17.5 14.7 14.4 25.8 43.1 26.8 35.2 30.7 28.7

Zr 60 66.8 33.8 12.5 175.4 105.9 76.7 132.3 112.8

Nb 1.99 3.11 1.1 5.03 20.35 6.73 12.84 9.52 8.43

Mo 0.17 0.32 0.13 0.44 0.83 0.23 0.4 0.87 0.67

Sn 0.65 0.75 0.54 1.44 5.2 1.26 2.14 1.64 1.42

Sb 0.28 0.18 0.08 0.07 0.48 0.27 0.17 0.41 1.3

Cs 2.27 0.53 0.36 1.17 2.71 0.25 0.62 1.46 2.02

La 7.1 10.2 7.6 18.2 39.6 13.2 30.9 23.9 26

Ce 16.1 20.8 15.9 36.8 86.7 30.7 64 53.9 56.5

Pr 2.24 2.76 2.22 4.68 9.98 4.34 7.91 7.25 7.57

Nd 10.1 11.6 10 18.6 37.9 19.4 30.5 30.6 30.7

Sm 2.63 2.71 2.54 4.2 7.64 4.7 6 6.55 6.43

Eu 1.04 0.94 0.88 1.49 0.99 1.65 1.43 1.64 1.74

Gd 2.64 2.86 2.54 4.55 8.74 4.71 6.85 6.83 6.86

Tb 0.55 0.51 0.5 0.83 1.48 0.97 1.19 1.21 1.16

Dy 3.42 3.07 3.01 5.1 8.84 5.77 7.17 6.98 6.57

Ho 0.67 0.56 0.56 0.96 1.55 1.09 1.28 1.16 1.06

Er 1.88 1.6 1.61 2.75 4.61 2.95 3.79 3.44 3.06

Tm 0.29 0.24 0.23 0.42 0.68 0.42 0.58 0.47 0.42

Yb 1.97 1.63 1.52 2.79 4.61 2.7 3.99 3.15 2.9

Lu 0.3 0.24 0.23 0.42 0.68 0.38 0.6 0.47 0.4

Hf 1.69 1.9 1.14 0.89 4.82 2.79 2.83 3.46 2.76

Ta 0.16 0.22 0.06 0.47 1.76 0.48 1.03 0.73 0.6

Tl 0.27 0.09 0.09 0.17 0.73 0.04 0.15 0.35 0.77

Pb 4.99 7.06 3.07 7.16 133.18 10.03 6.11 8.35 over range

Th 1.52 2.61 1.3 4.07 17.42 1.06 10.67 5.73 5.24

U 0.39 0.69 0.39 1.16 3.38 0.3 2.16 1.43 1.26

41 44C 7-2 58 73 64 68 69 42A

SiO2 56.8 57.1 57.6 57.9 62.5 63.7 68.4 69.8 74.2

TiO2 0.53 1.15 0.79 1.49 0.67 0.7 0.29 0.31 0.38

Al2O3 19.1 17.2 17.8 16 16.6 13.6 15.6 15.4 14

Fe2O3 7.59 8.68 7.73 10.2 5.39 7.32 2.89 2.84 2.05

MnO 0.16 0.14 0.13 0.16 0.1 0.09 0.09 0.09 0.03

MgO 4.36 6.38 5.98 2.89 2.57 5.52 1.01 0.92 0.7

(continued on next page)


Table 2

Sr, Nd and Pb isotope analyses of samples of Teru Volcanic Formation and a Kohistan batholith sample

206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 206Pb/204Pb(I) 207Pb/204Pb(I) 208Pb/204Pb(I) 87Sr/86Sr 87Sr/86Sr(I) 143Nd/144Nd Nd

44D 18.5827 15.59568 38.70315 18.52709 15.59307 38.64227 0.70443 0.70429 0.51281 3.3

44E 18.5154 15.60883 38.66461 18.44458 15.60551 38.58299 0.70409 0.70402 0.51284 3.99

71 18.5603 15.60712 38.76525 18.5472 15.6065 38.75001 0.70406 0.70398 0.51287 3.29

44B 18.429 15.55456 38.50014 18.35452 15.55107 38.40202 0.70417 0.70344 0.51285 4.17

LP-4 18.5436 15.59731 38.72463 No data No data No data 0.70412 0.70409 0.51286 4.45

44C 18.5807 15.5954 38.71614 18.55801 15.59434 38.69068 0.70404 0.70402 0.51292 5.42

58 18.5622 15.65523 38.89806 18.43433 15.64923 38.82986 0.70409 0.70243 0.51285 4.16

73 18.4564 15.55286 38.48754 18.37683 15.54913 38.37965 0.70401 0.70323 0.51289 4.97

68 18.5928 15.58712 38.72825 18.55568 15.58538 38.67892 0.705 0.70428 0.51285 4.1

987-7 18.4359 15.54985 38.47514 18.35825 15.54613 38.35556 0.70444 0.70381 0.512852 4.86

41 44C 7-2 58 73 64 68 69 42A

CaO 5.03 4.63 6.06 6.89 5.71 3.61 2.38 2.98 1.34

Na2O 4.92 2.63 4.4 3.24 3.76 3.48 5.12 4.58 5.02

K2O 1.88 1.83 0.64 2.42 2.32 1.15 2.57 2.15 2.23

P2O5 0.18 0.36 0.16 0.47 0.2 0.21 0.04 0.05 0.09

Sum 101 100 101 102 99.9 99.4 98.5 99.2 100

Sr 463 455 259 315 141 141 364 363 461

Ba 347 80 232 137 353 303 144 207 294

Li 42.6 24.7 5.6 33 16.1 16.4 16.3 20.7 17.6

Be 0.56 0.71 1.27 0.86 1.17 1.05 0.8 0.97 1.13

Rb 55.9 16.1 42 27.9 53.1 43.3 14 23.3 42.2

Y 15.1 23.7 40.5 16.1 20 20.3 26.2 29.3 14.7

Zr 38.9 75 228.3 86.6 101.4 99.9 94.7 105.3 114.5

Nb 1.59 2.8 7.26 3.92 4.82 4.82 3.38 3.88 8.31

Mo 0.29 0.33 1.58 0.3 0.19 0.16 0.72 1.44 1.75

Sn 0.61 0.84 2.05 0.73 1.32 1.75 1.07 2.02 0.97

Sb 0.19 0.18 0.17 0.15 0.18 0.33 0.09 0.25 0.22

Cs 1.88 2.33 3.88 0.13 1.27 0.99 1.53 1.49 0.87

La 6.3 10.9 19.4 11.1 16.9 14.9 12 15.7 18

Ce 14 24.3 45.2 22.3 34.7 26.6 26.1 32.2 33.7

Pr 2.06 3.27 6.13 3.04 4.04 3.8 3.65 4.32 4.1

Nd 9.8 15 27.4 13.2 14.8 14 16.5 19.3 15.3

Sm 2.6 3.79 6.73 3.02 3.21 3.11 4.3 4.85 3.04

Eu 1.07 1.36 1.65 0.95 0.94 0.95 1.55 1.62 1.03

Gd 2.49 3.75 6.68 3.15 3.65 3.58 4.27 4.88 3.57

Tb 0.53 0.79 1.43 0.62 0.66 0.67 0.94 1.03 0.58

Dy 3.25 4.79 8.42 3.52 4.03 3.95 5.54 6.08 3.29

Ho 0.6 0.89 1.52 0.62 0.74 0.74 1.04 1.11 0.55

Er 1.76 2.56 4.37 1.78 2.31 2.31 2.98 3.19 1.59

Tm 0.25 0.36 0.65 0.27 0.36 0.36 0.45 0.47 0.23

Yb 1.71 2.49 4.28 1.74 2.58 2.58 2.93 3.16 1.69

Lu 0.26 0.37 0.64 0.26 0.41 0.4 0.45 0.47 0.25

Hf 1.26 2.2 6.17 2.32 3.14 3.06 2.62 2.9 3.1

Ta 0.1 0.17 0.49 0.26 0.43 0.42 0.25 0.28 0.7

Tl 0.23 0.13 0.13 0.13 0.34 0.27 0.07 0.1 0.25

Pb 5.24 10.68 7.37 5.2 9.13 12.67 7.32 6.74 12.87

Th 1.03 1.91 5.49 2.45 6.89 6.63 2.51 3.37 5.28

U 0.26 0.56 1.58 1.51 1.67 1.64 0.62 0.83 1.49

Table 1 (continued)


Fig. 3. 40Ar/39Ar age data for Teru Volcanics.


processes. For this reason, the following discussion

about tectonic setting is based on non-mobile elements,

i.e. high field strength elements (HFSE).

On variation diagrams (Fig. 5), MgO, CaO, Fe2O3,

TiO2 and Al2O3 negatively covary. Whereas, Na2O

and K2O increases with increasing SiO2. In general, the

trends exhibited by the major oxides can be explained

in terms of continued fractionation of varying

amounts of plagioclase+clinopyroxene+amphibole+

Table 3

Rb and Sr concentration and 87Sr/86Sr ratios for a granite and

separated biotite from the Shunji Pluton

Rb Sr 87Rb/86Sr 87Sr/86Sr

Biotite 272.29 53.50 14.74 0.71760

Whole rock 71.53 180.57 1.15 0.70498

The apparent age is 65.4 Ma and (87Sr /86Sr)I = 0.70392.

spinel. The inflection in P2O5 trend at about 58%

SiO2 may reflect the onset of apatite fractionation.

Teru Volcanic Formation have typical subduction-

related trace element patterns on a normal mid-ocean

ridge basalt (N-MORB)-normalized diagram (Fig. 6).

LIL elements (Sr, K, Rb, Ba, Pb and Th) are enriched

relative to light rare earth elements (LREE; e.g. Ba/La

7–55) and are enriched relative to high field strength

elements (HFSE) such as Nb and Ta (e.g., Ba/Nb 13–

218, La/Nb 2–7). All samples of the Teru Volcanic

Formation exhibit LREE enrichments (Ce/Yb 8–20),

which are especially high in high K-samples. Most

samples have low HFSE abundances, which are

nevertheless greater than N-MORB values with the

exception of sample 44D (e.g. Nb/Yb=0.72–5.05

compared with 0.8 in average N-MORB) (Sun and

McDonough, 1989). LIL elements and LREE enrich-

Fig. 4. Teru Volcanics are plotted using (A) LeBas et al. (1986) and Le Maitre et al. (1989) and (B) Winchester and Flyod (1976).


ments and associated Nb–Ta depletions suggest chem-

ical affinities with arc-related rocks (Stolz et al., 1996).

Fig. 6 also shows the trace element patterns for the

composition of global subducting sediment composi-

tion (GLOSS; Plank and Langmuir, 1998). GLOSS

composition is determined from trench sediments

worldwide (70% of trenches). GLOSS is dominated

by terrigenous material (76 wt.% terrigenous, 7 wt.%

calciumcarbonate, 10wt.%opal, 7wt.%mineral-bound

H2O+) and is therefore similar to the composition of

upper continental crust (Plank and Langmuir, 1998).

The trace element pattern of the Teru Volcanic For-

mations is similar to GLOSS, suggesting influence by a

sediment component. Nevertheless, the trace element

pattern of the Teru Volcanic Formation has a greater

enrichment of most incompatible elements. As such,

the Teru Volcanic Formation samples have affinities

of lavas from other subduction-related environments.

4.3. Sr–Nd–Pb isotopes

Analyses of Sr, Nd and Pb isotopes for the Teru

Volcanic Formation are given in Table 2. Initial87Sr/86Sr and ENd values were calculated assuming

Fig. 5. Major elements (TiO2, A12O3, Fe2O3, MgO, CaO, Na2O, K2O and P2O5 wt.%) versus (SiO2 wt.% for Teru Volcanic Formation).


an emplacement age of 43 Ma. On a plot of initial87Sr/86Sr versus 143Nd/144Nd (Fig. 7), samples from

the Teru Volcanic Formation fall between the field

defined for present-day Indian MORB and that

representing the composition of Reunion plume

mantle at ~65 Ma (Dupre and Allegre, 1983; Fisk et

al., 1988). The Chalt Volcanics contain more radio-

genic Sr than samples from the Teru Volcanic

Formation. A diorite sample belonging to Stage 2

Kohistan batholith shows similar isotope ratios to the

Teru Volcanic Formation (Fig. 7). The Kamila

amphibolite enriched type has similar initial 87Sr/86Sr

but higher initial 143Nd/144Nd, which is comparable to

the Stage 2 batholith sample. The Chilas Complex as

well as D-type Kamila amphibolites on the other hand

show similar to lower ENd values compared to the

Teru Volcanic Formation. Fig. 6 also shows isotopic

ratio for GLOSS and sediments from the Ladakh

Suture Zone and the Indus margin (France-Lanord et

al., 1993). As can be seen, the Teru Volcanic

Fig. 6. N-MORB normalized (ICP-MS) trace element patterns for selected Teru Volcanic rocks (normalizing values Sun and McDonough,

1989). For comparison, GLOSS (Plank and Langmuir, 1998) composition is also plotted.


Formation form a quasi-linear array between Indian

MORB and Ladakh and Indus sediments.

Initial Pb isotopic ratios were calculated and are

shown in Table 2. A plot of initial 206Pb/204Pb versus

initial 207Pb/204Pb and 208Pb/204Pb (Fig. 8) shows the

data listed in Table 2, in addition to the fields for

Indian MORB and the Reunion plume component

(Oversby, 1972), and GLOSS (Plank and Langmuir,

1998). Compositions of various units from Kohistan

are also plotted. A few samples show higher ratios as

Fig. 7. Plot of 87Sr/86Sr versus 143Nd/144Nd for Teru Volcanics. Source of data Chalt Volcanics, Kamila Amphibolite, Chilas Complex = Khan et

al. (1997), Ladakh Suture Zone sediments, Indus margin sediments = France-Lanord et al. (1993), GLOSS = Plank and Langmuir (1998), Indian

MORB= Simonetti et al. (1998), Reunion Plume = Dupre and Allegre (1983) and Fisk et al. (1988).


compared to Indian MORB, as is typical for arc

volcanic rocks (Fig. 8). Samples 73 and 44B of the

Teru Volcanic Formation are less radiogenic and plot

in the field of Indian MORB. Sample 58 of Teru

Volcanic Formation shows the highest Pb isotopic

ratio. The Chilas complex samples show isotopic

compositions similar to samples of the Teru Volcanic

Formation, whereas the isotopic data from samples of

the Stage 2 Kohistan batholith overlaps sample 73 and

44B of the Teru Volcanic Formation. A similar

isotopic pattern can be seen on a 206Pb/204Pb and143Nd/144Nd plot, as shown in Fig. 9.

5. Discussion

There is a general consensus that the major source of

arc magmas lies in the mantle wedge (Perfit et al., 1980;

Gill, 1981; Plank and Langmuir, 1998; Davies and

Bickle, 1991) and that the subducting slab provides the

components that trigger the melting process. Compo-

nents from the subducting slab that can contribute to the

subarc mantle include hydrous fluids and silicate melts

from altered oceanic basalt and sediment. Subducted

basalt and sediment have different isotopic and

incompatible trace element compositions, and by

considering these differences it is possible to identify

contributions from different components.

The Teru Volcanic Formation show a large range of

chemical compositions, from basalt to rhyolite, which

can be explained by fractional crystallization (Khan,

2001). As expected, little variation is seen in the

isotopic compositions and isotopic variations do not

correlate with degree of fractionation. Instead, the

correlation between isotopes and trace elements

appears to be related to variable contributions of

sediment and mantle components to the magma

source, as will be discussed below.

5.1. Partial melting and composition of the mantle

wedge

The composition of the mantle wedge and details of

the melting process are best investigated using incom-

patible elements such as the HFSE and HREE and

certain transition metals (Pearce and Parkinson, 1993;

Fig. 8. Plots of Pb isotopes for Teru Volcanics. Source of data Chalt Volcanics, Kamila Amphibolite, Chilas Complex = Khan et al. (1997),

GLOSS = Plank and Langmuir (1998), Indian MORB= Simonetti et al. (1998), Reunion Plume =Oversby (1972).


Pearce and Peate, 1995). A plot of Nb/Yb versus Zr/Yb

(Fig. 10) demonstrates the key features of these patterns

for all the samples. Most of the Teru Volcanic

Formation is significantly enriched relative to average

N-MORB, although few samples have N-MORB-like

compositions. This, together with a generally enriched

isotopic mantle source (Fig. 7), appears to reflect the

increasing influence of a plume component.

The question then arises as to the causes of the

relative enrichments and depletions within the Teru

Volcanic Formation. A diagram of K/Yb against Nb/

Yb is informative (Fig. 11). There are three principal

types of trends, described in detail by Pearce and

Peate (1995). The vertical trend (Trend A) results

from variable addition of a subduction component to a

mantle wedge of constant composition. The extent of

displacement from the MORB array indicates the

percentage of potassium in the wedge that can be

attributed to subduction (%sz): the contours indicate

that most samples contain 85–95% subducted K.

Trend B results from the addition of a constant

subduction component to a variably enriched mantle

wedge. Trend C, which runs parallel to the MORB

array, can best be explained by dynamic melting

following addition of the subduction component. A

proportion of the overall variance for Teru Volcanic

Formation can be explained by this process, indicating

that the rocks of the Teru Volcanic Formation are a

Fig. 9. Plot of 206Pb/204Pb versus 143Nd/144Nd for Teru Volcanics. Source of data Chalt Volcanics, Kamila Amphibolite, Chilas Complex = Khan

et al. (1997), GLOSS = Plank and Langmuir (1998), Indian MORB= Simonetti et al. (1998), Reunion Plume =Dupre and Allegre (1983), Fisk

et al. (1988) and Oversby (1972).


combination of variable subduction component added

to a variable composition mantle wedge.

5.2. Constraints on the subduction component

Although sediment contribution to subduction-

related magmas is generally accepted, debate con-

Fig. 10. Nb/Yb versus Zr/Yb for Teru Volcanics (MORB array from

Pearce and Peate, 1995).

tinues whether the sediment component is trans-

ported from the slab as bulk sediment (Miller et al.,

1994), sediment melt (Miller, 1995) or fluid

extracted from sediment fluid (Miller, 1995). The

enriched 207Pb/204Pb and the Nd, Sr isotope

compositions of the Teru Volcanic Formation

samples, relative to Indian MORB domain compo-

Fig. 11. Nb/Yb versus K/Yb for Teru Volcanics (MORB array from

Pearce and Peate, 1995). A= Variable subduction component

B = variable enriched mantle wedge, C = variable melt extraction.

,

Fig. 12. 143Nd/144Nd versus 87Sr/86Sr variation diagram for the Teru

Volcanics showing the calculated two mixing lines, between Indian

MORB (Vroon et al., 1993), Indus margin sediments (France-

Lanord et al., 1993) and fluid from Indus margin Sediment

Composition of Ladakh Suture zone sediments (France-Lanord

et al., 1993) are also plotted for comparison.


sition, are consistent with contamination of their

mantle source by a sediment component. In addi-

tion, Ce/Pb is within the range 0.08–0.26 and

distinctly lower than in oceanic (MORB, OIB)

basalts (Hofmann et al., 1986). A sedimentary

signature is also reflected in the low Sr/Nd (0.76–

5.8), low Nb/La (0.16–0.55), elevated Ce/Sr (0.3–

2.95), high Th/Ta (2.43–23.83) and high Th/La

(1.67–9.27). Moreover, the presence of negative Ta–

Nb–Ti anomalies in spidergrams (Fig. 6) also

indicates the involvement of a subduction-related

component, although this is not necessarily due to

sediment involvement.

Island-arc basalts tend to have lower 143Nd/144Nd

than depleted MORB. This is generally explained in

terms of a small contribution from subducted sedi-

ment (e.g., Hawkesworth et al., 1993), as most

sediments have 143Nd/144Nd much less than in

MORB (Fig. 7). The variations in potential sediment

contributions to petrogenesis can be modeled

through the use of mixing calculations. The calcu-

lations were performed using end-member values as

shown in Table 4, the Indian MORB data is taken

from Vroon et al. (1993), while Indus marginal

sediments considered for these calculations are from

France-Lanord et al. (1993). Isotopic compositions of

sediment and their dewatering fluids are identical but

the total concentrations are different owing to the

differing mobility of these elements in hydrous fluid.

These sediments and fluid mixing lines are plotted in

Fig. 12. The mobility values of Tatsumi et al. (1986)

are used to calculate element concentrations for

fluids from Indus margin sediments. The result of

this calculation indicate that addition of 0.2–0.6% of

the Indus margin sediments and/or 2–4% of Indus

Table 4

Isotopic ratios used to determine mixing between mantle and

sediment sources

Sample 87Sr/86Sr(I) 143Nd/144Nd Sr Nd

Indian MORB 0.70300 0.51305 9 0.73

Indus margin

sediments

0.70981 0.51188 299 33.2

Fluid from Indus

margin sediment

0.70981 0.51188 56.81 3.652

Fluid/element concentration based on Tatsumi et al. (1986).

Mobility of Sr = 19% and of Nd = 11%.

.

margin sediment fluids can generate the Sr–Nd

isotopic compositions of the samples from Teru

Volcanic Formation’s composition. Moreover, there

is no isotopic evidence for the involvement of

ancient continental crust of India.

5.3. Tectonic implications

Collision of the Karakoram and Kohistan terranes

along the northern suture zone has been dated at 75–95

Ma (Petterson andWindley, 1985; Coward et al., 1987).

Thrusting of the Kohistan terrane southward over the

north Indian margin along the Main Mantle thrust

probably took place in late Cretaceous or Paleocene

time and was started 55 Ma (Searle et al., 1999).

According to Searle et al. (1999), the calc-alkaline

volcanism (Utror and Teru Volcanic Formations) also

continued until 55 Ma. Searle et al. further concluded

that these volcanic rocks in northern Kohistan were

then intruded by later, Stage 2 plutons of the Kohistan

batholith. Evidence for extensive granitoid emplace-

ment in Kohistan and Karakoram during and after the

initial collision of India with Kohistan comes from Rb–

Sr whole-rock age data of Debon et al. (1987). The

Fig. 13. (A) Ce/Y and Zr/Nb plot comparing Teru Volcanic

Formation with other similar units of Kohistan and Ladakh. (B

and of Teru Volcanic Formation compared with High Himalayas

(Allegre and Ben Othman, 1980; Searle et al., 1997), Trans

Himalayan (Allegre and Rousseau, 1988), Kohistan (Petterson et al.

1993; Khan et al., 1997) and western Ladakh (Rolland et al., 2002)


Batura unit in Karakoram has yielded ages of 45F7 and

43F3 Ma and the Gindai pluton has given an age of

59F2 Ma, suggesting that between 59 and 43 Ma a

second major intrusive phase of granitoids occurred,

into theweldedKohistan andKarakoram terranes. Thus

this subduction-related plutonism continued after

collision of India with Kohistan.

In contrast to the Searle et al. (1999) interpreta-

tion, our new 40Ar/39Ar age data for the Teru

Volcanic Formation suggests that volcanism contin-

ued until at least 33 Ma. During this period, the

Stage 2 Kohistan batholith were also emplaced. The

65 Ma Shunji Pluton is a good example of earlier

stage plutonic body on which the Teru Volcanic

Formation were extruded. The source of this

magmatic event seems to be the northward-dipping

Indian plate oceanic crust beneath Kohistan. Indirect

evidence for the subduction of Indian continental -

margin rocks beneath Kohistan exists in the form of

high and ultrahigh-pressure eclogite facies meta-

morphism of Indian plate rocks in the Western

Himalayas (metamorphic ages=ca. 55–44 Ma) (Pog-

nante and Spencer, 1991; Guillot and Le Fort,

1995). In the upper Kaghan Valley of Pakistan, the

eclogite-facies assemblages are developed in the

immediate footwall of the Main Mantle thrust

(Pognante and Spencer, 1991). The recent identifica-

tion of coesite inclusions in omphacite from one

sample of the upper Kaghan Valley eclogites by

O’Brien et al. (1999) was the first documentation of

ultrahigh-pressure metamorphism in the Himalayas

(~680 8C, 27 kbar). Sm–Nd and U–Pb geochronology

suggest a 44–49 Ma age for eclogite-facies meta-

morphism in Pakistan (Tonarini et al., 1993; Spencer

and Gebauer, 1996). These metamorphic ages all

predate the youngest Teru Volcanic Formation sample,

suggesting the arc-type volcanic activity continued

after the metamorphic events. This suggests that these

metamorphic ages would not represent the timing of

collision, but rather simply represent the ages when

the tectonic block last crossed the closure temper-

atures of analyzed elements. Instead, existence of 33

Ma arc-type volcanics in the Teru Volcanic Forma-

tions suggest strongly that convergent margin volcan-

ism survived until 33 Ma.

Evidence for subducted slab under Kohistan also

comes from recent tomographic imaging of the mantle

under Tibet, India and adjacent Indian Ocean, which

reveals several zones of high P-wave velocities at

various depths (Van der Voo et al., 1999). This work

revealed a regional northward-dipping slab, which is

still attached to the lithosphere of the Indian plate.

Under northern Pakistan, this slab show a roll-over

structure with the deeper portion overturned and

dipping southward (Van der Voo et al., 1999). This

)

-

,

.


roll-over structure, which supports the model of

continued northward convergence and indentation by

northeastern India over some 2000 km or more after

initial early Tertiary collision.

5.4. Comparison of Teru Volcanic Formation with

other units

Following Clift et al. (2002), Teru Volcanic

Formation is compared with other units of Kohistan

and Ladakh using Ce/Y and Zr/Nb plot and ENd (Fig.

13). The units selected for this comparison include

Chalt Volcanics (Petterson et al., 1991), sedimentary

Nindam Formation from Ladakh (Clift et al., 2000),

Dras 1 and Kardung volcanics (Clift et al., 2002),

volcanics from eastern Ladakh, western Ladakh and

Shyok Suture (Holland et al., 2002), and rocks from

Kohistan batholith (Honegger et al., 1982). On a Ce/Y

versus Zr/Nb plot Teru Volcanic Formation show

broad range and overlap with the fields of all the units,

but are similar to Kardung Volcanics. Fig. 12B shows

ENd composition for Teru Volcanic Formation and

other units of Kohistan and Ladakh. Teru Volcanic

Formation show good correlation with Kohistan,

Ladakh and volcanics of western Ladakh.

6. Conclusion

Our results giving evidence for post-collisional

volcanic event(s) in Kohistan might indicate pre-

viously unrecognized widespread volcanic activity

throughout the Himalayas after the collision of the

Indian plate with Asia. On the other hand, we find

that the isotopic (Sr–Nd–Pb) and trace element

compositions of the Teru Volcanic Formation indi-

cate a predominantly mantle signature. Even the

younger (33 Ma) volcanic sample shows a dominant

mantle signature. This is contrary to our expect-

ations that the post-collisional volcanic rocks should

have a strong geochemical signature from the

interaction with the Indian lithosphere. This may

indicate that final collision of Indian plate with

Kohistan occurred somewhat later than generally

accepted thus raising additional questions. We

recommend further investigation of the significance

of these ages for the Teru Volcanic Formation by

using U–Pb dating methods.

Acknowledgements

We thank S. Ahmad, F. Ali, M. Shamas, O.

Khattak (University of Peshawar), T. Plank (Boston

University), J. Shervais (Utah State) and P. Treloar

(Kingston University, London) for their suggestions

and criticism.

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