www.elsevier.com/locate/jvolgeores

Journal of Volcanology and Geotherm

Australia and Indonesia in collision:

geochemical sources of magmatism

M.A. Elburga,*,1, J.D. Fodena, M.J. van Bergenb, I. Zulkarnainc

aDepartment of Geology and Geophysics, University of Adelaide, Adelaide, SA 5005, AustraliabDepartment of Earth Sciences, Utrecht University, the Netherlands

cRDCG-LIPI, Bandung, Indonesia

Received 14 October 2003; received in revised form 26 January 2004; accepted 15 July 2004

Abstract

The islands of Alor, Lirang, Wetar and Romang are located in the extinct section of the Sunda–Banda arc, where the

collision with the Australian continent has brought subduction to a halt. Intrusive and extrusive igneous samples show a wide

range of Sr, Nd and Pb isotopic characteristics. Samples from the northeast coast of Alor extend the trend of increasing206Pb/204Pb ratios along the arc in an easterly direction, with values as high as 19.6. Samples from Alor’s south coast, Lirang,

Wetar and Romang have appreciably lower 206Pb/204Pb ratios (V19.1), and 143Nd/144Nd ratios down to 0.5119. The Pb isotope

data are interpreted as reflecting mixing between two internally variable end members within the subducting Australian

continent, either the upper and lower crust, or two upper crustal end members of different ages. These melts may come up

virtually unmodified, giving rise to the felsic, low 143Nd/144Nd samples, or may interact with the mantle, of which the partial

melts and the fractionation products thereof give rise to basalts to rhyodacites with more intermediate Nd isotopic

characteristics. Mixing modelling of the latter samples’ isotopic ratios constrains the amount of crustal material that has been

added to the mantle wedge to reach up to 9%. The isotopic and trace element heterogeneity in the samples studied is likely to

reflect inhomogeneity of the crustal sources contributing to magmatism.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Sunda–Banda arc; subduction; arc–continent collision; slab break-off; Pb; Sr; Nd isotopes

0377-0273/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jvolgeores.2004.07.014

* Corresponding author. Department of Isotope Geochemistry,

Free University, De Boelelaan 1085, Amsterdam 1081 HV, The

Netherlands. Tel.: +31 20 4447397; fax: +31 20 4449942.

E-mail address: marlina.elburg@falw.vu.nl (M.A. Elburg).1 Now at: Max Planck Institute for Chemistry, P.O. Box 3060,

55020 Mainz, Germany.

1. Introduction

The eastern part of the Indonesian Sunda–Banda

arc is the world’s prime example of an active arc–

continent collision zone, and provides us with a

unique natural laboratory to study the effects of such a

collision on the geochemistry of erupted magmas.

Documenting the geochemistry of magmas generated

al Research 140 (2005) 25–47

M.A. Elburg et al. / Journal of Volcanology and Geothermal Research 140 (2005) 25–4726

in a present-day arc–continent collision zone is not

only a prerequisite for recognising such deposits in the

geological record, but also places constraints on the

physical parameters describing the collision and

refines our knowledge of processes involved in arc

magma genesis.

Most studies of subduction-related volcanism are

concerned with steady state subduction of oceanic

lithosphere underneath oceanic or continental crust.

Within this context there are many studies of geo-

chemical changes in space, such as along (Hawkes-

worth and Ellam, 1989; Hilton and Craig, 1989;

Kersting et al., 1996; Turner et al., 1997) or across

arcs (Whitford and Nicholls, 1976; Woodhead and

Johnson, 1993; Hochstaedter et al., 2000; Churikova

et al., 2001), and their implications for geochemical

sources and physical slab–wedge transfer processes.

However, relatively little is known about magmatism

associated with the final stages of the subduction

process during arc–continent or continent–continent

collision, when the entrance of a buoyant piece of

crust brings subduction to a halt.

The northward movement of the Australian plate

beneath the Indonesian Sunda–Banda arc has led to

collision between the eastern portion of the arc and the

Australian continent. This has caused cessation of

magmatism, which first affected the islands of Wetar,

Lirang and Atauro (of which the youngest deposits are

3 Ma; Abbott and Chamalaun, 1981) and has since

spread to Romang, Alor and the Pantar Strait islands

(Fig. 1A). The volcanic deposits directly to the east

and west of the inactive segment have been the

subject of previous geochemical studies (Vroon et al.,

1993, 2001; Hoogewerff et al., 1997). However, the

inactive area itself has received comparatively little

attention. Early work includes the theses by De Jong

(1941) and Heering (1941), which focused on field

and petrological descriptions of the igneous rocks

from Alor, Wetar and Lirang. More recent studies on

the inactive segment have dealt with its geochronol-

ogy (Abbott and Chamalaun, 1981), or presented

limited geochemical data for some of the islands

(Whitford et al., 1977; McCulloch et al., 1983; Vroon

et al., 1993, 2001; Elburg et al., 2002).

In this contribution we report new major and trace

element and Sr, Nd and Pb isotopic data for samples

from the inactive islands of Alor, Lirang, Wetar and

Romang (Fig. 1). Rb–Sr dating was undertaken on

selected samples to constrain the temporal develop-

ment of the geochemical signature. The isotopic data

show a pronounced regionality, with striking north-to-

south Pb isotopic trends on the island of Alor. These

are interpreted as reflecting mixing between different

subducted crustal components and the sub-arc mantle,

whose Sr, Pb and Nd budget is swamped by the

crustal contributions.

2. Tectonic situation

An excellent review of the tectonic development of

the southeast Asian region during the past 50 Ma has

been given by Hall (2002), and the following

summary is largely based on this work.

The Indonesian subduction system has a long

history, with the Eocene–Oligocene arc being located

along the edge of Sundaland, from Sumatra through

Java, Sumba and western Sulawesi. Collision between

western Sulawesi and several continental slivers of

Australian origin caused a jump in the subduction

system to a more southerly position during the middle

Miocene, with volcanic activity commencing on the

oceanic plate to the east, giving rise to the islands of

Bali, Lombok and Sumbawa. Extension in the Flores

Sea started round 10 Ma as a result of southward

movement of the subduction hinge. This caused

movement of the island of Sumba to a fore-arc

position, and development of the south Banda Sea

(Honthaas et al., 1998). Progressive eastward develop-

ment of the subduction system gave rise to the eastern

Sunda and Banda arc. The South Banda Basin was

long thought to be a trapped piece of Mesozoic

oceanic crust (Bowin et al., 1980), but recent work has

shown that it formed by back-arc extension during

Late Miocene–Early Pliocene time (Hinschberger et

al., 2001; Honthaas et al., 1998). The age of the

subducting lithosphere along the arc increases from

50–90 Ma along Sumatra to 140–160 Ma near Flores

(Widiyantoro and van der Hilst, 1996). This age

variation has resulted in a distinct steepening of the

subducted slab towards the east (Widiyantoro and van

der Hilst, 1996; Schoffel and Das, 1999) and the onset

of tearing in the oldest part (Spence, 1986). The

boundary between the oceanic and continental part of

the downgoing plate is thought to lie in the vicinity of

Sumba (Lynnes and Lay, 1988). Seismic data suggest

Fig. 1. (A) Overview of the Sunda–Banda arc, including the location of Indonesian shelf sediments (track III and DSDP 262) from Vroon et al.

(1995). Triangles indicate active volcanoes. BA=Banda, MA=Manuk, SE=Serua, NI=Nila, TE=Teon, DA=Damar, BT=Batu Tara. Thin lines

with numbers denote depth to the Benioff zone after McCaffrey (1989). (B) Close-up of the islands from the inactive segment. Sample code for

Alor: circles: young volcanics (grey: NE coast, white: S coast), squares: older volcanics; triangles: intrusives. Intrusive samples from coastal

areas were collected as float.

M.A. Elburg et al. / Journal of Volcanology and Geothermal Research 140 (2005) 25–47 27

that the oceanic part of the plate may have started to

disconnect from the continental part in the collision

zone (McCaffrey et al., 1985; Charlton, 1991). The

active Banda arc has a strongly curved shape, leading

to the idea that two slabs with opposite subduction

direction may be involved (McCaffrey, 1988). More

recent interpretations of the seismic data argue that the

area is underlain by a single spoon-shaped subducted

oceanic slab which extends to a depth of more than

600 km, but which is faulted at shallow depth at the

eastern end (Widiyantoro and Van der Hilst, 1997;

Milsom, 2001).

The exact start of the collision between the Sunda–

Banda arc and the northward moving Australian

continent is unclear, but volcanism first ceased on the

island of Wetar, where the youngest volcanic deposits

having been dated at 3 Ma (Abbott and Chamalaun,

1981). Although this puts a minimum age on the start of

continental collision, someworkers believe it may have

been as early as 8 Ma, based on 40Ar/39Ar dating of the

Aileu formation in eastern Timor (Berry and McDou-

gall, 1986). The present volcanically inactive area

extends from the Pantar Strait islands in the west

through Alor, Atauro, Lirang and Wetar to Romang in

M.A. Elburg et al. / Journal of Volcanology and Geothermal Research 140 (2005) 25–4728

the east. Continuing deformation in the area is taken up

by the Wetar thrust to the north of the volcanically

extinct islands (Kreemer et al., 2000). There is no

seismicity between 50 and 380 km beneath Timor and

the volcanically inactive islands to its north (McCaf-

frey, 1989; Milsom, 2001).

It is generally thought that the eastern Sunda and

Banda arcs have been built on oceanic crust (Bowin et

al., 1980), but some workers have proposed the

presence of a piece of continental material in the

Wetar area on the basis of slightly elevated crustal

thicknesses (Richardson and Blundell, 1996) and the

limited Sr and Nd isotopic data available for the area

(Honthaas et al., 1998).

3. Sample description

Samples from Wetar and Lirang were derived from

pre-existing collections (De Jong, 1941; Heering,

1941) and some south coast samples (sample numbers

prefixed WE) came from the Snellius II expedition

(SOZ-LIPI, 1984–1985). The only age constraints that

exist are given by the K–Ar work of Abbott and

Chamalaun (1981). Extrusives from Wetar were dated

between 3 and 8 Ma on whole rocks, and hornblende

separates from intrusives on the south coast yielded

ages of 6 to 13 Ma. Although Abbott and Chamalaun

(1981) assume that the approximately 12 Ma age for

the intrusives is correct, and that the younger ages

reflect argon loss, it is equally well possible that

excess argon has influenced their results, since K

contents of the separates were low (0.16–0.2%). No

other igneous samples with ages of more than 8 Ma

have been reported from the area (Honthaas et al.,

1998). The younger end of the age range of the

extrusives is confirmed by our Rb–Sr isochron (Table

1) of biotite, apatite and whole rock on a completely

fresh rhyolite (H260) from the east of the island

(3.48F0.45 Ma; MSWD=0.16). We analysed four

coarse-grained intrusives (WE1E2, WE1K2, H110x,

H138x) and a dolerite (H135) from near the area

where Abbott and Chamalaun (1981) found their 6–13

Ma intrusives. Apart from a dolerite from a more

northerly area (H19x) all other Wetar samples

analysed were extrusives, among which were two

samples of cordierite-pseudomorph bearing rhyolites

(WE1A1 and WE1A2) that overlie the intrusives.

The samples from Lirang and the small island of

Pulau Babi to the east consist of quartz–feldspar

rhyolites, a granodiorite and a holocrystalline sample

of intermediate-fine grained plagioclase and horn-

blende, which is found as a dyke crosscutting the

granodiorite. This is described as a hornblende-

spessartite by De Jong (1941). Both the granodiorite

and the hornblende-spessartite are altered. Whole-

rock K–Ar dating of Lirang samples has been

performed by Honthaas et al. (1998) who obtained

an age of 4.1F0.2 Ma for a rhyolite and 6.7F0.4 Ma

for a granodiorite.

The interior of the island of Alor has reasonably

strong topographic relief with the highest peak

reaching 1700 m. No volcanic landforms can be

distinguished, and most relief is likely to be a

reflection of erosion and post-depositional uplift that

has been shown to be in the order of 1 mm/year

(Hantoro et al., 1994). Most exposed rock types are

igneous, but minor amounts of limestone, in the form

of uplifted coral reefs, outcrop on the northwest

peninsula. During our fieldwork, three major types of

igneous deposits have been sampled. The island is

blanketed by a cover of relatively fresh pyroclastics

and subordinate lava flows, which we will refer to as

the byounger volcanicsQ. Underneath these fresh

deposits is a layer of strongly altered volcanics, which

we have called the bolder volcanicsQ. The older

volcanics have been intruded and altered by dioritic

to granodioritic intrusives, and seem to be the oldest

exposed deposits on the island. The intrusives are only

exposed in deeply incised valleys in the interior of the

island.

The older volcanics include ignimbrites and

mingled mafic–felsic lavas. Plagioclase has been

extensively altered to sericite and pseudomorphs of

pyroxene consist of chlorite and fibrous amphibole.

Relatively large crystals of Fe–Ti-oxide may be

phenocrystic. Calcite and quartz occur as alteration

minerals.

The intrusives range from diorite to granodiorite,

with mafic enclaves being relatively common in the

granodioritic bodies. The distribution of the different

intrusive rock types cannot be deduced from the

limited outcrop that could be investigated. Their

mineralogy changes from clinopyroxene, plagioclase

and Fe–Ti-oxide in the more mafic samples to

hornblende, biotite, plagioclase, Fe–Ti-oxide with

Table 1

Rb–Sr isochron results

Sample Phase 87Rb/86Sr 87Sr/86Sr Rb (ppm) Sr (ppm) Result

H260 AP 0.0063 0.708166 0.5807 265.21 age: 3.48F0.45

Wetar WR LD 1.5025 0.708206 104.01 200.29 MSWD: 0.16

Extrusive BT 68.0124 0.711510 444.75 18.93 initial: 0.708145/82

ME00AL63 WR LD 0.6176 0.708044 102.14 478.49 age: 1.38F0.17

Alor AP 0.0044 0.708033 1.00 655.80 MSWD: 1.4

NE coast BT LD 4.1173 0.708142 14.70 10.33 initial: 0.708045/64

Extrusive BT LT 50.2370 0.708963 676.40 38.96

BT 36.4053 0.708867 495.21 39.36

ME00AL151 WR LD 0.6726 0.710888 82.34 354.30 age: 2.33F0.69

Alor AP 0.0111 0.711083 2.71 709.03 MSWD: 13

S coast BT LD 28.8235 0.711472 142.09 14.27 initial: 0.71099/45

Extrusive BT LT 85.9754 0.713973 604.95 20.37

BT 69.2973 0.713317 455.31 19.01

PL 0.0184 0.711309 4.84 760.60

ME00AL116 AP 0.0065 0.707460 3.08 1375.96 age: 1.51F0.63

Alor BT 55.4477 0.708822 686.98 36.828 MSWD 14

North BT LT 140.9202 0.710726 1188.24 24.40 initial: 0.70769/67

Tuff BT LD 0.1262 0.707931 17.33 397.37

ME00AL42 AP 0.0337 0.706532 2.72 233.44 age: 2.52F0.13

Alor AM+PX 0.7305 0.706633 4.33 17.15 MSWD: 0.29

Central BT 106.6672 0.710385 408.98 11.10 initial: 0.706593/89

Intrusive BT LD 3.4632 0.706890 2.49 2.08

BT LT 158.6575 0.712379 645.68 11.78

WR 0.6832 0.706628 87.59 370.87

RO8C5 WR 1.8490 0.709266 135.16 211.52 age: 1.72F0.26

Romang AP 0.0399 0.709234 12.14 881.38 MSWD: 0.1111

Extrusive PL 0.0403 0.709190 11.40 818.41 initial: 0.709210/73

BT 36.6794 0.710112 374.20 29.52

AP=apatite, BT=biotite, PL=plagioclase, AM=amphibole, PX=pyroxene, WR=whole rock, LD=leached, LT=leachate. Errors on the 87Rb/86Sr

ratio were taken as 1% for calculation of the isochrons; one standard deviation of the measured 87Sr/86Sr ratio was taken for the Sr isotope ratio.

M.A. Elburg et al. / Journal of Volcanology and Geothermal Research 140 (2005) 25–47 29

interstitial quartz and K-feldspar at the felsic end of

the spectrum. Apatite is a common accessory phase.

Clinopyroxene can be partially altered to green

fibrous amphibole, and biotite can be replaced by

chlorite. Rb–Sr dating of a float sample gave a reliable

age of 2.52F0.13 Ma (MSWD 0.029; Table 1).

The younger volcanics are lava flows, tuffs and

coarse pyroclastic flows with clast sizes varying

between 100 and 10 cm, with most in the 20 cm

range. Most tuffs and pyroclastic flows contain a

homogenous clast population, which is therefore

assumed to be juvenile. The clasts range from basalt

to rhyolite, but most are andesitic in composition.

Basalts contain olivine, clinopyroxene and plagio-

clase, while orthopyroxene replaces olivine and

hornblende clinopyroxene in the intermediate to

felsic samples. Quartz appears as a phenocryst phase

in the rhyolitic samples. Biotite is rare, and has

mainly been found in samples from the NE coast,

apart from sample ME00AL151, which comes from

the south coast. Loose blocks with biotite can also be

found on the NW coast, but these have been derived

from the neighbouring island of Ternate, and are not

included in this paper. Evidence for magma mixing is

common in the form of resorbed plagioclase phenoc-

rysts, olivine overgrowths of hypersthene, and the

occurrence of incompatible assemblages such as

olivine+ quartz. Several samples from the north coast

near Tanjung Babi (Fig. 1B) contain cm-sized

xenoliths of recrystallised holocrystalline material or

M.A. Elburg et al. / Journal of Volcanology and Geothermal Research 140 (2005) 25–4730

large, rounded plagioclase xenocrysts, which are

most likely derived from the underlying intrusives.

The younger volcanics have been divided into three

groups according to their geographical location

(south coast, northeast coast, central Alor) to facili-

tate discussion of their isotopic signature (see below).

Three samples were dated by Rb–Sr, with the most

reliable analysis (MSWD 1.4) providing an age of

1.38F0.17 Ma. The two other samples gave error-

chrons with an MSWD around 14 and ages of 1.5

and 2.3 Ma. The high MSWD indicates that there

was no isotopic equilibrium between the mineral

phases, and that these ages can be taken as being

approximate at best.

Samples from Romang have been analysed and

described by Vroon et al. (2001, 1993) and these

analyses have been incorporated into the database

used for this study. Three additional samples from the

same sample collection were analysed during the

present study. One sample was dated and gave an age

of 1.72F0.26 Ma (MSWD 0.111).

4. Analytical techniques

Samples were crushed in a stainless steel jaw

crusher after removal of weathered rims. A split was

ground to b2 Am grainsize in a tungsten carbide ring

mill, and this material was used for XRF, ICP-MS and

Sr, Nd and Pb isotopic analyses.

XRF analyses were performed at the Department of

Geology and Geophysics, Adelaide University, fol-

lowing procedures described by Elburg et al. (2002).

Most Sr, Nd and Pb isotopic compositions were

measured on a Finnigan MAT 262 Thermal Ionisation

Mass Spectrometer in static mode at Adelaide Uni-

versity. The average 87Sr/86Sr ratio for SRM987

during the time when the samples were run was

0.710278F26 (2r, n=45). Whole procedure blanks

for Sr are better than 1 ng. Rb and Sr concentrations

were measured for mineral separates and whole rocks

used for geochronology by splitting the dissolved

sample and spiking with 84Sr and 85Rb spike.

Reproducibility was monitored by analysis of the in-

house standard TasBas and is better than 1% for the

Rb/Sr ratio. Typical Rb blanks were less than 100 pg.

Leaching of biotite for the Rb–Sr isochrons was

performed in capped beakers with 6 N HCl for 24 h

on a hotplate at 150 8C. The supernatant was pipettedoff and the sample was washed twice with distilled

water, which was added to the leachate. Leaching of

whole-rock powders was done with hot 3N HCl for 30

min, with the rest of the procedure the same as for the

biotite. Leachates and residues were analysed for

isotopic composition only.

Nd isotopic ratios were monitored by measuring

J&M specpure Nd2O3, and this yielded a 143Nd/144Nd

ratio of 0.511569F22 (2r, n=25). The value for BCR-1 at the time was 0.512590F28, and LaJolla gave

0.511800F22. Nd blanks are better than 500 pg.

All Pb isotopic analyses were performed at

approximately the same temperature of 1150 8C, anda mass fractionation factor of 0.10% per amu was

used, based on replicate analyses of the NBS981 Pb

standard. Two sigma errors are 0.008 for 206Pb/204Pb,

0.0012 for 207Pb/204Pb and 0.030 for 208Pb/204Pb

based on repeated measurements of the NBS981

standard. This mainly reflects variation in the fractio-

nation factor.

One sample from Wetar (WE1A1) was analysed

at the Free University of Amsterdam for its major

and trace element and isotopic composition, follow-

ing the procedures described by Vroon et al.

(1993).

Rare Earth Elements (REE) and some other low

abundance elements were analysed by ICP-MS at the

Department of Earth Sciences, Monash University,

and the University of Queensland, using calibration

curves based on rock standard BHVO-1 following

Eggins et al. (1997) for trace element content. Drift

corrections were applied by the repeated analysis of

dummy standards and the use of an internal In

standard. Washing between samples was done with a

dilute HF solution, to minimise the memory effect for

elements like Hf. Reproducibility is better than 2% for

all elements, except for Cs and Hf, for which it is

better than 4%. Accuracy is in the order of 5% for all

elements.

5. Data presentation

5.1. Major and trace element geochemistry

Representative Harker diagrams are shown in Fig.

2. They include data from the Pantar Strait volcanoes

Fig. 2. Selected Harker variation diagrams. Shaded area is the field for the Pantar Strait volcanoes from Elburg et al. (2002).

M.A. Elburg et al. / Journal of Volcanology and Geothermal Research 140 (2005) 25–47 31

to the west of Alor (Elburg et al., 2002), Atauro (or

Pulau Kambing; Whitford et al., 1977), located

between Alor and Lirang, and the samples from

Romang analysed by Vroon et al. (1993). Alteration

is a significant problem for the older volcanics, as seen

in hand sample, and in high values for loss on ignition

(LOI); therefore, concentrations of mobile elements

such as K2O and the Large Ion Lithophile Elements

(LILE) must be regarded with caution. Alteration is not

judged to be a significant problem for the other

samples; minor alteration is, however, seen in the

Wetar intrusives and cordierite rhyolites; Alor intru-

sive ME99AL16 and south coast younger volcanic

ME00AL151; and Lirang spessartite J82x.

The samples analysed show a wide range of SiO2

(48–78%) and MgO (0–7%) contents (Table 2), and

most can be classified as belonging to the high-K

series. Most samples from Alor are andesites to

dacites, whereas the majority of the Wetar extrusives

are more silicic. The rhyolites with cordierite

pseudomorphs from the latter island are characterised

by very high potassium contents. The Romang

samples show a bimodal distribution of andesites

and rhyolites and three of the four Lirang samples have

SiO2 contents higher than 70%. No sample has TiO2

contents higher than 1.3%, as is typical for arc-type

magmas.

Harker diagrams for trace elements show signifi-

cant scatter. Highly compatible elements such as Cr

and Ni are generally low (b80 and b30 ppm,

respectively) except for some of the most magne-

sium-rich samples. Sr contents decrease with

increasing silica contents, as expected for crystal

fractionation of a plagioclase-bearing assemblage.

Average Sr contents in the studied samples are lower

than observed in the Pantar Strait volcanoes, which

are the westernmost volcanoes of the extinct section.

The Wetar intrusives have exceptionally low Sr

contents. These low contents are unlikely to be a

reflection of alteration, since contents of less mobile

Table 2

Whole-rock data for selected samples

Group

sample

Alor

old volc.

ME99AL25

Alor

old volc.

ME00AL88

Alor

intrusives

ME99AL16

Alor

intrusives

ME00AL42

Alor

intrusives

ME00AL94

Alor

young volc.

ME99AL13

Alor

young volc.

ME99AL29

Alor

young volc.

ME99AL35

Alor

young volc.

ME99AL40

Alor

young S.

ME00AL127

Alor

young S.

ME00AL147

SiO2 61.03 65.90 51.17 61.92 67.29 56.49 63.89 47.53 53.63 59.50 61.31

Al2O3 16.75 16.00 20.85 16.77 15.26 18.19 16.60 18.49 18.78 16.47 17.17

Fe2O3 6.17 4.51 8.95 5.71 4.50 8.08 5.64 12.22 8.92 8.68 6.81

MnO 0.15 0.04 0.14 0.08 0.07 0.14 0.07 0.19 0.16 0.14 0.12

MgO 3.92 2.55 3.05 2.50 1.93 3.11 1.84 5.97 4.40 2.26 3.02

CaO 5.24 3.30 10.46 5.48 4.23 7.46 5.10 11.02 8.90 6.55 6.66

Na2O 3.40 4.32 2.95 3.27 2.96 2.62 3.64 2.31 2.96 3.42 3.14

K2O 2.67 0.99 1.25 2.73 3.17 2.82 2.47 1.02 1.40 1.81 1.19

TiO2 0.57 0.48 0.95 0.59 0.49 0.89 0.60 1.04 0.72 1.05 0.53

P2O5 0.12 0.13 0.23 0.14 0.10 0.20 0.15 0.20 0.13 0.12 0.06

LOI 6.31 1.77 2.04 0.81 0.89 0.60 0.65 0.04 0.31 0.70 1.25

Total 99.92 99.64 99.69 99.89 99.37 99.66 99.35 99.96 99.97 100.19 99.71

Sc 19 8.8 21.8 17.7 12.5 23.1 15 27.2 25.5 26.6 21.6

V 148 89 231 128 91 216 127 333 220 245 146

Cr 58 15 19 20 9 8 4 4 13 7 39

Ni 23 6 17 8 4 9 4 12.81 1 6 13

Cu 3 7 74 7 6 66 24 42.697 52 129 22

Zn 70 33 68 29 19 66 47 76.924 76 83 46

Ga 15.8 15.5 20.9 18.2 14.3 20 16.3 16.565 20.5 18.9 18

Rb 76 26 44 90 87 111 75 20 46 66 16

Sr 231 366 609 375 338 478 472 769 498 274 238

Y 19 15 25 29 18 30 21 22 20 28 16

Zr 105 124 90 143 136 154 147 51 96 113 74

Nb 5 8 5.6 9.1 7.3 8.0 6.6 3.0 3.7 4.6 2.7

Cs 2.92 1.95 0.79 4.34 1.25 1.35 2.38 6.47 4.76

Ba 738 414 564 950 1381 793 1180 783 559 417 324

La 27 11 31.5 30.0 40.8 34.0 41.0 34.0 21.5 14.9 7.5

Ce 47 24 61.2 58.3 64.3 62.7 69.4 61.8 40.0 31.2 15.6

Pr 7.2 6.8 6.3 7.2 7.3 7.1 4.8 3.9 1.9

Nd 18 7 27.4 25.4 20.5 27.4 26.0 26.7 17.9 16.0 7.8

Sm 5.6 5.3 3.6 5.6 4.5 5.4 3.6 4.0 2.0

Eu 1.48 1.18 0.87 1.41 1.29 1.51 1.09 1.11 0.64

Gd 4.91 4.94 3.03 5.04 3.74 4.83 3.47 4.37 2.26

Tb 0.73 0.76 0.46 0.80 0.54 0.68 0.52 0.72 0.38

Dy 4.30 4.46 2.77 4.41 2.99 3.85 3.03 4.54 2.44

Ho 0.90 0.97 0.58 0.90 0.61 0.79 0.62 0.99 0.55

Er 2.45 2.81 1.67 2.64 1.82 2.16 1.84 2.88 1.61

Tm 0.37 0.43 0.26 0.39 0.28 0.32 0.27 0.44 0.26

Yb 2.31 2.78 1.74 2.47 1.83 1.99 1.73 2.85 1.71

Lu 0.34 0.42 0.27 0.37 0.29 0.29 0.26 0.43 0.26

Hf 2.57 4.33 3.40 3.91 3.36 1.59 2.94 3.09 1.95

Pb 9 3 5.36 11.04 8.50 21.25 20.05 14.45 16.51 24.03 14.97

Th 11 17 7.34 17.78 13.77 10.27 16.14 12.19 6.97 5.06 2.24

U 2.4 3.1 1.73 4.78 2.71 2.52 3.29 2.04 1.61 2.27 0.91

All oxides and most elements by X-ray fluorescence; Nb, Cs, REE, Hf, Pb, Th and U by ICP-MS for samples with full analyses.

M.A. Elburg et al. / Journal of Volcanology and Geothermal Research 140 (2005) 25–4732

incompatible elements such as Zr and the rare earth

elements (REE) are also very low. The felsic

samples from Romang are characterised by high

contents of Zr and Nb (not shown) compared to

samples of equivalent SiO2 contents from the other

islands. MORB-normalised trace element patterns of

nearly all samples show the characteristics of arc

volcanics with negative Nb and Ti anomalies and

positive K and Pb spikes (Fig. 3). However, the

magnitude of the anomalies varies between samples,

and nearly all show higher normalised Nb contents

than Zr. The relative enrichments of Ba and Rb also

vary in an unsystematic way, as shown by the three

mafic samples from Alor (Fig. 3B). As these

samples are completely fresh, alteration cannot be

responsible for these variations.

Alor

young S.

ME00AL151

Alor

young NE

ME00AL64

Alor

young NE

ME00AL72

Alor

young NE

ME00AL82

Romang

RO8B2

Romang

RO8C5

Wetar

crd-rhy

WE1A2

Wetar

H107

Wetar

intrusive

H110x

Wetar

J15x

Wetar

J27

Wetar

J101

Lirang

intrusive

J82x

63.09 52.17 78.27 52.81 60.86 72.09 69.07 76.21 50.55 65.23 73.36 55.21 52.77

18.08 17.65 12.32 18.37 17.24 14.85 14.98 13.27 17.14 16.11 14.40 18.38 16.93

4.37 7.27 0.67 9.27 5.71 2.23 2.57 1.54 10.68 5.29 2.23 7.81 9.73

0.06 0.12 0.01 0.15 0.10 0.03 0.05 0.02 0.15 0.11 0.05 0.13 0.10

2.02 6.91 0.23 5.11 3.24 0.40 1.31 0.15 7.01 1.51 0.54 4.60 7.47

5.86 9.67 1.47 9.35 7.10 2.10 2.02 2.14 9.28 4.32 3.33 8.93 6.80

3.49 2.88 3.11 2.77 2.80 3.94 2.89 3.41 3.24 3.18 3.13 2.49 4.61

2.54 1.39 3.77 1.31 2.17 3.87 6.61 3.08 0.84 3.29 2.65 1.50 0.25

0.41 0.69 0.12 0.73 0.67 0.41 0.29 0.14 1.05 0.74 0.26 0.76 1.15

0.10 0.13 0.02 0.13 0.10 0.07 0.20 0.02 0.06 0.21 0.05 0.19 0.20

1.00 1.09 0.70 0.34 1.39 0.35 2.71 1.91 1.37 2.18 1.85 1.15 1.91

99.76 100.12 99.57 100.22 99.73 99.39 99.65 99.14 100.12 99.50 99.31 99.90 99.58

10.7 24 1.2 24.7 19.7 8.7 8.8 7.3 42.1 15.3 7.4 26.6 37.2

84 196 9 229 183 22 29 5 434 54 27 255 333

9 174 8 33 24 2 12 3 24 1 8 45 127

6 63 2 20 10 1 7 2 20 1 4 20 54

15 33 6 54 43 6 2 7 5 4 12 29 537

42 45 14 75 60 28 32 34 56 70 28 66 51

18.3 16.4 12 18.1 18.4 18.8 17.1 14.9 17.4 18.2 15.5 17.6 16.2

97 61 185 44 83 141 234 92 25 118 47 44 3

365 526 218 493 362 213 46 97 152 338 168 588 173

11 17 8 20 25 32 15 26 16 33 17 27 23

112 85 92 106 134 215 93 151 27 186 100 86 76

8.2 4.9 7.2 3.6 15.2 20.3 14.2 7.1 1.3 11.0 6.6 4.4 2.3

3.66 4.45 11.83 2.59 4.69 4.62 1.79 11.37 1.10 5.40 2.10 2.71 0.11

464 680 1160 501 610 860 525 465 260 831 440 661 100

20.4 24.8 42.3 21.8 26.7 38.6 19.1 21.5 3.4 41.0 13.0 36.8 8.9

39.1 44.5 59.0 41.9 49.1 75.4 40.6 42.2 8.0 73.9 24.6 59.1 19.5

4.2 5.0 5.0 4.9 5.7 8.4 4.7 4.8 1.2 8.7 2.6 6.9 2.6

14.6 18.3 13.7 18.4 20.8 30.3 17.1 17.4 5.7 32.2 9.3 26.0 11.2

2.7 3.6 1.8 3.8 4.2 6.2 3.9 3.8 1.8 6.3 2.0 4.8 3.0

0.72 1.04 0.38 1.08 0.94 1.13 0.57 0.70 0.66 1.52 0.55 1.31 1.01

2.27 3.29 1.21 3.45 4.11 5.65 3.28 4.10 2.22 5.78 2.13 4.39 3.47

0.34 0.48 0.17 0.52 0.64 0.89 0.50 0.69 0.38 0.88 0.36 0.64 0.59

1.94 2.90 0.97 3.12 3.92 5.17 2.55 4.31 2.48 4.98 2.26 3.62 3.77

0.39 0.60 0.20 0.67 0.84 1.08 0.45 0.91 0.55 1.00 0.50 0.76 0.81

1.06 1.70 0.61 1.89 2.39 2.95 1.14 2.65 1.58 3.00 1.47 2.28 2.30

0.16 0.26 0.10 0.29 0.36 0.44 0.17 0.40 0.24 0.43 0.24 0.33 0.34

1.06 1.64 0.74 1.85 2.34 2.71 1.04 2.63 1.52 2.85 1.58 1.16 2.18

0.16 0.24 0.12 0.28 0.35 0.40 0.15 0.39 0.23 0.43 0.24 0.33 0.32

2.73 2.22 2.47 2.35 3.68 4.03 2.51 4.28 0.87 4.82 2.71 3.51 1.97

28.87 7.78 37.19 15.29 34.25 25.46 4.52 17.92 1.88 24.86 13.10 26.58 3.16

7.88 10.26 27.22 6.61 9.77 14.45 11.18 9.04 0.89 15.92 4.75 10.10 1.71

2.29 2.98 9.21 1.62 2.83 3.64 4.16 2.29 0.25 3.34 1.34 2.14 0.96

M.A. Elburg et al. / Journal of Volcanology and Geothermal Research 140 (2005) 25–47 33

Sample J82x, the hornblende-spessartite from

Lirang, is the only sample that does not show a

pronounced positive potassium anomaly (Fig. 3C),

and also has low contents of Ba, Rb and Cs. Although

petrographic inspection showed that the sample was

not as fresh as most others, it also has an unusually

high Nd isotopic ratio (see below), and this may

reflect a different source for this sample than for the

bulk of the magmas from this area of the Sunda–

Banda arc.

5.2. Sr, Nd and Pb isotopes

The samples analysed show a wide variation in

isotopic ratios (Table 3). Although there is the

expected negative correlation between Sr and Nd

Fig. 3. Normalised trace element diagrams for selected samples. MORB-normalising values from Sun and McDonough (1989). (A) Patterns for

Alor intrusives and older volcanics (ME00AL88). (B) Patterns for the three most mafic samples analysed of the younger volcanics from Alor’s

south coast (ME00AL127), central Alor (ME00AL35) and northeast coast (ME00AL64); shaded area for all younger volcanics from Alor. (C)

Patterns for Wetar cordierite–rhyolite (WE1A2), Wetar intrusive (H110x), Wetar volcanic (J15x), Lirang hornblende spessartite (J82x) and

Romang extrusive (RO8b2).

M.A. Elburg et al. / Journal of Volcanology and Geothermal Research 140 (2005) 25–4734

isotopic ratios when the full data set is considered,143Nd/144Nd ratios vary between 0.51265 and 0.51235

at 87Sr/86Sr ratios around 0.707 (Fig. 4A), and

samples from Alor’s south coast account for nearly

all of this spread. The highest Sr and lowest Nd

isotopic ratios are measured for the cordierite rhyolites

from the south coast of Wetar. Although these samples

are comparatively felsic, there is no clear-cut corre-

lation between an index of fractionation such as SiO2

content and isotopic composition for the data set as a

whole (Fig. 5). The spessartite from Lirang is the

sample with the most mantle-like Sr and Nd isotopic

compositions, although it still plots towards the field

of sediments compared to values for Indian Ocean

mid ocean ridge (I-MORB) or ocean island basalts (I-

OIB). Romang has, compared to the other islands, a

Table 3

Locations and Sr, Nd and Pb isotopic characteristics for samples from Alor, Lirang, Wetar and Romang

Sample Phase 87Sr/86Sr 2 S.E. 143Nd/144Nd

2 S.E. 206Pb/204Pb

207Pb/204Pb

208Pb/204Pb

Longitude

W

Latitude

S

Alor

ME99AL9 WR 0.706770 0.000009 0.512512 0.000008 19.222 15.703 39.552 124.37 8.37

ME99AL13 WR 0.706658 0.000014 0.512548 0.000010 19.160 15.719 39.613 124.73 8.31

ME99AL16 WR LD 0.706155 0.000014 0.512610 0.000014 19.159 15.689 39.541 124.71 8.27

ME99AL16 WR LT 0.706199 0.000016 0.512584 0.000007 19.115 15.684 39.487 124.71 8.27

ME99AL17 WR 0.706218 0.000008 0.512570 0.000006 19.141 15.687 39.537 124.71 8.27

ME99AL25 WR LD 0.706300 0.000013 0.512575 0.000008 19.125 15.684 39.479 124.80 8.22

ME99AL29 WR 0.706487 0.000010 0.512539 0.000005 19.124 15.669 39.418 124.79 8.17

ME99AL33 WR 0.706618 0.000017 0.512515 0.000005 19.188 15.713 39.561 124.41 8.26

ME99AL35 WR 0.706568 0.000011 0.512476 0.000006 19.190 15.717 39.554 124.41 8.28

ME99AL40 WR 0.707340 0.000009 0.512447 0.000005 19.273 15.713 39.645 124.49 8.26

ME00AL42 WR 0.706600 0.000013 0.512564 0.000007 19.127 15.691 39.522 124.88 8.17

ME00AL63ld WR LD 0.708044 0.000009 0.512399 0.000007 19.553 15.722 39.911 125.11 8.17

ME00AL64 WR 0.708008 0.000016 0.512411 0.000007 19.590 15.741 40.000 125.11 8.17

ME00AL72 WR 0.708474 0.000015 0.512379 0.000008 19.394 15.751 39.813 125.05 8.15

ME00AL82 WR 0.707296 0.000010 0.512433 0.000008 19.273 15.711 39.643 124.46 8.26

ME00AL88 WR LD 0.706387 0.000012 0.512544 0.000007 19.129 15.687 39.495 124.88 8.19

ME00AL88 WR LD

RPT

19.135 15.694 39.529 124.88 8.19

ME00AL88 WR LT 0.706504 0.000015 0.512586 0.000011 19.043 15.677 39.392 124.88 8.19

ME00AL94 AP 0.706476 0.000011 0.512518 0.000006 19.122 15.687 39.506 124.83 8.25

ME00AL116 AP 0.707460 0.000016 0.512524 0.000035 124.74 8.21

ME00AL127 WR 0.708533 0.000013 0.512530 0.000006 19.044 15.709 39.460 124.53 8.42

ME00AL128 WR LD 0.706002 0.000014 0.512607 0.000006 19.112 15.685 39.494 124.83 8.39

ME00AL128 WR LT 0.706045 0.000011 0.512618 0.000007 19.069 15.683 39.452 124.83 8.39

ME00AL135 WR 0.706240 0.000015 0.512608 0.000005 19.106 15.692 39.478 124.94 8.36

ME00AL138 WR 0.707190 0.000016 0.512451 0.000006 19.018 15.625 39.252 124.80 8.40

ME00AL143 WR 0.707268 0.000013 0.512593 0.000008 19.077 15.705 39.485 124.76 8.40

ME00AL147 WR 0.707334 0.000014 0.512640 0.000007 18.974 15.689 39.340 124.45 8.45

ME00AL151 WR LD 0.710888 0.000012 0.512226 0.000003 18.985 15.699 39.356 124.42 8.45

ME00AL151 WR LT 0.710084 0.000011 0.512264 0.000017 18.937 15.667 39.229 124.42 8.45

ME00AL152 WR 0.707152 0.000013 0.512366 0.000009 19.157 15.719 39.569 124.35 8.43

Lirang

J82x WR LD 0.705291 0.000013 0.512842 0.000005 19.082 15.674 39.445 125.74 8.02

J82x WR LT 0.707608 0.000012 0.512821 0.000010 18.833 15.646 39.142 125.74 8.02

J82x WR 0.705386 0.000011 0.512826 0.000024 19.013 15.647 39.310 125.74 8.02

J78 WR 0.707014 0.000010 0.512473 0.000004 19.088 15.678 39.483 125.75 8.03

J87 WR 0.707160 0.000012 0.512509 0.000007 19.093 15.683 39.481 125.75 8.03

J103x WR 0.709418 0.000011 0.512345 0.000007 19.123 15.707 39.527 125.77 7.97

Wetar

WE1A1a WR 0.716148 0.511998 19.023 15.702 39.482 126.44 7.94

J15x WR 0.708165 0.000010 0.512448 0.000005 19.079 15.697 39.497 126.18 7.81

J101 WR 0.707299 0.000008 0.512536 0.000011 19.077 15.704 39.484 125.89 7.77

J101 WR RPT 0.512534 0.000005 125.89 7.77

J27 WR 0.708491 0.000012 0.512440 0.000005 19.009 15.675 39.357 126.16 7.85

J27 WR RPT 0.512456 0.000009 126.16 7.85

H260 WR LD 0.708206 0.000011 0.512355 0.000011 18.973 15.670 39.332 126.80 7.71

WE1A2 WR LD 0.716732 0.000010 0.511937 0.000006 19.012 15.701 39.432 126.44 7.94

(continued on next page)

M.A. Elburg et al. / Journal of Volcanology and Geothermal Research 140 (2005) 25–47 35

Sample Phase 87Sr/86Sr 2 S.E. 143Nd/144Nd

2 S.E. 206Pb/204Pb

207Pb/204Pb

208Pb/204Pb

Longitude

W

Latitude

S

Wetar

WE1A2 WR LT 0.710673 0.000014 0.511947 0.000005 18.929 15.689 39.476 126.44 7.94

WE1K2 WR LD 0.707113 0.000013 0.512673 0.000006 18.948 15.685 39.332 126.44 7.94

H110x WR LD 0.708561 0.000015 0.512564 0.000006 18.953 15.680 39.308 126.40 7.88

H110x WR LT 0.707922 0.000014 0.512553 0.000021 18.653 15.652 38.952 126.40 7.88

H107 WR LD 0.712122 0.000012 0.512209 0.000005 19.096 15.716 39.516 126.40 7.90

J119x WR 0.706462 0.000010 0.512502 0.000007 19.119 15.706 39.578 125.88 7.94

Romang

RO6 WR 0.709048 0.000013 0.512384 0.000005 19.157 15.683 39.495 127.35 7.51

RO8B2 WR 0.708660 0.000012 0.512430 0.000005 19.145 15.667 39.443 127.35 7.55

RO8C5 WR 0.709266 0.000012 0.512406 0.000059 19.162 15.684 39.495 127.35 7.55

S.E.=standard error; WR=whole rock; LD=leached; LT=leachate, RPT=repeat; AP=apatite. The reported Pb isotope data have been corrected for

fractionation.a Analysed by Pieter Vroon, Free University, Amsterdam.

Table 3 (continued)

M.A. Elburg et al. / Journal of Volcanology and Geothermal Research 140 (2005) 25–4736

relatively restricted range of Sr and Nd isotopic

values, with little difference between the andesitic

and rhyolitic samples, which all fall around 87Sr/86Sr

of 0.709 and 143Nd/144Nd of 0.5124. The range in Sr

and Nd isotopic compositions of the samples studied

is far larger than seen in active volcanoes from the

eastern Banda (Vroon et al., 1993) or western Sunda

arc (Turner and Foden, 2001) (non-collisional Sunda–

Banda volcanics in Fig. 4B).

The data define a rather straight trend in a208Pb/204Pb versus 206Pb/204Pb diagram, but other

projections show more scatter. On the island of Alor,

samples from the south coast have the lower Pb

isotopic ratios, and those from the NE coast the

highest, with 206Pb/204Pb ratios reaching 19.6. One of

the south coast samples falls outside the field for the

rest of the group and has much lower 207Pb/204Pb

ratios. The samples from Wetar fall within the same

range as those from Alor’s south. Within a diagram of208Pb/204Pb versus 207Pb/204Pb, there appear to be two

groups on the low 207Pb/204Pb side of the diagram,

with the samples from Lirang and Romang defining

the high 208Pb/204Pb trend, and some from Alor’s

south and Wetar the lower 208Pb/204Pb group. How-

ever, these two groups do not adhere to strict

geographical control, since the low 207Pb/204Pb sample

from Alor’s south falls on the extension of the trend

defined by Lirang and Romang. The overall trend of

all samples in the Pb isotope diagrams runs between

the fields for North Australian sediments and that for I-

OIB. Compared to the Pantar Strait volcanoes, the

present data set shows a wider range in values, both on

the radiogenic and non-radiogenic sides of the Pb

isotope diagrams. The slope of the Pb data array for the

samples under discussion is significantly different

from that of the non-collisional Sunda–Banda vol-

canics in the 207Pb/204Pb and 208Pb/204Pb versus206Pb/204Pb diagrams; in the 208Pb/204Pb versus207Pb/204Pb diagrams, the slopes of the arrays are

similar, but the non-collisional volcanics have lower208Pb/204Pb for their 206Pb/204Pb ratio than the samples

from the Alor–Romang segment.

6. Discussion

Most models for the geochemical signature of arc

volcanics involve three main components: (1) the sub-

arc mantle, with isotopic and trace element character-

istics similar to, or more depleted than, a MORB

source (Pearce and Parkinson, 1993); (2) a fluid

component, which can be in equilibrium with the

oceanic slab (Miller et al., 1994), subducted sediments

(Class et al., 2000) or both (Hochstaedter et al., 2001);

(3) subducted sedimentary material, which is gener-

ally thought to be added to the mantle as a partial melt

(Elliott et al., 1997; Vroon et al., 2001). This type of

model has been applied successfully to the petro-

genesis of samples from the active parts of the Sunda–

Banda arc (Vroon et al., 1993; Turner and Foden,

2001), showing the increasing importance of sub-

ducted sediments towards the extinct area of the arc.

Fig. 4. Sr, Nd and Pb isotope diagrams for all samples, with fields for the Pantar Strait volcanoes (left) and I-MORB, I-OIB (data sources as in

Elburg and Foden, 1999), Indonesian shelf sediments and North Australian sediments (right). Range of 87Sr/86Sr for Atauro from Whitford et al.

(1977). Stars: Toba tuff (Sumatra) from Turner and Foden (2001). The samples identified in G are those that have the highest likelihood to have

suffered crustal contamination (silicic, high 87Sr/86Sr and low 143Nd/144Nd ratios).

M.A. Elburg et al. / Journal of Volcanology and Geothermal Research 140 (2005) 25–47 37

Fig. 5. 87Sr/86Sr versus SiO2 content of the samples. Stippled field is

for the Pantar Strait volcanoes. The data set as a whole does not

show any correlation between Sr isotopic ratio and indices of

fractionation, although the cordierite rhyolites from Wetar may

represent virtually pure crustal melts.

M.A. Elburg et al. / Journal of Volcanology and Geothermal Research 140 (2005) 25–4738

However, the difference in slope between the Pb

isotope arrays for the Alor–Romang segment, and

those for the non-collisional volcanics of the Sunda–

Banda arc shows that other end members must be

involved. In a collisional setting like the part of the

Sunda–Banda arc under discussion, the involvement

of (4) subducted continental crust and lithospheric

mantle must be considered rather than an oceanic slab

and entrained sediments. Helium isotope studies

(Hilton and Craig, 1989; Hilton et al., 1992) have

provided convincing evidence that the continental

crust itself, rather than subducted fine-grained sedi-

ment, has contributed to magmatism of the active and

recently extinct volcanoes in the area. Hilton et al.

(1992) did not incorporate the Neogene rocks from

the inactive segment from Alor to Wetar in their

helium isotopic studies, to avoid corrections necessary

to account for in-growth of 4He through time from

radioactive decay.

Our knowledge of the exact composition of the

north Australian continental crust is limited, and we

have used analyses of river sediments from north

Australia as a proxy (Elburg et al., 2002). However,

this will only give an approximation of the upper

crust, which is likely to be highly variable. No

information exists on the composition of the lower

crust or the Australian lithospheric mantle in the area.

We will now interpret the available isotopic and trace

element data to identify and constrain the relative

importance of sources involved in magmatism in the

Alor–Romang section, and compare this to data from

neighbouring volcanoes.

6.1. Upper crustal contamination

Before the merits of any source contamination

model can be discussed, it is necessary to assess

whether the geochemical characteristics of the sam-

ples have been influenced by upper crustal contam-

ination. This is especially important for the samples

with high 87Sr/86Sr, low 143Nd/144Nd ratios, and

intermediate to high silica contents, such as those

from Romang and Alor and Wetar’s south coast. Their

Sr and Nd isotopic characteristics resemble those of

the Lake Toba Tuff on Sumatra (Fig. 4B), which is

widely believed to have been formed by crustal fusion

(Turner and Foden, 2001). However, there is no good

correlation between the SiO2 content of magmatic

rocks and their isotopic ratio that would be expected if

assimilation were coupled with fractional crystallisa-

tion. (Fig. 5). Furthermore, the most mafic samples

already have values that show a significant crustal

influence, implying that the contamination originates

below the arc’s crust. An additional problem with the

idea of upper crustal contamination is that, unlike the

situation in Sumatra, there is no good evidence that

the volcanic arc in the area under discussion was built

upon continental crust. Although it has been sug-

gested that the crust in the Romang area is somewhat

thicker than normal oceanic crust (up to 20 km:

Richardson and Blundell, 1996), this has not been

described for the Alor–Wetar area. It is also clear from

Fig. 4 that the samples under discussion show no

resemblance to the Lake Toba Tuff in terms of their Pb

isotopic characteristics, so any potential upper crustal

contaminant cannot be located within the Sundaland

plate. Thereby, volcanic phenocrysts from Romang

have d18O values that are indistinguishable from

mantle values (Vroon et al., 2001), arguing against

significant upper crustal contamination in the area

where evidence exists for a thicker-than-normal crust.

In the absence of evidence for contamination by

continental crust from the overriding plate, an alter-

native assimilant could be the oceanic crust and

sediments of the upper plate. The sediments were

sampled during the Snellius II expedition and have

been included into the field for dshelf sedimentsT inFig. 4. It is clear from Fig. 4H that shelf sediments

M.A. Elburg et al. / Journal of Volcanology and Geothermal Research 140 (2005) 25–47 39

have a lower 208Pb/204Pb ratio for their 207Pb/204Pb

ratio than the more silicic samples from south Alor,

Lirang, Wetar and Romang, which are the most likely

to have experienced crustal contamination. On the

basis of the above considerations, we conclude that it

is unlikely that the samples with the most dcrustalTisotopic ratios obtained these characteristics by upper

crustal contamination.

6.2. Identification of Pb isotopic end members

Pb isotopic data can give us a first impression of

the identity of end members involved in magma

petrogenesis, since, unlike in Sr–Nd isotopic plots,

two-component mixing lines in Pb–Pb diagrams

should be straight, independent of slab-to-mantle

transfer processes. The wide scatter in the Pb isotope

data (Fig. 4), both on the unradiogenic and radiogenic

side of the diagrams, already shows that the sample

set cannot be modelled by simple mixing of two

homogeneous components. In a 208Pb/204Pb versus207Pb/204Pb diagram, it is obvious that the shelf

sediments analysed by Vroon et al. (1995) cannot

represent the radiogenic end member, since they do

not have high enough 208Pb/204Pb for their207Pb/204Pb. The north Australian river sediments

are a more suitable end member, although the scatter

in the most radiogenic samples from NE Alor shows

that this component is not homogeneous. It is,

however, quite likely that the Australian upper crust

represents the end member with high time-integrated

U/Pb and Th/Pb ratios. The other end of the Pb

isotopic array appears to point back towards the field

for I-OIB with significantly higher 208Pb/204Pb than

I-MORB at given 207Pb/204Pb ratios. This has been

the reason why previous studies have concluded that

the sub-arc mantle in the adjacent active section of the

Sunda–Banda arc should have Pb isotopic character-

istics similar to I-OIB rather than I-MORB (e.g.

Vroon et al., 1993), and that the arrays in Pb isotopic

space reflect mixing between mantle and subducted

continental material. However, when all available data

for the Sunda–Banda arc are considered, it becomes

obvious that the dcollision areaT, as defined by low3He/4He ratios relative to MORB (because of a

contribution of radiogenic helium from the subducting

continent), coincides with the area where the samples

point back towards high 208Pb/204Pb ratios on the less

radiogenic side of the array (Elburg et al., 2004). It is

hard to see why the sub-arc mantle should change

from being similar to I-MORB (as for the non-

collisional sector (Turner and Foden, 2001)) to

resembling I-OIB in its Pb isotopic composition in

the sector of the arc–continent collision. It is therefore

more logical that also the less-radiogenic end of the

Pb isotopic array represents a component brought into

the system during arc–continent collision. This agrees

with the conclusion by Miller et al. (1994) that Pb

isotopic compositions of arc magmas are dominated

by subducted components, and not by the mantle

source. From these considerations, we infer that the

less radiogenic end of the Pb isotopic array reflects

another component within the subducted Australian

plate, possibly the lower crust. Although no data are

available of the north Australian lower crust, existing

estimates of the lower crust in general indicate that it

should have U/Pb and Th/Pb ratios about half of those

of the upper crust (Taylor and McLennan, 1985). Over

time, this would lead to lower Pb isotopic ratios for

the lower crust than for the upper crust, which could

thus explain the array seen in the Pb isotope diagrams.

The scatter on both sides of the diagrams is likely to

reflect heterogeneity in both the lower and upper

crustal end members, as can be expected from the

diversity of crustal lithologies. Pb isotopic data for

west Australian granites indicate extreme heterogene-

ity, which in turn mimics the heterogeneity of the

lower crustal rocks from which they were derived

(Bickle et al., 1989).

The areal distribution of the Pb isotopic signature

is interesting to consider in relation to data from the

neighbouring volcanoes. Within the Alor–Romang

sector, the island of Alor shows by far the largest

range in isotopic signatures, and there is some

geographical grouping visible, with samples from

the northeast having the highest 206Pb/204Pb ratios

(V19.6), and those from the south coast the lowest.

The northeast coast samples fall within the trend of

increasing 206Pb/204Pb ratios from the west to the east

of the Sunda arc (Turner and Foden, 2001) (Fig. 6).

The samples from Alor’s south coast, Lirang, Wetar

and Romang, however, have lower 206Pb/204Pb iso-

topic ratios (V19.1). These values increase again whenpassing from the extinct sector into the active Banda

arc to the east (islands of Damar, Teon and Nila).

Within our previously discussed interpretation of the

Fig. 6. Variation of 206Pb/204Pb ratios along the oceanic Sunda–Banda arc, starting at Bali and ending at Banda. Data from Elburg et al. (2002),

Hoogewerff (1999), Stolz et al. (1990), Turner and Foden (2001), Turner et al. (2003), Van Bergen et al. (1992), Vroon et al. (1993). Symbols as

in Fig. 4 and closed diamonds: Pantar Strait volcanoes; small grey squares: active volcanoes Sunda–Banda arc. Arrows indicate the different Pb-

isotopic zones of the Sunda–Banda arc; Pb isotope arrays reflect mixing of (1) subducted oceanic crust and entrained sediments; (2) old

Australian upper crust and a low 206Pb/204Pb component of Australian derivation (probably lower crust), with upper crust dominant; (3) like 2,

but lower crust dominant.

M.A. Elburg et al. / Journal of Volcanology and Geothermal Research 140 (2005) 25–4740

Pb isotopic array of the extinct sector, this would

mean that magmatism in Alor’s south and on Lirang,

Wetar and Romang was dominated by a contribution

from the Australian lower crust, whereas the upper

crust would be the main contributor for neighbouring

islands.

6.3. Sr and Nd isotopes

Although Sr and Nd isotopes are potentially more

powerful indicators of the composition of the sub-arc

mantle and slab-to-wedge transfer processes, all

samples, apart from the Lirang spessartite, plot well

away from any possible mantle source and several plot

in the field for shelf sediments. Therefore, the

information they can convey about the composition

of the local mantle is highly limited. Even the

spessartite falls outside the field for any asthenospheric

mantle, and must contain a significant sedimentary

component.

The samples with the highest Sr and lowest Nd

isotope ratios are the two cordierite-bearing rhyolites,

of which the mineralogy already indicates the

importance of crustal material. Although high Sr

and low Nd isotopic ratios in subduction-related

magmas compared to MORB mantle are commonly

ascribed to subduction of continental material (either

sediment or continental crust) and its introduction into

the sub-arc mantle, this generally does not lead to the

extreme values seen on Wetar. Neither does this

process explain the presence of cordierite phenoc-

rysts, which have only been reported in magmas with

high d18O values (ambonites) on Ambon, in the

extinct, northern part of the Banda arc (Magaritz et

al., 1978; Vroon et al., 2001). Although the Sr

isotopic composition of these samples could be

influenced by alteration, Nd isotopic ratios are hardly

affected by most secondary processes, and these must

be close to primary magmatic values. They are

comparable to the most continental Nd isotopic values

reported previously for Wetar by McCulloch et al.

(1983). The isotopic composition, mineralogy and

felsic composition of these samples indicate that they

cannot have been in equilibrium with the mantle, or

have evolved from normal arc magmas by crystal

fractionation. Since we do not think that these

magmas represent upper crustal contamination of

mantle-derived magmas (see above; cf. Van Bemme-

len, 1949; Honthaas et al., 1999; Vroon et al., 2001

regarding the cordierite bearing bambonitesQ from the

island of Ambon), it is more likely that they are

partial melts of subducted continental material. It is

M.A. Elburg et al. / Journal of Volcanology and Geothermal Research 140 (2005) 25–47 41

important to note that samples from Romang have

similarly low 206Pb/204Pb ratios as the low-Nd

isotopic samples from Wetar, whereas the Sr–Nd–

Pb–O–Hf isotope data from Romang indicate magma

petrogenesis by modification of the sub-arc mantle by

partial melts of continental material, followed by

partial melting of this contaminated mantle (cf. Vroon

et al., 1993, 2001). This suggests that the material

subducted into the mantle wedge is similar in its Pb

isotopic ratios as the material that gave rise to the

cordierite rhyolites.

Another point to consider is that the samples with

the lowest Nd isotopic ratios are located on the south

coasts of the islands of Wetar and Alor, i.e. near the

front of the arc (Fig. 7). This is opposite to the, far

more restricted, decrease in Nd isotopic ratios across

the adjacent parts of the arc (Hoogewerff et al., 1997;

Elburg et al., 2002). The far greater magnitude of

variation in Nd isotopes in samples from the Alor–

Wetar section of the arc indicates the importance of

continental material in their origin. On Alor, there is a

clear trend in Pb isotopes, from low 206Pb/204Pb ratios

in the south to higher ratios in the northeast. Finally,

unlike the situation on Ambon, there is no unequiv-

ocal evidence for the presence of metamorphic base-

ment on the islands of Alor, Wetar or Lirang. Based

on these systematics, we propose that the cordierite

rhyolites are best explained as partial melts of

subducted continental crust. In one end member

Fig. 7. 143Nd/144Nd ratio versus estimated depth to the Benioff zone.

The samples with the lowest Nd isotopic ratios are found near the

front of the arc, in contrast to the more usual moderate decrease in

Nd isotopic ratio across the arc, as seen in the Pantar Strait

volcanoes (stippled field).

scenario, this crustal melt mixed with the mantle,

which subsequently melted, giving rise to the mag-

matism of Romang. On the other side of the spectrum

of possibilities, this crustal melt erupted in an almost

uncontaminated state, as seen in the cordierite

rhyolites on Wetar. If our interpretation that the low206Pb/204Pb isotope end member represents lower

crust, then it was mainly this component that was the

dominant non-mantle source to magmatism in south

Alor and Wetar. This contrasts with the situation in

northeast Alor, and neighbouring islands, where the

upper crust was the main non-mantle contributor.

During steady-state subduction of the continental

margin, the upper crust will always be closer to the

surrounding hot mantle wedge than the lower crust,

and it would logically be the main contributor. To

explain the presence of lower crustal melts at the front

of the arc, we propose that the leading, oceanic part of

the subducted slab has become disconnected from the

Australian continent, as has been shown by geo-

physical modelling to be the almost inevitable result

of arc–continent collision (Davies and von Blancken-

burg, 1995; Van de Zedde and Wortel, 2001). This

would then lead to upwelling of hot asthenospheric

mantle along the tear, thereby increasing the temper-

ature of the exposed lower crust, leading to melting of

this component. Evidence for slab detachment in this

particular part of the Sunda–Banda arc is given by

seismic data (McCaffrey et al., 1985; Charlton, 1991)

and modelling of the effective elastic thickness of the

north Australian lithosphere (Tandon et al., 2000).

The seismic quiescence between 50 and 380 km

underneath the inactive part of the arc (McCaffrey,

1989; Milsom, 2001) could also be interpreted as

signifying the absence of a slab. If slab break-off

happened at relatively shallow depths (cf. Van de

Zedde and Wortel, 2001), the resulting crustal melts

would be most visible near the arc trench, as is the

case on Wetar and Alor.

6.4. Alternative interpretation of the low 206Pb/204Pb

component

The Pb isotope data indicates that Sundaland crust,

similar to the material involved in the petrogenesis of

the Lake Toba Tuff, does not play a role in magma

genesis in the extinct section of the Sunda–Banda arc.

There is also little doubt from comparison with the

M.A. Elburg et al. / Journal of Volcanology and Geothermal Research 140 (2005) 25–4742

data for north Australian river sediments that the high206Pb/204Pb component represents the upper Austral-

ian crust. Unfortunately, the lack of constraints on the

composition of the Australian lower crust in the area

prohibits the unequivocal identification of the low206Pb/204Pb component as Australian lower crust.

However, considering the fact that this low 208Pb/204Pb also shows a relatively high 208Pb/204Pb for it208Pb/204Pb ratio, similar to the Australian upper

crust, and the fact that this component only appears

within the collisional section of the arc, it seems

certain that this component is also of Australian

derivation. An alternative to the idea of melting of the

lower crust is the presence of a different upper crustal

component in the sub-arc mantle. The fact that this

component has a less radiogenic Pb isotopic signature

than the high 206Pb/204Pb component could be a

reflection of its younger age, rather than a lower U/Pb

ratio. The existence of an underthrust Australian

microcontinent in the area to the east of Wetar has

been proposed by Richardson and Blundell (1996) to

explain the crustal thickness in this area, whereas

Snyder et al. (1996) suggests it could equally well be

an underthrust Palaeozoic basin. This postulated piece

of underthrust Australian continent or basin, with an

average age younger than the north Australian

continental rim, could be invoked as the source of

the low 206Pb/204Pb component. The area where the

low 206Pb/204Pb component dominates the Pb isotopic

budget (from south Alor to Romang) would in this

case reflect the size of the underthrust crust or basin.

The resemblance in Pb isotopic ratios between the

crustal melts that are represented by the south Wetar

cordierite rhyolites, and the mantle melts that formed

the Romang magmas (Vroon et al., 2001) still

necessitates a melting scenario similar to that depicted

in Section 6.3.

6.5. Trace element constraints

Both absolute elemental concentrations and the

ratios of trace elements are highly variable and far in

excess of what can be explained in a closed system by

crystal fractionation processes. Overall, ratios of

incompatible elements that are indicative of fluid

involvement in slab-to-wedge transfer processes (Pb/

Ce, Ba/Th; Fig. 8A) are rather low, as they are also in

the Pantar Strait volcanoes (Elburg et al., 2002).

Samples from south Alor, however, range to higher

values for Pb/Ce than most other samples. The

correlation between Pb/Ce and Ba/Th is much poorer

in this data set than in the Pantar Strait volcanoes.

Ratios involving Nb are heavily influenced by the

high concentrations of this element in samples from

Romang and, to a lesser extent, Wetar. The Th/Nb

versus 143Nd/144Nd ratios of Romang and some

south Alor samples can be modelled by simple

mixing between MORB source and a crustal melt

like the Wetar cordierite rhyolites (Fig. 8B). In this

model, 5% of crustal melt needs to be added to the

MORB source to explain the trace element and

isotopic characteristics of Romang. This is the same

value found by Vroon et al. (2001) to explain the

oxygen isotopic characteristics of the Romang

samples, using a less specific crustal melt (partial

melt of subducted sediments) as a contaminant.

Higher Th/Nb and 143Nd/144Nd ratios, more like

those in the Pantar Strait volcanoes, are seen in the

Wetar intrusives and the samples from central and

northeast Alor. The high Th/Nb ratios of the Pantar

Strait volcanoes have previously been modelled by

mixing between MORB source and partial melt of

subducted continental material, where rutile in the

residue retains Nb relative to Th. The lower Th/Nb

ratios seen in the Wetar cordierite rhyolites may

reflect larger degrees of melting at higher temper-

atures, where rutile is not a residual phase anymore.

One of the few trace element ratios that shows a

reasonable correlation with both Sr and Nd isotopic

composition is Rb/Ba (Fig. 8C), with the samples

with the highest Rb/Ba ratio having the lowest143Nd/144Nd and highest 87Sr/86Sr ratio. This corre-

lation is far better than that of Sm/Nd with143Nd/144Nd. The Wetar cordierite rhyolites form

the high Rb/Ba, low 143Nd/144Nd end of the trend,

and some Alor samples fall at the low Rb/Ba end of

the array. In spite of the good overall correlation,

several samples (a Wetar intrusive, the Lirang

spessartite and a rhyolite, and a basalt from central

Alor) fall off the main trend. Although it is

tempting to explain the observed correlation as

simple mixing between sub-arc mantle and a melt

similar to the Wetar cordierite rhyolites, it must be

noted that N-MORB-type mantle does not plot on

this trend, as it has a Rb/Ba ratio (~0.09) that is

higher than the low Rb/Ba side of the trend.

Fig. 8. (A) Pb/Ce versus Ba/Th for the analysed samples. Also shown is the field for the Pantar Strait volcanoes (light grey). Both elemental

ratios are indicative for the relative amount of fluid involved in slab-to-wedge transfer of element; the low values indicate that this transfer

mechanism was not very important in the extinct sector of the Sunda–Banda arc. The poor correlation between the ratios for the samples

analysed in this study may indicate that additional processes may have modified the ratios. (B) 143Nd/144Nd ratio versus Th/Nb, showing that the

least radiogenic Nd isotope sample from Alor’s south coast can be modelled by mixing between MORB-type mantle and 9% of a partial melt of

crustal material similar to the south Wetar cordierite rhyolites. Dark stippled field: North Australian river sediments (Elburg et al., 2002); light

stippled field: track III shelf sediments (Vroon et al., 1995). (C) Correlation between 143Nd/144Nd and Rb/Ba ratio. This is the best correlation

seen in the data, and appears to signify mixing between sub-arc mantle and a melt similar to the cordierite rhyolites. Fields as in A and B; striped

field: MORB. (D) 143Nd/144Nd versus 206Pb/204Pb ratio. The data set can be explained by mixing between MORB-type mantle, a component

similar to the Wetar cordierite rhyolites and Australian upper crust, for which the field of north Australian sediments acts as a proxy.

M.A. Elburg et al. / Journal of Volcanology and Geothermal Research 140 (2005) 25–47 43

Fractionation of Rb/Ba ratios in magmas with

respect to mantle and crustal end members has also

been observed for the Pantar Strait volcanoes

(Elburg et al., 2002), and this may reflect the

presence of phlogopite during partial melting of the

mantle.

6.6. Quantification of source contributions

It is obvious from the scatter in the isotope and

trace element diagrams that any attempt to model the

composition of the Alor–Romang samples must

involve a large number of end members, or a limited

number of end members with large internal variation.

In both cases, we do not have a handle on the exact

composition of these end members, neither in their

isotopic nor in their trace element characteristics. Such

large uncertainties on the input into the arc system

preclude any detailed modelling of the processes that

gave rise to magmatism in the Alor–Wetar area. A

crude estimate of the amount of continental material

involved in magma petrogenesis can be made on the

basis of several assumptions: (1) the sub-arc mantle is

similar to the source of I-MORB. This is based on

studies of Indonesian volcanics from the non-colli-

sional segment (Vroon et al., 1993; Turner and Foden,

2001). (2) One crustal end member (with relatively

low Pb isotopic ratios) is represented by the Wetar

cordierite rhyolites. (3) The other crustal end member

is the Australian upper crust. We took one of the

analyses of north Australian river sediments (ME1A;

Elburg et al., 2002) as a representative for this upper

crustal component. In a 143Nd/144Nd versus 206Pb/204Pb diagram (Fig. 8D) all samples fall between the

M.A. Elburg et al. / Journal of Volcanology and Geothermal Research 140 (2005) 25–4744

mixing curves between I-MORB and the two crustal

components. No more than 2% of the upper crustal

component is needed to explain the isotopic compo-

sition of the samples from Alor’s north coast, and up

to 9% to model the sample with the lowest Nd

isotopic composition from Alor’s south coast. The

extreme variability in the 87Sr/86Sr composition of the

upper crustal proxy (0.72–0.87) makes it hard to

constrain the Nd–Pb isotope results against models

involving Sr isotopes. The variability in Nd isotopic

composition at relatively constant Sr isotopic compo-

sition for the south Alor samples shows that a simple

two-component mixing model cannot explain their

isotopic ratios.

6.7. Involvement of the sub-continental lithospheric

mantle?

We have proposed that different crustal lithologies

may be involved in petrogenesis of the magmas in

the collision domain of the Banda–Sunda arc, but

previous studies (e.g. Varne, 1985) have invoked a

role for the sub-continental lithospheric mantle. This

was partially based on the extreme potassium

enriched signature of presumed collisional volcanics,

such as Batu Tara (Fig. 1A) and the active or

recently extinct volcanoes from Sumbawa (Tambora,

Sangeang Api, Sangenges and Soromundi). Our data

for the extinct segment show that most collisional

volcanoes do not have this shoshonitic signature, but

Fig. 9. (A) K2O versus SiO2 for the Sunda–Banda collisional volcanoes, an

and Soromundi. The data show that Batu Tara is clearly an exception in term

Sunda–Banda collisional volcanoes; lighter grey field: NW Sulawesi ultrap

collisional volcanics, Tambora, Sangeang Api, Sangenges, Soromundi and

represent melting of the Australian subcontinental lithospheric mantle (Elbu

isotopic evidence for involvement of old lithospheric mantle.

fall in the field of more normal dhigh-KT volcanics. Ifwe take the dcollision sectorT to comprise all

volcanoes in the area of low helium isotopic

signature (Hilton and Craig, 1989; Hilton et al.,

1992: east Flores to the Banda arc), Batu Tara is

clearly an exception in terms of its high potassium

contents (Fig. 9A). The shoshonitic volcanoes on

Sumbawa fall outside the collision area and at least

one of them, Sangeang Api, has a helium isotopic

signature that is close to that of MORB (Hilton and

Craig, 1989). The idea that Sumba is an Australian

microcontinent and has been involved in a collision

with the arc, thereby giving rise to the potassic

volcanism on Sumbawa (Varne, 1985; Varne and

Foden, 1986), is not supported by data from other

disciplines. Palaeomagnetic (Wensink, 1994; Wen-

sink and Van Bergen, 1995) and isotopic data (Vroon

et al., 1995) show that Sumba has come from a more

northerly position near western Sulawesi. It has

therefore never been part of the subducting plate

and cannot have been involved in magma genesis on

Sumbawa. Finally, shoshonitic samples from north-

west Sulawesi, which have been interpreted to

represent partial melts of underthrust Australian

lithospheric mantle (Elburg et al., 2003), show Pb

isotopic signatures that are different from either the

Sumbawa volcanoes or the collision segment (Fig.

9B).

We therefore see no evidence for the involvement

of Australian subcontinental lithospheric mantle in the

d the non-collisional volcanoes Tambora, Sangeang Api, Sangenges

s of K2O content among the collisional volcanoes. Grey field: other

otassics. (B) 208Pb/204Pb versus 206Pb/204Pb ratios for Sunda–Banda

Miocene samples from NW Sulawesi that have been interpreted to

rg et al., 2003). The data from the Sunda–Banda arc do not show Pb

M.A. Elburg et al. / Journal of Volcanology and Geothermal Research 140 (2005) 25–47 45

petrogenesis of any of the collisional or non-colli-

sional volcanoes of the Sunda–Banda arc.

7. Conclusions

Magmatism in the now extinct sector of the Sunda–

Banda arc has a high-K signature and variable trace

element and isotopic characteristics. The high206Pb/204Pb ratios in the Pantar Strait volcanoes and

most of Alor are interpreted to reflect a contribution

from the subducted Australian upper crust. The sudden

decline in 206Pb/204Pb ratios in samples from Alor’s

south coast, Lirang, Wetar and Romang is likely to

reflect a contribution from a different subducted

component of Australian derivation, probably the

lower crust. Melting of the lower crust may have been

induced by break-off of the oceanic lithosphere from

the continental part of the slab, thereby allowing hot

asthenospheric mantle to ascend through the slab

window and heat the lower crust to its melting point.

An alternative interpretation is that the low 206Pb/204Pb

component represents an underthrust promontory of

younger Australian upper crust. Virtually unmodified

partial melts of this low 206Pb/204Pb crustal component

have been found as rhyolites with cordierite pseudo-

morphs on Wetar’s south coast. Other samples from

the dlow 206Pb/204Pb zoneT represent partial melts of a

mixture of I-MORB mantle and up to 9% of the low206Pb/204Pb crustal component. No more than 2%

addition of upper crustal material to the mantle wedge

is needed to explain the Nd–Pb isotopic composition

of the high 206Pb/204Pb samples. However, these

constraints are not very precise, as the scatter in

isotopic and trace element data indicates pronounced

heterogeneity in the crustal end member. We cannot

comment on the homogeneity of the mantle end

member, as all samples show such a pronounced

influence of subducted continental material that the

mantle signature is obscured. Arrays in Pb isotopic

space, which have previously been interpreted as

mixing between mantle and crustal end members, are

now interpreted as reflecting mixing between sub-

ducted components only; in the case of the collision

segment, these may be the Australian upper and lower

continental crust. In case of the active areas to the east

and west, this is thought to be the subducted oceanic

slab and entrained sediments.

Acknowledgments

The field and analytical work for this paper was

carried out while ME was a recipient of an ARC APD,

and written during tenure of an EU Marie Curie

Fellowship. Pieter Vroon provided the analysis of

Wetar sample WE1A1. Jan Werner is acknowledged

for providing access to the collections housed at the

Geological Museum in Artis (Amsterdam). Ahmed

Karaing and Paul Bons provided invaluable help

during fieldwork. Julian Pearce and an anonymous

reviewer provided comments on an immature version

of part of this manuscript. Rhiannon George and an

anonymous reviewer are thanked for their formal

reviews.

Appendix A. Supplementary data

Supplementary data associated with this article can

be found, in the online version, at doi:10.1016/

j.jvolgeores.2004.07.014.

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