Spatial and temporal isotopic domains of contrasting igneous suites in Western and Northern...

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Chemical Geology 199 (2003) 243–276

Spatial and temporal isotopic domains of contrasting igneous suites

in Western and Northern Sulawesi, Indonesia

Marlina Elburga,*, Theo van Leeuwenb, John Fodena, Muhardjob

aDepartment of Geology and Geophysics, University of Adelaide, Adelaide SA 5005, AustraliabPT Rio Tinto, Jakarta, Indonesia

Received 30 July 2002; accepted 21 February 2003

Abstract

Palaeocene to Pliocene magmatism in NW Sulawesi shows a progression from an Older Series with calc-alkaline/tholeiitic

signatures (51–17 Ma) to a Younger Series of mafic-intermediate high-K magmas (f 14–5 Ma) and felsic K-rich calc-alkaline

(CAK) magmas (9–2 Ma). The isotopic and geochemical compositions of the Older Series samples indicate that the more

westerly samples have been generated in a continental arc setting and the more easterly samples in an oceanic arc; this

distinction defines the boundary between the Western and Northern Sulawesi tectonic terranes. The Younger Series high-K

magmas have unusual isotopic compositions, with variable but low 143Nd/144Nd, high 87Sr/86Sr values, and high 208Pb/204Pb for

their 206Pb/204Pb ratios compared to subduction-related magmas. The diversity of the isotopic compositions points towards a

source with a long and heterogeneous geochemical evolution, most likely located within the Australian subcontinental

lithospheric mantle. The Younger Series felsic CAK magmatism has a more homogeneous isotopic and geochemical signature

and reflects melting of continental crust of Australian origin.

This geochemical progression in time is very similar to that seen in central Sulawesi [Chem. Geol. 156 (1999a) 67], and is

best explained by normal subduction of an oceanic plate followed by melting of an underthrust sliver of Australian continent.

The size of this microcontinent can be estimated from the areal extent of low-Nd-isotope magmas in Western Sulawesi, ranging

from approximately 4jS to 1jN. Underthrusting must have happened prior to 14 Ma, indicating that this event cannot be

equated to the collision between Sulawesi and the Sula platform, which occurred at 5 Ma.

While subduction beneath western Sulawesi ceased prior to the onset of potassic magmatism in this region, it continued

beneath northern Sulawesi producing predominantly calc-alkaline suites.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Sr-Nd-Pb isotopes; Indonesia; Sulawesi; Subduction; Continent collision; Trace elements

1. Introduction

The Indonesian archipelago lies in a tectonically

complicated area, where three major plates, Philip-

0009-2541/03/$ - see front matter D 2003 Elsevier Science B.V. All right

doi:10.1016/S0009-2541(03)00084-6

* Corresponding author. Now at: Max-Planck Institute for

Chemistry, P.O. Box 3060, 55020 Mainz, Germany.

E-mail address: [email protected] (M. Elburg).

pine, Australian and Eurasian, interact. Tectonic

reconstructions indicate that the area has seen rapid

changes in plate geometry over the past 50 Ma (Hall,

1996, 2002), but precise definition of tectonic events

is still lacking.

Sedimentological, stratigraphic and structural data

are commonly used to unravel tectonic histories, but

the potential of igneous rocks to provide additional

s reserved.

Fig. 1. (a) Location of Sulawesi. (b) Location map of samples analysed and discussed. Samples of Pompangeo Schists fall outside the map area.

(c) Geological map of NW Sulawesi. (d) Map of Sulawesi, showing relief (1000- and 2000-m contours) and the main faults and thrusts. Solid

triangles indicate active volcanoes of the Sangihe Arc.

M. Elburg et al. / Chemical Geology 199 (2003) 243–276244

Fig. 1 (continued ).

M. Elburg et al. / Chemical Geology 199 (2003) 243–276 245

information is sometimes underutilised. Thus, although

it has been known for some time that the geochemistry

of igneous rocks is a reflection of their broad tectonic

situation (Pearce and Norry, 1979; Pearce et al., 1984;

Pearce and Peate, 1995), the potentially powerful

ability of geochemical variation in magmatic rocks to

track tectonic changes in time and space is often

neglected. The resolution of this tool is further

enhanced when, in addition to major and trace element

signatures, the radiogenic isotope compositions of

igneous rocks are also determined. Sr, Nd and Pb

isotopic signatures reflect the age and geological his-

tory of the mantle and crustal domains involved in

tectonic events (White and Patchett, 1984; Nelson et

al., 1986; McDonough and McCulloch, 1987).

In this contribution we will present new field,

geochemical and isotopic data for igneous rocks from

northwest Sulawesi in the now extinct part of the

Indonesian arc (Fig. 1a–d), which span an age range

of 51 to 2 Ma. The main focus will be on the Sr, Nd

and Pb isotopic characteristics of the various igneous

suites, which range from normal tholeiitic and calc-

alkaline arc volcanism to high-K magmatism. The

geochemistry of the different groups of igneous rocks

of Eocene to Pleistocene age will be compared, and

the implications for the tectonic regime in the area

discussed. These new data will be integrated with

existing data sets on central, south and north Sulawesi

(Kavalieris et al., 1992; Priadi et al., 1993, 1994;

Bergman et al., 1996; Polve et al., 1997, 2001; Elburg


and Foden, 1998, 1999a,b; Elburg et al., 2002b) to

achieve a better picture of Sulawesi’s geochemical

evolution in space and time.

2. Tectonic setting

Northwest Sulawesi occupies the central part of a

Cainozoic magmatic belt that encompasses the south

arm of Sulawesi, central Sulawesi, the neck and the

north arm (Fig. 1d), and is known as the West(ern)

Sulawesi (Plutono-Volcanic) Arc or Western Sulawesi

(Magmatic) Province. It straddles two disparate ter-

ranes referred to by Taylor and Van Leeuwen (1980)

as Western Sulawesi and Northern Sulawesi, which

represent continental margin and oceanic arc setting,

respectively. The boundary between these two ter-

ranes remains ill-defined.

The northern Sulawesi terrane consists of a Neo-

gene volcanic arc with associated sedimentary deposits

(North Sulawesi Arc) built on a highly deformed

Palaeogene basement composed dominantly of basal-

tic rocks, which is probably underlain by oceanic crust

(Kavalieris et al., 1992; Pearson and Caira, 1999). The

Neogene volcanic and intrusive rocks are dominantly

of calc-alkaline affinity (Polve et al., 1997; Elburg and

Foden, 1998). Radiometric age dating of these rocks

(Surmont et al., 1994; Polve et al., 1997; Elburg and

Foden, 1998; Pearson and Caira, 1999) suggests that

they were formed between 22 and 0.9 Ma. Active

volcanism presently takes place only at the eastern tip

of the north arm.

Western Sulawesi started to develop in the Late

Mesozoic at the southeast margin of Sundaland. Its

basement consists of several metamorphic complexes.

Those exposed in the southern half of the terrane,

including the Bantimala Complex (Wakita et al.,

1996), together with the Pompangeo Schist Complex,

which crops out in eastern Sulawesi (Parkinson,

1998), form part of an arcuate zone of dismembered

accretionary complexes that extends through south-

east Kalimantan into west Java. They were metamor-

phosed in a northerly dipping subduction system

during the mid-Cretaceous (Parkinson et al., 1998).

The metamorphic complexes are in thrust or deposi-

tional contact with weakly metamorphosed deep

marine clastics of Late Cretaceous age, which are

interpreted as fore-arc basin deposits related to a

northerly directed subduction system (Van Leeuwen,

1981; Hasan, 1991; Sukamto and Simanjuntak, 1983).

During the Early Tertiary, plate readjustments in SE

Asia caused widespread extension. Western Sulawesi

rifted away from Sundaland, resulting in the formation

of the Makassar Straits; this event was probably

connected to the formation of the Celebes Sea (Hall,

1996). Syn-rift siliciclastics deposited in graben

basins were succeeded by platform carbonates and

other neritic to deeper marine deposits during the Late

Eocene to Middle Miocene (Coffield et al., 1993;

Wilson and Bosence, 1996; Calvert, 2000). Calc-

alkaline arc volcanism commenced as early as the

Palaeocene (Van Leeuwen, 1981; Polve et al., 1997)

and continued into the Late Eocene in the south arm

and until mid-Oligocene times further to the north.

Following a possible cessation of magmatic activity,

the next recorded arc volcanism event took place

around 19–15 Ma (Elburg and Foden, 1999a; Priadi

et al., 1997; Yuwono, 1987; this study).

The Neogene geology of Western Sulawesi is

characterised by the widespread occurrence of potassic

to ultrapotassic volcanic and volcaniclastic deposits

and co-magmatic intrusives, which in the central and

northern parts are found together with high-K calc-

alkaline granitic intrusives and subordinate felsic vol-

canics (Leterrier et al., 1990; Priadi et al., 1993, 1994;

Bergman et al., 1996; Polve et al., 1997, 2001; Elburg

and Foden, 1999a,b; Elburg et al., 2002b). Neogene

potassic magmatism, which began to develop f 14

Ma ago, is thought to be related to a series of collision

events between the Western Sulawesi arc that com-

menced in the Late Oligocene–Early Miocene, involv-

ing oceanic and microcontinental fragments derived

from the Pacific Plate and the northern margin of

Australian Plate (Hamilton, 1979; Lee and Lawver,

1995; Bergman et al., 1996; Charlton, 1996; Hall,

1996, 2002). Which fragments were colliding and

the timing and location of these collisions are still

not well understood, although good evidence exists

that the last collision between Western Sulawesi and

the Sula platform occurred around 5 Ma (Hall and

Wilson, 2000; Hall, 2002).

In Western Sulawesi the effects of the collision

events become evident only towards the end of the

Miocene (Hall, 2002). These include west-verging

thrusting and folding (Bergman et al., 1996), the

development of a NNW-trending strike-slip fault sys-


tem, including the Palu-Koro Fault that has exhumed

parts of the lower crust (Helmers et al., 1990; Polve et

al., 2001), and rapid elevation of mountains to about

2.5 km with deposition of molasse deposits along their

flanks. During the Pliocene the north arm underwent a

20j to 25j clockwise rotation (Sasajima et al., 1980;

Surmont et al., 1994), causing subduction of the south-

ern part of the Celebes Sea beneath the north arm. The

only volcanic activity related to this event is Una-Una

volcano in the Gulf of Gorontalo (Vroon et al., 2000).

Some plate tectonic models assume that Northern

Sulawesi was originally located further to the east (e.g.

Daly et al., 1991). However, palaeomagnetic measure-

ments suggest that it remained more or less close to its

present day position throughout its history (Surmont et

al., 1994). This in turn suggests that during the

Cainozoic, Western and Northern Sulawesi formed a

continuous magmatic arc (Rangin et al., 1999), cross-

ing from continental to oceanic crust. Subduction

beneath this arc was in a northerly to westerly direc-

tion, with western Sulawesi initially orientated NE and

rotating counterclockwise about 40j, together with

Borneo, between 25 and 10 Ma (Hall, 2002).

3. Local geology

The study area from which samples were collected

falls approximately between 2jS and 1jN. Its geology(Fig. 1c) consists of a basement of two metamorphic

complexes (Malino and Palu Complexes) and a

sequence of Upper Cretaceous metasediments, which

are overlain by Palaeogene volcanic-sedimentary for-

mations (Papayato Volcanics and Tinombo Formation).

The Neogene to Quaternary stratigraphy includes sedi-

mentary sequences deposited in marine basins, syn-

orogenic deposits of Plio-Pleistocene age (Celebes

Molasse), and intrusive and volcanic rocks of high-K

calc-alkaline to shoshonitic affinity in the continental

margin segment, and of normal calc-alkaline affinity

(including the Bolano Andesite) in the oceanic arc

segment.

In this paper the Palaeogene and Neogene igneous

units that were sampled are referred to as the Older

Series and Younger Series, respectively. The Miocene

series of calc-alkaline rocks (Bolano Andesite), which

does not fit well into either group, will be referred to as

the CA suite. Appendices A and B give more infor-

mation on the lithologies, mineral content, age con-

straints, alteration and deformation of the units

studied.

3.1. Metamorphic complexes

The Malino Complex (Kavalieris et al., 1992; Fig.

1c) is located in the western part of the north arm. It

consists predominantly of muscoviteF biotite schists

and gneisses, with intercalations of quartzite, graphite

schist, marl and amphibolite. Greenschists form a

discontinuous selvage around the complex; they were

derived from the overlying Tinombo Formation in the

west and the Papayato Volcanics in the east (Van

Leeuwen et al., in preparation). The contact between

the complex and Palaeogene formations is likely to be

tectonic rather than depositional.

The Palu Complex (Van Leeuwen et al., in prepa-

ration) represents a medium-P metamorphic belt, rang-

ing from chlorite up to staurolite zone, which is

exposed in the southeastern part of the study area. Its

lithologies are dominated by biotite gneisses and

schists, with subordinate amphibolites, granulites,

migmatites, peridotites, calc-silicate rocks and meta-

granitoids. The medium-P metamorphic rocks have

been overprinted by low-P metamorphism that is

thought to be related to young magmatic activity

(Egeler, 1946). High-pressure metamorphic rocks

occur as blocks and lenses along the Palu-Koro fault

and include garnet peridotite, garnet-clinopyroxene

granulite and felsic granulite (Egeler, 1946; Parkinson

et al., 1998). The maximum pressure that these rocks

were exposed to varies from 6 to 38 kbar (Helmers et

al.,1990; Parkinson et al., 1998; Kadarusman and

Parkinson, 2000; Polve et al., 2001). Several samples

have been analysed by Polve et al. (2001) for their

geochemical and isotopic composition.

The two metamorphic complexes are the subject of

continuing research (Van Leeuwen et al., in prepara-

tion), but preliminary U-Pb zircon dating and Sr-Nd-

Pb isotope analysis indicate that their igneous and

sedimentary protoliths were old continental crust,

probably of Australian derivation. Reconnaissance

U/Pb, Ar/Ar and K/Ar dating indicates that the Malino

rocks were metamorphosed in the mid-Miocene (23–

11 Ma); predominantly Early Pliocene ages were

obtained for the Palu Complex, but Oligocene and

Mesozoic dates have also been reported (Siahaan et


al., 1994; Priadi et al., 1996, 1999; Kadarusman and

Parkinson, written comm., 2000).

3.2. Older series

The Papayato Volcanics (PV) (Kavalieris et al.,

1992) forms the basement of Northern Sulawesi. The

formation consists of a thick pile of basaltic lava flows

and breccias with rare intercalations of deep marine

sedimentary rocks, which are cut by dykes and stocks

of gabbroic to dioritic composition. Less voluminous

felsic rocks occur in bimodal association with the

mafic volcanics as dykes, lava flows and volcaniclas-

tic deposits. The PV extends beyond the eastern

boundary of the study area, having a total strike length

of about 275 km. Limited palaeontological and radio-

metric age dating (T. van Leeuwen, unpublished data;

Polve et al., 1997) suggests that they have a Middle

Eocene to Earliest Miocene age.

Within the study area the PV has been subdivided

into three units based on their geochemical character-

istics; ‘‘PV mafic’’ and ‘‘PV felsic’’, which are both of

tholeiitic affinity, and ‘‘PVKCA’’, which includes

both mafic and felsic rocks displaying medium to

high-K calc-alkaline features. Limestone intercala-

tions in PV mafic and PVKCA rocks dated at Mid-

dle–Late Eocene and Earliest Miocene, respectively,

could possibly indicate a change from tholeiitic to

calc-alkaline magmatism with time.

The Tinombo Formation (Brouwer, 1934) is a thick

sequence of weakly metamorphosed pelitic sediments

with interbedded greywacke, arkose, limestone, marl

and volcanic rocks, showing in places flysch-like

features. It is widely exposed in the neck, where it

overlies the Palu Complex (in the southern part) and in

the Toli-Toli area. It is also found in a fault block to

the west of the Palu-Koro Fault and in the Donggala

Peninsula. Brouwer (1934) and Sukamto (1973)

assigned a Late Cretaceous to Eocene age to the

formation, but recent palaeontological dating (T. van

Leeuwen, unpublished data) suggests a similar age

range as that of the PV, i.e. Middle Eocene to Earliest

Miocene.

The volcanic rocks associated with the Tinombo

Formation range in composition from basalt to rhyolite.

They occur as intercalations in the Tinombo Formation,

as dykes cutting this formation and the underlying Palu

Complex, and locally as volcanic piles with only minor

sedimentary rocks. Based on their geographic location

and geochemical characteristics, they have been sub-

divided into the following units: Tinombo Volcanics

(TV; exposed in the neck), Tinombo Volcanics D

(TVD; Donggala Peninsula) and Toli-Toli Volcanics

A and B (TTVA and TTVB; Toli-Toli area).

Co-magmatic intrusive stocks, ranging in composi-

tion from diorite to granodiorite, intrude the Tinombo

Formation and Palu Complex, and are referred to as

Tinombo Intrusives (TI). Limited available radiometric

age dates for TI, TV and TVD samples range between

about 51.5 and 30 Ma (Sukamto, 1973; Polve et al.,

1997; T. Van Leeuwen and C. Allen, unpublished data;

this study, Table 1). Limestones found as lenses within

or close to TV rocks contain Middle and Late Eocene

benthonic foraminifera assemblages (Brouwer, 1934;

T. van Leeuwen, unpublished data).

The Renangkali Granite (RG), a two mica granite,

is found as stocks and late dykes intruded in the Palu

Complex east of Tompe. Two dyke samples yielded K/

Ar ages of around 34 Ma (Table 1). Slightly older40Ar/39Ar ages (37–42 Ma) were obtained from three

samples collected from the main intrusive stock (T. van

Leeuwen and P. Vasconcelos, unpublished data).

Both the RG and TI rocks show evidence of

moderate to strong deformation. This in contrast to

the Tondopado Microdiorite (TM) that occurs as a

series of dykes in the neck to the east of Tompe and in

the Donggala Peninsula. A dyke sample from the

latter area yielded a whole rock K/Ar age of 17.7

Ma (Polve et al., 1997). Diorite rocks found to the

south of Palu, which have been dated at about 15.5

Ma (Siahaan et al., 1994), may belong to the same

magmatic event.

The Papayato Volcanics and Tinombo Formation

have both undergone low-grade greenschist facies

metamorphism. The latter shows strong isoclinal fold-

ing and thrusting in the northern neck and Toli-Toli

area (Brouwer, 1934; Ratman, 1976). The relationship

between these two units has not been clearly estab-

lished. Ratman (1976) thinks they possibly interfinger;

most authors (e.g. Polve et al., 1997) regard the PV to

be a volcanic member of the Tinombo Formation.

3.3. CA suite

Following Polve et al. (1997), we refer to the calc-

alkaline (CA) rocks that make up the bulk of the

Table 1

K/Ar dates of selected samples

Suite Sample Material 40Ar (� 10� 10 mol/g) 40Ar/40Artot %K Age (Ma)

TI NWS53 Hbl 4.076 0.39 0.527 44.1F1.0a

4.143 0.48 44.8F 0.9

TI NWS25 Hbl 0.2575 0.622 0.3913 37.5F 0.3a

0.3920

RG NWS11 Mu 5.2009 0.731 8.887 33.4F 0.2b

RG NWS14 Mu 39.654 0.84 6.737 33.7F 0.7a

41.317 0.83 35.1F 0.7

BA NWS40 WR 0.2102 0.32 1.045 11.6F 0.2b

0.30

HK NWS401 Bt 1.5728 0.201 6.503 14.1F 0.4c

1.6058 0.196 6.434

HK NWS414 Bt 1.4390 0.237 7.054 11.6F 0.3c

1.4063 0.233 7.044

HK NWS422 Kfsp 0.4698 0.141 5.985 4.6F 0.2c

0.4920 0.116 6.098

HK NWS424 Bt 0.5188 0.137 5.987 4.9F 0.2c

0.5025 0.136 6.076

HK NWS319 Bt 0.5804 0.54 4.32 7.7F 0.2a

HK MLP4 Bt 0.5267 0.59 4.53 6.7F 0.1a

HK MKE1 Bt 0.9598 0.43 6.20 8.9F 0.2a

O NWS35 WR 0.4121 0.12 3.541 6.7F 0.2a

DSS (L) NWS17 Mu 0.4860 0.476 8.151 3.4F 0.1b

8.134

HK=Younger Series High-K; BA=Bolano Andesite; O =Ongka Volcanics; D =Dondo; RG=Renangkali Granite; TI = Tinombo Intrusives;

Bt = biotite; Kfsp =K-feldspar; WR=whole rock; Mu =muscovite.a Analysed by Institute of Geological and Nuclear Sciences.b Analysed by AMDEL.c Analysed by Geochron Laboratories.


Neogene North Sulawesi Arc as the ‘‘CA Suite’’. In

the study area, their occurrence is restricted to the

eastern part where they are found as andesitic and

dacitic stocks, dykes and epiclastics associated with

Early–Middle Miocene marine deposits. The unit was

sampled only at one locality, south of the Malino

Complex, where a group of small orthopyroxene–

hornblende andesite stocks, given the name here of

Bolano Andesite (BA), intrude the Papayato Vol-

canics. The BA yielded a K/Ar age of 11.6 Ma (Table

1), which falls roughly in the middle of the age range

of the North Sulawesi Arc (see above).

3.4. Younger series

Magmatic activity during the Late Miocene to

Pleistocene produced two igneous suites in the con-

tinental margin part of the study area, which we

denote as the Younger Series igneous rocks, consist-

ing of an intermediate to felsic high-K suite and a

high-K to shoshonitic/ultrapotassic suite.

3.4.1. HK suite

The high-K to shoshonitic/ultrapotassic suite con-

sists of intrusives and extrusives with mafic to felsic

compositions of which outcrops and float have been

found in a number of scattered locations in NW

Sulawesi. These have not been previously described.

However, Polve et al. (1997) report a single location

of shoshonitic rocks to the east of the study area dated

at 11 Ma. Similar rocks are widespread in central

Sulawesi and the south arm where they have been

classified as shoshonitic, alkaline potassic or ultra-

potassic (SH, AK, UK; Polve et al., 1997), or simply

as high-potassium (HK; Priadi et al., 1994). We will

use the latter term to refer to this series.

The HK suite rocks have been observed as dykes

and stocks cutting the Palu Complex, Upper Creta-

Table 2

Whole rock analyses of representative samples

NWS74,

PVKCA

NWS252,

PVKCA

NWS41B,

PV mafic

NWS540,

PV felsic

NWS68A,

TTVA

NWS66B,

TTVB

NWS322,

TVD

NWS325,

TVD

SiO2 45.2 75.4 51.3 73.58 50.4 52.5 58.0 48.5

Al2O3 20.9 12.5 16.0 11.74 16.3 18.4 17.0 18.2

Fe2O3 8.90 2.34 11.40 4.55 7.68 8.08 5.80 9.12

MnO 0.16 0.05 0.35 0.14 0.16 0.15 0.15 0.17

MgO 5.77 0.47 7.40 0.63 7.75 4.49 4.51 6.01

CaO 9.82 0.27 7.70 3.03 8.75 5.56 7.48 11.24

Na2O 2.54 3.99 2.40 3.95 4.09 5.06 3.24 2.62

K2O 1.32 3.07 0.29 0.55 0.04 1.36 0.29 0.16

TiO2 0.65 0.12 0.84 0.54 0.95 1.33 0.49 0.72

P2O5 0.10 0.03 0.07 0.13 0.13 0.62 0.16 0.09

LOI 4.25 1.19 2.39 1.58 2.84 3.65 2.89 2.60

Total 99.56 97.72 100.14 100.42 99.06 100.70 100.04 99.42

Sc 38 13 47 37 35 31 15 38

V na 11 na 26 198 235 120 313

Cr 26 170 52 77 330 118 148 109

Co 37 3 35 2 23 22 15 31

Ni 16 14 13 nd 92 49 40 27

Cu 95 nd 44 nd 135 33 45 103

Zn 67 47 19 60 61 82 43 54

Ga 16 15 16 13 14 15 18 18

Rb 19 23 2 6 2 22 3 5

Sr 531 51 109 149 120 448 530 378

Y 12 85 16 38 20 16 15 17

Zr 26 156 32 93 26 125 64 41

Nb 1.4 1.1 1.1 nd 1.2 16.5 0.7 0.1

Cs 0.05 0.24 0.08 nd 0.48 1.20 1.02 0.70

Ba 130 198 21 53 28 207 62 70

La 3.36 12.38 1.85 4.7 4.26 9.83 5.27 2.20

Ce 7.77 17.64 4.98 13 11.95 16.80 13.66 6.40

Pr 0.95 3.87 0.72 2.29 1.87 2.17 1.67 0.90

Nd 5.14 22.15 4.22 12 9.36 9.32 9.16 5.90

Sm 1.60 7.60 1.61 3.9 2.74 2.23 2.17 1.90

Eu 0.62 1.05 0.63 1.35 0.90 0.93 0.77 0.64

Gd 1.90 9.93 2.27 5.0 3.03 2.51 2.24 2.40

Tb 0.32 1.91 0.41 0.9 0.54 0.48 0.37 0.40

Dy 2.14 13.63 2.87 6.3 3.29 2.98 2.37 2.90

Ho 0.44 2.95 0.62 1.4 0.74 0.61 0.48 0.60

Er 1.40 9.51 1.93 4.1 1.83 1.82 1.59 1.90

Tm 0.21 1.44 0.28 0.66 0.26 0.29 0.22 0.29

Yb 1.27 9.67 1.83 4.1 1.71 1.88 1.68 1.90

Lu 0.20 1.53 0.30 0.67 0.28 0.28 0.30 0.32

Hf 0.79 5.55 0.99 2.8 1.11 2.90 2.19 1.30

Ta 0.08 0.05 0.04 nd 0.11 1.30 0.07 0.03

Pb nd nd 5 nd 13 6 nd nd

Th 0.83 1.72 0.57 0.5 0.28 1.10 0.45 0.76

U 0.18 0.46 0.08 nd 0.15 0.35 0.18 0.10


Table 2 (continued)

NWS45,

TV

TNB3,

TV

TNB19,

TI

NWS25,

TI

NWS53,

TI

NWS9,

TM

NWS557,

RG

NWS40,

BA

SiO2 49.9 54.8 48.3 58.60 54.70 51.0 65.0 62.0

Al2O3 16.1 17.7 17.0 17.50 17.90 19.3 17.72 16.6

Fe2O3 8.24 8.43 5.97 7.00 7.90 8.30 3.12 4.30

MnO 0.17 0.90 0.11 0.14 0.14 0.10 0.06 0.10

MgO 8.35 4.68 8.85 3.40 4.00 4.40 1.6 2.24

CaO 6.95 1.67 11.79 7.00 8.20 10.50 4.33 4.75

Na2O 4.55 6.94 2.59 3.05 3.10 3.25 4.48 3.96

K2O 0.43 0.35 0.67 1.16 0.91 0.38 1.18 1.29

TiO2 0.77 0.82 0.53 0.80 0.97 0.97 0.33 0.32

P2O5 0.17 0.24 0.12 0.14 0.20 0.14 0.1 0.20

LOI 4.77 3.82 4.21 1.00 1.66 1.30 0.65 4.65

Total 100.40 100.33 100.90 99.79 99.68 99.64 98.57 100.35

Sc na na 33 na na 29 7 12

V na na 140 na na na 50 99

Cr 40 596 459 57 57 57 313 21

Co 36 26 32 17 17 27 7 10

Ni 56 61 128 13 13 15 19 nd

Cu 27 132 21 5 18 90 nd 12

Zn 51 123 37 80 85 26 40 nd

Ga na 16 15 na na 18 17 17

Rb 7 6 20 35 27 13 36 19

Sr 629 365 446 293 315 317 501 380

Y 19 17 13 35 23 20 10 7

Zr 66 73 52 125 137 72 113 44

Nb 3.0 4.6 3.2 6 7 3.5 2.0 nd

Cs 0.50 0.32 0.64 1.49 0.62 1.20 4.10 0.53

Ba 121 123 88 273 158 80 104 218

La 7.72 6.05 6.37 19 15.3 6.18 7.50 4.25

Ce 15.30 11.82 14.13 35.9 30.5 14.91 16.0 9.20

Pr na 1.51 1.56 4.59 na 1.82 1.96 1.08

Nd 10.80 7.94 7.85 20.5 18 9.22 8.10 4.90

Sm 2.88 2.26 2.10 5 3.93 2.74 1.80 1.20

Eu 1.02 0.72 0.73 2 1.15 0.97 0.74 0.50

Gd na 2.45 2.13 5.78 na 3.03 1.80 1.23

Tb 0.54 0.44 0.36 1.04 0.62 0.53 0.30 0.21

Dy na 2.89 2.43 5.99 na 3.45 1.70 1.27

Ho 0.78 0.62 0.50 1.34 0.76 0.71 0.40 0.27

Er na 1.83 1.43 3.44 na 2.21 1.00 0.78

Tm 0.38 0.29 0.20 0.61 0.23 0.32 0.15 0.11

Yb 2.15 1.87 1.43 3.49 2.64 2.12 1.10 0.79

Lu 0.25 0.27 0.24 0.31 nd 0.34 0.17 0.13

Hf 1.60 2.07 1.34 3.4 3.71 2.01 2.90 1.51

Ta 0.30 0.31 0.20 0.56 0.54 0.20 0.3 nd

Pb 24 8 nd 17 14 na nd nd

Th 1.45 1.13 1.23 6.45 3.95 2.14 3.5 1.17

U 0.36 0.79 0.28 0.62 0.46 0.29 1.7 0.41

(continued on next page)


Table 2 (continued)

NWS737,

HK

NWS217,

HK

NWS376,

HK

MKE1,

HK

NWS316,

HK

NWS411,

HK

NWS511,

HK

SiO2 55.7 57.1 49.9 50.9 51.6 56.9 50.0

Al2O3 18.1 17.1 9.6 11.4 12.5 14.1 16.0

Fe2O3 6.39 6.34 7.35 9.08 7.96 5.82 9.24

MnO 0.10 0.10 0.19 0.15 0.14 0.07 0.26

MgO 2.91 3.96 11.25 11.68 7.86 2.35 4.31

CaO 5.64 5.58 6.63 7.99 7.20 4.23 8.62

Na2O 3.64 2.64 0.93 1.72 1.70 1.95 2.54

K2O 4.59 3.62 5.87 4.72 5.83 7.57 4.36

TiO2 0.72 0.82 1.02 0.84 1.00 0.81 0.87

P2O5 0.52 0.21 1.03 0.59 1.12 0.56 0.57

LOI 1.85 1.17 5.03 0.50 1.69 4.46 1.82

Total 99.76 98.78 98.71 99.53 99.42 98.89 98.63

Sc 17 18 19 28 25 15 26

V 208 125 131 177 177 125 285

Cr 100 101 636 772 360 112 85

Co 13 21 40 58 32 13 29

Ni 223 33 433 399 118 17 44

Cu 76 24 90 79 10 35 155

Zn 53 59 99 70 44 74 297

Ga 15 20 16 19 19 21 19

Rb 88 154 313 308 201 353 121

Sr 1310 343 904 725 1192 597 1380

Y 8 25 21 26 33 23 23

Zr 106 198 350 157 264 291 152

Nb 7.3 14.2 17.0 11.2 15.5 17.0 12.0

Cs 1.00 9.00 10.70 17.10 3.40 3.30 2.60

Ba 2270 806 3913 1891 5075 1700 3740

La 10.80 40.20 50.60 47.10 101.60 47.00 55.00

Ce 17.50 75.20 112.20 94.00 194.20 94.00 99.00

Pr 2.12 6.99 11.65 9.73 18.62 10.70 11.50

Nd 8.60 31.10 50.20 48.20 85.40 41.00 42.00

Sm 1.86 5.70 9.40 8.70 14.30 7.90 7.00

Eu 0.76 1.29 3.83 1.91 2.13 1.65 1.80

Gd 1.75 4.90 9.20 7.00 10.50 6.20 6.60

Tb 0.28 0.70 0.90 0.90 1.30 0.80 0.80

Dy 1.57 4.40 4.10 4.80 6.50 4.20 3.90

Ho 0.32 0.80 0.70 0.90 1.10 0.80 0.80

Er 0.91 2.60 2.40 2.50 3.10 2.10 2.10

Tm 0.13 0.37 0.21 0.33 0.39 0.32 0.29

Yb 0.91 2.30 1.40 2.10 2.50 2.00 1.80

Lu 0.14 0.37 0.25 0.32 0.40 0.28 0.25

Hf 2.80 5.80 9.40 4.50 7.00 7.70 4.20

Ta 0.50 1.44 1.15 0.66 0.70 1.00 0.60

Pb 11 33 130 11 6 36 65

Th 4.21 20.64 31.00 24.00 34.52 33.00 26.00

U 0.79 5.18 6.40 6.33 5.68 5.60 3.90



Table 2 (continued)

NWS639,

HK

NWS910,

HK

NWS401,

HK

NWS414,

HK

NWS319,

HK

MLP4,

HK

SiO2 48.7 59.2 55.90 52.60 52.38 57.61

Al2O3 11.7 16.8 14.64 16.89 14.01 14.86

Fe2O3 10.55 6.67 5.89 8.80 9.12 6.61

MnO 0.17 0.12 0.12 0.14 0.15 0.12

MgO 10.17 2.98 3.23 3.89 6.86 6.66

CaO 10.85 5.50 5.28 5.08 9.24 6.37

Na2O 1.54 3.61 2.93 3.07 1.80 2.34

K2O 3.77 6.28 5.88 6.32 3.62 2.94

TiO2 1.09 0.74 0.60 0.86 0.80 0.63

P2O5 1.04 0.49 0.49 0.81 0.53 0.24

LOI 0.58 0.78 4.31 2.17 0.36 1.49

Total 100.17 100.15 99.27 100.64 99.00 99.87

Sc 39 17 18 20 37 18

V 239 132 124 178 233 131

Cr 431 112 135 56 235 409

Co 40 16 15 20 33 28

Ni 140 26 34 nd 23 125

Cu 71 30 41 61 55 44

Zn 71 62 61 83 29 42

Ga 15 18 18 20 17 19

Rb 192 224 323 325 204 119

Sr 933 1270 593 840 529 674

Y 26 28 23 28 19 19

Zr 132 268 264 270 97 132

Nb 10.0 15.0 17 17 5.2 10.57

Cs 12.30 18.80 13 5.7 4.7 8.07

Ba 2780 2180 981 1710 857 1165

La 49.20 51.80 51 50 20.2 40.93

Ce 100.00 98.60 93 97 42.4 74.46

Pr 12.60 12.20 10.5 11.5 4.42 6.67

Nd 52.00 47.00 39 47 22.4 27.72

Sm 10.40 9.10 7 9.2 4.6 5.27

Eu 2.42 2.74 1.5 1.96 1.18 1.23

Gd 8.20 7.80 5.9 7.1 4 4.22

Tb 1.10 1.00 0.8 1 0.6 0.59

Dy 5.60 5.00 3.9 5 3.5 3.42

Ho 1.00 0.90 0.7 0.9 0.7 0.66

Er 2.50 2.50 2.2 2.5 2.1 1.89

Tm 0.35 0.36 0.34 0.42 0.29 0.29

Yb 2.10 2.10 2 2.4 1.9 1.87

Lu 0.30 0.33 0.3 0.33 0.3 0.29

Hf 3.90 6.00 7.7 6.8 2.8 4.06

Ta 0.70 1.00 1.8 1.1 0.36 0.86

Pb 27 66 45 33 nd 33

Th 25.10 26.70 49 39 8.16 17.98

U 6.10 9.30 12 8.7 1.96 4.478


Table 2 (continued)

NWS35,

Ongka V

NWS238A,

Ongka V

NWS300,

DSS

M21,

DSS

NWS17,

DSS (L)

NWS357,

DSS (L)

NWS645,

Gimpu V

SiO2 66.90 69.1 65.7 70.4 75.60 70.4 68.85

Al2O3 16.00 13.3 15.3 14.2 14.20 14.8 15.28

Fe2O3 2.98 3.05 4.05 2.14 0.78 1.32 2.76

MnO 0.09 0.04 0.07 0.03 0.10 0.03 0.05

MgO 1.85 1.74 2.30 0.76 0.15 0.54 1.09

CaO 2.67 2.82 3.78 1.97 0.75 1.19 2.42

Na2O 3.75 2.86 2.99 3.93 2.85 4.82 4.25

K2O 4.83 4.23 4.17 4.08 4.80 5.35 4.34

TiO2 0.53 0.55 0.52 0.29 0.07 0.13 0.36

P2O5 0.26 0.26 0.19 0.12 0.09 0.10 0.21

LOI 0.70 1.14 0.38 0.97 0.68 0.49 0.62

Total 100.56 99.08 99.57 98.93 100.09 99.15 100.22

Sc 10 9 10 5 3 3 5

V 85 71 71 32 na 28 34

Cr 100 181 167 114 126 204 151

Co 17 8 10 5 2 3 3

Ni 26 26 16 60 6 32 nd

Cu 17 7 na 82 4 88 nd

Zn 49 33 40 nd 9 nd nd

Ga – 11 18 17 21 15 18

Rb 200 163 187 133 314 171 127

Sr 622 467 306 558 81 1039 904

Y 24 26 24 14 17 11 20

Zr 217 178 160 152 37 124 203

Nb na 15.2 13.4 9.0 21 19.0 14.0

Cs 6.07 4.60 9.20 1.60 20.7 7.50 3.1

Ba 1590 1377 677 1196 113 2028 1920

La 45.40 52.40 47.80 29.90 9.15 19.30 66.70

Ce 82.00 95.30 88.00 61.60 17.7 48.80 118.00

Pr na 9.07 7.95 5.53 na 4.93 12.6

Nd 36.70 39.60 34.10 21.40 8.42 20.20 41.90

Sm 6.48 6.70 5.90 3.90 2.19 4.00 6.60

Eu 1.37 1.17 1.11 1.43 0.27 1.93 1.44

Gd na 5.30 4.90 4.40 na 3.70 4.9

Tb 0.91 0.80 0.70 0.50 0.37 0.40 0.70

Dy na 4.40 4.10 2.40 na 1.90 3.7

Ho 0.61 0.90 0.80 0.50 0.38 0.30 0.70

Er na 2.70 2.50 1.50 na 1.10 2.00

Tm 0.48 0.40 0.38 0.20 0.23 0.12 0.32

Yb 2.53 2.60 2.40 1.40 1.25 0.80 2.10

Lu nd 0.42 0.39 0.22 nd 0.13 0.33

Hf 0.89 5.50 5.00 4.50 1.6 5.00 5.80

Ta 1.98 1.52 1.36 0.75 4.7 1.73 1.3

Pb 7.00 13 7 30 8 183 29

Th 21.50 28.40 30.61 19.50 11.3 51.00 33.80

U 4.45 8.52 6.82 6.10 3.83 10.90 7.80

Abbreviations as in text. Oxides in weight percent; elements in ppm. na = not analysed; nd = not detected. All iron as Fe2O3.


ceous metasediments, Tinombo Formation and RG.

In three localities extrusiveF intrusive complexes

were observed, consisting of various pyroclastics,

epiclastics and lavas. Intrusive lithologies of the

high-K suite include lamprophyre, nepheline gabbro,

quartz-syenite, melanosyenite and monzodiorite. Ex-

trusive rocks are mainly basanite, trachyte and latite.

K/Ar age dating of seven intrusive and extrusive

Table 3

Rb-Sr and Sm-Nd isotopic compositions (by TIMS) and elemental concentrations (by ICP-MS)

Sample Age 87Sr/86Sr 87Sr/86Sri Rb

(ppm)

Sr

(ppm)

143Nd/144Nd

Nd

(ppm)

Sm

(ppm)

147Sm/144Nd

Papayato KCA

NWS74 40 0.704453 0.704394 19.1 531 0.512874 5.36 1.57 0.1766

CSW III 40 0.704037 0.703807 43 305 0.512977 9.53 2.63 0.1670

NWS534 40 0.704058 0.703997 7.0 190 0.512952 8.78 2.71 0.1867

Papayato mafic

NWS41D 40 0.705508 0.705407 1.2 18.8 0.512984 1.97 0.65 0.1983

NWS249 40 0.705255 0.704941 27.6 144 0.513006 3.54 1.29 0.2202

NWS532 40 0.703906 0.703883 3.0 218 0.512933 9.38 2.60 0.2103

Toli-Toli Volcanics A

MB 2 40 0.706749 0.705884 20 38 0.512734 4.21 1.03 0.1475

NWS68A 40 0.706929 0.706905 1.7 120 0.512968 10.28 3.05 0.1794

Toli-Toli Volcanics B

NWS66B 40 0.704797 0.704716 22 448 0.512923 13.30 3.27 0.1487

Tinombo Volcanics D

NWS325 40 0.703559 0.703539 4.6 378 0.512990 5.36 1.86 0.2096

Tinombo Volcanics

NWS45 40 0.707404 0.707386 6.7 629 0.512766 9.99 2.73 0.1654

Tinombo Intrusives

TNB 19 40 0.703725 0.703651 20 446 0.512797 8.62 2.17 0.1520

NWS323 31 0.706210 0.706152 94 163 0.512602 14.97 3.51 0.1416

NWS53 40 0.704567 0.704457 27 315 0.512658 15.93 3.72 0.1414

Renangkali Granite

NWS557 40 0.703953 0.703926 36 501 0.512736 7.15 1.74 0.1472

Tondopado Microdiorite

NWS9 17.7 0.704482 0.704452 13 317 0.512864 9.50 2.63 0.1671

Bolano Andesite

NWS40 11.4 0.706114 0.706091 18.6 380 0.512919 4.55 1.08 0.1440

Gimpu Volcanics

NWS645 2 0.711138 0.711127 127 904 0.512181 41.90 6.60

NWS652 2 0.715907 0.715882 177 575 0.512108 32.90 5.50

Ongka Volcanics

NWS238A 6.7 0.712953 0.712857 163 467 0.512176 39.60 6.70

NWS238B 6.7 0.712045 0.711914 211 445 0.512220 36.30 6.40

NWS238B leacht 0.512203

Dondo Supersuite Volcanics

NWS300 3.4 0.716716 0.716631 187 306 0.512070 34.10 5.90

NWS301 3.4 0.719323 0.719274 204 578 0.512098 44.20 6.70

NWS69 3.4 0.710839 0.710804 152 606 0.512177 24.90 4.30

M21 3.4 0.710160 0.710127 133 558 0.512268 21.40 3.90



Table 3 (continued)


(ppm)

Sr

(ppm)

143Nd/144Nd

Nd

(ppm)

Sm

(ppm)

147Sm/144Nd

Dondo Supersuite Leucocratic

NWS357 3.4 0.712056 0.712033 171 1039 0.512188 20.20 4.00

High-K Suite

MLP 4 6.7 0.711459 0.711410 120 674 0.512215 27.70 5.30

NWS66 6.7 0.707509 0.707488 96 1250 0.512584 27.40 4.10

NWS66 leacht 0.707484 0.512594

NWS66 unl 0.707537 0.512581

NWS68B 6.7 0.711463 0.711424 148 1065 0.512132 38.40 7.30

NWS737 6.7 0.707463 0.707444 88 1310 0.512630 8.60 1.86

NWS217 9? 0.719141 0.719012 154 343 0.512159 31.10 5.70

NWS319 7.7 0.708749 0.708627 204 529 0.512365 22.40 4.60

NWS375 7.2 0.714047 0.713928 271 674 0.512092 38.80 7.10

NWS376 7.2 0.713820 0.713718 313 904 0.512124 50.20 9.40

NWS376 leacht 0.713715 0.512144

NWS385 7.2 0.714332 0.714268 286 1317 0.512079 49.80 9.90

NWS644 0.715269 0.715146 315 757 0.512017 51.30 9.20

NWS335 7? 0.713635 0.713599 132 1054 0.512097 47.50 8.30

NWS335 leacht 0.714030 0.512085

MKE 1 10 0.714420 0.714245 308 725 0.512133 48.20 8.70

NWS316 10 0.714071 0.714002 201 1192 0.512183 85.40 14.30

NWS411 7 0.716898 0.716728 353 597 0.511846 41.00 7.90

NWS411 leacht 0.714975 0.511878

NWS412 7 0.716196 0.716081 257 642 0.511935 49.00 9.30

NWS412 leacht 0.714966 0.511917

NWS414 7 0.714317 0.714206 325 840 0.512035 47.00 9.20

NWS511 7 0.713820 0.713795 121 1380 0.512058 42.00 7.00

NWS639 9 0.715316 0.715240 192 933 0.512085 52.00 10.40

NWS682 9 0.713340 0.713283 216 1390 0.512247 48.30 9.30

NWS910 9 0.713322 0.713256 224 1270 0.512264 47.00 9.10

Palu Complex

NWS203 0.736314 78 110 0.511790 30.99 5.72 0.1116

NWS584 0.709422 113 453 0.512248 33.46 5.97 0.1078

NWS602 0.717917 233 269 0.512097 23.60 4.59 0.1174

NWS604 0.712562 26 1100 0.512176 72.95 12.83 0.1063

NWS610 0.708420 63 612 0.512305 33.99 6.15 0.1093

NWS373 0.739167 46 34 0.511815 11.76 5.21 0.2679

NWS757 0.710111 3 214 0.512127 35.76 7.31 0.1235

NWS903 0.706775 na na 0.512393 21.39 4.16 0.1177

NWS612 0.703479 na na 0.513154 9.73 3.17 0.1972

NWS612 rep 0.703466

NWS580 0.704627 na na 0.512985 8.35 2.82 0.2042

NWS596 0.709494 na na 0.512277 37.46 6.06 0.0978

Malino Complex

IKA19 0.705054 5 142 0.512969 4.50 1.60

NWS941 0.704394 nd 181 0.512960 9.72 2.79 0.1732

NWS942 0.730166 47 173 0.511696 18.77 3.29 0.1060

NWS38 0.743278 na na 0.511685 39.24 6.62 0.1020

NWS63 0.781940 na na 0.511619 25.70 4.75 0.1117

NWS526 0.705204 na na 0.513009 13.04 4.43 0.2052

NWS523 0.704367 na na 0.513015 12.73 3.05 0.1449


Table 3 (continued)


(ppm)

Sr

(ppm)

143Nd/144Nd

Nd

(ppm)

Sm

(ppm)

147Sm/144Nd

Tinombo Fmt, Toli-Toli

NWS510 0.711449 na na 0.512352 0.1163

Tinombo fmt, Donggala

NWS568 0.710495 na na 0.512353 20.61 4.09 0.1199

Tinombo fmt, Palu

NWS590 0.707873 na na 0.512680 0.1478

Pompangeo Schists

NWS920 0.708479 44.0 162 0.512413 6.17 1.24 0.1217

NWS920 rep 0.708480

NWS921 0.705111 6.7 138 0.512918 1.15 0.43 0.2278147Sm/144Nd ratio given for samples for which this was determined by isotope dilution measurements. See Appendix A for the constraints on the

ages. unl = unleached; leacht = leachate; rep = repeat.


samples yielded ages ranging between 14 and 5 Ma

(Table 1).

3.4.2. CAK suite

The youngest suite consists of the Dondo Super

Suite (DSS) of intrusive rocks and co-magmatic vol-

canic rocks (Kavalieris et al., 1992; Polve et al., 1997,

2001; this study). These have been classified as being

high-K calc-alkaline (CAK) by previous workers, and

will be referred to as the CAK suite. They are found

along a N-NE-trending belt, 300-km long (which

extends into central Sulawesi), as stocks and batholiths

up to 2 km in diameter, commonly showing contact

metamorphic aureoles. Intrusive lithologies include

porphyritic and equigranular monzogranite and quartz

monzonite, and late leucocratic syenogranite dykes.

The DSS has ages ranging predominantly between 9

and 1.6 Ma (OCTA, 1971; Van Leeuwen et al., 1994;

Polve et al., 1997; Bellier et al., 1998; Widiasmoro et

al., 1997; this study, Table 1).

Volcanic equivalents are the Ongka Volcanics (Van

Leeuwen et al., 1994) and the Gimpu Volcanics

(Polve et al., 1997). The former is exposed near the

village of Ongka, where they cover f 200 km2, and

to the SW of Tinombo in outcrops of restricted size.

The Gimpu Volcanics is found along the Palu-Koro

Fault Zone in the vicinity of Gimpu and to the north of

Lake Lindu. Both units consist of pyroclastics includ-

ing ignimbrites, and subordinate lavas of predomi-

nantly rhyodacitic composition. K/Ar dating yielded

ages of 6–7 Ma for the Ongka Volcanics (Polve et al.,

1997; this study, Table 1), and 2–4 Ma for the Gimpu

Volcanics (Polve et al., 1997).

4. Analytical techniques

A selection of 184 (70 from the Older Series, 45

CAK and 69 HK) samples was analysed by Actlabs

(Canada) for major oxides and a wide range of trace

elements by fusion-ICP and fusion-ICP-MS methods,

respectively, using methods described by Elburg et al.

(2002b), with accuracy for major elements around 1%

(Table 2). Analysed samples were selected to obtain a

representative coverage of the geographical areas and

rock types involved.

Samples were selected for isotope analyses on the

basis of their freshness (low loss on ignition), low

silica content (when possible) and their being repre-

sentative of their suite. Sr, Nd and Pb isotope ratios

were analysed at the Department of Geology and

Geophysics at the University of Adelaide on a Fin-

nigan MAT 262 Thermal Ionisation Mass Spectrom-

eter in static mode. All ground samples were leached in

3 N HCl for 30 min at approximately 100 jC. Thesupernatant was pipetted off, the sample washed in

deionised water and the water pipetted off. The residue

was then analysed for its isotopic composition. In

selected cases, the leachate was analysed too (Tables

3 and 4). Nd and Pb isotopic compositions of leachate

Table 4

U-Th-Pb isotopic compositions (by TIMS) and elemental concentrations (by ICP-MS)

Sample 206Pb/204Pb corr 207Pb/204Pb corr 208Pb/204Pb corr U (ppm) Pb (ppm) Th (ppm)

Papayato KCA

NWS74 17.246 15.520 37.255 0.18 nd 0.83

CSW III 18.207 15.523 38.109 0.41 nd 1.11

NWS534 18.185 15.577 38.179 0.20 nd 0.60

Papayato Mafic

NWS41D 18.591 15.613 38.748 0.05 nd 0.26

NWS249 18.313 15.543 38.224 0.29 nd 0.34

NWS532 18.241 15.578 38.295 0.10 nd 0.80

Toli-Toli Volcanics A

MB 2 18.700 15.632 38.890 0.37 19 0.10

NWS68A 18.651 15.632 38.934 0.15 13.4 0.28

Toli-Toli Volcanics B

NWS66B 18.470 15.589 38.502 0.35 6 1.10

Tinombo Volcanics D

NWS325 18.216 15.535 38.151 0.10 nd 0.76

Tinombo Volcanics

NWS45 18.469 15.589 38.517 0.36 24 1.45

Tinombo Intrusives

TNB 19 18.440 15.606 38.589 0.28 nd 1.23

NWS323 18.668 15.633 38.802 1.60 nd 7.04

NWS53 18.617 15.638 38.795 0.46 14.00 3.95

Renangkali granite

NWS557 18.463 15.594 38.492 1.70 nd 3.50

Tondopado Microdiorite

NWS9 18.582 15.622 38.723 0.29 nd 2.14

Bolano Andesite

NWS40 18.414 15.576 38.606 0.41 nd 1.17

Gimpu Volcanics

NWS645 18.750 15.609 39.001 7.80 29.00 33.80

NWS652 18.771 15.682 39.038 7.60 10.00 31.60

Ongka Volcanics

NWS238A 18.845 15.631 39.029 8.52 13.00 28.40

NWS238B 18.822 15.649 39.051 6.50 8.00 24.62

Dondo Supersuite Volcanics

NWS300 18.822 15.666 39.169 6.82 7.00 30.61

NWS301 19.145 15.743 39.436 10.17 8.00 36.03

NWS69 18.546 15.607 38.889 2.10 7.00 14.20

M21 18.761 15.654 39.035 6.10 30.00 19.50

Dondo Supersuite Leucocratic

NWS357 18.745 15.668 39.022 10.9 183 51


Table 4 (continued)


High-K Suite

MLP 4 18.718 15.626 39.013 4.5 33 18.0

NWS66 17.894 15.541 38.828 0.6 34 12.1

NWS66 unl 17.907 15.576 38.931

NWS68B 18.456 15.605 38.943 2.1 36.6 11.9

NWS737 17.911 15.547 38.859 0.8 11 4.2

NWS217 19.009 15.670 39.266 5.2 33 20.6

NWS319 18.328 15.588 39.009 2.0 nd 8.2

NWS375 18.633 15.638 38.879 7.1 15 33.4

NWS376 18.651 15.636 38.861 6.4 130 31

NWS376 leacht 18.666 15.653 38.922

NWS385 18.651 15.654 38.890 4.4 99 21.7

NWS644 18.657 15.670 38.984 7.3 20 30.9

NWS335 18.621 15.640 38.842 3.1 12 17.8

NWS335 leacht 18.614 15.637 38.782

MKE 1 18.557 15.629 38.948 6.3 11 24

NWS316 18.917 15.668 39.012 5.7 6 34.5

NWS411 18.012 15.589 38.944 5.6 36 33.0

NWS411 leacht 18.043 15.607 38.960

NWS412 18.172 15.588 39.009 8.9 34 45

NWS412 leacht 18.149 15.600 39.007

NWS414 18.342 15.610 39.119 8.7 33 39

NWS511 17.329 15.501 38.718 3.9 65 26

NWS639 18.520 15.678 39.123 6.1 27 25.1

NWS682 19.074 15.713 39.230 14.4 44 48.2

NWS910 19.049 15.685 39.142 9.3 66 26.7

Palu Complex

NWS203 18.868 15.662 38.938 2.9 nd 15.2

NWS584 18.787 15.654 39.063 1.4 5 13.0

NWS602 18.830 15.650 38.858 13 45 36.2

NWS604 18.707 15.645 38.912 4.2 13 29.8

NWS610 18.671 15.624 38.775 2.1 nd 6.6

NWS373 17.508 15.543 37.308 0.3 nd 1.5

NWS757 18.761 15.633 38.778 0.23 15.58 0.39

NWS903 18.968 15.682 39.086 na na na

NWS612 17.825 15.545 37.600 na na na

NWS612 rep 17.712 15.548 37.577

NWS580 17.863 15.482 37.766 na na na

NWS596 18.737 15.626 38.973 na na na

Malino Complex

IKA19 18.253 15.554 38.296 0.10 nd 0.20

NWS941 18.394 15.592 38.444 0.26 nd 1.01

NWS942 18.182 15.610 38.845 2.07 20.84 9.85

NWS38 18.388 15.621 38.730 na na na

NWS63 19.044 15.756 39.059 na na na

NWS526 18.076 15.507 38.068 0.18 nd 0.81

NWS523 17.991 15.497 38.075 na na na

Tinombo fmt, Toli-Toli

NWS510 18.732 15.654 38.930 na na na



Table 4 (continued)


Tinombo fmt, Donggala

NWS568 18.775 15.653 38.947 na na na

Tinombo fmt, Palu

NWS590 18.633 15.648 38.813 na na na

Pompangeo Schists

NWS920 18.668 15.638 38.774 0.96 12.94 2.03

NWS921 18.601 15.643 38.700 nd nd 0.26

na = not analysed; nd = not detected. Pb isotopic ratios have been corrected for fractionation during mass spectrometry.


and leached sample are generally within error. Sr

isotopic compositions of the leachate can be both

higher and lower than that of the leached sample,

reflecting influences of alteration and selective leach-

ing. Separation procedures have been described by

Elburg and Foden (1998).

The long-term average for the in-house Nd stand-

ard (J&M specpure Nd2O3) is 0.511603F 9 (1r of

total population, n = 105). The LaJolla standard gave

0.511828F 11 (n = 9) and BCR-1 was 0.512593F 16

(n= 12). Typical blanks are in the order of 100–200

pg for Nd. The average for the NBS987 Sr standard is

0.710258F 18 (n = 56). Typical Sr blanks are better

than 1.5 ng, which is negligible compared to a typical

sample size of 10–100 Ag of Sr. All Pb isotopic

analyses were performed at approximately the same

temperature of 1150 jC, and a mass fractionation

factor of 0.08% per amu was used, based on replicate

analyses of the NBS981 Pb standard. Typical Pb

blanks are in the order of 500 pg.

K/Ar dating of some samples (Table 1) was per-

formed at AMDEL analytical laboratories, on a modi-

fied MS-10 mass spectrometer, following the techni-

ques described byWebb et al. (1986); at the Institute of

Geological and Nuclear Sciences Ltd, Rafter Labora-

tories, following the techniques described by Elburg et

al. (2002b); and at Geochron Laboratories, of which

the procedure is as follows. Potassium was analysed in

duplicate on a Beckman Model DU flame spectrom-

eter. For the argon analyses a measured amount of the

sample is fused in molybdenum crucible using an Rf

induction furnace. During the fusion very pure 38Ar

tracer gas is admitted to the system. The mixed

sample-tracer Ar is analysed in an AEI model MS-10

mass spectrometer. The reported uncertainties on the

measured K/Ar ages are based on the expected pre-

cisions on the K and Ar analyses, and a 1% error

allowing for uncertainties in the calibration.

5. Major and trace element geochemistry

The analyses for the Older Series of samples span a

wide range in SiO2 (40–76%) and MgO (0.3–32%)

concentrations (Figs. 2 and 3). These samples fall

within the low- to medium-K fields in a K2O versus

SiO2 diagram (Fig. 2), except some of the PVKCA

samples, which fall within the high-K field.

The Miocene HK suite of the Younger Series has

MgO contents between 0.7% and 17.8%, and SiO2

contents range between 45% and 65%. These samples

mainly fall within the shoshonitic field, with fewer

analyses in the high-K field.

The Younger Series CAK suite (Dondo Super

Suite, DSS Leucocratic, Gimpu and Ongka Volcanics)

is more felsic than the other two groups, with SiO2

contents between 62% and 76%, and MgO contents

not exceeding 2.8%. They plot on the boundary

between the high-K and shoshonitic fields. Only a

few samples of either the Younger or the Older Series

have TiO2 values greater than 1.3% (Fig. 3); these low

values of TiO2 are characteristic for arc-type lavas

(Pearce, 1982).

MORB-normalised trace element diagrams for

selected samples from the different groups are shown

in Fig. 4A–D. Most of the TV, TI and TVD samples

show distinct Nb-Ta anomalies and LREE-enriched

patterns (Fig. 4A). They also display geochemical

characteristics that are typical of continental arc rocks,

such as relatively high Zr/Y (>3; Fig. 5) and La/Yb

(3.5–6) ratios and elevated K, Sr, Rb, Zr and Th

contents. This is consistent with their setting (meta-

Fig. 2. K2O versus SiO2 diagram for the samples analysed. Divisions following Gill (1981). Note that the samples from the Eocene–Miocene

Older Series have significantly lower K2O content than those for the Miocene–Pliocene Younger Series.


morphic basement of largely continental derivation).

The RG and TM have arc-type patterns too. The TV

and TVD rocks have been previously classified by

Polve et al. (1997) as back-arc basin (BAB) basalts

(based on only three samples) because of their reduced

Nb-Ta anomalies, significant depletion in Hf, Zr or Y

and flat to slightly depleted LREE patterns. However,

Fig. 3. TiO2 versus MgO for the Older and

our more extensive data do not support this interpre-

tation.

The various PV samples mostly show very pro-

nounced negative Nb-Ta anomalies (Fig. 4A) and

strongly resemble oceanic island arc basalts, as is

indicated by, for instance, their low Zr/Y ( < 3; Fig. 5)

and La/Yb ( < 2) ratios.

Younger Series. Symbols as in Fig. 2.

Fig. 4. (A–D) N-MORB normalised trace element diagrams for selected samples from the different groups. Both the CAK and HK samples

from the Younger Series show strong enrichments in incompatible elements, relatively high Nb/Zr ratios, and the negative Nb-Ta and Ti

anomalies typical for arc-type rocks. The Older Series samples have lower levels of incompatible elements and lower Nb/Zr ratios. Normalising

values and order from Sun and McDonough (1989).


Fig. 5. Zr/Y versus Zr diagram (after Pearce, 1982) for the Older

Series, showing the mainly oceanic arc character of the Papayato

Volcanics, and the mainly continental arc character of the TV, TVD,

TTV and TM samples. Symbols as in Fig. 2.


The two groups of TTV, A and B, have very distinct

trace element patterns, unlike typical arc basalts,

neither showing well-developed negative Nb-Ta or Ti

anomalies (Fig. 4B). However, they do have strong

positive Pb anomalies, one of the other signatures of

subduction-related basalts. The TTVB samples show

distinct continental arc features whereas the TTVA

samples display more mixed signatures.

The trace element patterns for representative sam-

ples from the different groups of Miocene HK rocks

show very strong enrichments in the most incompat-

ible elements (on the left hand side of the diagram,

Fig. 4C), and most show the overall pattern of arc-

type rocks. MORB-normalised values of Nb are

generally higher than for Zr, a sign of either a

continental arc or a collisional signature (Pearce

and Peate, 1995; Elburg and Foden, 1999b). The

patterns for the three groups of Miocene–Pliocene

CAK samples are nearly identical (Fig. 4D), with the

leucocratic DSS intrusives showing a slightly more

fractionated pattern with strong depletions in P

(apatite fractionation) and Ti (Fe-Ti-oxide fractio-

nation). Our analyses for the CAK samples closely

resemble the samples analysed by Polve et al.

(2001).

The normalised trace element patterns of the HK

and CAK suites are surprisingly similar considering

the difference in fractionation stage. They are distin-

guished from the Older Series by their much greater

enrichment in incompatible elements, both the fluid-

mobile ones such as Rb and Ba, and the fluid-immo-

bile ones such as Nb and Ta.

6. Radiogenic isotopes

Isotope and elemental data for Rb-Sr, Sm-Nd and U-

Th-Pb are given in Tables 3 and 4. No age correction

has been applied for 143Nd/144Nd (close to being within

error of the analysis) or Pb isotopes (owing to Pb levels

below the detection limit for many of the samples from

the older series), but all Sr isotopic ratios have been

recalculated to their measured or assumed age (see

Appendix A for the age constraints).

In terms of Sr and Nd isotopes, there is a clear

distinction between samples of the Older and those of

the Younger Series, with the former having lower87Sr/86Sr and higher 143Nd/144Nd than the latter (Fig.

6). The Older Series encompasses the field for the

Celebes Sea basement (Serri et al., 1991) and that for

samples from the Sangihe Arc in NW Sulawesi

(Elburg and Foden, 1998). The range of Sr-Nd isotopic

compositions of the Younger Series broadly overlaps

with that of Miocene–Pliocene (18–1 Ma) igneous

samples from central Sulawesi (Bergman et al., 1996;

Elburg and Foden, 1999a). Within the Younger Series,

the CAK samples have a slightly more restricted range

in isotopic compositions, particularly in 143Nd/144Nd.

Interestingly, central Sulawesi also experienced mafic,

high-K magmatism in the Middle Miocene which gave

way to more felsic CAK magmatism in Late Mio-

cene–Pliocene times (Priadi et al., 1994; Polve et al.,

2001). The link between this CAK magmatism and the

present-day dacitic products of the volcano Una-Una

(or Colo) in the Gorontalo Basin, which was suggested

by Priadi et al. (1993), is not borne out by their Sr-Nd

isotopic data. Una-Una has very constant isotopic

characteristics at 87Sr/86Sr 0.7061 and 143Nd/144Nd =

0.5126 (Vroon et al., 2000), while all the analysed

samples from the CAK series have 143Nd/144Nd <

0.5123 and 87Sr/86Sr>0.709. Our analyses for the

CAK series overlap with those from Polve et al.

(2001). The isotopic values for the NW Sulawesi

Younger Series extend to more ‘continental’ compo-

sitions (higher 87Sr/86Sr, lower 143Nd/144Nd) than

those of analysed low-grade sedimentary rocks from

K

Fig. 6. 87Sr/86Sr versus 143Nd/144Nd of the igneous samples from NW Sulawesi (top) and the same groups of samples compared to other

igneous, sedimentary and metamorphic samples from the area (bottom). Data from: Serri et al. (1991): Celebes Sea; Elburg and Foden (1998):

NE Sulawesi; Elburg and Foden (1999b) and Elburg et al. (2002a,b): south Sulawesi and north Australian sediment; Bergman et al. (1996) and

Elburg and Foden (1999a,b): central Sulawesi; Housh and McMahon (2000): Upper Miocene–Pliocene intrusives from Irian Jaya; Vroon et al.

(1996): south Sulawesi sediment; Vroon et al. (2000): Una-Una volcano; Polve et al. (2001): CAK and basement (peridotites and granulites).

Note that the low-grade sedimentary rocks from Sulawesi (south Sulawesi sediment, Tinombo Formation Phyllite and Pompangeo Schists) have

relatively unradiogenic Sr and Pb, and radiogenic Nd isotopic ratios.


the Tinombo Formation or the Pompangeo Schists

(Table 3, Fig. 6), or from other sedimentary rocks in

central and south Sulawesi (Vroon et al., 1996; Elburg

and Foden, 1999a). Some of the samples trend towards

the field for north Australian sediments (Elburg et al.,

2002a), in which several metamorphic samples from

the Malino and Palu complexes fall as do the lower

crustal samples analysed by Polve et al. (2001). The

slightly steeper samples trend towards low 143Nd/

144Nd at intermediately high 87Sr/86Sr (towards

NWS411) that two of the Miocene high-K samples

follow a trend somewhat similar to the seen in

Miocene intrusive rocks from Irian Jaya (Housh and

McMahon, 2000).

The Older Series samples overlap with the Younger

Series on the unradiogenic end of their trends in a207Pb/204Pb versus 206Pb/204Pb diagram, but do not

reach significantly more radiogenic values than the NE


Sulawesi Sangihe arc. The samples from the Younger

Series show a wide span (Fig. 7), broadly ranging from

Indian Ocean Mid-Ocean Ridge Basalt (I-MORB)

towards the field for north Australian sediments. This

is a wider range than seen in samples from equivalent

age in central Sulawesi. Within the Younger Series, the

Fig. 7. 207Pb/204Pb versus 206Pb/204Pb for the same samples as in Fig. 6. F

from Nelson et al. (1986).

CAK samples fall on the more radiogenic end of the

trend.

There is a pronounced distinction between the

Younger and the Older Series in the 208Pb/204Pb

(Fig. 8) versus 206Pb/204Pb diagram, with the Older

Series running from the I-MORB field towards the

ield for Western Australia and New South Wales ultrapotassic rocks

Fig. 8. 208Pb/204Pb versus 206Pb/204Pb for the same samples as in Fig. 6. Note the relatively high 208Pb/204Pb ratios for the HK samples.


low-grade sediments from NW and south Sulawesi,

encompassing the range for the NE Sulawesi samples,

and overlapping on the lower side of the field for

igneous rocks from south Sulawesi. The high-K

samples from the younger series are characterised by

high 208Pb/204Pb ratios for their 206Pb/204Pb ratios,

indicating a source component with long-term enrich-

ment of Th over U. The trend towards low 206Pb/204Pb


values does not point towards any easily identifiable

end member, but the high 206Pb/204Pb side of the trend

overlaps with that of the CAK samples and points

towards the field of north Australian sediments. The

overlap between the CAK series and Australian sedi-

ments was also noted by Polve et al. (2001) for the

samples they analysed from central and northwest

Sulawesi.

7. Discussion

7.1. Source of the older series

The isotopic compositions of the Older Series are

generally similar to those of subduction-related vol-

canics. Most of the PV Group isotopic values resem-

ble those for oceanic arc rocks, such as the products

of the Sangihe Arc. Only one PVKCA sample falls

squarely in the field for MORB in terms of its Pb

isotopic composition. Most of the TV, TI and TTVA

samples have lower 143Nd/144Nd and higher 87Sr/86Sr

ratios than the PV samples. Although it is possible

that alteration has affected the Rb-Sr system, leading

to unreliable initial 87Sr/86Sr ratios, this generally

does not change the 143Nd/144Nd ratios. The lower

Nd isotopic ratios are more similar to rocks from

continental arcs, or magmas that have suffered con-

tamination with sedimentary materials, either within

the subduction zone, or in high-level magma cham-

bers. This is also reflected in their relatively high Zr/

Y (Fig. 5) and Nb/Zr ratios, and by 87Sr/86Sr ratios in

excess of 0.707870 shown by the associated Tinombo

sediments (Table 3), indicating a continental prove-

nance. The mixed oceanic–continental arc signature

of the TTV samples may represent subduction under-

neath an area of thin continental crust, or varying

degrees of sediment subduction underneath an oce-

anic arc.

The presence of the Older Series of igneous rocks,

with its subduction signature, in NW Sulawesi

extends the areal extent of subduction-related magma-

tism in Western Sulawesi. According to Polve et al.

(1997), subduction-related magmatism in this region

was largely restricted to the Palaeocene Alla/Bua

Volcanics in south Sulawesi. Our data show that the

TV, TI, TVD and TTV in NW Sulawesi are also

subduction-related. Moreover, Eocene–Lower Oligo-

cene subduction-related magmatism has also been

recognised in the south arm of Sulawesi (Langi Vol-

canics; Van Leeuwen, 1981; Elburg et al., 2002b) and

several locations in central Sulawesi (T. van Leeuwen,

unpublished data). It agrees with several plate recon-

structions (Lee and Lawver, 1995; Hall, 1996; Rangin

et al., 1999) that both Western and Northern Sulawesi

were part of a volcanic arc over a west dipping subduc-

tion zone.

The fact that the PV has isotopic and geochemical

signatures more similar to those of oceanic arc rocks,

while the samples from the more westerly TV and TI

show a more continental arc signature, is interpreted

as reflecting a break in the character of the basement

underlying the deposits. This supports Taylor and

Van Leeuwen’s (1980) division of the West Sulawesi

Magmatic Province into two terranes, the boundary

of which we interpret to lie between the TV/TI and

PV, with the TTV being located in the boundary area.

An apparent gap in subduction-related magmatism

in the continental (western) part of the study area

exists between the Late Oligocene and Early Miocene,

with the youngest ages of the TV/TI around 30 Ma,

while the next phase of arc-type volcanism (Tondo-

pado Microdiorite) occurring at 18 Ma. It is not clear

yet whether this gap in magmatic activity is real or

reflects the poor geological record for this period of

time. However, it is interesting to note that a similar

gap in magmatism may exist in the central and south-

ern parts of Western Sulawesi as is suggested by

available stratigraphic and radiometric age dating

evidence (Polve et al., 1997). This apparent absence

of arc-related magmatism in Western Sulawesi could

reflect plate reorganisations, as suggested by the

tectonic reconstructions of Hall (2002). Arc-type

magmatism of the Papayato Volcanics continued

probably without any major breaks until the Early

Miocene (approximately 22 Ma). At Labuanaki, about

220 km east of the study area, it was preceded by/

coincided with the production of Eocene BAB basalts

that show similar geochemical characteristics to Cel-

ebes Sea basalts (Priadi et al., 1997).

7.2. Source of the younger series high-K samples

The high-K samples of the Younger Series show an

exceptional range in isotopic signatures. A significant

number of analyses display isotopic characteristics


that appear to have been influenced by upper crustal

rocks or sediments, with 87Sr/86Sr z 0.713, 143Nd/144Nd V 0.5121, 206Pb/204Pb>18.6, 207Pb/204Pb>

15.64, 208Pb/204Pb around 39 (Figs. 6–8). Represen-

tatives of this group are samples NWS376 and

NWS217. Several other end members can be found

within the high-K set. One representative is sample

NWS411, which is extending towards rather low143Nd/144Nd for its 87Sr/86Sr content; this sample

has low 206Pb/204Pb (18), but 208Pb/204Pb relatively

high at 38.9. Samples NWS66 and 737 have the least

‘continental’ Sr and Nd (0.708, 0.5126) isotopic

signature, but their Pb isotopes are only slightly less

radiogenic than those of NWS411. Finally, sample

NWS511 has Sr and Nd isotopic ratios similar to

NWS376, but combines this with the lowest Pb

isotopic signature of all high-K samples.

The isotopic signature of the HK suite does not

overlap with arc suites, nor with values for magmas

derived from the asthenospheric mantle such as mid-

ocean ridge basalt (MORB) or ocean island basalt

(OIB). Not even the high-K volcanoes from the Sunda

Arc (Muriah or Ringgit-Beser; Edwards et al., 1991,

1994) or high-K syn- and post-collisional samples

from the south arm of Sulawesi (Elburg and Foden,

1999b; Elburg et al., 2002b) have 143Nd/144Nd ratios

that even approach the low values seen in the HK

samples from NW Sulawesi.

The isotopic signature of a significant group of

these samples points towards that of clastic sediment

derived from Proterozoic upper crust, such as that from

north Australia (Elburg et al., 2002a). Interestingly,

this signature is more ‘continental’ than that of low-

grade sedimentary rocks from Sulawesi, such as phyl-

lites from the Palaeogene Tinombo Formation, or the

Mesozoic Pompangeo Schists. Some metasediments

from the Pre-Tertiary Palu and Malino Metamorphic

Complexes, as well as the lower crustal samples

analysed by Polve et al. (2001), have appropriate Sr

and Nd isotopic compositions that have been involved

in the petrogenesis of these samples, but none of them

have suitably high 208Pb/204Pb to explain the most

radiogenic high-K samples in Pb isotopic space. All

the high-K samples that trend towards low 206Pb/204Pb

ratios have significantly higher 208Pb/204Pb ratios than

subduction-related rocks from south or NE Sulawesi,

and also somewhat higher 207Pb/204Pb ratios. This

feature is shared by some Miocene high-K samples

from the Sassak area in central Sulawesi (Elburg and

Foden, 1999a), although the latter samples do not

reach the same relative enrichment in 208Pb/204Pb,

nor similarly low 206Pb/204Pb ratios. Although upper

Miocene to Pliocene high-K rocks from Irian Jaya also

show relatively high 208Pb/204Pb values for their206Pb/204Pb, the enrichment is not as extreme as seen

in NW Sulawesi.

A commonly invoked reservoir with high 207Pb/204Pb and 208Pb/204Pb for its relatively unradiogenic206Pb/204Pb ratios is the subcontinental lithospheric

mantle, as sampled by kimberlites and lamproites.

Data from such ultrapotassic rocks in Western

Australia and New South Wales (McCulloch et

al., 1983; Nelson et al., 1986) do indeed show

enrichment in 207Pb/204Pb and 208Pb/204Pb ratios for

their 206Pb/204Pb ratios compared to subduction-

related rocks, although neither group is an exact

fit for our HK data set. Nevertheless, we think that

the great diversity of isotopic compositions can

only be explained by melting of material with a

long and diverse geological history, with variable

enrichments and depletions in certain elements at

certain times. The high magnesium and low silica

content for many of the samples and the lack of

correlation between isotopic composition and indi-

ces of fractionation indicate that the magmas must

have been derived from the mantle, and that their

isotopic composition is unlikely to reflect contam-

ination in an upper crustal magma chamber. We

therefore propose that the HK samples in NW

Sulawesi reflect small degree melts from the Aus-

tralian lithospheric mantle, similar to the HK sam-

ples from central Sulawesi (Bergman et al., 1996;

Elburg and Foden, 1999a). This interpretation dif-

fers from that by Polve et al. (1997, 2001) and

Macpherson and Hall (1999), who propose that the

HK magmatism in central Sulawesi reflects melting

of Sundaland lithosphere, metasomatised by pre-

vious subduction.

7.3. Source of the younger series CAK samples

The CAK samples distinguish themselves from the

HK samples by their higher SiO2 contents, and their

more homogeneous isotopic and trace element com-

position. They overlap in isotopic composition with

the HK samples that have the more ‘continental’


isotopic composition, with high 87Sr/86Sr, low 143Nd/144Nd and high Pb isotope ratios, plotting close to the

field for north Australian sediments (Figs. 6–8), as

also noticed by Polve et al. (2001). There are four

ways in which these samples could have obtained

this continental geochemical signature: (1) Fractiona-

tion of mafic, mantle derived magmas that obtained

their isotopic signature by subduction of sedimentary

material into the mantle. This idea is contradicted by

the absence of any mafic and intermediate CAK

magmas and the fact that none of the CAK samples

show the unusual isotopic signature observed in the

mantle-derived HK samples (low 206Pb/204Pb ratios

combined with relatively high 208Pb/204Pb ratios) or

MgO contents in equilibrium with mantle rocks; (2)

Assimilation of crustal materials by mantle-derived

magmas. If this theory were correct, we would expect

to see a correlation between the samples MgO or

SiO2 content (as a measure of fractionation) and their

isotopic signature, which is not the case; (3) Mixing

of crustal and mantle-derived melts. The absence of

mafic samples or a correlation between isotopes and

indices of fractionation argues against this option; (4)

Melting of continental crust. This is our preferred

model and that of previous authors (Priadi et al.

1993; Bergman et al., 1996; Polve et al., 1997,

2001; Elburg and Foden, 1999a). Supporting evi-

dence includes the fact that we see only felsic

materials and no parental magmas that are in equili-

brium with the mantle, the predominantly peralumi-

nous character of the CAK rocks, and the presence of

cordierite and garnet in some samples, minerals that

are typical for crustally derived granites (White and

Chappell, 1988).

The crust that melted to form the granites cannot

be represented by the upper, Sundaland plate, since

the analyses of sedimentary rocks (Tinombo For-

mation Phyllite, Pompangeo Schist) indicate that

this plate does not show suitably high Sr and Pb

isotopic signatures, or low enough Nd isotopic

signatures. The CAK series magma must have

originated by melting of the Australian upper crust

that has been thrust underneath the younger Sunda-

land plate. High-pressure rocks exposed along the

Palu-Koro fault (Helmers et al.,1990; Parkinson et

al., 1998; Kadarusman and Parkinson, 2000) are

likely to represent this underthrust piece of Austral-

ian material, since their isotopic composition (Polve

et al., 2001) is broadly similar to that of the CAK

samples.

7.4. Isotopic signature in space and time

Fig. 9 shows the Nd isotopic composition through

time for different areas in Sulawesi. Fig. 10 shows a

map view of this information, with samples classified

as being either ‘‘Pre-Miocene’’ (i.e. the Older Series

of NW Sulawesi, ‘‘pre-collisional’’ samples of south

Sulawesi), ‘‘Miocene’’ (the HK samples of NW and

central Sulawesi, the Bolano Andesite, syn-collisional

samples from south Sulawesi, 12–14 Ma samples

from NE Sulawesi) or ‘‘Miocene-Recent’’ (CAK

samples from NW and central Sulawesi; post-colli-

sional (Gunung Lompobatang) samples from south

Sulawesi, 4 Ma and recent samples from NE Sula-

wesi and the recent products from Una-Una Volcano).

Data sources are given in the figure caption. The

threefold grouping in Fig. 10 neglects the fact that the

different groups overlap in time, as can be gleaned

from Fig. 9, and in this way the figures are comple-

mentary. The size of the points in Fig. 10 represents

the Nd isotopic compositions, and this shows very

well that for central and NW Sulawesi, Nd isotopic

values decrease markedly in the Miocene, whereas

this is not the case for south or NE Sulawesi. This

clearly points towards a different source for Miocene

HK magmatism in south Sulawesi and that in central/

NW Sulawesi (Macpherson and Hall, 1999), with

south Sulawesi magmatism representing melting of

metasomatised Sundaland mantle (Yuwono et al.,

1988) with input of sediments derived from an

Australian microcontinent (Elburg et al., 2002b),

while the source in central/NW Sulawesi is under-

thrust Australian lithospheric mantle.

Normal subduction-related magmatism overlapped

in time with high-K magmatism, as seen from the age

range of North Sulawesi arc magmatism (22–0.9 Ma),

including the date of the calc-alkaline Bolano Ande-

site (11 Ma). The Bolano Andesite occurs to the east

of the area in which we have evidence for Miocene

HK magmatism with low-Nd isotopic signatures, and

Miocene–Pliocene CAK magmatism. We propose

that its spatial position defines the eastern boundary

of the underthrust Australian sliver. This would then

coincide with the boundary between Northern and

Western Sulawesi, defined by the Papayato Volcanics

Fig. 10. Map view of Nd isotopic compositions for the same samples as in Fig. 9. The size of the circle corresponds to the value of the143Nd/144Nd ratio, with small circles denoting low values, and the colour of the circle indicating its age group. The shaded area indicates the

position of the underthrust piece of Australian continent, inferred from the spatial distribution of low-143Nd/144Nd samples.


and igneous suites of the Tinombo Formation. There-

fore, the spatial extent of the piece of underthrust

Australian lithosphere and crust extends from approx-

imately 4jS to 1jN.

7.5. Timing of underthrusting and cause of melting

There are two possible scenarios for the timing of

underthrusting of the Australian sliver and the cause

of its melting.

(1) Underthrusting occurred shortly before the

onset of HK magmatism, which started around 14

Fig. 9. Nd isotopic composition versus time for different areas within Sula

the diagram indicate the range of Nd isotopic ratios of the Celebes Sea bas

NW Sulawesi show similar evolutionary trends, with the low-Nd isotopic

central Sulawesi, if a single analysis by Bergman et al., 1996 is taken into

samples. This strongly contrast with the trends seen in SW and NE Sulaw

Ma, apart from an isolated lamprophyre date at 17 Ma

from central Sulawesi (Bergman et al., 1996). Under-

thrusting of a piece of Australian continent would have

doubled the lithospheric thickness. Adjustment of the

isograds after underthrusting caused heating and melt-

ing, first of the Australian lithospheric mantle (HK

magmatism), and later of the overlying crust (CAK

magmatism). Melting may also have been promoted by

break-off of the leading oceanic portion of the sub-

ducted slab and subsequent influx of hot astheno-

spheric material (Sacks and Secor, 1990; Polve et al.,

2001). The area of central and NW Sulawesi where we

wesi. Data sources as in Fig. 6. Rectangles on the right hand side of

ement and several metamorphic and sedimentary suites. Central and

signature in HK rocks coming in around 11–14 Ma (or 17 Ma for

account). Both areas also have a Miocene–Pliocene series of CAK

esi, where Nd isotopic values do not decrease in the same manner.


find Miocene high-K magmatism with very low Nd

isotopic ratios, and Miocene–Pliocene felsic CAK

magmatism, roughly coincides with an area of high

topographic relief, with mountain ranges up to 3000-m

high (Fig. 1d). Rapid uplift of the area is also indicated

by the exposure of very young granites (3–6 Ma;

Polve et al., 1997; Bellier et al., 1998; Elburg and

Foden, 1999a; this study) and metamorphism ages

(Van Leeuwen et al., in preparation). This could be a

reflection of the underthrusting event.

There is only limited data from disciplines other

than geochemistry to support this underthrusting

event in the Late Oligocene–Early Miocene. As

Calvert (2000) and Hall and Wilson (2000) have

pointed out, the sedimentary record of the southern

part of NW Sulawesi and central Sulawesi shows

evidence for a tectonic event only around the Mio-

Pliocene boundary. However, some radiometric age

dating, structural and stratigraphic evidence from the

Palu Complex, the north arm, and Luwuk area in

the east arm could point towards Late Oligocene–

Early Miocene collision events in NW Sulawesi.

Kadarusman et al. (in preparation) obtained a Sm-

Nd date of 27 Ma for garnet from a peridotite

sample collected south of Palu, which they interpret

to represent the age of peridotite emplacement in the

Palu Complex as the result of a collision event

between Western Sulawesi and a microcontinent.

In the north arm, Neogene formations overlie with

a regional unconformity the more strongly deformed

Middle Eocene–Earliest Miocene Tinombo Forma-

tion and Papayato Volcanics (Brouwer, 1934; Rat-

man, 1976; Kavalieris et al., 1992); the Malino

Complex yielded metamorphic (uplift) ages ranging

between 23 and 14 Ma; and its contact with the

overlying formations is probably tectonic. In the

Luwuk area, Mesozoic sediments have been folded

and overthrust by an ophiolitic nappe, which is

overlain by less deformed Middle-Miocene molasse

(Villeneuve et al., 2000). Underthrusting of a con-

tinental fragment during the Late Oligocene–Early

Miocene may explain the apparent gap in magma-

tism in Western Sulawesi during this period.

(2) The Australian continental sliver was under-

thrust long before the onset of HK magmatism. In this

case, it could represent the Cretaceous collisional

event documented in some of the high-pressure com-

plexes around Western Sulawesi (Parkinson et al.,

1998; Wakita, 2000). This would agree with the

interpretation of Hall (2002) that Miocene-age mag-

matism in Western Sulawesi was a response to exten-

sion, resulting in melting of the subcontinental

lithospheric mantle. In the case of the low-Nd area

of Western Sulawesi, this lithospheric mantle and

overlying crust would have been tectonically

emplaced underneath the Sundaland plate during the

Cretaceous. If this scenario is correct, the area of high

relief in Western Sulawesi is likely to be a response to

Pliocene thrusting (Hall, 2002) and the fact that it

overlaps with that of the low-Nd isotopic signature

must be purely coincidental. The change from sub-

duction-related magmatism to HK magmatism would

only reflect a change in tectonic regime from con-

vergent to extensional.

Whatever the cause of melting in the Miocene and

the timing of underthrusting of the continental sliver,

the extent of the low-Nd signature in Western Sulawesi

represents the size of the underthrust Australian con-

tinental fragment. Its northeastern limit appears to

coincide with the limit of the Eocene continental arc

of Western Sulawesi.

8. Conclusions

New geochemical and isotopic data for samples

from NW Sulawesi indicate that the area was part of

an arc system between f 51 and f 18 Ma, with

eruption and intrusion of subduction-related magmas

of continental affinity in the western part of the area

and of oceanic affinity in the east. This defines the

boundary between Western and Northern Sulawesi.

High-K magmatism in Western Sulawesi started

around 14 Ma, and its isotopic signature can only

be explained by a source with a long and varied

geochemical history, probably located within the

Australian subcontinental lithospheric mantle. The

end of the high-K event overlapped in time with

the start of felsic CAK magmatism around 9 Ma,

which continued until approximately 2 Ma. This

magmatism reflects melting of continental crust of

Australian affinity, most likely the material overlying

the lithospheric mantle that melted to produce high-K

magmatism. This sequence of magmatic events, from

normal subduction-related volcanism through HK and

CAK magmatism, is also seen in central Sulawesi,


but not in the south arm or in Northern Sulawesi. It is

best explained by melting of an underthrust piece of

Australian continent. The areal extent of this continen-

tal sliver can be estimated from the area containing the

aforementioned HK and CAK magmatism, located

between 4jS and 1jN. The underthrusting event must

have taken place prior to the first occurrence of high-K

magmatism at 14 Ma, so it is not related to the collision

with the Sula platform around 5 Ma (Hall, 2002). It is

as yet unclear whether the underthrusting event took

place shortly before HK magmatism, or represents a

much earlier event.

Appendix A. Characteristics of the Older Series igneou

Unit Description Ages P

Tinombo Volcanics (D) basalt-rhyolite 34–533 pl

Tinombo Intrusives gabbro, diorite, qz-diorite,

granodiorite

37.5–44.51 pl

Papayato Volcanics diabase, spillite, pillow lava;

basalt, dacite– rhyolite

22–502,3 ba

pl

Toli-Toli Volcanics basalt–dacite Miocene5 pl

Renangkali Granite mu-bt granite, granodiorite 41.94 pl

Tondopado

Microdiorite

andesite–diorite

diorite porphyry

17.73 pl

Bolano Andesite andesite 11.47 pl

Appendix B. Characteristics of the Younger Series igne

Unit Description Ages Ph

Gimpu Volcanics pyroclastics, rhyodacitic lava 2–4 Ma2 pl,

Ongka Volcanics pyroclastics, rhyodacitic lava 6–7 Ma3 pl,

Dondo Supersuite

(DSS)

(qz diorite), monzonite,

monzogranite, syenogranite

1.6–73,6 pl,

(FHK Series lamprophyre–monzodiorite,

basanite– latite

14.1–4.63,7 ol,

ksp

Acknowledgements

ME was a recipient of an ARC APD Fellowship

when the analytical work was carried out, and was

supported by an EU Marie Curie Fellowship during

writing of the manuscript. David Bruce is acknowl-

edged for his help in the isotope lab. Al Hofmann is

thanked for supporting ME’s appointment in Germany

and Tuti Mariani for her secretarial support to TvL.

The manuscript benefited from comments by C.

Macpherson and R. Hall, and official reviews by M.

Polve, R. Maury and T. Housh. [RR]

s units

henocrysts Alteration Deformation

, cpx, opx greenschist facies strong folding

, hbl, qz ser, chl, ep, act, cb moderate– strong

salt: pl, cpx, (ol) felsic:

, hbl, px, bt

chl, ep, ab, cb, qz,

ill/ser, (ksp, qz), ze

strong folding

, cpx, hbl, bt moderate

, qz, ksp, bt, mu weak–moderate ser,

chl, ep, cb

weak–strong

, am (qz, ksp, bt) < 10%, chl, ep, py,

ser, cb, sph

minor

(microfracturing)

, hbl, opx, qz, op none none

Pl = plagioclase, hbl = hornblende, am = amphibole,

qz = quartz, cpx = clinopyroxene, px = pyroxene, opx =

orthopyroxene, bt = biotite, ksp =K-feldspar, op = opa-

ques, sph = sphene, ser = sericite, chl = chlorite, ep =

epidote, ab = albite, act = actinolite, cb = carbonate,

ill = illite, ser = sericite, ze = zeolite, py =pyrite,

crd =cordierite, gt = garnet. (1) Unpublished K/Ar

ages (Rio Tinto); (2) K/Ar age, Priadi et al.

(1993); (3) K/Ar age, Polve et al. (1997); (4)

unpublished 40Ar/39Ar date on muscovite (van

Leeuwen and Vasconcelos, 2001 unpublished data);

(5) paleontological data from penecontemporaneous

Buol Beds; (6) K/Ar age, Van Leeuwen et al.

(1994); (7) this study.

ous units

enocrysts Alteration Deformation

qz, bt, hbl, FK-sp very weak none

qz, bt, hbl, FK-sp very weak none

qz, bt, K-sp, F hbl

crd, gt: xenocryst?)

generally weak foliation only near

Palu-Koro Fault

cpx, bt,F hblFF pl

weak–moderate

lamprophyre: weak–

strong: act, chl, cb


Abbreviations as in Appendix A.

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