Sources for magmatism in Central Sulawesi ...

Chemical Geology 156 Ž1999. 67–93

Sources for magmatism in Central Sulawesi: geochemical and Sr–Nd–Pb isotopic constraints Marlina Elburg ) , John Foden Department of Geology and Geophysics, UniÕersity of Adelaide, Adelaide SA 5005, Australia Received 7 January 1998; accepted 30 September 1998

Abstract Middle Miocene to Quaternary magmatism in Central Sulawesi ŽIndonesia. is characterised by radiogenic Sr and Pb isotopic signatures, and unradiogenic Nd isotopic ratios. Although the geochemistry of the samples is heterogeneous, all samples show a distinct subduction signature Žnegative anomalies for Nb and Ti, positive for K and Pb. in their trace element patterns. Mafic magmatism is represented by lamprophyric magmas, and a suite of ‘gabbros’ Žranging from gabbro to clinopyroxenite.. The isotopic signature of the lamprophyres could be explained by simple mixing between a mantle source, similar to that for mid-ocean ridge basalts ŽMORB., and sediments, as is typical for subduction-related volcanics, but the Pb–Sr isotopic systematics of the gabbros preclude this interpretation. They are interpreted as containing an important contribution from an old sub-continental lithospheric source, located within a sliver of Australian continent that has been thrust underneath Central Sulawesi. Felsic magmatism is likely to reflect high degrees of crustal contamination or intracrustal melting. These interpretations suggests that contemporaneous subduction did not play a major role in determining the isotopic signature of Miocene–Pleistocene magmatism in the area, despite the ‘subduction signature’ seen in their trace element patterns. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Magmatic arc; Subduction; Assimilation; Isotope geochemistry; Trace elements

1. Introduction The present study shows that even in recently active island arcs like Sulawesi ŽIndonesia., the geochemistry of magmatic rocks may give evidence of quite complex variations in source materials over geologically short periods of time. Some of these source materials include ancient crustal material and thus impart an inherited rather than newly produced ‘arc’ signature to the magmas. Indeed the problem of )

Corresponding author. Department of Earth Sciences, Monash University, Clayton 3168, Australia. Fax: q61-3-99054903

distinguishing the geochemical fingerprint of truly magmatic rocks whose origin is a contemporary subduction setting, from those where melts are delivered from the mantle for other reasons Žin rifts, or associated with mantle plumes. and which are then contaminated during their ascent through the subcontinental lithospheric mantle or the continental crust, is a challenge to geochemistry ŽHergt et al., 1989.. Magmas produced in subduction settings are complex geochemical mixtures. The understanding of these systems and the origins of their complexity is an important quest because of the key role attributed

0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 8 . 0 0 1 7 5 - 2

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M. Elburg, J. Fodenr Chemical Geology 156 (1999) 67–93

to the growth and accretion of magmatic arcs as a critical step in the growth and chemical evolution of the continental crust through time Že.g., Taylor and

McLennan, 1985.. Subduction-related magmas have geochemical signatures which resemble those of the upper continental crust, characterised by high

Fig. 1. Maps of the area studied. Ža. Location of Sulawesi within the Indonesian archipelago, and location of plate boundaries. Žb. Local geology after Sukamto Ž1975, 1979., Bergman et al. Ž1996., Wilson and Bosence Ž1996.. Žc. Location of samples used for this study.


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Fig. 1 Žcontinued..

contents of LIL elements and by high ratios of LILErHFSE. The argument that arcs are the sites of continental growth is partly based on the premise that processes active during the petrogenesis of arc magmas are the origin of the geochemical signatures of the crust. On the other hand there is extensive recognition of the recycling to the mantle of the continental crust via subduction zones, both as an

explanation of the limited net accumulation of crust over geological time since the early to mid Archaean Že.g., Armstrong, 1991. and also as the main explanation of the restricted rate of the mantle’s depletion Že.g., Bennett et al., 1993; Woodhead et al., 1993.. As emphasised by Plank and Langmuir Ž1988, 1993. a fraction of the continental sedimentary material subducted is returned to the crust as a component of

70


Fig. 1 Žcontinued..

the arc lavas. This mechanism implies that arc magmas gain their geochemical character partly as a result of recycling of more ancient continental crust, emphasising source rather than process. Recent research on igneous rocks from Central Sulawesi has shown that the Miocene to Recent volcanics and intrusives are characterised by high 87 Srr86 Sr Ž) 0.710. and low 143 Ndr144 Nd isotopic ratios ŽBergman et al., 1996.. These extreme isotopic ratios were interpreted to reflect melting of old lithosphere that was subducted underneath relatively young ŽTriassic; Hutchison, 1989. Sundaland crust of Central Sulawesi. However, similar isotopic characteristics could potentially result from mixing between MORB mantle and subducted continental material, as has been argued for the Banda and Sunda Arc ŽVroon, 1992; Varekamp et al., 1989; Van

Bergen et al., 1993. and Italy ŽEllam et al., 1988., or from assimilation of continental crust or sediment during ascent of the magma ŽRogers et al., 1985; Davidson, 1987; Gasparon et al., 1994; Thirlwall et al., 1996; Davidson et al., 1990; Smith et al., 1996; Ort et al., 1996.. For the present study, samples from the more accessible areas of Central Sulawesi were collected and analysed for major and trace elements, as well as their Sr, Nd and Pb isotopic composition, to investigate whether the Pb isotopic composition of the samples can shed more light on the sources and processes responsible for the unusual geochemistry of magmatism in this region. KrAr dating was performed on these samples to relate the geochemical evolution of Central Sulawesi to the tectonic history for the area proposed by Hall Ž1996..

Fig. 2. Sequence of cartoons illustrating the tectonic evolution in the area near Sulawesi, after Hall Ž1996.. Blackrdiagonal stripes: Australian plate; dark greyrlight grey: Sundaland crust; hatched pattern: Sunda arc; light stipple: SulawesirHalmaherarKalimantan.


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2. Local geology The island of Sulawesi is located in the central part of the Indonesian archipelago, north of the Sunda volcanic arc ŽFig. 1a.. The island can be divided into an eastern and a western part on the basis of geological criteria. The eastern province comprises the east and southeast arms of Sulawesi, and consists of an ophiolite complex of Cretaceous age ŽSimandjuntak, 1992., and high-pressure metamorphic assemblages with ages of approximately 35 Ma ŽWijbrans et al., 1994.. These are locally overlain by Triassic to Cretaceous deep sea sediments ŽSukamto, 1979.. The western province consists of the South and North arms, and is dominated by igneous assemblages, associated with marine sedimentary rocks. The area discussed here is the central part of the western province ŽFig. 1a.. As the geochemical signature of the igneous rocks in this area is significantly different from that found in the currently active arc of North Sulawesi ŽSangihe Arc., and from that in the south arm of Sulawesi Že.g., Polve´ et al., 1997., we will contrast the Central Sulawesi samples with those from South and North Sulawesi. The oldest exposed rocks in the area are Cretaceous meta-sediments ŽSukamto, 1975., which may be related to the subduction complex of the same age described from Southern Sulawesi ŽSukamto, 1979; Parkinson, 1996; Wakita et al., 1996.. An unconformity separates these rocks from overlying Tertiary units ŽBergman et al., 1996.. Early Tertiary units are marine sedimentary rocks as well as Oligocene to Miocene basic volcanics. However, the exact age of these basic volcanics Žunit Tpv on Fig. 1b. is unclear: previously published ages of samples from this formation vary from 15–33 Ma ŽPriadi et al., 1994., 57–137 Ma ŽBergman et al., 1996. up to 291 Ma ŽPolve´ et al., 1997.. Miocene–Pliocene marine sedimentary rocks Žunit Tms. locally contain interbedded tuffs or volcaniclastics. The Celebes Molasse Žunit Tcm. which may

correlate with the Walanae formation described by Wilson and Bosence Ž1996. from Southern Sulawesi, consists of coarse clastic sediments. Miocene–Pliocene intrusive Žunit Tgd. and extrusive Žunit Tnv. igneous rocks are dominant in the mid-western part of the area under study, and most of the area north of Sasak is covered by a 1 Ma old tuff deposit, the Barupu Tuff Žunit Qtv..

3. Tectonic evolution of the area The most recent reconstruction of the tectonic evolution of Southeast Asia over the past 50 Ma has been given by Hall Ž1996., and Fig. 2 illustrates this for the past 30 Ma. The western side ŽSouth and North arms. of what we now know as Sulawesi was a volcanic arc from 50 Ma until 5 Ma, related to west-dipping subduction. The slab subducting underneath the Palaeogene–Neogene arc is likely to have been part of the Philippine plate ŽAli and Hall, 1995.. The East and part of the Southeast arm of Sulawesi consist of an ophiolite complex which was obducted approximately 25 Ma ago ŽHall, 1996; Sidimantjuk and Barber, 1996.. West dipping subduction continued after this event. During the middle to late Miocene the continental fragments of Buton and Sula collided with the arc in South and Central Sulawesi, respectively ŽFig. 2., with the ButonTukang Besi fragment arriving at 13–11 Ma and Sula at 5 Ma ŽHall, 1996; Smith and Silver, 1991.. As the stratigraphy of these microcontinents is similar to that of New Guinea ŽCharlton, 1996. they are thought to have been transported from New Guinea westwards along a left lateral fault of the Sorong system ŽHamilton, 1979; Silver et al., 1983; Audley-Charles et al., 1988; Rangin et al., 1990; Ali and Hall, 1995; Lee and Lawver, 1995; Charlton, 1996.. These events therefore represent the collision between the Eurasian ŽSundaland. and IndoAustralian ŽNew Guinea. plate.

Fig. 3. Selected Harker variation diagrams for Central Sulawesi samples Žlegend given in figure.. Fields are indicated for trends observed in contemporaneous samples from North Sulawesi Žmedium-grey., which is underlain by oceanic basement, and for South Sulawesi Žlight-grey., underlain by continental basement. Data for North and South Sulawesi from Elburg and Foden Ž1998. and Elburg and Foden Žsubmitted..


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74

Sample L POL9 SiO 2 Al 2 O 3 FeO) MnO MgO CaO Na 2 O K 2O TiO 2 P2 O5 LOI Total mg ASI Zr Nb Ni Sr Rb Y Ba U Th

L POL10

G TT13

49.42 49.68 47.81 11.71 13.24 11.99 9.39 10.68 11.70 0.17 0.19 0.19 8.10 5.22 8.29 11.39 9.74 11.57 0.93 1.23 1.82 6.16 6.79 4.11 1.27 1.54 1.09 1.31 1.51 1.19 2.45 1.83 0.56 98.95 98.48 99.33 60.6 46.5 55.8 0.40 0.49 0.42 334.8 449 99.5 24.9 30.7 5.3 82 299 112 1171.3 1546.1 1246.9 234 222.8 196.1 40.4 48.7 24.0 4034 5105 3449 12.3 21.6 2.5 47.7 70.1 13.4

G TT16

G TT17

G TT18

42.08 50.20 45.39 3.95 11.97 13.56 15.58 9.12 13.37 0.19 0.17 0.19 14.49 9.05 6.76 18.71 11.28 11.61 0.58 1.91 2.06 0.86 4.44 3.67 0.95 0.83 1.25 2.29 0.83 1.84 0.10 0.26 0.13 100.36 99.21 99.05 62.4 63.9 47.4 0.11 0.42 0.48 40.6 109.5 101.9 1.3 7.6 5.4 250 98 58 373.7 1039.8 1313.3 46.2 199.7 192.1 19.0 18.7 27.8 549 2930 3733 1.0 3.7 3.4 6.2 18 17.9

LK TT27

MK TT3

TA POL13

S GD POL27C POL18

55.76 55.72 56.71 60.19 16.07 19.32 13.24 16.02 8.93 6.63 5.52 4.81 0.17 0.19 0.09 0.09 6.07 3.27 9.95 3.31 8.17 8.09 5.91 5.02 2.55 3.98 1.96 3.49 0.86 1.61 5.32 5.80 0.97 0.71 0.75 0.77 0.28 0.37 0.44 0.40 1.45 2.44 0.86 1.05 100.12 99.37 99.47 99.66 54.8 46.8 76.3 55.1 0.80 0.84 0.67 0.76 160.9 102.0 248 364.3 7.5 7.5 19.7 31.2 46 12 277 33 229.4 1098.7 481.8 627.9 31.7 72.5 246.6 208.1 44.9 23.9 31.1 41.3 317 696 1781 1258 0.9 2.2 7.2 11.3 4.0 3.7 28.8 50

67.20 14.25 3.66 0.07 3.64 3.12 2.60 4.66 0.58 0.17 0.25 99.70 63.9 0.95 214.5 16.3 40 206 241 36.6 549 6.4 27.1

GD POL21 67.31 14.35 3.85 0.07 3.56 3.14 2.46 4.44 0.58 0.16 0.90 99.27 62.3 0.99 186.9 16.0 33 190.1 210.4 36.2 491 3.6 20.9

GD TT24 66.78 15.18 3.44 0.06 2.35 3.47 3.21 4.57 0.59 0.27 0.69 99.67 54.9 0.92 246.5 19.4 22 355.4 226.2 40.1 924 9.7 37.8

GD TT1c 68.55 16.23 2.23 0.04 1.60 3.42 3.49 3.87 0.39 0.13 0.37 99.40 56.1 1.01 183.2 13.0 9 440.1 177.8 14.8 691 7.3 19.3

GD TT2 66.20 16.74 2.88 0.05 2.16 3.74 3.27 4.22 0.52 0.17 0.41 99.43 57.2 1.00 219.3 17.1 14 441.7 169.3 18.3 726 8.0 25.2

R POL22 69.86 15.17 1.45 0.02 1.94 1.19 3.75 6.20 0.28 0.12 2.54 98.70 70.4 1.01 228.3 25.3 63 284.4 406.9 10.2 399 18.9 48.2

Sed. POL4 67.39 20.95 5.05 0.02 0.75 0.04 0.54 4.11 0.99 0.08 4.40 99.98 20.9 3.87 193.5 14.0 158 126.2 235.3 32.9 427 2.7 13.3


Table 1 Selected chemical analyses

82.4 27.9 263 217 12.7 118 70 76.9 160.0 19.74 77.2 14.93 2.95 1.49 12.59 7.27 1.30 3.47 0.46 2.76 0.39 9.08 1.81

87.3 22.3 335 863 16.6 151 89 101 204 96

44.2 33.5 404 200 13.6 352 92 36.5 74.0 9.19 36.8 7.40 1.83 0.81 5.96 4.20 0.81 2.20 0.30 1.88 0.28 3.14 0.43

6.8 70.6 427 510 9.4 35 60 23.9 53.7 7.52 33.9 7.22 1.71 0.73 5.79 3.62 0.66 1.68 0.21 1.22 0.18 1.47 0.29

51.8 33.9 287 310 14.1 86 75 31 74 31

41.2 31.4 448 17 16 141 95 52.0 104.6 13.07 52.4 10.03 2.25 0.98 8.94 4.98 0.94 2.50 0.33 1.95 0.29 2.84 0.51

7.1 17.4 123 190 25.3 31 146 15.9 37.5 4.84 20.4 6.66 1.37 1.34 8.05 7.80 1.54 4.10 0.59 3.58 0.52 3.62 0.68

9.0 15.4 164 17 19.5 26 85 12 34 16

42.2 15.1 102 571 13.4 48 51 53.3 110.4 13.21 50.5 9.90 1.71 1.06 7.71 5.39 1.01 2.73 0.38 2.28 0.34 6.28 1.42

50.1 14.8 98 114 21.3 15 37 91 182 69

34.2 10.7 58 197 17.5 11 53 48.3 100.4 11.60 42.0 8.21 1.04 1.06 6.76 5.90 1.19 3.34 0.49 2.96 0.44 4.59 1.82

27.0 11.9 67 189 17.4 22 53 42.1 87.6 10.17 37.2 7.40 1.02 1.00 6.32 5.78 1.17 3.31 0.49 2.96 0.44 2.46 1.73

40.0 11.4 63 65 18.2 20 54 67.5 124.8 14.30 50.3 9.34 1.53 1.16 7.58 6.36 1.28 3.52 0.50 3.07 0.45 5.19 1.95

52.5 6.4 41 69 18.9 5 42 23 52

38.3 9.0 62 50 21.8 20 49 31 68

16

25

76.0 3.5 21 112 25.5 11 55 59.3 117.8 12.90 42.2 6.54 0.74 0.41 3.45 1.84 0.32 0.90 0.12 0.74 0.11 6.46 3.20

13.7 18.6 165 585 21.7 48 108 33.8 70.5 8.26 31.3 6.53 1.44 0.92 5.83 5.31 1.09 3.12 0.47 2.93 0.45 4.67 1.03

L s lamprophyre; G s gabbro; LK s low-K Andesite; MK s medium-K andesite; TA s trachyandesite; Sssyenite; GDs granodiorite; R s rhyodacite; Sed.ssediment; mg s mg-number Ž100)MgrŽMgqFe)... Oxides and elements by XRF except for elemental analyses in bold, which have been determined by ICP-MS.


Pb Sc V Cr Ga Cu Zn La Ce Pr Nd Sm Eu Tb Gd Dy Ho Er Tm Yb Lu Hf Ta

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Subduction of a continental slice underneath Central Sulawesi at approximately 20 Ma has been proposed in the tectonic reconstruction of Hall Ž1996., to explain the enriched isotopic signatures of the area reported by Bergman et al. Ž1996..

4. Analytical techniques The analytical techniques followed for XRF, ICPMS, Sr, Nd and Pb isotope analyses and KrAr dating have been described recently by Elburg and Foden Ž1998.. All samples used for isotopic analyses were routinely leached in hot HCl before dissolution, to remove the effects of surface contamination or minor alteration. Analyses of leachates and residues indicated that this had a negligible result on the isotopic ratios measured.

5. Sample description, ages and whole rock geochemistry Our aims were to define potential multiple magmatic sources and to monitor changes in the way these different sources contribute to Central Sulawesi igneous suites in response to plate collisions and plate movements. Accordingly, sampling was under-

taken in order to furnish as complete a record of Neogene magmatic activity as possible, with particular reference to their relative age. The geographic locations of samples are given in Fig. 1c. The sampled suites are geochemically very diverse although most are characterised by a potassium-rich composition, as reflected by their mineralogy, with biotite and K-feldspar as important constituents of many rocks. Because the analysed samples have very variable characteristics we facilitate the description of their petrology and geochemistry by subdividing them into the following groups Žfrom oldest to youngest units.: Meta-sediment Ž96POL4.; Medium-K andesite Ž96TT3.; Gabbros Ž96TT13, 14, 16, 17, 18.; Low-K andesite Ž96TT26, 27.; Lamprophyres Ž96POL6, 7, 9, 10.; Trachy-andesite Ž96POL13.; ‘Granodiorites’ Ž96POL15, 18, 20, 21; 96TT1, 2, 24, 25.; Syenites Ž96TT27a, 27c.; Rhyodacites Ž96POL22, 24.. Although this appears to be an excessive number of groups for the number of samples analysed, the petrological and geochemical characteristics ŽFig. 3a. of the samples do not permit fewer subdivisions. Representative analyses are given in Table 1, and

Table 2 K–Ar data for Central Sulawesi samples Ar)r40Artotal

Sample

Description

%K

40

Ar) Ž10y10 molrg.

40

Age ŽMa.

1 s ŽMa.

TT3 amph TT3 mix TT16 bt TT18 bt TT18 ksp TT27 plag TT24 bt POL18 bt POL21 bt TT2 bt POL9 bt POL27c bt POL22 bt POL22 san TT8 bt

MK andesite MK andesite gabbro gabbro gabbro LK andesite granodiorite granodiorite granodiorite granodiorite lamprophyre syenite rhyodacite rhyodacite tuff

0.79 0.77 7.41 7.2 6.41 1.44 7.15 7.25 6.7 8.62 7.34 7.64 7.68 5.86 6.03

0.113 0.075 0.993 1.25 1.231 0.141 0.817 0.703 0.628 0.65 0.649 0.655 0.47 0.427 0.0122

0.43 0.39 0.7 0.91 0.84 0.66 0.82 0.801 0.75 0.76 0.75 0.753 0.6 0.74 0.108

19.24 14.91 11.05 11.02 13.1 8.59 8.01 7.7 7.16 5.68 6.84 6.55 5.91 5.68 1.08

0.22 0.15 0.1 0.09 0.11 0.07 0.07 0.08 0.06 0.05 0.06 0.06 0.05 0.04 0.07

bt s biotite; san s sanidine; ksp s K-feldspar; Amphs amphibole; plag s plagioclase; mix s mixture of amphibole and groundmass.


age information in Table 2. Selected Harker variation diagrams can be found in Fig. 3, and N-MORB normalised trace element patters in Fig. 4. The medium-K andesite is found as large boulders alongside the road near Palopo ŽFig. 1c.. The groundmass is altered, but the sample contains recognisable crystals of plagioclase and remarkably fresh amphibole and clinopyroxene, with subordinate

77

amounts of magnetite and apatite. The medium-K andesite is the oldest sample from the area, and belongs to the Lamasi volcanics, as defined by previous workers, on the basis of its geographic location ŽFig. 1b,c.. A minimum age of 14 Ma is given by a KrAr analysis of amphibole mixed with groundmass, and a pure amphibole separate yielded a 19 Ma age, which is the more reliable age due to

Fig. 4. MORB-normalised trace element patterns for representative samples from the different groups. Element ordering and normalising values from Sun and McDonough Ž1989.. All samples are characterised by negative Nb-anomalies, and positive K and Pb spikes. Notice that the only sample with a positive Sr anomaly—another feature typical of arc volcanics—is the medium-K andesite, the one sample from this study which also shows an isotopic signature within the range for normal arc volcanics.

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possible argon loss during alteration of the groundmass for the 14 Ma analysis. This compares well with the date of 17.8 Ma reported by Priadi et al. Ž1994.. The sample analysed has higher K 2 O and Na 2 O contents than the low-K andesite, and lower MgO, FeO) and TiO 2 contents. Pb, U, Th and REE contents are similar to the low-K andesite, but its Sr contents is five times higher. The ‘gabbros’ yield a KrAr age of 11 Ma for their biotite separates, but one K-feldspar separate gave an aberrant age of 13 Ma. The 11 Ma is accepted as the most reliable, because the same age has been obtained by other workers ŽI. Kavalieris, pers. comm... The samples belong to the unit of Miocene–Pliocene intermediate and basic volcanics. Not all samples are true gabbros: they range from gabbro to clinopyroxenite, and some are likely to represent cumulates. The most mafic varieties are rich in clinopyroxene, whereas the more felsic ones are dominated by alkali-feldspar. Other minerals are magnetite, phlogopite and apatite, whereas olivine is only present in the most mafic samples. They are found in the area near Sasak, where a Cu-porphyry is known to exist, associated with quartz monzonites ŽI. Kavalieris, pers. comm... Porphyritic rocks, with phenocrysts of clinopyroxene in a devitrified groundmass, also occur in this area. The samples are relatively low in Zr and Nb compared to the lamprophyres Žsee below., which have equivalent SiO 2 contents. They also contain lower abundances of K 2 O, Y, U, Th, Pb and REE. The cumulate character of the most mafic sample is exemplified by its high MgO, CaO Žclinopyroxene accumulation. and P2 O5 Žapatite. content. In a MORB-normalised trace element diagram, these samples show distinctly negative Nb and Ti anomalies, and positive Pb, K and Sr anomalies. These features are characteristic for subduction-related rocks. The positive Sr anomaly sets this pattern apart from the trace element signature of sedimentary rocks Žsee below.. The low-K andesites, which were found as lithics included within the Barupu Tuff, are fine-grained, vesicular Žalthough many of the vesicles are now filled with calcite. and contain microphenocrysts of plagioclase, ortho- and clinopyroxene set in a glassy groundmass. The low-K andesite has been dated at 8.4 Ma, whereas the tuff itself yields an age of only 1.1 Ma. The lithics therefore seem to be unrelated to

this tuff. Geochemical distinctions between analyses of the Barupu tuff by Priadi et al. Ž1994. and our low-K andesite also suggest that the samples are unrelated Že.g., Na 2 OrK 2 O ratios, which are around 3 for the low-K andesites and approximately 1 for the Barupu Tuff.. Comparatively low contents of K 2 O, Rb, Sr, Ba, Th, U, Pb, Y and Nb characterise the low-K andesites. The MORB-normalised trace element pattern for these samples is similar to that of the gabbros, although a positive Sr anomaly is absent. The lamprophyres occur as dykes crosscutting the Cretaceous sediment. They have not been classified as any separate formation, but are classed with the Miocene–Pliocene intrusives, found in the area 10– 20 km north of Polewali. They have been dated at 6.8 " 0.1 Ma ŽTable 2.. They are either homogeneously fine-grained, with some clots of secondary calcite, or contain euhedral crystals of olivine Žoften altered to iddingsite., finely zoned clinopyroxene and phlogopite and magnetite set in a groundmass of devitrified glass. They are characterised by low silica content, high potassium content and high K 2 Or Na 2 O ratios Ž) 4.. MgO contents are variable Ž4– 8%., and mg-numbers vary between 46 and 66. Zr and Nb contents are relatively high Žup to 450 and 32 ppm, respectively., as are Y, Th, U, Pb and REE. Sr, Ba and Rb are high too, but fall within the same range as the gabbros. Cr and Ni are variable, and reach values of 850 and 300 ppm, respectively. The characteristics that set their MORB-normalised trace element signature apart from a typical subduction signature is the absence of a positive Sr anomaly, and the enrichment of Nb compared to Zr, Ti or Y. The age of the trachy-andesite is unknown, but the sample was derived from the Miocene–Pliocene volcanics, so a late Miocene age is likely. The sample contains phenocrysts of variably altered olivine, and clinopyroxene in a groundmass of finegrained feldspar, pyroxene, magnetite, apatite and devitrified glass. Its K 2 O and Rb contents are appreciably higher than for the low- and medium-K andesites, and it has high Ni and Cr contents. Its MORB-normalised trace element pattern bears most resemblance to that of the lamprophyres. The age of the granodiorites varies between 5.7 and 8 Ma. Except for sample POL21, which is equigranular, the granodiorites are porphyritic and


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Table 3 Isotopic data for samples from Central Sulawesi Sample

Description

87 86

Srr Sr

1s

RbrSr

87 86

Srr Sri

143

1s

´ Nd

SmrNd

T DM ŽMa.

0.512356 0.512360 0.512352 0.512078 0.512177 0.512173 0.512105 0.512065 0.512157 0.512000 0.512971 0.512301

38 44 41 60 51 38 30 28 27 37 34 38

y5.49 y5.42 y5.58 y10.92 y8.99 y9.07 y10.40 y11.19 y9.38 y12.44 6.50 y6.57

0.2087 0.1939 0.1894 0.1961 0.1956 0.1988 0.1551

1266 1140 1124 1590 1435 1471 1232

0.2147

249

0.2009 0.2128

1385 1759

0.1914 0.1858

1593 1411

0.327

7763

144

Ndr Nd

POL4 POL9 POL10 POL13 POL18 POL21 POL22 POL24 POL27c TT2 TT3 TT8 Apatite TT13 TT16 TT16 Apatite TT17 TT18 TT24 TT26 TT27

Sed. L L TA GD GD R R S GD MK Barupu Tuff

0.710937 0.710049 0.709972 0.718484 0.722483 0.720001 0.719892 0.719562 0.716825 0.716333 0.703378 0.710250

57 99 109 36 53 46 62 46 69 61 64 41

1.8645 0.1998 0.1441 0.5118 1.1699 1.1068 1.4307 1.3104 0.3314 0.3833 0.0660

0.709993 0.709932 0.718358 0.72212 0.719682 0.719545 0.719244 0.716735 0.716243 0.703337

G G G

0.711609 0.712608 0.712357

62 57 38

0.1573 0.1236

0.711538 0.712552

0.512240 0.512089 0.512105

41 20 44

y7.76 y10.71 y10.41

G G GD LK LK

0.712813 0.713315 0.718321 0.711928 0.712011

49 72 44 36 39

0.1921 0.1463 0.6365 0.1429 0.1381

0.712726 0.713236 0.718111 0.711879 0.711963

0.512111 0.512046 0.512136 0.512138 0.512230

39 60 39 35 40

y10.27 y11.55 y9.79 y9.76 y7.96

Sample

Pb

U

Th

206

207

208

206

204

POL4 POL9 POL10 POL13 POL18 POL21 POL22 POL24 POL27c TT2 TT3 TT8 TT13 TT16 TT17 TT18 TT24 TT26 TT27

13.68 82.36 98.97 42.21 34.16 27.04 76.04 48.6 50.1 38.3 9.27

2.72 12.31 16.51 7.19 6.42 3.587 18.85 20.6 11.3 8 1.06

13.72 47.71 64.09 28.76 27.13 20.93 48.21 49.9 50.0 25.2 3.08

44.17 6.773 51.8 41.17 39.99 7.6 17.40

2.5 0.992 3.7 3.39 9.69 0.8 4.50

13.37 6.203 18 17.91 37.84 3.7 6.70

Pbr Pb

18.814 19.081 19.090 18.999 18.999 18.938 19.069 19.062 18.921 18.996 18.366 18.872 18.409 18.305 18.368 18.284 18.969 18.810 18.820

204

Pbr Pb

15.674 15.671 15.666 15.674 15.666 15.658 15.694 15.685 15.672 15.684 15.574 15.657 15.598 15.604 15.600 15.591 15.667 15.649 15.641

RbrSr ratio from XRF data, SmrNd ratios from ICP-MS. U–Th–Pb data from ICP-MS when in two decimals, otherwise from XRF. No age correction has been applied to the Nd isotopic ratios. Abbreviations of sample type the same as in Table 1.

204

Pbr Pb

39.025 39.163 39.276 39.139 39.082 39.029 39.325 39.299 39.056 39.151 38.434 39.018 38.874 38.805 38.895 38.842 39.076 38.988 38.991

204

Pbr Pbi

18.798 19.069 19.071 18.985 18.984 18.927 19.052 19.033 18.904 18.991 18.313 18.872 18.402 18.287 18.359 18.273 18.947 18.799 18.817

207 204

Pbr Pbi

15.673 15.670 15.665 15.674 15.665 15.658 15.693 15.683 15.671 15.683 15.572 15.657 15.598 15.603 15.599 15.590 15.666 15.648 15.641

208 204

Pbr Pbi

39.000 39.148 39.119 39.121 39.061 39.010 39.311 39.277 39.032 39.137 38.405 39.018 38.862 38.768 38.881 38.825 39.048 38.973 38.990

80


Fig. 5. Initial 87 Srr86 Sr– 143 Ndr144 Nd systematics for Central Sulawesi samples and contemporaneous volcanics from North Sulawesi Ždark grey field. and South Sulawesi Žlight grey.. The sediment sample overlapping with the lamprophyre is from this study, the other samples from Vroon et al. Ž1996., Vroon Žunpublished data. and Bergman et al. Ž1996.. The field for Miocene igneous rocks is from Bergman et al. Ž1996..

contain phenocrysts of plagioclase, biotite, amphibole with or without clinopyroxene. Their groundmass consists of medium grained plagioclase, quartz and K-feldspar or devitrified glass, indicating that these rocks were either shallow intrusives or extrusives. POL21 consists of an interlocking fabric of plagioclase, K-felspar, quartz, biotite and amphibole. The most interesting characteristic of the granodior-

ites is a rather fractionated trace element pattern, with obvious depletions in Ba, Sr and P, which may be related to the fractionation of K-feldspar, plagioclase and apatite. The syenite samples were found as boulders along the road east of Polewali and are slightly altered Žsericite and minor chlorite.. They are holocrystalline and contain large prismatic K-feldspar crystals and

Fig. 6. Initial 87 Srr86 Sr–SiO 2 systematics for the same samples as in Fig. 4. Although most samples show high Ž) 0.7095., only the more silicic samples have 87 Srr86 Sr ) 0.717.

87

Srr86 Sr ratios


smaller biotite, pyroxene, and plagioclase crystals. Quartz occurs as small, anhedral interstitial crystals. The syenites are slightly more felsic than the trachyandesite, and they have been dated at 6.6 Ma. They differ from the trachy-andesite with respect to their far lower MgO, Ni and Cr contents. Y, Nb, Zr and REE contents are higher. The rhyodacite samples contain euhedral plagioclase, K-feldspar and biotite phenocrysts in a slightly devitrified groundmass. They are derived from the units that were classified as Miocene–Pliocene volcanics and Miocene–Pliocene intrusives by Sukamto

81

Ž1975.. Inclusions of glass and apatite are common in the feldspars; zircon is scarce. The rhyodacites resemble the granodiorites by showing depletions in Ba, Sr and P, but they also have very low Y and HREE contents. Their age is 5.7–5.9 Ma. Although this age was older than expected on the basis of their unaltered appearance, analyses of a biotite and a sanidinergroundmass separate gave similar ages; excess argon is unlikely to cause major disturbances, due to the high-K character of the analysed separates. Further noticeable traits of these samples are their high U, Th and Pb contents, which fall in the

Fig. 7. Ža and b. Pb isotope systematics for the same samples as in Fig. 4. The gabbros have remarkably low 206 Pbr204 Pb ratios compared to their highly enriched Sr and Nd isotopic characteristics. Again, the medium-K andesite appears to be the only normal arc volcanic.

82


same range as the lamprophyres. Ni and Cr contents are surprisingly high for their evolved nature, and this is also the case for their mg-number. In order to estimate the local average crustal composition a meta-sediment sample was also analysed. It comes from the area north of Polewali, and is a graphite-rich shale. The stratigraphic age of the unit from which it is derived is Cretaceous ŽSukamto, 1975.. As is typical of post-Archaean shales, the sample has high Al 2 O 3 , Cr and Ni, low CaO, Na 2 O and Sr ŽTaylor and McLennan, 1985..

6. Isotope geochemistry The initial Sr–Nd isotopic data for the suite analysed in this study ŽTable 3; Fig. 5. shows that all samples, except the 19 Ma old medium-K andesite, have extremely enriched isotopic signatures Žwith ‘enriched’ meaning high 87 Srr86 Sr and low 143 Ndr144 Nd ratios.. Samples with lower silica contents have on average lower 87 Srr86 Sr ratios ŽFig. 6., and show a negative correlation between their 87 Srr86 Sr and their 143 Ndr144 Nd composition. These are the lamprophyres, gabbros and low-K andesites. The higher silica samples ŽSiO 2 ) 56%; trachyandesite, syenite, felsic intrusives and rhyodacites. do not show this correlation, and are displaced towards higher 87 Srr86 Sr ratios at similar 143 Ndr144 Nd ratios compared to the lower silica samples. Interestingly, the Sr and Nd isotopic ratios of exposed Cretaceous and Palaeogene sedimentary units from Central and Southern Sulawesi Ždata from this study; Vroon et al., 1996; Bergman et al., 1996. are significantly more primitive than the analyses for most igneous samples presented here. The data of Miocene igneous rocks from Bergman et al. Ž1996. include two samples with low 87 Srr86 Sr ratios ŽFigs. 5 and 6., similar to ratios seen in normal arc volcanics. Most samples are also enriched in their Pb isotopic compositions ŽFig. 7a,b., with 206 Pbr204 Pb G 18.8. Exceptions are the medium-K andesite with 206 Pbr204 Pb s 18.3, but also the gabbros, with 206 Pbr204 Pb of 18.3–18.4. Despite their relatively low 206 Pbr204 Pb, the 208 Pbr204 Pb and, to a lesser extent the 207 Pbr204 Pb ratios of the gabbros are only slightly lower than those of the high 206 Pbr204 Pb samples. These Pb isotopic ratios are similar to those

reported for galena from the Sasak porphyry deposit ŽFig. 1b,c; Doe and Zartman, 1979., found in 10.6 Ma monzodioritic intrusions ŽTaylor and Van Leeuwen, 1980..

7. Discussion It is important to note that the first occurrence of a relatively enriched Sr and Nd isotopic signature in the data set presented here occurs at 11 Ma ŽFig. 8.; this is prior to the onset of collision between Central Sulawesi and the Sula platform, which is interpreted as having occurred at 5 Ma, and certainly no earlier than 10 Ma ŽHall, 1996.. Moreover, if the data from Bergman et al. Ž1996. are included in this study, the first occurrence of this enriched signature could be as early as 17 Ma ŽFig. 8.. On the basis of the data published by Bergman et al. Ž1996., tectonic reconstructions have postulated the subduction of a piece of continental material underneath Central Sulawesi at approximately 20 Ma ŽHall, 1996.. The data from the present study are consistent with this interpretation. It is also compatible with the fission track study by Bergman et al. Ž1996., which suggests that rapid uplift was already occurring at 8–10 Ma, which may be the response to the subduction of a buoyant piece of continental material; unfortunately, the data from that study do not show when, prior to 10 Ma, uplift was initiated. We will now address the question to what extent the trace element and isotopic data for the most mafic samples from Central Sulawesi, the lamprophyres and gabbros, necessitate the involvement of this continental slice in their petrogenesis. We will then discuss the relationship between the mafic and silicic samples, and assess the importance of crustal contamination.

8. Constraints on mantle sources from lamprophyres and gabbros Although crustal contamination may have played an important role in the petrogenesis of the more silicic samples Žas addressed in Section 9., the isotopic signature of the lamprophyres and gabbros are unlikely to have been affected by this process. These


83

Fig. 8. Nd and Sr isotopic characteristics of the samples vs. age. The enriched isotopic signature is first seen at 11 Ma, or 17 Ma if the data from Bergman et al. Ž1996. are included. This is prior to the onset of collision between the arc and the Sula platform, which happened between 5 and 10 Ma. This suggests that the collision with Sula is not the reason for the development of this peculiar geochemical signature.

rocks are relatively unfractionated Žwith high MgO, Ni, and Cr concentrations and with high mg-numbers., and they have high Sr, Nd and Pb contents, making them quite insensitive to potential crustal contamination. Moreover, crustal contamination by the Cretaceous sedimentary rocks exposed in the area cannot explain the Pb isotopic signature of the lamprophyres, which is significantly more radiogenic than those of the sedimentary rocks. On this basis, it is assumed that the isotopic signature of the gabbros and lamprophyres is representative of the mantle– source from which the magma was derived. 8.1. Constraints from trace elements The gabbros and, to a lesser extent, the lamprophyres display trace element patterns with most characteristics of a subduction-zone signature on a MORB-normalised diagram: Nb, Zr and Hf, and Ti are depleted compared to their neighbouring elements, whereas K and Pb form positive anomalies. Strontium, an element which generally also shows a

positive anomaly in subduction-related magmas, does not appear to be significantly enriched compared to its neighbouring elements in the samples under discussion. Another feature that sets these rocks apart from ‘true’ arc volcanics is the fact that the negative Nb and Zr anomalies are not as large as in ‘true’ arc volcanics of equivalent age in South Sulawesi ŽFig. 9.. However, the trace element signature is similar to that of subduction related volcanics, and we will therefore attempt to model their isotopic signature by contributions from sources normally implicated in subduction-related magmatism: MORB-type mantle, fluids from the subducted slab and subducted sediments ŽHawkesworth et al., 1991; Turner et al., 1996; Peate et al., 1997., and, if necessary, the subcontinental lithospheric mantle ŽPearce, 1983.. 8.2. Modelling of the isotopic signature of the lamprophyres The isotopic signature of the lamprophyres approximately overlaps with that of dredged sediments

84


Fig. 9. NbrZr vs. ZrrY ratios for Central Sulawesi samples compared to samples from North and South Sulawesi. Except for the gabbros and the low-K andesite, the samples plot outside the fields for volcanics from the other areas in Sulawesi. It is unclear why the medium-K andesite, which appeared to be the most ‘normal’ arc volcanic in all other diagrams, plots in this diagram with the granodiorites and lamprophyres.

from the Banda Arc ŽVroon, 1992; Fig. 10a,b., which are the closest ocean floor sediments of which analyses are available. This may reflect addition of subducted continental material to a MORB source. We therefore have tried to model the isotopic signature of the lamprophyres by simple mixing between a mantle source and a sediment. The composition of potential mixed end members are given in Table 4. The mantle source is taken to be isotopically similar to Indian Ocean MORB as analysed by Serri et al. Ž1991. for the basement of the Celebes Sea, which is located immediately north of Sulawesi’s north arm. This is consistent with the observation by HickeyVargas et al. Ž1995. that MORB in this area is more like Indian Ocean MORB ŽI-MORB. than Atlantic or Pacific MORB. The Sr–Nd isotopic signature of the lamprophyres can be modelled by adding 2% of Australian-derived sediment to this MORB source, while an approximate fit for the Pb isotopic signature is achieved by adding - 1% of a similar sediment to the MORB source. 8.3. Modelling of the isotopic signature of the gabbros 8.3.1. Mixing between MORBr OIB and sediments The isotopic data for the gabbros cannot be modelled by simple mixing between a mantle source and

subducted continental material that falls within the range of compositions analysed for sediments from this region. This is best illustrated in a 87 Srr86 Sr– 206 Pbr204 Pb diagram ŽFig. 11.. By comparison with MORB or OIB, these gabbros show more enrichment in their 87 Srr86 Sr than in their 206 Pbr204 Pb ratios. Continental material with enriched Sr and Pb isotopic signatures, typical for mature, old sediments, always have relatively high Pb and low Sr contents, producing mixing curve A on the diagram. Considering the higher mobility of Pb in a water-rich fluid compared to Sr ŽKeppler, 1996., it is obvious that contamination of MORB by a fluid derived from similar sediments will show even greater enrichment in the composition of Pb relative to the Sr isotopic ratio. Although carbonate has the desired high SrrPb ratio, mixing between MORB and carbonate Žcurve B in Fig. 11. can never result in a 87 Srr86 Sr ratio much higher than .7093, the present-day ratio of sea water. The 87 Srr86 Sr ratio of the gabbros is higher than 0.711, and can therefore not be explained by this process. 8.3.2. Mixing between Sundaland lithospheric mantle, MORBr OIB and sediments As the rather unusual combination of isotopic ratios Žhigh 87 Srr86 Sr and low 143 Ndr144 Nd combined with relatively low 206 Pbr204 Pb, intermediate


85

Fig. 10. Ža and b. Sr–Nd and Pb–Pb isotopic models for the lamprophyres. Fields for Indian Ocean MORB and OIB from Dupre´ and Allegre ` Ž1983., Hamelin and Allegre ` Ž1985., Hamelin et al. Ž1985., Ito et al. Ž1987., Dosso et al. Ž1988., Mahoney et al. Ž1992., Rehkamper ¨ and Hofmann Ž1997., Schiano et al. Ž1997., Yang et al. Ž1998.. OIB samples from Kerguelen, Comores, Reunion, Crozet, Amsterdam, St. Paul and Mauritius. The ‘sediment’ field represents dredge sediments from the Banda Arc ŽVroon, 1992; Vroon et al., 1995., sedimentary rocks from South Sulawesi ŽVroon et al., 1996., Sula and river sediments from North Australia ŽVroon, unpublished data.. The simple mixing models between an Indian-MORB source and an isotopically enriched sediment Žsimilar to sediments derived from Australia. give a reasonable fit for the isotopic characteristics of the lamprophyres, although the amount of sediment added is different for the Sr–Nd and Pb–Pb isotopic systems.

207

Pbr204 Pb and reasonably high 208 Pbr204 Pb . cannot be explained by simple mixing between asthenospheric mantle and recently subducted material, it is most likely that the sub-continental lithospheric mantle has been Žone of. the sourceŽs. for magmatism. The isotopic signature of the Sundaland lithospheric mantle is constrained by data for Plio-Pleistocene

magmatism in South Sulawesi ŽElburg and Foden, submitted., which postdates the collision between Sulawesi and the microcontinent of Buton, and Kalimantan ŽElburg, unpublished data.. These data show that magmas from these areas, which have been interpreted to be formed by melting of the lithospheric mantle ŽElburg and Foden, submitted., have


86 Table 4 End members for the mixing models

MORB source Terrestrial sediment A Carbonate Terrestrial sediment C–D

Sr Žppm.

Nd Žppm.

Pb Žppm.

87

Srr86 Sr

9 100 2000 100

0.73 39

0.03 15 1 15

0.7028 0.7394 0.7093 0.73

143

Ndr144 Nd

0.5130 0.5119

206

Pbr204 Pb

18.0 19.57 19.0 19.0

207

Pbr204 Pb

15.45 15.78

Data for elemental concentrations for MORB source from Sun and McDonough Ž1989., assuming that MORB was formed by 10% melting. Terrestrial sediment end member A, used for curve A in Fig. 11, similar to the most enriched sediment from Vroon Ž1992.. Carbonate composition from the same source, but with enhanced contrast between Sr and Pb concentrations. Terrestrial sediment C–D used for curves C and D in Fig. 11.

relatively low Sr and Pb isotopic ratios Ž87 Srr86 Sr s 0.7042 – 0.7051; 206 Pbr 204 Pb s 18.35 – 18.51; 207 Pbr204 Pb s 15.57–15.58; 208 Pbr204 Pb s 38.35– 38.49. and high 143 Ndr144 Nd ratios Ž0.51274– 0.5129.. As these isotopic values are intermediate between those of MORB mantle and sediment, addition of this lithospheric mantle as a potential source to MORB and subducted continental material cannot explain the isotopic compositions of the Central Sulawesi gabbros either. 8.3.3. Melting of old lithospheric mantle The isotopic signature of the gabbros points towards long-term enrichment in Rb over Sr

compared to depleted mantle or bulk earth, and enrichment in Nd over Sm. The 208 Pbr204 Pb signature indicates an enrichment in Th compared to Pb with respect to present-day depleted mantle, whereas the 206 Pbr204 Pb ratio suggests only minor enrichments for U. It is interesting to note that the 207 Pbr204 Pb ratio of the gabbros is high relative to their 206 Pbr204 Pb ratio when compared to samples from other areas in Sulawesi, or from other volcanic arcs. Although some samples of OIB found within the Indian Ocean ŽI-OIB. overlap with the gabbros in Pb isotopic space, the 87 Srr86 Sr ratio of these OIB samples is significantly lower than those of the gabbros. An exact match for the isotopic composi-

Fig. 11. 87 Srr86 Sr vs. 206 Pbr204 Pb illustration of mixing models. Curve A represents mixing between MORB source and a high 87 Srr86 Sr terrestrial sediment from Australia; curve B represent mixing between MORB source and carbonate. Mixing between MORB source and a fluid Žfrom the subducting slab or sediment. would have the same shape as curve A. Neither curve A or B can explain the isotopic signature of the gabbros Žopen circles.. Although curve C, representing simple mixing between MORB and terrestrial sediment, can explain the Sr–Pb isotopic composition of the more felsic samples, it fails to explain their high d18 O values. An AFC model Žcurve D. is more realistic. See text for details.


tions of the gabbros is not known to the authors, but the relative enrichment of 207 Pbr204 Pb is likely to point towards a source with ancient UrPb enrichment ŽZindler and Hart, 1986., and the high 208 Pbr204 Pb ratio indicates enrichment of Th over Pb. The relatively low 206 Pbr204 Pb ratio of the samples indicate that the UrPb must have been lowered more recently. This kind of complex threestage history is similar to those suggested for lamproites from Western Australia ŽNelson et al., 1986.. The Pb isotopic composition of the Western Australian lamproites is more extreme than that of the Central Sulawesi gabbros, reflecting either the timing of events or differences in the amount of enrichment that has taken place. It is difficult to uniquely model the Pb isotopic composition of the gabbros by a three-stage history due to the uncertainties that surround the Pb isotopic signature of the mantle Že.g., Zindler and Hart, 1986; White, 1993; McCulloch and Bennett, 1994; Chauvel et al., 1995., but an illustrative attempt was made by assuming a single-stage history for the depleted mantle, with m s 8.85, so that the present-day value for the depleted mantle is

87

similar to that seen in I-MORB and the least enriched arc volcanics from Sulawesi. The lead isotopic signature of the gabbros can then be modelled by increasing m to 10.5 at 2.5 Ga, and decreasing it to 7 at 1.2 Ga ŽFig. 12.. This solution is not unique Žsee Nelson et al., 1986 for possible solutions to a similar equation., but it gives an indication of the time scale of events. If the age of the enrichment event is taken to be significantly younger, the m values need to be changed far more drastically. For a 1.5 Ga enrichment event, m must be increased to approximately 20, and then decreased to 0 at 0.9 Ga to achieve the appropriate present-day Pb isotopic composition. An additional constraint is given by the depleted mantle Nd-model age of the samples, which is 1.4–1.8 Ga. If the SmrNd ratio in the source is higher than that of the samples, which is likely, as SmrNd ratios decrease during partial melting, then the age of the source is even older. If any high 143 Ndr144 Nd component, such as sediment, was added to the source at any time, the age given here is a maximum age. It is likely that sediment has been involved in the petrogenesis of the gabbros as well

Fig. 12. Simplified model to explain the Pb isotopic composition of the gabbros. Their relatively high 207 Pbr204 Pb ratio for their 206 Pbr204 Pb ratio suggests that they were formed by a two Žor three. stage process. Assuming that they evolved from a mantle, which through time, were to involve into the source for MORB Žcurve for m s 8.85., they need to have undergone an increase in UrPb ratio Ž m s 10.5. early in time Ž2.5 Ga on this diagram.. A source with this kind of history would have a Pb isotopic composition similar to the lamprophyres from Central Sulawesi. To arrive at the Pb isotopic signature for the gabbros, the UrPb ratio need to be lowered again Ž m s 7., on this diagram at 1.2 Ga. This solution is not unique, but it gives an indication of the time scale on which events happened to the source of the gabbros.

88


as the lamprophyres, as both rock types show a negative Eu-anomaly ŽFig. 13., typical of involvement of sediments in their petrogenesis ŽSun and McDonough, 1989.. There are several ways in which the implied changes in m could be interpreted. It could reflect the addition of a partial melt to the source region Žincreasing the UrPb and ThrPb ratios, as U and Th are more incompatible than Pb during partial melting. followed by the extraction of partial melt from the source, which would lower the UrPb and ThrPb ratios, provided no partial melt remained behind. Alternatively, the increase in m could reflect Pb evolution in the continental crust ŽNelson and McCulloch, 1989., which is compositionally similar to a partial melt, with high UrPb and ThrPb. The decrease in UrPb could in this case be explained by the addition, to subcontinental lithospheric mantle, of crustally evolved Pb as a subduction zone component, which normally has a low UrPb and ThrPb ratios due to extreme enrichment of Pb. It is possible that the trace element signature of the gabbros reflects this old subduction zone component. This sequence of events would approach the m s 0 scenario and would point towards subduction approximately 900 Ma ago. Alternatively, if we are to interpret the trace element signature of the gabbros as a reflection of

the contemporary Miocene subduction-event, we must conclude that the Pb isotopic signature is dominated by that of the lithospheric mantle source, and that therefore these samples’ ‘subduction zone signature’ is not so much a reflection of additions from slab or sediment, but instead results from conditions of mantle melting unique to the subduction environment ŽHawkesworth et al., 1991.. The modelled age of the enrichment and depletion events indicates that the source for the gabbros is likely to be older than 1.5 Ga, unlike any known Sundaland mantle source. Considering that slivers of Australian continent are found within the Indonesian archipelago Ži.e., Bacan; Vroon et al., 1996., the interpretation by Hall Ž1996. that Australian continental crust was subducted underneath Central Sulawesi in the Early Miocene appears to be plausible and is consistent with the uplift history of Central Sulawesi. 8.4. Interpretation of the isotopic signature of the lamprophyres Although the isotopic composition of the lamprophyres does not necessitate the involvement of ancient enriched lithospheric mantle, it could be argued that they represent a mantle component with high time-integrated UrPb, ThrPb, RbrSr and low

Fig. 13. EurEu) vs. SiO 2 for Central Sulawesi samples. Compared to samples from North and South Sulawesi, all samples except the medium-K andesite display pronounced negative Eu anomalies, even before plagioclase fractionation is likely to have had an important effect. This may point towards the involvement of sediment in their petrogenesis, similar to the situation in the Banda Arc ŽVroon, 1992..


SmrNd, in which the UrPb history might be similar to the first two stages of the source for the gabbros. However, this is inconsistent with the present-day UrPb ratio of the lamprophyres, which, even though higher than the gabbros, are like those of MORB. Nevertheless, a more complex origin of the lamprophyres than that of simple sediment–mantle is implied by the isotope data. There is clearly a discrepancy between the amount of subducted continental material Ž‘sediment’. needed to explain the Sr and Nd isotopic data, and that required to explain the Pb isotopic data.

9. Importance of crustal contamination in the felsic samples The felsic samples from Central Sulawesi are characterised by higher 87 Srr86 Sr than the mafic rock types, but similarly low 143 Ndr144 Nd ratios. As Rb and Sr are fractionated from each other more effectively during weathering than in the mantle environment, sediments and sedimentary rocks are generally characterised by greater enrichment in 87 Srr86 Sr compared to their depletion in 143 Ndr144 Nd. The evolved character of the igneous rocks from Central Sulawesi in which these isotopic characteristics are found suggests a role for crustal contamination or intracrustal melting. However, a problem with this model is that the sedimentary rocks ŽCretaceous and Palaeogene. from Sulawesi itself do not have the extremely enriched Sr and reduced Nd isotopic ratios of the more silicic magmatic rocks. Therefore, if crustal contamination is responsible for the highly enriched isotopic signatures of the felsic rock types, the contaminant must be different from the exposed sediments on Sulawesi, and more similar to Precambrian crustal material of Australian derivation, which is the only likely source in the area for old Žand therefore isotopically enriched. sediments. Considering that we have already concluded that the isotopic characteristics of the mafic rocks Žgabbros. suggest that old lithospheric mantle was involved in their petrogenesis, the model that old sedimentary material was involved in generation of the isotopic signature of the more evolved felsic magmas appears to be logical. Preliminary oxygen isotope analyses ŽM. Elburg,

89

unpublished data. show that the felsic samples have high d18 O values relative to V-SMOW Žup to q12‰., clearly indicating a crustal contribution to magmatism. The radiogenic isotope composition of the more felsic samples can be explained by mixing between a MORB source and sediment Žcurve C in Fig. 11, sediment end member given in Table 4., in which case up to 20% of sediment needs to be added to a MORB source. Even if the sediment had d18 O as high as q20‰, this would only result in a magma with d18 O of - 9‰. A more realistic scenario is that the isotopic composition of the felsic magmas results from high-level assimilation concomitant with fractionation Žcurve D in Fig. 11., in which case more contaminant is needed Ž65% for the curve shown, where D mineralrliquid Sr s 1.5, D mineralrliquid Pb s 0.1, rate of assimilationrrate of crystallisations 0.5, and the parent magma is the low-K andesite., which agrees better with the high d18 O values for the felsic samples. Alternatively, the felsic samples could be interpreted to be purely crustal melts, as their isotopic composition falls within the field for sediments.

10. Low-K andesite The samples of low-K andesite are geochemically distinct from most other samples in the area because they display less enrichment in incompatible elements. The samples resemble andesites from N. Sulawesi in their MORB-normalised trace element pattern, except for a smaller depletion in Nb, and less relative enrichment in Sr. However, their isotopic characteristics fall within the same range as the other, incompatible element enriched samples from Central Sulawesi. This leads to depleted mantle extraction ages ŽT DM . which are older than the age of the Earth, due to the lack of significant LREE enrichment. At present, it is not clear how to explain this decoupling between trace element and isotopic characteristics.

11. Medium-K andesite Although the sample of medium-K andesite studied has experienced greenschist metamorphism, its

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trace element and isotopic composition is nevertheless like that of arc volcanics from South Sulawesi ŽElburg and Foden, submitted., suggesting that metamorphism did not significantly affect its geochemical composition. The data presented here suggest that this sample is subduction related and 19 Ma old, whereas previously published ages and trace element characteristics of samples from apparently the same formation Ž‘Lamasi volcanics’. vary from 15 to 33 ŽPriadi et al., 1994. or even 291 Ma ŽPolve´ et al., 1997., and are reported to have back-arc basin affinities. It is clear that more work needs to be done to resolve these discrepancies, but we interpret our data as evidence that normal subduction-related magmatism was occurring until 19 Ma, after which the ‘evolved’ isotopic signature became prevalent in Central Sulawesi magmatism. 12. Lateral extent of the enriched isotopic signature More detailed sampling needs to be done to assess the spatial distribution of the enriched isotopic signature in Sulawesi, but combining the data collected for this study and previously published isotopic ratios can give us some indication. The enriched isotopic signature appears to be present at least as far south as ParePare ŽElburg and Foden, unpublished data., and as far north as Malala, in northwest Sulawesi ŽVan Leeuwen et al., 1994.. This would constrain the minimum size of the subducted segment as being 550 km from north to south. 13. Conclusions Miocene to Pleistocene igneous rocks from Central Sulawesi have highly diverse geochemical and isotopic characteristics. However, all analysed samples younger than 19 Ma have high initial 87 Srr86 Sr values ŽG 0.710.. Although the geochemistry of the lamprophyres in the area could be explained by mixing between MORB source and old subducted continental material, the Pb isotopic composition of the gabbros precludes this interpretation. Their combined Sr, Nd and Pb isotopic signature indicates derivation from old subcontinental lithospheric mantle. The geochemical ‘subduction signature’ in these

samples probably represents an event far older than the contemporary Miocene subduction responsible for magmatism in the area. This implies that there is no reason, based on geochemical composition of the magmatic products, to postulate that subduction was still occurring in the Central Sulawesi area after 19 Ma. Nevertheless, it is unlikely that Miocene–Pliocene magmatism in the area is purely a result of crustal thickening for two reasons: maximum temperatures in the thickened crust are generally reached 20–30 Ma after crustal thickening; and temperatures are unlikely to reach temperatures higher than 7008C in areas with normal heat flow, and no more than 10008C if both conductive heat flow and internal heat production are higher than usual ŽEngland and Thompson, 1986.. Considering that kimberlite-type rocks, such as the lamprophyres in Central Sulawesi, have liquidus temperatures in excess of 12008C ŽEggler, 1989., it is unlikely they can be formed by crustal thickening alone. Therefore, a mantle heat source, potentially in the form of subduction-related magmatism, is still necessary to explain their petrogenesis. Although the more silicic rocks could potentially be the result of intracrustal melting, it is uncertain whether the time-lag between crustal thickening and magmatism is sufficient to achieve the necessary increase in temperature. Data on the uplift history of Central Sulawesi prior to 10 Ma could shed more light on this problem. Acknowledgements Foremost, ME would like to thank Pieter Vroon for making unpublished data available, and for his general good advice on geochemistry and arc magmatism. Logistic assistance in the field was provided by the employees from North Mining in Polewali, notably Malcolm Wilson, to whom we are greatly indebted. Drs. Alan Webb and Keith Turnbull provided invaluable help with KrAr analyses, while David Bruce assisted with isotopic analyses. Jon Dougherty-Page is acknowledged for setting up the procedure for Pb isotope analysis in Adelaide. Alan Greig performed the ICP-MS analyses, and John Stanley did the XRF. The manuscript had benefited from a constructive review by David Peate and further criticism from Nick Arndt, but not much from the remarks of one anonymous reviewer. [NA]


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