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Journal of Petrology | Volume 43 | Number 4 | Pages 581-606 | 2002
© Oxford University Press 2002
Origin of Geochemical Variability by Arc–Continent Collision in the Biru Area, Southern Sulawesi (Indonesia)
1DEPARTMENT OF GEOLOGY AND GEOPHYSICS, ADELAIDE UNIVERSITY, ADELAIDE, S.A. 5005, AUSTRALIA
2RIO TINTO EXPLORATION, JAKARTA, INDONESIA
Received August 31, 2000; Revised typescript accepted September 28, 2001
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ABSTRACT |
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Analyses of igneous rocks from the Eocene calc-alkaline and Miocene potassic volcanic arc in southwest Sulawesi indicate that magmas became more heterogeneous in their trace element and Pb–Sr–Nd isotopic signature following the collision of the Buton microcontinent with the arc at
15 Ma. Isotopic ratios become more ‘continental’ 4 my after the collisional event (87Sr/86Sr 
0·7085, 143Nd/144Nd 
0·5125, 206Pb/204Pb 19·2, 207Pb/204Pb 15·73, 208Pb/204Pb 39·4). As the overriding plate consists of young Sundaland crust, whereas the subducted sediment is likely to have been shed from a compositionally distinct microcontinent of Australian derivation, we can be certain that the continental isotopic signature reflects subduction of continental material rather than crustal contamination. The isotopic compositions of the magmas can be explained by the melting of a mixed mantle wedge, consisting of fluid-fluxed and sediment-modified MORB mantle. In this model, the maximum amount of sediment added to the mantle source is 10%. KEY WORDS: arc volcanism; continental collision; Indonesia; radiogenic isotopes; subduction
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INTRODUCTION |
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Our understanding of the sources contributing to arc magmatism has been greatly increased by detailed trace element and isotope analysis of subduction-related rocks. There is a broad consensus that much of island-arc geochemistry can be modelled by varying contributions from the mantle wedge, fluid from the subducted slab, and the presence or absence of a subducted sedimentary component (White & Patchett, 1984
; Gill & Williams, 1990; Woodhead & Johnson, 1993; Kepezhinskas et al., 1997; Peate et al., 1997; Alves et al., 1999; Hoogewerff, 1999; Pearce et al., 1999; Eiler et al., 2000). Despite agreement on the importance of these components, there is still some dispute about the importance of an ocean island basalt (OIB) component in the wedge (Stolz et al., 1990; Edwards et al., 1991) and the implied role of residual rutile (Ryerson & Watson, 1987; Foley et al., 2000). If an arc is built upon continental crust, or overlain by a thick layer of sediment, there is a potential contribution from the upper plate via assimilation–fractional crystallization (AFC) processes, which can sometimes be difficult to distinguish from the signature of subducted sediment (Davidson, 1987; Thirlwall et al., 1996; Gasparon & Varne, 1998). The only unambiguous way to distinguish between subducted sediment and crustal assimilation is offered by arcs where the isotopic composition of the upper plate is very different from that of the subducted sediment, or where lateral variation is clearly controlled by crustal structures.
Additional problems in identifying source components in magmatism are encountered in collisional situations, where the potential exists for subduction of the continental crust, and addition of material to the region of magma generation from that source (Hilton et al., 1992; Van Bergen et al., 1993).
The extinct arc of western Sulawesi (Brouwer, 1947; Sukamto, 1975; Katili, 1978) offers us the chance of improving our knowledge of source components in collisional magmatism by studying the geochemistry of pre- to syn-collisional deposits and identifying changes in magma geochemistry. This study follows on from the work by Elburg & Foden (1999a) in which the studied collection of samples was divided into pre-, syn- and post-collisional groups, based on K/Ar age and the perceived timing of the collision between the arc and the microcontinent of Buton. The present study concerns a stratigraphically well-defined suite of samples from the Biru area (Van Leeuwen, 1981) (Fig. 1), belonging to the Eocene (50 Ma) calc-alkaline arc and the Miocene (15–6·3 Ma) potassic volcanic arc. The advantage of knowing the stratigraphic order of the deposits is that we are not solely reliant on K/Ar dating, which cannot always distinguish between sample suites with similar ages.
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GEOLOGICAL SETTING |
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As the tectonic history of the area has already been discussed by Elburg & Foden (1999a)
, we will give only a brief overview, and identify where our interpretation has changed since the previous publication.
The oldest exposed rocks are represented by two inliers of tectonically stacked metamorphic, ultrabasic and sedimentary lithologies of Jurassic to mid-Cretaceous age (Sukamto, 1982, 1986; Wakita et al., 1996). They are overlain with an angular unconformity by deep marine flysch deposits of the Upper Cretaceous Balangbaru Formation (Hasan, 1991). A contemporaneous distal flysch sequence (Marada Formation; Van Leeuwen, 1981) occurs further to the east. These Mesozoic formations are interpreted to have been formed in a fore-arc basin on a trench slope (Van Leeuwen, 1981; Hasan, 1991), related to a west-dipping subduction zone (Hamilton, 1979; Parkinson, 1991) that generated magmatism in south–central Kalimantan (Parkinson et al., 1998).
This pattern of subduction ended when a continental fragment was thrust underneath Sulawesi at 115–120 Ma. Remnants of this underthrust plate are preserved as ultra-high-pressure metamorphic complexes in SW, central and SE Sulawesi (Parkinson et al., 1998). Subduction started to affect SW Sulawesi again in the Paleocene. At Bantimala this produced a series of basaltic to rhyolitic rocks and associated intrusions, named the Alla or Bua Formation (Sukamto, 1986; Yuwono, 1987). These grade upwards into an Early to Middle Eocene sequence of marginal marine, coal-bearing siliciclastic rocks of the Malawa Formation (Sukamto, 1982).
In the Biru area, magmatic activity started during the Middle Eocene, or possibly earlier, and continued into the Late Eocene, producing a thick pile of andesitic volcanics named the Langi Volcanics (Van Leeuwen, 1981). They formed as a response to west-dipping subduction (Van Leeuwen, 1981; Yuwono et al., 1985). The upper parts of the Malawa Formation and Langi Volcanics interdigitate with the Tonasa Limestone Formation, a sequence of up to 1100 m thickness of platform carbonates, marls and redeposited carbonates of Middle Eocene to early Middle Miocene age (Van Leeuwen, 1981; Sukamto, 1982; Wilson, 2000; Wilson & Bosence, 1996). Towards the end of the Early Miocene, magmatic activity of calc-alkaline affinity resumed (Yuwono, 1987; Yuwono et al. 1988). However, it was soon succeeded by potassic volcanism in the early Middle Miocene, the onset of which coincided with an Early to Middle Miocene tectonic event involving a phase of faulting that resulted in reactivation of earlier faults, and localized tilting and/or subaerial exposure of fault blocks (Van Leeuwen, 1981; Wilson & Bosence, 1996; Wilson, 2000). At Biru, this tectonic event can be placed at 14–15 Ma on the basis of palaeontological evidence (Van Leeuwen, 1981) and is likely to have been caused by the collision between western Sulawesi and the northern margin of Australia (with subsequent left-lateral translation of the entire collision complex; Daly et al., 1991; Charlton, 2000), or independently moving terranes that had been detached from Australia during Mesozoic rifting (e.g. Pigram & Panggabean, 1984; Smith & Silver, 1991). The collision, which consisted of a series of highly complex, as yet poorly understood events, was initiated during the Late Oligocene–Early Miocene and continues to the present day (Hall & Wilson, 2000). The collision event probably involved Buton, located in the southern part of the collision complex (Fig. 1), either as a small separate microcontinent (Smith & Silver, 1991; Milsom et al., 1999) or forming part of a larger fragment (Milsom et al., 2000). Buton docked with Western Sulawesi 15–13 my ago, as evidenced by a regional unconformity on the island (Davidson, 1991). This is slightly older than the age of 13–11 Ma we previously assigned to the ‘Buton collision’ (Elburg & Foden, 1999a). A sketch map of the tectonic situation just before the collision as envisaged by Hall (1996) is shown in Fig. 2. Potassic magmatism continued well after this collisional event, with the youngest deposits in the Biru area of 6·3 Ma. Further south, the Lompobatang Volcano was active as recently as 1·8 Ma (Polvé et al., 1997; Elburg & Foden, 1999a), apparently unrelated to subduction. The tectonic history of the area is further complicated by a second collision, between Buton and the submerged Tukang Besi platform, in the Pliocene around 2–3 Ma (Fortuin et al., 1990; Ali et al., 1996). It is unclear whether subduction occurred underneath Buton between 15 and 3 Ma, or whether westward movement of the Tukang Besi platform was accommodated by strike-slip faulting. As no record exists of subduction-related volcanism in this area around this time, the latter alternative seems more likely.
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The tectonic history of the area therefore indicates that South Sulawesi is part of Sundaland and consists of continental crust that was cratonized in the Mesozoic (Hamilton, 1979), whereas the colliding microcontinent of Buton is derived from the much older Australia–New Guinea continent (Daly et al., 1987). Therefore, the isotopic signature of the upper plate and the collider are likely to be very different, and this situation gives us the opportunity to clearly distinguish between upper-crustal contamination and subduction of the microcontinent or sedimentary material derived from it into the source of magmatism during continental collision.
Geology of the Biru area
The geology of the Biru area has been described by Van Leeuwen (1981) and is shown in Fig. 1. The oldest rocks exposed in the area are a flysch-like sequence of greywackes, arkosic sandstones and shales of Late Cretaceous age (Marada Sandstone Formation). These are overlain (presumably above an angular conformity) by the Langi Volcanics, a series of propylitized andesitic lavas and volcaniclastics that were formed in Paleocene(?) to Eocene times and pre-date the collisional event (see below). The Tonasa Limestone Formation, which ranges in age between Late Eocene and earliest Middle Miocene, conformably overlies the volcanic series. The top part of this formation contains significant amounts of volcanic material, including lava intercalations, which are referred to in this paper as the Pake Volcanics. These are the only syn-collisional samples analysed from this area. The three formations were folded and block faulted during the early Middle Miocene and subjected to erosion before being covered by a thick sequence of sedimentary and potassic volcanic rocks during the Middle to Late Miocene. Block faulting continued during this period, resulting in local tilting of the strata. The oldest part of the cover sequence is a sedimentary unit belonging to the lower member of the Camba Formation, which crops out in the northern part of the Biru unit. This unit contains a middle to late Middle Miocene nannoplankton fauna (Hasibuan, 1996). Higher in the sequence conglomerates are found containing material derived from the late Middle Miocene Sopo Volcanics (see below) and older formations.
The potassic volcanic rocks can be divided into six units (Table 1), each having distinct lithologic, petrological (Table 2) and geochemical (Table 4, see below) characteristics. These are the Sopo Volcanics (SV; alternating basalt to basaltic andesite lavas and volcaniclastics, and associated dykes), Marara Ignimbrite (MI; pyroclastic flow deposits of dacitic composition), Bila Volcanics (BV; polymict volcaniclastics with intercalations of leucite-bearing tephrite lava flows in the middle part of the sequence), Kahu Volcanics (KV; volcaniclastics and interbedded lavas of basaltic to andesitic composition), Ulubila Volcanics (UV; andesitic to dacitic volcaniclastics and lavas), and Lemo Volcanics (LV; breccias, subordinate lavas and dykes of andesitic composition). K/Ar dating results indicate that the six units fall into two main age groups, namely, 11·2–10·3 Ma (SV, MI and BV) and 7·6–6·2 Ma (KV, UV and LV). The intervening period was characterized by differential block-faulting and erosion. We will refer to the older group as ‘early post-collisional’ and the younger group as ‘late post-collisional’.
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The Marara Ignimbrite and the lower and middle parts of the Bila Volcanics are the equivalent of, respectively, the E, F and G members of the Pammesurang Volcanics of Van Leeuwen (1981)
. The Kahu Volcanics and Ulubila Volcanics correspond to his Walanae Volcanics, excluding the occurrences found in the Walanae Depression, which we have grouped with the Walanae Formation.
Intrusive rocks are extensively exposed in the catchment areas of the Biru and Bulubuluk rivers. These were called Biru Granodiorite by Van Leeuwen (1981), but renamed the Biru Intrusive Complex (BIC) in this paper as our investigations have shown that the intrusive body consists of at least two suites: a Middle Eocene hornblende–biotite monzonite–tonalite suite, which forms the bulk of the intrusive complex and is genetically related to the Langi Volcanics, and an Upper Miocene clinopyroxene–biotite syenite–monzonite suite, named here the Biru Syenite. A K/Ar cooling age of 8·4 Ma was obtained, but the actual emplacement age could be older, which could explain the isotopic similarities between the Biru Syenite and the Sopo Volcanics. Nishimura (cited by Van Leeuwen, 1981) obtained a zircon fission-track age of 19 ± 3·4 Ma for a granodiorite sample, suggesting that the intrusive complex may contain a third suite of Early–Middle Miocene age.
The structure of the Biru area (Van Leeuwen, 1981) is dominated by a series of steeply dipping to vertical north- to NW-trending faults, including the West Walanae Fault, which show both vertical and horizontal displacements. A second, less prominent fault system consists of a series of steeply dipping to vertical normal faults with northeasterly to easterly trends. Both fault systems were probably already active in early Middle Miocene times and continued to be active intermittently up to the Pliocene.
The field and petrographic characteristics of the units studied have been tabulated in Tables 1 and 2. K–Ar age data are given in Table 3.
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ANALYTICAL TECHNIQUES |
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A selection of samples from the Eocene to Upper Miocene units were analysed by Actlabs (Canada) for major oxides and a wide range of trace elements by fusion–inductively coupled plasma and fusion–inductively coupled plasma-mass spectrometry methods, respectively, using a lithium metaborate fusion rather than acid digestion to place the sample into solution. This fusion technique ensures that all major elements can be determined. Accuracy for the major element oxides is better than 0·01%. Analyses of international standards show that accuracy for the trace elements is better than 20% for Tb and Nb at levels lower than 8 ppm, better than 15% for Cr, Ce, Pr, Hf, Ta and Th, better than 10% for V, Cu, Cs, Ba, Er, Tm and Lu, and better than 5% for Ni, Rb, Sr, Y, Zr, Nd, Sm, Eu, Gd, Dy, Ho, Yb, Pb, U and Nb at levels higher than 100 ppm. Trace element analyses of standards are included in Table 4.
Sr, Nd and Pb isotope ratios were analysed at the Department of Geology and Geophysics by thermal ionization mass spectrometry (TIMS) on a Finnigan MAT 262 system in static mode. All ground samples were leached in 3N HCl for 30 min at 100°C. The supernatant was pipetted off, the sample washed in deionized water and the water pipetted off. The residue was then analysed for its isotopic composition. In selected cases, the leachate was analysed too (Table 5).
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The long-term average for the in-house Nd standard (J&M specpure Nd2O3) is 0·511603 ± 9 (1
of total population, n = 105). The La Jolla standard gave 0·511828 ± 11 (n = 9) and BCR-1 gave 0·512593 ± 16 (n = 12). Typical blanks are of the order of 100–200 pg for Nd. The average for the NBS987 Sr standard is 0·710258 ± 18 (n = 56). Typical Sr blanks are better than 2 ng. Although this is a high blank, it is negligible compared with a typical sample size of 10–100 µg of Sr. All Pb isotopic analyses were performed at approximately the same temperature of 1150°C, and a mass fractionation factor of 0·08% per a.m.u. was used, based on replicate analyses of the NBS981 Pb standard. Typical Pb blanks are of the order of 300 pg.
K–Ar dating of samples B360, B370, O1 and T was performed at AMDEL analytical laboratories, on a modified MS-10 mass spectrometer, following techniques described by Webb et al. (1986). Duplicate K analyses agree within 0·5% and duplicate Ar analyses within 1%. The errors given are for the analytical uncertainty at 1 SD. The other samples were dated at the Institute of Geological and Nuclear Sciences Ltd, Rafter Laboratories, on a modified MS-10 mass spectrometer, following in-house methods developed by C. Adams. Errors of age measurement are 2 SD and combine individual errors of potassium (±0·5%) 38Ar spike calibration (±0·3%), Ar isotope measurement (<0·1%) and instrument mass discrimination variation (for 40Ar/36Ar <0·3%).
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MAJOR AND TRACE ELEMENT GEOCHEMISTRY |
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Whole-rock analyses of the Biru samples cover the petrographic spectrum from basaltic to dacitic rocks (SiO2 contents between 45 and 67 wt %; Fig. 3, Table 4). The Miocene samples are characterized by high total alkalis (Na2O + K2O > 5 wt %), high K2O/Na2O ratios (>0·6 at 50 wt % SiO2, >1·0 at 55 wt % SiO2), high but variable Al2O3 (14·3–20·8 wt %), low TiO2 (<1·5 wt %), and low mg-numbers (<54). Most of the Biru samples plot in the shoshonitic field of the K2O–SiO2 classification diagram of orogenic lavas of Peccerillo & Taylor (1976
; Fig. 3). Only the Eocene Langi samples fall within the field for low- to medium-K rocks.
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The Miocene suites have the following trace element characteristics in common. Compatible elements occur in relatively low concentrations (Cr <220 ppm, Ni <70 ppm, Co <40 ppm). Normal mid-ocean ridge basalt (N-MORB)-normalized trace element diagrams (Fig. 4) show well-defined negative anomalies for Ta, Nb and Ti, and patterns that are both steep and enriched in large ion lithophile elements (LILE). The Nb–Ta anomaly is least pronounced in samples from the Ulubila Volcanics, and in this respect they resemble the Pliocene Lompobatang Volcanics (Elburg & Foden, 1999a). Positive anomalies are observed for K, Pb and Sr (except for the Marara Ignimbrite and Ulubila Volcanics).
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Chondrite-normalized rare earth element (REE) patterns (Fig. 5) display moderate to high enrichment of light REE (LREE) over heavy REE (HREE). The Langi Volcanics show the least LREE enrichment, and the degree of enrichment increases from BV basalt and SV samples, through KV and LV samples to BV leucite tephrite, MI and UV samples. None of the suites show significant HREE fractionation. The MI, LV, UV and some of the KV samples display negative Eu anomalies, whereas the other KV samples and samples from the SV and BV units show smooth patterns. The one sample of the Pake Volcanics shows a positive Eu anomaly. As this sample is altered, with epidote and albite as secondary minerals (Table 2), we think that this positive anomaly is more likely to be related to alteration than to reflect a primary feature of the sample.
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The Langi Volcanics and Intrusives display the geochemical characteristics most typical of arc volcanics, with low Nb and Ti contents, and a calc-alkaline trend in an SiO2 vs FeO*/MgO diagram. In many variation diagrams, the Langi samples overlap with those from the oceanic Sangihe arc of northern Sulawesi.
Our dataset includes a single, altered, basaltic rock from the Pake Volcanics and it is unclear how representative of the suite is this sample. Immobile elements such as Ti, Nb, Zr and Y overlap the fields of those of the slightly younger Sopo Volcanics.
The Sopo Volcanics are geochemically fairly similar to the ‘pre-collisional’ samples analysed by Elburg & Foden (1999a), with which they are now thought to be contemporaneous. Sr contents are very high (often >2000 ppm), and Ba contents are also elevated. With respect to Nb, Ti, Zr and Y, the Sopo Volcanics do not differ much from the Langi Volcanics and Intrusives.
The Marara Ignimbrite stands out in many Harker variation diagrams, by having rather variable contents of Na2O, K2O, Ba, Rb and Sr, and low CaO contents. These elements are mobile during alteration, and there appears to be a broad correlation between loss on ignition and values for these elements. We therefore think that these variations are more related to alteration than to the primary igneous composition of these samples.
The Bila Tephrite is unusual with respect to its very low SiO2 content, combined with relatively low contents of MgO and compatible trace elements such as Ni and Cr. Its TiO2, V, P2O5, Cu and Ba content, on the other hand, are much higher than for the other suites, and Th, Sr and Zn are also high for the silica content of the samples.
The Biru Syenite and Kahu Volcanics do not show any particularly unusual geochemical features. Like all post-Eocene suites from this area, they are fairly rich in K2O, Rb, Ba and Th. Levels of Nb and Y are only slightly higher than in the Eocene Langi Volcanics and Intrusives.
The Ulubila Volcanics contain high but variable contents of Nb and Zr, elements that are typically low in abundance in arc volcanics. Some of the enrichment of Nb and Zr may result from the relatively fractionated character of the samples, but they are also high in these elements when compared with samples of equivalent SiO2 content from other units. Rb and especially Th are also high, whereas Ba and Sr occur at levels comparable with those of the other Miocene suites. Although the overall pattern of enrichments is similar to that seen in the Pliocene Lompobatang volcano further south, the Ulubila volcanics show more extreme enrichments in Nb and Zr. The two suites also differ in their Rb and K2O contents (higher in the Ulubila) and Sr and P2O5 contents (higher in the Lompobatang)
The youngest suite in the Biru area, the Lemo Volcanics, does not show any of the unusual Nb, Zr or Th enrichments seen in the slightly older Ulubila Volcanics. With respect to most elements, the Lemo Volcanics fall within the same trend as the Biru Syenite or Kahu Volcanics.
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ISOTOPE GEOCHEMISTRY |
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In terms of Sr and Nd isotopic signature (Table 5, Fig. 6), the Biru samples fall roughly in the same range as the samples from South Sulawesi analysed by Elburg & Foden (1999a)
. The one sample of the syn-collisional Pake Volcanics has a relatively high 87Sr/86Sr ratio for its Nd isotopic ratio when compared with other samples from South Sulawesi. Although this sample was leached before analysis, alteration was severe, and it is possible that the elevated Sr isotopic ratio reflects alteration rather than a primary igneous value. The Sopo Volcanics are the most primitive in terms of Nd isotopic composition, although the Langi samples extend to somewhat lower 87Sr/86Sr ratios. The early post-collisional Sopo Volcanics plot very close to the field for the ‘pre-collisional’ samples of Elburg & Foden (1999a), of which the age is poorly known. The Biru Syenite is also fairly primitive in its Sr and Nd isotope ratios, but the other post-collisional samples from Biru fall within the same range as the 7–12 Ma [termed ‘syn-collisional’ by Elburg & Foden (1999a)] samples from elsewhere in South Sulawesi. The samples with the most ‘continental’ Nd and Sr isotopic signature overlap with sediments from South Sulawesi (Vroon et al., 1996; Elburg & Foden, 1999a).
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In terms of Pb isotopes (Fig. 7), the Biru samples extend the trend defined by the ‘syn-collisional’ samples from further south, with differences most pronounced in 207Pb/204Pb ratios. The Eocene Langi Volcanics and Intrusives stand apart from the other samples because of their elevated 207Pb/204Pb ratios relative to their 206Pb/204Pb ratios. In this respect they show more similarity to the igneous rocks from North Sulawesi (Elburg & Foden, 1998). It is interesting to note that the Biru samples extend to more radiogenic Pb isotope signatures than sedimentary rocks from South and Central Sulawesi, especially with respect to 206Pb/204Pb and 208Pb/204Pb. This means that exposed sedimentary rocks in Sulawesi are not viable end members to explain the ‘continental’ isotopic signatures of the Biru samples. Another argument against crustal contamination (in an AFC-type process) is the lack of correlation between any index of fractionation and isotopic signature. Within several suites, the samples with the most radiogenic Sr isotopic signature are those with the highest content of MgO (Tables 4 and 5), which is the opposite from what would be expected for assimilation combined with fractional crystallization.
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ISOTOPIC VARIATION THROUGH TIME |
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The Biru samples define a broad envelope of isotopic change through time, with 143Nd/144Nd ratios decreasing to their lowest values at 6 Ma. The decidedly post-collisional samples of Lompobatang have higher values again. Sr isotopes mirror the trend of Nd, rising to their highest values at 6 Ma and then falling (Fig. 8). These broad temporal changes are also reflected in some aspects of geochemical change. If Nd isotopic ratios are plotted against the age of the samples it is obvious that the late post-collisional samples from Biru have more variable, and, on average, lower 143Nd/144Nd ratios than the pre-, syn- and early post-collisional samples. However, although there is a broad trend of isotopic change with time, it is not one of simple unidirectional change towards more ‘continental’ isotopic ratios. For example, the Biru syenite, which falls between the Bila Tephrite and the Kahu Volcanics with respect to its K/Ar age, has higher 143Nd/144Nd ratios than either of these deposits. This discrepancy could also be explained by the emplacement age of the syenite being older than its K/Ar cooling age, although it might be excessive to postulate a gap of 3 my between intrusion and cooling through the biotite closing temperature. If data from the Lompobatang volcano are incorporated into this diagram we can see that 143Nd/144Nd values return to appreciably higher values >13 my after the collisional event. The trends seen in Sr and Pb isotopes are very similar, with ‘continental’ values becoming more important just after the collisional event, whereas the much later Lompobatang samples show a return to values close to those for syn- and pre-collisional suites.
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The isotopic trend through time in the Biru area contrasts markedly with the trend seen in North Sulawesi (Fig. 8), indicating that the collision with the Buton microcontinent and other continental fragments did not have an effect on the northerly area. The isotopic composition of samples from North Sulawesi remains virtually unchanged from 14 Ma until recent times, when the isotopic signature becomes more variable, probably because of cessation of active subduction of the Molucca Sea plate (Elburg & Foden, 1998). This is marked by a decrease in the Nd isotopic signature, but the northern Sulawesi samples never approach the very ‘continental’ values seen in the post-collisional rocks from the Biru area.
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GEOCHEMICAL VARIATION THROUGH TIME |
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 TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESMAJOR AND TRACE ELEMENT...ISOTOPE GEOCHEMISTRYISOTOPIC VARIATION THROUGH TIME GEOCHEMICAL VARIATION THROUGH... DISCUSSIONCONCLUSIONSREFERENCES |
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Comparison of the geochemical characteristics of the various units is hampered by their difference in fractionation stage, and we have therefore attempted to determine crucial elemental concentrations and ratios at 55% SiO2 (Fig. 9). This is in some cases only a rough estimate, as some units do not extend to these silica contents (i.e. Bila Tephrite), and it is not always possible to unambiguously interpret the trend of each element or elemental ratio with SiO2 content. However, if we attempt this exercise, we see that there is, again, no unambiguous progression in time, although all Miocene volcanics have higher K2O contents than the Eocene Langi Volcanics and Intrusives at 55% SiO2. On average, Nb/Y and Ce/Yb ratios are higher too. La/Nb ratios can be both higher (Bila Tephrite, Kahu Volcanics; even when taking the error on the Nb analyses into account) and lower (Ulubila Volcanics) than in the Langi samples.
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) and are therefore younger.
Changes in isotopic and chemical compositions are often observed in across-arc transects, such as described for the Sunda Arc by Hoogewerff et al. (1997). It could therefore be argued that the position of the Biru area changed with respect to the subducting slab through time, and that this caused the geochemical and isotopic changes observed. We do not think that this is a satisfactory explanation, as the across-arc changes described by previous workers (Leeman et al., 1990; Woodhead & Johnson, 1993; Hoogewerff et al., 1997) do not induce more pronounced isotopic and chemical variability, but rather a unidirectional change. This scenario would also necessitate very sudden changes in slab geometry in a very short time, as units that have virtually indistinguishable K/Ar ages (Ulubila Volcanics and Lemo Volcanics) have vastly different isotopic and geochemical signatures. We therefore contend that the observed geochemical changes reflect a time progression, and thereby a response to arc–continent collision.
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DISCUSSION |
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Nature of the mixing end members
The discrepancy in isotopic compositions between sediments from South Sulawesi and the Biru samples is also clearly visible in a diagram of 143Nd/144Nd vs 206Pb/204Pb (Fig. 10). It is often envisaged that some mixing processes take place in the mantle wedge between a mantle component similar to that of a MORB source, and a fluid, melt-like or solid sedimentary end member. If this mixing process were the cause of the ‘continental’ isotopic signature in some of the Biru samples, the Nd–Pb isotope mixing arrays would be strongly curved (Fig. 11), with first an increase in Pb isotope values at high and near-constant Nd isotope ratios, followed by a decrease in 143Nd/144Nd at high and nearly constant Pb isotope ratios (e.g. Vroon et al., 1993
). This results from the very high Pb/Nd ratios of fluids or sediments compared with a MORB source (Miller et al., 1994). To explain the array of, for instance, the Lemo Volcanics by this type of process, we need two crustal end members with distinct Pb isotopic characteristics. Another option is that the array seen in the Lemo Volcanics represents a single mixing line. In that case, a MORB source is unlikely to be the isotopically primitive end member for this mixing array, as its elemental Pb/Nd ratios are too low to give rise to this kind of array when sedimentary material (with very high Pb/Nd ratios compared with a MORB source) is mixed with it. In that case we would need an end member with isotopic characteristics similar to a MORB source, but with Pb/Nd ratios closer to those of sediments or the melts or fluids derived therefrom. Arc volcanics themselves have suitable Pb/Nd ratios to be one end member for this kind of process. The mixing process could therefore be envisaged as taking place between an arc-type magma or fluid-fluxed mantle component (which are themselves a two-component mixture of MORB source and fluid), and a sedimentary end member. However, this sedimentary end member cannot be represented by the exposed sedimentary rocks in South Sulawesi, as they do not have the appropriate Pb isotopic signature.
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To explain the mixing array seen in the Pb isotope diagrams we need a component that has higher Pb isotope ratios than exposed sedimentary rocks from Sulawesi. Although we could postulate that this component is present in the Sulawesi crust at a deeper level, this solution invokes something like a deus ex machina. Material that has the appropriate Pb isotope composition is found among sediments of the Banda Arc (Vroon et al., 1995), which have an Australian cratonic origin, instead of the Sundaland origin of the exposed Sulawesi sedimentary rocks. Although we have no data on the sediments that were present in the trench of the West Sulawesi subduction zone, there are reasons to think that these contained a significant component of Australian origin. The main reason is that these sediments are likely to have been shed from the microcontinents of Australian origin that collided with the volcanic arc at a later stage. Tectonic reconstructions, based on stratigraphic similarities between Buton and the islands on the Australian continental shelf, such as Timor, generally classify Buton as being part of the Australian plate (e.g. Hall, 1996). It is therefore probable that the sediments that were subducted shortly before, and during collision with the Buton microcontinent had an Australian origin, and could have had the appropriate Pb isotopic signature to explain the Pb–Pb mixing array.
The Eocene Langi samples have relatively high 207Pb/204Pb ratios for their 206Pb/204Pb ratios compared with the other samples from South Sulawesi, or from other Indonesian areas such as the Banda Arc (Vroon et al., 1993). They overlap in Pb isotopic composition with 12–14 Ma igneous samples from the oceanic Sangihe arc in North Sulawesi (Elburg & Foden, 1998). The shape of the array for the other Biru volcanics and for the Banda Arc samples is characterized by a great variation in 206Pb/204Pb ratios for a limited variation in 207Pb/204Pb, thereby creating a relatively flat trend (Fig. 12). This kind of trend reflects involvement of a sedimentary end member of Australian origin. The opposite trend is observed in igneous suites from Halmahera and the Philippines, where there is significantly more variation in 207Pb/204Pb ratios than in 206Pb/204Pb, thereby creating a steep to almost vertical field. This is generally taken to reflect involvement of a sedimentary end member with high 207Pb/204Pb ratios for their 206Pb/204Pb ratios, being sediment from the Pacific or South China Sea (Fig. 12) rather than from the Australian continent. We propose that the distinction seen between the Eocene and Miocene–Pliocene Biru samples has a similar origin, with Pacific-type sediment being subducted early in the history of the volcanic arc, and Australian-type sediment becoming dominant as the Buton microcontinent approaches the arc.
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Involvement of an OIB component?
The high Nb contents, high Nb/Zr and relatively low La/Nb ratios of the Ulubila Volcanics compared with the other samples could be explained by the involvement of an OIB-type component in magma petrogenesis. This end member has been invoked in several studies of Indonesian volcanics (Wheller et al., 1987; Vroon et al., 1993), most notably to explain the geochemical signature of the high-K samples of the volcano Muriah in Java (Edwards et al., 1991). The high-K samples of Muriah are characterized by relatively high 208Pb/204Pb ratios at a given 207Pb/204Pb ratio, an observation that can also be made for Indian Ocean OIB (Fig. 13). This therefore makes a very strong case for the involvement of an OIB-type component in magma genesis. The situation in the Biru sector is different, as all samples, irrespective of their Nb/Zr ratio, fall on a straight mixing line between MORB-type mantle and a sedimentary component, which could be represented by Australian continental material, such as has been dredged in the Banda Arc (Vroon et al., 1995) or collected in North Australia (Hoogewerff, 1999). If the elevated Nb contents of the Ulubila Volcanics were the result of involvement of an OIB-type component, we would expect to observe a shift towards the field for OIB and the Muriah high-K volcanics. As this is not the case, we do not think that an OIB-type mantle has been involved in the petrogenesis of any of the igneous rocks from the Biru area.
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The Lompobatang volcanics, which also display relatively high Nb/Zr ratios (Fig. 14), do not show any shift in Pb isotopic compositions towards the field for OIB either. For these volcanics Elburg & Foden (1999a) argued that they represented melting of the subcontinental lithospheric mantle. An alternative option, that high Nb/Zr contents could represent addition of a partial melt of sediment to the mantle wedge, is not considered likely as the Lompobatang volcanics show isotopically less influence of a sedimentary component than the syn-collisional samples from the same area, which have lower Nb/Zr ratios (Fig. 14). The Ulubila Volcanics and the Lompobatang volcanics display some important differences: trace element and isotopic ratios are distinctly more variable within the Ulubila Volcanics, and their isotopic signature trends towards more ‘continental’ values. Nb/Zr ratios in the Ulubila are higher (0·09–0·14) than in the Lompobatang volcanics, but also higher than in global sediments (average of 0·09, Taylor & McLennan, 1985), or those from the Banda trough (Fig. 14: 0·07–0·1, Vroon et al., 1995) or the North Australian continent (0·02–0·05, P. Z. Vroon & M. A. Elburg, unpublished data, 1994). The isotopic data could be interpreted to mean that the Ulubila Volcanics contain a variable amount of a sediment in their source. The fact that Nb/Zr ratios are higher than those for average sediment would imply that the sediment was added to the mantle wedge as a partial melt, rather than as a wholesale addition. High Nb/Zr ratios in partial melts can be achieved only if rutile is not a residual phase, as this mineral has a significantly higher solid–melt distribution coefficient for Nb than for Zr (Jenner et al., 1994). Partial melting experiments on pelagic clay show that rutile is not a residual phase (Nichols et al., 1996). It is possible that rutile would be stable in more silicic environments (Ryerson & Watson, 1987), thereby lowering the Nb/Zr ratio of the partial melt, but there is also evidence that zircon would be stable in these circumstances (Johnson & Plank, 1999), and this would result in an increase in the Nb/Zr ratio of the melt. It is therefore possible that addition of partial melt of sediments to the source of the volcanics is responsible for the high Nb/Zr and ‘continental’ isotopic values of the Ulubila Volcanics.
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Quantification of the mixing process
If our ideas about the mixing process and end members (simple mixing between fluid-modified mantle wedge and Australian sediment, based on within-suite trends for Pb and Nd isotopes; see above) are correct, we should be able to quantify the amount of sediment that has been mixed into the mantle wedge, and constrain its isotopic composition. It is indeed possible to construct simple mixing curves that follow the isotopic trends seen in the Biru volcanics (Fig. 15a and b; end members given in Table 6a). For the fluid-modified mantle wedge, we took the isotopic composition of the Sopo Volcanics, of which the trace element data show they were mainly influenced by a fluid component. The trace element composition was taken as being 10 times less than that of the Sopo Volcanics, roughly equivalent to assuming that the volcanics were formed by 10% of melting. A satisfactory mixing curve was achieved only if the sediment was taken to be of North Australian derivation (P. Z. Vroon & M. A. Elburg, unpublished data, 1994). The mixing models constrain the maximum amount of sediment added to the source as being 10%. This is fairly high, but this follows from the assumption that the mantle wedge was already modified by addition of a fluid before sediment was added, thereby significantly increasing the Pb and Sr concentrations in the mantle.
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At present, we do not have any constraints on the composition of the sediment that was subducted. Tectonic reconstructions suggest that the Buton microcontinent came from the New Guinea area of the Australian plate (Hall, 1996
), and is younger, and therefore isotopically less evolved, than the sedimentary end member with which the modelling was achieved. We are therefore not convinced that the proposed mixing model is correct. To circumvent the necessity of postulating this isotopically very evolved sedimentary end member, we made a mixing model between fluid-modified and sediment-modified (perhaps best envisaged as resulting from fault-controlled interleaving) mantle before melting (Fig. 16), with the sedimentary end member being similar to that of sediments from the Banda Trough. The end members of this mixing process are given in Table 6b. Although this is a process that can explain the isotopic variation in the Biru samples, the physical aspects of this process are poorly constrained. We suggest that melting of sediment, and thereby transfer into the sub-arc mantle, may be facilitated by collision-induced stalling of the subducted slab, thereby raising the temperature of slab and subducted sediments at a given depth. The isotopic variation we observe in the Biru samples may result from melting of a multiply contaminated mantle, or by mixing of melts from parts of the mantle that have had distinct contamination histories.
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CONCLUSIONS |
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Analyses of pre- to post-collisional samples from the Biru area show that the compositions of post-collisional samples show greater isotopic and geochemical variability. The timing of the onset of the geochemical variability is hard to determine as we do not have complete time coverage. If the collisional event is indeed taken to have occurred around 13–15 Ma, coincident with faulting in the Biru area, the apparent geochemical impact postdates this event by several million years, as the Marara Ignimbrite, at 10·5 Ma, is the first unit to show more continental isotopic ratios. A time lag between the subduction of a geochemical tracer into the arc and the expression thereof in arc volcanics has also been observed by Turner et al. (1997)
, who found that 2–4 my elapsed before the geochemical signature was seen in erupted volcanics.
The dataset presented here conforms to the idea by Turner et al. (1997) that samples that show the greatest influence of a fluid component in their trace element ratios (e.g. high Ba/Th ratios) have isotopic compositions that deviate least from those expected for the mantle wedge. This is taken to be evidence that the fluid has been derived by dewatering of the subducted slab, rather than any entrained sedimentary component. Samples with lower Ba/Th ratios show more continental isotopic values, indicative of a sedimentary component in magma genesis. The high Pb isotopic values of some Biru units preclude that this sedimentary component is of Sundaland origin. Although it is possible that Australian crust is present in the upper plate owing to underthrusting in the Cretaceous (Parkinson et al., 1998), our preferred interpretation is that this sediment was subducted and added to the mantle wedge. Simple mixing models of MORB source and sediment can explain the observed isotopic variations, but necessitate the involvement of sedimentary end members with different isotopic compositions to explain the Pb isotopic variation within in a single magmatic unit. Mixing models between fluid-modified MORB source and sediment can explain the observed isotopic variations with a single sedimentary end member, but this sediment must have very evolved isotopic ratios, similar to sediment from North Australia. Perhaps the best way to explain the observed variations is by melting of a mixture of fluid-modified and sediment-modified mantle.
The question of whether the continental isotopic signature that is observed reflects subduction of sediment or of the leading edge of the Buton microcontinent itself cannot be resolved with this dataset. In the case of the Sunda–Banda Arc, several workers have argued that the continental signature in the volcanics reflects subduction of the continental crust itself (Hilton et al., 1992; Van Bergen et al., 1993) on the basis of helium isotopes and the uplift of the arc. Helium isotope data are not available from South Sulawesi, but it is obvious that uplift in the area has not been pronounced. This contrasts with Central Sulawesi, where there is clear evidence for high rates of uplift and isotopic signatures characteristic of the Australian subcontinental lithospheric mantle (Bergman et al., 1996; Elburg & Foden, 1999b). In the absence of this evidence in South Sulawesi, we would suggest that the continental signature here is related to subduction of sediment rather than the continental crust itself. The reason for this sedimentary signature to become more pronounced in the post-collisional volcanics may be related to halting of the subduction process, thereby allowing the slab and subducted sediment more time to heat up at a given depth in the mantle. This could cause melting of subducted sediment and allow for more effective transport of sediment from the slab to the mantle wedge. Evidence for partial melting of sediments is sometimes found by an increase in Nb/Zr ratios of the erupted volcanics (Vroon et al., 1993), but in the case of South Sulawesi there is no clear correlation between Nb/Zr ratios and the isotopic signature of the volcanics. It is therefore unclear whether the high Nb/Zr ratios of the Ulubila Volcanics reflect partial melting of sediment or the addition of a component from the subcontinental lithospheric mantle, as has also been argued for the Lompobatang Volcano.
It is interesting to note that continental collision does not impose unidirectional geochemical change in the post-collisional volcanics, but rather introduces greater geochemical variability. This would argue for the semi-contemporaneous tapping of discrete mantle reservoirs, rather than a wholesale change in the geochemistry of the magma source. A similar collisional event may also explain the extreme geochemical variability seen in Central Sulawesi (Bergman et al., 1996; Elburg & Foden, 1999b).
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ACKNOWLEDGEMENTS |
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David Bruce is acknowledged for his assistance in the isotope laboratory. This work was carried out while M.A.E. was a recipient of an ARC Australian Postdoctoral Fellowship. Helpful reviews were provided by Simon Turner and Manfred van Bergen.
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FOOTNOTES |
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*Corresponding author. E-mail: marlina.elburg{at}adelaide.edu.au
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