geochemical and tectonic relationships in the east ...searg.rhul.ac.uk/pubs/vanbergen_etal_1993...

Tectonophysics, 223 (1993) 97-116 Elsevier Science Publishers B.V., Amsterdam

97

Geochemical and tectonic relationships in the east Indonesian arc-continent collision region: implications for the subduction

of the Australian passive margin

M.J. van Bergen a, P.Z. Vroon alb and J.A. Hoogewerff a a Faculty of Earth Sciences, Utrecht University, P.O. Box 80.021, 3508 TA, Utrecht, The Netherlands

b Geology, Royal Holloway University of London, Egham, TW20 OEX, UK

(Received April 30, 1993; revised version accepted May 5, 1993)

ABSTRACT

Van Bergen, M.J., Vroon, P.Z. and Hoogewerff, J.A., 1993. Geochemical and tectonic relationships in the east Indonesian arc-continent collision region: implications for the subduction of the Australian passive margin. In: M.J.R. Wortel, U. Hansen and R. Sabadini (Editors), Relationships between Mantle Processes and Geological Processes at or near The Earth’s Surface. Tectonophysics, 223: 97-116.

Variations in the isotopic signatures of volcanics along the East Sunda Banda Arc reflect changes in the nature and amount of sedimentary material supplied by the northeast Indian Ocean floor and the adjacent Australran passive continental margin, which form the two major domains of the Indian Ocean plate that approach the arc system. A compilation of isotopic data for 200-SOO-km-long arc sectors shows that the trend in magmatic signatures folkvs distinct subduction/collision stages reached by the corresponding oceanic and continental-margin sections entering the trench system. Maximum amounts of magma source contamination are inferred for volcanics near an extinct sector north of Timor, where the Australian continent started to collide with the arc first. Pb-Nd isotopic source mixing models point to contamination by sediments with variations in composition, similar to observed along-arc changes in sediments entering the trench. The results indicate an increasing contribution of subducted continental material in the direction of the collision region. Mass-balance calculations, considering the magmatic output and minimum input of subducted continental material required to generate the composition of the volcanic arc in the collision region, are difficult to reconcile with subduction of ocean-floor sediments alone. Thicknesses of sediments presently covering oceanic crust near the margin are close to calculated thicknesses of the sediments fluxed into the trench and magmatically returned to the arc crust. but cannot account for the additional volumes of material accreted on the overriding plate in the same period of time. It IS mferred that leading portions of the Australian continental margin have reached magma generation zones in the easternmost Sunda arc and western Banda arc, which implies subduction to depths greater than 100 km.

Introduction

When a passive continental margin approaches a subduction zone along an island arc or active margin, the resulting collision causes tectonic dis- turbances which often lead to the cessation of the subduction process. Arc-continent and continent-continent collisions have occurred through-

Correspondence to M.J. van Bergen.

out large parts of the Earth’s history, as for example within the Tethys region from the western Mediterranean to southeast Asia. The extent to which underthrusting/subduction of continental crust proceeds in these settings is not only an important control for tectonic presses at the surface, but also provides fundatnental constraints on the budgets of recycled crustal material to the mantle.

The depth reached by a passive margin below the overriding plate before convergence ceases, is

0040-1951/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

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GEOCHEMICAL AND TECTONIC RELATIONSHIPS IN THE EAST INDONESIAN ARC-CONTINENT COLLISION REGION 99

difficult to assess, given the general complexity of collision settings. The discovery of coesite-bearing assemblages in high-pressure metasediments of Alpine regions (Chopin, 1984, 1987; Schreyer et al., 1987) supports the idea that continental crust

can be subducted to at least 100 km depth. Physi- cal properties of the subducting slab are not necessarily prohibitive (Molnar and Gray, 1979; Van den Beukel, 1992), and evidence for the presence of continental crust at depths of more than 150 km has been obtained from seismic velocities in the Pamir-Hindu Kush region (Roecker, 1982).

Here we use geochemical signatures of volcanic arc rocks to address the problem of passive margin subduction beneath the Indonesian East Sunda-Banda Arc, which is in collision with the northern margin of the Australian continent (Hamilton, 1979). Isotopic studies have demon- strated the involvement of subducted sedimentary/continental material in magma genesis for various parts of the arc (Whitford et al., 1977, 1981; Whitford and Jezek, 1979, 1982; Hilton and Craig, 1989; Stolz et al., 1990; Hilton et al., 1992; Vroon et al., 1992; Van Bergen et al., 1992). This paper compares along-strike variations in magma-source contamination with subduction/ collision-related controls governing the supply of “continental” material. From mass-balance considerations it is suggested that a substantial portion of the Australian margin has been subducted to at least the depths of magma generation beneath the volcanic arc, that is > 100 km.

The east-Indonesian collision setting

The Indian-Australian plate approaches the Sunda-Banda Arc, situated on the Eurasian plate, at a velocity of 7-8 cm/yr in a NNE direction (Minster and Jordan, 1978). While oceanic lithosphere subducts beneath the arc along the Sunda Trench, Australian continental crust has entered the subduction region in the eastern part of the Sunda Arc and the Banda Arc (Hamilton, 1979; Fig. 1). The Banda Arc is also enclosed by continental lithosphere to the east, due to the westward movement of the Pacific

plate. This interaction between the Eurasian, In-

dian-Australian and Pacific plates (and associated microcontinents) has resulted in the complex geological pattern in the Banda Sea region (Ham- ilton, 1979; Silver et al., 19851, and may have caused the strong curvature of the Banda Arc (Katili, 1975). The Banda Sea floor and fore-arc basins are essentially oceanic (Bowin ct al.. 1980), but the surrounding troughs are underlain by continental crust, which is up to 40 km thick near Timor (Jacobson et al., 1979).

Seismically, the slab of the Indian-Australian plate can be followed down to more than 600 km

(Cardwell and Isacks, 1978; McCaffrcy, 1988, 1989). It has a steep dip at depths greater than 200 km in the Eastern Sunda Arc (60-70 degrees) and flattens towards the eastern end 01 the Banda Arc. According to these authors a separate slab enters the Seram Trough with a relatively shallow southwestward dip.

Absence of earthquake foci at rclativcly shallow depth near Timor, one of the large islands in the non-volcanic outer arc (Fig. 11, coincides with the stretch of the arc where collision probably began first. The exact locus, style and timing of the collision, the distribution of deformation, and hence the extent to which subduction has proceeded after the onset of collision, have been the subject of considerable debate (see Barber. 1981; Charlton et al., 1991). Contrasting views stem from structural interpretations of t be complex geology of Timor and other islands in the outer arc, and from marine geological/geophysical observations in the bordering fore-arc basins and troughs. Imbricate or accretionary wedge models are favoured by several authors (e.g., Hamilton, 1979; Silver et al., 1983; Karig et al.. 19871, and contrast with overthrust (Carter et al.. 1976; Bar- ber et al., 1977; Audley-Charles et al.. 1979) and upthrust models (Chamalaun and Grady. 19781. Harris (1991) suggested that much of the contro- versy can be reconciled when temporal differences in the deformational stages of the structural evolution along the orogen are taken into account. Important factors causing t hcse differences in the East Sunda-SW Banda outer arc are the oblique convergence angle and the presumed outward “bulge” of the original Australian mar-

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gin where Timor is situated now (Fig. 2; cf., Harris, 1991).

Geological arguments against subduction/underthrusting have emphasized the importance of crustal shortening and deformation in the fore- arc, arc and back-arc (e.g., Audley-Charles, 1981), but several observations suggest that the continental crust has reached at least the inner margin of the outer arc (cf. Hamilton, 1979):

(1) The strong recent uplift in the outer arc (Chappell and Veeh, 1978; De Smet et al., 1989) which can be interpreted to reflect the presence of buoyant subducted continental material.

(2) From seismic evidence McCaffrey et al. (1985) suggested that continental crust could have been subducted down to 150 km.

(3) Gravimetric interpretations which point to the presence of subducted continental crust below Timor and the adjacent oceanic Savu Sea, while the presence of thin crust to depths of more than 100 km cannot be excluded (Chamalaun et al., 1976; McBride and Karig, 1987).

(4) Geometric considerations based on the convergence rate of 7-8 cm/yr and the time elapsed since the collision started. If this occurred about 3 Myr ago (e.g., Abbott and Chamalaun, 1981>, more than 200 km of continental crust could have entered the subduction zone, whereas if collision events began 5 Myr (e.g., Harris, 1991) or even 8 Myr ago (Berry and McDougall, 19861, a much greater length is con- ceivable (McCaffrey, 1989; and see McCaffrey, 1988 for a discussion of additional mechanisms accommodating (part of) the northward motion of Australia). Charlton et al. (1991) associated the presence of Australian margin sediments on Timor with two periods of collision. During the Late Miocene, the basal Palaeozoic-?Mesozoic parts of distal sequences were added to the collision complex by underplating. Proximal parts of the margin covering continental crust entered the trench during the Early Pliocene collision, as witnessed by exposed frontally accreted and under-plated sequences.

(5) Detachment of the old oceanic part of the slab from the buoyant continental part (Eva et al., 1988); if the latter had been subducted to a depth of at least some 100 km, rebound forces

could explain the rapid recent uplift and exten-

sion of the fore arc (Charlton, 19911. Although unequivocal evidence is lacking, several of these points would imply that the continental crust has reached the magma generation zone in some parts of the eastern Sunda and southwestern Banda Arc.

To the west of Timor collisional deformation decreases (Harris, 1991). South of Flares the continental-type Scott Plateau is colliding with the arc. Here the transition occurs from the continental margin to the Jurassic Indian Ocean floor of the Argo Abyssal plane. The oceanic crust becomes younger to the west. South of Bali- Sumbawa, a few hundred meters of pelagic sediments cover the oceanic basement, and an imbricate melange wedge is present along the trench (Hamilton, 1979). Close geological similarities ex- ist between Seram, at the northeastern end of the arc, and Timor (Audley-Charles et al., 1979). According to Hamilton (1979) the geology of Seram can be accounted for largely by subduction of the New Guinea continental shelf.

The East Sunda-Banda volcanic arc

The volcanic Sunda-Banda arc east of Java is constructed on a basement which becomes progressively more oceanic in nature and thinner towards the Banda Sea (Jacobson et al., 1979). The oldest igneous rocks cropping out in the East Sunda Arc and in the extinct sector between the active East Sunda and Banda arcs are Miocene in age (cf. Van Bemmelen, 1949; Katili, 1975; Ab- bott and Chamalaun, 1981). No such ages are known from the much smaller islands of the volcanically active Banda Arc which probably have a more recent origin. With a few exceptions, the active volcanoes are situated on Wadati-Benioff zone depth contours between 100 and 200 km (Hamilton, 1979). Volcanoes in the Adonara- Pantar sector form an across-arc array (h = lOO- 250 km) which allows detecting the penetration of continental material from down-di changes in the contributions of subducted co ponents to magma sources (Hilton et al., 1992). 1 ccording to the seismic interpretation of McCajffrey (1989),

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the volcanoes in the NE Banda sector, are situated on contours of less than 100 km.

Rock types of active and Quaternary volcanoes in the East Sunda-Banda Arc show typical island-arc characteristics. In the Banda Arc they range from low-K tholeiitic in the NE to high-K calcalkaline in the SW (Jezek and Hutchison, 1978; Whitford and Jezek, 1979; Van Bergen et al., 1989; Vroon, 1992). Volcanics of the East Sunda Arc show a large compositional variation (Whitford et al., 1979; Foden and Varne, 1980; Varne and Foden, 1986; Wheller et al., 1987; Stolz et al., 1988, 1990). An exceptional wide range is found in the across-arc array mentioned above: from low-K tholeiitic at the front to ultrapotassic behind the arc (Varekamp et al., 1989; Van Bergen et al., 1992). Acidic intrusives occur on Wetar in the inactive sector, where they are associated with predominantly extrusive and py- roclastic rocks (Van Bemmelen, 1949). The igneous activity lasted from 12 to 3 Myr ago (Ab- bott and Chamalaun, 1981). It should be noted, however, that dredging has revealed the presence of only 400~kyr-old high-Mg diabase rocks off the north coast of Wetar (Schwartz et al., 1984; Silver et al., 1985). In a puzzling position behind this island, a small active volcanic island (Gunung Api North of Wetar) rises from the 5000-m-deep ocean floor. The depth to the Benioff zone (h) is some 400 km, but its relation to the subduction process is not evident (e.g., Silver et al., 1985).

Magma-source contamination

The East Sunda-Banda arc is particularly suit- able to study contamination of magma sources by

subducted material. The transition from an oceanic to a continental character of the huhduct-. ing plate is coupled with changes in the nature and quantity of sediments that enter the trench along the arc. Moreover, the supply of detrital sediments by the erosion of ancient (Prc- Cambrian) rocks of the Australian craton, facili- tates detection in the volcanics, given their highly radiogenic Sr and Pb, and unradiogenic Nd isotopic ratios. Absence of continental crust in the overriding plate east of Java minimizes the chance that primary signatures of mantle-derived magmas were modified by contamination a~ shallow depths. Finally, the spatial distribution of volcanoes is favourable for geometrical considerations based on along- and across arc systematics.

Using isotopic evidence. Whitford and co- workers were the first to point out the differences between the western and eastern parts of the arc system, and proposed that magma sources in the easternmost Sunda Arc and Banda Arc were contaminated by subducted continent-derived material (Whitford and Jezek, 1979, 1982. and rcfcr- ences therein). Subsequent work established the view that the Banda Arc is isotopically perhaps the world’s most “continental” island arc (e.g., Magaritz et al., 1978; Morris. 1984; Vroon ct al., 1992). Compelling evidence for sediment subduction was recently provided by the along-arc parallelism in isotopic variations of volcanics and sediments in front of the arc (Vroon, 1992; Vroon ct al., 1992).

A significant degree of source contamination has also been documented in the easternmost part of the Sunda Arc (Whitford et al., 1977; Stolz et al., 1990; Vroon et al., 1990; Van Bergen

Fig. 3. Along-arc variations in Sr-Nd-Pb-He isotope signatures of sectors in the volcanic arc, indicative of changes in the contamination of magma sources due to the subduction of continental material. Data are from active and young volcanoes in all sectors, except for extinct Alor-Romang which includes vokanics with up to Miocene ages. The largest deviations from uncontaminated mantle values occur in the Alor-Romang sector with a decrease on both sides in “‘Sr/%Gr, z’aPb/“MPb and an increase in 143Nd/‘44Nd and Rc/Ra (= measured 3He/4He value normalized to the ratio in air). Extremely large ranges in this sector encompass most of the values in the active actors. Smalt fields added to the Flares and A-P sectors in the laJNd/‘“Nd plot indicate low-K tholeiitic volcanoes close to the front where transfer of Nd from subducted material to magma Sources is believed to be less efficient than behind the front (Hoogewerff, in prep.). Data sources: Whitford (1975), Whitford et al. (IY77. 19811, Whitford and Jezek (19791, McCulloch et al. (1982), Varne and Foden (19861, Stolz et al. (1990). Vroon et al. (1990, lY92),

Vroon (19921, Van Bergen et al. (19921, Hoogewerff and Vroon (unpubl. data, Utrecht. Egham).

GEOCHEMICAL AND TECTONIC RELATIONSHIPS IN THE EAST INDONESIA ~~~~~~~~~~~ MLLISION REGION

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et al., 1992). Further west, the situation is less

unambiguous. For the Bali-Sumbawa part of the arc, situated outside the collision area, argclments against contributions from recently subducted upper-crustal sialic material have been discussed by Varne and Foden (1986) and Wheller et al. (1987). Based on the systematic deviation of several isotope systems from mantle values, and on analo- gies with the more eastern parts of the arc, we believe that recent source contamination cannot be ruled out. It should be noted, however, that the involvement of different mantle components in the East Sunda Arc, probably including an enriched type (e.g., Stolz et al., 19901, poses limits on the detection of small amounts of sediments.

Along-arc systematics

A large data set on volcanics and sediments is currently available (see Fig. 3 for references), which allows evaluating along-arc systematics from Bali to the NE Banda Arc for various isotope systems in considerable detail. For conve- nience, we distinguish a number of sectors which roughly coincide with changes in nature and volume of sedimentary/continental material currently present in front of the arc, or -with changes in the extent to which collision has proceeded; from west to east: Bali-Sumbawa (B-F), Flores (F), Adonara-Pantar (A-P), Alor-Romang (A-R), SW Banda Arc (SW BA), central Banda Arc and NE Banda Arc (NE BA). Each sector contains a number of active volcanoes, except for the Alor- Romang sector, where volcanism is probably extinct since 3 Ma, the youngest age of dated rocks from Wetar (Abbott and Chamalaun, 1981).

The variations in isotope ratios of the sectors along the volcanic arc are shown in Figure 3, and can be compared with changes in relevant sedimentary and geophysical parameters along the

trench in Figure 4. The Sr, Nd, Pb, He isotope ratios show a roughly symmetrical pattern with the most extreme values near the eldtinct Alor- Romang sector. Here 87Sr/86Sr, 206Pb/204Pb and 20*Pb/204Pb reach the highest values and decrease on both sides, while 143Nd/144Nd and 3He/4He are lowest and increase on both sides. These isotopic systematics indicate maximum deviations from uncontaminated mantle <values near Timor, and are consistent with a progressively increasing role of subducted continental material in magma genesis towards the extinct sector. The following additional observations are worthwhile to note:

(1) The extinct sector shows a wide range in isotopic signatures (Whitford et al., 1977; McCul- loch et al., 1982; Hoogewerff and Vroon, unpubl. data), which encompasses much of the variation in the other sectors of the East Sunda and Banda Arcs. The data from the latter are from active and Quaternary volcanoes only, while the range in the extinct sector might reflect changes in subduction-related controls with time, in view of the up to lZMyr-old ages (Abbott and Chama- laun, 1981).

(2) The volcanics at the eastern end of the Banda Arc do not fall back to the “least contaminated” values of the Bali-Sumbawa sector in the East Sunda Arc.

(3) Assimilation of arc crustal material in the SW and central Banda-Arc sectors may have modified Sr- and Nd isotope signatuxs obtained at greater depth to some extent, but cannot explain the overall “continental” compositions nor a slight deviation from the along-arc trends in the central sector (Vroon et al., 1992).

(4) Some of the variations within individual active sectors can be attributed to across-arc ef- fects. For example, volcanoes in the Adonara- Pantar sector show a trend of increasing 3He/4He

Fig. 4. Along-trench variations in some geophysical and sedimentary parameters. Sediment thicknesses, volumes of the accretionary wedge (estimates are rough and use the 6-km depth contour as base level) and arc-trench gaps are based on tiamilton’s (1979) map. A presumed break between the Indian-Australian plate and the plate entering the Seram Trough lies between the southwest Banda Arc (SW BA) and northwest Banda Arc (NW BA). Note that the latter sector is defined to include the northernmost active volcano only. Lithospheric stresses in the Indian Ocean plate close to the trench are from numerical modelling by Cloetingh and Wortel (1986) and crustal thicknesses of the overriding and subducting plates from Jacobson et al. (1979) and Bowh et al. (1980).

Pb isotope data for sediments in front of the arc are from Ben Othman et al. (1989) and Vroon (1992).

away from the trench, which can be explained by a decreasing contribution from subducted continental material (Hilton et al., 1992). High 14’Nd/ ‘44Nd ratios of some potassium-poor tholeiitic volcanoes closest to the trench in the Flores and Adonara-Pantar sectors can be ascribed to a less efficient transfer of Nd from subducted material to mantle sources than behind the front (Hoogewerff, in prep.). Presumably, the mobility of this element at relatively shallow depths of the slab remains reduced as long as the transfer is largely controlled by fluids (cf. Tatsumi et al., 1986) rather than by melts.

(5) The NE Banda Arc sector in Figure 3 includes Banda Api and Manuk, the two centres at the extreme end. It should be noted, however, that these volcanoes may genetically belong to different subduction systems if they are separated by the break between the slabs entering the Timor and Seram troughs (cf. Cardwell and Isacks, 1978; McCaffrey, 1988). Nevertheless, compositions of both show a significant imprint of subducted continental material.

The along-arc parallelism between the isotopic signatures and the eastward transition from an oceanic to continental nature of the Indian- Australian lithosphere is accompanied by increasing thicknesses of the crust and sediment sequences in front of the Sunda Trench-Timor Trough (Fig. 4). Numerical modelling of lithospheric stresses in the Indian-Australian plate near the subduction zone indicates an eastward change from N-S extension to compression (Cloetingh and Wortel, 1986). On the scale of the arc sectors, the isotopic shifts in the volcanic arc coincide with changes in volume of the corresponding parts of the accretionary wedge along the Sunda Trench-Timor Trough and with the thicknesses of outboard sediment sequences. The rapid increase of the arc-trench gap at the Flores sector, where thinned continental crust of the Scott Plateau has collided with the arc, coincides with a jump in the isotope signatures of the volcanics. Another maximum in the gap marks the bordering region between the two subduction systems. The width of the gap, together with a lower convergence rate at the Seram Trough, may explain why source contamination in the NE

Banda Arc is less than in other sectors, dcspitc the greater thickness of sediments in front of the trough.

Conventional bulk-mixing models have indi- cated that the addition of relatively small amounts of “continental” material is sufficient to generatc the deviations from uncontaminated mantle values in the Banda Arc and easternmost sectors of the active Sunda Arc (e.g., Stolz et al.. 1990; Vroon et al., 1992). Changes in quantity seem the most obvious explanation for the overall along-arc trend. However, variations in the isotopic signatures of co-eval volcanics along an arc-continent collision zone may also be caused by changes in the composition of subducted sediments. Hypo- thetically, this can be predicted when the sediments display:

(al Lateral compositional variations, which may be controlled by the provenance of elastic terrigenous material supplied by erosion, or by variations in the relative amounts of terrigenous and biogenic fractions.

(b) Vertical compositional variations, combined with along-arc changes in subduction/collision-related controls (e.g., frontal accretion, underplating, delamination) deEermining how “in- complete” stratigraphic sequences arrive at magma source regions.

(cl Compositional variations with distance to the passive margin (e.g., differences between shelf, slope and rise facie& combined with along- arc temporal variations for each sediment facies to become involved in magma genesis.

All three options may be applicable in the East Sunda-Banda Arc. Lateral changes in sediment compositions (a) have been documented, but cannot account for the isotopic variations alone (Vroon, 1992; and see below). Options (b) and (c), which require an oblique convergence or ir- regularities in the geometry of the continental margin (outward bulges, embayments) and/or of the arc (e.g., strong curvature) prior to collision, are potentially valid as well, but are more difficult to verify.

Alternative explanations for the isotopic variation could be inhomogeneity of the mantle component, or assimilation of arc crust material by magmas en route to the surface. Involvement of


different mantle components in the East Sunda- Banda Arc is likely (e.g., Wheller et al., 1987; Stolz et al., 1990; Vroon et al., 1992; Van Bergen et al., 19921, but the strong contrast between sedimenta~/continental material and most mantle types in terms of Sr, Nd, Pb contents and isotope ratios makes magma signatures more sus- ceptible to the subducted component. It would further be very coincidental if the along-arc changes in uncontaminated sub-arc mantle would independently follow the same trend as the tectonically controlled supply of sedimentary/continental material approaching the arc. Assimila- tion at shallow levels is visible in some of the Banda Arc volcanoes, but its effect seems less important than source contamination (Vroon et al., 1992).

Subducted continental crust or continent-derived sediments?

143Nd /‘44Nd_ 206Pb /204Pb bulk mixing models and estimates of sediment fluxes

The contaminating components involved in magma genesis must originate from the passive margin of the NW Australian continent, which formed by rifting during the Middle-Late Juras- sic and subsequent ocean floor spreading in the adjacent parts of the NE Indian Ocean (Veevers and Heirtzler, 1974; Larson, 1975). Precambrian basement rock is thought to underlay pre- and post-rift sediment sequences of the outer slope of the Timor Trough. Pre-rift sediments, found in grabens running roughly parallel to the present margin, include Cambrian to Jurassic sequences rich in siliciclastics. Post-rift sequences are up to 15~-m-thick distal slope and rise facies chalks of Late Cretaceous-Pliocene age, and up to 4000- m-thick shelf sediments consisting of Cretaceous sandstone and shale which are covered by Ceno- zoic carbonates (Powell, 1982; Von Rad and Exon, 1983).

Using Sr-Nd-Pb isotope and trace-element compositions it is difficult to assess whether subducted continent-derived sediments covering oceanic lithosphere or continental crust con- tributes to magma genesis. It has been shown that

the terrigenous fraction of sediments of the Aus- tralian shelf around the Banda Sea (vroon, 1992) and in ODP core 765 (Plank and Ludden, 1992) is compositionally close to average Australian upper crust. The following mass-balance considerations provide constraints on the amount ot’ continental material that must have been subducted. The approach is based on the requisite that estimated volumes of sedimentary/continental material which contributed to the formation of the arc crust through magmatic recycling correspond to minimum volumes of material that must have been supplied to the trench. We assume that sediments of the Argo Abyssal Plain. the Scott Plateau and the adjacent shelf are volumetrically and com~sitionally equivalent to the sediments of the leading ocean floor and portions of the margin that have entered the subduction/collision zone to the north. From a comparison between the thicknesses of these sediments and the calculated minimum thicknesses of subducted material required to generate the isotopic signatures of the arc volcanics, it can be inferred how much of the continental margin has been subducted.

Time-averaged minimum thicknesses of the s~dimenta~/continental sequence that has reached magma source regions in the distinct arc sectors were estimated as follows (cf.. White and DuprB, 1986). We first calculated the total volume of contaminated mantle source from which the total volume of the arc crust has originated since the onset of volcanism. The arc-crust was assumed to consist entirely of volcano-plutonic complexes with a total volume twice that of the volume above the sea floor. A melting percentage of 10% was adopted, based on trace-element models for magma genesis in the Handa Arc (Vroon, 1992). Subsequently, the share of Sub- ducted Continental Material (SCM) in this volume was calculated from ‘43Nd/ “’ Nd- “‘Pb/ 204Pb bulk-mixing models for source contamination, assuming that the density of the mantle is twice that of SCM. Finally, the SCM volume was converted to an average thickness. depending upon the convergence rate, the period of subduction during which the arc sector was built cap- proximate age of the sector), and the Icngth along

TABLE 1

Sediment and mantle compositions used as end members in ‘J’Nd/144Nd-2”hPb/2’14Pb mixing models of Fig. 5. For tix

Bali-Sumbawa sector, values are based on averaged ~om~sitjons of pelagic sediments from Ben Othman et al. (1989). Other isotopic data for sediments are based on DSDP262 (average of five samples for Flares and A-P; one selected sample tot- A-R),

SN3 (one selected sample for SW BA) and SNI (average of 5 samples) from Vroon (1992). Concentrations of Pb and Nd were calculated on a carbonate-free average of SNI-2-3 and DSDP262 sediments, and represent values of the continental crust. MORB-source mantle is a local mantle composition, taken from Vroon (1992) who used isotopic data from Morris et al. (1984).

_. __-_- 206Pb/2e“ Pb r43Nd/‘44Nd Pb (ppm) Nd @pm)

sediments

Bali-Sumbawa 18.84 0.51226 37 48 Flares lY.04 0.51212 21 27

Adonara-Pantar 19.04 0.51212 21 27 Alor-Romang 19.23 0.51200 21 27 SW Banda Arc 19.57 0.51195 21 27 NE Banda Arc 18.75 0.51219 21 27

MORB-source mantle 18.28 0.51297 0.03 0.73

the trench over which SCM was subducted. Con- vergence rates used are 20 km/Myr for the NE Banda Arc sector, derived from a discussion in McCaffrey and Abet-s (19911, and 75 km/Myr for the other sectors (Minster and Jordan, 1978).

Results of the source-Ming models from Nd- Pb isotopes, based on end-member com~sitions of Table 1, are illustrated in Figure 5. An important point to note is that isotapic variation in the sedimentary end member has to be invoked in order to model the volcanics along the entire East Sunda-Banda arc (cf. Ben Othman et al., 1989; Vroon et al., 1992). The shift in isotopic compositions of the volcanics corresponds with that of the sediments entering the trench, which is considered as compelling evidence for source contamination by recent subduction, White and Dupre (1986) documented a similar relation for the Lesser Antilles. According to Vroon (1992) the variations along the Banda Arc reflect differences in provenance of detrital components from

Australia and New Guinea. Sediments at ODP site 765 in the Argo Abyssal Plain still contain detrital Australian components (Plank and Lud- den, 1992), but in general the distal end of elastic deposits, derived from the NW Australian margin, remains relatively close to the continental slope (Hamilton, 1979). The mixing models further indicate an increasing contribution of sedimentary material in the direction of the inactive sector, which is consistent with evidence from the other isotopic systems shown in Figure 3. Acidic rocks from Wetar in the Alor-Romang sector have isotopic compositions falling in the sediment field, which suggests that they may represent al- most pure SCM melts.

The calculated average thicknesses of SCM and of the Total volume of Sedimentary Material (TSM) fluxed into the trench (contributing to SCM and the formation of the accretionary wedge) are given in Table 2 and are schematically illustrated in Figure 6. In the absence of sufft-

Fig. 5. 143Nd/‘44Nd-aaePb/2Pb mixing diagrams for the East-Sunda Arc (a) and Banda Arc cb) illustrating the percentages of sediment required to generate the isotopic compositions in each sector from contamination of the sub-arc mantle, assuming it has an Indian Ocean MORB source affinity. Compositions of end members are listed in Table 1. Note that along-arc changes in the sediment compositions match those of the voicanics, and that the sediment contributions increase towards the inactive Alor-Romang sector (A-R). V= pelagic Indian Ocean sediment from Ben Othman et al. (1989); see Fig. 1 for other sediment locations. The Flores (F) and Adonara-Pantar (A-P) fields do not include data from Ija and Werung, two very low-potassic volcanoes (see text). In order to avoid problems of dilution with the (variable) carbonate fraction in the sediments, the mixing models were based on

carbonate-free concentrations of Pb and Nd. Data sources for the volcanin are given in Fig. 3.

GEOCHEMICAL AND TECTONIC RELATIONSHIPS IN THE EAST INDONESIAN ARC-CONTINENT COLLISION REGION

0.5132,

0.5130'

0.5128'

2 0.5126,

=: '\

z" 0.5124

a

0.5122

0.5120

0.5118 1 7.l

I I I I I I I I I I -

I 17.5 18.0 18.5 19.0 19.5 2

206Pb~04Pb

0.5132

0.5130

0.5128

is 0.5126

$ '\

2 0.5124

a

0.5122

0.5120 Sediments

0.5118 I I I 1 I I I I 1 17.5 18.0 18.5 19.0 19.5 2

206Pb/204Pb

17.c

a

,F Volcanics

109

I.0

1.0

cicnt chronological constraints, different ages for each sector were assumed. The older ages are preferred for the western sectors and the younger for the eastern sectors (Van Bemmelen, 1949; Katili, 1975). Taking uncertainties into account, the calculated thicknesses serve to assess which portions of the margin have subducted to depths greater than 100 km, corresponding to the front of the volcanic arc.

For this purpose, a comparison is matic with the sediment sequences covering the oceanic crusi

of the Argo Abyssal Plain in front of the trench just west of the collision zone, which arc 5(N)-YOO m thick (DSDP Site 261. Vecvers ct al.. lY74; ODP Site 765, Gradstein et ai., 1902; Dumoulin and Bown, 1992). Lithologies arc divcrsc. hut ;I reasonable estimate of maximum t hickncsscs 01’ the continent-derived fraction is some 400 m. the

TABLE 2

Average thicknesses of subducted sediments required to generate contaminated magmas in arc sectors (SCM), and comparison with

estimated total sediment fluxes to the trench (TSM). % SCM: percentage of sediment in magma sources. obtained from the

‘43Nd/‘44Nd-2n6Pb/2”4Pb isotope mixing models of Fig. 5. SCM thicknesses: thicknesses of the carbonate-free I’raction of

sediments, which was subducted and returned to the arc crust through magmatism. TSM thicknesses: thicknesses of total \edimcnt

sequences having entered the trench, based on the sum of the accreted volume (including all sediment types) ;Ind SCM

(carbonate-free). SCM and TSM were calculated for different ages of the arc sectors). Ranges in parentheses are ha\ctl on the

variation in % SCM. Lengths and volumes of wedge and arc are based on Hamilton’s (1979) map and on e~timatcr. from

bathymetry. Arc volumes were assumed to be twice the volumes of volcanics above the ocean floor. Convergence rates used are 20

km/My for the NE Banda Arc sector, derived from the discussion in McCaffrey and Abers (19911, and 75 km/My for the other

sectors (Minster and Jordan, 1978). Calculations assume that the sediment density is half that of the mantle. Extreme maximum

thicknesses in the Alor-Romang sector are probably unrealistic, as the rocks on which they are based are not rcprescnt;ctivc for the

entire duration of magma production. High TSM and low SCM/TSM values for the NE Banda Arc sector may he hiabcd hy dn

overestimate of accreted material.

Sector

Bali-Sumbawa 520 85000 163 564000 Flores 400 186000 465 370000 Adonara-Pantar 205 137000 668 102000 Alor-Romang 510 350000 686 162000 SW Banda arc 170 92000 541 47200 NE Banda arc 125 72000 57b 30000

Length along Wedge volume

trench (km) (km’)

Wedge volume (km’/trench km)

Arc volume

(km’)

‘3 SCM

.-.. 0.5 (0. I - I 3)

I (0.5-3)

2 (l-2)

i (I-loll)

2 (l--7.5)

I (().I-I.$)

SCM thickness (m)

1 MY 5 MY 10My 20 My

Bali-Sumbawa 290 (60-870) 145 (30-435) 70 (15-220)

Flores 490 (250-l 500) 245 (125-745) 120 (60-370)

Adonara-Pantar 530 (270-800) 265 (U-400) 130 (70-200)

Alor-Romang 8.50 (170-17000) 425 (85-8400) 210 (40-4200)

SW Banda arc 1500 (750-1850) 300 (150-370) 150 (75-185)

NE Banda arc 2400 (250-3600) 480 (50-720) 240 (25-360)

TSM thickness (m)

1 MY

SCM/TSM

5 MY 10 My 20 My (X 100)

Bali-Sumbawa 720 (490- 1300) 360 (250-650) 180 (125-325) 40 (12-67)

Flares 1750 (1500-2700) 870 (740- 1400) 430 (370-680) 2x (17-54)

Adonara-Pantar 2300 (2000-2600) 1150 (1000-1300) 580 (510-640) 23 (13-31)

Alor-Romang 2700 (2000- 19000) 1300 (1000-9300) 670 (500-4700) 31 (8-YO)

SW Banda arc 8700 (8OCG-9000) 1750 (1600-1800) 870 @X-900) I7 (9. 20)

NE Banda arc 31000 (29000-32500) 6200 (5800-6500) 3100 (2900-3250) X(1-- II) -.-.._ ._ .

GEOCHEMICAL AND TECTONIC RELATIONSHIPS IN THE EAST INDONESIAN ARC-CONTINENT COLUSION REGION 111

average non-carbonate content in the ODP hole being some 60% (Plank and Ludden, 1992). Both sites are situated close to the Scott Plateau, probably already a continental crustal mass where sediment thicknesses rapidly increase (Hamilton, 1979). Thicknesses of pelagic sediments at abyssal depths decrease in westward direction.

This 400 m estimate is close to what can be

accounted for by subducted and magmatically recycled continental material alone, in the sectors east of Bali-Sumbawa for ages less than 20 My and more than 1 My (Table 2, Fig. 6). However, the actual thicknesses of sediments fluxed to the trench must have been much greater, considering the additional volumes required to build the wedge system in the same period of time. These average TSM thicknesses are in the order of one

KKiOO Sediment 7 thickness (m)

TSM

to several kilometres, exceeding the 7(W) m average of the complete DSDP261 and ODP765 sequences. M~mum SCM and TSM values are found for the extinct Alor-Romang sector.

The absolute values of calculated thicknesses should be considered with care, given their sensi- tivity for several input parameters, hut it should be noted that, except for the extreme values, the results listed in Table 2 are conservative estimates for the following reasons:

(a) Volumes lost by erosion of the Islands have not been incorporated in the volume estimates for the arc and the emerged parts td the accretionary wedge. Therefore, thicknesses may have been significantly greater than calculated for the sectors near Timor, where uplift rates are high. On the other hand, the volumes of any rock

B-S (=Myf

Fig. 6. Schematic illustration of along-arc variations in calculated thicknesses of subducted continental material (.S<‘.W 1 and of the total volumes of sedimentary material (TSM) having entered the trench (see Table 2 for details on calculations. assumptions and comments). Ranges are based on the variation in the inferred contribution of SCM to magma sources in each arc sector. Shown for comparison is the average thickness of DSDP 261 and ODP 765 sediments (Dumoulin and Bown, 1992) which cover oceanic crust (I = complete sequence; 2 = carbonate-free). Ages were arbitrarily selected in accordance with an eastward dccrcasc. Note that SCM thicknesses east of the Bali-Sumbawa sector are close to (2), but that TSM thicknesses even exceed (11, induating sediment fluxes to the subduction/collision system which imply involvement of the continental parts of the margin. Extreme thrcknesses in

the A-R (Alor-Romang) and NE BA (northeast Banda Arc) sectors are probably too large (see text).

types, present in the outer arc for reasons other than accretion, could not be corrected for.

(b) Total volumes of igneous rocks in the arc sectors could be larger than assumed if intrusives in the deeper parts of the arc are more volumi-

nous than the volcanics above the ocean floor. (c) The unknown amount of subducted mate-

rial which was not returned to the upper plate but was recycled to the upper mantle, is not accounted for.

(d) The convergence rate of 7.5 cm/yr adopted is probably an upper limit. If it has slowed down since collision(s) started (e.g., by more than half, cf. Hamilton, 19791, sediment thicknesses must have been proportionally larger.

(e) The Australian shelf sediments contain substantial amounts of carbonate. These carbonate sediments are included in the estimates of accreted volumes but not in the SCM estimates, as these were calculated on a carbonate-free ba- sis, and thus yield thicknesses of the detrital continent-derived fraction only. Hence, TSM values lack the possible portion of subducted carbonate.

(f) If a mantle more enriched in incompatible trace-elements than MORB-source mantle is lo- cally involved, the amount of sediment required to generate isotopic shifts by source contamination must be higher (see Vroon et al., 1992).

We conclude, therefore, that the thickness of Argo Abyssal Plain-type sediments is insufficient, and that the subduction of Scott Plateau-type continental crust down to magma-source regions is more likely for the arc sectors in the collision region near Timor. When taking the maximum estimates of SCM and TSM (Table 21, involvement of the shelf may be required.

For the Bali-Sumbawa sector, the estimated total thickness of accreted and subducted sediments is less than 200 m for an age of 20 Myr, which corresponds with the sediments currently covering the ocean floor in front of the trench. For the NE Banda sector, the lower convergence rate and shorter subduction period yield large average SCM thicknesses, despite a lower SCM contribution to magma sources. The calculated TSM thickness is extremely high, but the results are probably biased by an overestimate of the

accreted volume, since continental slices, ritted from Australia and New Guinea and tectonically emplaced in this area, may form part 01‘ the basement of Seram and other islands in the outer arc (Silver et al., 1985).

Concluding remarks

Acknowledging that uncertainties are inherent to mass-balance approaches of material fluxes at subduction systems, we believe that modelling the SCM flux from isotopic signatures of the East Sunda-Banda Arc magmas yields sufficiently ro- bust results to pose constraints on the subduction of continental margin material. The magmatic signals not only correspond to the large-scale along-arc change from an oceanic to a continental supply to the trench, but also tend to follow the geometry of the margin on the scale of 2otl- 500~km-long arc sectors.

Sediment fluxes in East Indonesia are larger than in the Lesser Antilles, where subduction of approximately 90-m-thick sediments would be required to account for about 1% contamination in the arc magmas, while some 250 m is being un- derthrust beneath the large Barbados Ridge accretionary prism (White and DuprC, 19861. With- in-section decollements above the igneous basement confirm that portions of non-accreting sediment may bypass this prism (Van Huene and Scholl, 1991, and references therein) and reach deep levels in the upper mantle.

Charlton et al. (3991) have suggested that the basal decollement of the Timor fold belt is underlain by Pre-Permian rocks, and that the prism above the decollement for a large part consists of underplated Permo-Triassic sequences and Juras- sic-Neogene frontally accreted material. The SCM in the region of the inactive sector may thus consist of Palaeozoic sediments or even older (Precambrian?) crystalline basement.

Accretion of the younger sediments, which on average contain more carbonate than the older sequences, may explain why volcanic gases in the SW Banda Arc and easternmost Sunda Arc do not contain anomalously high CO, contents, despite the amount of sediment involved in magma genesis. CO,/‘He ratios do not deviate signifi-


cantly from other island arcs, while the carbon isotope signatures are probably not sufficiently sensitive to detect subducted carbonate (Poorter et al., 1991; Varekamp et al., 1992; Van Bergen, 1992).

Supporting evidence for this mechanism is provided by “Be data of volcanics from Java and Bali. This isotope is a powerful tracer to detect recently subducted young sediments, but there is a lack of “Be enrichment in the volcanics along the Sunda trench (Tera et al., 1986) despite the evidence for sediment subduction from Pb isotopes (Whitford and Jezek, 1982; Fig. 5). As Plank and Ludden (1992) have pointed out, this may be due to the fact that only sediments older than Neogene (in which any “Be must have decayed) are being subducted.

This effect of combined off-scraping and sub-

duction is feasible along the entire length of the Sunda-Banda Arc, given its accretionary-wedge forming character (Van Huene and Scholl, 1991). According to these authors 20-30% of sediments entering subduction zones with accretionary prisms is frontally accreted, whereas the remain- ing 70-80% subducts and may either underplate older parts of the prism or bypass it entirely. Our crude estimates for the Bali-Sumbawa sector suggest that the volume fraction of bypassed sediments (carbonate-free) returned to the arc crust (SCM) is some 40% of the total sediment influx (Table 2), assuming the latter to be the sum of the accreted volume and SCM. The SCM share in the total volume tends to decrease eastward, which seems consistent with the structural change towards the collision regime (cf., Harris, 1991).

The inferred subduction of continental crust agrees with He isotope results (Hilton et al., 1992) and trace-gas compositions of fumaroles (Poorter et al., 1991) of the Eastern Sunda and Banda Arcs. Hilton et al. (1992) explained the unusually low 3He/4He ratios of the volcanics in the Adonara-Pantar sector by the degassing of the subducted continental crust, since fine- grained sediments are probably not capable of retaining sufficient helium after entering the subduction zone. The 3He/4He ratio increases in the Banda Arc but remains lower than is common in island arcs, whereas to the west the first “normal”

values are found in the Bali-Sumbawa sector (Hilton and Craig, 19891, apparently after cross- ing the transition to the oceanic environment. In the Adonara-Pantar sector, the 3He/4He ratios increase from extremely radiogenic values in the south to higher values in the north. Still, they remain relatively low as far as Batu Tara, an ultrapotassic volcano situated behind the arc. Us- ing the Wadati-Benioff zone depth contours of Hamilton (19791, this may imply that pieces of solid crystalline crust have been subducted down to about 250 km. From seismological evidence for slab pull forces in this area (Spence, 19861, it can be speculated that the down-dip leading portion of old oceanic crust has facilitated the penetration of relatively buoyant continental material into the mantle. Indications for slab detachment (Charlton, 1991) would imply that the limits of

margin subduction have been reached. The significant volumes of subducted conti-

nental material, inferred to have reached the magma sources in the volcanic arc, support the view (e.g., Berry and McDougall, 1986) that collision(s) near Timor started before, rather than during the Pliocene. Potentially, the timing of collision(s) and the kinematics of the Australian margin subduction can be further constrained by detailed studies of the older rocks in the East Sunda Banda volcanic arc, which focus on changes in isotopic signals of subducted continental mate-

rial with time.

Acknowledgements

We are grateful to Rinus Wortel, Ulli Hansen and Roberto Sabadini for inviting us lo partici- pate in the 1993 EUG Symposium on the “Rela- tionship between mantle processes aad geological processes at or near the surface”, and for pa- tience. We thank Serge Lallemant fur sharing his views on the evolution of wedges and Adrian Richardson for kindly making his maps available. The manuscript benefitted from critical comments by Rinus Wortel. This work would not have been possible without the generous support of Ir. Soebroto Modjo and colleagues at the Vol- canological Survey of Indonesia (Bandung) during joint field studies. Research was financially

supported by SOZ-NW0 (MJvB), AWON-NW0 (JAH) and the Faculty of Earth Sciences, Utrecht (PZV).

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