q 2004 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.Geology; January 2004; v. 32; no. 1; p. 4144; DOI 10.1130/G19941.1; 3 figures; 1 table; Data Repository item 2004005. 41

Subducted upper and lower continental crust contributes tomagmatism in the collision sector of the Sunda-Bandaarc, IndonesiaM.A. Elburg* Max Planck Institute for Chemistry, Geochemistry Division, P.O. Box 3060, 55020 Mainz, Germany, and

Department of Geology and Geophysics, University of Adelaide, Adelaide, SA 5005, AustraliaM.J. van Bergen Faculty of Earth Sciences, Utrecht University, P.O. Box 80021, 3508 TA Utrecht, The NetherlandsJ.D. Foden Department of Geology and Geophysics, University of Adelaide, Adelaide, SA 5005, Australia

ABSTRACTPb isotopes in igneous rocks from the Banda-Sunda arc show extreme along-arc vari-

ations, which correspond to major lithologic changes in crustal components entering thesubduction system. An increase in 206Pb/204Pb ratios toward the zone of collision with theAustralian continent reflects input of subducted upper-crustal material; maximum valuescoincide with anomalously radiogenic 3He/4He ratios that have been earlier attributed tothe involvement of the continental margin. The collision zone is further characterized by208Pb/204Pb ratios that are higher for a given 207Pb/204Pb value than observed in thenoncollisional sectors, and in the central part of the collision zone, the 206Pb/204Pb ratiosare lower than the most radiogenic values in the adjacent areas. We propose that thesePb isotope signatures reflect input of subducted lower crust, mobilized as a result of slab-window formation during arc-continent collision. Variations in Pb isotopes in the collisionzone are solely determined by variations in the nature and proportions of different sub-ducted components. The Pb isotope arrays in the noncollision area may be dominated byslab components as well and could reflect mixing between subducted oceanic crust andentrained sediments, rather than between subarc mantle and subducted sediments. Ournew interpretation of the Indonesian Pb isotope data does not call for involvement ofocean-island basalt (OIB)type mantle or Australian subcontinental lithospheric mantle,as has been suggested previously.

Keywords: subduction, collision, Pb-206/Pb-204, Indonesia, Sunda-Banda arc.

Figure 1. Map of Sunda-Banda arc with its volcanoes. BTBatu Tara; PAPantar; ALAlor;LILirang; WEWetar; RORomang; DADamar; TETeon; NINila; SESerua; MAManuk; BABanda. DSDP 262Deep Sea Drilling Project Hole 262. Track IIIdredge sed-iments. Inset maps show enlargements of Flores, Adonara, and Lomblen islands andSumbawa island. Dashed lines indicate the boundaries of the collision area, solid lines thedepth to the Benioff zone.

INTRODUCTIONThe Indonesian Sunda-Banda arc is the

worlds foremost example of an active arc-continent collision zone. The progressive col-lision of the Australian continent has broughtsubduction and accompanying magmatism inthe Pantar StraitRomang area to a halt (Fig.1). Active volcanism in the Sunda arc extendsfrom Sumatra in the northwest to Pantar in theeast and from Damar to Banda in the northeastalong the Banda arc. Since 3 Ma (Abbott andChamalaun, 1981), cessation of volcanism hasspread from Wetar to the neighboring islands,but arc-continent collision probably started asearly as 8 m.y. ago (Berry and McDougall,1986).

Trends in Pb isotope data from the Sunda-Banda arc have previously been modeled asrepresenting mixtures of subarc mantle andsubducted continental material (Stolz et al.,1990; van Bergen et al., 1992, 1993; Vroon etal., 1993; Hoogewerff et al., 1997; Elburg etal., 2002; Turner et al., 2003). This modelingcontrasts with other interpretations of Pb iso-topes in arc volcanic rocks, which regard Pbto be solely a tracer of the slab component,with or without entrained sediments (Miller etal., 1994).

Our compilation of existing Pb isotope dataon the entire along-arc section from Bali toBanda, in combination with new results fromthe extinct sector and published He isotopedata (Hilton and Craig, 1989; Hilton et al.,1992; Gasparon et al., 1994), shows that pre-vious interpretations of Pb isotopes in theSunda-Banda arc need to be revised and thatthe Pb isotope signatures are entirely domi-nated by subducted material of different crust-al derivation. Variations in the relative contri-butions of distinct subducted componentsclearly correspond to spatial changes in thesubduction-collision regime.

ALONG-ARC VARIATION IN Pb ANDHe ISOTOPES

In order to minimize potential effects ofshallow-level contamination, we have restrict-ed our evaluation to the area between Bali and

*E-mail: elburg@mpch-mainz.mpg.de.

42 GEOLOGY, January 2004

TABLE 1. CONSTITUENTS OF SAMPLE GROUPINGS USED IN FIGURES 2 AND 3

West noncollisional Batur, Agung (Bali), Rinjani (Lombok), Wai Sano, Ebulobo, Inerie, Iya, Rokatenda(Flores)

Sumbawa high-K Sangenges, Soromundi, Tambora, Sangeang Api (Sumbawa)Sumbawa low-K Intrusive rocks of Batu Hijau, dikes, lahars (Sumbawa, 42 Ma)West collisional Egon, Lewotobi, Mandiri (Flores), Boleng (Adonara), Lewotolo, Werung, Kedang

(Lomblen), Batu Tara, Pantar Strait volcanoes, Alor central and northCentral collisional Alor south, Lirang, Wetar, RomangEast collisional Damar, Teon, NilaEast noncollisional Banda, Manuk, Serua

Figure 2. 206Pb/204Pb, gamma 8/4 (deviationfrom line 208Pb/204Pb 5 215.84 1 3.5 3 207Pb/204Pb) and Rc/Ra with distance east from Balialong arc. See Table 1 for volcanoes and is-lands that belong to each group. Data fromHilton and Craig (1989), Hilton et al. (1992),Gasparon et al. (1994), Stolz et al. (1990), vanBergen et al. (1992), Vroon et al. (1993), Hoo-gewerff et al. (1997), Hoogewerff (1999), El-burg et al. (2002), Turner et al. (2003), andElburg and Foden (see GSA Data RepositoryTable DR2; see text footnote one).

the Banda archipelago where volcanoes are lo-cated on oceanic crust, thus excluding the is-lands of Sumatra and Java. The arc has beendivided into five sectors, which are, from westto east: west noncollisional, west collisional(partially inactive), central collisional (inac-tive), east collisional, and east noncollisional(Table 1). For the active sectors, data fromQuaternary volcanic rocks were used, apartfrom Sumbawa (west noncollisional) wheresome 24 Ma low-K intrusive and extrusiveunits were included, which have been distin-guished from the younger samples in Figures2 and 3. Analyzed samples from Lirang andWetar are presumed to have ages between 7and 3 Ma on the basis of age dating by Abbottand Chamalaun (1981) and Honthaas et al.(1998). Rb-Sr mineral isochrons give ages forsamples from Alor from 2.5 to 1.3 Ma, 1.7Ma for a single sample from Romang, and lessthan 1 Ma for the Pantar Strait volcanoes(GSA Data Repository Table DR11).

The first-order control on the variation inPb and He isotope characteristics is the loca-tion along the arc, and values for 206Pb/204Pb,gamma 8/4 (see the next section), and Rc/Raratios show strikingly similar trends. The206Pb/204Pb ratios increase toward the colli-sion sector (Fig. 2) and reach a maximum onAlors northeast coast (19.6). These high206Pb/204Pb ratios are not maintained through-out the extinct sector: samples from the southcoast of Alor and from Lirang, Wetar, and Ro-mang have 206Pb/204Pb ratios between 18.9and 19.2. The ratios increase again toward theeastern active islands of the Banda arc (Dam-ar, Teon, and Nila), to sharply fall toward low206Pb/204Pb ratios in Banda, similar to thoseseen in Bali and Lombok. Along-arc changesin Pb isotopes are not limited to the absoluteratios, but also to the 208Pb/204Pb ratio relativeto the 207Pb/204Pb ratio. We have defined abaseline in the 208Pb/204Pb versus 207Pb/204Pb diagram (Fig. 3), consisting of Bali,Lombok, and Banda (on which line volcanicrocks from Java and Sumatra also plot; Turner

1GSA Data Repository item 2004005, Rb-Sr datafor Pantar Strait samples and Pb isotope data, is avail-able online at www.geosociety.org/pubs/ft2004.htm,or on request from editing@geosociety.org or Doc-uments Secretary, GSA, P.O. Box 9140, Boulder, CO80301-9140, USA.

and Foden, 2001), for which the equation is208Pb/204Pb 5 215.84 1 3.5 3 207Pb/204Pb.We denote the vertical deviation from thisbaseline (208Pb/204Pbmeasured 2 208Pb/204Pbcal-culated) as gamma 8/4, analogous to the delta7/4 and delta 8/4 values defined by Hart(1984). The variation in gamma 8/4 is similarto that seen in the 206Pb/204Pb ratios. He iso-tope studies by Hilton and Craig (1989), Hil-ton et al. (1992), and Gasparon et al. (1994)show that Rc/Ra ratios (5 3He/4Hesample 43He/4Heair) decrease from mid-oceanic-ridgebasalt (MORB)like values of 8 6 1 in Java,Bali, and Sumbawa to values as low as 0.0075(Fig. 2), typical of continental crust, in thecollision area. The westernmost center withrelatively low Rc/Ra values is Iya at the vol-canic front in central Flores. Kelimutu, locatedbehind the volcanic front between Iya andEgon, both with crustal Rc/Ra values, shows aMORB-like value of 6, indicating the addi-tional across-arc control on He isotope sys-tematics. The low Rc/Ra values persist east-ward all the way up to the Banda islands,which have values of 3.6. These continentalRc/Ra values have been interpreted to reflect acontribution from subducted continental crustin the region. Hence, the area of involvementof subducted continental crust extends fromthe Banda islands to central Flores. This is asomewhat larger area than the area with con-trasting Pb isotope ratios, because Iya andBanda still have the low gamma 8/4 ratios thatcharacterize the low- to medium-K rocks ofthe noncollisional zone to the west ofSumbawa.

A second-order influence on the Pb isotoperatios is seen on Sumbawa, where the recentshoshonitic to high-K volcanoes have higher206Pb/204Pb and gamma 8/4 values than thesurrounding volcanoes or the 42 Ma low-Ksamples from this island.

Pb ISOTOPE ARRAYSThe clearest distinction between the differ-

ent geographic groups is obtained in a 208Pb/204Pb versus 207Pb/204Pb diagram (Fig. 3). Thenoncollision samples form an array be-tween Indian mid-oceanic-ridge basalt (I-MORB) and dredge sediments from the fore-arc, whereas those from the collision sectorvary between values for north Australian river

sediments (taken as a proxy for the local Aus-tralian upper crust), and the field for Indianoceanic-island basalt (I-OIB). This has beenthe basis for previous workers to model vol-canic rocks from the collision zone as mix-tures between subarc mantle with OIB-type Pbisotope characteristics and Australian uppercrust (Stolz et al., 1990; Hoogewerff et al.,1997; Elburg et al., 2002). However, inspec-tion of all available data for the Banda-Sundaarc shows that samples from the collision zoneare characterized by both low 3He/4He ratiosand high gamma 8/4 values, typical for I-OIBand Australian crust. Because the He isotopevalues have been explained by subduction ofthe leading edge of the Australian continent(Hilton et al., 1992), the Australian uppercrust is likely to represent the radiogenic endof the collisional Pb isotope array. It is, how-ever, difficult to argue that in the same sector

GEOLOGY, January 2004 43

Figure 3. 208Pb/204Pb vs. 207Pb/204Pb for (A) Sunda-Banda volcanoes compared to (B) Indianoceanic-island basalt (I-OIB), Indian mid-oceanic-ridge basalt (I-MORB), Christmas Island(Hart, 1988), north Australian sediment (Elburg et al., 2002), and dredge sediments (Vroonet al., 1995).

where upper-crustal material is introduced, thesubarc mantle wedge would have an OIB-likePb isotope signature as opposed to MORB-like characteristics outside the collision area.Low Nb/Zr ratios of front-arc collisional vol-canoes (Hoogewerff et al., 1997) also supportthe argument against involvement of a typicalOIB source (Elburg et al., 2002). It is there-fore more likely that the less radiogenic Pbend member of the collisional lavas is alsointroduced by the collision process and that itssource is located within the Australian litho-sphere. Compared to the Australian uppercrust, this end member has a similarly hightime-integrated Th/U ratio, but a lower U/Pbratio. The only plausible source for this com-ponent is the Australian lower crust, whichtypically has lower U/Pb ratios than the uppercrust (Rudnick and Goldstein, 1990). Pb iso-tope data of possible lower-crustal compo-nents in northwest Australia are not available,but those from granites formed by melting ofthe lower crust in western Australia (Bickle etal., 1989) show extreme Pb isotope heteroge-neity and encompass the unradiogenic end ofthe high gamma 8/4 array (not shown). As3He/4He ratios are the same for upper andlower crustal materials, the low 3He/4He ratiosin this segment are consistent with the in-volvement of two types of crustal materials.The suggested involvement of the Australiansubcontinental lithospheric mantle (Varne,1985), exemplified by northwest Australiankimberlites, is unlikely, as it does not have theappropriate Pb isotope composition (Nelson etal., 1986).

DISCUSSIONWe interpret the parallel spatial behavior of

Pb and He isotopes along the Sunda-Bandaarc as signifying the dominant importance ofthe subducted slab on these isotopic systems.Within the collision zone, the array of Pb iso-

topes reflects variation in the proportion of ra-diogenic Australian upper crust and less ra-diogenic Australian lower crust. The former ischaracterized by high 206Pb/204Pb and is dom-inant in the western and eastern parts. The lat-ter source with lower 206Pb/204Pb is dominantin the central collision zone. We propose thatthe increased contribution of lower crust in thecentral sector is a direct result of the collisionevent. In this area, the leading oceanic part ofthe slab may have started to detach from theAustralian continent (McCaffrey et al., 1985;Charlton, 1991), causing upwelling of hot as-thenospheric material and melting of the ex-posed continental domain. Detachment of theoceanic part of the slab from the continent hasbeen modeled to be the inevitable result ofarc-continent collision (Davies and vonBlanckenburg, 1995; Van de Zedde and Wor-tel, 2001). Upwelling of hot asthenospherethrough this slab window causes melting ofthe lower crust, whereas normal subductionof continental crust may at best only causemelting of upper-crustal material. The crustalmelts interact with the mantle, and partialmelting of this metasomatized mantle causesthe observed magmatism. The isotopic diver-sity in the collision zone is consistent with theinvolvement of melts from diverse small-scaledomains in the partially subducted continentalcrustal source.

Our interpretationthat the array of Pb iso-tope data from the collision area represents amixture of subducted components onlymayimply that the Pb isotope arrays seen outsidethis sector also reflect mixing between severalsubducted components, rather than mixing be-tween MORB-type mantle and subducted sed-iments, as proposed by other researchers, e.g.,Turner and Foden (2001). If so, the nonradi-ogenic Pb component of the samples that plotin between the fields for MORB and dredge

sediments is likely to be derived from the sub-ducted basalts of the oceanic slab, and the ra-diogenic component from subducted continen-tal material, equivalent to sediments from thefront of the arc (Vroon et al., 1995). The in-crease in 206Pb/204Pb ratios toward the colli-sion sector of the arc points toward theincreasing importance of subducted upper-crustal material with proximity to the locus ofcollision (cf. van Bergen et al., 1993).

The Quaternary high-K volcanism on Sum-bawa (Foden and Varne, 1980) does not fitinto the trend of unidirectionally increasinggamma 8/4 toward the collision zone, but hashigher gamma 8/4 than neighboring volcanicislands or older (42 Ma), low-K samplesfrom the same area. This isotopic signaturemust reflect the introduction of a differentsubducted component into the subarc environ-ment. Varne (1985) proposed that this newcomponent was derived from the island ofSumba, but tectonic reconstructions (Hall,2002; Rutherford et al., 2001) and paleomag-netic (Wensink, 1994) and Nd-Pb isotope data(Vroon et al., 1996) suggest that Sumba hasmigrated from a more northern position nearor on the Sundaland plate, rather than from asouthern, Australian position. It is thereforehighly unlikely that Sumba was ever part ofthe subducting Australian plate and couldhave contributed material to the source of thehigh-K volcanic rocks.

We propose that, instead of continental lith-osphere, the material that was introduced inthe Sumbawa subarc mantle was OIB-typeoceanic crust. The absence of a trace elementOIB-type signature in the high-K volcanicrocks (such as low Ba/Nb ratios; Varne, 1985)reflects the immobility of high field strengthelements like Nb and Zr during slab dehydra-tion (Stolz et al., 1996), whereas Pb is a highlymobile element. The Pb isotope data for theSumbawa Quaternary volcanoes overlap withthose of the nearest OIB-type crust of Christ-mas Island (Hart, 1988), and OIB-type crustis common in the Indian Ocean domain (Weisand Frey, 1996). It is therefore conceivablethat the Pb isotope array of the Sumbawahigh-K magmatism reflects mixing betweenIndian OIB-type crust and entrainedsediments.

CONCLUSIONSOur compilation of Pb data along the

Sunda-Banda arc shows that the samples fromthe arc-continent collision area have higher208Pb/204Pb ratios for their 207Pb/204Pb ratiosthan neighboring volcanoes. This isotopic sig-nature has previously been interpreted as mix-ing between a mantle component with Pb iso-tope ratios similar to I-OIB, and Australiansubducted continental material. We propose

44 GEOLOGY, January 2004

instead that the Pb isotope arrays show mixingbetween subducted materials only, which, inthe case of the collision zone, are the Austra-lian upper and lower crust. The main argu-ments for this new interpretation are: (1) Highgamma 8/4 values are geographically boundto the collision zone. (2) High gamma 8/4 val-ues correlate with He isotopes, which pointtoward involvement of continental crust. (3)Geophysical models predict influx of heat byasthenospheric upwelling induced by slab de-tachment. (4) There is no trace element evi-dence for the involvement of an OIB-typesource.

We therefore reinterpret the observed Pbisotope data arrays as reflecting mixing be-tween Australian upper and lower crust. Out-side the collision area, the Pb isotope arraysprobably reflect a mixture between subductedMORB-type crust and sediments, whereas thedata for the high-K volcanic rocks from Sum-bawa can be explained by mixing betweensubducted OIB-type crust and sediments.

ACKNOWLEDGMENTSElburg acknowledges support from an Australian

Research Council Postdoctoral Fellowship and aEuropean Union Marie Curie Fellowship. PieterVroon is thanked for providing unpublished data forBatur volcano and Wetar.

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Manuscript received 14 June 2003Revised manuscript received 10 September 2003Manuscript accepted 10 September 2003

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