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    7 M. Polvd e t a l . /Tec tono phys i cs 272 1997) 69-9 2

    Th e mos t striking tectonic implication of this magmatic evolution is that West Sulawesi can no longer be considered

    as a typical magmatic arc as previously assum ed. With the exception of the Manado area beneath w hich subduction is still

    active, calc-alkaline and island-arc tholeiitic lavas and plutonics are volumetrically m inor with respect to K-rich magm as.

    Their occurrence through time is also fairly restricted, mostly to the period between 30 and 15 Ma. Another important

    feature is the occurrence o f island-arc tholeiitic and calc-alkaline m agmas crosscutting an older terrane o f BAB affinity, the

    Tinom bo Form ation (M anado, Tolitoli and Palu areas). As this formation is being regarded as an equivalent to the C elebes

    Sea floor, the most likely explanation for this feature is the hypothesis of tectonic erosion linked to the NW-dipping

    subduction beneath the North Arm.

    Th e Late Miocene high-K m agmatic activity in Central and South Sulawesi reflects the prevalence of a post-collisional

    tectonic regime following the docking of microcontinents of Australian origin to Central Sulawesi during Neogene

    times. The incompatible element-enriched character of these high-K rocks might reflect their derivation from a mantle

    source enriched through metasomatism related to a previous subduction event. Such a model cannot account for the

    Plio-Pleistocene CA K magm atism o f Central Sulawesi, the acidic composition of which does not support a derivation from

    an ultrabasic source. The trace-element patterns of the CAK rocks are very similar to those o f the high-grade metamo rphics

    of Central Sulawesi, suggesting that the latter might represent their possible source. Such an anatectic model implies a

    collisional to po st-collisional tectonic regime limited to Central Su lawesi, while a post-subduction regime prevailed in the

    south.

    Keywords SE Asia; subdu ction; collision; magm atism; trace elements

    1 I n t r o d u c t i o n

    A c c o r d i n g t o g e o d y n a m i c r e c o n s t r u c t i o n s , t h e

    s o u th e a s t e r n e d g e o f t h e E u r a s i a n p l a t e , i . e . S u n -

    d a l a n d , h a s b e e n a r e g io n o f s u c c e s s iv e s u b d u c t i o n s ,

    c o l l i s i o n s a n d b a c k - a r c b a s in o p e n in g s d u r in g t h e

    C e n o z o i c ( R a n g in e t a l ., 1 9 9 0 a ,b ; D a ly e t a l ., 1 9 9 1 ) .

    T h e b e s t d o c u m e n t e d g e o d y n a m i c e v e n t s o c c u r r e d

    d u r i n g t h e N e o g e n e a n d w e r e l i n k e d t o t h e n o r t h -

    w a r d mig r a t i o n o f mic r o p l a t e s o f A u s t r a l i a n o r i g in .

    F o r i n s t a n c e , t h e B i r d s H e a d m ic r o c o n t i n e n t ( Ir i a n

    J a y a ) w a s d i s m e m b e r e d c a. 1 7 M a a g o a n d s o m e o f

    i t s fr a g m e n t s ( B u to n , S u l a ) c o l l i d e d w i th t h e S u n d a -

    l a n d ma r g in b e tw e e n 1 1 a n d 5 Ma ( S m i th a n d S il v e r ,

    1 9 9 1 ; H a l l , 1 9 9 6 ) . E a r l i e r g e o d y n a mic e v e n t s a r e

    o f t e n d i f f i c u l t t o r e c o n s t r u c t f r o m k in e m a t i c a n d t e c -

    t o n i c c o n s id e r a t i o n s d u e t o t h e g e o lo g i c a l c o mp le x -

    i t y a n d t h e h i g h r a te o f p l a te m o v e m e n t s . O n e w a y

    t o o v e r c o m e t h i s d i f f ic u l t y i s t o d o c u m e n t t h e g e o -

    c h e m ic a l a f fi n i t ie s a n d t h e p o s s ib l e g e o d y n a m ic s i g -

    n i f ic a n c e o f t h e c o p i o u s m a g m a t i c p r o d u c t s e r u p t e d

    a l o n g t h e S u n d a l a n d m a r g i n . T h e K - s h a p e d i s la n d o f

    S u l a w e s i ( e a s t e r n I n d o n e s i a ; F ig . 1 ) i s a c h a l l e n g in g

    t a r g e t f o r s u c h a n i n v e s t i g a t i o n , d u e t o i t s l o c a t i o n

    a t t h e j u n c t i o n o f t h e E u r a s i a n , I n d o - A u s t r a l i a n a n d

    Pac i f ic p la te s .

    T h e i s l a n d m a y b e s u b d i v i d e d i n t o t h r e e m a i n

    g e o lo g i c a l p r o v in c e s ( S u k a mto , 1 9 7 5 ; H a mi l t o n ,

    1 9 7 9 ) , n a me ly : ( 1 ) W e s t S u l a w e s i w h e r e p r o mi -

    n e n t m a g m a t i c r o c k s a n d T e r t ia r y s e d im e n t s o v e r l ie

    a h i g h -g r a d e m e t a m o r p h i c b a s e m e n t ; ( 2 ) C e n t ra l a n d

    S o u t h - E a s t S u l a w e s i m a d e u p m a i n l y o f E a r l y C r e -

    t a c e o u s me ta mo r p h i c r o c k s ; a n d ( 3 ) E a s t S u l a w e s i

    w h e r e a h u g e o p h i o l it i c n a p p e r e s t s o n M e s o z o i c a n d

    P a l a e o z o i c s e d i m e n t a r y r o c k s . T h e W e s t S u l a w e s i

    m a g m a t i c p r o v i n c e i n c l u d e s t h e S o u t h A r m o f S u -

    l a w e s i ( U ju n g P a n d a n g a r e a ) , t h e w e s t e r n p a r t o f

    C e n t r a l S u l a w e s i ( T o r a j a a n d P a lu a r e a s ) a n d f i n a l l y ,

    t h e N o r th A r m w h i c h g o e s f r o m P a l u to M a n a d o a n d

    in c lu d e s t h e T o l i t o li a n d M a n a d o a r e a s ( F ig . 1 ). W i th

    t h e e x c e p t i o n o f th e M a n a d o a c t iv e v o l c a n o e s ( J e z e k

    e t a l . , 1981; Mor r ice e t a l . , 1983; Mor r ice and Gi l l ,

    1 9 8 6 ; T a t s u mi e t a l . , 1 9 9 1 ) , t h e r e h a d b e e n n o e x -

    t e n s iv e g e o c h e m i c a l s t u d y o f t h e v a r io u s m a g m a t i c

    r o c k s u i t e s u n t i l r e c e n t l y ( Y u w o n o e t a l . , 1 9 8 8 a ;

    Pr iad i e t a l . , 1993 , 1994; Bergman e t a l . , 1996) .

    T h e b u l k o f W e s t S u l a w e s i h a s o f t e n b e e n c o n s i d -

    e r e d a s a c a l c - a lk a l i n e T e r t i a r y ma g m a t i c a r c ( K a t i l i,

    1 9 7 5 , 1 9 7 8 ; H a mi l t o n , 1 9 7 9 ) , a l t h o u g h l e u c i t e - b e a r -

    i n g u l t r a p o t a s s ic l a v as w e r e r e p o r t e d f r o m th e U ju n g

    P a n d a n g a r e a b y I d d i n g s a n d M o r l e y ( 1 9 1 5 ) . O u r

    n e w d a t a o n N o r t h e r n a n d C e n t r a l S u l a w e s i a l l o w

    u s t o c h a r a c t e r i z e t h e g e o c h e m ic a l a f f i n it i e s a n d t h e

    t ime d i s t r i b u t i o n o f t h e ma in ma g r n a t i c u n i t s a n d t o

    d i s c u s s t h e c o n s t r a i n t s t h e y i m p o s e o n g e o d y n a m i c

    r e c o n s t r u c t i o n s .

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    M. Polvd et al . / Tectonophysics 272 1997) 69 -92 71

    S H A K U K

    magmatism

    CAK magmatism

    I ~ C E L E B E SE

    P L

    CA magmatism

    IA T magmatism

    M N M a n a d o a r ea

    TT Tol i to li a r ea

    P L P a l u a r e a

    TJ Tana Tora j a

    U P U j u n g P a n d a n g a r e a

    U

    BAB magmatism

    MN.

    Gorontalo

    F i g . 1 . L o c a t i o n o f t h e m a i n m a __ gm a ti c t y p e s i n W e s t S u l a w e s i . T h e b o u n d a r i e s b e t w e e n t h e d i f f e r e n t a re a s a n d t h e m a j o r t e c t o n i c

    f e a t u r e s a c c o r d i n g to M o n n i e r e t a l . 1 9 9 5 ) a r e s h o w n a s i n s e t s .

    2 . A n a l y t i c a l m e t h o d s

    Th i r ty - s ix new s amples w ere da ted by the

    4K-4Ar method e i ther on whole- rock or on min-

    eral separates , a t the Labo rato ire de G6ochimie,

    G6achronologie , Univers i t6 de Bretagne Oeeiden-

    ta le , fo l low ing the procedure deta i led in Bel lon and

    Rangin (1991) . Age calcula t ions were carded out

    us ing the cons tants recommended by S te iger and

    Jliger (1977). Errors were calculated with the error

    equat ion descr ibed in Defant e t a l. (1992) w hich de -

    f ives f rom M ahoo d and Drake (1982) . These resul ts

    are presented in Tables 1-6 , together wi th 27 dates

    from the Palu and Toraja areas (Priadi et al . , 1993,

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    7

    M . P o l v E e t al . / T e c t o n o p h y s ic s 2 7 2 1 9 9 7 ) 6 9 - 9 2

    o

    < I

    o ~

    ;

    5 0 M a .

    5 . 1 . P re se n t - d a y t e c to n i c r e g im e o f S u la w e s i

    Sulawes i i s sur rounded by Ter t iary marginal

    bas in s Ce lebes S ea bas in o f M idd le Eocene age ,

    M akas s a r bas in , N or th B anda bas in o f La te M iocene

    age) w h ich a r e be ing cons um ed a long s mal l tr enches

    Hamil ton , 1979) . The only act ive volcanic mani-

    fes ta t ions on Sulawes i are located in the Manado

    area and represent the southern terminat ion of the

    Sang ihe arc Tatsumi et al ., 1991). This magm a-

    t ism is c lear ly l inked to the NW-dipping subduet ion

    zone re la ted to the la tes t s tage of subduct ion of the

    M o lueca S ea S i lver and M oore , 1978 ; Cardw ell e t

    a l . , 1980) . In the Manado area , the SE-NW polar i ty

    o f m agmat i s m i s ind ica ted by the inc reas e o f incom -

    pat ib le e lements and the more radiogenic Sr and Pb

    isotopic s ignature away f rom the trench Tatsumi e t

    al., 1991).

    The southward subduct ion zone beneath the Nor th

    An n o f Sulawes i d ies out eas twards S i lver e t a l .,

    1983) and involves the wes tern par t of the Celebes

    S ea bas in w here the magne t i c anomal ie s s how an

    asymmetr ic pat tern 0Veissel , 1980) . The act ive vol-

    cano of Una-Una, in the Gorontalo Gulf , i s the only

    potent ia l product of th is subduct ion , a l though i t i s

    located 250 km awa y f rom the t rench.

    Other par ts of Sulawes i are tec tonical ly linked by

    strike-slip faults Fig. 1, inset) which exten d east-

    ward in the Nor th Banda Sea Rth anl t e t a l. , 1991).

    The Nor th Banda Sea is a marginal pul l -apar t bas in

    which s tar ted i t s opening in a t rans tens ional envi-

    ronment dur ing the Late Miocene and is present ly

    consumed along the Tolo t rench, below Eas tern Su-

    lawes i Rtha ul t e t al ., 1994) . The B one Gulf , a l-

    though present ly poor ly known, might be a s imilar

    example of a young pul l -apar t bas in . To the wes t ,

    Sulawes i i s separated f rom Borneo by the Makassar

    bas in w hich is cur rent ly in terpreted e i ther as a fore-

    land bas in Bergman et a l. , 1996) or a par t of the

    Ce lebes Sea marginal basin Hall , 1996).

    5 . 2. 0 - -1 5 M a : t h e c o l l i s i o n a n d i t s m a g m a t i c e f f e c t s

    Centra l Sulawes i was the s i te of docking of Aus-

    tralian microblocks with the Eurasian active margin.

    F irs t impact of col l is ion can be t raced back to 23

    Ma ago f rom geodynamic recons truct ions and cor-

    responds to the onset of a compress ional regime in

    the Southw es t Pacif ic marginal bas ins . As d iscussed

    be low , the younges t CA magm at ic ac t iv i ty s topped

    at 18 M a in the Ujun g Pandang area Yuwo no et

    al. , 1988a) and even earlier in Central Sulawesi, in

    f ront of the apex o f the col l is ion zone, wh ereas in the

    Nor th Arm, IAT/CA magrnat ism cont inued f rom the

    Mio--Pliocene up to the p resent day.

    In the Toraja and Ujung Pandang areas , ma gma tic

    ac t iv i ty re s umed a t 13 M a w i th the emplaceme n t o f

    h igh-K magnms ranging f rom shoshoni tes to u l t ra-

    potass ic basani tes. In Centra l Sulaw es i Toraja and

    Palu areas ) th is event was very shor t 13-11 M a)

    al though impor tant volumes of lavas and in t rus ive

    rocks were emplaced. In South Sulawes i the h igh-K

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    88 M. Po lvg et al. /Tectonophysics 272 1997) 69-92

    magmatic act iv i ty cont inued unt i l the Quaternary

    Lompobatang Volcano) . This h igh-K magmatism

    shows obvious s imilar it ies wi th the Rom an M ag-

    ma tic Provin ce Pecc eril lo et al ., 1984; Pecceril lo,

    1985; Be cca luva et al ., 1991). Yuw ono et al. 1988a)

    ascr ibed the South Sulawes i magmatism to mel t ing

    in a within-plate dis tensional regime of a mantle

    wedge which was metasomat ized dur ing a previous

    subduct ion event . Our new t race e lements and age

    data allow us to extend this hypothesis to Central Su-

    lawes i where there is no typical IAT/CA magm atism

    pos tdat ing the beg inning o f col l is ion .

    CA K magmat i s m i s w e l l know n to be d i r ec t ly

    re la ted to act ive subduct ion . I t i s a lso documented

    in collis ion zones although both its origin either

    mantle-related with crustal contamination or crustal

    anatexis) and its precis e geo dyn am ic contex t either

    syn- or pos t -col l ision) are of ten debated Pearce e t

    al., 1990; Harris et al. , 1990). In Sulaw esi, petrologic

    and geoch emical arguments sugges t that the young

    6 .5 -0 .6 M a) CA K m agmat i sm f rom the Tora ja and

    Palu areas wa s produced b y the pos t -col l is ional mel t -

    ing of continen tal crust Priadi et al ., 1993, 1994).

    The pos tu la ted th ickening of cont inenta l crus t i s

    sugges ted by the upl i f t of Wes t Sulawes i and the

    exposure, along the Palu graben, of granulite facies

    rocks garnet peridotites , pyroxe nites , metab asalts) ,

    displaying local anatectic features. Other high-grade

    metamorphic rocks have been mapped in the Tol i -

    to l i area Ratman, 1976) . M os t of these rocks have

    REE and multi-element patterns s tr ikingly s imilar to

    those of the CA K group. For th is reason and a lso

    bec ause of i ts acidic, Al-rich character, Priadi 1993)

    proposed that the CAK magmatism resul ted f rom

    pos t-col l is ional mel t ing o f h igh-grade metam orphic

    rocks . But th is m odel does no t explain why t race-ele-

    men t s ignatures are so s imilar for CA K and h igh-K

    magmatic rocks, which, as already mentioned, dis-

    p lay evidenc e of a m ant le- re la ted or igin . In con tras t

    wi th th is model , Coff ie ld e t a l . 1993) and Bergman

    et a l . 1996) argued that both CAK and h igh-K mag-

    matism originated in a collis ion setting involving

    micro plates of Au stralian origin.

    The most l ikely explanation for the crustal thick-

    ening beneath Central Sulawesi is the underthrusting

    of buoyan t continental crust. Althoug h the collis ion

    w as s ea led by M iocene mo las s e in Eas t S u law es i ,

    there is some ev idence that the convergence process

    betw een Sula and Sulawe si is s til l continuing, induc-

    ing a shor tening with in the microblock s in the Banda

    Sea along the Tolo thrust R6hault et al . , 1994).

    Howev er , the precise meaning of the in it ia t ion of

    CAK magmatic act iv i ty a t 6 .5 MA with respect to

    the recent collis ion events is s til l uncertain. I t could

    correlate with a crustal thickening even t linked to the

    docking of the Sula b lock a t ca . 5 Ma Hal l , 1996) .

    Al ternat ively i t could resul t from decom press ion and

    melt ing fo l lowing the previous crus ta l th ickening

    event l inked to the docking of the Buton-Tukang

    Bes i microblock a t 11 M a Hal l , 1996).

    In contras t wi th the o ther par ts of Wes tern Su-

    lawes i , subduct ion-re la ted magmatism appears to be

    cont inuous a long the Nor th Arm for the las t 15 Ma

    Table 1). In the Tolitoli and Manado areas, IAT/CA

    magmas a r e documen ted a t ca . 22 -14 M a , w hereas

    CA act iv i ty resume d at 8.8 Ma Tol ito li area) and

    4.3 Ma Man ado area), respect ively , and is wel l doc-

    umen ted f rom ca. 4 Ma Pinogu volcanics ) to the

    present Mana do act ive volcanoes) . In o ther words ,

    there seems to be an eas tward migrat ion o f th is ma g-

    mat ism with t ime, f rom the centra l par t of the Nor th

    Arm Marisa--Gorontalo area) to i ts eastern termi-

    nation where calc-alkaline volcanoes are s ti l l active.

    Volcanic rocks re la ted to th is m agm atism dated a t ca .

    4 Ma in the Gorontalo area were ro ta ted c lockwise

    20, as are the ca . 30 Ma volcanic rocks in Bi lun-

    gala Surm ont et al ., 1994). This imp lies that the

    subduction causing this rotation is younger than 4

    Ma and thus cannot be respons ib le for the emplace-

    men t o f the La te M iocene-P l iocene CA m agmat i sm.

    The m ajor par t of th is 15-4 M a subduct ion-re la ted

    magmatism might be associa ted wi th nor th-d ipping

    subduction, which is progress ively young er towards

    the east.

    5 .3 . 1 5 - 5 0 M a : t h e p r o m i n e n t s u b d u c t i o n r e g i m e

    CA magmat i s m o lde r than 15 M A i s p robab ly

    rela ted to the general ly accepted subduct ion of the

    Indian Oc ean p la te below the Sundaland. Our data

    r evea l on ly l imi ted occu r r ences o f CA magmat i s m

    younger than 22 Ma. This may be explained by

    trench-to-trench transform faulting or very obliq ue

    subduct ion) a long South and Centra l Sulawes i Ran-

    gin et al., 1990b). Alternatively, tect oni c ero sion e.g.

    Von Huene and Lal lemand, 1990; Lal lemand et a l . ,

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    M. Polv et al . /Tectonophysics 272 (1997) 69- 92 9

    1992) could have ca used the arc in Centra l and South

    Sulawes i to re t reat wes twards . Consequent ly major

    par ts of the Palaeogene volcanic arc would have

    been co nsum ed through the progress ive tectonic ero-

    s ion process of the f ront of the upper p la te , leading

    the volcanic arc to re t reat over i t s former back arc

    bas in . Some suppor t for th is model is provided by

    the p r esence o f BA B bas a l ts T inombo F ormat ion ) in

    mos t areas where they are in t ruded and/or over la in

    by younger IA T o r CA magm as .

    5.4. Th e Paleo cene subduc t ion event

    The o ldes t CA magmat i s m r eco rded in S u law es i

    is present in the Bua Format ion Ujung Pandang

    area) where andes i tes have been dated as ca . 60

    M a Van Lee uwe n, 1981; Yuwo no et a l ., 1988a; Ta-

    ble 5) . They are younger than the CA lavas f rom

    the Cretaceous Al ino and Manunggul format ions in

    the Me ratus Range , southeas t Kal imantan Yuwono

    et al. , 1988b), bu t a Pale oce ne age 59.1-4-3.0 M a)

    was obta ined on a basal t ic dyke crosscut t ing the

    Manunggul lava p i le in neighbour ing Pulau Laut is -

    land Pr iyomarsono, 1985) . Fragments of CA lavas

    o f L a te Cre taceous --Eocene ages w ere a l s o found in

    the melange- typ e format ions of Karansambung, C en-

    t ra l Java Suparka and Soer ia-Atmadja , 1991). These

    scat tered occurrences indicate a subduct ion-re la ted

    mag matism al l a long the southeas tern margin of the

    Euras ian cont inent dur ing the Paleocene.

    5.5 . En igm at ic pre Ter t iary basal t s

    BA B bas a l t s and gabbros o f p r es umed M es ozo ic

    crys ta l l isa t ion ages outcrop in the L amas i Form at ion

    of Centra l Sulawes i see a lso Bergm an et al ., 1996) .

    The Kalam iseng Forma t ion South Sulawes i) basal ts

    d isp lay s imilar chem ical aff in i ties and their Mioc ene

    K -A t ages m ay r ep resen t t ec ton ic emplacem en t ages

    Yuw ono et al ., 1988a) rather than crystallization

    ages . In both cases the contacts wi th the o ther geo-

    logical uni ts of W es t Sulawes i are e i ther tec tonic or

    hidden.

    The t ec ton ic f r amew ork o f the fo rmat ion o f the

    Lamas i and Kalamiseng basal ts remains h ighly con-

    jectural, given the uncertainties on their crystaUisa-

    t ion ages . They might represent f ragments f rom the

    upper volcanic uni t of the Eas t Sulawes i Ophiol i t ic

    Napp e which is thrus t on to the wes tern arm, as sug-

    ges ted by Yuwono et a l . 1988a) and Bergrnan e t

    a l . 1996) . Ba sal ts d isp laying negat ive Nb and Ta

    anomalies are a lso repor ted in the Eas t Sulawes i

    Oph iolite Mo nnier et al ., 1994; M onn ier et al .,

    1995) . How ever the crys ta l l isa t ion age o f amph ibole

    f rom gabbros belonging to th is ophiol i t ic sequence

    is Middle Eocen e Mon nier e t a l ., 1994) and thus ap-

    pears to be incom patible with this interpretation. Al-

    ternat ively the Lamas i and Kalamiseng Format ions

    might cor respond to f ragments o f back-arc crus t cre-

    a ted during the M esozoic d is locat ion of As ian b locks

    of Gondw ana or ig in. Indeed, carbonate p la t form sed-

    iments of pos tu la ted Jurass ic and Cretaceous ages

    in Sulawes i have been in terpreted as par ts of the

    Australian margin r if ted during the Jurassic Aud -

    ley-Char les e t a l ., 1988) . Som e of these carbonate

    depos i ts have been dated as Late Tr iass ic Com 6e

    et al ., 1994). Th ey are overlain by Jurassic tu rbiditic

    l imes tones and radio lari tes , the occurrence o f which

    sugges ts a rapid subs idence dur ing Ear ly and M iddle

    Jurassic t imes.

    6 Co n c l u s i o n

    The o ldes t ev idence o f magm at ic ac t iv i ty in W es t

    Sulawes i i s present in the Lamas i Format ion but the

    or ig in of these BA B basal ts i s s t il l unknown.

    The o ldes t a r c magmat i s m r eco rded in S ou th

    Sulawes i i s ca . 60 Ma old and might be re la ted

    to the same subduct ion that lef t remnants in Java

    Karangsambung) and Eas t Kal imantan Pulau Laut

    and Meratus, among others) .

    The m ain occurrences of typical IAT and CA sub-

    duct ion-re la ted magm atism are l imi ted both in space

    and time rough ly 30 -14 M a) pos s ib ly becaus e o f

    tectonic eros ion that consum ed the o ld CA m agrnat ic

    arc and bui l t up the recent one over the o ld BAB

    mater ia l Tinom bo Format ion) .

    The only wel l represented CA act iv i ty began 15

    Ma ago in the Nor th Arm and is s t i l l ac t ive in

    the Manado area . I t i s re la ted to the nor th-d ipping

    subduct ion which migrated eas tward through t ime

    from the Toli to l i area to the Mana do a rea and then to

    the Sangihe arc.

    While the Nor th Arm was involved in th is nor th-

    d ipping subduct ion , Centra l Sulawes i magmatism

    began to record the ef fects of col l is ions involving

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    90 M. Po lv~ e t a l . / Tec tonophys ics 272 1997) 69 -92

    m icroblo cks of Australian origin (e.g. the Sula and

    Buton-Tukang Bes i b locks) , which were in i t ia ted

    dur ing the Miocene. I t produced h igh-K magmas in

    South Sulaw es i and, more recently , cop ious amounts

    of acid ic CAK products in Centra l Sulawes i . The

    latter could be related to crustal thickening and sub-

    sequent mel t ing dur ing decom press ion .

    Sulawes i can no longer be cons idered as a typi-

    cal i s land arc : subdu ct ion-re la ted magm atism is not

    common enough. But this area il lustrates the con-

    sequ ence s of collis ion and crustal thickening on

    magma genes is . Coupl ing between tectonic obser -

    vat ions and g eochem ical character is t ics ra ises ques-

    t ions about the respect ive meanings of ' tec tonic age '

    of the col l is ion and i ts 'geochemical age ' as the

    t ime-span betw een the m is not fu l ly unders tood.

    cknowledgements

    The authors thank J .C. Phi l ippet and Dr J . Cot ten

    for their analytical contribution to this s tudy. Field

    w ork w as s upported by the IN S U -M A E jo in t p ro -

    g ram P ICS 46 and the F rench - Indones ian ( IN S U -

    DGGMR) agreement . B.P . ' s thes is grant was pro-

    vided by the French Minis t ry of Foreign Affai rs .

    Labora to ry w ork has been s uppor ted by a CN RS -

    DBT grant on CAK magmatism. This paper ben-

    ef i ted great ly f rom informal d iscuss ions wi th in the

    PICS group and especia l ly f rom Dr J . Malod ' s syn-

    thet ic v iews . Rev iews by Drs R . Hal l e t B.C. Burch-

    field help ed greatly to improve the manuscript. R.

    Hal l i s par t icu lar ly thanked for h is com men ts on the

    tectonic h istory and t iming o f events in Sulawes i .

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