Pompangeo Schist Complex of Central Sulawesi, Chris Parkinson

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rtu rclina Arc (1998) 7,nL-245

Thematic Article

An outline ol the petrology, structure and age ol the Pompangeo SchistGomplex ol central Sulawesi, lndonesia

Cr*,rs PamrNsoN

Departmcnt of Ear-th and, Ptanetaru Scienc";;;?!r;J!;!i:f of rechnotogv, ootaavama 2-12-1,

lnmrr6 fVuri"Uty dismembered and metamorphosed accretionary complexes constitute'the basehent of mueh of the Indonesian island of Sulawesi. The most extensive of these

is the Pompangeo Schist Complex, which crops out over -5000 lmr2 in central Sulawesi,

and is predominantly composed of interbanded phyllitic marble, calcareous phyllite'graphitic schist and quartnle; rocks of terrigenous to shallow marine origin. Along the

"r.i"* mariin of the complex, schists are interthrust with unmetamorphosed Jurassic

sandstone, *fiictr may represent parental material of the complex. The schists are un-"

conformably overlainiy pelagic s6diments with an Albian-Cenomanian biostratigraphy'lf,surmetamorphic progressive deformatio:r of the Pompangeo Schist Complex has re-*.rrlt"d

in repeated'iso-elinal folding and a strong transposition foliation striking north-northwest/south-southeast and dipping west, subparallel to the compositional banding

of the complex; microstmctural fabrics indicate a top-to-east sense of shear. On a re-gional scale the Pompangeo Schist Complex is lithostratigraphically coherent and an

east-to-west metamorphic field gradient is recognizable, which, if continuous, repre-sents a relatively low ihermal gradient of - 15 "C/lsn. K-Ar dating yielded ages of ca

111 Ma. Correlative metamorphic roeks appear to underlie the entire Neogene mag-

matic province, since they occur sporadically throughout western Sulawesi, ineluding

the Bantimala region of the South Arm. The Pompangeo schist metamorphism cannot

be eorrelated with arc magmatism in western Sulawesi, which is of Neogene age. The

Pompangeo and Bantimala sehists, as well as other aceretionary complexes in westernSulawesi, were probably generated in the same subduction system that was responsiblefor the extensive Mesozoic eontinental arc in central Kalimantan, at the eastern marginof Sundaland.f

Key words: blueschist, Cretaceous, Indonesia, Pompangeo Schist Complex,Sundaland.

Sulawesi, O-J

rl!NTR0DUCTI0N

To metamorphic petrologists and plate tecton-icians the island of Sulawesi (formerly Celebes)occupies a prominent position in the panoply ofcircum-Pacific high-pressure metamorphic belts.The recognition that the minerals jadeite (de

Roever 1955) and ferroearpholite (de Roever1951) are important indieators of high-pressuremetamorphism was first described in rocks fromcentral Sulawesi. More significantly, and in

Accepted for publication August 1997.

common with the Sanbagawa-Ryoke system ofJapan, the juxtaposition of an inner plutono-vol-canic ate associated with high-temperaturemetamorphie rocks against an outer belt ofblueschists has for many years been cited as at;pe example of a paired metamorphic belt sys-tem (Miyashiro 1961, 1973). Thus, the rocks ofSulawesi have, in a broad sense, been imporbantin formulating models of subduction zone dy-namics and metamorphism. Our knowledge ofthese rocks has not, however, been commensu-rate with the importance afforded them. ]

r"

232 C. Pa,rldrlson

fThe eartiest investigations of the rocks ofcentral Sulawesi were conducted by Dutch ge-ologists and explorers before World War II(entertainingly summarized in Rutten (1927), andby van Bemmelen (1949)). These studies sug-gested that the metamorphic rocks of the islanddisplay a broadly bipartite distribution. A vari-ety of staurolite and sillimanite + andalusite+ cordierite-bearing amphibolites crop outthroughout western central Sulawesi (Egeler1947), where they are generally intimately as-sociated with granodioritie and monzonitic bod-ies. By contrast, Brouwer (1947) noted thatglaucophane-bearing'crystalline schists' consti-tute the metamorphic basement in central andsoutheast Sulawesi, where they are associatedwith massive ultramafic bodies. Comparison with

the Sanbagawa-Ryoke paired metamorphic beltsystem led some geologists to infer the presenceof a clear structural demarcation, analogous tothe Median Tectonic Line (MTL), separating thetwo belts. Accordingly, the authors of manypaleogeographie reconstructions of the centralIndonesian region (Audley-Charles 1974; Su-kamto 1975; Hamilton 1979; Katili 1989) implythat the contrasting metamorphic belts of Sul-awesi were generated and juxtaposed in a single,simple west-dipping subduction zone- Evidencefor a direct causal relationship between the twobelts. however. has never been more than cir-cumstantial.l

[ttre apparent simplicity of the configuration ofmetamorphic rocks in Sulawesi has been chal-lenged by recent advances in our knowledge of

Fig. 1 Simplified distribution ollithotectonic units in Sulawesi.

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their exaet distribution and nature (Fig. 1). Theextension of directly eorrelative glaucophane-bearing and other sehists into the Southeast Arrnand the island of Kabaena was recoglized andreported by de Roever (1950, 1953). Graphite-bearing schists ur- the Rumbia Mountains in theSoutheast Arm (Fig. 1) have been the subject ofrecent petrological study by Helmers et al-(1939). In easterrt central Sulawesi, Pompangeosehists are overthrust by a serpentinite-matrixm6lange eontaining blocks of amphibolite, meta-gabbro, jadeite-bearing metamafite and meta-cherb with Oligoeene metamorphic ages(Parkinson 1996)- Furthermore, glaueophane-bearing rocks are not restricted to eastern andcentral Sulawesi but occur, with amphibolite andeclogite, in two small tectonic windows in theBarm and Bantimala areas of the South Arm,where they are interealated with slabs of ultra-mafic rocks and broken formations (togetherlcnown as the Bantimala Complex)- The field re-lations, petrolog5r and age of these rocks havebeen reported by Wakita et aL- (1996) andIVliyazaki et aL. (19%). Glaucophane- and lawsenite-.bearing metabasites, schists and marblesalso crop out in the Latimodjong Mountains(Gisolf 19L?; C, Parkinson unpubl. data, 1993),

where they constitute part of the LatimodjongComplex. Rocks of this complex are apparentlydistributed throughout western central Sulawesi(Simandjuntak et al. 1991a, 1991b). Granulite,garnet peridotite and eclogite of uncertain age

-ind origin crop out in the floor of the Palu-Koto @Farilt valley, south of Palu (Helmers et al.1990; vA. Kadarusman pers- eornm-, 1996). ] j

F

ITHE P0MPANGE0 SCHIST G0MPLEX

Most of the major mountain ranges and high-lands with elevations exceeding 1000 m in cen-tral Sulawesi are composed of rocks of thePompangeo Schist Complex. Sehists crop out in aseries of north=south-trending ranges, whichcomprise, from east to west: the Tokorondo,Koro IJe, K"oyt, Wanaripalu,. and PompangeoMountains,(Fig. 2). Large areas of these moun-tains are cbvered by impenetrable jungle, tropi-cal rainforest or eloudforest; this factor, alliedwith limited aecessibility and poor quality ofexposure resulting from pervasive tropicalweathering, severely hampers any detailed studyof the Pompangeo sehist's. Furthermore, the na-ture of the predominating protolithologies

Fig. 2 Highly simplified geological map of central Sulawesi.

Numerals represent localities mentioned in the text. 1, S. Saatu,

northern Tokorondo Mountains; 2, S. Malei, northern Pompangeo

Mountains; 3, S. Bombalo, northern Pompangeo Mountains; 4,

S. Tomasa and S. Tompakoe, eastern margin of Poso depression;

5, Laa basin; 6, Iowi Mountains.

(limestone and quartz-rich clastic sediments) andthe small grain size of their recrystallized prod-ucts ensure that the schists reveal little infor-mation about metamorphic conditions. Theevident structural and lithologieal complexityhinders interpretation of the fragmentary dataavailable]llA.ccordingly, here I present no morethan a simple overview of the petrology andstructure of the Pompangeo Schist Complexdominantly derived from a 50 km traverse acrossthe strike of the complex, in the Tokorondo,Kruyb and Wanaripalu Mountains (Fig. 3), butalso including data from schists exposed in theS. Saatu (5., sungai (river)) of the northernTokorondo Mountains, S. Malei and S. Bdmbaloof the Pompangeo Mountains, the Majumba areaof the Laa basin and the S. Tomasa and

g"

234 C. Parhinson

Fig. 3 Mineral assemblages in samples and representative field data for the Pompangeo Schist Complex along an -50 km traverse in the

Lake Poso region of central Sulawesi. For location see Fig. 2.

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S. Tompakoe of the Poso depression. Previouswork on the schists is restricted to the recon-naissance expeditions of the Dutch geologist H.A. Brouwer and his coworkers (Brouwer 1934,1947) and the petrographic investigations ofmaterial collected during these campaigns(Willems 1937; de Roever 1947). The GeologicalResearch and Development Centre in Bandunghas recently produced geological maps of centralSulawesi on a scale of 1:250 000 (Poso Sheet;

rocks which constitute the base of the East Sul-awesi Ophiolite. Overthmsting and emplacementof the ophiolite on to the Pompangeo SchistComplex occurred in the Oligocene (Parkinson1998). Inthe Pompangeo Mountains (S. Malei andS. Bombalo), ophiolitic tectonic m6lange occursas small klippen atop highly deformed rocks ofthe Pompangeo Schist Complex.

Throughout northern central Sulawesi, andespecially in the Poso depression, the Pomp-

1villag€y'kampung iunsealsd road/tmcl {fault \s*earn/riwr iS. = sqai . .iva) |

ffiple l,ocation

Sr foliatis52 foliationl,ll linealionFl folds

F2 fdds

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Simandjuntak et al. t99La; Malili Sheet; Sim-r.-. angeo Schist Complex is directly overlain byandjuntak et at. t}9Lb)l - W Mio-Pliocene clastic sediments of the Puna For-

--l\ mation and Plio-Pleistoeene reef limestones of

[nre rorunr GE'L'G' ffifJ:iflffffi'i!"J unconrormably overlie theAt the western margin in the Tokorondo Moun-tains, the Pompangeo Schist Complex over-thmsts the Kambuno granodiorite of the westernSulawesi magmatic province. Since the intrusiverocks are of Mio-Pliocene age (Simandjuntaket al.199l), overthrusting must have occurred nolater than ca 10 Ma. Rocks at the eastern ex-tremity of the Pompangeo Schist Complex areunderthrust beneath an extensive nappe ofvariably disrupted and metamorphosed ophiolitic

Pompangeo Schist Complex are the unmeta-morphosed but severely teetonized bituminouslimestones, calcilutites and shales of the MatanoBroken Formation in eastern central Sulawesi(Koolhoven 1930;Sukamto & Simandjuntak 1983;

Parkinson 1991). The depositional ages of theserocks delimits the timing of exhumation of theschists. Heteroheli.n and Globotru'ncan,L havebeen reported (Sukamto & Simandjuntak 1983),

and preliminary biostratigraphic analysis of

planktonic foraminifera in tectonized limestonesfrom the Kolonodale region has indicated Albian

Pompangeo Schist Complex, Indonesi,a 235

meters. East of Lake Poso the complexis characterized, in descending order ofabundance,

and Campanian to Maastrichtian ages (Corne6 by variably micaceous and graphitic marble, cal-

et aL. 1994). As well as pelagic carbonates, ra- careous phyllite, quartz-mica schist, phyllite, and

diolarian cherts and manganese nodules are rel- metaeonglomerate with thin (generally < 10 m)

atively common in the Matano Formation, and discontinuous metabasic intmsions and meta-radiolarian assemblages in cherts from the Sor- tuffaceous horizons. Lithological variations, espe-

oako and Tentena regions indicate an Albian- cially those involving phyllite, calcareous phylliteCenomanian age (Silver et at. 1983; D. L. Jones and marble, are sometimes gradational; in mostpers. comm., 1989). Significantly, a similar field eases, however, disparate lithologies are juxta-

disposition of petagic sediments atop a schistose posed across west-dipping thmsts. The relativebasement has-also been recognized in the Ban- proportions of lithologies west of the lake istimala area of the South Arm (Haile et aI. 1979), ^ slightly different: gpphitic marble and calcareous

where radiolarian biostratigraphy also yields_an (4/ phyl[te are repeatedly interthrust with quart'z-

Albian-Cenomanian age (Wakita et aI.7OSe\f, [ mica schist, quartzo-feldlpathic schist and gra-- ) phitic quartzite (Fig.4)J[MetaserpenLinite and

r metabasite are interfolded and intercalated withLLITHOLOGICAL

CHARACTERISTTCS quartzite in the western extremity of the complex.

The Pompangeo Schist Complex consists of a di- In general,lithological gradations and boundaries

verse range of lithotogius generally interlayered are orientated subparallel to the dominant north-on a scale ranging foom centimeters to tens of northwest/south-southeaststructuralgrainof the

dffigffietb{rtite

drlqikid€rphdb

albiteoligtrl*e

titanitedle

Caffiphi*

ertdG

picng{!eg6phibkaoiinite

High-grade Low-grade

Fig. 4 Generalized geological map of the Lake Poso region showing lithological variations and metamorphic field gradient in the Pompangeo

Schist Complex.

F"

236 C. Parkinson

" Poles to Sr foliation (n = 115)

" Lr mineral stretching lineations (n = 19)

* Fr fold hinges (n= 17)

complex (Fig.5). Thus, prognessively higherstmetural levels are encountered upon movingeastwards and, ignoring interlayering, the grosssequence of predominatinglithologies, from low tohigh structural levels (i.e. from east to west acrossthe complex) comprises: weakly recrystallizedphyllite and metaclastic ---+ semipelitic sehist -'metaconglomerate ---+ variably micaceous andgraphitic marble - q'uartzite ---+ metase{pentini-tg * metabasite. JLMaterial parental to the Pompangeo Schist

Complex presumably includes variably argilla-ceous limestone, tuff, grit, sandstone and con-glomerate: rocks of a shaliow marine orcontinental margin origin. The abundance ofsmall basic igneous intrusions (particularlywithin the quartzite) and serpentinitic interca-lations increases westwards. Significantly, roeks

not identified in the complex.]

F

LSTRUCTURAL SWLES

The Pompangeo Schist Complex has suffered aprotraeted and complicated polyphase defor-mational history, and it is often difficult to dif-ferentiate discrete episodes. The earliest andmost pervasive phase is a strongly developedwest-dipping foliation, orientated subparallel tothe compositional layering, and readily apparenton satellite and aerial photographic images as

strongly developed parallel lineaments whichcorrespond to ridges eroded along planes of

of a clear ophiolitic association or oceanic origin, fr)such as pillow basalt and pelagic sediment, were )/

Fig. 5 Stereo plots ot variousstructural features of the Pomp-angeo Schist Complex.

compositional layering (Fig. 6). Lineaments areparticularly well developed in the Pompangeoand Kruy't Mountains, where they are consis-tently orientated north-northwest-south-south-east (N330-340"E). On the mesoscopic scale (inthe field), foliation swfaces are defined by thepreferred orientation of mica, graphite, chloriteand occasionally, amphiboleJfn finer grainedgraphitic and micaceous schists the foliation isnot strictly planar but deviates around ubiqui-tous as;rmmetric qtartz phacoids (1-10 cmacross) and rootless, intrafolial isoclinal folds(Fr). The latter are generally defined try thinqtrafiz or mica,/graphite laminae, having ampli-tudes of < 10 cm and, where sufficiently wellexposed, can be seen to be asymmetric. Thelimbs of the folds are tmncated at very low an-gles by mica- and graphite-rich bands. In theWanaripalu and Kruyl Mountains the foliationencloses at least two generations of disharmonicisoclinal folds, which interfere to produce Type-Sinterference patterns (Fig. 7b). The presence ofthese folds and the layer-parallel orientation ofthe foliation indicates that the foliation is theproduct of repeated isoclinal folding and trans-position, and as such should not be considered as

strictly an Sr surface. This surfaee is designatedSr, the transposition foliation. Fold styles andasymmetries suggest that the early stages ofprogressive deformation (Fr folding, transposi-tion) resulted from non-coaxial deformation.][fft" Sr foliation of the schists east of Lake

Poso displays a relatively consistent north-northeast-south-southwest strike, but dips can

" Poles to 52 foliation {n=14)+ Fz fold hinges (n = 30)

e F:fold hinges (n = 10)

Fig. 6 nerial pnotogr;r'**. oi a part of the PompangecMountains northeast of Tentena. Strong north-northwest-south-southeast orientated lineations (dashed lines) corespond to ridgesand valleys eroded along compositional layering. Ridges are trun-cated by easl-dipping low-angle thrusts (Dz).

vary considerably over small distances (tens ofmeters), espeeially in the Kruyt Mountainswhere the schists have been strongly refoldedduring later deformational episodes. West ofLake Poso, however, the Sr: foliation strikesnorth-northwest-south-southeast, and dips arepredominantly to the west. Locally, the 31 foli-ation assumes a mylonitic character, where sig-moidal quarLz shear pods (often surrounded byanastomosing mylonitic zones), asymmetricmineral augen (quartz, calcite, feldspar), S-Cfabrics and displaced broken grains invariablyindicate a top-to-east sense of shear. In less in-tensely deformed rocks a strong stretchinglineation (MLr) is developed parallel to F1 in-trafolial fold axes (i.e. generally north-north-west-south-southeast). The lineation lies in theplane of the S.1 foliation and in most cases givesthese rocks a strong lineated-sehistose (L-S)tectonite fabric. The lineation is usually definedby small rods of quartz or calcite;in some schistson the eastern and western shores of Lake Posothe stretching lineation is defined by microbou-dinaged needles of crossite (Fig. 8).1

[A second discrete phase of deformation, D2, isrestricted to the schists east ofLake Poso, and is

Pompungeo Schist Compler, Ivtdonesia 237

eharadertzed by coaxial norlh-south to north-northeast-south-southwest-trending folds (F),which deform the Sa foliation. These folds areasymmetrie, tight-isoclinal, recumbent or gentlyinclined and generally have wavelengths of a fewmeters or more. Axial planes dip at quite shallowangles (5-30") to the east, and all have vergencesto the west (Fig. 7a). An axial planar cleavage(Sz) is often developed in hinge regions. Syntec-tonic M2 minerals (ineluding sporadic develop-ment of blue amphibole in schists) are sometimesaligned with the 52 srndace, and they elearlycross-cut the Sr metamorphie fabric. The distri-bution of thin conglomeratie marker beds in theKruyb Mountains indicates that these folds maybe parasitie on much larger recumbent folds, on akilometre seale, as suggested by Brouwer (1947).

F2 folds are usually associated with low-anglethrusts, which lie parallel to the axial planes ofthe folds. In the Kamba region of the Kru)-tMountains and in the S. Wimbi of the WanaripaluMountains, thrust planes dip consistently to theeast-southeast, at angles between 10 and 25".Where well exposed, some thrust planes can beseen to be eurved (concave upwards); some havebeen folded by a later F3 eyent. Aerial photo-graphic images of the Pompangeo Mountainssuggest that the thrusts may be large-scale fea-tures, continuous over tens of kilometers; ridgeseroded along Sa are imbricated by linear faults,whose sur{ace traces are orientated north-northeast-south-southwest (Fig. 6). ]

fThe latest phase of (post-metamorphic) defor-mation manifests as folds which can be tracedfrom the Pompangeo Schist Complex into theoverlying Mio-Pliocene cover units (Puna andPoso Formations). These F3 folds can be geo-metrically related to currently active norlhwest-southeast sinistral wrench faults and relateddeformation of the Palu-Koro, Matano, Poso andother fault systems. Fs folds in the PompangeoSchists generally occur in en echelon fold trainswith consistent north-south trends; Most arehighly cylindrical and s}'rnmetrical, steeply in-clined to upright, open structures.]

rIPETROL0G

rCAL CHARACTER ISTICS

The areal extent and relatively coherent natureof the Pompangeo Schist Complex enables re-gronal metamorphic patterns to be traced, andprogressive metamorphic gradient to be bfoadlydelineated. However, determination of equilibri-um assemblages and estimation of pressure and

v

238 C. Ps,rkinson

Fig.7 (a) Fz folding of well-bedded metasandstone of the

Pompangeo Schist Complex; S. Jayakita, Pompangeo Mountains.(b) Sr foliation deviating around F1 rootless intrafolial fold in

chloritic quartz-mica schist of the Pompangeo Schist Complex;S Toka, Wanaripalu Mountains.

temperatures of metamorphism are protrlematic.Parageneses diagnostic of most low- or moder-ate-grade metamorphic facies are largely lackingin the predominating lithologies (quartnLe,marble, serpentinite). Furthermore, althoughmetamorphic grade clearly increases from eastto west across the complex, detailed investiga-tion of this field gradient is made difficult by the

rapid changes in lithology across the strike(Fig. a).1

fParageneses in the schist and marble of theKruJ,t, Wanaripalu and Pompangeo Mountainsare almost exclusively characterized by varyingmodal proportions of quartz, calcite, muscovite,chlorite, albite, iron oxide and graphite. Inmetapelite and metaelastic rocks brownish-green-to-green stilpnomelane is of sporadic butwidespread occril'rence. Some quartz schists onthe track between Kamba and Majumba containthe assemblage: Qtz + Fe-carpholite + Ms + Kln+ Chl (Fig. 9a)- Electron microprobe analyses oftwo samples of carpholite in sehists from the S.Laa region yielded compositions of: Fe-earpho-liteTe, Mg-carpholite2s, Mn-carpholitel and Fe-carpholite6, Mg-carpholite31, Mn-carpholite1.Further west, on the eastern shore of Lake Posoand in the Poso depression the assemblage inmetapelites comprises: Qtz + erossite + Ep+ Ms + Chi + Ttn + Cal + Ab + Lws * Sps. Py-rite and apatite are common accessory minerals.Lawsonite laths are turbid due to incipient al-teration to zoisitelcalcite. Amphibole needles liein the 31 plane and are consistently orientatednorth-south. All display 'reverse' optical zona-tion (cf. Misch 1969), that is, zonation from coreto rim comprises: colorless glaucophane ---+ bluecrossite ---+ greenish-blue sodic actinolite. Syn-kinematic recrystallization slmchronous withelongation parallel to Sa is demonstrated bymicroboudinage of the amphiboles. Fine-grained,discontinuous metabasic intercalations occur inthe S. Tompakoe and S. Tomasa, on the easternmargin of the Poso depression. These rockscontain Gln + Lws + Sps + Ttn + Qtz (Fig. 9b),with accessory hematite and zircon.f

[Ott ttre western shore of Lake Poso, crossiticamphibole is associated with epidote, and occa-

Fig. I 0riented photomicro-graph of zoned sodic amphiboledisplaying synmetamorphic mic-roboudinage; zonation comprises a

colorless glaucophanitic core (GL)

and light blue (CR1) and dark blue(CR2) crossitic rim. Groundmassconsists of spessartine, epidote,phengite and quartz. Sample LP8;

Lake Poso.

Fig. I (a) Plane-polarized light (PPL) photomicrograph of quarlz

schist (PE3A) displaying aggregate o{ ferrocarpholite in quartz. East

of Kamba, Pompangeo Mountains. (b) PPL photomicrograph o{

metabasite (KKB2A) displaying alternating bands of glaucophane(dark) and lawsonite + quar[ (light). Equant grain in the bottom left

is spessartine-rich garnet. S. Tompakoe, Poso depression.

sionally pale pink spessarbine garnet in metape-iite, and with lawsonite in marble. Albite, chlorite,muscorrite and graphite are virtually ubiquitous inall lithologies, and titanite, tourmaline, zircon andpy'r'ite are common accessory minerals. Thin(< 5 m) metamorphosed dykes in the qaartzite ofthe Tokorondo Mountains provide better indica-tions of metamorphic grade at those points in thelithotectonic sequence. The metabasic rocks aregenerally composed of finely crystallineGln + Lws + Chl + Ms + T[n + Rt. Alignment ofglaucophane fibres and phyllosilicates define aweak S.1. schistosity. Porphyroblastie almandineand epidote are eorumon. Epidote prisms, whichmay attain lengths of several centimetres, arealigned parallel to the foliation (- north-south)and display evidence of microboudinage. Alman-dine-rich garnet is of similar size, highly euhedraland poikiloblastic, generally containing inclusionsof epidote, glaucophane and lawsonite. In a fewrocks there is little or no distortion of the sur-rounding fabric, indicating that the garnet growth

Pompangeo Schi,st Com,plex, Ind,onesi,a 239

was post-tectonic. Metapelites at the westernextremity of the complex are eharaef,efized by theassemblage Cld + AIm + Ms + Chl; quarbzo-feldspathic schists contain Qtz + oiigoclase + Bt+-Ms.][systematic changes in parageneses developed

in the range of lithologies that constitute thePompangeo Schist Complex define a broad, pro-gressive metamorphic field gradient. Metamor-phie grade increases from east to west, that is,with decreasing depth in the west-dipping meta-morphic pile; thus, the field gradient is inverbed.Characterization of the gradient in terms of theappearanee/disappearance of index minerals isdiffieult for the reasons already outlined. There isno field evidenee for large discontinuities inmetamorphic grade across the complex, but eastof Lake Poso the schists have elearly been imbri-cated by a post-metamorphic (D/ eventJ

[e-r corvnmorus oF METAMoRPHTsM

The absence of compositional data and appro-priate geothermobarometers precludes detailedP-T determinations, but parageneses and sta-bility flelds can be used to obtain approximateP-T conditions.

Parageneses in the schists of the chlorite andstilpnomelane-bearing schists in the east aresimilar to those developed in Oboke Formationmetapelites of the Sanbagawa Belt of Japan, andchlorite zone of the Haast Schist Terrane of NewZealand. Pressure-temperature conditions forthe common Ab + Qtz + Cal + ChI + Ms + Stp+ Gr (+Ep) assemblage are considered to be

-200-300 "C at 44 kbar (Banno & Sakai 1989).The assemblage Qtz + Fe-carpholite + Ms + Kln+ Chl in the schists of the S. Laa region is stableat temperatures -230-260 "C and pressuresabove -4 kbar, but not exceeding 7 kbar, be-cause of the increasing substitution of Mg for Feat high pressures (Viswanathan & Seidel 1979;CJropin & Schreyer 1983; Goff6 et aL t98$.J[The position of the fust appearance of sodic

amphibole in metapelites approximately coincideswith that of lawsonite (in marbles). Significantiy,aragonite was not detected in any lithologies. Atthis point the equilibrium assemblage in metape-lites is Qtz + Ab + Ms + Ep + crossite + Chl +Gr + Ttn + Sps. Stability of this assemblage isconfined to pressures in excess of 4 kbar andtemperatures below -400 "C. Further" westglaucophane decreases, rutile increases at theexpense of titanite, and a Fe-Cld + Alm + Ms +

$i'.to'.].:-tff-_:=:=t lJ."J rnfi

r

240 C. Parkinson

Chl + Qtz + Gr + Rt + Ep assemblage is stablein metapelites. Where ferrochloritoid eoexistswith glaucophane, temperatures are probably inthe region 35H50 "C (Maresch 19?7; Chopin &Schreyer 1983).J

l_Increasing grade at higher stmctural levels, inthe Tokorondo Mountains, is indicated by ap-pearance and increasing abundance of oligoclase,biotite and actinolite, and decreasing ehlorite inmetapelitic lithologies. This assemblage is eom-parable to those developed in lower parts of theoligoclase-biotite zone of the Sambagawa Belt.Estimated P-T conditions of these rocks are

- 10 kbar at 600 "C (Banno & Sakai 1989). Al-though peak P-T conditions of the PompangeoSchists are clearly significantly lower than thoserecorded in the Sambagawa schists (both horn-blende and barroisitie amphibole are absent fromthe former), the schists in both tenains have z-rdeveloped comparable intennediate high-pres-(!sure metamorphie facies series along geothermalgradients of -15 "C/lan (Fig. 10).] _f

rlK-Ar DATING 0F SCHISTS

In order to accurately determine the age of themain phase of Pompangeo metamorphism, sann-ples that (i) contained no relict phases or detritalgrains; (ii) were firlly recrystallized; and (iii)displayed no evidence of a thermal overprintwere selected. The latter criterion disqualifledschists from east of Lake Poso, some of whiehhave clearly suffered a synteetonic Mz meta-morphic event manifest as recrystrllization in Fzfold hinges. Three samples of pelitic, semipeliticand metabasic schist from the TokorondoMountains yielded white mica (FM62B, FM66C)and whole rock (FM68A) K-Ar radiometrie ages z\of 112.0 + 3.9 Ma, 108.0 + 2.5Ma and, fla.O +(l)3.0 Ma (rable 1).I

-fr[srGNrHcAilGE 0F JURASSTC SEDTMENTARY RoCKSIN EASTERN CENTRAL SULAWESI

Underlying the East Sulawesi Ophiolite in theKolonodale area and throughout the TowiMountains northeast of the Gulf of Tomori, low-grade Pompangeo schists are interthrust or inindeterminate fault contact with unmetamor-phosed Jurassic sedimentary rocks of the Nan-aka and Tetambahu Formations (Fig. 11). Theformer consists of thinly bedded sandstone andshale intercalated with thin limestone and con-

300 4ao 500Temoerature f"Cl

Fig. 10 Estimated metamorphic tield gradient (broad dark stip-pled line) for the Pompangeo Schist Complex in the Lake Poso area

ol central Sulawesi. Boundaries between metamorphic facies fieldsshown as broad lighi stippled lines. Am EC, amphibole eclogitefield; GR, granulite; GS, greenschist; PrA, prehnite-actinolite; PP,

prehnit*-pumpellyite; Pu--Act, pumpellyite-actinolite; ZEO, zeolite.

Reaction curves: aragonit+-calcite transformation from Carlson and

Rosenfeld (1981); albite : jadeite + quarD from Newton and

Smith {1967); actinolite, hornblende, barroisite stability coexistingwith chlorite, epidote, muscovite, albite, quarh and hematite fromOtsuki and Banno (1990); [1] chlorite + kaolinite : ferro-carpholite + water: Goff6 el aL (1988); [2] kaolinite + quarE :pyrophyllite + water: Goffe et al. (1988), [3] glaucophane stabilitylield from Maresch (1977); t4l lawsonite stability from Liou (.1971),

Nitsch (.1971); [5] fenocarpholite : ferrochloritoid + quartz +

water from Chapin & Schreyer (1983). 81 is the approximate P-Tfield of peak metamorphism for Bantimala eclogite and related rocks,

82 that of retrograde metamorphism for Baniimala eclogite and peak

metamorphism of regional Bantimala schists (Miyazaki ef a/. 1996;Parkinson ef a/. 1998).

glomerate layers and coal horizons, whereas thelatter comprises limestones, marls and sand-stones. Both are clearly terrestrial-to-shallowmarine sequences. Jurassic molluscs have beenidentified (Simandjuntak et al- 1991a). Similarrocks (the Meluhu Formation) crop out in theSoutheast Arm, where they are incipiently re-crystallized and interbhrust with PompangeoSchists. Bivalves and belemnites indicate'" a LaLeTriassic-Early Jurassic age (Sukamto & Wes-terman 1992). It may be more than fortuitous

15

t110

J--trlEl s'l

I

B1Am EC

_ G..l.L

uo// Pr*,

iC

iL,f,

82

Pompangeo Schast Complex

Pompangeo Sch,ist Com,plen, Indonesia 241

Table I K-Ar age data for Pompangeo schists

Sample Rock VoK RadiogenicaoAr 1nl,/g)

Error Age (Ma * o)RadiogenicaoLr (oqo)

FM62BPE66CPE68A

Mica schistMica schistMetabasite

MicaMicaWhole rock

0.3401.3070.810

1.5055.6333.708

3.081.942.65

112 + 3.9108 * 2.5114 + 3.0

that, both in the field and in thin section, dis-tinction between the very low-grade Pompangeoschists and the unmetamorphosed but induratedNanaka and Tetambahu Formations is oftendifficult. These prononnced lithological similari- .-.ties indicate that the latter may eonstitute the(g/parental material of the former.] 4

I

Fig. 11 Schematic lime chart 0f major geological events affecting

the Pompangeo Schist Complex and correlative units. Pompangeo

metamorphism delimited by K-Ar radiometric dating: (1)Wakita etal.(1996); (2) Hasan (1990); (3) the present study.

lrrcrouc sETTTNG 0F METAMoRpHTsM'nrun upurt

The presence of high P/T assemblages in rocksthat record high shear strains has been widelyinterpreted as evidence for subduction (Dewey &Bird 1970; Ernst 1975). Pressure and tempera-ture estimates and the non-coaxial geometry ofductile strain are all consistent with generationof the Pompangeo Schist Complex in a subduc-tion zone environment.]

[Mylonites and localized zones of intense defor-mation imply that shear strains were not uniformthroughout the complex, and this is consistentwith the interpretation of the complex as a para-autochthonous imbricate subduction package withsome rocks being metamorphosed and deformedat different depths and temperatures (and, pre-sumably times) to others. In simple terms, theprogression of metamorphic grade (from pum-pellyite-actinolite through greenschist to epidoteamphibolite facies) from low to high structurallevels along a relatively high thermal gradient isconsistent with successive underthrusting ofslices of downgoing material in a mature sub-duction. In this way the direction of teetonictransport would emplace higher-grade, moredeeply subducted sections over the more feeblyrecrystallized, near-surface sections of rock, acorollary being that metamorphic grade system-atically decreases away from the old plate junc-tion. When compared with other circum-PaciflcHP metamorphic belts, the Pompangeo SchistComplex is anomalous in that parental rocks ap-pear to be predominantly of shallow marine orcontinental margin parentage. Indeed unmeta-morphosed Jurassic sandstones (Nanaka, Tetam-bahu and Meluhu Formations) are intimatelyassociated with the schists in the eastern ex-tremity of the complex. Consequently, thePompangeo Schist Complex may represent anexhumed relic of a partially subducted microcon-tinental fragment which collided with aird at-tempted to subduct beneath the Sundalandmargin in mid-Cretaceous times. Termination of

rlt

242 C. Parkinson

underflow by collision and buoyancy of the conti- Peak P-T conditions have been estimated to be

nentalfragmentmayhavefacilitatedupliftof the -350-450 "C at 5-8 kbar (Miyazals et al. 1996:.

stranded, iecrystallized accretionary complex.] C. Parkinson unpubl. data, 1993). They have

[Syn-kinematic sequential 'reverse' mineral yielded white mica K-Ar radiometric ages ofgrowth (zonation and microboudinage) in am- 1L1 Ma (J. D. Obradovich in Hamilton 1979),

phiboles indicates that high shear strains may 113 + 6 Ma,114 + 6 Ma,115 t 6 Ma(Walstaetal.Lave prevailed during metamorphie conditions 19,96) and 111 t 3 Ma (Hasan 1990).Iof declining P-T (retrograde metamorphism) lThe Bantimala schists are assbciated withand uplifb, and that the transition from pro- te?tonic blocks of eclogite, garnet-glaucophanegrade &urial) to retrograde (uplifb) metamor- rock, garnet amphibolite and jadeite-garnet-phism occurred with iittle change in the strain qaarl,zrock.Someoftheserocksunderwentpeakregime. This interpretation presupposes that metamorphism at pressures in excess of 20 kbarthe amphibole (and other) lineations grew at 600-750'C (Miyazaki et aL. 1996; Parkinsonpreferentially in an orientation parallel to the et al. 1998). K-Ar dating of phengite from eclo-

extension axis of the mean strain ellipsoid. The gite has yielded ages of 132 t 7 Ma, 113 I 6 Ma,stretching lineations indicate that 'tectonic 127 =6 Ma & 137 t SlVIa (Wakita et aL. 1996;

transpori during the uplift phase was directed Parkinson ef ol. 1998).]appro'ximately north-south

-(i.e. perpendicular [In striking concordalce with the Pompangeoto that during burial). The occurrence of ra- Schist Complex, Bantimala schists are also in-diolarian cherts with a Cenomarrian biostrati- terthrust with unmetamorphosed Jurassic sand-graphy (i.e. ca 15 Ma after metamor-phism as stones (the Paremba Sandstone of Sukamto &recorded by the K-Ar ages) lying directly upon Westerman 1992) and unconformably overlain bythe Pompangeo schists may be expiicable in cherts with Albian-Cenomanian radiolarian as-

terms of rapid post-metamorphic subsidence nV 6D semblages (Wal<ttaetal-1996). Hence, I interpretgravitational collapse and eollision-related ex- Y the Pompangeo Schist Complex and the Ban-tensional tectonics. ] f timata Complex to constitute fragments of the

I same accretionary assemblage, metamorphosed

[connrLnnoil wrrn orHEH ilrFTAMoRpHrcCOMPLEXES II{ SULAWESI

Phyllite, graphitic quarlzite and marble petro-logically identical to the Pompangeo SchistComplex crop out throughout the Southeast Armin the Mekongga, Mendoke and Rumbia Moun-tains. Sehists from the latter mountains havebeen described by Helmers et al. (1989), whoreported Fe-Cld + Gln + Lws + Alm in meta-pelite and Omp + Fe-Gln + Lws (+ Grt + Ep) inmetabasic intercalations. Thus, they appear to beequivalent to the Pompangeo schists of the To-korondo Mountains, rather than the relativelylo-w-grade schists east of Lake Poso.l

lRocks of the Pompangeo Sehist Complex arealso petrologically and geochronologically com-parable to some metamorphie rocks of the norLh-west-southeast-trending Bantimala Complex inthe South Arm. Metamorphic eonstituents of thiscomplex prineipally comprise slab-like micaceousand graphitic schist eontaining discrete conglom-eratic and quartzitic bands. Parageneses are in-dicative of transitional greenschisVblueschistconditions and are identical to those developed inthe Lake Poso region of the Pompangeo Schists.

and uplifted in a west-dipping subduction zone atthe eastern Sundaland margin in the Early Cre-taceous. Mid-Cretaceous glaucophane and otherschists in the Latimodjong Mountains of west ./\central Sulawesi, and eclogite in the Palu-Koro (tz)fault valley may also constitute other parts of this 7(Cretaceous accretionary assemblagel )

[rnrnro ffiETnMoRpHrc Brrrs rN suLArtrErr?

The notion that the configuration of metamorphicroeks in Sulawesi represents a metamorphic beltsystem is refuted by the new data for thePompangeo Schist Complex.

The Pompangeo schists of central Sulawesiyield Aptian isotopic ages, whereas K-Ar radio-metric dating of staurolite and/or cordierite +andalusite + sillimanite bearing amphiboliticmaterial in western Sulawesi has yielded ages of15.1 r 1.6 Ma for white mica (C. Parkinson un-publ. data, 1988), 3.0 Ma for biotite (Sukamto1975) and a range of 5-14 Ma for unspecifiedmetamorphic material (J. D. Obradovieh inHamilton 19?9). These ages are contemfiorane-ous with igneous ages for adjacent granitoidbodies, and coincide with well-{oeumented Mid-

244 C. Parkinson

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r

Pornpangeo Schi,st Complex, Ind,ones'ia 243

dle Miocene and Pliocene phases of magmatic ACKN0WLEDGEMEIIITS

activity throughout western s'lawesi (Priadi This work formed a minor parb of a NERC/

et at. iggs:Beigman et at. teelJ |tl"y]I Aftt9ug.h .ror'-npangeo schists overthrust 9llt^i"tearch studentship conducted some

granodiorites in the rokorondo Mountains, ilJie t?ffirHr#tihli:-if*"ilj"ffi'"*::T:is no fleld evidence for a profound tectonic sutureor'median tectonic rine' in this region. ryl*.i H,i1fi:H:fffifrXf"::,Ti-Hfff#H:more, because schists correlative with the ;;::--:-^::.pompangeo schist comprex appear t" r"'i"t"""- Hffi;ilfftlffifff-iT:,"*ffi *.ffiiiffilally continuous into western Sulawesi, there is no -:-;.--,--

""a.o,, to inrer such a demareation .*;#;; ff:::Tj"i"XXT'#n,il:#*tJJ":l; i*lff'-l

disparate metamorphic provinces- ,..,- Reviews by professors s. Maruyama andAn extensive continental arc d t?TTi: i. Balr; improved the manuscript.granodioritic and granitic rocks with igneousages (130-95 Ma) broadly eoeval with metamor-phic ages of the Pompangeo and Bantimala

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