Van Leeuw en 2016

Journal Jour nal of Asian Earth Sciences Sciences 115 (20 (2016) 16) 133–152 133–152

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Journa Journall of Asian Asian Earth Earth Scienc Sciences es j o u r n a l h o m e p a g e : : w w w . e l s e v i e r . c o m / l o c a t e / j s e a e s

The Palu Metamorphic Complex, NW Sulawesi, Indonesia: Origin and evolution of a young metamorphic terrane with links to Gondwana and Sundaland Theo van Leeuwen a, , Charlotte M Charlotte M.. Allen b,1, Marlina Elburg c,2, Hans-Joachim Massonne d, J. Michael Palin b, Juliane Hennig e ⇑

a

Jl. H. Naim IIIB No. 8, Jakarta, 12150, Indonesia Research School of Earth Sciences, Australian National University, Canberra, Australia c Department of Geology and Geophysics, University of Adelaide, Australia d Institut für Mineralogie und Kristallchemie, Universitaet Stuttgart, Germany e SE Asia Research Group, Department of Earth Sciences, Royal Holloway University of London, United Kingdom

b

a r t i c l e

i n f o

Article history: Received 19 July 2014 Received in revised form 13 September 2015 Accepted 21 September 2015 Available online 25 September 2015 Keywords: Indonesia Sulawesi Metamorphic core complex Thermochronology Radiogenic isotopes

a b s t r a c t

The Palu Metamo Metamorph rphic ic Complex Complex (PMC) is exposed exposed in a late Cenozoic Cenozoic orogenic orogenic belt in NW Sulawesi, Sulawesi, Indonesia. It is a composite terrane comprising a gneiss unit of Gondwana origin, a schist unit composed of meta-sediments deposited along the SE Sundaland margin in the Late Cretaceous and Early Tertiary, and one or more slivers of amphibolite with oceanic crust characteristics. The gneiss unit forms part of the West Sulawe Sulawesi si block block underly underlying ing the northern northern and central sections of the Wester Western n Sulawe Sulawesi si Province. The presence of Late Triassic granitoids and recycled Proterozoic zircons in this unit combined with with its isotopic isotopic signatu signature re suggest suggestss that the the West West Sulawe Sulawesi si block block has its its origin origin in the New Guinea Guinea margi margin n from from which which it rifted rifted in the the late late Mesoz Mesozoic oic.. It docke docked d with with Sund Sundala aland nd someti sometime me durin during g the Late Late Creta Cretaceo ceous us.. U–Th–Pb dating results for monazite suggest that another continental fragment may have collided with the Sundaland margin in the earliest Miocene. High-pressure (HP) and ultrahigh-pressure (UHP) rocks (granulite, peridotite, eclogite) are found as tectonic slices within the PMC, mostly along the Palu–Koro Fault Zone, a major strike-slip fault that cuts the complex. Mineralogical and textural features suggest that some of these rocks resided resided at depths of 60–120 km during a part of their histories. histories. Thermo Thermochro chronolo nologica gicall data (U–Th–P (U–Th–Pb b zircon zircon and 40Ar/39Ar) fromthe metam metamorph orphic ic rocks rocks indicate indicate a latest latest Miocene to mid-Pliocene mid-Pliocene metamorphic event, which was accompanied by widespread widespread granitoid magmamagmatism and took place place in an extensio extensional nal tectonic tectonic setting. setting. It caused caused recrystall recrystallizat ization ion of, and new overgro overgrowth wthss on, pre-existing pre-existing zircon crystals, and producedandalusite–cordier producedandalusite–cordierite–silliman ite–sillimanite–stauroliteassemblages ite–stauroliteassemblages in pelitic pelitic protolith protoliths, s, indicating indicating HT–LP HT–LP (Buchan (Buchan-typ -type) e) metamorph metamorphism ism.. The PMC was exhumed exhumed as a core comcomplex at moderate rates (c. 0.7–1.0 mm/yr) mm/yr) accompanied by rapid cooling in the Plio-Pleistocene. Plio-Pleistocene. Some of the UHP rocks rocks were were transpor transported ted to the surface surface at signifi significant cantly ly higher higher rates rates (P16 mm/yr) mm/yr).. Theresults of ourstudy do not supportrecent supportrecent plate plate tectonicreconst tectonicreconstruct ructions ions that that propose propose a NWAustralia NWAustralia margin margin origin origin for the West West Sulawes Sulawesii block (e.g. (e.g. Hall et al., 2009). 2009).  2015 Published by Elsevier Ltd.

1. Introduction Early plate tectonic reconstructio reconstructions ns of the Indonesian region suggested that the Western Sulawesi Province ( Fig. 1 inset), or a Corresponding author at: Jl. H. Naim IIIb No. 8, Jakarta 12150, Indonesia. [email protected] (T. van Leeuwen). E-mail address: [email protected] (T. 1 Present address: Queensland University of Technology, 2 George Street, Brisbane, QLD 4000, Australia. 2 Present address: University of Johannesburg, PO Box 524, Auckland Park, 2006 Johannesburg, Johannesburg, South Africa. ⇑

http://dx.doi.org/10.1016/j.jseaes.2015.09.025 1367-9120/  2015 Published by Elsevier Ltd.

part thereof, might might be underlain underlain by a continental continental fragment fragment derived from Gondwanala Gondwanaland nd (Aud Audleyley-Ch Charle arless et al., 1988 1988;; Hut Hutchis chison, on, 1989 1989;; Metcalfe, Metca lfe, 1990 1990). ). Indi Indire rect ct evide evidenc nce e in suppo support rt of this this hypo hypoth thesi esiss was was subsequ subsequent ently ly provid provided ed by radiog radiogenic enic isotope isotope data data and limitedU–Pb limitedU–Pb inherit inh erited ed zircon zircon ages ages obtain obtained ed from from widespr widespread ead Late Cenozo Cenozoic ic igneou igneouss suites, suites, interp interprete reted d to indicat indicate e that that the central central and northe northern rn parts of the province are underlain by continental crust of Australian tralian origin origin (Pr Priad iadii et al. al.,, 199 1994; 4; Be Berg rgma man n et al. al.,, 19 1996 96;; Po Polv lvé é et al., 1997, 2001; Elburg and Foden, 1999; Elburg et al., 2003). 2003 ). Polvé et al. (2001) interpreted granulites granulites exposed in tectonic slices

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T. van Leeuwen et al. / Journal of Asian Earth Sciences 115 (2016) 133–152









T. van Leeuwen et al./ Journal of Asian Earth Sciences 115 (2016) (2016) 133–152

along along the Palu–Ko Palu–Koro ro Fault Fault Zone Zone (PK (PKFZ) FZ),, a major major strike-s strike-slip lip fault fault that that cuts the Palu Metamorphic Complex (PMC, Fig. 1), 1), to be derived from this crustal fragment and brought to the surface from great depth depth along along the fault fault system. system. It was general generally ly assumedthe assumedthe complex complex was was comp compos osed ed of rocks rocks belon belongi ging ng to the the Sund Sundala aland nd marg margin in under under which which the contin continent ental al fragmen fragmentt had been thrust (e.g., (e.g., Kadarusman and Sopaheluwakan, 1995; Polvé et al., 2001). 2001 ). We carried carried out a reconnais reconnaissance sance study of of the PMC PMC and a similar similar study of the Malino Metamorphic Metamorphic Complex (MMC) (MMC) to the north (Fi Fig. g. 1 inset). inset). They involved petrographic petrographic,, electron microscope, microscope, geochem geochemical ical and radiog radiogeni enic c isotope isotope analyse analysess combin combined ed with with U–Pb zircon, U–Th–Pb monazite, and 40Ar/39Ar age dating of outcrop and float samples. The results demonstrate both complexes partly formed from rocks derived from Gondwana, although they differ significantly in several aspects. The MMC results have been presented by van Leeuwen Leeuwen et al. (20 (2007) 07),, who conclude that the MMC cons consis ists ts of a bloc block k of mica ica schi schist stss and and gnei gneiss sses es of Devoni Devonian– an–Ear Early ly Carbon Carbonifer iferous ous protol protolith ith age, age, surrou surrounde nded d by a caracarapace of autochthono autochthonous us Eocene greenschists, greenschists, which docked with Sund Sundala aland nd in the the late late Olig Oligoce ocene ne and and soon soon after afterwa ward rdss was was exhum exhumed ed as a metam metamorp orphic hic core complex complex.. van Lee Leeuwe uwen n and Mu Muhar hardjo djo (2005),, who name (2005) named d the the PMC, PMC, highl highligh ighte ted d the the main main findi finding ngss for this this complex: it is a composite terrane (including a continental fragment with Permo-Triassic Permo-Triassic granitoids granitoids derived from the northern northern Australia–Ne Australia–New w Guinea Guinea margin, margin, and rocks of Sundaland Sundaland affinity) that underwent underwent a Pliocene metamorphic metamorphic event accompanied by voluminous felsic magmatism and rapid exhumation. In this paper we present the results of our study of the PMC with the objective of (1) describing the setting and nature of the PMC, PMC, (2) presenting presenting our analytical analytical data and interpretation interpretations, s, and (3) (3) discu discussi ssing ng the the possi possible ble orig origin in and and tecto tectoni nic c histo history ry of the the complex. A total total of 72 outcr outcrop op,, float float and and drill drill sampl samples es were were colle collecte cted d from from diffe differe rent nt part partss of the the PMC PMC betw between een 1998 1998 and and 2001 2001,, and and 11 sampl samples es were taken from the Karossa Metamorphic Complex (KMC), to the west of the PMC (Figs. (Figs. 1 and 2) 2) and are thought to form a part of the same terrane. Sample locations are shown in Fig. 2. 2. Selected samples samples were subjected to the following analyses: petrographic petrographic (63 samples samples), ), whole whole rock and trace trace element element geoche geochemi mistry stry (22), (22), 40 39 Sr, Nd and Pb isotopic analyses (12), and Ar/ Ar (7), U–Th–Pb monaz monazite ite (1) and U/Pb U/Pb zircon zircon geochr geochrono onolog logy y (12) (12) analyse analyses. s. Anal Analytytical methods for the geochemistry, isotope and 40 Ar/39Ar analyses are the same as described by van by van Leeuwen et al. (2007). (2007) .

2. Geological setting Indone Indonesia sia lies along along plate plate bounda boundary ry zone zone where where the India– India– Australia and Pacific–Philippine Sea Plates converge on Eurasia. It comprises comprises a complex complex assembly of continental continental blocks, suture zones, arc terranes, and accretionary complexes. Although the region has a long long hist histor ory y of subd subduc ucti tion on goin going g back back as far far as the the Perm Permia ian, n, it has has grown grown mainly mainly by additio addition n of contin continent ental al fragme fragments nts ( Fig Fig.. 1a). A relatively small amount of material has been added by subduction accretion, arc magmatism and other subduction-related processes, with with some some arcs ceasing ceasing activity activity during collisions, collisions, shifting shifting their

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Australia–New Guinea, including one or more fragments that form the basement of large parts of E. Java, the Makassar Straits and Western Western Sulawes Sulawesi, i, referred referred to as the East Java–W Java–West est Sulawe Sulawesi si block block by Hall et al. (2009). (2009). Opening Opening of the Makassar Makassar Straits Straits caused rifting of the the easte eastern rn Sund Sundala aland nd marg margin in and and led to the the form format ation ion of Western Western Sulawesi Sulawesi (e.g. Hamilton, Hamilton, 1979 1979), ), which which started started around around the the begin beginnin ning g of the the Midd Middle le Eocen Eocene e and and accele accelera rated ted in the the Middle–Late Middle–Late Eocene (Lun Luntt and van Gor Gorsel, sel, 2013). 2013). In the Early Early Miocene Miocene,, the protoproto-Sula Sula Spur, Spur, a contine continenta ntall promo promonto ntory ry that that extended west from New Guinea, collided with the margin of SE Asia and was subsequ subsequent ently ly fragme fragmente nted d by subduc subduction tion-dr -drive iven n extension (Spakman (Spakman and Hall, 2010). 2010). Sulawes Sulawesii consist consistss of four four distinc distinctt tectono tectono-str -stratig atigrap raphic hic proprovinces: Western Sulawesi, Northern Northern Sulawesi (a Cenozoic Cenozoic island arc built on oceanic crust), Eastern Sulawesi (tectonically intercalated metamorphic, ophiolitic and Mesozoic–Paleogene sedimentary tary rocks), rocks), and the Bangga Banggai-Su i-Sula la microco microcontin ntinent ent ( Fi Fig. g. 1 inset). The basement of Western Sulawesi includes several metamorphic complex complexes, es, includi including ng the PMC PMC and KMC, which which are overlai overlain n by Upper Cretaceous and Cenozoic sedimentary–volcanic sequences. During the Late Cenozoic magmatism was exclusively potassic to ultra-potassic in nature (e.g. Polvé et al., 1997). 1997). The PMC forms the backbone of the ‘‘Neck of Sulawesi” over a strike strike length of 95 km as a narrow narrow isthmus isthmus connectin connecting g the North Arm with the rest of Sulawesi, and continues further to the south where it occurs predominantly as roof pendants to young granitoids (Fig. (Fig. 1b). 1b). The complex is characterized by rugged mountains with elevations elevations up to 2.5 km. In the the Neck Neck,, the the PMC PMC is flank flanked ed by Middl iddle– e–La Late te Eoce Eocene ne sedimentary and volcanic rocks of the Tinombo Formation, which have have underg undergone one lower-g lower-green reenschi schist st facies facies metamo metamorph rphism ism,, and Plio-Ple Plio-Pleisto istocen cene e syn-oro syn-orogen genic ic sedime sedimenta ntary ry deposit depositss (Celebe (Celebess Molasse) that unconformably overlie older units. The complex is intruded by stocks and dykes of dioritic to granodioritic composition, which are co-magmatic with the Tinombo volcanics, as suggest gested ed by sim similar ilarit itie iess in geoc geoche hemi mist stry ry and and K/Ar K/Ar ages ages (van Leeuwen Leeuw en and Muh Muhardjo, ardjo, 2005). 2005). The actual contact contact betwee between n the PMC PMC and Tinom Tinombo bo Forma Formation tion has nowher nowhere e been observed observed and could be an angular unconformity (Sukamto, ( Sukamto, 1973) 1973) and/or have a tectonic tectonic origin. The The sout southe hern rn part part of the the PMC PMC is boun bounde ded d and and local locally ly overl overlain ain by the Upper Cretaceous Latimojong Latimojong Formation, Formation, a thick succession consisting consisting predominan predominantly tly of weakly metamorphosed metamorphosed pelitic and fine-grained fine-grained psammitic psammitic rocks with subordinate intercalations intercalations of volca volcani nic c rocks rocks (Sim Siman andj djun unta tak k et al. al.,, 199 1991; 1; Ha Hadiw diwijo ijoyo yo et al. al.,, 1993; van Leeuwen and Muhardjo, 2005). 2005 ). Again, there is no information in the literature on the nature of the contact, although it is generally generally assumed to be an angular unconformity unconformity ( OTCA, 1971; Simandjuntak et al., 1991; Hadiwijoyo et al., 1993). 1993). The complex is also locally overlain by late Miocene–Quaternary sedimentary and volcanic deposits. In places these have undergone low grade thermal thermal metamorphis metamorphism, m, mainly mainly manifested manifested in the occurrence of new biotite growth without development of foliation. The The KM KMC C is expo expose sed d in two two bodi bodies es (Fig Figs. s. 1b and and 2). The The north northern ern body, body, measurin measuring g 20 km by 25 km, km, is in faulted faulted contact contact with the Latimojong and Tinombo/Budungbudung Formations. It comprises









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Fig. 2. Sample locations. Samples referred to in the text are numbered. Outlines of the Palu Metamorphic Complex (PMC) and Karossa Metamorphic Complex (KMC) are shown schematically.

present. Available geological evidence and radiometric ages suggest that the shoshonitic to ultrapotassic alkaline magmatism initiated tiated around around 13–14 Ma and ceased ceased by the earliest earliest Pliocene Pliocene,, and that that the felsic magmat magmatism ism started about 4 Myr later, continu continuing ing into the Quaternary Quaternary and peaking in intensity between between 7 and 4 Ma (Elburg et al., 2003). 2003 ). The main structural feature of the region is the NNW-trending PKFZ that cuts the PMC and links with the North Sulawesi Trench (Fig. 1 inset). 1 inset). Movement along the fault zone has significant horizontal and vertical components (e.g. Katili, 1970). 1970). A 200– 200–25 250 0 km sinistra sinistrall displace displacemen mentt during during the past past 5 Myr Myr has been deduced deduced from geological and paleomagnetic studies (Surmont ( Surmont et al., 1994; Bellier Bell ier et al., 200 2001 1). Isocli Isoclina nall foldi folding ng of Tin Tinom ombo bo rocks rocks in the the nort northh-

metamo metamorph rphic ic and high high pressure pressure/ult /ultrara-hig high h pressure pressure (HP/UH (HP/UHP) P) rocks.

3.1. Regionally-metamorphosed Regionally-metamorphosed rocks This group consists predominantly predominantly of various various biotite-bearing biotite-bearing schists, schists, which commonl commonly y contain contain andalusit andalusite e ± cordier cordierite ite ± sillisillimani manite te ± garn garnet, et, and and bioti biotite te gneis gneisses ses.. Subo Subord rdina inate te litho litholo logie giess include include amphib amphibolit olite, e, amphi amphiboli bolite te schist, schist, amphib amphibole/ ole/pyr pyroxen oxene e gneiss, greenschist, greenschist, various various types of migmatite, migmatite, and metagranimetagranitoids, with minor marble marble and calc-silicate rocks. Biotite-bearin Biotite-bearing g schists schists dominat dominate e in the northw northweste estern rn part part of the PMC PMC and gneisses gneisses










The dominan dominantt gneiss gneiss is biotite biotite orthog orthogneis neiss, s, in which which the origina originall granitic granitic texture is commonly well-preserved well-preserved,, including plagioclase plagioclase showing showing well-developed well-developed zonary structures. structures. Biotite is either the sole mafic mineral or is accompanied by hornblende. In addition to the mafic minerals, the rocks are composed of varying amounts of quartz, quartz, K-feldsp K-feldspar ar and plagiocl plagioclase. ase. Garnet Garnet usually usually occurs occurs in minor amounts or is absent. Biotite paragneisses are particularly well well deve develo lope ped d in the the Gim Gimpu and and Lake Lake Lind Lindu u area areass ( Egeler, 1946; OTCA, 1973). 1973). Sillimanite–cordie Sillimanite–cordierite–bio rite–biotite tite ± garnet gneiss gneiss is the dominant dominant rock type type and contains contains significa significant nt amount amountss of K-feldspar K-feldspar and plagioclase. plagioclase. The gneiss gneiss uni unitt contain containss graniti granitic, c, monzo monzoniti nitic, c, syenitic syenitic and dioritic intrusives that have been metamorpho metamorphosed sed evidenced evidenced by the following features: (1) a variably developed penetrative fabric represe represente nted d by oriente oriented d biotite biotite grains grains and aggreg aggregate ates; s; (2) flattene flattened d quart quartz z and and feldsp feldspar ar crysta crystals; ls; and and (3) (3) the the prese presenc nce e of garn garnet et,, cord cordier ierite ite and and kyan kyanite ite.. These These rocks rocks are herei herein n refer referred red to as metagranite, metagranite, etc. Contacts Contacts with the metamorphic metamorphic host rocks are either gradational or sharp. Some meta-granitoids show anatectic features. Biotite schist is the most common schist, formed after pelitic or psamm psammitic itic sedime sediments nts and locally locally grades grades into into quartz quartzitic itic schists. schists. The main main rock rock forming forming minera minerals ls are quartz, quartz, plagioc plagioclase lase and biotite, biotite, which which are usually usually accomp accompanie anied d by andalu andalusite site,, silliman sillimanite ite that that commonly replaces biotite, and/or cordierite. Other minerals present in subordinate subordinate amounts amounts include monazite, monazite, garnet, garnet, tourmaline, tourmaline, K-feldspar, K-feldspar, and muscovite. Hornblende Hornblende and/or diopside diopside are relatively tively abunda abundant nt in some some varieti varieties. es. An interes interestin ting g feature feature of the schists schists is the presence presence of stauroli staurolite te that that has been describe described d in some some detail detail by Egeler (1946). (1946). It commo commonly nly occurs as relicts relicts or pseudomorphs, and is enclosed by later andalusite or its alteration products (principally sericite). All transitions are found from very small corroded remnants remnants to more or less homogeneo homogeneous us crystals up to 2.5 mm in diameter. diameter. The structure structure of the schists varies from well-developed schistosity to more hornfelsic. True hornfelses are found along along the margins of some granite granite bodies, up to 30 m. thick, and are clearly of contact-metamorphic origin. The amphibolites and amphibole schists, of both igneous and sedimentary origin, consist of varying amounts of amphibole and plagioc plagioclase lase (domin (dominant antly ly oligocla oligoclase). se). They They may contain contain quartz quartz and/or clinopyroxene (mostly augite), and less commonly biotite, garnet garnet and/or epidote. Egeler Egeler (1946) divided divided the amphibolites amphibolites and amphibole schists into a normal type and a hornfelsic type based on textural differences. The latter type is characterized by a remarkably remarkably equigranular and fine-grained fine-grained crystalline texture, and and chan change gess in the the chem chemica icall compo composit sition ion of amph amphibo ibole le and and plagioclase.

3.2. High pressure and ultra-high pressure metamorphic rocks HP and UHP rocks include mafic and felsic granulites, garnet and spinel spinel perido peridotite tite,, eclogite eclogite,, and pyroxen pyroxenite. ite. The majori majority ty of known outcrop and float occurrences are located in the Palu and Koro valleys along the surface trace of the PKFZ (Brouw ( Brouwer, er, 1934; Egele Eg eler, r, 19 1946; 46; So Sopa pahe heluw luwak akan an et al. al.,, 19 1995; 95; Ka Kada daru rusm sman an an and d Parkinson, 2000), 2000), but peridotite has also been observed as outcrop

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leucocr leucocrati atic c granul granulite ite contains contains up to 15 cm wide bands bands of darker darker material, and locally kyanite-rich bands alternating with garnetrich bands. bands. This This and other other largerlarger-size sized d lenses, lenses, which which predom predomiinantly have NW to NNW trends and are fault bounded, suggesting they they were were tectoni tectonicall cally y emplace emplaced, d, while the smaller smaller pieces pieces found in the granitoids are probably xenoliths. Eclogite composed of garnet, kyanite and talc is reported from two localities in the Palu valley and occurs as xenoliths in young granites granites (Sopahe Sopaheluwak luwakan an et al., 1995 1995). ). We collected collected an eclogite eclogite float float samp sample le (NW (NWS 209) 209) 10 km nort north h of Tobo Toboli li.. It is a nonnonfoliated, foliated, medium-g medium-grained rained rock of equigranular equigranular texture, made up of garnet (40%), omphacite (45%), and quartz (5%) with secondary augite, augite, plagiocl plagioclase, ase, amphib amphibole, ole, and ilmeni ilmenite. te. A more more detaile detailed d description is given by Kadarusman et al. (2005). (2005).

4. Structural aspects The The struct structur ure e of the the PMC PMC is comp complexand lexand poor poorly ly unde underst rstoo ood. d. The The rocks rocks commo commonl nly y have have wellwell-de deve velop loped ed foliat foliation ion,, defin defined ed by comp compos ositi ition onal al bandin banding, g, align alignme ment nt of micas micas,, amph amphibo ibole le and quartzo-feldspathic segregations, or lensoid leucosomes as a result of anatexis. The foliation foliation shows a wide range of orientation. orientation. MyloniMylonitization is a common feature. Recent studies by Watkinson (2011) and and Hennig Hennig et al. (2012, 2014) 2014) indicate indicate that stretching lineations lineations and shear zones vary in their orientation across the complex, with some some rocks rocks showing showing multip multiple le stages stages of deform deformatio ation, n, includi including ng recumbent folding. In the central part shear fabrics predominantly record record low-an low-angle gle extensio extension n toward toward the west. west. South-, South-, west- and north-directed reverse shear bands also occur in close proximity to westwa westward rd extensi extension. on. Furthe Furtherr south, south, extensio extension n is directed directed toward toward the SW together with gently-dipping top-to-SE thrust fault fabrics. In the northern northern part of the PMC lineatio lineations ns vary considerab considerably, ly, but suggest suggest a domina dominant nt N–NE N–NE – directe directed d orienta orientation tion.. The complex complex is cut by late-stage brittle faults (normal and strike-slip). The Tawaeli–Toboli Tawaeli–Toboli road section is the best-studied traverse traverse acros acrosss the the PMC PMC (Fi Fig. g. 3; Brouwer Brouwer,, 193 1934; 4; Sop Sopahe aheluw luwaka akan n et al., 1995; Watkinson, 2011, 2011, this study). From west–east, biotite schist and quartz quartzitic itic biotite biotite schist, schist, amphib amphibolit olite e and amphib amphibole ole schist, schist, and granitic gneiss with schist and amphibolite are present, and have been been intrude intruded d by granit granite e and granod granodiori iorite. te. Strikes Strikes observe observed d in the schists are dominan dominantly tly NNE NNE to NNW; NNW; dips dips are domina dominantly ntly wester westerly ly directed directed.. Kadaru Kadarusma sman n and Sop Sopahe aheluw luwaka akan n (199 (1995) 5) obser observe ved d thre three e defor deforma matio tion n phase phasess in this this unit: unit: D1 with with schistosity schistosity plan planes es more more or less less para paralle llell to the the bedd beddin ing g plan planes es cause caused d by regional regional metamorph metamorphism ism and probably probably shortly followed by local layer-parallel layer-parallel shearing, D2 resultin resulting g in local local crenula crenulatio tions ns of the schistosity, schistosity, D3 believed to have caused a regional buckling of D 1 cleavag cleavages. es. The amphib amphibolit olite e uni unitt is bound bound on either either side by strongl strongly y mylonitized shear zones. Drilling Drilling at the Paboya Paboya gold gold prospe prospect ct to the north indicates indicates a major low-angle brittle–ductile shear zone. It is characterized by footwall footwall ductile shearing that transformed transformed gneiss and amphibolite protoliths protoliths into phyllonitic phyllonitic and gneissic schist. The hanging hanging wall displays pervasive brittle fracturing and faulting in biotite schists. The fault dips 25–40 to the SW, SW, is 10–15 m thick and consists consists of cataclasi cataclasite/f te/faul aultt breccia. breccia. Its charact character er sugges suggests ts that that it is a lowlow-









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Fig. 3. Cross Cross section section through through the Palu Metamo Metamorphic rphic Complex Complex between between Tawaeli Tawaeli and Toboli. Toboli. Location Location shown shown in inset. inset. Modified Modified after Brou Brouwer wer et al. (1947) (1947) and Kadarusman and Sopaheluwakan Sopaheluw akan (1995) (1995)..

and a wide range of trace elements. Analytical results are presented in Supplementary in Supplementary Data File 1. 1. The most interesting results are those for the five amphibolite samples that were analyzed. On a primitive primitive mantle-norm mantle-normalized alized spider diagram they show depletion in the more incompatible elements, similar to an N-MORB pattern, suggesting an oceanic ridge origin. The exception is NWS 901, which is relatively enriched in these elements, a feature that is more typical of an oceanic island basalt (OIB) or enriched (E-) MORB pattern (Fig. ( Fig. 4). 4). The metagranitoi metagranitoid–orth d–orthogneis ogneisss (11 samples) samples) are metaluminou metaluminouss granitoids characterized by enrichment in Sr, K, Rb and Th with negativ negative e Nb anomal anomalies ies in mantle mantle-no -norma rmalize lized d diagra diagrams ms (not (not shown) shown).. They They have have a volcan volcanic ic arc (VAG) affinity, affinity, as defined defined by Pearce Pear ce et al. (198 (1986) 6).. Most Most of the samples samples are high-K high-K (3.5–5.0 (3.5–5.0% % K2O). A noticeable exception is NWS 203, that has a K 2O content of 1.02%, a low Na2O content (0.53% versus 3.5–4.3%), and distinct isotopic signature (see Section 6 Section 6), ), suggesting a different source.

6. Radiogenic isotopes Twelve samples were subjected to isotopic analysis for Sr, Nd and Pb. They They consist consist of ten samples samples from the PMC, PMC, compri comprising sing amphibolite, granulite, gneiss and meta-granitoid, and two gneiss samples from the KMC (NWS 773 and 903). The Sr, Nd and Pb isotopic compositions as well as the 147Sm/144Nd ratios are given in Table 1 and 1 and plotted in Fig. 5; 5; some of these results were already presented in Elburg in Elburg et al. (2003). (2003) . Based on isotopic characteristics the samples can be divided into three groups. Group Group 1, amphi amphiboli bolite te sample sampless NWS NWS 580 and 612, 612, shows shows the lowest lowest Sr, and highest highest Nd isotopi isotopic c values values (0.7035 (0.7035–0. –0.704 7046 6 and 0.5130–0.5132 respectively). They also have the lowest Pb isotope ratios. The Nd and Pb isotopic ratios are close to those of present-

respecti respectivel vely) y) and low 143Nd/144Nd ratios ratios (around (around 0.5118). 0.5118). The granul granulite ite is charact characteri erized zed by having having the lowest lowest 206Pb/204Pb and 208 Pb/204Pb of all samples samples analyzed analyzed (17.5 (17.5 and 37.3 respecti respectively vely). ). It also also diff differ erss from from the the othe otherr sam samples ples by not not fall fallin ing g on a positively-corre positively-correlated lated trend of 147Sm/144Nd versus versus 143Nd/144Nd ratios, because it has a very high 147Sm/ Sm/144Nd ratio (0.27) for its 143 144 Nd/ Nd ratio. We interpret interpret the isotopic features features of Group 1 as typical of mafic igneous igneous rocks without without a prolonged prolonged crustal history, which is consistent tent with with the MORBMORB-like like geoche geochemis mistry try shown shown by the samples samples (5.0). (5.0). The third third sample sample in this group, group, a muscov muscovite– ite–hor hornbl nblend ende e gneiss gneiss (NWS 773) with slightly higher Sr and Pb and lower Nd isotope ratios ratios may have have been an immatu immature re sedime sediment, nt, or subducti subductiononrelated igneous rock. Group 2 samples are likely upper-crustal rocks, and this is reinforced forced by their their Sm–Nd Sm–Nd mantle mantle extracti extraction on ages, ages, which which vary vary between between 0.6 0.6 and and 1.1 1.1 Ga for for TCHUR and and 1.0 1.0 and and 1.5 1.5 for for TDM. This This contra contrasts sts with with the Group 1 samples, for which these ages cannot be calculated as a result of their high 143 Nd/144 Nd ratios. Polvé ratios. Polvé et al. (2001) report (2001) report Sr, Nd and Pb isotope results for three peridotite samples and one granulite sample from the PMC that fall within the ranges of our Group 2 samples. Our interpretation for the granulite sample of Group 3 is that it unde underw rwen entt recen recentt melt melt extra extracti ction on,, leavin leaving g a light light rare rare earth earth element-dep element-depleted leted residue, which is consistent consistent with the common common presence of garnet in this sample. This melt extraction event must have have been relatively relatively recent, recent, since since the sample sample would would otherw otherwise ise 143 144 143 144 have developed a higher Nd/ Nd ratio. If the Nd/ Nd/ Nd ratio at the time of melt extraction was higher than 0.5116, this event must must have have happen happened ed in the past 100 million million years. years. Sample Sample NWS 203 probably represents, like the Group 2 samples, an upper crustal rock rock but but is likely likely to have have had had a long longer er crusta crustall histo history ry as refle reflecte cted d by its mantle extraction age (TCHUR and TDM 1516 1516 and 1823 1823 Ma












139

Fig. 4. Primitive Primitive Mantle Mantle (PM) (PM) normalize normalized d diagram diagram for amphibo amphibolite lite samples, samples, showing showing mainly mainly N-MORB N-MORB-like -like characteri characteristics. stics. Only Only sample sample NWS901 shows shows a pattern pattern that is more more similar to an ocean island basalt or E-MORB. Normalizing values from Sun and McDonough (1989) .

Table 1

Whole rock TIM Sr, Nd and Pb isotopic compositions for selected samples. The Nd mantle extraction ages were calculated using a present-day composition for the Chondrite Uniform Reservoir (CHUR) of 143 Nd/144Nd = 0.512638 with 147 Sm/144Nd = 0.1967 and Depleted Mantle (DM) of 143 Nd/144Nd = 0.52324 with 147 Sm/144Nd = 0.222. KMC = Karossa Metamorphic Complex. Sr/86Sr

Group

a

87

143

Nd/144Nd

206

Pb/204Pb

207

Pb/204Pb

208

Pb/204Pb

147

Sm/144Nd

TCHUR (Ga)

TDM (Ga)

NWS612 NWS580 NWS206a NWS207a NWS 584 NWS 602

1 1 2 2 2 2

Plag rich amphibolite Amphibolite Bt metagranite Bt schist Meta-monzonite Meta-monzonite/bt gneiss

0.703479 0.704627 0.715023 0.710134 0.709422 0. 0.717917

0. 0.513154 0.512985

17.83 17.86 18.76

15.55 15.48 15.64

37.60 37.77 38.96

0.1972 0.2042

0.512248 0. 0.512097

18.79 18.83

15.65 15.65

39.06 38.86

0.1078 0.1174

0.67 1.04

1.16 1.48

NWS 604 NWS 610 NWS 757 NWS596 NWS 203 NWS 373 NWS773 NWS903

2 2 2 2 3 3 KMC KM KMC

Porphyroid Hbl bt gneiss Granulite Bt gneiss Foliated granite Granulite Musc-hbl gneiss Bt-fsp-crd gneiss

0.712562 0.708420 0.710111 0.709494 0.736314 0.739167 0.705116 0.706775

0.512176 0.512305 0.512127 0.512277 0.511790 0.511815 0.512792 0.512393

18.71 18.67 18.76 18.74 18.87 17.51 18.54 18.97

15.64 15.62 15.63 15.63 15.66 15.54 15.57 15.68

38.91 38.77 38.78 38.97 38.94 37.31 38.47 39.09

0.1063 0.1093 0.1235 0.0978 0.1116 0.2679 0.1526 0.1177

0.78 0.58 1.06 0.56 1.52

1.24 1.10 1.53 1.03 1.82

0.47

1.05

Pb isotopes measured on K-feldspar separates, Sr isotopes on apatite separates.

similar to isotope values obtained from gneiss samples from the Malino Complex, a continental fragment comprising mostly Paleozoic quartz–muscovite quartz–muscovite schists and gneisses gneisses (van Leeuwen Leeuwen et al., 2007). 2007 ). The Group 2 samples have no equivalent within this complex, plex, but show similariti similarities es to igneou igneouss rocks rocks from the island of Ambon, Ambon, interpreted interpreted to have assimilated basement material with

connected connected to an Agilent 7500 ICP-MS ICP-MS at the Australian National National University similar to that described by Bryan et al. (2004). (2004) . Seventeen teen isotope isotopess are collected, collected, six for dating purposes purposes and the rest to explore zircon composition and/or document inclusions (particularly ularly P and Ti). Zircons Zircons were epoxy mount mounted ed and polished polished to expose expose their their cores. cores. Such a laser laser ablation ablation drilling drilling techniqu technique e propro-









140


Complex (PMC), and the Karossa Metamorphic Complex Complex (KMC). Data fields from Fig. 5. Present-day isotopic ratios for Group 1, 2 and 3 samples from the Palu Metamorphic Complex van Leeuwen et al. (2007): (2007) : Malino; Polvé et al. (2001): (2001) : PMC; Vroon PMC; Vroon et al. (1996) : Bacan and Ambon, and Elburg et al. (2002) : North Australian sediment. (A) 143 Nd/144Nd versus 87 Sr/86Sr. (B) 143 Nd/144Nd versus 147 Sm/144Nd. (C) 207 Pb/204Pb versus 207 Pb/204Pb. (D) 208 Pb/204Pb versus 206 Pb/204Pb. For details see text.










catho cathodo dolu lumi mine nesce scenc nce e imag images es are show shown n in Fi Figs. 6 and 7, respectively. NWS NWS 373 (isotope (isotope Group Group 3) is a kyanite kyanite + garnet garnet + plagiocl plagioclase ase granulite containing zircon with distinctive morphology ( Fig. 7a). 7a). The zircon zirconss are equant equant,, their their cathodo cathodolum lumine inescen scence ce respons response e is generally unbanded to weakly unbanded (Fig. ( Fig. 7a) 7a) inclusio inclusions ns are very very spar sparse se,, U cont conten ents ts are are low low (60– (60–25 250 0 ppm ppm), and and Th/U Th/U is betw between een 0.03 0.03 and and 2 but but most most are are betwe between en 1 and and 2 (rela (relativ tively ely high high), ), and and P conten contents ts are 150–25 150–250 0 ppm, ppm, which which is a norm normal al range range for many many zircons zircons.. Common Common Pb conten contents ts (%206Pb that is common in 206 all Pb) for individual analyses ranges from 0% to 13% but most are are less less than than 5%. 5%. Thir Thirty ty anal analys yses es yiel yielde ded d 208Pb-corrected 206 238 Pb/ U ages ages from 4.1 to 0.6 Ma and the population population mean mean age 3.84 ± 0.09 0.09 Ma (MSWD (MSWD of 2.3) if two grains grains with very very high common Pb contents are excluded (Fig. ( Fig. 6a). 6a). Restricting the population to those those analyse analysess concord concordant ant within within uncert uncertaint ainty y leaves leaves eight eight grain grainss with with a popu populat lation ion age of 3.74 3.74 ± 0.14( 0.14(MS MSW W = 1.73) 1.73) if one one grai grain n with with a 2.5 2.5 Ma age age is excl exclud uded ed fromthe fromthe mean mean.. Most ost grai grains ns fail fail the concordancy test because the 208 Pb-based correction does not adequately move 23 of the 30 grains to concordia. We interpret that that the zircons zircons grew grew during during granul granuliteite-faci facies es metam metamorp orphis hism. m. The lack of inh inherit erited ed zircon zircon probabl probably y reflects reflects the efficiency efficiency of metam metamorp orphic hic and/or and/or fluid-co fluid-contr ntrolle olled d reaction reactionss in consum consuming ing prot protol olith ith zirco zircon n (if it existe existed) d),, and and prod produci ucing ng new new zirco zircon n at 3.8 3.8 ± 0.1 Ma. NWS NWS 922 922 is a horn hornble blend nde– e–bi biot otite ite orth orthog ogne neiss iss.. Grain Grainss are equa equant nt to sube subequ quan antt and and in catho cathodo dolu lumi mine nesce scenc nce, e, dark darker er broadly-zoned cores are rimmed by brighter, broadly-zoned concentric banding (Fig. (Fig. 7b). 7b). They yielded a limited age range for the 31 total total analy analyses ses (8.5– (8.5–7.3 7.3 Ma) Ma) and and 19 are are conco concord rdan antt with within in uncer uncer-tainty (Fig. (Fig. 6b). 6b). These 11 concordant grains form a roughly normal dist distri ribu buti tion on with with a mean mean age age of 7.83 7.83 ± 0.12 0.12 Ma (MSW (MSWD D of 3.1) 3.1).. This This MSWD MSWD is somewh somewhat at high, high, and the popula population tion distribu distribution tion sugges suggests ts the two oldest oldest ages do not belong belong to the populatio population. n. Exclud Excluding ing these these yields 7.80 ± 0.09 0.09 Ma (MSWD (MSWD of 1.9). Uranium Uranium content contentss for the the 31 grain grainss rang range e from from 90 to 650 650 ppm, ppm, and aver averag age e Th/U Th/U is 0.57 (±0.10, 1 std dev) with common Pb contents less than 3.5%. Phosphorus Phosphorus ranges from below detection detection ( 10 ppm) ppm) to 50 ppm. ppm. Except for the possible older population represented by the two grains, this uniform uniform population could be interpreted interpreted as igneous igneous or metamorphic. However based on the cathodoluminescence pattern, tern, and lack of oscillat oscillatory ory zoning zoning typical typical of igneou igneouss rocks, rocks, we favor a metamorphic origin. NWS NWS 1026 is a metased metasedime imenta ntary ry biotite biotite schist, schist, containi containing ng kyanite kyanite 

141

2007; Hermann, 2002). 2002). We interpret that the protolith was a clastic rock containing containing detrital zircons zircons with ages as old as 365 Ma and including including possibly Cretaceous–T Cretaceous–Tertiary ertiary ones. NWS 207 is a biotite schist from Group 2. Eighty spots were ablated, a few complicated patterns were not processed, and two wildly discordant parts of analyses are ignored. Many grains gave both both core core and and rim ages, ages, resu resulti lting ng in the the inter interpr preta etatio tion n of 84 segments segments (Fi Fig. g. 6d). Concen Concentra tratin ting g on cores cores or ‘‘inher ‘‘inherited ited ages” ages” (>10 Ma), 18 gave concordant concordant results ranging in age from 1830 to 55 Ma. Ma. Except Except for the two 1730 and 1830 1830 ages, all are less than 500 Ma. Ma. Most Most of the apparen apparentt ages are between between 55 and 140 Ma. These appear to be a range of truly concordant ages representing detrital material of that age as opposed to a group of grains with variable Pb loss especially given the large gap in age from the oldest of 1830 1830 Ma to 500 Ma. Ma. Concen Concentrat trating ing on the young grains, grains, 51 segments segments yield 3–9 Ma, clearly not a single population. population. There is a cluste clusterr of grains grains between between 7 and and 8 Ma, Ma, then then an age age gap gap to 3 Ma. Ma. These These youn young g grain grainss have have Th/U Th/U of 0.04 0.04 or less, less, consis consiste tent nt with with metametamorphic zircons formed contemporaneously with a Th-rich phase as abov above. e. We inter interpr pret et this this as a clasti clastic c sedim sedimen enta tary ry rock rock cont contain aining ing a range range of zirconages zirconages that that was was metam metamor orph phos osed ed more more recen recently tly than than 9 Ma. The remaining 6 PMC samples are all meta-igneous rocks. NWS 206 is a meta-granite from Group 2. Of 60 ablations, five gave patterns that are impossible to interpret and two wildly discordant grains (both short ablation plateau segments) are ignored. About half the grains gave patterns such that both rim and core ages ages coul could d be obta obtain ined ed.. The The olde oldest st core core age age obta obtain ined ed was was 226 226 Ma, Ma, and and the younge youngest st rim, rim, 3 Ma (Fig. ( Fig. 6e). 6e). We interpret interpret this this sample sample is a Trias Triassic sic intru intrusio sion n that that was was metam metamor orph phos osed ed at 3–4 Ma, and any concordant intermedia intermediate te ages are the product of Pb loss. loss. The The basis basis of this this inter interpr pret etati ation on is that that the the seve seven n oldest oldest conconcordan cordantt grains grains yield a weight weighted ed mean age age of 218 ± 5 Ma with an MSWD of 2.0. This is a 206Pb/238U Concord Concordia ia age. age. There There are 11 208 206 rim analyses that yield concordant concordant Pb-corrected Pb/238U conconcordia cordia ages ages to 3–4.6 Ma, Ma, giving giving a weighted weighted mean mean of 3.54 ± 0.3 Ma with a MSWD of 2.7. The remaining spots are interpreted to have suffered suffered partial Pb loss. loss. The constra constraint int on maximu maximum m protoli protolith th age is the oldest oldest concord concordant ant grain grain at 226 Ma. Ma. Eighte Eighteen en of the 21 young young analyse analysess <10 Ma have P concen concentra tration tionss <100 <100 ppm, ppm, unu unususually low for igneou igneouss zircon zircons. s. These same same grains grains have have Th/U between between 0.01 0.01 and and 0.1 0.1 and and U betw between een 250 and and 1000 1000 ppm, ppm, i.e. their their Th cont conten ents ts are low for igne igneou ouss orig origin in.. These These are meta metamo morp rphi hic c overg overgro rowt wths hs.. In this this sample sample low Th in the the rims rims can be tied tied to









142











143

Fig. 6 ( continued)

are concordant within uncertainty. Six grains with ages less than 4 Ma have 0.4 to 18% common Pb, Th/U of 0.01–0.65, 0.01–0.65, and variable U conten contents ts (50–10 (50–1000 00 ppm). ppm). The 3 grains grains which which are concordan concordantt

concor concordan dantt at 230 ± 3 Ma, Ma, but abundan abundantt core core materia materiall is indicated indicated in the cathodoluminescence image (Fig. ( Fig. 7g). 7g). The other two have apparent ages of Proterozoic ( 1730 Ma), Ma), and Mesozoic Mesozoic.. The age 











144











the occurren occurrence ce of grains grains of 210–230 210–230 Ma age, althoug although h older older proprotolith tolith ages ages cannot cannot be fully fully dismissed dismissed.. Three Three samples samples (NWS 207, 207, 927, 927, 1026 1026)) appe appear ar to recor record d a some somewh what at older older even eventt arou around nd 8 Ma (NWS 207 also records the Pliocene event. The most interestinteresting of the ten samples is UHP granulite sample NWS 373, which as explain explained ed in 8.4 gives a precise precise age of its metamorp metamorphis hism, m, only only 3.8 Ma, Ma, indicat indicating ing very rapid rapid exhum exhumatio ation. n. Also of interes interestt is the presen presence ce of Protero Proterozoic zoic grains in three three samples samples (NWS 203, 207, 207, 1026), 1026), sugges suggestin ting g involve involveme ment nt of Protero Proterozoi zoic c crust crust or detritu detrituss deriv derived ed from from such such crust crust in the the gene genesis sis of at least least some some of the the PMC PMC rocks. rocks. Nd isotope isotope data data availab available le for one of these these sample sampless (NWS 203) are in agreement with this interpretation. The Nd isotope tope signatu signature re of NWS NWS 373 also indicat indicates es a prolong prolonged ed crustal crustal history.

7.2. SHRIMP U–Pb zircon dating SHRIMP U–Pb zircon dating was undertaken on sample NWS 2, a fine-gr fine-grain ained ed garnet garnet-bea -bearin ring g biotite biotite schist. schist. The zircons zircons vary vary in size from 50 to 250 lm and contain a few inclusions. Cathodoluminescence imaging shows oscillatory zoned grains and some with distinctiv tinctive e cores. cores. U conten contents ts are variabl variable e (85–217 (85–2175 5 ppm) ppm) and Th/U is relat relativ ively ely high high (0.48 (0.48–2 –2.3 .32). 2). Comm Common on Pb cont conten ents ts are low (<0.34%), apart from one analysis at 1.96%. Fifteen core and three rim analy analyses ses yield yielded ed 204Pb-corrected 206Pb/238U age ages from rom 120 120.7 ± 1.7 1.7 Ma to 67.5 67.5 ± 0.9 0.9 Ma (Fig ( Fig.. 8a). a). One One analy analysis sis from from an inherit inh erited ed core yielded yielded an age of 797.4 797.4 ± 9.9 Ma. Ma. We interpr interpret et the zircons to be igneous igneous grains that were subsequently subsequently reworked into the schist protolith.

7.3. U–Th–Pb monazite dating

145









146

T. van Leeuwen et al. / Journal of Asian Earth Sciences 115 (2016) 133–152 Table 2

Summary results for the 40 Ar/39Ar analysis of seven PMC samples. Sample

Rock type

Material

Plateau age (Ma)

±

Best est est estimat imate e age (Ma)

NWS NWS 602 602

Meta Meta monzonite

Biotite

7.00

0.40

3.2

Biotite Biotite Biotite

3.10 3.28 3.10

0.10 0.03 0.10

K-feldspar

2.40

0.10

K-feldspar K-feldspar

2 .6 .60 2 .5 .50

0.10 0.10

NWS NWS 604 604

2.5

NWS 605

Mica schist

Muscovite Muscovite Muscovite Muscovite

2.00 2.00 2. 2.30 2.00

0.20 0.60 0.20 0.10

2.0

NWS 606

Mica gneiss

Muscovite Muscovite Muscovite

2.50 2. 2.40 2. 2.50

0.10 0.10 0.10

2.5

NWS NWS 607 607

Feldsp Feldspar ar gneiss

K-feldspar

3.90

0.10

3.8

K-feldspar K-feldspar K-feldspar

3 .2 .20 3 .7 .70 3 .8 .80

0.10 0.20 0.10

Biotite Biotite Biotite Biotite

4.50 3.50 3.40 5.30

0.30 0.20 0.20 0.30

Biotite

3.40

0.20

Biotite

4.11

0.04

Hornblende Hornblende Hornblende

6.10 4.20 5.50

0.50 0.10 0.50

NWS NWS 610 610

Top – histo histogra gram m relate related d to the53 mona monazit zite e ages ages determ determine ined d with with the the EMP. EMP. A Fig. Fig. 9. Top curve was fitted to the age columns considering the two populations with average ages ages of 19.8± 19.8± 4.7 4.7 (2 r) Ma and and 41.8 41.8 ± 4.9 4.9 (2r) Ma (see text). text). The back-scat back-scattered tered

Feldsp Feldspar ar gneiss

Biotit Biotite e gneiss

3.5










147









148


parentage, parentage, to be remnants of Meso-Tethys Meso-Tethys crust to which the West Sulawesi block was attached when it separated from Gondwana. The schist unit is considerably younger than the gneiss unit, as indicated by the presence of a distinct Cretaceous zircon population in sample sample NW 2, Cretace Cretaceous ous–Pa –Paleog leogene ene zircons zircons in samples samples NWS 207 and 1026, and a Late Eocene population obtained from a schist sample by Hennig et al. (in preparation). The Cretaceous zircon zircon popu populat latio ion n in NWS NWS 2 is simila similarr to that that obtain obtained ed from from a comcomposite greywacke sample, NWS 1075 (Fig. (Fig. 8b), 8b), collected from the Latimojong Latimojong Format Formation ion about 20 km southwest southwest of Palu Palu (Fig. ( Fig. 2). 2). Neither sample contains younger zircons. This suggests that the schist unit is in part the metamorphosed equivalent of the Latimojong Form Format atio ion, n, as prev previo ious usly ly prop propos osed ed by Kadaru Kadarusm sman an an and d Sopaheluwak Sopahe luwakan an (1995) and and van Leeuwen and Muh Muhardjo ardjo (2005). (2005). Litho Litholo logi gical cal simila similarit rity y betw betwee een n schist schistss expo exposed sed along along the the Towalae Towalae-To -Tobali bali road road and low-gr low-grade ade metam metamorp orphic hic Latimo Latimojon jong g rocks occurring further west (well-bedded pelitic and psammitic protoliths) supports this interpretation. The rocks form part of an Upper Cretaceous turbidite sequence sequence found throughout throughout Western Western Sulawes Sulawesi, i, interp interprete reted d to have have been deposited deposited in a deep deep marine marine fore-arc basin setting associated with a westerly dipping subduction zone zone beneat beneath h the Sundal Sundaland and margin margin (e.g. (e.g. van Leeuw Leeuwen, en, 1981;; Cal 1981 Calver vert, t, 200 2000 0). The The Creta Cretaceo ceous us zirco zircons ns may, may, in part, part, be derived from a magmatic belt that developed further to the west over over the subductio subduction n zone, zone, the remnant remnantss of which which are found in the Meratus, Meratus, giving giving igneou igneouss K/Ar K/Ar ages ages of 110–67 110–67 Ma ( Hartono, 2012). 2012 ). The belt probabl probably y continu continued ed in a northe northerly rly directio direction, n, as sugge suggeste sted d by the the prese presenc nce e of volca volcani nic c inte interca rcalat latio ions ns in the the Latim Latimojo ojong ng Form Formati ation on ( Br Brou ouwe wer, r, 19 1934 34;; va van n Le Leeu euwe wen n an and d Muhardjo, Muh ardjo, 2005 2005). ). The The prese presenc nce e of Paleo Paleoge gene ne zircon zirconss in some some samples samples implies implies that that rocks rocks of the Tinombo Tinombo Format Formation ion are also incorporated in the schist unit. The granulites may have more than one origin. Some are spa-

stratigraphy of the East Java Sea basement (i.e. the southern part of the block) and the Arafura Shelf (Granath ( Granath et al., 2011). 2011).

8.2. When did the West Sulawesi block arrive at the Sundaland margin? The The arriv arrival al time time of the the West West Sulaw Sulawesi esi block block is poor poorly ly conconstrained. strained. Greywackes Greywackes and sandstones sandstones of the Tinombo Formation adjacent adjacent to the PMC PMC contain contain abundant abundant K-feldsp K-feldspar ar and biotite biotite together with granitic and metamorphic fragments, which are virtually tually absent absent in Tinomb Tinombo o sedime sedimenta ntary ry rocks rocks exposed exposed further further to the north (van (van Leeuwen and Muhardjo, 2005). 2005 ). Furthermore, the gneiss unit is intruded by dykes and stocks, which based on their composition and age appear to be co-magmatic with the Tinombo volcanic rocks (van (van Leeuwen and Muh Muhardjo, ardjo, 2005). 2005). These These feature featuress indicate the fragment had arrived prior to the Middle Eocene. A strong angular unconformity on seismic sections in the East Java Sea–south Sea–south Makassar Makassar Straits Straits region region is interpr interpreted eted by Granath et al. (2 (201 011) 1) to mark mark the the docki docking ng of a cont contin inen enta tall fragm fragmen entt (inclu (includding crust crust underly underlying ing this this region region)) with with the Sundala Sundaland nd margin margin around around 100 Ma. We We believe believe that that the West West Sulawesi Sulawesi block block has a different origin and tectonic history from this fragment and suggests that that its arrival arrival time time is likely to be differen differentt too, but not earlier earlier than 100 Ma given its outboa outboard rd position position in relation relation to the East Java Sea–Makassar Straits crust. We further propose that the dockingexhumation process had been completed by the end of the Cretaceous based on the presence of a regional angular unconformity at the top of the Upper Cretaceou Cretaceouss formations formations througho throughout ut Western Western Sulawesi Sulawesi (e.g. van (e.g. van Leeuwen and Muhardjo, 2005). 2005 ).

8.3. Metamorphic events Geochronology and other evidence indicate that the PMC rocks












suggests that this may have taken place before the separation of the West Sulawesi block from Gondwana. Triassic successions in other other eastern eastern Indone Indonesian sian terrane terraness of Gondwa Gondwana na origin, origin, like the Bird Bird’s ’s Head Head and and Bang Bangga gai-S i-Sul ula, a, are are not not metam metamor orph phos osed ed (e.g. (e.g. Gunawan Gun awan et al., 2012; Garrard et al., 1988), 1988), so it is more likely that the first metamo metamorph rphism ism was during during accretio accretion n of the West West Sulawe Sulawesi si block onto Sundaland and its subsequent uplift. Andalusite Andalusite and cordierite cordierite porphyrobla porphyroblasts sts containing containing biotite and quart quartz z grain grainss that that are align aligned ed in direct direction ionss obliq oblique ue to the the domi domina nant nt folia foliatio tion n in the the host host rock, rock, or folia foliatio tion n over overgr grow owin ing g relic relictt garn garnet et and and K-felds K-feldspar par observe observed d in some some schist schist samples samples (Egeler, ( Egeler, 1946, 1946, this this study) also indicate more than one regional metamorphic event. Our Our Tert Tertiar iary y mona monazit zite e ages, ages, which which show two peaks peaks (42 ± 5 Ma and 20 ± 5 Ma), Ma), may provide provide a clue as to the timing timing of the earlie earlierr event(s). The older peak represents an event that preceded garnet formation as indicated by high Y contents and low La/Gd ratios in the associat associated ed monazit monazite e popula population tion.. The younge youngerr peak peak can be explained explained by a melting melting event in the presence of garnet (low Y contents in monazite). Interestingly, the younger ages are similar to Sm–Nd Sm–Nd ages of 27–20 Ma obtained from a PMC garnet garnet peridotite sample (Kadaru (Kadarusman sman et al., 2002 2002), ), an 40Ar/39A e of 22 ± 3 M

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8–4.5 8–4.5 Myr Myr and of 15–20 C/Ma for the final stage of exhumation, and and exhum exhumati ation on rates rates of 0.75– 0.75–1.0 1.0 mm/yr mm/yr and and 0.9– 0.9–4.0 4.0 mm/yr mm/yr based based on several several geochr geochrono onomet meters ers applied applied to two granit granitoid oid samples samples from from the Neck. The exhumation exhumation estimates estimates indicate that at least c. 2 km of upper crust has been removed since the mid-Pliocene ( Hennig et al., 2014). 2014). Some of the (U)HP rocks may have experienced faster exhumation rates rates than the bulk bulk of the the PMC rocks rocks as sugges suggested ted by granul granulite ite sample NWS 373 and peridotite sample RT5. The granulite is most likely likely a partial partial melt melt residue residue given given its alumino aluminous us compos compositio ition, n, resulting in an abundance of kyanite and garnet, and absence of orthopyroxen orthopyroxene. e. The young U/Pb zircon age (3.8 Ma) obtained from this this sample sample reflects reflects the zircons zircons’’ formatio formation n age under under high high P–T conconditions ditions,, and not a reset reset age because: because: (1) the sample sample contain containss a single population of strongly recrystallized zircons (equant crystal shapes, shapes, no inclusi inclusions ons)) and no inh inherit erited ed zircons zircons.. And And (2) the the zircons zircons exhibit exhibit flat HREE HREE pattern patterns, s, while while usually usually zircons zircons are HREE HREE enriche enriched d because of the high distribution distribution coefficients for zircon/bulk. Flat patterns are found only where zircon has co-crystallized with garnet, which is the only mineral mineral with a higher mineral/bulk mineral/bulk distribudistribution (Rubatto, (Rubatto, 2002). 2002). This implies that zircon grew in equilibrium 










150

Banda Sea region (Spa ( Spakma kman n and Hal Hall, l, 201 2010 0.) Hot asthenosphere asthenosphere started started to ascend ascend into into the sub-con sub-contin tinent ental al lith lithosph osphere, ere, as sugsuggested by tomographic modelling, which shows hot mantle material beneath beneath Wester Western n Sulawes Sulawesii (W. Spakma Spakman, n, in Meere Meeren, n, 2009 2009), ), and and a heating event recorded in some of the Palu granulites ( Helmers et al., 1990; Meeren, 2009; 2009; H.-J.M., H.-J.M., unpubl. unpubl. data). Decompressiona Decompressionall heating of the subcontinental mantle, which had been metasomatized by previous subduction, initially caused partially melting of the the mant mantle. le. The The melt melt ascen ascende ded d into into the the lowe lowerr crust crust and and was was tapp tapped ed by deep-seated faults, giving rise to shoshonitic to ultrapotassic ultrapotassic magmatism (van (van Leeuwen and Muh Muhardjo, ardjo, 2005). 2005). Peridot Peridotite ite was transported transported from depths of 65 km (or more) more) and incorpo incorporate rated d in the continental crust (Meeren ( Meeren and van Roermund, 2009). 2009 ). About 4–6 Ma later the lower crust, consisting predominan predominantly tly of metaigneous rocks, also began to melt partially, as a result of heat provided vided by continu continued ed astheno asthenosph spheric eric upwellin upwelling g and ascendi ascending ng mantle-derived magma. It generated voluminous felsic magma of high-K calc-alkaline composition (Elburg ( Elburg and Foden, 1999; Elburg et al., 2003). 2003). Felsic Felsic magmat magmatism ism contin continued ued in the latest latest Miocene Miocene to midmidPliocene (Elbu (Elburg rg et al., 2003) 2003) and was anied anied by ional ional 

proto-Sula Spur or related continental fragment. Rocks belonging to this fragment, that was thrust beneath the West Sulawesi block, are exposed as tectonic slices in the PMC. The results of our study of the PMC represent only a few pieces of what appears to be a highly complex puzzle. We hope it will stimula stimulate te further further research research,, which which may lead to a better better underst understand and-ing of the tectonic evolution of the PMC.

Acknowledgements We thank Laura Webb for her extensive review, which greatly improved improved this manuscript, manuscript, John Carlile and Andrew Andrew Cuthbertson Cuthbertson for editing the near-final version, and Supriyadi for creating most of the figures. We gratefully acknowledge the 40Ar/39Ar analyses provided provided by Paulo Vasconcelos Vasconcelos of The University University of Queensland. Queensland. MAE is grateful for the access granted to the Adelaide University isotope lab by John Foden, and the analytical assistance by David Bruce.

Appendix A. Supplementary material









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