j Mg 96 Miyazaki Banti Mala

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  • 7/29/2019 j Mg 96 Miyazaki Banti Mala


    J. metamorphic Geol., 1996, 14, 549563

    Pressuretemperature conditions and retrograde paths of eclogites,garnetglaucophane rocks and schists from South Sulawesi, Indonesia

    K . M I YA Z A K I ,1 I . Z U L K AR N A I N ,2 J . S O P A H E L U WA K A N 2 A N D K . WA K I TA 1

    1Geological Survey of Japan, 11-3 Higashi, Tsukuba, Ibaraki 305, Japan2Research and Development Centre for Geotechnology, Jl. Cisitu, 21/154D, Bandung, 40135 Indonesia

    A B S T R A C T High-pressure metamorphic rocks exposed in the Bantimala area, c. 40 km north-east of Ujung Pandang,were formed as a Cretaceous subduction complex with fault-bounded slices of melange, chert, basalt,turbidite, shallow-marine sedimentary rocks and ultrabasic rocks. Eclogites, garnetglaucophane rocksand schists of the Bantimala complex have estimated peak temperatures of T=580630 C at 18 kbarand T=590640 C at 24 kbar, using the garnetclinopyroxene geothermometer. The garnetomphacite

    phengite equilibrium is used to estimate pressures. The distribution coefficient KD1=[(Xpyr)3


    ( Xdi



    ]3 among omphacite, garnet and phengite is a good index for metamor-phic pressures. The K

    D1values of the Bantimala eclogites were compared with those of eclogites with

    reliable PTestimates. This comparison suggests that peak pressures of the Bantimala eclogites were P=1824 kbar at T=580640 C. These results are consistent with the PTrange calculated using garnetrutileepidotequartz and lawsoniteomphaciteglaucophaneepidote equilibria.

    The estimated PTconditions indicate that these metamorphic rocks were subducted to c. 6585 kmdepth, and that the overall geothermal gradient was c. 8 C km1. This low geothermal gradient can beexplained with a high subduction rate of a cold oceanic plate. The retrograde paths of eclogite andgarnetglaucophane rocks suggest that these units were refrigerated during exhumation, consistent withdecoupling of the high-P rocks and ascent due to buoyancy force during continued underflow of the coldoceanic plate.

    Key words: eclogite; high-pressure metamorphism; Indonesia; PTconditions; retrograde metamorphism.

    plate subducted toward the West KalimantanI N T R O D U C T I O N

    Continent.A Cretaceous subduction complex, the BantimalaComplex, is exposed in the Bantimala area, east of

    G E O L O G I C A L S E T T I N GPankajene, South Sulawesi (Figs 1 and 2). It is madeup of fault-bounded slices of Cretaceous accretionary Cretaceous subduction complexes of Indonesia are

    distributed in West and Central Java, Southsediments, ultrabasic rocks and Cretaceous high-pressure metamorphic rocks (Sukamto, 1986). Wakita Kalimantan, and South Sulawesi (Fig. 1). Before the

    opening of the Makassar Strait, the Bantimalaet al. (1994, 1996) presented the following scenario ofthe evolution of the Bantimala Complex. The high- Complex constituted a single subduction complex

    with the subduction complexes in Java and Southpressure metamorphic rocks were formed in the LateJurassic or earliest Cretaceous by subduction of an Kalimantan (Hamilton, 1979). Cretaceous plutons

    occur in West Kalimantan and the basement of theoceanic plate toward the West Kalimantan Continent.Subduction ceased in the Albian, and the high- western Java Sea (Hamilton, 1979). The eastern and

    southern arms of the Sulawesi subduction complexpressure metamorphic rocks were exhumed beforeand during the deposition of middle Cretaceous are underlain by a Tertiary complex consisting mainly

    of high-pressure metamorphic rocks and ophiolitesradiolarian chert.This paper describes the occurrence, mineral assem- (Parkinson, 1991). These rocks are structurally

    overlain by the BanggaiSula continental fragmentsblages, mineral chemistry, peak pressure and tempera-ture conditions, and retrograde metamorphism of (Hartono, 1990), as a result of eastward-directed

    subduction.eclogites, garnetglaucophane rocks and schists of theBantimala Complex. These results contribute to anunderstanding of the evolution of the palaeo-oceanic

    Correspondence: Kazuhiro Miyazaki (email: g8707@gsj.go.jp)


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    550 K . M I YA Z A K I E T A L .

    Fig. 1. Tectonic map of the Indonesian region (modified from Wakita et al., 1994).

    The lawsonite-bearing and hematite-bearing glauco-O U T L I N E O F T H E G E O L O G Y O F T H E

    phane schists are repectively interlayered with lawson-B A N T I M A L A C O M P L E X

    ite-bearing chloritemica schists or albiteactinolitechlorite schists. The garnetglaucophane schists areThe Bantimala area is located about 40 km north-east

    of Ujung Pandang, South Sulawesi (Fig. 2). The interlayered with garnetchloritoidglaucophanequartz schists or garnetglaucophanequartz schistsdetailed geology of this area was investigated by

    Sukamto (1975, 1978, 1982, 1986). The Bantimala (Fig. 3). All three types of glaucophane schists are infault contact with each other. Eclogite and garnetComplex is about 10 km wide in the Bantimala area;

    it is surrounded by Tertiary and Quaternary sedimen- glaucophane rock occur as tectonic blocks withinsheared serpentinite (Figs 4 and 5). KAr ages oftary and volcanic rocks, and unconformably covered

    by Late Cretaceous to Palaeocene sedimentary rocks. phengite from these rocks (Wakita et al., 1994, 1996)

    are as follows: garnetglaucophane rocks (1327,The complex is intruded by Palaeogene diorite.The Bantimala Complex is composed of tectonic 1136 Ma); mica-rich part intercalated with garnet

    glaucophane rock (1246 Ma); and micaquartzslices of high-pressure metamorphic rocks, sedimentaryrocks and ultrabasic rocks (Fig. 2). The boundary schists intercalated with hematite-bearing glaucophane

    schists (1146, 1156 Ma).faults were active before the Palaeocene, and some ofthem were partly reactivated in Cenozoic time. The The sedimentary rocks are identified as melange,

    turbidite and shallow-marine clastic rocks. Melangesmetamorphic rocks in the Bantimala Complex consistof glaucophane schist, albiteactinolitechlorite include clasts and blocks of sandstone, siliceous shale,

    chert, basalt and schist in a sheared shale matrix. Aschist, chloritemica schist, garnetglaucophanequartz schist, garnetchloritoidglaucophanequartz radiolarian assemblage from chert is assigned a middle

    Cretaceous (late Albianearly Cenomanian) age, andschist, serpentinite, garnetglaucophane rock and eclo-gite. Predominant lithologies are glaucophane schists the chert unconformably overlies the high-pressure

    metamorphic rocks (Wakita et al., 1996).that are divided into three types: very fine-grainedlawsonite-bearing glaucophane schist; hematite-bearing The ultrabasic rocks are mostly serpentinized peri-

    dotite, locally including chromite lenses.glaucophane schist; and garnetglaucophane schist.

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    P- T C O N D I T I O NS , S U L A WE S I 551

    Fig. 2. Simplified geological map of theBantimala Complex, South Sulawesi(modified from Sukamto, 1986).

    epidote, phengite, rutile, quartz and, in very rare cases,chloritoid. The matrix contains subordinate amountsof epidote, phengite, rutile and quartz. Idioblasticglaucophane occurs rarely in the matrix. Magnesio-hornblende occurs as a matrix mineral in one sample(P-04).

    The garnetglaucophane rocks are characterized bymodally abundant glaucophane. Garnet porphyrob-lasts (up to 5 mm) are set in a matrix of glaucophane(0.20.75 mm), containing subordinate amounts ofepidote, omphacite, phengite, rutile and quartz. In veryrare cases, the matrix contains no omphacite.

    The mineral paragenesis of the eclogites and garnet

    glaucophane rocks are as follows (abbreviations afterKretz, 1983): eclogites, Omp+Grt+Ep+Phengite+

    Fig. 3. Outcrop of garnetglaucophane schist (dark coloured) Qtz+Rt; Omp+Grt+Gln+Ep+Phengite+Qtz+Rt;intercalated with garnetchloritoidglaucophanequartz schist Omp+Grt+Gln+Hbl+Ep+Phengite+Rt; and( light coloured). This outcrop occurs along the Cempaga

    Omp+Grt+Ep+Phengite+Rt; and garnetglauco-River.phane rocks, Gln+Grt+Omp+Ep+Phengite+Qtz+Rt; Gln+Grt+Ep+Phengite+Qtz+Rt.

    P ET R OG R AP H YGarnetglaucophane schists and their associated rocks

    Eclogites and garnetglaucophane rocksGarnetglaucophane schists show distinct schistosityand compositional banding of garnet-rich and garnet-The eclogites are made up essentially of garnet

    porphyroblasts (up to 1 cm) set in a matrix of a fine- poor layers. The euhedral garnet ranges from 0.1to 1 mm. The matrix consists of glaucophanegrained omphacite (0.010.05 mm). Garnet porphyro-

    blasts have inclusions of omphacite, glaucophane, (0.10.75 mm), epidote, phengite and quartz with

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    552 K . M I YA Z A K I E T A L .

    Fig. 4. Geological map along the CempagaRiver. This figure shows occurrence ofeclogites, garnetglaucophane rocks andschists.

    R E T R O G R A D E M I N E R A L P A R A G E N E S I S

    Some of the eclogites, garnetglaucophane rocks andschists underwent variable degrees of retrograde meta-morphism. In general, the garnetglaucophane schistssuffered more extensive retrograde metamorphism thanthe other rock types.

    Chlorite and lawsonite are found in some eclogitesand garnetglaucophane rocks. In sample P-04 (eclo-gite), these phases occur in particular domains showingwell-developed chlorite aggregates and coarse-grained lawsonite patches (1 2 mm). Outside thesedomains the mineral assemblage is garnet+epidote+omphacite+hornblende+glaucophane+rutile(Fig. 6a). Lawsonite has inclusions of omphacite,garnet, epidote, glaucophane, h

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